A Bi-Directional DC Solid-State Circuit Breaker Based on Flipped Γ -Source

: As the continuous consumption of non-renewable energy leads to resource shortages, distributed PV generation technology has received widespread attention. The DC microgrid is an effective way to connect distributed PV generation. However, DC microgrids have numerous challenges, such as the absence of zero current crossing point and high fault current rising rate. In the paper, a bi-directional DC solid-state circuit breaker based on ﬂipped Γ -source is proposed to overcome some of the challenges in DC microgrid. The topology uses the mutual inductance current generated by the transformer to force the SCR naturally commutates off. Which can rapidly interrupt and isolate the faulty part, and at the same time, there will be no circulating current impact on the source side. The diode bridges allow the response to faults on either the source or load side with only a single controlled switch. Therefore, the topology is simpliﬁed while enabling it to obtain the capability of bidirectional operation protection. Finally, using the simulation software PSCAD/EMTDC and experimental platform veriﬁes the effectiveness of the topology in this paper.


Introduction
In recent years, as the consumption of non-renewable energy leads to resource shortages and the deterioration of environmental pollution, distributed renewable energy power generation technologies such as wind power generation and solar power generation have received widespread attention. DC microgrid serves as effective access to distributed power generation technologies. Compared with the AC microgrid, the DC microgrid can more efficiently and stably receive distributed renewable energy power generation systems such as wind energy and photovoltaics, energy storage devices, electric vehicles, and other DC loads. Although the DC microgrid has many advantages, the low system impedance of the DC microgrid will cause the short-circuit current to rise rapidly when the DC side fails. At the same time, the current in the DC system cannot naturally cross the zero point, and it is difficult to shut down. These problems have become the key to restricting the wide application of DC microgrids [1][2][3][4].
DC circuit breakers are divided into pure mechanical DC circuit breakers broken off by traditional mechanical switches. Solid-state circuit breakers perform interruption by power electronic devices and hybrid circuit breakers combining the above two structures [5][6][7][8][9]. Compared with the pure mechanical DC breakers, the solid-state circuit breakers have no mechanical switch part, and power electronic devices have good functional characteristics. The solid-state circuit breakers use the advantage of power electronic devices to realize arc-free shutdown and rapidly isolate the fault current. At the same time, its usual time is significantly increased, which is suitable for operation protection of medium and low voltage DC microgrid [10][11][12][13].

