Transcript
1)
Why ELCB cannot work if Neutral input of ELCB does not connect to ground? ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that same current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current. What is the difference between MCB & MCCB, Where it can be used?
2)
MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. Normally it is used where normal current is less than 100A. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A. Why in a three pin plug the earth pin is thicker and longer than the other pins?
3)
It depends upon R=ρL/A where area (A) is inversely proportional to resistance (R), so if area (A) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument. Why Delta Star Transformers are used for Lighting Loads?
4)
For lighting loads, neutral conductor is must and hence the secondary must be star winding and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary So delta / star transformer is used for lighting loads. What are the advantages of star-delta starter with induction motor?
5)
The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced. What is meant by regenerative braking?
6)
When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as ―Regenerative Breaking‖.
7) When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?
No. We cannot sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay. If one lamp connects between two phases it will glow or not?
8)
If the voltage between the two phases is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp. What are HRC fuses and where it is used?
9)
HRC stand for ―high rupturing capacity‖ fuse and it is used in distribution system for electrical transformers
10) Mention the methods for starting an induction motor?
The different methods of starting an induction motor DOL:direct online starter Star delta starter Auto transformer starter Resistance starter Series reactor starter
11) What is the difference between earth resistance and earth electrode resistance?
Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
The resistance of the electrode has the following components: (A) the resistance of the metal and that of the connection to it. (B) The contact resistance of the surrounding earth to the electrode.
12) Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?
4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation. For example: A pressure instrument gives output 4mA to indicate 0 psi up to 20 mA to indicate 100 psi or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.
13) Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?
Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the Bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.
14) What happen if we give 220 volts dc supply to bulb or tube light?
Bulbs or devices for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element
15) What is meant by knee point voltage?
Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.
16) What is reverse power relay?
Reverse Power flow relay are used in generating stations‘ protection. A generating station is supposed to feed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.
17) What will happen if DC supply is given on the primary of a transformer?
Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out
When AC current flow to primary winding it induced alternating flux which also link to secondary winding so secondary current flow in secondary winding according to primary current.Secondary current also induced emf (Back emf) in secondary winding which oppose induced emf of primary winding and thus control primary current also. If DC current apply to Primary winding than alternating flux is not produced so no secondary emf induced in secondary winding so primary current may goes high and burn transformer winding.
18) Different between megger and contact resistance meter?
Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance meter used to measure low resistance like relays, contactors.
19) When we connect the capacitor bank in series?
We connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is considerable voltage drop along the transmission line due to impedance of the line. so in order to bring the voltage at the load terminals within its limits i.e (+ or – %6 )of the rated terminal voltage the capacitor bank is used in series
20) What is Diversity factor in electrical installations?
Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Diversity factor is usually more than one.
21) Why humming sound occurred in HT transmission line?
This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.
22) Why frequency is 50 hz only & why should we maintain the frequency constant?
We can have the frequency at any frequency we like, but then we must also make our own motors, transformers or any other equipment we want to use. We maintain the frequency at 50 Hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are made to operate at these frequency.
23) If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?
As we know the Voltage & current relation for transformer-V1/V2 = I2/I1 We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps. By putting these value on Relation-
540/11000= I2/2334 So,I2 = 114.5 Amps 24) What are the points to be considered for MCB (miniature circuit breaker selection)?
I(L)x1.25=I(MAX) maximum current. Mcb specification is done on maximum current flow in circuit.
25) How can we start-up the 40w tube light with 230v AC/DC without using any choke/Coil?
It is possible by means of Electronic choke. Otherwise it‘s not possible to ionize the particles in tube. Light, with normal voltage.
26) What is “pu” in electrical engineering?
Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).
27) Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off. If Fuse is provided in Neutral and if it is blowout and at the same time Supply is on than due to open or break Neutral Voltage is increase and equipment may be damage.
28) If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.
Each bulb when independently working will have currents (W/V= I) For 200 Watt Bulb current (I200) =200/230=0.8696 A For 100 Watt Bulb current (I100) =100/230=0.4348 A For 60 Watt Bulb current (I60) =60/230=0.2609 A Resistance of each bulb filament is (V/I = R) For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively
Now, when in series, current flowing in all bulbs will be same. The energy released will be I2R Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest. The 60W lamp as it has highest resistance & minimum current requirement. Highest voltage drop across it X I [which is common for all lamps] =s highest power. Note to remember: Lowest power-lamp has highest element resistance. And highest resistance will drop highest voltage drop across it in a Series circuit And highest resistance in a parallel circuit will pass minimum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation
29) How to check Capacitor with use of Multi meter.
Most troubles with Capacitors either open or short. An ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly show very low resistance. A open Capacitor will not show any movement on ohmmeter. A good capacitor will show low resistance initially, and resistance gradually increases. This shows that Capacitor is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To know the value and other parameters, you need better instruments
30) What is the difference between Electronic regulator and ordinary rheostat regulator for fans?
The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator give the power needed for that particular speed .But in case of ordinary rheostat type regulator the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.
31) What will happen when power factor is leading in distribution of power?
If there is high power factor, i.e if the power factor is close to one: Losses in form of heat will be reduced, Cable becomes less bulky and easy to carry, and very cheap to afford. It also reduces over heating of transformers.
32) What the main difference between UPS & inverter?
Uninterrupted power supply is mainly use for short time. Means according to ups VA it gives backup. Ups is also two types: on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But ups start with 12v dc with 7 amps.
but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup 33) Which type of A.C motor is used in the fan?
It is Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.
34) What is the difference between synchronous generator and asynchronous generator?
In simple, synchronous generator supplies‘ both active and reactive power but asynchronous generator (induction generator) supply‘s only active power and observe reactive power for magnetizing. This type of generators is used in windmills.
35) What is the Polarization index value?
Its ratio between insulation resistance (IR)i.e meager value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 .
36) What is Automatic Voltage regulator (AVR)?
AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
37) Difference between a four point starter and three point starters?
The shunt connection in four point starter is provided separately from the line where as in three point starter it is connected with line which is the drawback in three point starter
38) What happens if we connect a capacitor to a generator load?
Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Don‘t connect a capacitor across an alternator while it is picking up or without any other load
39) Why the capacitors work on ac only?
Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.
40) Why the up to dia 70mm² live conductor, the earth cable must be same size but above dia 70mm² live conductor the earth conductor need to be only dia 70mm²?
The current carrying capacity of a cable refers to it carrying a continuous load. An earth cable normally carries no load, and under fault conditions will carry a significant instantaneous current but only for a short time most Regulations define 0.1 to 5 sec before the fuse or breaker trips. Its size therefore is defined by different calculating parameters. The magnitude of earth fault current depends on: (a) the external earth loop impedance of the installation (i.e. beyond the supply terminals) (b) the impedance of the active conductor in fault (c) the impedance of the earth cable. i.e. Fault current = voltage / a + b + c Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth loop impedance (a); when It is quite large its impedance can be ignored. At this point there is no merit in increasing the earth cable size i.e. Fault current = voltage / a + c (c) is also very small so the fault current peaks out. The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for continuous full load. In a 3-phase system, If balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables are available with say half-size neutrals (remember a neutral is always necessary to provide single phase voltages). However the increasing use of nonlinear loads which produce harmonics has made this practice dangerous, so for example the current in some standard require full size neutrals. Indeed, in big UPS installations I install double neutrals and earths for this reason.
Electrical Q&A Part-2
1) Why We use of Stones/Gravel in electrical Switch Yard
Reducing Step and Touch potentials during Short Circuit Faults Eliminates the growth of weeds and small plants in the yard Improves yard working condition Protects from fire which cause due to oil spillage from transformer and also protects from wild habitat.
