Method and system for discriminating short circuit of CT secondary circuit

Abstract

The invention relates to a method and a system for judging short circuit of a CT secondary circuit, and belongs to the technical field of CT secondary circuit fault judgment. S1, if the differential current of a certain phase of the bus is greater than the differential current constant value, the phase is an abnormal phase, and S2 is continued, otherwise S1 is repeated; s2, if the braking current of the abnormal phase of the current current sampling period meets the condition corresponding to the braking current less than the braking current before N periods, continuing to execute S3, otherwise, repeating S1; s3, if the negative-sequence current of only one branch in the operation branches of the abnormal phase at the current moment is larger than the negative-sequence current constant value, the branch serves as an abnormal branch, S4 continues to be carried out, and otherwise S1 is repeated; s4, if the abnormal phase current of the abnormal branch in the current current sampling period meets the condition corresponding to the phase current before M periods, continuing to execute S5, otherwise, repeating S1; s5, if the abnormal phase current of the abnormal branch is greater than a current threshold value, judging that the abnormal branch is short-circuited, otherwise, repeating S1; n and M are greater than or equal to 1 and less than or equal to 3.

Description

Technical Field

[0001] The invention relates to a method and a system for determining a short circuit in a secondary circuit of a CT, and belongs to the technical field of CT secondary circuit fault determination. Background Art

[0002] During the operation of the secondary circuit of the current transformer, faults such as poor or damaged terminal contact, artificial disconnection or short circuit, and transformer damage can all be attributed to wire breakage or short circuit in the current circuit.

[0003] With the advancement of relay protection technology, CT disconnection logic has become increasingly perfect, and the domestic relay protection industry has formed standard technical specifications. CT disconnection is very dangerous, so it is generally not necessary to lock the differential protection mechanism. The CT secondary circuit is prone to phase-to-ground short circuit and two-phase short circuit due to insulation damage, terminal contamination, and terminal short circuit. CT short circuit generally locks the differential protection mechanism. The CT secondary circuit short circuit faults include phase-to-phase short circuit and phase-to-ground short circuit. These faults cause CT secondary circuit shunting, resulting in inaccurate secondary current sampling of the relay protection device, increased unbalanced current, differential current, etc., which leads to false operation or refusal of the relay protection device, and even causes accidents such as unit shutdown and line tripping, which seriously affects the safe and reliable operation of the power grid. Therefore, it is necessary to accurately judge the short circuit of the CT secondary circuit, and in the event of a short circuit, lock the differential protection action in time to avoid false operation of the differential protection.

[0004] At present, the methods for judging CT broken wires are relatively complete. Most of the methods for judging CT broken wires are judged by the magnitude of phase current, differential current and zero-sequence current during the analysis process. The Chinese invention patent with application publication number CN117117786A discloses a CT secondary circuit fault detection anti-mistake locking device. The short-circuit fault detection method of the device is to judge whether the CT secondary circuit has a short-circuit fault based on the zero-sequence current at the grounding wire of the sampled CT secondary circuit, the neutral point zero-sequence current and the phase current; however, when a two-phase short-circuit fault occurs in the CT secondary circuit, no zero-sequence current is generated. This judgment method is difficult to judge the two-phase short-circuit fault of the CT secondary circuit. Therefore, this judgment method is difficult to accurately and comprehensively judge the CT secondary circuit short-circuit fault. Summary of the invention

[0005] The purpose of the present invention is to provide a method and system for determining whether a CT secondary circuit is short-circuited, so as to solve the problem that the existing method of determining whether a CT secondary circuit has a short-circuit fault can only determine whether part of the CT secondary circuit is short-circuited, making it difficult to accurately and comprehensively determine the CT secondary circuit short-circuit fault.

[0006] In order to solve the above technical problems, the present invention provides a method for determining a short circuit in a CT secondary circuit, the method comprising the following steps: S1: if the differential current of a phase of the bus is greater than the differential current constant, the phase is regarded as an abnormal phase and S2 is continued, otherwise S1 is repeated;

[0007] S2: If the braking current of the abnormal phase in the current current sampling cycle satisfies the condition corresponding to being less than the braking current N cycles ago, then proceed to S3, otherwise repeat S1;

[0008] S3: If the negative sequence current of one and only one branch of the running branches of the abnormal phase at the current moment is greater than the negative sequence current set value, then this branch is regarded as an abnormal branch and S4 is continued, otherwise S1 is repeated;

[0009] S4: If the abnormal phase current of the abnormal branch in the current current sampling period satisfies the condition corresponding to being less than the phase current M periods ago, then proceed to S5, otherwise repeat S1;

[0010] S5: If the abnormal phase current of the abnormal branch is greater than the current threshold value, it is determined that the abnormal branch is short-circuited, otherwise S1 is repeated; N and M are greater than or equal to 1 and less than or equal to 3.

