Time-normalized voltage gradient protection for fault detection in MMC-HVDC grids

Abstract

Travelling wave fault detection is the mature and fastest protection scheme for HVDC grids. However, it faces the challenge of low resistance coverage. Unlike conventional voltage gradient-based travelling wave protections, this paper explores the new property of the initial fault travelling waves. It is found that gradient properties of initial fault travelling waves are affected by fault location as well as resistance but not in the same way as the fault severity. However, time to reach the maximum voltage gradient is only affected by fault location in case of internal faults. Based on these investigations, this paper proposes a new protection scheme termed as time-normalized voltage gradient protection. In this scheme, the locally available gradients of modal voltages are averaged out over the time to maximum gradient with proper power weightage to the timing factor. The proposed scheme can discriminate the internal and external faults in both forward as well as backward directions. At the same time, it is robust to fault resistance as high as 300 Ω, as fast as practically mature traditional gradient-based methods. The proposed protection is achievable at practically common sampling frequency and, therefore, simple for practical implementation. Validation studies on four terminal meshed MMC-HVDC grid simulated in PSCAD/EMTDC proves the proposed strategy’s effectiveness, adaptive rapidity and accuracy.