A Traveling Wave Based Method for Protection of Shunt Capacitor Bank

This paper proposes a traveling wave based scheme for wye-connected shunt capacitor bank protection. The proposed method provides high sensitivity and fast response, assisting in mitigating capacitor damage during internal cascading failure. The available protection schemes for capacitor bank use RMS values of voltage and current phasors for trip decision. Threshold selection is required in these methods which needs accurate estimation of equivalent capacitance of the bank. In addition, phasor being estimated from one-cycle running data window, such a method is slow. The proposed traveling wave based method identifies an internal fault in case of opposite polarities of traveling waves seen in the terminal voltage and current signals of the faulted phase. If the mid-point tapping is available in capacitor bank, the faulted half can be identified using the traveling wave of tap voltage. The magnitude of the traveling wave in terminal voltage reveals the number of failed capacitor elements. Such information assists in expediting maintenance. The performance of the proposed method is tested using PSCAD/EMTDC simulation data for various situations, including internal short circuit, external, and identical faults in the shunt capacitor bank. Comparative analysis highlights its higher sensitivity and better accuracy in comparison to available protection methods.

I. Introduction

Shunt capacitor banks (SCBs) are used in electric power systems for reactive power support, harmonic filter, and power factor improvement [1], [2]. The size of SCB depends on the system voltage level and its reactive power rating. Several capacitor elements (typically in few hundreds) are connected in series and parallel to form a bank [3]. The dielectric breakdown of a capacitor element in the SCB causes an internal short circuit. As a result, remaining capacitors experience voltage stress and overcurrent, which may lead to internal cascading failure of the SCB. From IEEE standard (C37.99-2012), a capacitor element can withstand up to 110% of rated voltage and 135% of rated current [4] for continuous operation. Therefore, fast protection is desirable to minimize and limit further damage to the capacitor bank.