[0011] Further, the method of judging whether the braking current of the abnormal phase in the current sampling period satisfies the condition corresponding to the braking current N periods ago includes:

[0012] Determine whether the difference between the braking current of the abnormal phase in the current sampling period and the braking current N periods ago is greater than the braking current constant, or determine whether the braking current of the abnormal phase in the current sampling period is less than the first set multiple of the braking current N periods ago; if the difference is greater than the braking current constant, or the braking current is less than the first set multiple of the braking current N periods ago, then determine that the braking current of the abnormal phase in the current sampling period satisfies the condition corresponding to being less than the braking current of the previously set number of periods ago, otherwise determine that it is not satisfied.

[0013] Further, the method of judging whether the abnormal phase current of the abnormal branch in the current sampling period satisfies the condition corresponding to the phase current less than M periods ago includes:

[0014] If the abnormal phase current of the abnormal branch in the current sampling period is less than the second set multiple of the phase current M periods ago, it is determined that the abnormal phase current of the abnormal branch in the current sampling period meets the condition corresponding to being less than the phase current M periods ago, otherwise it is determined that it does not meet the condition.

[0015] Furthermore, the magnitude of the negative sequence current is determined according to the magnitude of the three-phase current of each branch.

[0016] Furthermore, the value range of the first set multiple is [0.65, 0.85].

[0017] Furthermore, the value range of the second set multiple is [0.65, 0.85].

[0018] Beneficial effect: The present invention provides a new method for determining whether a CT secondary circuit is short-circuited. The method utilizes the fact that negative sequence current exists in the CT secondary circuit regardless of phase-to-ground short circuit or two-phase short circuit, and when a short circuit occurs, the short-circuited phase current will decrease and produce an unbalanced differential current. The method combines the characteristics of differential current, braking current, negative sequence current of branch current and phase current as criteria for determining whether a short circuit has occurred in the CT secondary circuit, and judges in turn based on these characteristics. When all the criteria are met, a short circuit fault occurs. That is, the method first selects the differential current of each phase of the busbar that is greater than the differential current according to the size of the differential current of each phase busbar. If the differential current of a certain phase of the bus is greater than the differential current setting (i.e., the CT abnormal blocking setting, which is usually 1.2 times the maximum unbalanced current flowing through the bus protection during normal operation), it means that the phase may have a secondary short circuit, a disconnection, a primary system fault, or other faults that cause an unbalanced power system. This phase can be preliminarily regarded as an abnormal phase. The braking current of the abnormal phase in the current sampling cycle is analyzed and compared with the previous braking current. If the braking current of the abnormal phase in the current sampling cycle satisfies the condition that it is less than the braking current corresponding to N cycles ago, it means that the braking current in the phase is reduced, and the possibility of a primary system fault is reduced ( When a primary system fault occurs, the braking current tends to increase), and the possibility of a secondary short-circuit fault further increases; if at the current moment, the negative sequence current of one and only one of the operating branches of the abnormal phase is greater than the negative sequence current constant, it means that there is a certain negative sequence current in the branch, which meets the characteristics of the branch where a secondary short circuit fault occurs, and the possibility of a primary system fault is reduced (in the current sampling period, the negative sequence current of two or more branches of the abnormal phase is greater than the negative sequence current constant, which is more likely to be due to a primary system fault). At this time, the possibility of a secondary short circuit fault is further increased. Finally, it is judged that the abnormal phase current of the abnormal branch is Whether it meets the conditions corresponding to the corresponding phase current less than M cycles ago, if it does, it means that the abnormal phase current of the abnormal branch has decreased, and it also meets the characteristics of the branch where a short-circuit fault occurs, and judges whether the phase current of the abnormal branch is greater than the current threshold. If it is greater, the possibility that the abnormal phase current reduction of the abnormal branch is caused by a circuit breaker in the branch is excluded, and it is determined that a short-circuit fault occurs in the branch; if one of the criteria is not met, the differential current of each phase bus is repeatedly detected to be greater than the differential current constant until the branches that meet all the criteria are screened out. Therefore, this method can more reliably, accurately and comprehensively identify the branch with a short circuit in the CT secondary circuit.