2) What is service factor?
Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10-hp motor has a 1.25 service factor, it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven
above its rated load in this manner, the motor must be supplied with rated voltage and frequency. However a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions Why transmission line 11KV OR 33KV, 66KV not in 10KV 20KV?
3)
The miss concept is Line voltage is in multiple of 11 due to Form Factor. The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is 1.11. The Main reason is something historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor (1.11). Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V. Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise. Now a days when, we have voltage correction systems, power factor improving capacitors, which can boost/correct voltage to desired level, we are using the exact voltages like 400KV in spite of 444KV
4) What is electrical corona?
Corona is the ionization of the nitrogen in the air, caused by an intense electrical field. Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night. Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8″ at 3500 volts).
5) What are the indications of electrical corona?
A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.
6) What damage does corona do?
The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-over.
The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.
7) How long does corona require creating visible damage?
In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey.
8) What voltage are corona rings typically installed at?
It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in the strings.
9) How do we select transformers?
Determine primary voltage and frequency. Determine secondary voltage required. Determine the capacity required in volt-amperes. This is done by multiplying the load current (amperes) by the load voltage (volts) for single phase. For example: if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required. Always select Transformer Larger than Actual Load. This is done for safety purposes and allows for expansion, in case more loads is added at a later date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3) then divide by 1000. Determine whether taps are required. Taps are usually specified on larger transformers.
10) Why Small Distribution Transformers not used for Industrial Applications?
Industrial control equipment demands a momentary overload capacity of three to eight times‘ normal capacity. This is most prevalent in solenoid or magnetic contactor applications where inrush currents can be three to eight times as high as normal sealed or holding currents but still maintain normal voltage at this momentary overloaded condition. Distribution transformers are designed for good regulation up to 100 percent loading, but their output voltage will drop rapidly on momentary overloads of this type making them unsuitable for high inrush applications. Industrial control transformers are designed especially for maintaining a high degree of regulation even at eight time‘s normal load. This results in a larger and generally more expensive transformer.
11) Can 60 Hz transformers be used at higher frequencies?
Transformers can be used at frequencies above 60 Hz up through 400 Hz with no limitations provided nameplate voltages are not exceeded. However, 60 Hz transformers will have less voltage regulation at 400 Hz than 60 Hz.
12) What is meant by regulation in a transformer?
Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage. For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the regulation would be 5%. Distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used.
13) Why is impedance important?
It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer. Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source. Calculate: Normal Full Load Current = Nameplate Volt Amps / Line Volts = 10,000 VA / 480 V = 20.8 Amperes Maximum Short Circuit Amps = Full Load Amps / 4% =20.8 Amps / 4%= 520 Amp The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts. Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 KVA, three phase transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the secondary is short circuited (faulted), the following capacities are required: Normal Full Load Current =Volt Amps / √ 3 x Line Volts= 75,000 VA / √ 3 x Line Volts √ 3 x 480 V =90 Amps Maximum Short Circuit Line Current = Full Load Amps / 5%= 90 Amps / 5% =1,800 Amps The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts. Note: The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is the only winding being interrupted.
14) What causes flash-over?
Flash-over causes are not always easily explained, can be cumulative or stepping stone like, and usually result in an outage and destruction. The first flash-over components are available voltage and the configuration of the energized parts, corona may be present in many areas where the flash-over occurs, and flash-over can be excited by stepping stone defects in the insulating path.
15) What are taps and when are they used?
Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Taps are generally set at two and a half and five percent above and below the rated primary voltage.
16) Can Transformers be reverse connected?
Dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phases can be reverse connected without any adverse effects or loss in KVA capacity. The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio. Example: A transformer with a 480 volt input, 240 volt output— can have the output connected to a 240 volt source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or 480 volt secondary. On transformers rated below 1 KVA single phase, there is a turn‘s ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load. For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be approximately 460 volts at no load and approximately 442 volts at full load. As the KVA becomes smaller, the compensation is greater—resulting in lower output voltages. When one attempts to use these transformers in reverse, the transformer will not be harmed; however, the output voltage will be lower than is indicated by the nameplate.
17) What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers?
Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary windings to attenuate (lessen) transient noise.
18) How many BTU’s of heat does a transformer generate?
The heat a transformer generates is dependent upon the transformer losses. To determine air conditioning requirements multiply the sum of the full load losses (obtained from factory or test report) of all transformers in the room by 3.41 to obtain the BTUs/hour. For example: A transformer with losses of 2000 watts will generate 6820 BTUs/hour.
19) What is a transformer and how does it work?
A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to ―step up‖ or ―step down‖ voltages and works on the magnetic induction principle. A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.
20) Factors Affecting Corona Discharge Effect:
Corona Discharge Effect occurs because of ionization if the atmospheric air surrounding the voltage conductors, so Corona Discharge Effect is affected by the physical state of the atmosphere as well as by the condition of the lines. (1) Conductor: Corona Discharge Effect is considerably affected by the shape, size and surface conditions of the conductor .Corona Discharge Effect decreases with increases in the size (diameter) of the conductor, this effect is less for the conductors having round conductors compared to flat conductors and Corona Discharge Effect is concentrated on that places more where the conductor surface is not smooth. (2) Line Voltage: Corona Discharge effect is not present when the applied line voltages are less. When the Voltage of the system increases (In EHV system) corona Effect will be more. (3) Atmosphere: Breakdown voltage directly proportional to the density of the atmosphere present in between the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather condition So Corona Breakdown voltage occurs at low voltages in the stormy weather condition compared to normal conditions (4)Spacing between the Conductors: Electro static stresses are reduced with increase in the spacing between the conductors. Corona Discharge Effect takes place at much higher voltage when the distance between the power conductors increases.
21) Will a transformer change Three Phases to Single Phase?
A transformer will not act as a phase changing device when attempting to change three phase to single phase. There is no way that a transformer will take three phase in and deliver single phase out while at the same time presenting a balanced load to the three phase supply system. There are, however, circuits available to change three phase to two phase or vice versa using standard dual wound transformers. Please contact the factory for two phase applications.
22) Can 60 Hz transformers be operated at 50 Hz?
Transformers rated below 1 KVA can be used on 50 Hz service. Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due to the higher losses and resultant heat rise. Special designs are required for this service. However, any 50 Hz transformer will operate on a 60 Hz service.
23) Can transformers be used in parallel?
Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other. For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.
24) What are causes of insulator failure?
Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona rings, … This corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid expansion causing fracture or rupture of the material.
25) Causes of Corona
Corona is causes by the following reasons: The natural electric field caused by the flow of electrons in the conductor. Interaction with surrounding air. Poor or no insulation is not a major cause but increases corona. The use of D.C (Direct Current) for transmission.(Reason why most transmission is done in form of AC)
26) Effects of Corona 1) Line Loss – Loss of energy because some energy is used up to cause vibration of the air particles. 2) 3) Long term exposure to these radiations may not be good to health (yet to be proven). Audible Noise
4) 5) 6)
Electromagnetic Interference to telecommunication systems Ozone Gas production Damage to insulation of conductor.
27) What is polarity, when associated with a transformer?
Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below. This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.
28) What is exciting current?
Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR.
29) What is Boucholz relay and the significance of it in to the transformer?
Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.
30) Why we do two types of earthing on transformer (Body earthing & neutral earthing)
The two types of earthing are Familiar as Equipment earthing and system earthing. In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human beings. The System Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted. So that unbalanced voltages will not arise. We can protect the equipment also. With size
of the equipment ( transformer or alternator)and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing. 31) Conductor corona is caused by?
