Then research prospects are presented in Section 5. Section 4 reviews proposed protection schemes and discusses the design of a protection system.
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In Section 3, current status is depicted in detail, including fault current response, protection devices, grounding options and simulation software of DC microgrids. Section 2 gives an introduction about typical DC microgrids. The organization of the paper is as follows. As a result, this paper intends to depict the current research status in order to help researchers to have a clear understanding on the protection issues of DC microgrids. The design of protection schemes for DC microgrids has not drawn enough attention. In addition, the implementation of protection has to restrain the transient voltage and requires operating the devices in a coordinated way.Īt present, studies on DC microgrids are primarily concerned with the topology structure, control method and energy management. Previous researches indicate that typical voltage sourced converters (VSCs) can only tolerate twice the converter full-load current rating and the fault clearing time should be around 2ms. Due to the high magnitude and significant changing rate of the fault currents, fault detection and isolation should be evaluated as soon as possible.
However, they did not reach a consensus and the widespread acceptance of standards still have a way to come. Some organizations have developed practical standards in specific fields such as the European standard ETSI EN 300 132-3-1, IEEE Standard 946 and IEC SG4, which are respectively designed for data/telecom systems, DC auxiliary power systems and LVDC distribution systems. In addition, protection standards and guidelines that can be widely accepted in industrial and commercial applications have not been proposed. In spite of the excellent characteristics of DC microgrids, effective fault protection schemes for DC microgrids still remain a challenge. They all aim to promote this technology at a new research level. Realizing this potential, academic and industrial institutions are conducting projects around the world, for example FREEDM, UNIFLEX and HEART. DC microgrids have been investigated in specific applications, including: ① LV (up to 1.5 kV DC) data centers and LVDC distribution systems ② MV (up to 35 kV DC) systems on electric ships and also offshore wind power plants. High-voltage DC (HVDC) technologies have been utilized extensively, while the practical application of medium-voltage (MV) and low-voltage (LV) DC microgrids is just beginning.
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Moreover, DC systems are inherently efficient without any skin effect and can decrease line losses. Compared to AC microgrids, DC microgrids require less conversion stages and transmit more DC power through a given cable. Nowadays, the advantages shown within DC microgrids arouse increasing interests of scholars around the world.
AC microgrids have been comprehensively researched because of the similarities to the traditional AC power system. According to the power properties, microgrids can be divided into AC and DC microgrids. The development of DERs can effectively reduce the carbon emissions, while enhancing the power quality and reliability. The concept of microgrids has been proposed in order to make good use of distributed energy resources (DERs). In addition, the novel concept of coordinated strategy of control and protection of the DC microgrids is explained. The future protection research directions lie in the development of novel protection devices, which are based on electronic technology to provide loose protection constraints and the improvement of suitable protection schemes. Meanwhile, to solve the protection issues and promote the development of the DC microgrid, this paper points out the key areas of future research. Therefore, this paper intends to depict the current research status in different relative areas and review the proposed protection strategies in order to help researchers to have a clear understanding on DC microgrid protection. At present, researches on DC microgrids primarily focus on the topology structure, control method and energy control, while researches on fault analysis, detection and isolation have not drawn enough attention. However, a well-designed protection scheme still remains a challenge for DC microgrids. The DC microgrid has become a typical distribution network due to its excellent performance.
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