[0019] The present invention also provides a CT secondary circuit short-circuit determination system, comprising a processor, wherein the processor is used to execute a computer program to implement the steps of the above-mentioned CT secondary circuit short-circuit determination method.

[0020] The CT secondary circuit short circuit determination system can achieve the same beneficial effects as the above-mentioned CT secondary circuit short circuit determination method. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flow chart of a method for determining a short circuit in a CT secondary circuit in an embodiment of the method for determining a short circuit in a CT secondary circuit of the present invention;

[0022] Figure 2 It is an actual circuit diagram of a CT secondary circuit short-circuit fault occurring in the embodiment of the method for determining a CT secondary circuit short-circuit of the present invention and its corresponding equivalent circuit diagram;

[0023] Figure 3 It is an actual circuit diagram of a phase-to-phase short circuit fault occurring in a CT secondary circuit in an embodiment of the method for determining a short circuit in a CT secondary circuit of the present invention and its corresponding equivalent circuit diagram;

[0024] Figure 4 It is a logic principle diagram of the method for determining a short circuit in a CT secondary circuit in an embodiment of the method for determining a short circuit in a CT secondary circuit of the present invention. DETAILED DESCRIPTION

[0025] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings.

[0026] Example of a method for determining a short circuit in a CT secondary circuit

[0027] This embodiment provides a technical solution for a method for determining a short circuit in a CT secondary circuit. The method is based on the fact that when a CT secondary circuit has a negative sequence current regardless of a phase-to-ground short circuit or a two-phase short circuit, and when a short circuit occurs, the phase current will decrease and an unbalanced differential current will be generated. The method combines the characteristics of the differential current, the braking current, the negative sequence current of the branch current, and the phase current as the criteria for determining whether a short circuit has occurred in the CT secondary circuit, and judges in turn based on these characteristics. When all the criteria are met, a short circuit fault occurs. The flowchart of the method for determining a short circuit in a CT secondary circuit is shown in FIG. Figure 1 As shown, the method specifically comprises the following steps:

[0028] S1: If it is detected that the differential current of a certain phase of the bus is greater than the differential current set value, then this phase is regarded as an abnormal phase and S2 is continued, otherwise S1 is repeated;

[0029] S2: If the braking current of the abnormal phase in the current sampling period satisfies the condition that it is less than the braking current of the abnormal phase N periods ago, then proceed to S3, otherwise repeat S1;

[0030] S3: If the negative sequence current of one and only one branch of the running branches of the abnormal phase at the current moment is greater than the negative sequence current set value, then this branch is regarded as an abnormal branch and S4 is continued, otherwise S1 is repeated;

[0031] S4: If the abnormal phase current of the abnormal branch in the current sampling period satisfies the condition corresponding to being less than the phase current M periods ago, then proceed to S5, otherwise repeat S1;

[0032] S5: If the abnormal phase current of the abnormal branch is greater than the current threshold value, it is determined that the abnormal branch is short-circuited, otherwise S1 is repeated; N and M are greater than or equal to 1 and less than or equal to 3, that is, the braking current before the first N cycles and the braking current before the first M cycles used for comparison are both historical data that are close to the current sampling cycle in time, and try to ensure that the real-time changes of the braking current can be reflected.