Overview of Z-Source Circuit Breakers
The DC microgrid has promoted the development of DC solid-state circuit breakers. The topology of traditional Z-source DC circuit breakers is divided into crossed topology, parallel-connected topology and series-connected topology. Table 1 explains the type of circuit breaker represented by the symbol. As shown in Figure 1, the three types of DC circuit breakers are mainly composed of SCR, capacitor, inductance, and RD buffer circuits. The red line in Figure 1 is the working state when the fault occurs. The fault current on the load side passes through the Z source capacitor and flows to the short-circuit point. Since the current in the line inductance cannot change suddenly when the reverse current in the SCR equals the steady-state load current of the line, the SCR is automatically closed, and the energy in the line decays to zero through the buffer circuit, thereby achieving the effect of quickly isolating the fault current.
Circuit structures, such as Γ-source and Z-source, are first applied to the research of inverters. With the development of PV Generation, they are applied to the field of solid-state DC circuit breakers. The crossed topology, shown in Figure 1a, fails to provide a common ground point between the source and load, so the actual application of the project is affected. The parallel-connected topology is shown in Figure 1b. The structure is transformed on the basis of the crossed type so that there is a common point between the source and the load, but when a fault occurs, the fault current will have a circulating current impact on the source side- Figure 1c shows the series-connected topology. Along with the common point, the structural characteristics of the Z-source capacitor greatly reduce the impact of the fault current on the source side, so it has become a relatively ideal solution at present. As shown in Figure 2, Three topology structures of bidirectional Z-source DC circuit are proposed by references [13,14]. Figure 2a,b shows that the characteristics of diode and SCR need to be used to achieve bidirectional operation, but the number of capacitors and inductances needs to be increased, and the current of LC branch will still have an impact on the source side. As shown in Figure 2c, the bidirectional Z-source DC circuit breaker adopting coupled inductors uses the mutual inductance current in the coupled inductors to realize automatic shutdown of the SCR, but the two sets of coupled inductors increase the volume and cost. Circuit structures, such as Γ-source and Z-source, are first applied to the research of inverters. With the development of PV Generation, they are applied to the field of solidstate DC circuit breakers. The crossed topology, shown in Figure 1a, fails to provide a common ground point between the source and load, so the actual application of the project is affected. The parallel-connected topology is shown in Figure 1b. The structure is transformed on the basis of the crossed type so that there is a common point between the source and the load, but when a fault occurs, the fault current will have a circulating current impact on the source side- Figure 1c shows the series-connected topology. Along with the common point, the structural characteristics of the Z-source capacitor greatly reduce the impact of the fault current on the source side, so it has become a relatively ideal solution at present. As shown in Figure 2, Three topology structures of bidirectional Zsource DC circuit are proposed by references [13,14]. Figure 2a,b shows that the characteristics of diode and SCR need to be used to achieve bidirectional operation, but the number of capacitors and inductances needs to be increased, and the current of LC branch will still have an impact on the source side. As shown in Figure 2c, the bidirectional Z-source DC circuit breaker adopting coupled inductors uses the mutual inductance current in the coupled inductors to realize automatic shutdown of the SCR, but the two sets of coupled inductors increase the volume and cost. Circuit structures, such as Γ-source and Z-source, are first applied to the research of inverters. With the development of PV Generation, they are applied to the field of solidstate DC circuit breakers. The crossed topology, shown in Figure 1a, fails to provide a common ground point between the source and load, so the actual application of the project is affected. The parallel-connected topology is shown in Figure 1b. The structure is transformed on the basis of the crossed type so that there is a common point between the source and the load, but when a fault occurs, the fault current will have a circulating current impact on the source side- Figure 1c shows the series-connected topology. Along with the common point, the structural characteristics of the Z-source capacitor greatly reduce the impact of the fault current on the source side, so it has become a relatively ideal solution at present. As shown in Figure 2, Three topology structures of bidirectional Zsource DC circuit are proposed by references [13,14]. Figure 2a,b shows that the characteristics of diode and SCR need to be used to achieve bidirectional operation, but the number of capacitors and inductances needs to be increased, and the current of LC branch will still have an impact on the source side. As shown in Figure 2c, the bidirectional Z-source DC circuit breaker adopting coupled inductors uses the mutual inductance current in the coupled inductors to realize automatic shutdown of the SCR, but the two sets of coupled inductors increase the volume and cost.

Design of Bidirectional DC Circuit Breaker Based on Flipped Γ-Source
The bidirectional DC circuit breaker based on flipped Γ-source is shown in Figure 3, which is composed of a transformer, an SCR, capacitors, a diode bridge, a load capacitor and a load. Compared with the topology in Figure 1, the topology in this paper uses the mutual inductance current produced by the transformer to force the SCR naturally com-

Design of Bidirectional DC Circuit Breaker Based on Flipped Γ-Source
The bidirectional DC circuit breaker based on flipped Γ-source is shown in Figure 3, which is composed of a transformer, an SCR, capacitors, a diode bridge, a load capacitor and a load. Compared with the topology in Figure 1, the topology in this paper uses the mutual inductance current produced by the transformer to force the SCR naturally commutate off, which replaces the LC branch of the traditional Z-source topology solves the problem of circulating current impact on the source side. The bidirectional dc circuit breaker proposed in this paper has full-bridge diodes. The full-bridge arrangement of diodes allows load current to flow in either direction while retaining the same conduction path through SCR. Thus, the bidirectional dc circuit breaker limits and interrupts fault current regardless of the direction of power flow. Owing to the diode bridge circuit in the topology, only one controlled switch can realize bidirectional operation protection. Compared with the topology in Figure 2, only an SCR and a two-winding transformer are utilized to reduce the number RD buffer circuit. Thus, the topology is simplified while enabling it to have the capability of bidirectional operation protection.
To figure out the characteristics of different types of DC circuit breakers, the characteristics of various circuit breakers are analyzed in detail in Table 2. The advantages of bi-based on flipped ΓSB proposed in this paper are discussed by comparing the common ground, bidirectional operation protection capability, fault current clearing capability, and the number of SCR. Compared with traditional ZSCBs, the ΓSCB using a transformer instead of two separate inductors can offer three advantages. The first advantage is a smaller volume, the second advantage is a settable turns ratio to optimize the protection, and the third advantage is the anti-saturation characteristic. The proposed topology can achieve bidirectional energy flow and achieve bidirectional fault interruption and isolation. Obviously, this topology has a simpler circuit, fewer components, and common ground compared with the previous approach.

