A Modified Impedance-Based Loss of Field Detection Method of Synchronous Generator in the Presence of Series Capacitor and its Over-voltage Devices

Loss of field (LOF) detection of synchronous generators in power networks compensated with series capacitors (SCs) create challenging conditions. Fault currents passing through the capacitors can generate over-voltages that exceed the SC ratings, necessitating protection by Metal Oxide Varistors (MOVs). However, when the MOV and capacitor are part of the fault impedance loop, unfavorable conditions arise for LOF relays, including current and voltage fluctuations, high-frequency oscillations from MOVs, and sub-harmonic oscillations. Therefore, this work presents a new modified impedance-based function for LOF protection in the presence of a series capacitor and its over-voltage devices. An analytical solution is proposed to correct the impacts of SCs on LOF protection, which has not been documented previously. The non-linear behavior of the varistor in parallel with the series capacitor is approximated using a linear series R-X impedance model. The operation of the modified LOF function is considered with various capacitor placements, and power network variations such as compensation levels and load changes under complete and partial LOF scenarios. Simulations conducted in MATLAB/Simulink demonstrate the effectiveness and reliability of the proposed method, highlighting its independence from the power system structure while addressing existing relay challenges.