[0033] The actual circuit diagram of the CT secondary circuit ground short circuit fault and its corresponding equivalent circuit diagram are as follows Figure 2 As shown in the figure, the actual circuit diagram of the CT secondary circuit with phase-to-phase short circuit fault and its corresponding equivalent circuit diagram are as follows Figure 3 As shown, Figure 2 The above is the actual circuit diagram of the A phase ground short circuit fault. Figure 2 Below is the equivalent circuit diagram corresponding to the actual circuit diagram of the A phase ground short circuit fault. Figure 3 The above is the actual circuit diagram of the phase-to-phase short circuit fault between phases B and C. Figure 3 Below is the equivalent circuit diagram corresponding to the actual circuit diagram of the phase-to-phase short circuit fault between phases B and C; Figure 2 and Figure 3 middle, I A ,I B ,I C are the currents on the secondary side of the A, B, and C phase CTs respectively; R LA , R LB , R LC I is the equivalent resistance of the three-phase current loop inside the protection device; a ,I b ,I c is the three-phase current actually flowing into the protection device. f is the short-circuit transition resistance, I f is the current flowing through the transition resistor. When the CT secondary circuit is short-circuited, the short-circuit transition resistor R f In the presence of short circuit, part of the current will flow through, that is, the shunt I f, which results in the actual current I flowing into the protection device a ,I b ,I c The short-circuited branch generates a negative sequence current, and the phase current of the short-circuited branch also decreases, so that the short-circuited phase generates a differential current. This method finds through analysis that the CT secondary circuit has a negative sequence current regardless of phase-to-ground short circuit or two-phase short circuit, and when a short circuit occurs, the phase current will decrease and generate an unbalanced differential current. Therefore, this method combines the characteristics of differential current, braking current, negative sequence current of branch current and phase current as the criterion for determining whether a short circuit occurs in the CT secondary circuit, and judges in turn according to these characteristics. When all the criteria are met, a short circuit fault occurs, which can more reliably, accurately and comprehensively identify the branches in the CT secondary circuit that may short-circuit.

[0034] The logical principle diagram of the method for determining whether the CT secondary circuit is short-circuited is as follows: Figure 4 As shown below, combined Figure 4 The following is a specific description of this method:

[0035] S1: If the differential current I of a phase of the bus is detected d Greater than the differential current setting value (also called CT abnormal blocking setting value or Figure 4 Medium CT short circuit lockout threshold I bsset ), then this phase is regarded as an abnormal phase (the abnormal phase is Figure 4 X phase in the process), proceed to S2, otherwise repeat S1;

[0036] In this embodiment, before executing S1, the currents of all running branches of each phase bus are collected, and the differential currents of each phase are determined according to the currents of all running branches of each phase of the bus;

[0037] Determine the differential current I d The method is:

[0038]

[0039] In the formula, I d is the differential current of the phase, j is the branch number; n is the number of branches of the phase, is the vector current of the jth branch. In other embodiments, the differential current of each phase can be obtained by other existing technologies.

[0040] Among them, the differential current setting value may be the CT abnormal closing setting value, and the differential current setting value is determined according to the unbalanced situation of the actual power system. Specifically, the magnitude of the differential current setting value is greater than the maximum unbalanced current flowing through the bus protection during actual normal operation. In this embodiment, the differential current setting value is 1.2 times the maximum unbalanced current flowing through the bus protection during normal operation. If the differential current of a phase of the bus is greater than the differential current setting value, it means that the phase may have a secondary short circuit, a broken line, a primary system fault or other faults that cause an unbalanced power system. After the judgment of step S1, the phases that may have a secondary short circuit, a broken line, a primary system fault or other faults that cause an unbalanced power system can be screened out.

[0041] S2: If the braking current I of the abnormal phase in the current sampling period r If the condition corresponding to the braking current of the abnormal phase N cycles ago is met, then continue to S3, otherwise repeat S1;

[0042] The braking current is determined based on the absolute value of the current of all running branches on the phase;

[0043] Determine the braking current I r The method is:

[0044]

[0045] In the formula, I r is the braking current of the phase; j is the branch number; n is the number of branches of the phase, is the vector current of the jth branch.

[0046] The method for judging whether the braking current of the abnormal phase in the current sampling period satisfies the condition corresponding to the braking current of the abnormal phase N periods ago is specifically as follows:

[0047] Determine whether the difference between the braking current of the abnormal phase in the current sampling period and the braking current N periods ago is greater than the braking current constant value or determine whether the braking current of the abnormal phase in the current sampling period is less than the first set multiple of the braking current N periods ago;

[0048] Among them, N is greater than or equal to 1 and less than or equal to 3. In this embodiment, N is taken as 2. In other embodiments, N can also be 1 or 3. The braking current constant is determined according to the imbalance of the actual power system. Specifically, the magnitude of the differential current constant is greater than the braking current change value when the actual power system is unbalanced (the change value of the braking current when the power system is unbalanced compared to when the system is stable); the value range of the first set multiple is [0.65, 0.85].

[0049] If the difference between the braking current of the abnormal phase in the current current sampling cycle and the braking current of the abnormal phase N (in this embodiment, N is 2) cycles ago is greater than the braking current constant value:

[0050] |I r[k-2T] |-|I r[k] |>I set1

[0051] Where k is the current current sampling period corresponding to the current moment, T is the period length, and I set1 is the braking current constant, I r[k] is the braking current of the abnormal phase in the current sampling period, I r[k-2T] is the braking current 2 cycles ago; in this embodiment, the current sampling period is generally 20ms (it can be flexibly adjusted in other embodiments).