Working Principle
Different operating states of ΓSCB are presented in Figure 4. The forward energy flow channel consists of an SCR, the diode D 1 and D 2 , capacitor C, the primary coil L 1 of the transformer. Similarly, the SCR, the diode D 2 and D 3 , capacitor C, the primary coil L 1 of the transformer, constitute the channel of the reverse energy flow. The normal operating states of the circuit are shown in Figure 4a. At th left side is the source, and the right side is the load. In the operating state, the path from the source side to the load side through L1 and SCR is shown in th the Figure. The voltage across the capacitor C can be regarded as equal to the age under the premise of neglecting the voltage drop of SCR. When the fau the load side, the working state of ΓSCB is shown in Figure 4b. Part of the f generated by capacitor C flows through the second coil of the transformer L compensate for the fault current. The other part flows through load capacito pensate for the fault current. During this process, the current in the secondar transformer L2 will rise rapidly. Due to the mutual inductance of the transform mary coil of the transformer L1 will generate an induced current opposite to current is, and when the two are equal, the SCR is turned off. After the SCR i the fault is completely isolated from the source side, and the current flows in ing path on the load side until the remaining energy decay to zero, as shown Due to the diode bridge circuit, ΓSCB does not need to add a buffer circuit, an ing principle of the forward energy flow is equal to the reverse energy flow.
Vs CL RL The normal operating states of the circuit are shown in Figure 4a. At this time, the left side is the source, and the right side is the load. In the operating state, the conduction path from the source side to the load side through L1 and SCR is shown in the red line of the Figure. The voltage across the capacitor C can be regarded as equal to the supply voltage under the premise of neglecting the voltage drop of SCR. When the fault occurs on the load side, the working state of ΓSCB is shown in Figure 4b. Part of the fault current generated by capacitor C flows through the second coil of the transformer L2 and D4 to compensate for the fault current. The other part flows through load capacitor CL to compensate for the fault current. During this process, the current in the secondary coil of the transformer L2 will rise rapidly. Due to the mutual inductance of the transformer, the primary coil of the transformer L1 will generate an induced current opposite to the original current is, and when the two are equal, the SCR is turned off. After the SCR is turned off, the fault is completely isolated from the source side, and the current flows in the circulating path on the load side until the remaining energy decay to zero, as shown in Figure 4c. Due to the diode bridge circuit, ΓSCB does not need to add a buffer circuit, and the working principle of the forward energy flow is equal to the reverse energy flow.  The normal operating states of the circuit are shown in Figure 4a. At this time, the left side is the source, and the right side is the load. In the operating state, the conduction path from the source side to the load side through L 1 and SCR is shown in the red line of the Figure. The voltage across the capacitor C can be regarded as equal to the supply voltage under the premise of neglecting the voltage drop of SCR. When the fault occurs on the load side, the working state of ΓSCB is shown in Figure 4b. Part of the fault current generated by capacitor C flows through the second coil of the transformer L 2 and D 4 to compensate for the fault current. The other part flows through load capacitor C L to compensate for the fault current. During this process, the current in the secondary coil of the transformer L 2 will rise rapidly. Due to the mutual inductance of the transformer, the primary coil of the transformer L 1 will generate an induced current opposite to the original current i s , and when the two are equal, the SCR is turned off. After the SCR is turned off, the fault is completely isolated from the source side, and the current flows in the circulating path on the load side until the remaining energy decay to zero, as shown in Figure 4c. Due to the diode bridge circuit, ΓSCB does not need to add a buffer circuit, and the working principle of the forward energy flow is equal to the reverse energy flow.

