29-06-2012, 12:09 PM
SUB-STATION AND DISTANCE PROTECTION
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ABSTRACT
The power system network basically consists of three sub-network, viz. generation sub-network, transmission sub-network, distribution sub-network. For the proper operation of power system, an effective, efficient and reliable protective scheme is desirable. The power system components which include, bus bar, transformer, transmission line, distribution system consisting of complex and composite loads are designed to operate under normal conditions of voltage, frequency power factor etc. However due to any reason, say some fault, these conditions become abnormal – say voltage becomes very high or very low, current becomes very high, power factor becomes very poor or line flows become abnormally very high-it is necessary that there should be a device which senses these abnormal conditions and if so, the element or component where such abnormality has taken place is removed i.e., deleted from the rest of the system as soon as possible. This is necessary because the power system components can never be designed to with stand the worst possible conditions, as it will make the whole system highly un economical. Therefore if such abnormal condition takes place in any element or component of the power system network, it is desired that the affected component is removed from the rest of the system reliably and quickly so as to restore power to the remaining system under normal conditions as soon as possible. This report mainly deals about various protective equipment in a sub-station mainly concentrated on the different protection schemes of transformer and its importance.
1. INTRODUCTION
1.1 OVERVIEW:
The 400/220KV sub-station, Vemagiri was commissioned at 16:35 hrs on 10/09/2005 by energizing the bus 1 from 400KV kalapaka vemagiri circuit-1, it is located near to NH-5 opposite to Andhra Bhoomi press, at Vemagiri village in Kadiyam mandal, East-Godavari District which is at a distance of 10KM from the RJY Railway station. The construction of the project was taken up under the scheme of the SIMHADRI-VIZAG power transmission systems with financial assistance of Govt of Japan through JBIC. Initially the sub-station was energized with 24 no bays with 14no’s 400KV feeders and 4no’s 200KV feeders.
The sub-station is connected with 4no INDEPENDENT POWER PRODUCERS (IPP) and these stations are connected with 4no double circuit 400KV feeder for evacuation of the power from the plants. The sub-station is connected to 400KV grid and it is meeting 1/6th of Andhra Pradesh power demand.
Recently, the 400KV construction wing, Rajahmundry completed the execution of 6no bays in the 400KV by acquiring 11 acres of land adjacent to the sub-substation out of these 6 bays, 2no’s of bays were allocated for connecting 400KV GVK double circuit feeder for which there is no bay initially.
At present, the sub-station consist of
400KV BAYS: 30
220KV BAYS: 12
And nearly handling 1800 MW of daily demand. Now, the total area of the sub-station becomes 80 acres.
1.2 PURPOSE OF THE SUB-STATION:
Substation serve as sources of energy supply for the local areas of distribution in which these are located. Their main functions are to receive energy transmitted at high voltages from the generating stations, reduce the voltage to a value appropriate for local distribution and provide facilities for switching. Some sub-stations are switching stations where different connections between various transmission lines are made, others are converting sub-stations which either convert ac to dc or vice-versa, convert high frequency to low frequency or vice-versa.
A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution.
An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".
1.1 Figure shows the entire parts of the Sub-Station
1.Primary power lines 2.Ground wire 3.Overhead lines 4.Transformer for measurement of electric voltage 5.Disconnect switch 6.Circuit breaker 7.Current transformer 8.Lightning arrester 9.Main transformer 10.Control building 11.Security fence 12.Secondary power lines
1.3 POWER TRANSMISSION FROM THE SUB-STATION:
Electric power is normally generated at 11-25kV in a power station. To transmit over long distances, it is then stepped-up to 400kV, 220kV or 132kV as necessary. Power is carried through a transmission network of high voltage lines. Usually, these lines run into hundreds of kilometers and deliver the power into a common power pool called the grid. The grid is connected to load centers (cities) through a sub-transmission network of normally 33kV (or sometimes 66kV) lines. These lines terminate into a 33kV (or 66kV) substation, where the voltage is stepped-down to 11kV for power distribution to load points through a distribution network of lines at 11kV and lower.
The power network, which generally concerns the common man, is the distribution network of 11kV lines or feeders downstream of the 33kV substation. Each 11kV feeder which emanates from the 33kV substation branches further into several subsidiary 11kV feeders to carry power close to the load points (localities, industrial areas, villages, etc.,). At these load points, a transformer further reduces the voltage from 11kV to 415V to provide the last-mile connection through 415V feeders (also called as Low Tension (LT) feeders) to individual customers, either at 240V (as single-phase supply) or at 415V (as three-phase supply). A feeder could be either an overhead line or an underground cable. In urban areas, owing to the density of customers, the length of an 11kV feeder is generally up to 3 km. On the other hand, in rural areas, the feeder length is much larger (up to 20 km). A 415V feeder should normally be restricted to about 0.5-1.0 km. Unduly long feeders lead to low voltage at the consumer end.
1.4 CONSTRUCTIONAL FEATURES OF SUB-STATION:
A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-stations are classified as :
(i) Indoor sub-station (ii) Outdoor sub-station
(iii) Underground sub-station (iv) Pole-mounted sub-station
(i) Indoor sub-stations:
For voltages up to 11 kV, the equipment of the sub-station is installed indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages up to 66 kV.
(ii) Outdoor sub-stations:
For voltages beyond 66 kV, equipment is invariably installed out-door. It is because for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.
(iii) Underground sub-stations:
In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub-station is created underground. The reader may find further discussion on underground sub-stations
(iv) Pole-mounted sub-stations:
This is an outdoor sub-station with equipment installed over-head on H-pole or 4-pole structure. It is the cheapest form of sub-station for voltages not exceeding 11kV (or 33 kV in some cases). Electric power is almost distributed in localities through such sub-stations.