[0052] Or when the braking current of the abnormal phase in the current sampling period is less than the first set multiple of the braking current N (in this embodiment, N is 2) periods ago, that is:

[0053] |I r[k] | <k1*|I r[k-2T] |

[0054] Where k is the current current sampling period corresponding to the current moment, T is the period length, and I r[k] is the braking current of the abnormal phase in the current sampling period, I r[k-2T] is the braking current 2 cycles ago, k1 is the first set multiple;

[0055] It is determined that the braking current of the abnormal phase in the current sampling period meets the condition corresponding to the braking current less than the previously set number of periods ago, otherwise it is determined that it is not satisfied.

[0056] If the braking current of the abnormal phase in the current sampling period satisfies the condition that it is less than the braking current corresponding to the abnormal phase N periods ago, it means that the braking current in the phase is reduced. When a primary system fault occurs in the CT, the braking current tends to increase, and when a secondary short circuit occurs in the CT, the braking current decreases. Therefore, the phase whose braking current satisfies the condition that it is less than the braking current corresponding to the abnormal phase N periods ago is screened out, so that the possibility of a short circuit in the CT at this time is further increased, and the possibility of a primary system fault in the CT is reduced. The accuracy of the judgment can be improved by step S2.

[0057] S3: If the negative sequence current of one and only one branch of the running branches of the abnormal phase at the current moment is greater than the negative sequence current fixed value, then this branch is regarded as an abnormal branch (i.e. Figure 4 branch m), and continue to S4, otherwise repeat S1;

[0058] Here, when the negative sequence current of one and only one branch is greater than the negative sequence current constant, the branch is determined to be an abnormal branch. This is because generally at the same time, only one branch in a phase may be short-circuited, and there is almost no situation where multiple branches are short-circuited at the same time. Therefore, if it is detected that more than two branches simultaneously satisfy the negative sequence current greater than the negative sequence current constant, then compared with the situation where two or more branches actually satisfy the negative sequence current greater than the negative sequence current constant, it is more likely that other faults have occurred (such as a primary system fault). Therefore, the situation where only one branch is short-circuited is used as the condition that must be met for a real CT secondary circuit short circuit to occur, and the situation where two or more branches simultaneously satisfy the negative sequence current greater than the negative sequence current constant is regarded as a primary system fault, and is not regarded as a CT secondary circuit short circuit. The method of "when the negative sequence current of one and only one branch is greater than the negative sequence current constant, the branch is regarded as an abnormal branch" can reduce the possibility of a primary system fault in the CT, and the possibility of a CT secondary circuit short circuit is further increased. Therefore, the accuracy of distinguishing a CT secondary circuit short circuit is improved through step S3.

[0059] The magnitude of the negative sequence current is determined according to the magnitude of the three-phase current of each branch.

[0060] S4: If the abnormal phase current of the abnormal branch in the current sampling period satisfies the condition corresponding to the abnormal phase current of the abnormal branch M periods ago, then proceed to S5, otherwise repeat S1; wherein M is greater than or equal to 1 and less than or equal to 3, in this embodiment, M is taken to be equal to 2, and in other embodiments, M may also be 1 or 3;

[0061] The method for judging whether the abnormal phase current of the abnormal branch in the current sampling period satisfies the condition corresponding to the abnormal phase current M periods ago is specifically as follows:

[0062] If the abnormal phase current of the abnormal branch in the current sampling period is less than the second set multiple of the abnormal phase current of the abnormal branch M periods (in this embodiment, M is 2) before, that is:

[0063] |I j[k] | <k2*|I j[k-2T] |,

[0064] Where k is the current time, T is the cycle length, k2 is the second set multiple, I j[k] is the abnormal phase current of the abnormal branch in the current sampling period, I j[k-2T] is the abnormal phase current of the abnormal branch 2 cycles ago;

[0065] It is determined that the abnormal phase current of the abnormal branch in the current sampling period satisfies the condition that it is less than the condition corresponding to the abnormal phase current M cycles ago, otherwise it is determined that it is not satisfied. In this embodiment, the value range of the second set multiple is [0.65, 0.85]. In other embodiments, the method for judging whether the abnormal phase current of the abnormal branch in the current sampling period satisfies the condition that it is less than the condition corresponding to the abnormal phase current M cycles ago can also be: judging whether the difference between the abnormal phase current of the abnormal branch in the current sampling period and the abnormal phase current M cycles ago is greater than the set abnormal phase current constant (if greater, satisfied) or judging whether the abnormal phase current of the abnormal branch in the current sampling period is less than the second set multiple of the abnormal phase current M cycles ago (if less, satisfied); or it can also be other judgment conditions that can reflect that the abnormal phase current of the abnormal branch in the current sampling period is less than the abnormal phase current M cycles ago.