Analysis of the Impedance Network
k is the coupling coefficient of the two coupling coils, in this paper, k = 1. When the DC circuit breaker is in normal working condition, the mutual inductance value of the two-winding transformer is M.
The inductance of the primary and secondary coils of the transformer is L 1 and L 2 .
According to the normal working equivalent circuit of DC circuit breakers in Figure 5, the input and output voltage transfer function is determined, and the steady-state characteristics of the circuit are analyzed. In the case of a resistive load, through theoretical derivation, the output and input voltage transfer function can be obtained as: The inductance of the primary and secondary coils of the transformer is According to the normal working equivalent circuit of DC circuit breake 5, the input and output voltage transfer function is determined, and the stead acteristics of the circuit are analyzed. In the case of a resistive load, through derivation, the output and input voltage transfer function can be obtained as  Substituting the relevant data of the simulation parameters into Formula cific Bode diagram can be obtained, as shown in Figure 6, in the normal worki gain of the transfer function in the low-frequency range is approximately un the phase angle is close to zero degrees, which does not affect the normal ope system. In the high frequency range, it is very close to the function of the low Therefore, the topology structure plays a filtering role under the premise of pr system's normal operation and guarantees the operating efficiency of the DC  Substituting the relevant data of the simulation parameters into Formula (3), the specific Bode diagram can be obtained, as shown in Figure 6, in the normal working state, the gain of the transfer function in the low-frequency range is approximately unity gain and the phase angle is close to zero degrees, which does not affect the normal operation of the system. In the high frequency range, it is very close to the function of the low-pass filter. Therefore, the topology structure plays a filtering role under the premise of protecting the system's normal operation and guarantees the operating efficiency of the DC microgrid.

Analysis of Fault Current Change Rate
For the parameter design of the topological structure of the reversed ΓSCB, when a fault occurs on the load side, the SCR can force automatic commutation only when the fault current rises to a critical value to achieve the purpose of fault isolation, so the rate o rising of the fault current and the sum current are important parameters that determine the automatic shutdown of the SCR. According to Figure 4b, the fault current ifault, the sec ondary coil of the transformer L2, capacitor C and the load capacitor CL form a loop. The current path of a short-circuit fault in the DC circuit breaker shows that the fault curren ifault can be expressed by the two capacitor line currents as: In the initial state of short-circuit fault, ifault can indicate: In Formula (6), Vo is the output voltage of the load, G is the fault conductance. Ne glecting the system line voltage drop, according to Formulas (4) and (6): In Formula (7), Gmin expresses as the smallest detectable fault conductance. The rate of rising of the short-circuit fault current determines the detection capability of the DC circuit breaker. The following analysis assumes that the change in fault con ductance with time is a linear relationship, and the fault change rate K can be expressed as: where Δt is a time interval for the ramp. The fault current in terms of K can be expressed

Analysis of Fault Current Change Rate
For the parameter design of the topological structure of the reversed ΓSCB, when a fault occurs on the load side, the SCR can force automatic commutation only when the fault current rises to a critical value to achieve the purpose of fault isolation, so the rate of rising of the fault current and the sum current are important parameters that determine the automatic shutdown of the SCR. According to Figure 4b, the fault current i fault, the secondary coil of the transformer L 2 , capacitor C and the load capacitor C L form a loop. The current path of a short-circuit fault in the DC circuit breaker shows that the fault current i fault can be expressed by the two capacitor line currents as: In the initial state of short-circuit fault, i fault can indicate: In Formula (6), V o is the output voltage of the load, G is the fault conductance. Neglecting the system line voltage drop, according to Formulas (4) and (6): In Formula (7), G min expresses as the smallest detectable fault conductance. The rate of rising of the short-circuit fault current determines the detection capability of the DC circuit breaker. The following analysis assumes that the change in fault conductance with time is a linear relationship, and the fault change rate K can be expressed as: where ∆t is a time interval for the ramp. The fault current in terms of K can be expressed as According to Formula (5), we can receive In the initial state when a fault occurs, V o = V s , from Formulas (4)-(6), we obtain V s is the source input voltage, from Formulas (9) and (11), the fault current i fault and the current through Z-source capacitor are rewritten as By solving Formula (13), the time at which capacitor current becomes maximum can be derived as Substituting (14) into (13), maximum capacitor current can be obtained as N is the turn of the winding L 1 and L 2 of the transformer, respectively. The ratio of the induced current between the primary coil and the secondary coil of the two-winding transformer is i L1 :i L2 = 1:N, and the induced current i L2 on the secondary coil equals to capacitor current i c , so when a fault occurs, the SCR can be automatically turned off when 1 N i C ≥ i s , i s is the steady-state load current, so it can be obtained from Formula (15): Therefore, when the change rate of the short-circuit fault conductance satisfies Formula (17), ΓSCB can work normally. Otherwise, the DC circuit breaker will not work normally. When determining the size of the system parameters, it needs to be based on the minimum fault current. It is required to select the appropriate C L /C ratio, set the minimum fault change rate by changing the size of the capacitor value, and also consider the short-term fault current level of the SCR and the reverse recovery time to ensure the normal operation of the DC circuit breaker In order to achieve the appropriate minimum fault ramp rate K min and to ensure the safe commutation of the SCR in sufficient time, the optimal value of the inductor must be selected, and the inductor current can be obtained: According to Formulas (13) and (18), the current flow SCR can be obtained: The inductor current should remain constant under fault conditions to minimize the detectable change rate of short-circuit faults. Therefore, the value of the inductance should be large enough so that the influence of the inductor current on the capacitor current is kept to a minimum. At the same time, the current through the SCR will not be affected.
In this paper, N = 2, substituting (20) into (17), we obtain Formula (21) shows the important relationship between the load R L and the system parameters, and the result also shows that the two-winding transformer is proportional to the capacitance C.

