Protection of Direct-Current Systems

Abstract

The overwhelming advancement in power electronics converters throughout the past few decades is leading to an increasing interest in the integration of Direct-Current (DC) systems to the existing AC ones on the generation, Low Voltage DC (LVDC), Medium Voltage DC (MVDC), and High Voltage DC (HVDC) levels. The utilization of DC systems offer many benefits over their AC counterparts such as the significant reduction in power losses and costs as well as the minimization of reactive current component. Nevertheless, DC systems still face many challenges among which protection is the most salient.

This dissertation investigates and addresses the protection challenges posed by DC faults’ behaviour in five DC systems. On the generation level, it explores the nature of various faults and partial shading conditions in utility-scale Photovoltaic (PV) arrays. The unique PV modules’ voltage behaviour during faults and partial shading conditions is scrutinized to identify distinctive characteristics. These voltage features are utilized to propose a new time-domain voltage-based protection scheme. The proposed scheme’s underlying concepts are analytically proved for generic PV modules, validated using detailed time-domain model of PV panels, and verified experimentally using polycrystalline-silicon panels.

On the LVDC level, the dissertation examines the behaviour of low- and high-resistance faults. The analysis are founded upon a detailed time-domain simulation of a meshed LVDC microgrid. The failure of conventional protection methods in the presence of even small amounts of fault resistance are demonstrated. An effective method is proposed to detect such faults by using the resonance frequency generated from passive oscillators installed on the line terminals.

The protection of MVDC microgrids is a major challenge as very high fault current magnitudes are attained within a couple of milliseconds. This dissertation reveals unique fault-launched Travelling wave (TW) waveform and polarity properties. These properties are exploited to propose an adequate time-domain TW-based protection scheme that detects, classifies, and locates DC faults in a timely manner.

The impediments to reliable protection of hybrid AC/DC microgrids are twofold: (i) the very low AC fault current magnitudes in the AC-side due to the current control capability of inverter-based Distributed Generation (DG)s, and (ii) the very high DC fault current magnitudes attained within few milliseconds in the DC-side due to the uncontrollable discharge of the converters’ DC link capacitors. A unified discriminant function TW-based protection scheme is proposed for hybrid AC/DC microgrids to detect, classify, and locate both AC and DC faults.

DC faults in HVDC grids can cause severe damage to the converter stations and large loss of infeed within few milliseconds. Ensuring selectivity and sensitivity of the protection system within a short time window is a major challenge. This dissertation analyze the frequency spectra of the TWs initiated by faults on HVDC grids. Using the spectral content and polarity of the current TWs, a novel frequency-domain TW-based scheme is proposed to detect and locate faults within the required timeframe.