Solid-state circuit breakers (SSCBs) are key devices in Flexible AC Transmission Systems (FACTS) and Distribution FACTS (DFACTS) that enable rapid, flexible, and accurate control of power system parameters and network structure. They are also crucial for ensuring the safe and reliable operation of modern power systems. With the increasing capacity of distribution networks, the short-circuit capacity of the system is also continuously increasing, placing higher demands on the breaking capacity of switching equipment. Simultaneously, as users' requirements for power quality continue to rise, it is also crucial to quickly disconnect short-circuit currents to suppress voltage drops during faults. Existing mechanical circuit breakers, limited by their physical structure, have limited breaking capacity, and the arcing caused by the separation of moving and stationary contacts prolongs the fault current disconnection time, making it difficult to meet the speed requirements of some power users for fault current interruption. Therefore, limiting and rapidly interrupting fault currents is becoming increasingly important. Solid-state circuit breakers based on power electronic devices have attracted widespread attention since their introduction due to their superior current interruption performance. 1. Working Principle of Solid State Circuit Breakers (SSCBs): SSCBs consist of thyristors connected back-to-back in parallel to form an AC switching module, which are then connected in series at rated voltage. If the thyristors are conventional silicon rectifiers (SCRs), the switch breaks the circuit when the AC current first crosses zero. This introduces a delay of a few milliseconds, which is acceptable for most applications. If a gate turn-off thyristor (GTO) is used, the current can be interrupted instantaneously. ZnO surge arresters and buffers are used to absorb overvoltages generated when the thyristors turn off. SSCBs used in 6kV and above systems mostly employ GTO devices. During normal operation, the GTO is on, and the load current flows through it. Once a fault is detected, the control system sends a turn-off pulse to the GTO gate, and the SSCB can disconnect the fault current within a few hundred milliseconds before it rises to a significant value. Therefore, it can effectively suppress voltage dips in the mains and the adverse consequences of large fault currents. If a set of reactors is connected in parallel to the SSCB, it forms a fault current limiter (FCL). Its working principle is as follows: Under normal load conditions, the GTO switch is closed and in a fully conducting state. The GTO switch is driven by the magnitude of the current or the rate of current rise, and can be switched to the open state within tens of 1Ls, and can interrupt a large current, so that the fault current is quickly limited before it reaches a destructive value. At the moment of GTO turn-off, the current flows to the buffer, and the buffer limits the voltage rise rate before the thyristor voltage reaches the ZnO surge arrester operating voltage. This voltage is also applied to the reactor, and due to the current limiting effect of the reactor, the fault current is limited. When the fault is cleared and the line current returns to normal, the GTO switch is turned on when the voltage is zero or close to zero, which can avoid the capacitor discharge current of the buffer being too large, and the current on the reactor will decay within a fraction of a second. 2. Application of solid-state circuit breakers (SSCBs) can improve the breaking speed and stability; they can accurately control the switching time, select to operate when the current crosses zero, and reduce overvoltage. By using SSCBs at two mutually redundant feeder ends, the power outage or abnormal time can be limited to within one cycle in the event of a fault in either feeder. A more efficient solution is to use conventional mechanical circuit breakers at the locations of SSCB1 and SSCB2, while SSCB3 and one Statcon will ensure normal power supply (with a response time of only milliseconds). Statcon ensures constant voltage during normal system voltage flicker and protects bus voltage and current from harmonics. If a fault occurs on the system side of SSCB3, it will disconnect within less than one cycle, and Statcon will begin supplying power to the sensitive load, providing sufficient time for conventional switches to clear the fault or for the load to be transferred to another fault-free feeder. SSCB3 can detect and disconnect the fault, while Statcon maintains the voltage level of the sensitive user's bus, effectively protecting the load from the impact of the entire AC system or other load failures. Even in severe cases, there is still time to activate backup power or transfer the load to a third fault-free feeder. A forced-air-cooled outdoor SSCB (13.8kV, 675A) developed by Westinghouse Electric