Tripping Of Bus Coupler In Substation

On Friday, 8 January 2021 at 14:05 CET, the Continental Europe Synchronous Area was separated into two areas (the North-West area and the South-East area) due to cascaded trips of several transmission network elements. 

The match between the IDCF values of the power flows and the actual flow for 13:30 was relatively good and only led to a mismatch on the flow of the busbar coupler of about 100 MW. The mismatch of the actual power flow on the busbar coupler compared to the forecasted value was about 150A at the time of 13:30. Therefore, the tripping of the busbar coupler was not included as a possible event in the N-1 contingency simulations in DACF, IDCF or in real time.

At 14:00:59 the current of the busbar coupler reached the value of 1931 A (note: at 14:00:00 the current was 1736 A). Thus the second threshold of 1,920 A was met and an alarm occurred. At 14:01:06 the power flow value fell again under 1,920 A and fluctuated between 1,830 A and 1,920 A. At 14:04:16 the value again exceeded the threshold of 1,920 A.

At 14:04:21 the current through the busbar coupler reached a value of 1,989 A.

The trip of the busbar coupler led to a shift of the flows, i.e. the current through the busbar coupler was now flowing through the transformers TR1 and TR2. Those two transformers were interconnected via the underlying 110 kV busbars and were therefore still connected to the two 400 kV busbars in SS Ernestinovo. Due to the resulting overcurrent both transformers TR1 and TR2 tripped at 14:04:28 and led to a complete separation of the two busbars in SS Ernestinovo. 

In summary, the protection setting on the bus coupler was too low, so it tripped in real time when the current went beyond the threshold setting.