Corona on a conductor can be due to conductor configuration (design) such as diameter too small for the applied voltage will have corona year-around and extreme losses during wet weather, the opposite occurs during dry weather as the corona produces nitric acid which accumulates and destroys the steel reinforcing cable (ACSR) resulting in the line dropping. Road salts and contaminants can also contribute to starting this deterioration.
32) What is flash-over and arcing?
Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc, an arc can have the grid fault current behind it and sustain until the voltage decreases below 50% or until a protective device opens. Flash-over can also occur due to induced voltages in unbounded (loose bolts, washers, etc) power pole or substation hardware, this can create RFI/TVI or radio/TV interference. Arcing can begin at 5 volts on a printed circuit board or as the insulation increases it may require 80kVAC to create flash-over on a good cap and pin insulator.
33) How to Minimizing Corona Effects
Installing corona rings at the end of transmission lines. A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in the vicinity of a terminal of a high voltage device. It is electrically insulated. Stacks of more spaced rings are often used. The role of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing the corona discharge.
34) What is BIL and how does it apply to transformers?
BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown. All transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV. This assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar surge protection devices.
35) The difference between Ground and Neutral?
NEUTRAL is the origin of all current flow. In a poly-phase system, as its phase relationship with all the three phases is the same, (i.e.) as it is not biased towards any one phase, thus remaining neutral, that‘s why it is called neutral. Whereas, GROUND is the EARTH on which we stand. It was perceived to utilize this vast, omnipresent conductor of electricity, in case of fault, so that the fault current returns to the source neutral through this conductor given by nature which is available free of cost. If earth is not used for this purpose, then one has to lay a long. long metallic conductor for the purpose, thus increasing the cost. Ground should never be used as neutral. The protection devices (eg ELCB, RCD etc) work basically on principle that the phase currects are balanced with neutral current. In case you use ground wire as the neutral, these are bound to trip if they are there – and they must be there. at least at substations. And these are kept very sensitive i.e. even minute currents are supposed to trip these. One aspect is safety – when someone touches a neutral, you don‘t want him to be electrocuted – do you? Usually if you see the switches at home are on the phase and not neutral (except at the MCB stage). Any one assumes the once the switch is off, it is safe (the safety is taken care of in 3 wire system, but again most of the fixtures are on 2 wire) – he will be shocked at the accidental touching of wire in case the floating neutral is floating too much.
36) What is impedance of a transformer?
If you mean the percentage impedance of the transformed it means the ratio of the voltage( that if you applied it to one side of the transformer while the other side of the transformer is short cuitcuted, a full load current shall flow in the short circuits side), to the full load current. More the %Z of transformer, more Copper used for winding, increasing cost of the unit. But short circuit levels will reduce, mechanical damages to windings during short circuit shall also reduce. However, cost increases significantly with increase in %Z. Lower %Z means economical designs. But short circuit fault levels shall increase tremendously, damaging the winding & core. The high value of %Z helps to reduce short circuit current but it causes more voltage dip for motor starting and more voltage regulation (% change of voltage variation) from no load to full load.
37) How are transformers sized to operate Three Phase induction type squirrel cage motors?
The minimum transformer KVA rating required to operate a motor is calculated as follows: Minimum Transformer KVA =Running Load Amperes x 1.73x Motor Operating Voltage / 1000 NOTE: If motor is to be started more than once per hour add 20% additional KVA. Care should be exercised in sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is approximately 5 to 7 times the running load
amperage. This severe starting overload will result in a drop of the transformer output voltage. When the voltage is low the torque and the horsepower of the motor will drop proportionately to the square of the voltage. For example: If the voltage were to drop to 70% of nominal, then motor horsepower and torque would drop to 70 % squared or 49% of the motor nameplate rating. If the motor is used for starting a high torque load, the motor may stay at approximately 50% of normal running speed The underlying problem is low voltage at the motor terminals. If the ampere rating of the motor and transformer over current device falls within the motor‘s 50% RPM draw requirements, a problem is likely to develop. The over current device may not open under intermediate motor ampere loading conditions. Overheating of the motor and/or transformer would occur, possibly causing failure of either component. This condition is more pronounced when one transformer is used to power one motor and the running amperes of the motor is in the vicinity of the full load ampere rating of the transformer. The following precautions should be followed: (1)When one transformer is used to operate one motor, the running amperes of the motor should not exceed 65% of the transformer‘s full load ampere rating. (2) If several motors are being operated from one transformer, avoid having all motors start at the same time. If this is impractical, then size the transformer so that the total running current does not exceed 65% of the transformer‘s full load ampere rating.
38) Which Point need to be consider while Neutral Earthing of Transformer?
The following points need to check before going for Neutral Grounding Resistance. Fault current passing through ground, step and touch potential. Capacity of transformer to sustain ground fault current, w.r.t winding, core burning. Relay co-ordination and fault clearing time. Standard practice of limiting earth fault current. In case no data or calculation is possible, go for limiting E/F current to 300A or 500A, depending on sensivity of relay.
39) Why a neutral grounding contactor is needed in diesel generator?
There would not be any current flow in neutral if DG is loaded equally in 3 phases , if there any fault(earth fault or over load) in any one of the phase ,then there will be un balanced load in DG . at that time heavy current flow through the neutral ,it is sensed by CT and trips the DG. so neutral in grounded to give low resistance path to fault current. An electrical system consisting of more than two low voltage Diesel Generator sets intended for parallel operation shall meet the following conditions: (i) Neutral of only one generator needs to be earthed to avoid the flow of zero sequence current. (ii) During independent operation, neutrals of both generators are required in low voltage switchboard to obtain three phases, 4 wire system including phase to neutral voltage. (iii) required to achieve restricted earth fault protection (REF) for both the generators whilst in operation. Solution:
Considering the requirement of earthing neutral of only one generator, a contactor of suitable rating shall be provided in neutral to earth circuit of each generator. This contactor can be termed as ―neutral contactor‖. Neutral contactors shall be interlocked in such a way that only one contactor shall remain closed during parallel operation of generators. During independent operation of any generator its neutral contactor shall be closed. Operation of neutral contactors shall be preferably made automatic using breaker auxiliary contacts.
40) Neutral grounded system vs solidly grounded system
In India, at low voltage level (433V) we MUST do only Solid Earthing of the system neutral. This is by IE Rules 1956, Rule No. 61 (1) (a). Because, if we option for impedance earthing, during an earth fault, there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being determined by the fault current magnitude and the impedance value. This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment, as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too. In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral potential and the high fault current would invariably cause the Over current or short circuit protection device to operate in sufficiently quick time before any harm could be done
Electrical Q&A Part-3
1) What is the reason of grounding or earthing of equipment?
with a ground path, in case of short circuit the short circuit current goes to the body of the equipment & then to the ground through the ground wire. Hence if at the moment of fault if a person touches the equipment body he will not get a shock cause his body resistance (in thousands of ohms) will offer a high resistance path in comparison to the ground wire. Hence the fault current will flow thru the ground wire & not thru human body. Providing a ground path helps in clearing the fault. A CT in the ground connection detects the high value fault current hence the relay connected to the CT gives breaker a trip command. Grounding helps in avoiding arcing faults. IF there would have been no ground then a fault with the outer body can cause a arcing to the ground by breaking the air. This is dangerous both for the equipment & the human beings.
2) A type-C MCB has thermo magnetic capability 5In to 10In that means a short circuit current will be interrupted as the value will reach between 5In to 10In but the MCB breaking capacity is (for example) define as 10kA.
5In to 10In is the pickup threshold for the magnetic trip element. The MCB will trip instantaneously when the current is between these limits. 10kA is the short circuit withstands capacity of the MCB. Under normal condition, a current limiting type MCB will trip on short circuit (magnetic trip) and the current during circuit interruption will be much less than the prospective current. However, the MCBs have to have a short circuit capacity more than or equal to the fault level at the location where it is installed.
