DC protection method for DC Microgrids. Protection plays a

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, marine
and other sensitive loads and industrial applications 1, 2. due to the
advantages of DC Microgrids over AC traditional power systems, according to the
their power density and efficiency3. On the other hand, the lack of effective
protection scheme for DC Microgrids represents a significant obstacle to more
adoption of DC systems. The particular challenge associated with DC Microgrids
include protection against short circuit faults. Therefore, there has been
considerable attention on developing a protection method for DC
Microgrids.  

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Protection plays a crucial role in the power systems, and one of the main
purposes of introducing Microgrids is increasing the reliability. Hence, one of the main problems of using Microgrids is related to
protection issues, because the protection of Microgrids may not be solved by
conventional methods for the several reasons 4 such as, bidirectional power
flow of Microgrids, dynamic characteristics of renewable resources, changing the
fault current during islanded mode, number, and type of the resources.
Therefore, proposing an adequate protection method is essential for the
Microgrids in both grid and islanded mode.  

Because both loads and sources can interface
to a common DC bus with fewer redundant stages of power conversion, the result
is less waste heat and potentially lower cost than AC based implementations of
DER. Moreover, DC transmission lines can flow more power than AC. The positive
aspects of using DC Microgrids is that loads, power sources and storage devices
can be connected simpler and more efficient. DC Microgrids provides great
advantages in terms of resilience, efficiency, reliability, and flexibility 5.
In addition, the DC Microgrids have the following advantageous:

·      
Most residential
loads are DC or can operate with DC voltage.

·      
Due to the lack
of skin effect in DC cables, the cable losses can decrease of 15% to 20% 6

·      
DC systems are
more safe for human bodies, the damages to the human body in the DC system
discharges is lower than AC 7.

·      
High power
transfer capacity 7.

·      
Fewer redundant
stages of converters can reduce the losses and heat, due to the both loads and
resources are DC 8.

·      
Multiple
resources can connect to a bus, therefore, power flow is more robust 8.

·      
Most of storage
devices such as battery and ultra-capacitors are DC 9. 

·       Synchronization problems are eliminated in DC Microgrids 10.

DC Microgrids are being considered in several
applications, and they are
divided into two voltage level, Medium Voltage DC (MVDC) and Low Voltage DC
(LVDC). MVDC Microgrids are of wide interest for marine and shipboard power
systems, because this will be able to prepare the power and energy density of
marine systems 11,12, and MVDC systems is rated from 1500v to 22kV 13. Also
different application of MVDC systems are introduced in 14-17. On the other
hand, LVDC Microgrids is suitable to use because most loads in the low voltage
rate 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 connect
to the AC power system using converters, and power flow of the system will be
bi-directional and therefore a different protection scheme is required for the
DC Microgrid 19. Fig. 1 shows a typical DC Microgrid.

 On the
other hand, due to the differences between the protection methods of DC
Microgrids and AC, fault location, classification and detection are the
protection challenges in these systems. Therefore, DC system must be
effectively protected against short circuit faults. A protection method must
propose a sensitive, reliable and selective protection in the DC Microgrids.

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