[0066] S5: If the abnormal phase current of the abnormal branch is greater than the current threshold value, it is determined that the abnormal branch is short-circuited, otherwise S1 is repeated. By executing step S5, the possibility that the decrease in the abnormal phase current of the branch is caused by a short circuit in the branch can be ruled out.

[0067] Implementation example of a system for determining short circuit in CT secondary circuit

[0068] This embodiment provides a technical solution for a system for determining a short circuit in a CT secondary circuit. The system includes a processor, and the processor is used to execute a computer program to implement the steps of the method for determining a short circuit in a CT secondary circuit in an embodiment of the method for determining a short circuit in a CT secondary circuit.

[0069] Since the specific implementation process and principle of the CT secondary circuit short-circuit determination system of this embodiment have been described in detail in the embodiment of the CT secondary circuit short-circuit determination method, no further details will be given here.

Claims

1. A method for determining whether a CT secondary circuit is short-circuited, characterized in that: The following steps are involved: S1: If the differential current of a certain phase of the bus is greater than the differential current setting value, then this phase is regarded as an abnormal phase and S2 is continued, otherwise S1 is repeated; S2: If the braking current of the abnormal phase in the current current sampling cycle satisfies the condition corresponding to being less than the braking current N cycles ago, then proceed to S3, otherwise repeat S1; S3: If the negative sequence current of one and only one branch of the running branches of the abnormal phase at the current moment is greater than the negative sequence current set value, then this branch is regarded as an abnormal branch and S4 is continued, otherwise S1 is repeated; S4: If the abnormal phase current of the abnormal branch in the current current sampling period satisfies the condition corresponding to being less than the phase current M periods ago, then proceed to S5, otherwise repeat S1; S5: If the abnormal phase current of the abnormal branch is greater than the current threshold value, it is determined that the abnormal branch is short-circuited, otherwise S1 is repeated; N and M are greater than or equal to 1 and less than or equal to 3.

2. The method for determining whether a CT secondary circuit is short-circuited according to claim 1, characterized in that: Methods for judging whether the braking current of the abnormal phase in the current current sampling period satisfies the condition corresponding to being less than the braking current N periods ago include: Determine whether the difference between the braking current of the abnormal phase in the current current sampling cycle and the braking current N cycles ago is greater than the braking current constant, or determine whether the braking current of the abnormal phase in the current current sampling cycle is less than the first set multiple of the braking current N cycles ago; if the difference is greater than the braking current constant, or the braking current is less than the first set multiple of the braking current N cycles ago, then determine that the braking current of the abnormal phase in the current current sampling cycle meets the condition corresponding to being less than the braking current of the previously set number of cycles ago, otherwise determine that it is not satisfied.

3. The method for determining whether a CT secondary circuit is short-circuited according to claim 1, characterized in that: The method of judging whether the abnormal phase current of the abnormal branch in the current current sampling period satisfies the condition corresponding to the phase current M periods ago includes: If the abnormal phase current of the abnormal branch in the current current sampling cycle is less than the second set multiple of the phase current M cycles ago, it is determined that the abnormal phase current of the abnormal branch in the current current sampling cycle meets the condition corresponding to being less than the phase current M cycles ago, otherwise it is determined that it is not satisfied.

4. The method for determining whether a CT secondary circuit is short-circuited according to claim 1, characterized in that: The magnitude of the negative sequence current is determined according to the magnitude of the three-phase current of each branch.

5. The method for determining whether a CT secondary circuit is short-circuited according to claim 2, characterized in that: The value range of the first setting multiple is [0.65, 0.85].

6. The method for determining whether a CT secondary circuit is short-circuited according to claim 3, characterized in that: The value range of the second setting multiple is [0.65, 0.85].

7. A CT secondary circuit short circuit determination system, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the method for determining whether a CT secondary circuit is short-circuited as described in any one of claims 1 to 6.

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