Extended the ΓSCB Simulation
In order to verify the effectiveness of the DC microgrid protection design based on the bidirectional DC circuit breaker based on flipped Γ-source in this paper. A simulation model with source voltage U DC = 1 kV, the steady-state load current i s = 0.1 kA, and the short-circuit fault current peak value are 0.65 kA is established in the PSCAD/EMTDC software, and the working status of the DC circuit breaker when a short-circuit fault occurs on the load side is also analyzed. The two-way topology adopts two operating modes of the forward power flow and reverse power flow in the simulation. The mutual inductance value of the two-winding transformer is M = 400 µH, and other relevant component parameters are shown in Table 3. The simulation result of the topology of the ΓSCB is shown in Figure 7. When the forward energy flow channel is running, the fault occurs on the load side when the simulation is set for 0.5 s, and the capacitor C and the load capacitor C L begin to discharge. The fault current rises rapidly, the magnetic flux of the secondary coil L 2 begins to increase, and the line current i L2 rises.
Due to the mutual inductance between the transformer windings, the line current of the primary coil L 1 continues to rise, forcing the SCR current to cross zero. Due to the symmetrical structure of the diode bridge circuit, there is no resonance effect, and the fault current is cleared after 400 µs. The voltage V SCR on both sides of the SCR is stabilized at 1.8 kV, the voltage V c on both sides of the capacitor C is stabilized at 0.75 kV, the voltages V L1 and V L2 on both sides of the primary and secondary windings are reduced to zero after a period of sine, and the load-side output voltage V o drops to zero. However, as the simulation time increases, the voltage on both sides of the SCR and capacitor C will slowly drop to zero after about 5.5 s.
As a result of the design of the diode bridge circuit, the ΓSCB is a symmetrical structure, so the simulation results under the reverse power flow operation of the bidirectional topology are consistent with the forward power flow operation. In the process of clearing the short-circuit fault current, it can be seen from the analysis of the simulation results that the bidirectional DC circuit breaker is designed to be suitable for the bidirectional operation of energy between different nodes and loads in the DC microgrid. Owing to the characteristics of the circuit structure, there is no need to add a buffer circuit and the ability to clear fault currents in a short time, which verifies the rationality of the topology of the ΓSCB. Due to the mutual inductance between the transformer windings, the line current of the primary coil L1 continues to rise, forcing the SCR current to cross zero. Due to the symmetrical structure of the diode bridge circuit, there is no resonance effect, and the fault current is cleared after 400 μs. The voltage VSCR on both sides of the SCR is stabilized at 1.8 kV, the voltage Vc on both sides of the capacitor C is stabilized at 0.75 kV, the voltages VL1 and VL2 on both sides of the primary and secondary windings are reduced to zero after a period of sine, and the load-side output voltage Vo drops to zero. However, as the simulation time increases, the voltage on both sides of the SCR and capacitor C will slowly drop to zero after about 5.5 s.
As a result of the design of the diode bridge circuit, the ΓSCB is a symmetrical structure, so the simulation results under the reverse power flow operation of the bidirectional topology are consistent with the forward power flow operation. In the process of clearing the short-circuit fault current, it can be seen from the analysis of the simulation results that the bidirectional DC circuit breaker is designed to be suitable for the bidirectional operation of energy between different nodes and loads in the DC microgrid. Owing to the characteristics of the circuit structure, there is no need to add a buffer circuit and the ability to clear fault currents in a short time, which verifies the rationality of the topology of the ΓSCB.
The comparison simulation results of ΓSCB under different transformer parameters are shown in Figure 8.

