A method and system for judging CT secondary circuit short circuit

Abstract

This invention relates to a method and system for determining short circuits in the secondary circuit of a CT (Continuous Transmission Center), belonging to the technical field of CT secondary circuit fault determination. S1: If the differential current of a phase of the busbar is greater than the differential current setting, then that phase is an abnormal phase and proceed to S2; otherwise, repeat S1. S2: If the braking current of the abnormal phase in the current current sampling period meets the condition corresponding to the braking current N cycles ago, proceed to S3; otherwise, repeat S1. S3: If at the current moment, only one branch of the abnormal phase has a negative sequence current greater than the negative sequence current setting, then that branch is considered an abnormal branch and proceed to S4; otherwise, repeat S1. S4: If the abnormal phase current of the abnormal branch in the current current sampling period meets the condition corresponding to the phase current M cycles ago, 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 has a short circuit; otherwise, repeat S1. N and M are greater than or equal to 1 and less than or equal to 3.

Description

Technical Field

[0001] This invention relates to a method and system for detecting short circuits in the secondary circuit of a CT scanner, belonging to the field of CT secondary circuit fault detection technology. Background Technology

[0002] Faults in the secondary circuit of a current transformer, such as poor terminal contact or damage, human-caused disconnection or short circuit, or transformer damage, can all be attributed to open circuits or short circuits in the current circuit.

[0003] With the advancement of relay protection technology, the logic for CT disconnection has become increasingly sophisticated, and the domestic relay protection industry has established standardized technical specifications. CT disconnection is extremely dangerous, therefore, it generally does not require blocking the differential protection mechanism. However, the CT secondary circuit is prone to phase-to-ground short circuits and two-phase-to-phase short circuits due to insulation damage, terminal contamination, and terminal short circuits. CT short circuits typically block the differential protection mechanism. CT secondary circuit short circuit faults include phase-to-phase and phase-to-ground short circuits. These faults cause current shunting in the CT secondary circuit, leading to inaccurate secondary current sampling of the relay protection device, increased unbalanced current, and differential current, resulting in maloperation or failure to operate of the relay protection device, and even causing unit shutdowns, line trips, and other accidents, seriously affecting the safe and reliable operation of the power grid. Therefore, it is necessary to accurately determine CT secondary circuit short circuits and promptly block differential protection operation during short circuits to avoid maloperation of the differential protection.

[0004] Currently, methods for detecting CT (Continuous Transmission Unit) disconnections are relatively well-developed. Most methods rely on the magnitudes of phase current, differential current, and zero-sequence current during the analysis process. Chinese invention patent application CN117117786A discloses a CT secondary circuit fault detection and anti-misoperation interlocking device. This device detects short-circuit faults by judging whether a short-circuit fault has occurred in the CT secondary circuit based on the sampled zero-sequence current at the grounding wire of the CT secondary circuit, the zero-sequence current at the neutral point, 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 method is insufficient to detect two-phase short-circuit faults in the CT secondary circuit, thus making it difficult to accurately and comprehensively determine CT secondary circuit short-circuit faults. Summary of the Invention

[0005] The purpose of this invention is to provide a method and system for identifying short circuits in the secondary circuit of a CT scanner, in order to solve the problem that existing methods for determining whether a short circuit fault has occurred in the secondary circuit of a CT scanner can only identify some short circuit situations, making it difficult to accurately and comprehensively determine short circuit faults in the secondary circuit of a CT scanner.

[0006] To solve the above-mentioned technical problems, the present invention provides a method for determining short circuit in the secondary circuit of a CT. The method includes the following steps: S1: If the differential current of a certain phase of the busbar is greater than the differential current setting value, then 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 period meets the condition corresponding to the braking current N periods ago, then continue to S3; otherwise, repeat S1.

[0008] S3: If at the current moment, only one branch of the abnormal phase has a negative sequence current greater than the negative sequence current setting, then the branch is considered an abnormal branch and S4 continues; otherwise, S1 is repeated.

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

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

[0011] Furthermore, the methods for determining whether the braking current of the abnormal phase in the current sampling period meets the condition corresponding to the braking current less than N periods ago include:

[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 setpoint, or determine whether the braking current of the abnormal phase in the current sampling period is less than a first set multiple of the braking current N periods ago; if the difference is greater than the braking current setpoint, 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 meets the condition of being less than the braking current of a previously set number of periods ago, otherwise determine that it does not meet the condition.

[0013] Furthermore, the methods for determining whether the abnormal phase current of the abnormal branch in the current sampling period meets the condition corresponding to the phase current less than M periods ago include:

[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, then it is determined that the abnormal phase current of the abnormal branch in the current sampling period meets the condition of 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 based on the magnitude of the three-phase current in each branch.

