Transcript
Introduction
The Subhasgram substation under WBSETCL is situated a few km away from the Subhasgram station on the way to Champahati. It is a 220/132/33 kv substation. It is stretched over 22.59 acres of land. It was commissioned at 18th August 2009. At present the 132 kV feeders are still under construction. Currently it feeds 220 kV to Lakshmikantapur & KLC and 33 kV to Madarhat. The substation has 8 transformers: 160 MVA, 220/132 kv Autotransformer – 2 31.5MVA, 132/33 kv Double Winding Transformer – 2 630KVA, 33/0.4 kv Station Service Transformer – 2 100KVA, 33/0.4 kv Earthing Transformer – 2 Incoming Voltage: - 220 kV Outgoing voltage: - 132 kV & 33 kV Power source: - Nearby Power Grid Corporation of India Limited (PGCIL) 400/220 kV substation. Sub-Station type: - Outdoor Primary Grid Sub-Station
Sub-station
Definition of sub-station: The assembly of apparatus used to change some characteristics of electric supply is called sub-station. Introduction: The present day electrical power is generated, transmitted, and distributed in the form of AC. The electric power is produce at the power station, which are located at favorable places, generally quite away from the consumers. It is delivered to the consumer through a large network of transmission and distribution. At many place in the line of power system, it may be desirable
and necessary to change some characteristic (e.g. Voltage, ac to dc, frequency p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-station for example, generation voltage (11KV or 6.6KV) at the power station is stepped up to high voltage (Say 220KV to 132KV) for transmission of electrical power. Similarly, near the consumer’s localities, the voltage may have to be stepped down to utilization level. Site Selection & Layout 220 KV Substation: 220KVSub-Station forms an important link between Transmission network and Distribution network. It has a vital Influence of reliability of service. Apart from ensuring efficient transmission and distribution of power, the sub-station configuration should be such that it enables easy maintenance of equipment and minimum interruptions in power supply. Sub-station is constructed as near as possible to the load center. The voltage level of power transmission is decided on the quantum of power to be transmitted to the load center. Transmission is decided on the quantum of power to be transmitted to the load center.
Classifications of different types of substations: Substations are classified into the following types according to the Service requirement: Transformer substation Switching substations Frequency Changer substations Power Factor Correction Substations Converting substations Industrial substations.
Substations are classified into the following types according to the Constructional features:o o o o Indoor Substations Outdoor Substations Underground Substations Pole Mounted Substations.
Substations are classified into the following types based on the Purpose served:-
o o o o
Step-Up Substation Primary Grid Substation Secondary Substation Distribution Substation
Transformers & Its Parts
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic
flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling. If a load is connected to the secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:
By appropriate selection of the ratio of turns, a transformer thus enables an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np. The windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.
160 MVA transformer of Subhasgram Substation
Different parts of a transformer:CORE AND WINDING:-The core of the transformer may be of various shapes i.e. core, shell. It is made by cold-rolled grain oriented silicon-steel lamination. Laminated sheets are insulated from each other by applying a thick layer of varnish insulation on the lamination. The core is laminated to reduce eddy current losses. The laminations are made in steps and try to give circular cross section. Bolts and nuts secure the lamination. The core is placed at the bottom of the tank. The tanks are constructed from welded sheet steel for small tanks and boiler sheet steel for large tanks. There are thermometer pockets, radiators tubes for increasing cooling surface. A three phase transformer has six separate windings- three primary and three secondary wounded on iron core. Enameled copper with insulation is used for winding. Insulated papers are used for interlayer insulation. Paper in the form of tape may be used for taping winding leads and other parts. Press boards are used as insulation between windings and cores. Press boards are
also used to separate H.V. windings from L.V. windings input nearer the core. TRANSFORMER OIL:-The tank filled with transformer oil and tank is sealed. It is a mineral oil obtained by refining crude petroleum. It serves the following purposes:
the core and oils. Good transformer oil should have:-
rm slugging under normal operating conditions. It is necessary to check the oil in regular intervals. Bushing: - The bushing is a hollow insulating liner that fits through a hole in a wall or metal case, allowing a conductor to pass along its centre and connect at both ends to other equipment. The purpose of the bushing is to keep the conductor insulated from the surface it is passing through. Bushings are often made of wet-process fired porcelain, and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing.
The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation. Bushings for mediumvoltage and low-voltage apparatus may be made of resins reinforced with paper. The use of polymer bushings for high voltage applications is becoming more common. The largest high-voltage bushings made are usually associated with high-voltage direct-current converters.
