In this work engineers from ABB and PD Drinsko-Limske HE used the output from a standard CT connected to the excitation transformer, which feeds the static exciter. That provides a measure of AC current, and from that information the team was able to program an ABB IDE to calculate the DC current of the rotor.
That information provides a hitherto unseen insight into the performance of the generator, enabling better understanding of its operation, but also to monitor for thermal overload, over-current protection, and under-current protection, of the rotor winding.
That was the theory, but to test it a measurement system was installed on one of a pair of 315MVA, 11kV, 426.6rpm, generator-motor units. Connections were made to both the LV and HV CTs, to maximize the data coming into the IED, which then used both measurements to calculate the DC current at the rotor twice, enabling the two sources of data to be compared.
A resistive shunt was also available, to check on the accuracy of the new system.
Not only did the new system match the DC voltage recorded by the shunt, but it also proved that CTs connected to the LV and HV sources produced the same calculated DC voltage. That is going to make for much cheaper and more accurate monitoring of rotor DC systems, and logging of detailed performance data for future analysis which should lead to improve generator operation.