Simulations of open phase faults for generating station auxiliary transformers using EMTP

This paper simulates and evaluates the terminal behavior of generating station auxiliary transformer under an open phase condition at its primary side using electromagnetic transient simulations. Open phase conditions on the primary winding of a generating station auxiliary transformers can cause issues with plant operations especially in Nuclear Stations. Existing protection schemes, typically set for overcurrent and fault current operation, may not detect primary open phase conditions until heavy loads are energized by the transformer. Detection at this point will usually result in tripping of load due to phase imbalance, and as a direct consequence, unintended plant operation, which can adversely affect safety related systems.The system model used in the study should represent the plant electrical system as well as the transformer, and different loads inside the plant electrical system. The transformer model implemented in PSCAD/EMTDC the magnetic and electric coupling between the phases on both primary and secondary side as well as the magnetic core. The parameters of the model were obtained from the transformer manufacturer test reports. The simulation results provides insight into the per phase voltage magnitude and angles as well as positive, negative, and zero sequence voltages and currents generated during the loss of one or two phases of the transformer primary side. These values are fundamental to the application of protection schemes designed to detect this condition. Different test cases with different transformer loading were conducted and each phase was open circuited. The terminal voltages and sequence voltages on all terminals of the transformer were monitored and recorded.