Substation Testing and Commissioning

Monday, January 9, 2017


1. Core identification is very impotent process during CT primary injection 

2. CT 1 core -1 is used for line protection. Some times Core 2 and Core 1  will cores connect each other and can not identify during CT secondary or CT testing .

3. This can be only identified in Primary Injection.

4. Inject some current in primary side and measure the secondary current.

5. Short the used Tap say S1& S4 and inject the current in primary side. Now the current will be reduced in the relay side means current will dived. Some current will flow through shorted path and minimum current will follow through relay path. If the relay had more impedance the current will be zero.

Thursday, November 17, 2016

Tertiary winding in Power Transformer

The Tertiary winding with DELTA connection is normally used in Yy connection power & Auto transformers for following purpose.

1. The Tertiary winding used as a path for the Zero sequence current  during secondary loaded with unbalanced load. Normally the primary neutral point would not be connected with system  ground. Tertiary winding one DELTA point is connected with system ground which is prevent the shifting of secondary neutral point and  the voltages become unsymmetrical during unbalanced load condition. Other wise the zero sequence current will flow through the secondary neutral connected with Transformer Tank and tank will get heat up.

2. When an external Earth Fault occurs, the resulting zero sequence currents  flow through the OPEN DELTA winding to ground.This prevents the zero sequence currents from flowing through the Transformer. This also prevents abnormal DIFFERENTIAL RELAY operation of the Transformer for external faults.

3. Some times a external resistor is used in the path of Delta to ground connection to limit the Current circulation. 

4. Some times this winding is used as a station supply and surge arrester are used to prevent the surges from the load side. 

Protection Arrangement

The CT in the closed conductor will detect asymmetrical-faults such as turn-to-turn, or winding-to-winding and connected with instantaneous over current 51 and other and one core is connected with winding Temperature meter.

The CT in the Grounding conductor will detect ground-faults in the delta winding. 

Tuesday, August 16, 2016


1. GIS surge arrester was tested by injecting nominal voltage injection during HV test 

2.In 380 kV GIS the single phase nominal voltage 220kV was injected in Surge Arrester Through bus.

3. The leakage current mili amps measurement was measured in surge counter and Multi meter.

4. There by the paper connections are conformed.

5. Some cause the outgoing cable bus link and Surge arrester link will be same. So arrester should dismantle separately and Tan Delta test should contact on the table.

Saturday, August 6, 2016

REC 670 Minimum Current Measurement Issue

1. 380 kV line  Metering CT was connected with BCU ( Bay control Unit) in GIS LCC.

2. The metering CT Ratio is 1500/1.

3. During SAS Pre commissioning measurement was checked like current injected 5% , 10% , 50%.

4. 5% Primary current 75 amps and REC 670 not read and current shows 0 in the display.

5. The secondary of 75 Amps is 0.05 amps which is not seance  by REC 670 relay.

6. MW also shows zero because of current seances as 0.

7. REC 670 seance above 100 amps primary that means above 0.061 amps.

8. Client not accept this condition.

9. After long discusses the Relay setting was change as below and relay start to read from Zero current.

10. This setting was changed for Reactive and Active & Apparent Power


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