OMICRON Magazine

Figure 2: Voltage curves during unit transformer stepping trigger machine line R. tion is usually linked logically with the island operation of the power plant and used to control over-voltages in the nearest substation at Pulgar. As the voltage threshold was exceeded and the signal link was missing, the generator transformers in the power plant were stepped up, which, in the case of a “missing” generator, increased the already high voltage in the on-site power further. As shown in Figure 2, the over-voltages tripped the 380 kV machine line between Lippendorf and Pulgar. The discovery of this defect confirmed «The occurence of these unexpected results confirmed that a practical run-through of the test offers significant advantages over pure simulation and is therefore necessary.» when the operating mode was changed. Switching to the “Manual excitation current control” operating mode and ramping up the voltage again was necessary (time point 2). Approximately 3 minutes after the start of the voltage run, the controller was switched to “Automatic voltage control” again (time point 3). A much less significant drop in voltage then occurred, which could be corrected by the automatic system. Ramping up the test network corresponded with expectations and demonstrated similar behavior with simulations carried out during preparation. However, the initial change in the excitation system operating mode should occur with a delay in the future, so it doesn’t have to leave its stable operating state. Remaining at a low rate of active power delivery for too long and the connection of large transformers is critical in this state. On the other hand, timely loading of the generator, e. g., through starting up a storage pump, appears to be useful. Connecting the load In the second part of the test, the required switching operations for load pickup should be implemented from the distribution network and assumptions from the preliminary stage should be confirmed. However, this part of the test did not quite proceed as expected. Firstly, after increasing the voltage level, the automatic stepping of the unit transformers in the Lippendorf power plant became active. This func380 215 225 235 255 265 275 245 390 400 410 430 420 440 450 460 470 10:32:53 10:33:10 10:33:27 10:33:45 10:34:02 10:34:19 10:34:36 10:34:54 Stepping of unit transformer Trip machine line R ATF PMU V PhA-G ATF PMU V PhB-G ATF PMU V PhC-G VPhPh (kV) VPhG (kV) 32