Direct current (DC) microgrids have elicited increasing attention in recent years, because they have a simple structure and are easy to control [1
]. However, the safe and stable operation of a DC microgrid is inseparable from the effective protection technology [3
]. When a short circuit occurs, fault current will rise instantly to a considerable extent. The equipment in the power system will suffer from huge electro-thermal stress, which seriously affects the operational reliability of the system [4
]. Therefore, a circuit breaker is required to isolate the fault. Existing circuit breaker techniques can be classified into three types: mechanical circuit breaker, hybrid circuit breaker, and solid state circuit breaker [6
]. The function of a mechanical circuit breaker is achieved by a mechanical switch. Although the mechanical circuit breaker can handle high current, electric arcing caused contact erosion reduces the lifetime and the disadvantage of long break time further limits its application. A hybrid circuit breaker is composed of a mechanical switch and parallel power devices. The current flows into the mechanical switch under normal working condition and transfers into the semiconductor switch under fault condition. The loss of a hybrid circuit breaker is minimal, but the control of a hybrid circuit breaker is complex. A solid state circuit breaker (SSCB) is composed of power devices and related circuits to realize the interruption of fault current. The fault clearing time of an SSCB is short, but the drawback of an SSCB lies in its loss. By comparison, an SSCB responds rapidly to the fault and produces no arc when cutting off the current. Besides, without the interaction of mechanical switch and power switch, the control of an SSCB is not complex and the reliability is relatively high. Therefore, an SSCB can better meet the fast and reliable protection requirements in DC microgrid, and has received extensive concern.
The commonly used devices in SSCBs are Silicon (Si)-based power devices [9
]. However, the performance of traditional Si devices has reached its limitation [15
]. With the improvement of wide-bandgap semiconductor technology and its application in SSCBs, the performance of SSCBs can be further improved. Compared with Si devices, wide-bandgap semiconductor Silicon Carbide (SiC) devices exhibit further excellent properties. The comparison of material properties is provided in Table 1
]. Wider bandgap and higher thermal conductivity indicate that SiC devices can withstand higher temperature than Si devices, which reduces heat dissipation requirement of an SSCB. Higher electron velocity guarantees faster switching speed, higher frequency characteristics, and higher current density of SiC-based power devices. The aforementioned advantages enable SiC devices to operate faster and withstand higher temperature than Si devices, which break through the limitation and improve the fault response capability of Si-based SSCB [17
Among SiC power devices, SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) exhibit the most promising prospects [19
], but study on SSCBs with SiC MOSFET is still in its infancy. Zhou et al. [21
] introduced a digital SSCB with SiC MOSFET to identify inrush current, but the fault clearing time was long, which is adverse to the equipment protection in the power system. Ren et al. [22
] focused on solving the inconsistency voltage distribution on cascaded SiC MOSFETs in SSCB in high voltage direct current (HVDC) transmission application, while the detailed design of SSCB was not given. Zhang et al. [23
] proposed a changeable delay time protection method for SSCB with SiC MOSFET. However, the high requirements to parameter calculation and complicated design make the SSCB hard to realize. Therefore, to achieve quick and reliable action of the SSCB and to make the SSCB easy to realize in application, detailed design of the SSCB with SiC MOSFET should be carried out.
In this study, a 400 V DC microgrid was adopted as the application background. The topology and structure of an SSCB with SiC MOSFET were introduced. The design of the SSCB was described in detail, including device selection, gate driver, fault detection circuit, energy absorption circuit, and snubber circuit. Finally, a prototype of the DC SSCB with SiC MOSFET was developed, tested and compared with Si insulated gate bipolar transistor (IGBT). Experiment results proved that the designed SSCB with SiC MOSFET can operate reliably in case of failure and can reduce voltage spike during the turn-off period. In addition, the designed SSCB exhibits fast interrupting characteristics, which provide guidance for improving the performance of SSCB and the application of SSCB with SiC MOSFET.
2. Topology of the SSCB with SiC MOSFET
a demonstrates the topology of the SSCB with SiC MOSFET. As illustrated in the figure, the SSCB with SiC MOSFET is composed of a SiC MOSFET switch, a drive circuit, a fault detection circuit, an energy absorption circuit, and a snubber circuit. The functions of the power device are to conduct current and to cut off the circuit when fault occurs. The drive circuit is used to send control signals to the power device to turn it on or off. The fault detection circuit can immediately detect the faults and react. The energy generated during a fault period is absorbed and dissipated by the energy absorption circuit. The snubber circuit is used to suppress the voltage spike induced by circuit inductance in the initial turn-off stage to protect the voltage on the device from exceeding the rated voltage. The structure of the SSCB is depicted in Figure 1
b to specifically show its components and further explain the working principle of the SSCB. The gate driver is connected to the SiC MOSFET by gate resistance Rg
. The fault detection circuit detects variations in gate voltage. R1
are divider resistances. The function of energy absorption circuit is realized by using a metal-oxygen-varistor (MOV), and the snubber circuit is achieved by simply using MOV_in instead of the commonly used resistance-capacitor (RC) or resistance-capacitor-diode (RCD) circuit [24
are the parasitic inductances.
