Power Quality Impacts of Series and Shunt Compensated Lines on Digital Protective Relays

The proliferation of compensators, due to their nonlinear characteristics raises concern over the distortion levels they cause in voltages and currents at the transmission level. In this paper, series compensation by Thyristor Controlled Series Capacitor (TCSC) and shunt compensation by STATCOM are investigated in terms of power quality and their effects on protective relays behavior. For each issue, the discussion will review how power quality can positively or negatively impact power system protection. Since Thyristor controlled equipments generate fast transients, the power quality is evaluated by EMTP/ATP simulations for a sample network equipped by TCSC or STATCOM. The simulation considers various conducting angles and the amount of current distortion produced. Since, the relay receives data through current and voltage transformers, their effect on the relay performance is considered in the simulation. The relay function is investigated by a digital relay simulator prepared in the MATLAB environment. The generated data would be transferred from EMTP/ATP to MATLAB. It is shown that in normal operation the content of harmonics is not high and the relay operates as expected. The distortion increases rapidly with further increase in compensation level, and hence the operation of the relay is affected.