DC supply systems in 400 kV sub stations

1. DC supply systems in 400 kV sub stations Why a DC source ? Why not AC source ? What for DC source is used in 400 kv sub stations How many levels of DC sources available ? Where from DC source is obtained?

2. DC supply systems in 400 kV sub stations • DC source is more stable , Local source, • not dependent on grid • 2. AC source is prone to disturbances, dependent • on grid condition • 3. DC source is mainly used for relays, Breakers, • Isolators • 4. Levels of DC source are 220 V DC and 50 V DC • 5. DC source is obtained from Batteries.

3. Charger Load DC supply systems in 400 kV sub stations DC Switch Boards AC source Charger Battery Load • Requirement for supervision Relays: • Relay are meant for supervision of Power system 24*7 *52 • No interruption is permitted • Two DC sources are designed from 220 KV power system level and above • All protection relays are divided into two groups • 5. One group of relays are fed from DC source –I and other group from • Second Source

4. DC supply systems in 400 kV sub stations DC source-2 DC Source –I Feeder Protection distribution Feeder Protection distribution

5. DC supply systems in 400 kV sub stations • Critical Equipment in DC Source systems : • Chargers --- Full wave rectifiers , Half wave rectifiers, with 1 % ripples • Batteries --- 109 Nos for 220 V system with 88th cell tapping , • 24 Nos for 50 V systems with 18th cell tapping. • C. DC distribution boards ---- DC switches , Fuses • Maintenance of DC systems • Voltage measurement between + Pole to Ground and - ve pole to Ground. • Monitoring of leakage current in DC systems -- 4-6 milli amps • Less leakage more healthiness of DC systems

6. + ve pole + ve pole - Ve pole - Ve pole DC supply systems in 400 kV sub stations • DC Earth faults and its effects : • Any One pole earthing – No problem but to be rectified. • Same pole earthing in both source ? • Two different pole earthing in different sources ? DC Source –II DC Source –I Earth fault relays shall give alarm

7. + ve pole + ve pole + ve pole + ve pole - Ve pole - Ve pole - Ve pole - Ve pole DC supply systems in 400 kV sub stations Circuit breaker tripping is possible in this case. Fuse blows out

8. + ve pole + ve pole - Ve pole - Ve pole DC supply systems in 400 kV sub stations

9. + Ve + Ve - Ve - Ve Relay contact Relay contact Trip coil Trip coil CB aux CB aux DC supply systems in 400 kV sub stations Source –I Source -II

10. DC supply systems in 400 kV sub stations • There is possibility of insulation damage in Source CB trip circuits • B. There is always possible for discharge of capacitance that is • present with DC cables insulation • C. The moment double earthing happens, then discharge is possible • through the trip coil and breaker can trip. This can happen for the breakers • where cable insulation is week and damaged. • To prevent such mal operation of breakers, when ever ingle pole • earthing occurs, same is to be identified and rectified.

11. AC supply systems in 400 kV sub stations • AC supply is for • Transformer Cooling systems • Circuit breaker compressors • Batteries • Control room lighting • AC plant • Fire fighting systems • Switch yard lighting • Street lighting • Colony power supply • Stores yard lighting

12. DC supply systems in 400 kV sub stations Tertiary supply 11/33 kV source DG source F/F ACP SWYD ACP F/F SWYD Lighting Colony DB DB DB DB DB DB DB DB 440 V Main Distribution system

13. AC supply systems in 400 kV sub stations • AC systems – Problems • Major problems are power cable failure from Transformer / • DG set to Main distribution board. • 2. Failure is due to improper laying of power cables • 3. Other problems are in Lighting system

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