2.1. Working Principle of FSFCL
As shown in
Figure 1, the FSFCL mainly consists of a current-limiting reactor, a fast switch, a controller, and current transformers [
19]. Under normal operating conditions, the fast switch remains closed with extremely low device loss, having no negative impact on the power grid operation. When the current transformer detects a short-circuit current, the fast switch quickly opens and the current-limiting reactor is engaged, effectively controlling the fault current within the safe bearing capacity of the transformer. After the fault is cleared, the fast switch closes again to ensure the stable operation of the power grid.
Based on the characteristics of the FSFCL, as shown in
Figure 2, the control process can be summarized as follows: At system startup, the fast switch is closed, and the current-limiting reactor is in a short-circuited state. The controller collects current signals using current transformers to detect faults and predict the current zero-crossing point. Upon detecting a short circuit, the controller triggers the fast switch to open at the current zero-crossing, engaging the current-limiting reactor. After approximately 2 s of current limitation, the controller reassesses the current status. If the current returns to normal, indicating a transient fault, the fast switch will close, the current-limiting reactor will be short-circuited again, and the circuit breaker will attempt to reclose. If the current remains abnormal, the controller determines it to be a permanent fault, and the circuit breaker will remain open.
Installing an FSFCL at the neutral point also requires consideration of its integration with existing neutral point equipment. To ensure the proper operation and maintenance of the FSFCL, this paper proposes a neutral point installation scheme for the FSFCL: as shown in
Figure 3, one end of the FSFCL is connected to the grounding switch, and the other end is connected to the transformer neutral point via a series isolation switch, while the other end of the grounding switch is grounded. The FSFCL branch is connected in parallel with the existing neutral point equipment, such as the grounding switch, surge arrester, and spark gap. This configuration allows for flexible adjustment of the transformer’s neutral point grounding state, enabling switching among non-direct grounding, direct grounding, or grounding through the limiter, which facilitates changes in the transformer’s operating mode and neutral point maintenance. Additionally, if the FSFCL experiences a fault, it can be quickly isolated and repaired, ensuring continuous system operation and reliability.
2.2. Short-Circuit Current Calculation Method
To study the current-limiting characteristics of the FSFCL, it is first necessary to calculate the maximum asymmetrical short-circuit current that may occur on the neutral point and the windings on each side of the transformer. To achieve this, this paper employs the symmetrical component method for short-circuit current calculation. This approach decomposes the asymmetrical state of the power system into three independent symmetrical systems: positive-sequence, negative-sequence, and zero-sequence systems. First, each independent system is analyzed; secondly, a composite-sequence network is constructed according to the type of fault, and the sequence components are calculated, with the results of each combined to obtain the current and voltage values at the fault point; finally, the currents in the transformer windings and at the neutral point are obtained based on the impedance distribution. Taking the short-circuit current at the fault point during a single-phase ground fault as an example, the mathematical expression is as follows:
In the formula, , , and represent the three-phase currents at the fault point; , , and represent the positive, negative, and zero-sequence current components at the fault point; and a is the three-phase phase shift operator, a = ej120.
Taking the No. 2 main transformer in a 220 kV substation as an example, this paper analyzes the asymmetric short circuit on the medium-voltage side of the transformer. The high-voltage side of the transformer supplies power separately, and the high-voltage side and medium-voltage side operate side by side with another transformer with basically the same parameters, while the low-voltage side operates independently.
The basic parameters of the transformer are as follows: the rated voltage is 220/121/10.5 kV; the rated capacity is 120/120/60 MVA; the short-circuit impedance is 14.52%, 23.98%, and 7.40%; the connection group is YNyn0D11; and the transformer core is a three-phase five-column type.
To accurately assess the severe short-circuit faults that the medium-voltage side may encounter under the most unfavorable conditions, this paper assumes that the transformer and its various side systems are all operating at maximum capacity, and the fault location is set at the near end of the medium-voltage side of the transformer. The simplified system topology is shown in
Figure 4.
The equivalent impedance standard values of the transformer and its sides are shown in
Table 1.
When a single-phase ground fault occurs at the near end of the medium-voltage side, the sequence network diagrams for each sequence are shown in
Figure 5.
In the diagram above, X11, X12, and X10 represent the positive-sequence, negative-sequence, and zero-sequence impedances of the high-voltage side winding, respectively. Similarly, X21, X22, and X20, and X31, X32, and X30 correspond to the positive-sequence, negative-sequence, and zero-sequence impedances of the medium-voltage and low-voltage side windings, respectively; Xg1, Xg2, and Xg0 represent the positive-sequence, negative-sequence, and zero-sequence impedances of the high-voltage side system.
Based on the sequence network diagrams, the total impedance for each sequence is calculated as follows:
The sequence short-circuit currents at the fault point are as follows:
Upon analyzing the neutral point on the medium-voltage side of the transformer, the current flowing through the neutral point is determined to be:
By analyzing the equivalent circuits for each sequence, the fault phase currents on the high-, medium-, and low-voltage windings of the transformer are found to be as follows:
The rated values of the fault phase currents for the high-, medium-, and low-voltage windings are as follows: IgA = 1969 A, IzA = 5449 A, and IdA = 12,428 A; the rated value of the fault current passing through the neutral point is I0 = 8174 A.