Single-ended limiting inductor voltage-ratio-derivative protection scheme for VSC-HVDC grids

Abstract

Single-ended protection systems present the characteristics needed to fulfil the restrictive speed requirement related to high voltage direct current grids. This paper proposes a novel single-ended protection algorithm based on the DC voltage across the limiting inductors placed at each link end. Voltage measurements are taken at both terminals of the limiting inductors. The ratio between the link- and bus-side voltages enables fault discrimination between forward and backward fault conditions while providing directionality. Moreover, the derivative of the voltage-ratio is calculated in order to enable fast fault detection. The combined operation of these two algorithms avoids nuisance operations against fluctuations and close-up external faults. A trip signal is only issued to the circuit breakers when the individual criteria of both algorithms are simultaneously fulfilled. According to this, a protection scheme is developed, which covers link primary and backup as well as busbar protections. The performance of the proposed protection scheme is evaluated through simulations in a four-terminal grid. The selective, sensitive and accurate performance of the proposed protection scheme is demonstrated against faults up to 250 Ω while employing only local measurements, common limiting inductor sizes and a relatively low sampling frequency. Thus, the proposed protection scheme overcomes the sensitivity limitations related to high-resistance fault detection presented in derivative-based algorithms. Its proper operation under noise disturbances is also demonstrated.