The working principle of the SSCB of SiC MOSFET can be explained as below: S1 conducts and current flows through SiC MOSFET under normal working condition. The resistance of MOV_in and MOV_ex are high, and no current flows into the snubber circuit and the energy absorption circuit. When abnormal state occurs, the fault current increases rapidly and causes the change of gate voltage. Through logical judgement and comparison, a turn-off signal is sent to gate driver to turn off the SiC MOSFET. Simultaneously, the voltage on SSCB reaches to the breakdown voltage of MOV, the resistance of MOV immediately drops to a very small value and the voltage on SSCB is clamped. First, MOV_in is triggered to operate to suppress the voltage spike in the early turn-off stage. Then, MOV_ex operates and because of the voltage difference between MOV_in and MOV_ex, the fault current transfers to energy absorption circuit. During this time, the voltage on MOV_ex is higher than the DC-link voltage. MOV_ex in absorbs the energy and fault current attenuates gradually. When the current reaches to zero, the SSCB stops working and fault is cleared.
4. Testing on the SSCB Prototype with SiC MOSFET
Based on the structure of Figure 1
b, the prototype of the SSCB with SiC MOSFET was developed and is shown in Figure 6
a. The SSCB is composed of a SiC MOSFET, a gate driver, a fault detection circuit, an energy absorption circuit, and a snubber circuit. Gate driver uses the ACPL-W346 driver chip. Fault detection circuit is based on the schematic diagram in Figure 3
. Energy absorption circuit is composed of MOV (14D511K) to absorb the energy. Snubber circuit is composed of MOV (05D681K) to suppress the voltage spike in the early turn-off period. The fault simulation circuit is shown in Figure 6
b, where S1 is the SSCB prototype. Switch S2, which is parallel with the load, is another SiC MOSFET that controls on and off of the circuit. Load is represented by a 100 Ω resistance. Voltage-stabilizing capacitor C
is 560 μF, and DC-link voltage is 400 V. First, S1 is on while S2 is off. The system is under normal working condition. After 5 μs, a turn-on signal is sent to S2, placing the system in a short circuit state, and the current rapidly increases. To prevent the abnormal state caused by the malfunction of SSCB, S1 is forcibly turned off after 3 μs of short circuit. The sequence of the control signals is shown in Figure 7
presents the test results of the fault detection circuit. When short circuit happens, gate voltage evidently rises. The differential operational amplifier can detect the change and react immediately. A turn-off signal is sent to the gate driver subsequently.
shows the test results of the SSCB under the fault. The purple curve is the current waveform, whereas the blue curve is the voltage waveform. Figure 9
a,b illustrate the cases where the snubber circuit is disconnected and connected to the circuit, respectively. When the snubber circuit is not connected, the peak voltage of SSCB during short circuit is 800 V, and oscillation is acute. Voltage spike can be suppressed to 750 V and the oscillation can be greatly restrained due to the existence of the snubber circuit. In addition, the peak fault current is approximately 80 A and the fault clearing time is about 720 ns. Therefore, the SSCB with SiC MOSFET can interrupt the current rapidly, and prevent the device from being exposed to high energy, long time shock, which provides protection for the device and improves the reliability of the system.
This study also does a test on an SSCB with Si IGBT, whose driver chip is TX-DA962D6 by LMY electronics, integrated with the desaturation detection function. Figure 10
shows the experimental result. The purple and blue curves represent the current waveform and voltage waveform, respectively. As shown in the figure, the fault clearing time is 2.8 μs and the fault current is up to 300 A. Both of these values are nearly four-fold those of the SSCB with SiC MOSFET. Such high current with such long time will exert huge electro-thermal stress on device and equipment in the system.
In summary, by using an SiC MOSFET as the switching device, the designed SSCB prototype is evidently faster than SSCB with Si IGBT, which can cut off the fault current within 1 μs. The proposed SSCB with SiC MOSFET is distinguished by the fault detection circuit and the simplicity of the snubber circuit. On one hand, the fault detection method used in an Si-based SSCB is extended to an SSCB with SiC MOSFET because of the existence of a Miller capacitor in the SiC MOSFET. The fault can be detected rapidly and effectively based on the variation in gate voltage. On the other hand, by replacing the RC or RCD snubber circuit with MOV, the complicated design of parameters can be avoided and voltage spike can be suppressed as well. The detailed design in this study makes the realization of SSCB with SiC MOSFET easier and less complicated, providing reference for design and improvement of SSCB.
In order to realize the fast and reliable fault isolation in DC microgrid, the topology and structure of a 400 V miniature DC SSCB with SiC MOSFET are introduced. The design of the SSCB prototype is described in detail, including the selection of the device, drive circuit, current detection circuit, energy absorption circuit, and snubber circuit. The experimental results demonstrate that the SSCB with SiC MOSFET can immediately detect and interrupt the fault within 720 ns, peak current value of 80 A. Compared with the SSCB with Si IGBT, the proposed SSCB evidently has shorter interruption time and causes less thermal stress on the device.