Earthing schemes enabling effective protection and fault location techniques for low voltage direct current microgrids

  • Ahmad Makkieh

Student thesis: Doctoral Thesis

Abstract

Growing public awareness and concern about the pollution caused by traditional power generation has sparked a clean energy revolution that is challenging the current centuryold power system networks. Existing LV distribution networks are being overstretched to accommodate an increasing number of low-carbon technologies such as electric vehicles (EVs), heat pumps, energy storage, and solar PV generation. This radical shift necessitates significant work to support the systems in order to alleviate the pressure and growth in power demand that are fundamentally challenging the capacity of the existing LV distribution networks. Alternatively, Low Voltage Direct Current (LVDC) microgrids have been identified by a number of industrial and research organisations as one of the most beneficial approaches for alleviating congestion and expanding capacity on existing LV distribution networks in order to meet the anticipated increases in transportation and heat demand. Along with advancements in power electronics and converters, a rising number of applications that operate predominantly with DC is a significant indication of the adoption of the LVDC microgrid infrastructure, with a number of applications, both industrial and consumer-based, that operate largely with DC as a primary power source. However, the lack of existing standards, accurate islanding detection, effective and reliable earthing systems, and DC location and protection solutions have contributed to a challenge to the transition to a fully operated DC distribution architecture. This thesis is devoted to the development of a reliable and accurate fault location technique, as well as a selective protection scheme that will ensure secure and reliable LVDC microgrid operations, thereby facilitating the transition to widespread implementation of LVDC microgrids. Typically, the LVDC microgrid’s network is interfaced to the AC grid through a two-level voltage source converter (VSC) that regulates the bus voltage. When the VSC converter is disconnected from the utility, the system goes into islanded mode. Rapid and accurate islanding detection (ID) is critical to ensuring that the system disconnects or switches to islanding operation mode. This is particularly challenging in DC systems because some of the variables that are typically used in AC systems to differentiate islanding events (such as phase and frequency) are absent in DC. Thus, the ROCOV and ROCOC methods are developed in this thesis for the detection of LOM (islanding) events, which in turn allows for the discrimination between islanding and non-islanding events (i.e. Faults). The detection performance of these schemes is evaluated in a variety of simulation scenarios. Besides that, system earthing is a technical challenge in LVDC microgrids, and therefore should be carefully designed to ensure device and user safety while also increasing the microgrid’s effectiveness. In this thesis, numerous earthing schemes, including multiple earthing points, have been scrutinised in order to determine the most reliable and effective earthing scheme capable of enabling safe and secure operation in LVDC microgrids. The corresponding understanding of the system’s behaviour when the LVDC microgrids are earthed with multiple earthing points during grid-connected and islanded modes, allowing the design of effective DC fault location and protection solutions. Leveraging these understandings facilitated the development of a method for locating DC faults through the use of multiple capacitive earthing schemes. This proposed technique is capable of estimating the fault location with high accuracy regardless of whether the remote end is connected to DC sources or a load. The enhanced performance of the proposed fault location technique has been validated through simulation studies and laboratory experiments. This enhanced accuracy and reliability facilitates DC faults to be accurately located and enables rapid network reconfiguration and postfault cable maintenance to take place. In addition, a novel current-based fault detection and isolation technique for LVDC microgrids has been developed and validated through simulation studies and laboratory experiments. The proposed technique is communication-less and relies only on local measurements. The proposed protection scheme has the ability to effectively protect against both solid and highly resistive faults and is capable of discriminating between internal and external faults under both grid connected and islanded modes. Thus, it eliminates the need for selection of different protection settings for different LVDC topologies.
Date of Award29 Sept 2022
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorGraeme Burt (Supervisor) & Rafael Pena Alzola (Supervisor)

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