3) What is Ferrari Effect?
Ferranti Effect is due to the rise in voltage at the receiving end than that of the sending end. This occurs when the load on the system reduces suddenly. Transmission line usually consists of line inductance, line to earth capacitance and resistance. Resistance can be neglected with respect to the line inductance .When the load on the system falls the energy stored in the capacitance gets discharged. The charging current causes inductive reactance voltage drop. This gets added vector ally to the sending end voltage and hence causes the voltage at the receiving end to raise A Long transmission line draws significant amount of charging current. If such line is open circuited or very lightly loaded at the receiving end, the voltage at the receiving end may become greater than sending end voltage. This effect is known Ferranti effect and is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance is responsible to produce this phenomenon. The capacitance (charging current) is negligible in short lines, but significant in medium and long transmission line. Hence, this phenomenon is applicable for medium and long transmission line. The main impact of this phenomenon is on over voltage protection system, surge protection system, insulation level etc.
4) Can single phase transformers be used for three phase applications?
Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer.
5) What is BIL and how does it apply to transformers?
BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The BIL of a transformer is a method of expressing the voltage surge that a transformer will tolerate without breakdown.
6) Where Auto-recloser is used?
For Generator protection / Transformer Protection / Transmission Line / Bus bar protection. Many faults on overhead transmission lines are transient in nature 90% of faults are used by birds, tree branches. These condition results in arching faults and the arc in the fault can be extinguished by de-energizing the lines by opening of CB on the both ends of the lines. Open-0.3 second-Close-3minute-Close this is the sequence of AR. i.e.-OPEN,CCLOSED whenever faults occurs CB opens, then after 0.3 sec it closes automatically, if faults persists then it will open after 3 min it closes and if still fault persists. It remains in open condition. Auto reclosure is generally used for Transmission lines where the general types of faults are transient in nature. It can be three phase auto-reclosure or single pole auto-reclosure. The single pole auto reclosures are generally for 400kV line below this three pole autoreclosures are used. The reason for a line the single pole reclosures provides a better stability of the system since some part of power is still transferred through the healthy phases. Also 400kV breaker till date has a independent drive/ trip/ close coils for the three poles, below that all breakers have common drive/ trip / closing coils for the three poles.
7) What is difference between power transformers & distribution transformers?
Distribution Transformers are designed for a maximum efficiency at 50% of load. Whereas power transformers are designed to deliver max efficiency ay 90% and above loads. The distributions transformers have low impedance so as to have a better regulation power transformers have higher so as to limit the SC current. Power transformers are used to step up voltages from 11 KV which is the generating voltage to 132 or whatever will be the transmission voltage levels. Power transformers are having Star-Delta connection. It will be located at power generating stations. Distribution transformers are used to step down voltages from transformer levels to 11 KV/415 V. Will be having Delta-Star. It will be located in substations near load centers. The main basic difference lies in the Design stage itself as power transformer are to operate at near full load so there sensing is such that they achieve equal. of copper losses & iron losses at full loads whereas this is achieved in the design itself at about 50% loading in dist transformer but friends there is a dilemma as our dist. transformer are almost fully loaded & beyond so they never go operate at their full eff. & also poor voltage regulation. The difference between power and distribution transformers refers to size & input voltage. Distribution transformers vary between 25 kVA and 10 MVA, with input voltage between 1 and 36 kV. Power transformers are typically units from 5 to 500 MVA, with input voltage above 36 kV. Distribution transformer design to have a max efficiency at a load lower than full load. Power transformer design to have a max efficiency at full load
8) What will be happen if the neutral isolator will be open or close during the running condition of power?
During normal condition the neutral isolating switch should be kept close. In case it is kept open, under balanced load conditions the current through neutral will not flow & nothing harmful will take place but in case an earth fault takes place then there will be no earth fault current flowing through the system & the generator will run as a ungrounded generator. Thus the earth fault will not be cleared. If more number of generators are connected parallel. We will have a close loop and hence negative sequence current will flow. This will increase the rotor temperature. Hence if more number of generators are connected then only one is earthed and others are open. In case of Two or more generators connected to a common bus without a transformer in between, basically in hydro stations, one of the Neutral Isolation Switch(NIS) is kept closed & rest are opened to prevent circulating currents to flow between generators. Hence the above explanation will not be valid for such systems. Sometime we may want to test generator and may want to isolate the neutral from ground. like for example meggaring etc. In such case we would like to open ground connection cable in case we want to remove the NIS? we will certainly not like to open all the bolted connections for just a small test like checking insulation with a meggar etc. for such things we need a NIS. Neutral isolator is required if we have delta transmission system and at the time to connection with the Grid Neutral isolation is required. If we ungrounded the neutral then the generator is connected to the ground via Phase to earth capacitances. Hence during faults arcing grounds can take place. Which are dangerous both to human & equipment. When we provide earthed neutral, for a fault, earth fault current will start flowing through the neutral, which we can sense thru a CT & relay & hence can immediately identify & clear the fault in about 100 ms by opening the associated breaker/prime mover/excitation. Quicker the fault clearance less is the damage.
9) Why shorting type terminal required for CT?
During maintenance or secondary injection you will need to bypass the CT & for the same you need shorting link. During sec. injection you will short circuit the main CT & bypass it. Open circuiting the CT will saturate it & damage it.
10) Why fuse is given for only PT and not CT?
Fuse if given for CT blows off due to a fault then rather than protecting the CT it will make it open circuited hence it will be saturated & damaged. For PT it gives overload & SC protection.
11) Why is insulating base required for LA?
The LA is provided with a dedicated Prper earthing which may be in the form of a buried treated electrode next to it.LA connection is securely made with the electrode via a surge
counter. If we directly earth the LA through structure then the surge counter will not be able to measure the no of surges. For lesser rating the counter is not provided, hence we can bypass the insulated base. But then proper earthing has to be assured. 12) Can 60 Hz transformers be operated at 50 Hz?
Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service.
13) Can transformers be used in parallel?
Single phase transformers can be used in parallel only when their voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers which will cause excess heating and result in a shorter life of the transformer. In addition impedance values of each transformer must be within 7.5% of each other.
14) Can Transformers be reverse connected?
Dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phases can be reverse connected without any adverse effects or loss in KVA capacity.
15) Why short circuit do not take place when electrode is touched to ground.
Basically during welding we force a short-circuit at the electrode tip. The fault condition produces large magnitude currents. Greater the Current value have greater I2R heat produced. The arcing energy elevates the temperature & hence melts the electrode material over the joint. The transformer is designed to withstand such high currents. But welding is a very complex & detailed phenomenon. Besides there are many principles on which welding operates. Some may be a welding, dc welding, arc, constant voltage, constant current etc
16) What’s the difference between generator breaker and simple breaker?
Breaker is one which disconnects the circuit in fault condition and It is similar for all equipment. Based on the equipment voltage and maximum short circuit current the ratings will be decided. For better understanding we call generator or transformer or line etc breakers.
17) What is accuracy Class of the instrument?
Generally the class indicates the accuracy with which the meter will indicate or equipment will measure with respect to its input.
The accuracy of different equipment will depend on number of factors. For example for a PT accuracy will depend on its leakage reactance & winding resistance. For a PT accuracy gives the voltage & phase error & it varies with the VA burden of secondary. Also better core material will give better heat dissipation & reduce error. class of accuracy will give the voltage error for a PT different type of PTs available are:0.1, 0.2, 0.5, 1, 5 & error values will be: class% voltage error(+/_) phase displacement Similarly indicating instruments shall have accuracies & accordingly application as depicted below for testing the following values are generally used: for routine tests : accuracy class 1 for type tests : accuracy class 0.5 or better. indicating meters generally will have accuracy of 1.
