Improvement of loss of excitation protection of an alternator in the presence of shunt-FACTS – A comprehensive review

Abstract

Among the different Loss of Excitation (LOE) relaying schemes in practice today, the most famous is the mho relay. During the loss of excitation, the impedance as seen by the relay enters the zones of protection that are set up according to the generator and system specifications. It then generates a trip signal to separate the affected alternator from the system in case the fault persists. Modern-day Power Systems incorporate FACTS devices to deal with certain tasks like voltage regulation, improvement of transient stability, prevention of voltage instability and damping Power oscillation. It has been shown that the reactive power injected by the shunt-FACTS devices prevents the terminal voltage of the alternator from dropping during LOE, which causes a delay in the operation of the LOE relay. Under certain conditions such as the event of partial LOE or during condenser mode of operation of an alternator, the presence of shunt-FACTS devices may lead to under-reach of the relay. This paper analyses the effects that shunt FACTS devices, such as Static VAR Compensators (SVCs) have on the loss of excitation (LOE) protection of an alternator. Simulations were carried out in the MATLAB/Simulink environment to simulate various LOE relay schemes during excitation system failures. The results give us an insight into the advantages and disadvantages of various LOE protection schemes and serve as a reference for selecting a particular scheme for any system.