Experimental Validation
In order to verify the principle of the DC microgrid protection design based on the reversed Γ source bidirectional DC solid-state circuit breaker proposed in this paper, this section builds an experimental platform with a power supply voltage of 48 V, a steadystate load current of 1.2 A. The experimental platform verifies the load-side short-circuit fault detection and breaking process. A two-way experimental circuit is built for the diode bridge structure, and the experimental circuit schematic diagram of the ΓSCB turned over is shown in Figure 9. In the diode bridge structure in Figure 9, L1 and SCR are long-term current-passing branches, RL simulates load resistance, Ro simulates load-side short-circuit fault resistance, short-circuit fault occurs after the air switch Ksw is closed, capacitor C starts to discharge, and the magnetic flux of L2 rises until the SCR Automatic shutdown. Because the voltage level of the experiment platform is relatively low, the currentlimiting module does not need to be added. Because the circuit current rises rapidly after the Ro resistance is connected, the circuit breaker is turned off according to the expected effect. In selecting experimental devices, full consideration should be given to the withstand voltage level and rated current to avoid breakdown and damage of the experimental devices and ensure the experimenters' personal safety. The physical diagram of ΓSCB turned over is shown in Figure 10.
After the system works normally, Ksw is closed, Ro is connected to the circuit to simulate a short-circuit fault, and the SCR reaches the critical value to force automatic commutation, and the entire experimental operation process ends. Part of the experimental

Experimental Validation
In order to verify the principle of the DC microgrid protection design based on the reversed Γ source bidirectional DC solid-state circuit breaker proposed in this paper, this section builds an experimental platform with a power supply voltage of 48 V, a steady-state load current of 1.2 A. The experimental platform verifies the load-side short-circuit fault detection and breaking process. A two-way experimental circuit is built for the diode bridge structure, and the experimental circuit schematic diagram of the ΓSCB turned over is shown in Figure 9. In the diode bridge structure in Figure 9, L 1 and SCR are long-term current-passing branches, RL simulates load resistance, Ro simulates load-side short-circuit fault resistance, short-circuit fault occurs after the air switch K sw is closed, capacitor C starts to discharge, and the magnetic flux of L 2 rises until the SCR Automatic shutdown.

Experimental Validation
In order to verify the principle of the DC microgrid protection design base reversed Γ source bidirectional DC solid-state circuit breaker proposed in this pa section builds an experimental platform with a power supply voltage of 48 V, a state load current of 1.2 A. The experimental platform verifies the load-side sho fault detection and breaking process. A two-way experimental circuit is built for t bridge structure, and the experimental circuit schematic diagram of the ΓSCB turn is shown in Figure 9. In the diode bridge structure in Figure 9, L1 and SCR are lo current-passing branches, RL simulates load resistance, Ro simulates load-side s cuit fault resistance, short-circuit fault occurs after the air switch Ksw is closed, cap starts to discharge, and the magnetic flux of L2 rises until the SCR Automatic shu Because the voltage level of the experiment platform is relatively low, the limiting module does not need to be added. Because the circuit current rises rapi the Ro resistance is connected, the circuit breaker is turned off according to the e effect. In selecting experimental devices, full consideration should be given to t stand voltage level and rated current to avoid breakdown and damage of the expe devices and ensure the experimenters' personal safety. The physical diagram turned over is shown in Figure 10.
After the system works normally, Ksw is closed, Ro is connected to the circui ulate a short-circuit fault, and the SCR reaches the critical value to force automa mutation, and the entire experimental operation process ends. Part of the expe Because the voltage level of the experiment platform is relatively low, the currentlimiting module does not need to be added. Because the circuit current rises rapidly after the Ro resistance is connected, the circuit breaker is turned off according to the expected effect. In selecting experimental devices, full consideration should be given to the withstand voltage level and rated current to avoid breakdown and damage of the experimental devices and ensure the experimenters' personal safety. The physical diagram of ΓSCB turned over is shown in Figure 10.
waveform diagrams that under the condition of the source voltage of 48 V, the norm operating current of the entire DC system is 1.2 A, and the interruption fault current o the entire DC circuit breaker is about 400 μs. The circuit breaker topology designed in th paper reduces the number of SCR used and simplifies the overall structure on the basis o quickly shutting down the fault current. The experiment verifies the effectiveness of th topology of the flip Γ source DC circuit breaker.    Figure 11. Part of bidirectional DC circuit breaker test waveform.