18) First pole to clear factor-Circuit breakers
The first pole to clear factor (kpp) is depending on the earthing system of the network. The first pole to clear factor is used to calculating the transient recovery voltage for three phase faults. In general following cases apply:1. kpp = 1.3 corresponds to three phase faults in system with an earthed neutral. 2. kpp = 1.5 corresponds to three phase faults in isolated or resonant earthed system. 3. kpp = 1.0 corresponds to special cases e.g. railway systems. A special case is when there is a three phase fault without involving earth in a system with earthed neutral. This case responds to kpp = 1.5 . This special case is however not normally considered in the standards.
19) Why we use a resistance to ground the neutral when we need always low resistivity for the grounding?
If we ground the generator directly then whenever a fault will take place at any phase with ground the fault current flowing throw the faulted phase-to ground-to neutral will be very high cause there will be no resistance to limit the value of fault current. Hence we insert a resistance in the neutral circuit to limit this fault current. Also we need to reduce the fault current to such a value that the protection CTs are able to identify the fault current without saturating the CTs. Communicate it to the protection relays & hence the relays can then isolate the system from the fault; so that the system is isolated from the fault before the harm is done by the fault current. That is the reason that all the equipment will be designed for fault KA values for 1 sec so that the total operation(CT sensing-relay functioning-circuit breaker operation ) time will be less than 1 sec. hence the Breakers will isolate the fault before 1 sec i.e. within the time period the equipment are designed to carry the fault current. Thus all your objectives of: preventing the arcing. limiting the fault current. isolating the faulted system are achieved
20) Why are NGR’s rated for 10sec?
NGR are placed in the neutral circuit & hence will be energized only in the fault conditions thus their continuous loading is not expected. Hence they are selected for intermittent rating. Similarly when we place a transformer in the neutral grounding circuit the KVA rating obtained after the calculation is multiplied by a diversity factor to obtain smaller rating cause the therefore It will not be continuously rated. NIS is also provided to cut the circulating negative sequence current in 2 more generators connected in parallel. in some grid conditions they ask to keep neutral isolated after being connected to grid.
21) How to calculate knee point voltage and significance of knee point voltage?
Knee point voltage: That point on the magnetizing curve (BH curve) where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current). Its significance lies mainly in PS class core of CTs used for diff protection
22) Design method for neutral grounding resistor?
NGR design basics: Capacitive coupling of generator, equipment and the ground -Generator to ground capacitance. -Generator cable to ground capacitance (or bus duct as the case may be) -Low voltage winding of trafo & ground capacitance. -Surge arrestor capacitance. The total capacitance is then obtained from the above values & then we calculate from that the capacitive reactance. The capacitive current then produced is calculated from the generator voltage & the capacitive reactance obtained above. Once the current is obtained we can then calculate the electrostatic KVA from the current multiplied with voltage.
23) Criterion is there for selection of Insulation Disc in Transmission and Distribution Line.
11kV is the phase to earth voltage for 220kV =220/ (sqrt(3)*11)=12 No‘s of disc are suitable.The number can be increased to increase the creep age distance. While selecting the disc insulators one has to keep in mind the following things: 1. EM-strength of the string. All the forces coming on to the string & the ability of the string to withstand them. 2. Sufficient Cree page distance so as not to cause a flashover . 3. Interface with the type of conductor used (moose, tarantula, zebra etc) So we will get the value of no of discs by dividing the phase to earth voltage with 1.732. Once that is done then we need to see its suitability with respect to EM strength. After this we need to consider the force that the stack has to bear. If we have a strain type of fitting i.e. the stack has to bear horizontal conductor tension, weight load of the conductor, wind load, ice load etc then the number of insulator discs required may be more.
But for a suspension type system which has to bear only the weight then number of discs required may be less than what we get by dividing by 11. That is the reason we have seen only 23/24 discs in 400 kv line cause in that case the creep age obtained must have been enough & also the strain requirement. 33kv insulators are generally used in a vertical installation & are not stacked together because that will make the suspension very rigid
24) Do taps work the same when a transformer is reverse fed?
Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed, the taps are on the output side and can be used to adjust the output voltage.
25) Why may I get the wrong output voltage when installing a step up transformer?
Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H terminals are primary and X terminals secondary. This is true for step down transformers, but in a step up transformer the connections should be reversed. Low voltage primary would connect to X terminals while high voltage secondary would connect on the H terminals.
26) Can a single phase transformer be used on a three phase source?
Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.
27) Why in Double circuit wire are transposed (R – B, Y – Y, B – R)
This is done to avoid 1. Proximity effect 2. Skin effect 3. Radio interference 4. Reduction in noise in communication Signals
28) Selection of LA
The voltage rating of LA is selected as: Line voltage x sqrt(2)/ sqrt(3) so for 11kV line its 9kV In that case also the values would not differ much if We takes the TOV factor as 1.4. However, we can take the value of 1.56 as TOV to be more precise.
29) Which is more dangerous AC or DC
Low frequency (50 – 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death. However any practical distribution system will use voltage levels quite sufficient to ensure a dangerous amount of current will flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard.
30) What all are the applications where high speed grounding switches are used.
Generator neutral is earthed directly or through distribution transformer. This neutral earthing is through done through a switch. This is general practice for only one generator. For two generators in parallel to a bus the neutral earthing is different. If both the neutral earthing is closed the negative sequence current will be flowing though both the generator taking earth as path. This leads to increase in loss and increase in temperature (This may leads to false tripping also). Hence once the second generator is synchronized with the bus or grid the neutral is isolated. Neutral grounding switch we do not need a high speed grounding switch. A normal switch with the correct rating capacity would also work.
31) What is Skin Effect and how does it happen??
According to faradays law of electromagnetic induction, a conductor placed in a changing magnetic field induces an emf. The effect of back emf is maximum at the centre because of maximum lines of field there. Hence the maximum opposition of current at inner side of conductor and minimum opposition at the surface. Hence the current tries to follow at the surface. It is due to this reason that we take hollow tube conductors in bus duct. Taking into account the inductance effect, its simple consider the DC current. Since its constant & not varying hence no back emf but if we gradually start increasing the frequency then the flux cutting the conductor goes on increasing, hence greater the frequency greater the alternating flux cutting the conductor & hence greater the back emf & therefore greater the skin effect.
32) Why we ground the sheath of single core power cables and to avoid grounded at both the ends?
A single core cable with a sheath is nothing but a conductor carrying current surrounded by another conductor (sheath). Hence the Alternative current in the conductor induces voltages in the sheath or the armour. Hence grounding these cables at both ends will cause the potential of the armour to be same as ground potential & hence shall become safe for the personnel. But grounding the cables at both the end will cause a problem. In that case the circulating currents will start flowing with the armor, the ground & with the two ends of the grounding completing the circuit. This will also provide path for the fault currents to
flow. Hence this whole thing will cause the cable to produce some I2R losses, hence heating & hence the current carrying capacity will be de rated. This system of cable earthing is called both-end bonding. This system is suggested only when one wants to avoid the voltage development because can either go with the de rated cable or if one updates the cable in advance. When only one end of the cable sheath is grounded then there is no path for the circulating current to flow. Hence the current carrying capacity of the cable will be good. But in this case potential will be induced between sheath & ground. This potential is proportional to the length of the cable & hence this will limit the length of the cable used. This method is called single point bonding. This is thus used only for short lengths. There is another system called the cross bonding system in which the sheath are sectionaliosed & cross connected so that the circulating currents are minimized. Although some potential will also exist between sheath & ground, the same being maximum at the link boxes where bonding is done. This method provides maximum possible current carrying capacity with the maximum possible lengths.
