Research on Bridge-Type Fault Current Limiters

1 Bridge-Type Superconducting Fault Current Limiter
1.1 Structure and Operating Principle of the Bridge-Type SFCL
Figure 1 shows the single-phase circuit diagram of the bridge-type SFCL, which consists of four diodes D₁ to D₄, a DC bias voltage source V_b, and a superconducting coil L. A circuit breaker CB is connected in series with the limiter to interrupt the fault current after it has been limited. The bias source V_b provides a bias current i_b to the superconducting coil L. The voltage of V_b is set high enough to overcome the forward voltage drop of the diode pairs (D₁ and D₃, or D₂ and D₄), establishing a bias current i₀. The value of i₀ is set greater than the peak value of the line current i_max, with allowance for overload conditions.

Therefore, under normal conditions, the diode bridge remains continuously conducting, and the SFCL does not exhibit any impedance to the line current i, ignoring the minor forward voltage drop across the bridge. Assuming that during normal operation the currents passing through diodes D₁ to D₄ are iD1 to iD4 respectively, the line current is:

It is obtained according to Kirchhoff's Current Law (KCL):

When a short-circuit fault occurs on the line, the line current rapidly increases to i₀. During the positive and negative half-cycles, one pair of diodes becomes reverse-biased and turns off, thereby automatically inserting the coil L into the circuit. The short-circuit current is thus limited by the inductive reactance of the coil.

By setting the critical current of the superconducting coil appropriately, the coil remains in the superconducting state during the fault, avoiding the effects of response time and recovery from quenching. However, as the fault persists, the current through the superconducting inductor continues to rise, eventually approaching the steady-state short-circuit current value that would exist without the limiter. Therefore, the fault source must be timely interrupted by a circuit breaker within a specified time. For simplicity, it is assumed that the short-circuit fault occurs at the instant when the source voltage passes through zero (t = t₀). According to Kirchhoff's Voltage Law (KVL), the following equation is obtained:

Initial condition iL​(t0​)=I0​,solving this differential equation yields:

Figure 2 shows the waveforms of the inductor current and line current during normal operation and after a fault occurs, with the fault initiated at t = 0.1 s. The simulation results indicate that the short-circuit current rises slowly due to the current-limiting effect of the superconducting inductor. The current-limiting process is essentially the magnetization of the superconducting inductor. Once the fault current stabilizes, the limiter ceases to be effective. Therefore, the fault must be cleared by the circuit breaker before the short-circuit current reaches its steady-state value. In the figure, the fault is cleared by the circuit breaker at t = 0.2 s.

1.2 Structural Improvement of Bridge-Type Superconducting Fault Current Limiters
A conventional bridge-type superconducting fault current limiter (SFCL) can only suppress the rate of rise of short-circuit currents but is ineffective in controlling their steady-state values. To limit the steady-state value of short-circuit currents, a hybrid SFCL combines the characteristics of zero resistance in the superconducting state and the rapid increase in resistance during a quench of superconductors. This is achieved by integrating resistive superconducting fault current limiters with bridge-type SFCLs. The schematic diagram of this hybrid approach is shown in Figure 3.

Under normal operating conditions, switch K is open, so the resistive SFCL does not exhibit any external impedance, allowing current i_L to pass through it without resistance. Upon the occurrence of a fault, the resistive SFCL immediately presents high impedance and works in series with the superconducting inductor to jointly suppress the fault current. After the fault is cleared, switch K is closed; at this point, due to its own high impedance, the resistive SFCL is short-circuited and quickly returns to the superconducting state.

Since switch K has on-state resistance, it will be short-circuited by the recovered resistive SFCL, thereby making the entire hybrid bridge-type limiter appear as low impedance externally. At this moment, opening K concludes the entire current-limiting process. To enhance the capacity of the resistive SFCL, series and parallel connections of resistive SFCL units are typically employed to improve the voltage and current ratings of the device. Figure 4 illustrates the circuit schematic of the resistive superconducting limiter, where R₁ to R₆ represent superconducting resistors, and R serves as a bypass resistor that can prompt simultaneous quenching of two superconductors in the same series branch during a short-circuit fault.

The role of the inter-phase coupling transformer is to ensure iL1 = iL2 = iL3, so that SFCL units across different parallel branches can simultaneously quench after a short-circuit fault occurs . The hybrid bridge-type SFCL effectively limits the steady-state value of short-circuit currents by utilizing the superconductor's transition characteristics from superconducting to normal state (S/N), automatically engaging the current-limiting resistor upon fault detection without requiring additional fault detection mechanisms. However, the addition of the resistive superconducting fault limiting device increases overall operational costs and prolongs the recovery time from quench, complicating coordination with system reclosing operations.

2 Bridge-Type Non-Superconducting Fault Current Limiter
2.1 Solid-State Current Limiter
In recent years, rapid advancements in power electronics technology and high-capacity power semiconductor devices—such as SCR, GTO, GTR, and IGBT—along with their widespread application in practical systems, have made fault current limiters composed of inductors, resistors, capacitors, and power electronic components a research hotspot. The non-superconducting bridge-type fault current limiter is constructed from conventional components, avoiding complex superconducting technology, and offers advantages of high reliability and good cost-effectiveness.

