Analysis of capacitive voltage transformer transients with wind farm integration

Wind energy has gained much attention with the development of conversion technologies, concerns on fossil fuel use and associated environmental impacts. As a result, an increasing number of large wind farms are coming into the power system at high voltage (HV) grid level. Voltage transformers (VT) are used for obtaining secondary voltages suitable for metering and protection devices in a wind farm at collector feeders, medium voltage bus and HV side of the main step-up transformer(s). Capacitive Voltage Transformers (CVT) are preferred at HV level over the conventional electromagnetic VTs. In large wind farms, CVTs are installed on the HV side of the step-up transformer. However, during line faults, due to the collapse of primary voltage, CVT will dissipate its stored energy in stack capacitors and tuning reactor, which will introduce a transient component to the voltage inputs to protection and control equipments. This will have a significant impact on the distance and directional protective element performance in multi function numerical intelligent electronic devices. This paper considers a CVT placed at the HV side of a step-up transformer in a large wind farm. CVT and the wind farm employing Doubly Fed Induction Generators (DFIG) are modeled in Matlab/Simulink. Faults and voltage dips are created in the power system and within the wind farm, and CVT secondary voltage profiles are simulated and analyzed.