Impact of K-factor and active current reduction during fault-ride-through of generating units connected via voltage-sourced converters on power system stability

This study investigates the impact of the injection of additional reactive current and of active current reduction during fault-ride-through (FRT) of generating units connected to the grid via fully rated voltage-sourced converter (VSC generating units) on the behaviour and stability of the power system. The primary focus is on voltage support, transient stability and frequency stability. The investigation is carried out via computer simulations using a fictitious transmission system, based on German grid code requirements. The K-factor of the dependence of additional reactive current with voltage deviation, the method of active current limitation during FRT and the rate of active power recovery after fault clearance are varied. Results and influences are illustrated and discussed. They show that with an increasing number of converter-connected generating units in power systems, a careful selection of the parameters becomes very important. Insufficient grid code requirements may increase the risk of system instability. Conclusions are drawn and recommendations for optimal settings with respect to future development of grid codes are provided. The mechanism of loss of synchronism of VSC generating units because of inadequate current injection (current angle instability) is explained.