Coordinated under frequency load and capacitor shedding for bulk power systems

Under frequency load shedding (UFLS) scheme has been widely implemented as a safety net to prevent system collapse following major disturbances which results in large mismatch between load and generation. However, voltage related issues post activation of UFLS stages are also vital concerns in preserving system stability and should be properly analysed. Fixed and switched shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve can generate surplus reactive power post operation of UFLS relays which may result in over voltage issues, generator under excitation and some undesirable conditions such as transformer saturation, ferro-resonance etc. This study addresses the need for coordinated under frequency load and capacitor shedding and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and under frequency capacitor shedding (UFCS) schemes for a real and actual power network. In addition, the proposed coordinated UFLS and UFCS scheme has been combined with automatic switching of shunt reactors to optimise the performance of the scheme.