Testing of ground fault relay response during the energisation of megawatt range electric boilers in thermal power plants

Large controllable loads may support power systems with an increased penetration of fluctuating renewable energy, by providing a rapid response to a change in the power production. Megawatt range electric boilers are an example of such controllable loads, capable of change rapidly, with the advantage that the warmed water can be reused in a thermal power plant or at regional heating, thus, minimising the overall losses.

However, one problem was raised by those purchasing the boilers, mainly the possibility of an unwanted triggering of the protections relays, especially ground fault protection, during the energisation of a boiler. A special case for concern was the presence of an electric arc between the electrodes of the boiler and the water in the boiler during approximately 2s at the energisation, which can in theory be seen as a ground fault by the relay.

The voltage and current transient waveforms were measured for two separate energisation transients of the boiler and the results used to build a PSCAD model able to replicate the results for different system configurations.

The results of various PSCAD simulation, as well as the original measurements, are used as input signals for the testing of two ground fault protection relays, in order to assure that they are not triggered by the energisation of the boiler.

The test is performed via an OMICRON CMC 256 with Advanced TransPlay SW, which generates the signals that would be present at the secondary of the instrumentation transformers, resulting in a realistic simulation environment. The test of different cases demonstrates that the relays will not present unwanted triggering.