Conservator: - To allow room for oil expansion and contraction. The transformer is completely filled with the oil and when it heats up under load or due to ambient temperatures, the oil has to have a place to go. In the event of colder weather or if the transformer is not under heavy load the oil cools and contracts creating a slight vacuum inside the tank. The conservator acts as a reservoir of oil that can then flow back into the tank so that no air enters it. It is connected by piping to the main transformer tank that is completely filled with oil. The conservator also is filled with oil and contains an expandable bladder or diaphragm between the oil and air to prevent air from contacting the oil.
Air enters and exits the space above the bladder/ diaphragm as the oil level in the main tank goes up and down with temperature. Air typically enters and exits through a desiccant-type air dryer that must have the desiccant replaced periodically. The main parts of the system are the expansion tank, bladder or diaphragm, breather, vent valves, liquid-level gauge and alarm switch. Vent valves are used to vent air from the system when filling the unit with oil. A liquid-level gauge indicates the need for adding or removing transformer oil to maintain the proper oil level and permit flexing of the diaphragm.
Conservator Tank
Silica Gel Breather
Silica Gel Breather:- A transformer breather is an accessory of an oil filled type transformer which is attached into the oil conservator tank; this serves as the breathing point of the unit, that when the insulating oil of the transformer gets heated up, it expands and goes back to the conservator tank and subsequently pushes the dry air out of the conservator tank through the breather which is filled with silica gel, when the oil cools down, it retracts and sucks fresh air from the atmosphere through the breather and from this point, the silica gel dries up the air that goes back in to the conservator tank.
Buccholz Relay: -
It is a gas activated relay inside in oil immersed
transformers for protection against all types of fault. Any fault produces heat and in course the evaluation of gases. It mainly consists of two float switches and placed in the connecting pipe between he main tank and the conservator. Under normal condition the main tank and the Buccholz relay is completely filled up with oil and the conservator tank is half full. When the fault occurs, it produces gas which is collected in the container. So the oil level falls and closing the alarm circuit. If no attention is paid to it the gas collection will be more and close another circuit, which will cut out the transformer from the line.
Buccholz Relay PRD (Pressure Release Device):- The pressure relief valve plays a significant role in the protection of Power transformer systems. As mentioned before, a major fault inside the transformer causes instantaneous vaporization of the oil, leading to extremely rapid build-up of gaseous
pressure. If this pressure is not relieved within a few milliseconds, the transformer tank can get ruptured, spilling oil over a wide area. The consequent damage and fire hazard possibilities are obvious. A pressure relief device provides instantaneous relieving of dangerous pressure.
PRD Radiator: - Radiators are used in a transformer to cool the transformer oil through natural air or forced air flowing in these radiator fins. As the transformer oil temperature goes down due to cooling it goes to the transformer tank from bottom, cool the windings and gets heated, and then returns to the radiator for next cooling. This cycle repeats as the oil flow is also natural due difference in temperature of oil on bottom and top. In big power transformers this oil circulation is forced by oil pumps for effective cooling. The radiator has many small fins and there are 4-10 radiator banks in a transformer depending on capacity and make of the transformer.
Transformer radiator
160 MVA TRANSFORMER
31.5 MVA transformer
630 KVA STATION SERVICE TRANSFORMER
100 KVA EARTHING CUM STATION SERVICE TRANSFORMER
Substation Equipments
Lightning Arrester: - A lightning arrester is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal oxide varistors (MOVs) have been used for power system protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth. The lightning arrestor protects the structure from damage by intercepting flashes of lightning and transmitting their current to the ground. Since lightning strikes tend to strike the highest object in the vicinity, the rod is placed at the apex of a tall structure. It is connected to the ground by lowresistance cables. In the case of a building, the soil is used as the ground, and on a ship, water is used. A lightning rod provides a cone of protection, which has a ground radius approximately, equal to its height above the ground.
Lightning Arrester
Isolator: - In electrical engineering is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance. Often the isolation switch is not intended for normal control of the circuit and is used only for isolation; in such a case, it functions as a second, usually physically distant
master switch (wired in series with the primary one) that can independently disable the circuit even if the master switch used in everyday operation is turned on. Isolator switches have provisions for a padlock so that inadvertent operation is not possible (see: Lockout-Tag out). In high voltage or complex systems, these padlocks may be part of a trapped-key interlock system to ensure proper sequence of operation. In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems where both end of the circuit need to be isolated. The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit. Standards in some countries for safety may require either local motor isolators or lockable overloads (which can be padlocked).
Horizontal Center Break Isolator Pantograph Isolator
Current Transformer: - A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need be insulated only for a low voltage. The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite of the primary current. But, as in the voltage transformer, some of the primary current or the primary
ampere-turns are utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be "transformed" into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into two-the current or ratio error and the phase error.
