Protection of future power systems with high amounts of power electronic converters

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2023
Thesis identifier
  • T16579
Person Identifier (Local)
  • 201879806
Qualification Level
Qualification Name
Department, School or Faculty
  • In recent decades, power systems worldwide have seen a significant increase in power electronic converters to facilitate the integration of renewable energy, e.g., solar and wind energy, and also for the transmission of electrical power between different areas, e.g., High-Voltage Direct Current (HVDC) links. Unlike Synchronous Generators (SGs), Converter-based Resources (CBRs) cannot generate high magnitude fault currents and their fault characteristics are dependent on implemented control strategies, which are governed by the associated grid codes. Those features of CBRs can pose severe challenges to the reliable operation of protection systems designed based on the fault characteristic of SGs. Two typical application scenarios with high penetrations of CBRs are investigated in this thesis, which includes the HVDC system connected to the transmission system and the low-voltage (LV) microgrid connected to the distribution system. In this thesis, the performance of distance protection is evaluated systematically using the Hardware-in-the-Loop (HIL) approach on two commercially available relays. Based on HIL test results and further supported by the theoretical analysis, it was found that the performance of the distance relay is compromised significantly after the connection of CBRs such as HVDC systems. The observed issues include the distance under/over-reach, incorrect faulted phase selection, problematic impedance measurement and oscillating impedance locus. A novel sequence component-based distance measuring element is proposed in this thesis to accurately reflect the fault distance and thus address the under/over-reach issue of the conventional impedance based distance algorithm, whose performance is verified by the simulation conducted using the Real-Time Digital Simulator (RTDS) under a wide range of system and fault conditions including varied fault types, fault resistances, fault positions, system fault levels and protected line length. Based on simulation results, the proposed algorithm can measure the fault distance accurately and trip faults within the time required by the GB grid code. Except for HVDC systems in the transmission system, significant amounts of inverter-interfaced distributed generators (IIDGs) are also connected to the distribution system, which poses severe challenges to the existing overcurrent protection relays because of the reduced fault contribution of IIDGs and the bi-directional power flow. Therefore, some solutions must be designed to protect the future distribution system. As an emerging type of distribution system, the microgrid, a compact network comprising generators, loads and storage devices, is selected in this thesis to investigate the protection challenges of the future distribution system and demonstrate the development of a new energy-based protection scheme. In normal conditions, a microgrid is typically connected to the main grid, i.e., grid-connected microgrid, but when severe disturbances occur, e.g., loss of major generation or faults, it can be safely disconnected from the main grid and run as an independent islanded system, i.e., islanded microgrid. This dual mode capability results in the dynamic variation of fault levels in different operating modes, where the fault level is reduced substantially in the islanded mode due to the loss of the fault infeed from the main network and the limited fault current contribution from IIDGs. Additionally, different fault responses of IIDGs in a microgrid, governed by embedded controllers of inverters, also increase the risk of protection failure. To address issues of fault level variation of two operating modes, reduced fault current infeed in the islanded mode, and non-uniform fault characteristics of CBRs, an energy-based protection scheme is designed in this thesis to protect LV microgrids, where the energy of high-frequency transients resulting from the reflections of travelling waves are extracted using the Maximal Overlap Discrete Wavelet Transform (MODWT) algorithm, and relays in microgrids are coordinated by a developed algorithm. The performance of the proposed protection algorithm is evaluated using a 400-V CIGRE benchmark microgrid implemented in MATLAB/SIMULINK. In summary, this thesis evaluates the challenges of conventional distance and overcurrent protection in transmission and distribution systems (i.e., microgrids) where high amounts of CBRs/IIDGs are expected. Solutions proposed in the literature are reviewed, along with a comprehensive discussion of their benefits and limitations. Additionally, a sequence components-based distance protection algorithm and a novel high-frequency transient-based protection scheme are developed in this thesis to protect transmission lines connected to CBRs and microgrids dominated by IIDGs.
Advisor / supervisor
  • Campbell, Booth
  • Dyśko, Adam
Resource Type