Feasibility study of medium voltage AC fast current limiter

When a short-circuit fault occurs, it is important to avoid the problem of excessive short-circuit current. This paper offers a current limiter scheme which considers both flow capacity and current limiting capacity. The feasibility of the current limiting scheme is verified by building a simulation model of the power grid around 110/11 kV substation. In addition, the optimal performance parameters of each component of the scheme are determined.


Introduction
At present, there are several current limiting measures in the grid.The first method is to adjust the structure of the power grid and change the operation mode of the system.This adjustment considers the operation of the entire grid, which is complex to implement and sometimes requires higher costs [1] .The second one is connecting reactance in series to the line.Although this method can effectively limit the fault current, this will do great harm to the safe and lasting operation of our communication system [2] [3] .At the same time, reactance will bring large fluctuation to the system, cause system instability, and produce active loss, which increases cost of the system operation [6] [7] .
The third method is to install a small reactance at the neutral point of the transformer.Although this method can greatly limit the current of Single-phase ground short circuit, it is ineffective for the current of three-phase short circuit [4] .
The AC limiter studied in this paper can not only avoid unnecessary losses in steady-state operation, but also effectively suppress various fault current.

Current limiter scheme
The current limiter is applied to the 11 kV AC side.It mainly consists of two parts, one part mainly contains high-speed circuit breakers and parallel current limiting reactance, and the other part includes detection device and control device.The topology structure is shown in Figure 1.

High-speed circuit breakers
Control signals The topology structure of the current limiter When a system short-circuit fault occurs, the detection device quickly detects the system fault, and the control system sends a breaking signal to the switch.After a few milliseconds, the ends of circuit breaker will separate to the effective insulation gap [10] .When the fault current crosses zero point, The energy of the burning arc between the contacts is gradually used up and begins to die out and the current is thus directed to the inductor branch.

Simulation models.
Build a simulation model of the power grid around the 110/11 kV substation.The relevant parameters of transformer are shown as follows.

Parameters of the circuit breaker
During normal operation, the breaker should be able to bear the long-term stable working current of the system [5] .From the results of the simulation, the rated current of the line is about 2.2 kA.Considering a certain margin, the rated current of the breaker under long-term operation should be more than 2.5 kA.
The simulated peak currents for various fault conditions are shown in Table 2.The separated speed of the breaker is an important factor about the successful operation of the current limiter.Before the fault current naturally crosses the zero point, the breaker needs to reach an effective insulation distance, otherwise, the recovery voltage across the breaker will lead to arc re-ignition and failure of current limiting [9] .The relevant parameters are shown as follows.Based on Table 3, the fault current can be precisely limited by adjusting the time of breaking signal [8] .

Parameters of current limiting reactor
During the steady state operation, no current flows over the current limiting reactor.The current waveforms at different reactance values are shown in Figure 3.The relationship between the peak current and the reactance is shown in Figure 4.According to Figure 3 and Figure 4, it can be obtained that when the reactance value exceeds 5 mH, the current limiting effect will not change significantly with the increase of the reactance.In addition, the manufacture of large reactance also requires more cost and space.In view of above considerations, the optimal value of reactance is 5 mH.

Single-phase ground short circuit
Set a single-phase ground fault in phase A. Since the fault occurs on the 11 kV side and the 11 kV side of the transformer is Y-shaped connection, there is no short-circuit current path in system.The simulation results are shown in Figure 5.According to Figure 5, we can see that the voltage of phase A decreases to near zero but the voltage of phase B and C rises to √3 times the original.The three-phases current is unchanged.The line voltage remains symmetrical.In this case, the system can operate for a short period of time without current limiting.

Three-phase short circuit
This paper focuses on the application effect of current limiter in three-phase short circuit fault.Equivalent the original system parameters to the 11 kV side.
In Equation ( 1),  represents the amplitude of the AC periodic component,  represents the voltage amplitude of the AC power supply,  and  represents the resistance and reactance of the line respectively,  and  represents the resistance and reactance of the transformer respectively, τ represents the decay time constant of the DC component after a short circuit, and  represents the shortcircuit inrush current.We can get the results as shown in Equation ( 2).By controlling whether the current limiter is involved in the operation, the simulation results can be obtained.Figure 6 (a) shows the results without the current limiter.According to Figure 6 (a), it can be obtained that the value of the inrush current is 31.4kA and the amplitude of the AC periodic component is 19.33 kA.The simulation results are almost equal to those in Equation ( 2).From Figure 6 (b), We can see that the fault peak current is limited to 5.85 kA after the current limiting, which proves the current limiting effect is significant.
Taking the A-phase current-limiting reactance as an example, the voltage and current waveform of the system is shown in Figure 7.According to Figure 7 (a), it can be obtained that the inrush voltage value of reactance is less than 10kV and the current value is less than 6kA.

Conclusion
Through theoretical calculation and simulation, it can be concluded as follows:  Compared with the traditional methods, this current limiting device based on high-speed circuit breaker has superior steady-state current flow ability and fault current limiting ability. The effectiveness of the current limiter scheme is verified.The fault peak current can be limited from 31.4 kA to 5.85 kA in three-phase short-circuit fault.

Figure 2 .
Figure 2. Simulation model of infinite power supply system

Figure 3 .
Figure 3. Short-circuit current limiting waveform with different reactance.

Figure 4 .
Figure 4.The relationship between peak current and reactance

Figure 5 .
Figure 5. Three-phase voltage and current waveform of single-phase ground fault

Figure 7 .
Figure 7.The voltage and current waveform of a current-limiting reactor

Table 1
According to Table1, we can get the resistance and reactance of transformer.Then, set a simulation model as shown in Figure2.It is assumed that all faults occur at 0.5 km of the 11 kV side outlet of the substation.

Table 2
According to the table 2, the rated breaking current value should be more than 35 kA.

Table 3 .
Main parameters of circuit breaker