Corporation was installed and put into operation at the PSE&G substation in New Jersey, USA, in February 1995. The wiring diagram is shown in Figure 3 (in combination with the fault current limiter FCL). The rated current of the GTO circuit of the SSCB is 675A, and the fault current through the SCR is 8kA. The GTO is the main switch, used to conduct normal load current; the SCR connected in parallel is an auxiliary switch, open during normal operation and conducting fault current during a fault. The current-limiting reactor ensures that the system fault current is lower than the breaking capacity of the SCR and reduces the breaking pressure on downstream circuit breakers. When a system fault is detected, the control system sends a shutdown pulse to the GTO. After the GTO is turned off, it remains closed for half a cycle before reclosing. Simultaneously, it checks whether the line current returns to normal within the next half cycle. If it does, the GTO remains on. Therefore, in this situation, the power interruption lasts for a maximum of half a cycle. If the fault persists, the GTO disconnects again, and the SCR turns on, activating the current-limiting reactor branch, ultimately limiting the fault current below a certain value. If the fault still exists after more than 10 cycles, the SCR disconnects, finally clearing the fault. The Power System Library (PSB) in MATLAB is a tool for visual modeling and simulation of power systems. Within the PSB's Power Electronics library, there is an ideal switch model that simulates a simplified power electronic device, such as a GTO or a solid-state circuit breaker for current interruption. The ideal switch model consists of a resistor, an inductor, and a switch controlled by a logic signal C. The ideal switch is used to connect or disconnect a reactor (L = 1 H, R = 50 Ω, stray capacitance C = 50 nF). The switch parameters are: R = 1 Ωm, L = 1 x 10⁻⁹ H, with no buffer circuit. The switch is initially closed, opened at 36 ms, and then closed again at t = 0.15 s. The simulation is started, and the current flowing through the reactor and the voltage across it, as well as the switch voltage Uswitch and the switch current Iswitch, are observed. At t = 36 ms, the switch opens, and the current flowing through the reactor is 0.185 A. When the current on the reactor is disconnected, a high-frequency overvoltage (711HZ) is generated on the reactor. Its maximum voltage can be calculated by the following formula: Ulmax=IL =0.185 =830V (1) When t=0.15s, the switch is closed again. 3 Discussion Although foreign countries have made great breakthroughs in the research and development of high-voltage solid circuit breakers and have practical engineering applications over the past ten years, solid circuit breakers have not been widely used to date due to the following inherent shortcomings. 1) The rated voltage and rated current of power electronic devices are currently low. In high-voltage power grids, multiple GTOs need to be connected in series and parallel to improve the breaking capacity and reliability of solid circuit breakers. Therefore, it is necessary to solve the problem of synchronous control of each driving pulse to ensure that each GTO connected in series and parallel has good voltage and current sharing characteristics when opening and closing. Otherwise, uneven voltage drop or uneven current shunting will lead to damage to GTO components and threaten the safety of the entire series module. However, due to the very small gain of the GTO during interruption, only 4 to 5 times, interrupting large currents requires a large gate drive pulse current with a steep rise rate. Furthermore, as the number of series and parallel devices increases, controlling the consistent triggering of these pulses becomes very difficult. 2) The GTO has high conduction losses. Currently, the conduction voltage drop of a high-power GTO is approximately 3.2V, while the conduction voltage drop of a mechanical circuit breaker is only a few mV. Therefore, the excessively high conduction losses necessitate special cooling devices for solid-state circuit breakers to ensure their safe and reliable operation. This not only increases operating costs but also complicates the device and reduces its reliability. 3) The GTO has low overload capacity and cannot continuously conduct fault current; it can only turn off before the fault current reaches its maximum breaking value. Therefore, solid-state circuit breakers cannot coordinate with the settings of downstream circuit breakers. Due to these drawbacks, the application range of solid-state circuit breakers is greatly limited. They are generally used only in some special applications, such as interrupting devices that generate high-power pulse power supplies, grounding circuit breakers in high-voltage direct current transmission systems, and transfer switches for some important loads.