Figure 5 shows the schematic diagram of an ideal single-phase bridge-type current limiter, consisting of a single-phase bridge circuit and a current-limiting inductor L. Under normal operation, continuous trigger pulses are applied to the four thyristors. After a brief magnetizing process, the current in the inductor reaches the peak value of the load current. When the voltage drop across thyristors T₁ to T₄ is neglected, the limiter exhibits no external impedance.

If a short-circuit fault occurs during the positive half-cycle of the supply voltage, T₃ is forced to turn off, inserting the current-limiting inductor into the circuit to suppress the fault current. By properly setting the value of inductor L, the short-circuit current can be limited to any desired level. Additionally, this limiter has the capability to instantly interrupt the short-circuit current. However, due to the use of four controllable switches, the control logic for instantaneous interruption is relatively complex. During fault current limiting, significant harmonics are generated; these can be effectively mitigated by connecting bypass inductors in parallel across the bridge arms.

2.2 Semi-Controlled Bridge Short-Circuit Fault Current Limiter
Figure 6 illustrates the topology of a single-phase short-circuit fault current limiter based on a semi-controlled bridge and self-turn-off devices. This system comprises diodes D₁ to D₄, self-turn-off devices T₁ and T₂, a superconducting inductor L, a current-limiting inductor Llim, and a ZnO overvoltage absorber, with us representing the AC power source and CB serving as the line circuit breaker.

Under normal operating conditions, the two self-turn-off devices T₁ and T₂ are continuously triggered. Upon initial power-up, the current in the superconducting inductor gradually increases to the peak value of the line current under the influence of the voltage source. Once the load stabilizes, iL remains constant. Neglecting the forward voltage drops across diodes D₁ to D₄ and self-turn-off devices T₁ and T₂, the voltage across the bridge is zero, and the voltage across the current-limiting inductor Llim is also zero. Consequently, the current limiter exhibits no external impedance and has no impact on the system.

When a short-circuit fault occurs in the system, the current iL in the superconducting inductor increases. Upon detection of the short-circuit fault, T₁ and T₂ are immediately turned off, causing the bridge to exit operation. The short-circuit current then transfers to the bypass current-limiting inductor Llim, while the current in the superconducting inductor continues to flow through diodes D₁ and D₄ until it decays to zero. Figure 7 shows the steady-state and fault-state current and voltage curves of a single-phase short-circuit fault current limiter based on a semi-controlled bridge.

The system powers up at t=0.02 seconds and reaches steady state within one cycle. A short-circuit fault occurs at t=0.1 seconds, and T₁ is turned off within a quarter cycle after the fault is detected. The circuit parameters used for the simulation are as follows: the peak phase voltage of the power supply is 100V/50Hz; the peak rated load current is 10A; the load resistance is 10Ω; the superconducting DC inductor L is 10mH; the forward voltage drop across the diodes and controllable switches is 0.8V; and the current-limiting inductor Llim is 10mH.

One of the primary purposes of employing superconducting fault current limiters (SFCLs) in power systems is to limit fault currents so that they do not exceed the instantaneous interrupting capacity of line circuit breakers. In analysis, the fault current reduction ratio $D$ ($0<D<1$) is commonly used to represent the percentage reduction in peak fault current, and the expression for $D$ is:

ip​ is the peak inrush current during a short circuit without the SFCL installed, and its value is related to the system's equivalent $X/R$ ratio.

In Equation (7), IpIp​ denotes the amplitude of the periodic component of the short-circuit current, and TaTa​ is the time constant. ilimilim​ represents the peak value of the limited short-circuit current, which depends on the magnitude of the current-limiting inductor LlimLlim​. By appropriately selecting the value of LlimLlim​, the desired percentage reduction in peak fault current can be achieved. Simulations were conducted with LlimLlim​ set to 10 mH, 15 mH, and 20 mH, and the results are shown in Figure 8. It can be observed that a larger LlimLlim​ provides better current-limiting performance, but also leads to higher operational costs.

2.3 Improvement of the Semi-Controlled Bridge Short-Circuit Fault Current Limiter
In the configuration shown in Figure 6, T₁ and T₂ are continuously triggered under normal operating conditions. Once a short-circuit fault is detected, the control circuit turns off both T₁ and T₂. By placing a single controllable switch T in the common path of the bridge to replace T₁ and T₂, similar current-limiting effectiveness can be achieved. This modification reduces the number of controllable switch components, lowers costs, and simplifies the circuit complexity. The schematic diagram is shown in Figure 9.

3 Conclusion
This paper presents several types of bridge-type short-circuit current limiters. By cascading a conventional superconducting bridge-type fault current limiter with a resistive superconducting fault current limiter, both the peak and steady-state values of short-circuit currents can be effectively limited. Moreover, leveraging the S/N (superconducting-to-normal) transition characteristics of superconducting materials, the system integrates fault detection, triggering, and current limiting into a single unit, offering fast response and high reliability.

In recent years, with the rapid development and practical application of power electronics technology and high-capacity power electronic devices, non-superconducting bridge-type short-circuit current limiters—comprising conventional power electronic switches and inductors—have gained advantages in reliability and cost-effectiveness due to the absence of complex superconducting technology. Simulation results demonstrate that both types of current limiters achieve excellent current-limiting performance, confirming the feasibility of the proposed current-limiting approaches.