Conclusions
This paper proposed a DC microgrid protection design based on the flip Γ source D circuit breaker, expounds the working principle of the flip Γ source DC circuit breake topology and the analysis method of the failure rate. Finally, it is verified by simulatio and experimental platform, and the following conclusions are obtained: (1) When the ΓSCB works normally, by deriving the input and output voltage transfe function, it can be known that it is about unity gain in the low-frequency range an does not affect the normal operation of the system. Its effect is similar to the low pas filter and improves the operating efficiency of the DC microgrid. After the system works normally, K sw is closed, Ro is connected to the circuit to simulate a short-circuit fault, and the SCR reaches the critical value to force automatic commutation, and the entire experimental operation process ends. Part of the experimental waveforms of the ΓSCB is shown in Figure 11. The experimental waveforms are SCR current waveform, output current waveform, SCR voltage waveform and output voltage waveform, and the time scale is 2 ms/div. It can be seen from the current and overvoltage waveform diagrams that under the condition of the source voltage of 48 V, the normal operating current of the entire DC system is 1.2 A, and the interruption fault current of the entire DC circuit breaker is about 400 µs. The circuit breaker topology designed in this paper reduces the number of SCR used and simplifies the overall structure on the basis of quickly shutting down the fault current. The experiment verifies the effectiveness of the topology of the flip Γ source DC circuit breaker.
Energies 2022, 14, x FOR PEER REVIEW waveforms of the ΓSCB is shown in Figure 11. The experimental waveforms ar rent waveform, output current waveform, SCR voltage waveform and outp waveform, and the time scale is 2 ms/div. It can be seen from the current and o waveform diagrams that under the condition of the source voltage of 48 V, operating current of the entire DC system is 1.2 A, and the interruption fault the entire DC circuit breaker is about 400 μs. The circuit breaker topology desig paper reduces the number of SCR used and simplifies the overall structure on quickly shutting down the fault current. The experiment verifies the effective topology of the flip Γ source DC circuit breaker.

Conclusions
This paper proposed a DC microgrid protection design based on the flip Γ circuit breaker, expounds the working principle of the flip Γ source DC circ topology and the analysis method of the failure rate. Finally, it is verified by Figure 11. Part of bidirectional DC circuit breaker test waveform.

Conclusions
This paper proposed a DC microgrid protection design based on the flip Γ source DC circuit breaker, expounds the working principle of the flip Γ source DC circuit breaker topology and the analysis method of the failure rate. Finally, it is verified by simulation and experimental platform, and the following conclusions are obtained: (1) When the ΓSCB works normally, by deriving the input and output voltage transfer function, it can be known that it is about unity gain in the low-frequency range and does not affect the normal operation of the system. Its effect is similar to the low pass filter and improves the operating efficiency of the DC microgrid. (2) When a short-circuit fault occurs on the load side, this topology does not require an external control circuit, and the SCR can be quickly turned off through the mutual inductance of the transformer. At the same time, compared with the series-type Z-source DC circuit breaker using LC circuit, the source side will not produce circulating current impact after fault isolation; (3) The topology not only has the ability to isolate currents rapidly but also meets the needs of bidirectional protection of DC microgrid. As a result of the structural characteristics of ΓSCB, only an SCR and two-wingding transformers are needed. The existence of mutual inductance of transformers also greatly reduces the demand for total inductance, thereby reducing the overall size and cost of the topology.