Current Transformer
Capacitor Voltage Transformer: - A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT) is a transformer used in
power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a transformer to isolate and further step down the voltage for the instrumentation or protective relay. The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices. The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay. CVTs are typically singlephase devices used for measuring voltages in excess of one hundred kilovolts where the use of wound primary voltage transformers would be uneconomical. In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2.
The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high frequency communication signals from
power frequency. This forms a carrier communication network throughout the transmission network.
CVT Potential Transformer:-
Potential Transformer
Potential transformers are instrument transformers. They have a large number of primary turns and a few numbers of secondary turns. It is used to control the large value of voltage. The potential transformer works along the same principle of other transformers. It converts voltages from high to low. It will take the thousands of volts behind power transmission systems and step the voltage down to something that meters can handle. These transformers work for
single and three phase systems, and are attached at a point where it is convenient to measure the voltage. Potential Transformer is designed for monitoring single-phase and threephase power line voltages in power metering applications. The primary terminals can be connected either in line-to-line or in line-to-neutral configuration. Fused transformer models are designated by a suffix of "F" for one fuse or "FF" for two fuses. A Potential Transformer is a special type of transformer that allows meters to take readings from electrical service connections with higher voltage (potential) than the meter is normally capable of handling without at potential transformer. Wave Trap: - The wave traps are used to block the high frequency currents with values of frequencies between 50 kHz and 300 kHz, but to allow the power frequency current to pass without losses. The blocking effect depends on the value of inductance in the wave trap. Higher the inductance in the wave trap, the larger the blocking effect will occur. Wave traps are designed as resonant circuit tuned to block the carrier wave. In the wave trap, inductance and condenser are connected in parallel. Arrangement can be made such that a wave trap is able to tune two different earner frequencies. The addition of a second inductance and a second condenser can do this. The wave traps are connected directly in one phase of the power transmission line. It is also essential to design the inductances in such a way that they can carry the current flowing in the line. Wave traps are standardized for currents of 200 ampere. 400 ampere and 700 ampere. The variation in the tuning frequency in the wave trap is possible due to the variable condensers. The inductance unit is made of copper or aluminum windings. The condenser units arc mounted on a moisture proof casing.
Wave Trap Circuit Breaker: - A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery,
although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.
Circuit Breaker
Conductors:Conductor Material Properties
Materials commonly used in conductors are aluminum, copper, and steel. The properties of these common materials fabricated as wires are summarized in Table 1.1. Galvanized steel wires are combined with aluminum in the most common type of overhead conductor -- Aluminum Conductor Steel Reinforced (ACSR). The use of copper is uncommon in modern transmission lines since it weighs and usually costs considerably more than aluminum conductor of the same resistance. Conductor Design & Construction "Standard" bare overhead conductors consist of round strands helically laid about a core in one or more layers. In a
homogeneous conductor - all aluminum conductor (AAC), hard drawn copper conductor (CU), or all aluminum alloy conductors (AAAC5005 or AAAC6201) - the core consists of a single strand identical to the outer strands. Since all the strands are the same diameter, one can show that the innermost layer always consists of 6 strands, the second layer of 12 strands, etc., making conductors having 1, 7, 19, 37, 61, 91, or 128 strands. In a non-homogeneous conductor - aluminum conductor steel reinforced (ACSR), aluminum conductor alum weld steel reinforced (ACSR/AW), or hard drawn copper conductor copper weld steel reinforced (CU/CW), or aluminum conductor aluminum alloy reinforced - the strands in the core may or may not be of the same diameter. In a 30/7 ACSR conductor the aluminum and steel strands are of the same diameter. In a 30/19 ACSR they are not. Within the core or within the outer layers, however, the number of strands always increases by 6 in each succeeding layer. The most common type of transmission conductor is ACSR. ACSR consists of one or more layers of aluminum strands surrounding a core of 1, 7, 19, or 37
galvanized steel strands. Certain strandings are stronger than others. 36/1 ACSR is the weakest stranding (1/37 of the cross-sectional area is steel). 30/7 is the strongest (7/37 of the cross-section is steel). Conductors used in Subhasgram sub-station: DOG (6/7) PANTHER (30/7) DEER (30/7) MOOSE (54/7) ZEBRA (54/7)
Tower
The supporting structures for line conductors are called towers. In general towers have the following properties: 1) High mechanical strength to withstand the weight of conductors and wind loads. 2) Light in weight without the loss of mechanical strength.