33) What is EDO & MDO type breaker?
In the Breakers for the operation spring charging is must. In EDO breaker the spring charging is done with a motor and draw out manually by hand. so EDO means Electrically spring charged Draw Out breaker In MDO breaker the spring charging is also done by hand manually and draw out about also by hand only. so MDO means Manual spring charge Draw Out breaker
34) Why transformer rating is in KVA or KW?
Because power factor of the load is not defined in case of transformer that‘s why it is not possible to rate transformer in KW. The losses (cu loss and iron loss) of the transformer depends on current and voltage purely, not on load i.e, phase angle between the current and voltage i.e. why transformer rated in kVA Transformer is not a load and having no effect on P.F (that‘s why no change in its power factor) and it only transfer the constant power from one voltage level to another voltage level without changing frequency. since both the losses viz copper loss(depends on current) and iron loss(depends on voltage) are independent of power factor, that is why a Transformers rating is not on kW, but on KVA
35) Why the secondary of CT never open when burden is connected on the CT.?
secondary of CT is never opened as because CT is always connected to the line so opening the secondary will mean there will be no counter mmf to balance the primary current as a result of which a very high induced emf may appear in the secondary as flux is very high and no counter mmf and this will be dangerous for the personnel in the secondary side and for pt if it is shorted then with full voltage applied to the primary.
If we short the secondary then much high current will circulate in the secondary due to high induced emf much higher than the actual full load current as a result of which the transformer‘s secondary winding may burn out.
36) Distance relay setting
Step1: Get the conductor Details (i.e Positive Sequence Impedance (Z), Zero Sequence Impedance(Z0)) which is in Primary value. Convert in terms of secondary values. Step 2 : Based upon the calculated value divide into various zones Zone 1 (Forward) means 80% of your protected line length. Zone 2 (Forward) means 100% of protected line length + 20% Adjacent Shortest line Zone 3 (Forward) means 100% of protected line length + 50% Adjacent Longest line. Zone 4 (Reverse) means 10% of protected line.
37) Difference between CT class 0.2 and 0.2S?
0.2S & 0.5S: Special type of measurement CTs they guarantee the declared accuracy, even with 20% loading. And some definite error can be defined even with a load as low as 1%. Thus they are suitable for industries where loads are commissioned in steps or stages. Also for tariff metering purposes. 0.2S: Special class for metering. It is more accurate than 0.2 classes. Generally if we use 0.2s class CT than VA burden of core is also come down. In 0.2 classes CT, ratio & phase angle errors must be within the specified limits at 5%, 20%, 100% & 120% of rated secondary current. Whereas in 0.2s class CT, ratio & phase angle errors must be within the specified limits at 1%, 5%, 20%, 100% & 120% of rated secondary current. Also in 0.2s class, Ratio & Phase angle errors limits are lower than 0.2 classes.
38) Why we use inductors
Inductors have the property to oppose sudden changes in Current. When connected to the primary side of transformer, if any sudden short circuit (very high) current flows due to some fault in the system, the inductor will oppose the flow of that current saving the transformer. Secondly, for the problem of lagging current. Capacitors are connected across the inductor to improve the lagging current. So Mainly Inductor is used to (i) protected the transformer, (ii) solved the problem of lagging current.
39) Why do we need a bigger breaker when reverse feeding a transformer?
Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is
reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online. 40) How many types of Neutral grounding system?
There are primarily three types of grounding system which are: (1)Solid grounding – The neutral point of the system is grounded without any resistance. If the ground fault occurs, high ground current passes through the fault. Its use is very common in low voltage system, where line to neutral voltage is used for single phase loads. (2) Low Resistance grounding (LRG) – This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited. The ground fault current is limited to in the rage from 25A to 600A. (3) High Resistance Grounding (HRG) – It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn‘t trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted. The ground fault current is limited to 10A or less. There are other two types such as Corner Grounding (for Delta system) and ungrounded system but they are not commonly used
Electrical Q&A Part-4
1) What value AC meters show, is it the RMS or peak voltage?
AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value when connected to varying DC providing the DC is varying quickly, if the frequency is less than about 10Hz you will see the meter reading fluctuating instead.
2) Why in the transmission tower construction Middle arm is longer than the upper and lower Arm.
Conductor of Upper Arm and Lower Arm will stay apart. To prevent big birds (Ostriches etc) from bumping their heads against the conductor above when they sit on the wire below.
Designed to maintain the mechanical requirement to prevent arching between conductors while maintaining a tower height that is manageable, and of course preventing head injuries to birds The arms are of different links to prevent a broken upper line from falling on one or more of the phase lines below. The clearance from other phase. Mutual inductance minimization. Preventing droplet of water/ice to fall on bottom conductor.
3) What is the difference between Surge Arrester & Lightning Arrestor
LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge. Transmission Line Lightning Protection: The transmission line towers would normally be higher than a substation structure, unless you have a multi-storey structure at your substation. Earth Mats are essential in all substation areas, along with driven earth electrodes (unless in a dry sandy desert site). It is likewise normal to run catenaries‘ (aerial earth conductors) for at least 1kM out from all substation structures. Those earth wires to be properly electrically to each supporting transmission tower, and bonded back to the substation earth system. It is important to have the catenaries‘ earth conductors above the power conductor lines, at a sufficient distance and position that a lightning strike will not hit the power conductors. In some cases it is thus an advantage to have two catenary earth conductors, one each side of the transmission tower as they protect the power lines below in a better manner. In lightning-prone areas it is often necessary to have catenary earthing along the full distance of the transmission line. Without specifics, (and you could not presently give tower pictures in a Post because of a CR4 Server graphics upload problem), specifics would include: Structure Lightning Protection: At the Substation, it is normal to have vertical electrodes bonded to the structure, and projecting up from the highest points of the structure, with the location and number of those electrodes to be sufficient that if a lightning strike arrived, it would always be a vertical earthed electrode which would be struck, rather than any electrical equipment. In some older outdoor substation structures, air-break isolator switches are often at a very high point in the structure, and in those cases small structure extension towers are installed, with electrodes at the tapered peak of those extension towers. The extension towers are normally 600mm square approximately until the extension tower changes shape at the tapered peak, and in some cases project upwards from the general structure 2 to 6 metres, with the electrode some 2 to 3 metres projecting upwards from the top of the extension tower. The substation normally has a Lightning Counter – which registers a strike on the structure or connected to earth conductors, and the gathering of that information (Lightning Days, number per Day/Month/Year, Amperage of each strike)
4) How Corona Discharge Effect Occur in Transmission Line?
In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters. In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision. The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet color appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.
5) Methods to reduce Corona Discharge Effect:
Critical Breakdown voltage can be increased by following factors By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases. By increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved. By using Bundled Conductors: By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors. By Using Corona Rings or Grading Rings: This is of having no greater significance but i presented here to understand the Corona Ring in the Power system. Corona Rings or Grading Rings are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.
6) How to test insulators?
Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to contain an insulator that may shatter, due to steam build-up from moisture in a cavity, arcing produces intense heat, an AM radio is a good
RFI/arcing detection device, a bucket truck AC dielectric test set (130KV) is a good test set for most pin and cap type insulators. A recent article said the DC voltage required to ―search out defects can be 1.9 times the AC voltage. Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-over that are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or other transients happen, stop when the line is de-energized or dropped below 50% of arc ignition voltage. Using a meg-ohm-meter can eliminate defective insulators that will immediately arc-over tripping the test set current overload.
