DC microgrids (MGs) are a modern form of electricity distribution system that use DC instead of AC to transmit and distribute electrical energy. In a DC MG, various distributed energy resources (DERs) such as photovoltaic (PV) systems, wind turbines, and energy storage devices are connected to a common DC bus through power electronics interfaces. DC MGs are becoming increasingly popular due to their numerous advantages over traditional AC grids, including improved energy efficiency, higher power quality, greater flexibility, and economical reasons in integrating renewable energy sources ^[1][2][3][1,2,3]. They are also considered to be an important solution for addressing the challenges of the increasing demand for electricity, energy security, and climate change mitigation.

However, the design and operation of DC MGs pose unique challenges, such as controlling power flow and maintaining stability and reliability, which require innovative solutions. As a result, research and development efforts in the field of DC MGs are ongoing, with the aim of improving their performance and expanding their application ^[4][5][6][4,5,6].

An overview of a typical DC MG is presented in Figure 1. DC circuit breakers are placed at various locations in the grid, near the renewable energy resources, transmission line, main grid, battery bank, and load sides to ensure microgrid protection and maintenance. The DC circuit breaker (CB) types vary due to the presence of different levels of voltage and current paths within the network, ranging from generation to load. The primary objective of having a DCCB in DC systems is to protect the system against intentional or unintentional faults and voltage or current switching surges ^{[7][8][9][10]}[7,8,9,10]. Table 1 provides a brief comparison between the DCCB specifications of some manufacturers ^{[11][12][13][14][15]}[11,12,13,14,15]. The selection of DCCBs will be based on the working conditions, the voltage and current level, and the thermal capacity, which is substantially affected by 𝐼2𝑡 of the beaker. The AC system can endure the fault current for a slightly longer period of time when it is experiencing a thermal overload or overcurrent, since the fault current rise rate is comparatively slow. Due to the DC system’s low short-circuit impedance and the rapid rising time of the fault current, it must be stopped immediately ^[16][17][18][16,17,18].

Furthermore, DCCBs are also utilized for maintaining the devices within the DC system. When a component of the system requires maintenance, the DCCB can safely interrupt the circuit, allowing the maintenance work to be carried out without posing a risk to personnel or damaging the equipment.

The DCCB system has enabled some researchers ^[19][20][21][19,20,21] to focus on DC MG fault current limiting, control, and clearing. These areas of study have been widened to include DC MG clusters.

For DCCBs based on MOVs, the circuit breaker’s embedded MOV completely handles surge absorption. Different designs define the strategy for absorbing the released energy ^{[22][23][24][25]}[49,50,51,52].

The significant drawback of these designs is that the MOVs deteriorate over time when exposed to surges ^[26][27][22,30].

The separated MOV technique shown in Table 2 illustrates an approach that involves two distinct MOVs placed in specific locations within a circuit, to isolate the two functions of the MOV, namely voltage clamping and energy absorption. This proposed approach aims to separate the two functions of MOVs, allowing them to operate independently and more efficiently. The result shows successful test and surge absorption through the paralleled MOV’s circuit. However, the test is restricted to a small range of voltage and current amplitudes.

^46][68,69,70,71].

All designs shown in Table 4 effectively perform fast through different techniques. The capacitor discharge path is considered for all models, and the voltage and current levels in the circuits are different.

The ground clamping strategy illustrates a new DCCB design that uses a current limiter to absorb the surge voltage ^[29][54]. This SSCB design consists of several components including a main switch (S₂) that conducts the line current and prevents source voltage before and after breaker operation. Additionally, the design includes a MOV to demagnetize the energy stored in the system inductor, a ground clamping switch (S₁) that bypasses the DC bus, and a current-limiting inductor (L₂) with its resistive energy absorber. In this design, minimum current limiting inductance could be determined by Equation (21) according to the bus voltage, $V_{DC{}_{}}$ , the breaking time, 𝑇𝐵𝑟𝑒𝑎𝑘, the zero current detection time, 𝑇𝑑𝑒𝑡, the saturation current of the inductor, 𝐼𝐿𝑠𝑎𝑡, and the threshold current of 𝐼0:

Furthermore, the magnetic coupling fault current limiter (MC FCL) technique proposes a fault current limiting design with a magnetic coupling auxiliary circuit in the input of the circuit breaker to limit the severity of the current shocks.  In this study, the released energy is coordinated to be dissipated in both the MOV and the resistance in the secondary of the transformer.

Overall, while various techniques for surge absorption have been successful in damping surges and fault currents, there are design issues that need to be addressed. For example, many designs rely on metal oxide varistors (MOVs) for surge absorption, but often fail to consider their limitations, as their ability to damp surges can weaken, and they will degrade over time.

To enhance the design of DCCBs, some studies have explored the use of capacitor-based technology for both commutating and surge absorption purposes. This approach involves a bridge-type capacitor-commutation unit that serves to buffer the device voltage and is considered an independent method for improving DCCB design ^[31][32][56,57]. Table 3 provides a concise overview of the recent designs, highlighting their points of comparison.

All three capacitor-based DCCB designs mentioned in Table 3 redirect surges to a subcircuit to reduce the impact of energy released during DC system faults or surges. They effectively absorb energy using specific techniques. However, the designs differ in terms of their response time, their voltage and current levels, and the components used in the circuit.

Other studies have employed unidirectional and bidirectional Z-source DCCB (Z-s DCCB) designs, whose strategy is focused on a capacitor-based design ^{[36][37][38][39][40][41]}[61,62,63,64,65,66]. Z-s DCCBs show potential as suitable options for protect low- and medium-voltage distribution networks, along with DC equipment, among the various configurations available, because of their uncomplicated structure, control mechanism, and economical price ^[42][67]. 

In these designs, electrolytic capacitors for surge absorption are utilized, but these components have limited capacity and cannot dissipate the energy released by high-voltage faults with a longer duration and have a limited lifespan due to their chemical structure.

Several studies have proposed novel designs of hybrid MOV–Cap DCCBs to overcome the weakness of MOV degradation and the restricted capability of capacitors to absorb energy ^{[43][44][45][}

In the active thyristor CB (AT-CB) technique, a bidirectional, low-loss DCCB with a reliable opening process based on a simple hybrid design for a capacitor and a MOV is implemented. This technique is more suitable for medium-voltage DC systems ^[47][72].

Therefore, the value of the bypass capacitor can be approximated using Equation (23), which involves determining the recovery time, 𝑡𝑞, which can be calculated using Equation (22), where the values for the maximum allowable fault, 𝐼𝑓𝑚𝑎𝑥, and the desired coefficient, α, are inserted.

Other techniques involving active injection circuits (LCC-AICs) based on TIM-Pack (Thyristor–IGBT–MOV) and on an inductor capacitor-capacitor, shown in Table 4, switch fault currents into the designed subcircuits within a couple of microseconds to improve reliability.