11 KV Substation.

1 1 11 KV Substation. Pole-Mounted Sub-Station: 1. It is a distribution sub-station placed overhead on a pole. It is the cheapest form of substation as it does not involve any building work. Fig (i) shows the layout of pole-mounted sub-station 2. Whereas Fig (ii) shows the schematic connections. The transformer and other equipment are mounted on H-type pole (or 4-pole structure). 3. The 11 kV line is connected to the transformer (11kV / 400 V) through gang isolator and fuses. 4. The lightning arresters are installed on the side to protect the sub-station from lightning strokes. 5. The transformer steps down the voltage to 400V, 3-phase, 4-wire supply. The voltage between any two lines is 400V whereas the voltage between any line and neutral is 230 V. 6. The oil circuit breaker ( ) installed on the side automatically isolates the transformer from the consumers in the event of any fault. 7. The pole-mounted sub-stations are generally used for transformer capacity up to *200 kVA.

2 8. The following points may be noted about pole-mounted sub-stations : A. There should be periodical check-up of the dielectric strength of oil in the transformer and B. In case of repair of transformer or , both gang isolator and should be shut off. Distribution substation consists of: 1. Pin type insulator 2 2. AB switch 3. Lightning arrester 4. Circuit Breaker 5. Distribution Transformer 6. Earthing 7. DO Fuse 8. Stay Wire 9. MV cable 10. LV cable Fig shows the single line diagram of 11KV/440V substation DP structure. The single line diagram contains the 11KV distribution line, lightning arrester , A B switches, drop out fuse, 11KV/440V transformer, LT CB, etc. First the 11KV supply is coming from State Electricity Board to the DP structure through cable via metering unit at the AB switch then drop out fuse then pin type insulator and then it goes to the transformer HT bushing. In this structure the lightening arrestors are connected at the top.

3 It is used to protect the substation equipment from lightening strokes. AB switch is use to isolate the supply from system. If AB switch is open, then contacts are open and supply could not come to the transformer. Hence, if fault occur at consumer side then by opening the AB switch it can safely repair the fault. But before operating a fault, care should be taken that the line should be discharge properly. AB switch works as an isolator. After that it contains Drop Out fuse. Then supply come to H T bushing of transformer. The transformer is 11KV/440V deltastar connected. A step down transformer which step down the 11KV to 440V to main panel of the college or industry. From this main panel the supply is distributed the main area through cables, MCB and other switches. 3 B. Lightning arrester : A lightning arrester is a device used on Electrical power systems from the damaging effects of Lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal.

4 When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester , the current from the surge is diverted through the arrestor, in most cases to earth. If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the distribution lines, and can also cause severe damage to transformers and other electrical devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical appliances. C. Air Break Switch: A. Pin Type Insula tor : A pin insulator consists of a non - conducting material such as porcelain, glass, plastic, polymer, or wood that is formed into a shape that will isolate a wire from a physical support on a utility pole or other structure, provide a means to hold the insulat or to the pin, and provide a means to secure the conductor to the insulator. By contrast to a Strain insulator, the pin insulator is directly connected to the supporting pole.

5 The pin insulator is designed to secure the conductor to itself. 4 An air break switch disconnectors are the vital part of any overhead line network, providing crucial points of isolation. Most overhead line network designed so that when a fault occurs or maintenance work needs to be carried out it is relatively simple, by means of a systematic series switching operations, to isolate the certain section of overhead line. When this switching process is carried out it is absolutely imperative that the air break switch disconnector is reliable and effective. D. Distribution Transformer: A distribution transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882.

6 Distribution transformers normally have ratings less than 500 kVA, although some national standards can describe up to 5000 kVA as distribution transformers. Since distribution transformers are energized for 24 hours a day (even when they don't carry any load), reducing iron losses has an important role in their design. As they usually don't operate at full load, they are designed to have maximum efficiency at lower loads. To have a better efficiency, voltage 5 regulation in these transformers should be kept to a minimum. Hence they are designed to have small leakage reactance. E. Drop Out Fuse: What are they In the utilities industry, a fuse cutout is a combination of a fuse and a switch. These units are used primarily on overhead feeder lines and are designed to protect distribution transformers from any current spikes or surges that can overload equipment. A cutout consists of three major components: Body: The frame which supports the fuse tub/blade and is mounted to the cross arm or bracket.

7 The insulator body on this frame can be either polymer or porcelain material. The live connector parts are also mounted to the ends of this frame. Fuse Holder: Known as the fuse tube or door that contains the fuse link. This piece acts as a simple switch. When the fuse operates, the fuse holder will drop open disengaging the switch from the line. This ensures any downstream circuits are electrically isolated. Fuse Link: Also known as an element is the replaceable portion of the product that extinguishes due to higher than normal current transfers. How do they work? A current surge from a customer circuit or a transformer will cause the fuse inside the tube to expand and melt. Once the fuse reaches maximum current capacity it breaks and this energy is thrown out of the bottom of the tube and disconnects the transformer from the line by the tube dropping out of the upper contact and swinging down on the hinge. The physical indication that the fuse has been extinguished and needs to be replaced is seen when the tube swings open and remains in a downward orientation.

8 Instrument transformers: The lines in sub-stations operate at high voltages and carry current of thousands of amperes. The measuring instruments and protective devices are designed for low voltages (generally 110 V) 6 and currents (about 5 A). Therefore, they will not work satisfactorily if mounted directly on the power lines. This difficulty is overcome by installing instrument transformers on the power lines. The function of these instrument transformers is to transfer voltages or currents in the power lines to values which are convenient for the operation of measuring instruments and relays. There are two types of instrument transformers viz. (i) Current transformer ( ) (ii) Potential transformer ( ) Current transformer ( ): A current transformer in essentially a step-up transformer which steps down the current to a known ratio. The primary of this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of a large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of the current in the line.

9 Suppose a current transformer rated at 100/5 A is connected in the line to measure current. If the current in the line is 100 A, then current in the secondary will be 5A. Similarly, if current in the line is 50A, then secondary of will have a current of 2 5 A. Thus the under consideration will step down the line current by a factor of 20. Voltage transformer: It is essentially a step down transformer and steps down the voltage to a known ratio. The primary of this transformer consists of a large number of turns of fine wire connected across the line. The secondary winding consists of a few turns and provides for measuring instruments and relays a voltage which is a known fraction of the line voltage. Suppose a potential transformer rated at 66kV/110V is connected to a power line. If line voltage is 66kV, then voltage across the secondary will be 110 V. Metering and Indicating Instruments: There are several metering and indicating instruments ( ammeters, voltmeters, energy meters etc.)

10 Installed in a sub-station to maintain watch over the circuit quantities. The instrument transformers are invariably used with them for satisfactory operation. 7 PURPOSE OF SUBSTATION EARTHING SYSTEM The object of an earthing system in a substation is to provide under and around the substation a surface that shall be at a uniform potential and near zero or absolute earth potential as possible. The provision of such a surface of uniform potential under and around the substation ensure that no human being in the substation subject in shock of injury on the occurrence of a short circuit or development of other abnormal conditions in the equipment installed in the yard. The primary requirements of a good earthing system in a substation are: 1. It is stabilize circuit potentials with respect to ground and limit the overall potential rise. 2. It is protect life and property from over voltage. 3. It is provide low impedance path to fault currents to ensure prompt and consistent operation of protective devices during ground faults.