7) How to identify the starting and ending leads of winding in a motor which is having 6 leads in the terminal box
If it is a single speed motor then we have to identify 6 leads. Use IR tester to identify 3 windings and their 6 leads. Then connect any two leads of two winding and apply small voltage across it and measure the current. Then again connect alternate windings of same two windings and apply small amount of voltage (same as before) and measure current. Check in which mode you get the max current and then mark it as a1-a2 & b1-b2. You get max current when a2-b1 will be connected and voltage applied between a1-b2. Follow the same process to identify a1-a2, b1-b2, c1-c2.now we will be able to connect it in delta or star.
8) How to measure Transformer Impedance?
Follow the steps below: (1) Short the secondary side of the transformer with current measuring devices (Ammeter) (2) Apply low voltage in primary side and increase the voltage so that the secondary current is the rated secondary current of the transformer. Measure the primary voltage (V1). (3) Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage impedance of the transformer. When we divide the primary voltage V1 with the full load voltage we will get the short circuit impedance of the transformer with refereed to primary or Z01. For getting the percentage impedance we need to use the formula = Z01*Transformer MVA /(Square of Primary line voltage).
9) Why Bus Couplers are normally 4-Pole. Or When Neutral Isolation is required?
Neutral Isolation is mandatory when you have a Mains Supply Source and a Stand-by Power Supply Source. This is necessary because if you do not have neutral isolation and the neutrals of both the sources are linked, then when only one source is feeding and the other source is OFF, during an earth fault, the potential of the OFF Source‘s Neutral with respect to earth will increase, which might harm any maintenance personnel working on
the OFF source. It is for this reason that PCC Incomers & Bus Couplers are normally 4Pole. (Note that only either the incomer or the bus coupler needs to be 4-pole and not both). 3pole or 4pole switches are used in changing over two independant sources ,where the neutral of one source and the neutral of another source should not mix the examples are electricity board power supply and standalone generator supply etc. the neutral return current from one source should not mix with or return to another source. as a mandatory point the neutral of any transformer etc are to be earthed, similarly the neutral of a generator also has to be earthed. While paralling (under uncontrolled condition) the neutral current between the 2 sources will crises cross and create tripping of anyone source breakers. also as per IEC standard the neutral of a distribution system shall not be earthed more than once. means earthing the neutral further downstream is not correct,
10) Why Three No’s of Current transformer in 3 phase Star point is grounded.
For CT‘s either you use for 3 phase or 2 phase or even if you use only 1 CT‘s for the Over current Protection or for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as what you explain as above but actually it is for the safety of the CT‘s when the current is passing thru the CT‘s. In generally, tripping of Earth faults and Over current Protection has nothing to do with the earthing the neutral of the CT‘s. Even these CT‘s are not Grounded or Earthed, these Over current and the Earth Faults Protection Relay still can operated. Operating of the Over current Protection and the Earth Faults Relays are by the Kirchhoff Law Principle where the total current flowing into the points is equal to the total of current flowing out from the point. Therefore, for the earth faults protection relays operating, it is that, if the total current flowing in to the CT‘s is NOT equal total current flowing back out of the CT‘s then with the differences of the leakage current, the Earth Faults Relays will operated.
11) What is tertiary winding of Transformer?
Providing a tertiary winding for a transformer may be a costly affair. However, there are certain constraints in a system which calls for a tertiary transformer winding especially in the case of considerable harmonic levels in the distribution system. Following is an excerpt from the book ―The J&P Transformer Book‖. Tertiary winding is may be used for any of the following purposes: (A)To limit the fault level on the LV system by subdividing the infeed that is, double secondary transformers. (B)The interconnection of several power systems operating at different supply voltages. (C) The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to the terminals of one winding. It is desirable that a three-phase transformer should have one set of three-phase windings connected in delta thus providing a low-impedance path for third-harmonic currents. The presence of a delta connected winding also allows current to circulate around the delta in
the event of unbalance in the loading between phases, so that this unbalance is reduced and not so greatly fed back through the system. Since the third-order harmonic components in each phase of a three-phase system are in phase, there can be no third-order harmonic voltages between lines. The third-order harmonic component of the magnetising current must thus flow through the neutral of a star-connected winding, where the neutral of the supply and the star-connected winding are both earthed, or around any delta-connected winding. If there is no delta winding on a star/star transformer, or the neutral of the transformer and the supply are not both connected to earth, then line to earth capacitance currents in the supply system lines can supply the necessary harmonic component. If the harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion. Even if the neutral of the supply and the star-connected winding are both earthed, then although the transformer output waveform will be undistorted, the circulating third-order harmonic currents flowing in the neutral can cause interference with telecommunications circuits and other electronic equipment as well as unacceptable heating in any liquid neutral earthing resistors, so this provides an added reason for the use of a delta connected tertiary winding. If the neutral of the star-connected winding is unearthed then, without the use of a delta tertiary, this neutral point can oscillate above and below earth at a voltage equal in magnitude to the third-order harmonic component. Because the use of a delta tertiary prevents this it is sometimes referred to as a stabilizing winding. When specifying a transformer which is to have a tertiary the intending purchaser should ideally provide sufficient information to enable the transformer designer to determine the worst possible external fault currents that may flow in service. This information (which should include the system characteristics and details of the earthing arrangements) together with a knowledge of the impedance values between the various windings, will permit an accurate assessment to be made of the fault currents and of the magnitude of currents that will flow in the tertiary winding. This is far preferable to the purchaser arbitrarily specifying a rating of, say, 33.3%, of that of the main windings.
12) Why do transformers hum?
Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization. The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbour, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a
magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level Why not reduce the noise in the core by reducing the amount of flux? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear. 13) How can we reduce airborne noise?
Put the transformer in a room in which the walls and floor are massive enough to reduce the noise to a person listening on the other side. Noise is usually reduced (attenuated) as it tries to pass through a massive wall. Walls can be of brick, steel, concrete, lead, or most other dense building materials. Put the object inside an enclosure which uses a limp wall technique. This is a method which uses two thin plates separated by viscous (rubbery) material. As the noise hits the inner sheet some of its energy is used up inside the viscous material. The outer sheet should not vibrate. Build a screen wall around the unit. This is cheaper than a full room. It will reduce the noise to those near the wall, but the noise will get over the screen and fall elsewhere (at a lower level). Screens have been made from wood, concrete, brick and with dense bushes (although the latter becomes psychological) Do not make any reflecting surface coincident with half the wave length of the frequency. What does this mean? Well, every frequency has a wave length. To find the wave length in air, for instance, you divide the speed of sound, in air (generally understood as 1130 feet per second) by the frequency. If a noise hits a reflecting surface at these dimensions it will produce what is called a standing wave. Standing waves will cause reverberations (echoes) and an increase in the sound level. If you hit these dimensions and get echoes you should apply absorbent materials to the offending walls (fibreglass, wool, etc.)
14) What is polarity, when associated with a transformer?
Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity. This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.
15) What is exciting current?
Exciting current is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and less to approximately 2% on larger sizes of 750 KVA.
16) Can a three phase transformer be loaded as a single phase transformer?
Yes, but the load cannot exceed the rating per phase and the load must be balanced. (KVA/3 per phase) For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.
17) What are taps and when are they used?
Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Standard tap arrangements are at two-and-one-half and five percent of the rated primary voltage for both high and low voltage conditions. For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating.
18) What is the difference between “Insulating,” “Isolating,”and“Shielded Winding” transformers?
Insulating and isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two. A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise. This is especially important in critical applications such as computers, process controllers and many other microprocessor controlled devices. All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.
19) Can transformers be operated at voltages other than nameplate voltages?