3) Cheap in cost. 4) Longer life 5) Easy accessibility of conductors for maintenance. For transmission purpose we only use steel towers. Various types of steel towers: 1) A TYPE tower: It’s a suspension type tower and it has angle of deviation of 0-2degree between the conductors. 2) B TYPE tower: These are tension towers having an angle of deviation of 2 -15 degree. 3) C TYPE tower: These are tension towers having an angle of deviation of 15 -30degree. 4) D TYPE tower: These are also tension type tower having angle of deviation of 30-60 degree.
Transmission Tower
ROW (Right Of Way) ROW or Right Of Way is the minimum permissible distance from an electrical tower where a building can be build.
RELAY
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contractor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".
A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB. When an electric current is passed through the coil it generates a magnetic field that activates the armature and the consequent movement of the movable contact either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing. When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to
semiconductor circuit components. Some automotive relays include a diode inside the relay case. Alternatively, a contact protection network consisting of a capacitor and resistor in series (snubber circuit) may absorb the surge. If the coil is designed to be energized with alternating current (AC), a small copper "shading ring" can be crimped to the end of the solenoid, creating a small out-of-phase current which increases the minimum pull on the armature during the AC cycle. A solid-state relay uses a thyristor or other solid-state switching device, activated by the control signal, to switch the controlled load, instead of a solenoid. An optocoupler (a light-emitting diode (LED) coupled with a photo transistor) can be used to isolate control and controlled circuits There are many types of relays:
Directional Over current Relays Auxiliary relays High Impedance Differential relay Directional relays Numerical relays
COMMUNICATION
In Subhasgram substation, we have basically three types of communication. They are as follows: VHF (Very High Frequency) Communication: The frequency used for this purpose is about 167 megahertz. It is a simple system which does not require plcc. Communication can be done both ways but one at a time i.e. we can either speak or listen at a single 1. Instance. It is used to connect to Madarhat substation. The signal is communicated through 33kv line. 2. Hotline connection through PLCC: We don’t require EPAX in this system since the connection is made as soon as we pick up the receiver end. This type of connection is used for high priority communication. It is implied to connect Subhasgram substation to power grid from where it gets the input power. The connection sequence: phase wire (blue in general) – wave trap- LMU box – Carrier (PLCC) – Receiver. The incoming is done through 220kv line.
3. PLCC connection: It is the general configuration which is used to connect our substation to Lakshmikantapur, Kasba, Bantala substations and the output is conveyed through 220kv line. The outline of the communication system: phase wire (blue in general) – wave trap- LMU box – Carrier (PLCC) - EPAX- Receiver end. The Communication room which consists of Carrier, Epax and other equipments are operated in 48 volt dc. There is a backup room consisting of 24 batteries for backup to the communication room. An example of frequency range for interconnection of our substation to others: Frequency- KASBA- SINGLE CHANNEL TX (P) =143.57 KHz RX (P) =147.57 KHz Where TX represents Transmission end frequency and RX represents receiving end frequency.
CONCLUSION
The vocational training at WBSETCL has been a practical experience for all of us from Subhasgram subdivision. We have been extremely fortunate to have got a scope to learn the various intricate details of power transmission from the engineers and the officers of WBSETCL, Subhasgram. It was a firsthand experience for all of us to see the various transmission methods that are employed at a transmission sub-station. We got to understand how power from various generating station is transmitted to a transmitting substation and the several processes that are undergone before transmitting the power to a distributing sub-station. While consuming power we hardly care about the fact that it is the effort of so many tireless people that we get to enjoy power at our home and other commercial places. We realize that power transmission is not an easy task and that it comes after a lot of hard work and loss of innumerable energy. We also got to see transformers and understand its functioning from very close quarters. We saw the functioning of circuit breakers, isolators and capacitor banks all that we had been only reading in textbooks. It was an enlightening experience to interact with the engineers who has helped to enrich our knowledge. We also learnt about PLCC and the various models of communication that are employed in power plants and sub-stations. In all we had an overall understanding and knowledge of the functions of a transmission substations. We as students feel proud to have been associated with the WBSETCL and hope to strengthen our relationship in the near future. We sincerely thank all the staff of WBSETCL, Subhasgram for making our training a truly enriching and enjoyable. We had a wonderful time in WBSETCL.
Acknowledgement
I would like to thank Mr. Soumen Maity, Divisional Engineer, Mr. Bipul singha, Assistant Engineer & other employees of Subhasgram Sub-station, WBSETCL, for supporting me in every possible manner during my training.
Finally I would also like to thank each and everyone responsible for having made my training days at WBSETCL, Subhasgram a pleasant learning experience.