DC microgrids have attracted significant attention over the last decade in both academia and industry. DC microgrids have demonstrated superiority over AC microgrids with respect to reliability, efficiency, control simplicity, integration of renewable energy sources, and connection of dc loads. Despite these numerous advantages, designing and implementing an appropriate protection system for dc microgrids remains a significant challenge. The challenge stems from the rapid rise of dc fault current which must be extinguished in the absence of naturally occurring zero crossings, potentially leading to sustained arcs. In this paper, the challenges of DC microgrid protection are investigated from various aspects including, dc fault current characteristics, ground systems, fault detection methods, protective devices, and fault location methods. In each part, a comprehensive review has been carried out. Finally, future trends in the protection of DC microgrids are briefly discussed.

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DC Microgrid Protection: A Comprehensive Review

Due to inherent advantages of DC system over AC system such as compatibility with renewable energy sources, storage devices and modern loads, Direct Current Microgrid (DCMG) has been one of the key research areas from last few years. The power and energy management in the DCMG system has been a challenge for the researchers. MG structure and control strategies are the integrated part of the power and energy management system. This paper covers all the aspects of the control of DCMG, whether it is DC bus voltage, power or energy related. Different MG Structures with their comparative analysis has been given in this paper. Various control schemes: Basic control schemes like centralized, decentralized and distributed control and multilevel control scheme such as hierarchal control has been discussed. The Power management in grid-connected, Islanded mode and transition mode has been presented. Different energy management strategies have been presented as energy management plays very important role in optimizing the size and rating of energy storage system and their maximum utilization. The energy management of a battery and super capacitor based HESS in all configurations has also been discussed and finally, future trends in further research are presented.

A review on overall control of DC microgrids

As power semiconductor devices are the key components of hybrid DC circuit breakers (HCBs), how to select suitable devices is critical for the whole HCB design. First, this paper compared the general characteristics of press pack insulated gate bipolar transistor (IGBT), injection-enhanced gate transistor (IEGT), and integrated gate commutated thyristor (IGCT). Then working conditions of devices were analyzed from the perspective of HCBs. According to these, vital features of devices including maximum turn-off current, on-state voltage, conduction ability, and robustness of power semiconductor devices were carefully analyzed and compared. Related experiments were implemented to compare maximum turn-off capability, and the 4.5 kV/3 kA IGBT/IEGT was able to turn off 19 and 22 kA respectively after 5-ms current conduction. Analysis and experiments indicated that IGCT was suitable for natural commutation low-voltage and low-current hybrid circuit breakers, due to its low on-state voltage and huge surge current conduction ability. Comparatively, IGBT and IEGT were more suitable for very high current interruption, and IEGT was the superior selection among available commercial devices. However, the interference should be considered in applications.

Analysis and Experiments for IGBT, IEGT, and IGCT in Hybrid DC Circuit Breaker

Microgrid, which is one of the main foundations of the future grid, inherits many properties of the smart grid such as, self-healing capability, real-time monitoring, advanced two-way communication systems, low voltage ride through capability of distributed generator (DG) units, and high penetration of DGs. These substantial changes in properties and capabilities of the future grid result in significant protection challenges such as bidirectional fault current, various levels of fault current under different operating conditions, necessity of standards for automation system, cyber security issues, as well as, designing an appropriate grounding system, fast fault detection/location method, the need for an efficient circuit breaker for DC microgrids. Due to these new challenges in microgrid protection, the conventional protection strategies have to be either modified or substituted with new ones. This study aims to provide a comprehensive review of the protection challenges in AC and DC microgrids and available solutions to deal with them. Future trends in microgrid protection are also briefly discussed
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https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-gtd.2018.5212

Review on microgrids protection

The hybrid direct-current circuit breaker (DCCB) consists of a mechanical switch (MS) branch and a static DCCB branch. Once a short-circuit fault is detected, the current should commutate from the MS branch to the static DCCB branch first. In this paper, two current commutation measures have been investigated. A hybrid DCCB prototype with current commutated by the arc voltage of the SF ${}_{6}$ switch is developed. Based on the prototype, current commutation and interruption tests have been carried out. The results show the current of 3.4 kA can be commutated within 2.4 ms, and be interrupted within 6 ms. Then, a current commutation drive circuit (CCDC) is proposed as the other current commutation measure. Equivalent current commutation tests have been carried out. The results show with the CCDC, both positive current and negative current can be commutated within tens of microseconds. Using a CCDC in series with a mechanical vacuum switch as the MS branch, a 44 kV/3.4 kA hybrid DCCB prototype is developed. Based on the prototype, current commutation and interruption tests are carried out. According to the results, the current of 3.4 kA can be commutated within 100 $\mu $ s, and be interrupted within 2 ms.

