Magazine | Issue 1 2022 Figure 3: Voltage curves in the Altenfeld substation; switch-on attempt transformer 411 and subsequent events. that a practical run-through of the test offers significant advantages over pure simulation and is therefore necessary. Secondly, after switching on the unloaded 380/110 kV transformer, a further anomaly was revealed: due to the severe voltage drop after closing the transformer circuit breaker, the excitation system of the Goldisthal pumped storage plant switched to Fault-RideThrough (FRT) mode. This means that in the event of a network fault, the voltage is supported by the generator. While the target values of the excitation system remained at 18 kV nominal generator voltage, the voltage at the generator terminal dropped significantly due to the engagement of transformer inductance. The excitation system delivered reactive power to the idling island network to support the voltage. Approximately 6 seconds after switching on the transformer, a change in the excitation system operating mode can be detected from the curves in Figure 3. Consequently, the voltage rose further, which led to over 420 kV on the nodes in the test network and finally to the protective tripping of the 380 kV Pulgar–Vieselbach line. In the final step, the now loaded island network was synchronized with the integrated European power grid. To do so, the speed and frequency target value for the turbine controller in the pumped 100 58 108 208 258 158 150 200 300 250 350 400 450 11:00:30 11:00:40 11:00:50 11:01:00 11:01:10 11:01:20 11:01:30 Switch-on transformer 411 Trip line 589 VPhPh (kV) VPhG (kV) Changing the operating mode of the excitation system PhA-G PhB-G PhC-G storage station was adjusted to 50.06 Hz. Thus, the ramp-up grid ran asynchronously to the European power grid and the synchronizing device in the Vieselbach substation could be activated. Following successful synchronization, the control type of the turbine controller in the pumped storage station was switched from speed-dependent to power-dependent. Summary and outlook Grid restoration using a ramp-up network was tested successfully in 50Hertz's territory. A prepared area of the network consisting of several lines and substations was ramped up to nominal voltage within minutes. During an emergency, this delivers a significant time advantage compared with grid restoration strategies that use sequential connection and loading. There are also plans for further comparable field tests using the Markersbach pumped storage power plant. The aim of these tests is to supply extensive parts of the network with voltage to secure the on-site power structure of power plants. Connecting several ramp-up networks established independently of one another at selected sites in the network forms a part of this concept. Grid restoration with generators producing electricity from renewable energy sources will play a key role in the changing generation industry as a result of the energy revolution. 33
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