An active protection scheme for fault isolation and post-fault recovery in microgrids

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2024
Thesis identifier
  • T16949
Person Identifier (Local)
  • 201891997
Qualification Level
Qualification Name
Department, School or Faculty
  • The growing penetration of converter-interfaced Distributed Generation (DG) creates unprecedented challenges to protection strategies at all voltage levels. Microgrid, where DG is typically utilised, is an essential formation within the modern power system. The conventional protection methods are mainly passive, and often suffer from bi-directional power flow or high cost when applied in a small area. Although a few active protection methods, such as harmonic injection method, are proposed and studied, the implementation of such methods can be complicated and quite costly compared to the conventional passive methods. This thesis proposes a novel NegativeSequence Current Injection (NSCI)-based active protection scheme for islanded microgrids, which is designed to have high performance in terms of fault detection and discrimination capability, and at the same time, remain cost-effective and flexible in practical applications. Firstly, the challenges in microgrid protection are pointed out, such as the difficulty in relay coordination, the bi-directional power flow during a fault due to multiple DG sources, and higher cost of implementing advanced methods such astravelling-wave or high frequency-based protection. Through systematic literature review of existing passive and active protection methods, the research gap is identified and new, NSCI-based active protection scheme is proposed. The operating principle of the proposed protection scheme is demonstrated in detail, including fault detection, discrimination, NSCI control algorithm, and the fault isolation strategy. Additionally, the protection setting strategy for the entire protection scheme is introduced. The fault direction identification method based on the negative sequence current increment between the pre-injection and current generation steady state conditions enables the scheme to achieve an excellent High Impedance Fault (HIF)detection capability (around 18 Ohm). The proposed NSCI control algorithmmaintains the phase angle of the negative sequence current fixed during injection progress, thus providing a highly discriminative feature that facilitates the correct identification of the fault direction. In addition, a systematic simulation-based validation is undertaken considering avariety of influencing factors such as fault type, resistance, and position, as well as the impact of load distribution under HIFs. The results show that the scheme has an excellent detection and discrimination ability, especially during unbalanced faults, and is not affected by load distribution or behaviour of other sources. The performance of the proposed protection method is also evaluated under the possible presence of different types of energy sources, including Voltage Source Converter (VSC), Current Source Converter (CSC), Synchronous Generator, and the main grid (i.e. ideal voltage source). The setting strategy enables the reliable operation of the protection systemtaking into account certain physical limitations, such as the control algorithm or capacity of the energy source. The implementation of the scheme under different network topologies is also demonstrated. To complement the fault isolation method, the thesis also proposes two novel post-fault recovery strategies for a dedicated VSC to mitigate the power unbalance byinjecting positive sequence current after the fault clearance. Well-designed recovery strategy can enhance the transient dynamic response effectively due to small overshoots and less steady-state oscillations following a major fault. The performance relies on optimal values of several key parameters inside the converter controller. The post-fault dynamics are carefully compared and analysed to develop the optimal control solution. Furthermore, the EMT simulation based testing of both protection and post-fault recovery systems operating together is implemented in the network with multiple DGs.It is demonstrated through comprehensive simulation case studies that both high performing protection and effective post-fault recovery can be provided by the same piece of equipment without the requirement for communication. This is particularly advantageous and cost-effective when considering protection system design for the future converter-dominated microgrid. Further work should focus on the co-ordination of the proposed method with other protection and/or control strategies, such as load shedding and power sharing.
Advisor / supervisor
  • Egea-Àlvarez, Agusti
  • Dyśko, Adam
Resource Type
Embargo Note
  • This thesis is currently held under moratorium due to a third party copyright issue. If you are the author of this thesis please contact the library to resolve this issue.