Research on Current Commutation Measures for Hybrid DC Circuit Breakers

The fast dc line protection is one of the key techniques for the multi-terminal flexible dc grid whose main technical difficulty lies in quick and reliable fault identification. The traditional protection principles for the ac system are not suitable for the flexible dc grid, and those for the conventional dc transmission system can also not be applied directly. Therefore, the frequency-domain traveling-wave boundary characteristics after faults are analyzed first in this paper to find the critical condition for distinguishing the internal and external faults as well as the forward and backward faults. And then, a novel single-ended transient-voltage-based protection strategy for flexible dc grid was proposed, which mainly consists of the fault identification criterion and the direction criterion. The protection scheme used wavelet transform to extract transient voltage energy. In order to eliminate the influences of the wavelet transform boundary effect and converter blocking, the principle to select the proper data window was proposed. Compared with the existing traveling-wave-based dc protection, the proposed method has a higher reliability and stronger ability to endure high transition resistance, which was verified by lots of simulation cases based on the PSCAD/EMTDC.

A Novel Single-Ended Transient-Voltage-Based Protection Strategy for Flexible DC Grid

The continuous development of the voltage-source-converter-based multiterminal high-voltage direct-current transmission system has led to a great need for dc circuit breakers to isolate short-circuit faults in the dc side. The operating time of dc breakers is mainly dependent on the mechanical contacts separation time. In this paper, three key units of the ultra-fast operating mechanism, including drive unit, buffer unit, and holding unit were investigated. A prototype of the 40.5-kV ultra-fast vacuum switch has been developed where the Thomson-coil actuator (TCA) is used to fulfill the function of the drive unit and the buffer unit, and a bistable spring mechanism is used as the holding unit. The state equations of the bidirectional TCA used as the drive unit and buffer unit in the operating process are completed based on the equivalent circuit method and finite-element method. The experimental and simulation results of the prototype are compared and they are in good agreement as expected. Therefore, the mathematical model is verified. The factors that influence the drive unit and the buffer unit performances are discussed. The achieved operating time of the prototype is about 2.3 ms.

Research on Operating Mechanism for Ultra-Fast 40.5-kV Vacuum Switches

Voltage source converter based multiterminal HVDC (VSC-MTDC) technology has shown many advantages. However, following a large disturbance, the dc voltage and the output power of the VSC station with fixed droop control may hit their limits, thereby reducing the dc voltage control ability of the whole MTDC system. Here an adaptive droop control scheme is proposed on the basis of both the dc voltage deviation factor and the power sharing factor. It contributes to ensure the dc voltage and the power loading rate of each converter within their limits during large disturbances, and the power sharing capability of the whole MTDC system remain high. The working principle of the proposed adaptive droop control is discussed in details. Case study and simulations are performed based on the PSCAD/EMTDC model of a ±320 kV four-terminal VSC-HVDC system to validate the effectiveness of the proposed adaptive droop method.

Adaptive Voltage Droop Method of Multiterminal VSC-HVDC Systems for DC Voltage Deviation and Power Sharing

The overhead line will be widely applied in the future flexible high-voltage dc (HVDC) grid. Correspondingly, effective reclosing strategy which can identify the fault property must be configured for the dc circuit breaker (DCCB) to improve the system power supply reliability. However, if the DCCB is reclosed directly, large second overcurrent may occur to damage the vulnerable devices in the system. Therefore, the inherent voltage characteristic difference between different dc faults (nonpermanent or permanent) is analyzed in the paper. And then a novel DCCB reclosing strategy for the flexible HVDC grid is proposed. Compared with the method which recloses DCCB directly, the proposed strategy can avoid the second damage under permanent fault condition, because it can identify the fault property without completely reclosing the DCCB (only the residual dc current breaker is reclosed before fault property identification). Moreover, compared with the typical sequential auto-reclosing strategy, the proposed strategy significantly reduces the additional control and design requirements on the DCCB. Finally, simulation cases based on the PSCAD/EMTDC are carried out to verify the correctness of the theoretical analysis and the superiorities of the proposed reclosing strategy.

Weijie Wen

Papers

Research on Current Commutation Measures for Hybrid DC Circuit Breakers

A Novel Single-Ended Transient-Voltage-Based Protection Strategy for Flexible DC Grid

Research on Operating Mechanism for Ultra-Fast 40.5-kV Vacuum Switches

Adaptive Voltage Droop Method of Multiterminal VSC-HVDC Systems for DC Voltage Deviation and Power Sharing

A Novel DCCB Reclosing Strategy for the Flexible HVDC Grid