The Intermountain Power Project commissioning-subsynchronous torsional interaction tests

Subsynchronous torsional interaction (SSTI) tests were performed during the commissioning of the Intermountain Power Project (IPP) Unit 1 and the HVDC transmission system. It was found that full-scale SSTI tests are essential to evaluate and ascertain the adequacy of the electrical damping contribution for an HVDC system that is designed to operate radially with a turbine-generator. A subsynchronous damping controller must be incorporated into the HVDC control to avoid subsynchronous interaction with a turbine-generator near a rectifier and with a weak connecting AC system. Mode 1 interacts significantly with the HVDC control. Higher modes (2, 3 and 4) do not interact as much with the HVDC control, because the frequencies are above the HVDC current control bandwidth and the generator mode shape factors are smaller. The damping of the higher modes is dominated by the inherent mechanical damping of the machine. In the islanded operating condition, mode 1 damping is dominated by the electrical damping contribution from the HVDC control, which is sensitive to the DC voltage. Torsional damping is lower for monopolar operation than for bipolar operation but it adequate for safe and stable operation