DC Microgrid has become a superior power system in recent years due to development of DC loads and higher efficiency of DC systems. One of the challenging problems of Microgrids is protection.
Protection of DC Microgrids is still a particular concern associated with the challenges of developing a proper protection scheme owing to its characteristics and lack of adequate standards in DC protection. Due to the significantly increasing interest on DC power systems; this paper investigates protection problems and schemes that need to be considered in modern power systems involving DC microgrids. This paper analyzes and presents a comprehensive review of the most recent growth in the DC Microgrids protection. Additionally, the fault characteristics of DC power systems and some of the proposed methods to overcome the protection problems are discussed. The differences between the proposed protection methods for the DC Microgrids are also discussed.DC Microgrids have the wide potential for different power applications,such as small scale generation, backup of energy storages, data centers, marineand other sensitive loads and industrial applications 1, 2. due to theadvantages of DC Microgrids over AC traditional power systems, according to thetheir power density and efficiency3. On the other hand, the lack of effectiveprotection scheme for DC Microgrids represents a significant obstacle to moreadoption of DC systems.
The particular challenge associated with DC Microgridsinclude protection against short circuit faults. Therefore, there has beenconsiderable attention on developing a protection method for DCMicrogrids. Protection plays a crucial role in the power systems, and one of the mainpurposes of introducing Microgrids is increasing the reliability. Hence, one of the main problems of using Microgrids is related toprotection issues, because the protection of Microgrids may not be solved byconventional methods for the several reasons 4 such as, bidirectional powerflow of Microgrids, dynamic characteristics of renewable resources, changing thefault current during islanded mode, number, and type of the resources.Therefore, proposing an adequate protection method is essential for theMicrogrids in both grid and islanded mode. Because both loads and sources can interfaceto a common DC bus with fewer redundant stages of power conversion, the resultis less waste heat and potentially lower cost than AC based implementations ofDER. Moreover, DC transmission lines can flow more power than AC. The positiveaspects of using DC Microgrids is that loads, power sources and storage devicescan be connected simpler and more efficient.
DC Microgrids provides greatadvantages in terms of resilience, efficiency, reliability, and flexibility 5.In addition, the DC Microgrids have the following advantageous: · Most residentialloads are DC or can operate with DC voltage.· Due to the lackof skin effect in DC cables, the cable losses can decrease of 15% to 20% 6· DC systems aremore safe for human bodies, the damages to the human body in the DC systemdischarges is lower than AC 7. · High powertransfer capacity 7.· Fewer redundantstages of converters can reduce the losses and heat, due to the both loads andresources are DC 8. · Multipleresources can connect to a bus, therefore, power flow is more robust 8.
· Most of storagedevices such as battery and ultra-capacitors are DC 9. · Synchronization problems are eliminated in DC Microgrids 10.DC Microgrids are being considered in severalapplications, and they aredivided into two voltage level, Medium Voltage DC (MVDC) and Low Voltage DC(LVDC). MVDC Microgrids are of wide interest for marine and shipboard powersystems, because this will be able to prepare the power and energy density ofmarine systems 11,12, and MVDC systems is rated from 1500v to 22kV 13. Alsodifferent application of MVDC systems are introduced in 14-17. On the otherhand, LVDC Microgrids is suitable to use because most loads in the low voltagerate are DC.
LVDC Microgrids can be used in the wide applications such as,telecom power systems, power system controls, protection systems and residential 18. LVDC Microgrids must connectto the AC power system using converters, and power flow of the system will bebi-directional and therefore a different protection scheme is required for theDC Microgrid 19. Fig. 1 shows a typical DC Microgrid. On theother hand, due to the differences between the protection methods of DCMicrogrids and AC, fault location, classification and detection are theprotection challenges in these systems. Therefore, DC system must beeffectively protected against short circuit faults. A protection method mustpropose a sensitive, reliable and selective protection in the DC Microgrids.