In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated at a voltage in excess of its nameplate rating, unless taps are provided for this purpose. When operating below the rated voltage, the KVA capacity is reduced correspondingly. For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts. If the transformer was originally
rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage. 20) Can a Single Phase Transformer be used on a Three Phase source?
Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.
21) Can Transformers develop Three Phase power from a Single Phase source?
No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phases.
22) Can Single Phase Transformers be used for Three Phase applications?
Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer. For example: Three 10 KVA single phase transformers will accommodate a 30 KVA three phase load
23) Difference between Restricted Earth Fault & Unrestricted Earth Fault protections?
Restricted earth fault is normally given to on star connected end of power equipment like generators, transformers etc. mostly on low voltage side. For REF protection 4 no‘s CTs are using one each on phase and one in neutral. It is working on the principle of balanced currents between phases and neutral. Unrestricted E/F protection working on the principle of comparing the unbalance on the phases only. For REF protection PX class CT are using but for UREF 5P20 Cts using. For Differential Protection CTs using on both side HT & LV side each phase, and comparing the unbalance current for this protection also PX class CTs are using.
24) Can transformers be operated at voltages other than nameplate voltages?
In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly.
25) How many types of cooling system it transformers?
ONAN (oil natural,air natural)
ONAF (oil natural,air forced) OFAF (oil forced,air forced) ODWF (oil direct,water forced) OFAN (oil forced,air natural)
26) What is the function of anti-pumping in circuit breaker?
when breaker is close at one time by close push button, the anti pumping contactor prevent re close the breaker by close push button after if it already close.
27) There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum?
The motor has max load current compare to that of transformer because the motor consumes real power.. and the transformer is only producing the working flux and it‘s not consuming. Hence the load current in the transformer is because of core loss so it is minimum.
28) Where the lighting arrestor should be placed in distribution lines?
Near distribution transformers and out going feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.
29) Why Delta Star Transformers are used for Lighting Loads?
For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.
30) NGR grounded system vs. solidly grounded system
In India, at low voltage level (433V) we must do only Solid Earthing of the system neutral. This is by IE Rules 1956, Rule No. 61 (1) (a).Because, if we have opt for impedance earthing, during an earth fault, there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being determined by the fault current magnitude and the impedance value. This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment, as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too. In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral potential and the high fault current would invariably cause the Over current or short circuit protection device to operate in sufficiently quick time before any harm could be done.
31) Why Do not We Break Neutral in AC Circuits?
Neutral is connected to earth at some point, thus it has some value as a return path in the event of say and equipment earth being faulty. It‘s a bit like asking ‗why don‘t we break the Earth connection‘ It was stupid and dangerous, as it was possible for the neutral fuse to blow; giving the appearance of ‗no power‘ when in fact the equipment was still live.
32) What is Minimum Value of Insulation Resistance / Polarization Index?
Motor Insulation Resistance: The acceptable meg-ohm value = motor KV rating value + 1 (For LV and MV Motor). Example, for a 5 KV motor, the minimum phase to ground (motor body) insulation is 5 + 1 = 6 meg-ohm. Panel Bus Insulation Resistance: The acceptable meg-ohm value = 2 x KV rating of the panel. Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 meg-ohm IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX (min IR at 400C in MΩ)
Minimum Insulation Resistance R1 min = kV+1 R1 min = 100 R1 min = 5
TEST SPECIMEN For most windings made before about 1970, all field windings, and others not described below For most dc armature and ac windings built after about 1970 (form wound coils) For most machines with random -wound stator coils and form-wound coils rated below 1kV
33) What is service factor?
Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10-hp motor has a 1.25 service factor; it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied with rated voltage and frequency. Keep in mind, however, that a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions.
34) Calculate the size the CT on the neutral point of the secondary side of 11/0.415 kV Transformer
For high impedance relays (differential or restricted earth fault relays), ‗Class X‘ current transformers are recommended to be used. Please note that both CTs (neutral & phase) shall have the same characteristics. The following is an example to size the CT:
Input data: 11/0.415 kV ,2500 KVA Power transformer ,Transformer impedance is 6% ,Length of cable from neutral CT to the relay is 200 m ,Cross section of CT cable to be used is 6 mm² -copper and resistance is 0.0032 Ω/m Step 1: Calculation of CT Rated Primary Current I = kVA/ (0.415×1.732) = 2500/ (0.415×1.732) = 3478.11 A, CT with primary current of 4000 A to be selected. Select the secondary current of the CT 1 or 5 A. selecting 1 A secondary current, as the cross section and length of pilot wires can have a significant effect on the required knee voltage of the CT and therefore the size and cost of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the pilot wires. Step 2: Calculation of maximum Fault Current Ift = kVA/ (0.415×1.732x Z) Ift = 2500/ (0.415×1.732×0.06) = 57968.59 A (say 58000 A) Step 3: Calculation of the Knee Voltage of the CT (Vkp) Vkp = (2x Iftx (Rct+Rw)/CT transformation ratio) Where: Rct is the CT resistance (to be given by the manufacturer), Here Rct is1.02 Ω. Rw: total CT cable resistance= 2x cable length (200 m) x wire resistance= 2x200x0.0032= 1.28 Ω CT transformation ratio = CT Primary Current/CT Secondary Current CT transformation ratio = 4000/5= 800 A, for CT with 5 A secondary current; or, CT transformation ratio = 4000/1= 4000 A, for CT with 1 A secondary current. We will use 1 A in this example. Vkp = (2x58000x (1.02+1.28)/4000)= 66.7 V. The Vkp of the CT should be higher than the setting of relay stability voltage (Vs), to ensure stability of the protection during maximum Through fault current. To calculate the stability voltage,we should follow the related formula given by the relay manufacturer, as each relay manufacturer has its own formula. we may calculate the Vkp as above using a CT with secondary current of 5 A, and you will notice the difference in the Vkp.
35) When should we use Molded Case Circuit Breakers and Mini Circuit Breakers?
MCB is Miniature Circuit Breaker, since it is miniature it has limitation for Short Circuit Current and Amp Rating MCB: MCB are available as Singe module and used for :Number of Pole :- 1,2,3,4 – 1+ N , & 3 + N Usually Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63 Amp. Also available 80A, 100A, and 125 Amp. SC are limited 10 KA Applications are as: – Industrial, Commercial and Residential application. Tripping Curve: (1) B Resistive and lighting load, (2) C Motor Load, (3) D Highly inductive load. MCCB: MCCB: – Moulded Case Circuit Breaker.
MCCB are available as Singe module and used for: Number of Pole :- 3 pole , & 4 Pole Current range for A.C: For 3.2 /6.3/12.5/25/50/100/125/160 Amp and Short Circuit Capacity 25/35/65 KA. For 200 250 Amp and Short Circuit Capacity 25/35/65 KA For 400 630/800 Amp and Short Circuit Capacity 50 KA Protection release : Static Trip :- Continuous adjustable overload protection range 50 to 100 % of the rated current Earth fault protection can be add on with adjustable earth fault pick up setting 15 to 80 % of the current. Micro processor Based release: Over load rated current 0.4 to1.0 in steps of o.1 of in trip time at 600 % Ir (sec) 0.2.0.5,1, 1.5 , 2 ,3 Short Circuit :-2 to10 in steps of 1 lr , short time delay (sec) 0.02.0.05,0.1, 0.2 ,0.3 Instantaneous pick up :2 to10 in steps of 1 in Ground fault pick up Disable: 0.2 to 0.8 in steps of 0.1 of in Ground fault delay (sec): 0.1 to 0.4 in steps of 0.1 MCB (Miniature Circuit Breaker) Trip characteristics normally not adjustable, factory set but in case of MCCB (Moulded Case Circuit Breaker) Trip current field adjustable.