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Abstract: 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.
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1 Introduction
A power system can experience blackout for various reasons such as frequency instability, voltage instability, excessive load shedding etc. Frequency collapse can be initiated by a large imbalance between load and generation which may lead to cascaded loss of generation and system collapse because of under frequency, over frequency, under voltage or over voltage condition. The main reason of frequency instability is that generators are equipped with under frequency relays which trip the generation units in case of under frequency condition. Therefore under frequency load shedding (UFLS) is used as a special protection scheme to preserve system stability following severe disturbances [1]. The UFLS is activated when frequency declines to predefined values and the relay automatically disconnects some load feeders in several stages to return the system frequency to acceptable limit.
Capacitor banks are normally switched on to supply reactive power to the load during heavy load condition. The issue is that post operation of UFLS stages, shunt capacitors that were in service to support system voltage remain connected to the network which may cause overvoltage condition particularly at generators...