Generator motoring protection - Are you protected?

Steam turbines can be damaged upon loss of steam flow while the generator remains connected to the power system. Reverse power relays are used to detect the motoring power into the electrical machine and open the generator main breaker to prevent damage from this abnormal operating condition. The motoring power for large steam turbines can be a very small fraction of the power rating, leading to sensitivity issues and a failure to detect and trip for this condition. Reverse power protection systems that have been proven to work dependably over many normal shutdowns may not provide the expected protection during an inadvertent motoring event. During normal sequential trip shutdown sequences, the active and reactive power outputs are ramped down to zero prior to tripping the turbine. In this case, the power factor is near unity and the relay has little difficulty measuring the active power component of the signal. However, during a true inadvertent motoring event, the reactive power output may remain at near pre-event levels, resulting in a significant MVA output at nearzero power factor. During these conditions, small angle errors in the instrument transformers and measuring devices can result in large errors in measuring the very small active component of the apparent power flow. This paper discusses motoring events and the resulting active and reactive power flows. It reviews current transformer (CT) and voltage transformer (VT) accuracy as defined by ANSI IEEE C57.13 and IEC 61869. It examines existing reverse power relays used for generator protection and presents a new algorithm for dependable motoring detection. It also describes the implementation and testing of the algorithm on an embedded processing platform.