Fault location in DC microgrids based on a multiple capacitive earthing scheme

This paper presents a new method for locating faults along feeders in a DC microgrid using a multiple capacitive earthing scheme. During fault conditions, capacitors within the earthing scheme are charging by transient currents that correlate to the fault distance and resistance. Therefore, by assessing the response of the capacitive earthing scheme during the fault, the distance to fault is estimated. The proposed method uti- lizes instantaneous current and voltage measurements (obtained from the feeder terminals and earthing capacitors) applied to an analytical mathematical model of the faulted feeder. The proposed method has been found to accurately estimate the fault position along the faulted feeder and systematic evaluation has been carried out to further scrutinize its performance under different loading scenarios and highly-resistive faults. Addition- ally, the performance and practical feasibility of the proposed method has been experimentally validated by developing a low- voltage laboratory prototype and testing it under a series of test conditions

Investigation of different system earthing schemes for protection of low voltage DC microgrids

DC microgrids are expected to play an important role in maximising the benefits of distributed energy resources in future low carbon smart power systems. One of the remaining complex challenges is the requirement for effective DC protection solutions. The advancement of DC protection is hindered by the lack of good understanding and development of reliable and effective earthing schemes which can enable safe and secure operation of DC microgrids in both on-grid and off-grid modes. Therefore, this paper discusses different DC microgrid earthing opportunities, and comprehensively evaluates through detailed simulation studies the influence of different earthing methods on the fault behaviour of DC microgrid. A transient model of an active DC microgrid is developed in PSCAD/EMTDC and used for the paper studies

Capacitive earthing charge-based method for locating faults within a DC microgrid

This paper presents a new fault location method using capacitive earthing charge current combined with moving average and Savitzky-Golay filters. Locating a DC fault in a DC microgrid can be challenging due to reduced fault current magnitudes, resulting either from high resistive faults, or during the transition between grid-connected and islanded modes. The capacitive earthing method is proposed for earthing DC systems to avoid the corrosion of earthed metallic surfaces. Under different fault conditions and at different locations, the capacitive earthing with the earth path, charges a transient current with a peak value that depends on the initial voltage of the capacitor and the fault loop between the capacitor and the fault point. Therefore, this paper utilises earth capacitor pre-fault voltages, transient current peak and the derivative current of the capacitive earthing to estimate the total inductance of the fault loop. This in turn can be used to determine the location of DC faults. This paper also quantifies the impact that resistive faults have on the accuracy of the method, especially when the resistance of the fault dominates the total fault loop. The ability to distinguish between downstream and upstream faults with respect to the earthing point location also adds significant value to the proposed method. The proposed fault location technique is tested against pole-to-earth fault at different locations using Matlab-Simulink

Fault characterisation of a DC microgrid with multiple earthing under grid connected and islanded operations

Direct Current (DC) microgrids are gaining an international interest to improve the flexibility and the reliability of future distribution systems. DC microgrids are more suitable and efficient infrastructure for connecting and controlling distributed energy resources. However, they do present new challenges under different fault conditions. To design appropriate protection solutions to handle such challenges and ensure DC microgrid secure operation under different operation modes (grid-connected and islanded), there is a need to fully understand their fault behaviour under each DC microgrid mode. Therefore, this paper investigates in detail the fault behaviours associated with DC microgrids with local distributed energy resources under different fault conditions (e.g. DC pole to ground and pole to pole faults). A DC microgrid modelled in PSCAD/EMTDC with the appropriate layout, multiple earthing and control structures is used as a test network in the simulation fault studies

Assessment of passive islanding detection methods for DC microgrids

This paper provides an assessment of passive islanding detection methods in DC microgrids. In order to analyse the response of a DC microgrid to an islanding event, DC voltage and current signatures are captured locally at the terminals of Distributed Energy Resources (DERs). Further analysis on DC voltage and current measurements is carried out to derive the Rate of Change of Voltage (ROCOV) and the Rate of Change of Current (ROCOC), to distinguish between genuine islanding events and other disturbances. A detailed DC microgrid has been developed in MATLAB/Simulink to analyse the response of the DERs within a DC microgrid during intentional or unintentional islanding events. The results show that these approaches cannot be used to classify and characterise between islanding and non-islanding events caused by high resistive faults for DC microgrids, as the response of the DERs are dependent on the technology and associated control systems,which influences post event analysis in distinguishing between events

DC Networks on the Distribution Level – New Trend or Vision?

"DC networks on Distribution Level – are they a new trend or a Vision?" That is the question that has focused the efforts of the Working Group the last two years, and whose consideration is summarized in this report. This report represents the first phase evaluation of this topic and is focused primarily on medium (MVDC) and low voltage (LVDC) level applications

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

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.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