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

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

[0018] Beneficial Effects: This invention provides a novel method for determining short circuits in the secondary circuit of a current transformer (CT). This method utilizes the characteristics discovered through analysis that negative sequence currents exist in both phase-to-ground and two-phase-to-phase short circuits in the CT secondary circuit. Furthermore, during a short circuit, the short-circuit phase current decreases, generating an unbalanced differential current. The method combines the negative sequence currents of differential current, braking current, and branch currents, along with the characteristics of phase currents, as criteria for determining whether a short circuit has occurred in the CT secondary circuit. Based on these characteristics, the method sequentially judges the situation, and a short circuit fault occurs when all criteria are met. Specifically, this method first filters out phases of the busbar where the differential current is greater than the differential current of each phase. For an abnormal phase (i.e., the CT abnormal blocking value, usually taken as 1.2 times the maximum unbalanced current flowing through the bus protection during normal operation), if the differential current of a certain phase of the bus is greater than the differential current setting, it indicates that a secondary short circuit, open circuit, primary system fault, or other fault causing power system imbalance may occur in that phase. This phase can then be preliminarily considered an abnormal phase. Further analysis and comparison are then performed on the braking current of the abnormal phase in the current sampling period with the previous braking current. If the braking current of the abnormal phase in the current sampling period meets the condition of being less than the braking current corresponding to N periods ago, it indicates that the braking current in that phase has decreased, and the probability of a primary system fault has decreased. When a primary system fault occurs, the braking current often increases, further increasing the likelihood of a secondary short-circuit fault. If, at the current moment, only one branch of the abnormal phase has a negative sequence current greater than the negative sequence current setting, it indicates the presence of a certain negative sequence current in that branch, satisfying the characteristics of a branch experiencing a secondary short-circuit fault, thus reducing the likelihood of a primary system fault (if, during the current sampling period, two or more branches of the abnormal phase have negative sequence currents greater than the negative sequence current setting, it is more likely due to a primary system fault). In this case, the likelihood of a secondary short-circuit fault further increases. Finally, the abnormal phase current of the abnormal branch is determined to be... If the condition corresponding to the phase current less than M cycles ago is met, it indicates that the abnormal phase current of the abnormal branch has decreased, which also meets the characteristics of a branch with a short circuit fault. It is then determined whether the phase current of the abnormal branch is greater than the current threshold. If it is greater, the possibility that the decrease in the abnormal phase current of the abnormal branch is caused by an open circuit in the branch is ruled out, and the branch is determined to have a short circuit fault. If one of the criteria is not met, the detection of the differential current of each phase bus being greater than the differential current setting is repeated until a branch that meets all the criteria is selected. Therefore, this method can more reliably, accurately and comprehensively identify branches with short circuits in the CT secondary circuit.

[0019] The present invention also provides a CT secondary circuit short-circuit detection system, including a processor, the processor being used to execute a computer program to implement the steps of the above-described CT secondary circuit short-circuit detection method.

[0020] The CT secondary circuit short circuit detection system can achieve the same beneficial effects as the CT secondary circuit short circuit detection method described above. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method for determining short circuits in the CT secondary circuit in an embodiment of the present invention.

[0022] Figure 2 This is the actual circuit diagram and its corresponding equivalent circuit diagram of the CT secondary circuit ground short circuit fault in the embodiment of the CT secondary circuit short circuit discrimination method of the present invention;

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

[0024] Figure 4 This is a logic diagram of the method for determining short circuits in the CT secondary circuit in an embodiment of the present invention. Detailed Implementation

[0025] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0026] Example of a method for detecting short circuits in the secondary circuit of a CT scanner.

[0027] This embodiment provides a technical solution for determining short circuits in the secondary circuit of a current transformer (CT). This method is based on the analysis that a negative sequence current exists in the CT secondary circuit regardless of whether it is a phase-to-ground short circuit or a two-phase-to-phase short circuit. Furthermore, when a short circuit occurs, the phase current decreases, generating an unbalanced differential current. The method combines the characteristics of the differential current, braking current, negative sequence current of the branch current, and phase current as criteria for determining whether a short circuit has occurred in the CT secondary circuit. These characteristics are judged sequentially, and a short circuit fault occurs when all criteria are met. The flowchart of the CT secondary circuit short circuit determination method is shown below. Figure 1 As shown, the method specifically includes the following steps:

[0028] S1: If the differential current of a certain phase of the busbar is detected to be greater than the differential current setting, then the 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 meets the condition that it is less than the braking current of the abnormal phase N periods ago, then continue to S3; otherwise, repeat S1.

[0030] S3: If at the current moment, only one branch of the abnormal phase has a negative sequence current greater than the negative sequence current setting, then the branch is considered an abnormal branch and S4 continues; otherwise, S1 is repeated.

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

[0032] S5: If the abnormal phase current of the abnormal branch is greater than the current threshold, the abnormal branch is determined to be short-circuited; otherwise, repeat S1. N and M are greater than or equal to 1 and less than or equal to 3. That is, the braking currents of the previous N cycles and the previous M cycles used for comparison are historical data that are close to the current sampling period in time, so as to ensure that the real-time changes of the braking current can be reflected as much as possible.

[0033] The actual circuit diagram and its corresponding equivalent circuit diagram for a ground fault in the secondary circuit of a CT are as follows: Figure 2 As shown, the actual circuit diagram and its corresponding equivalent circuit diagram for a phase-to-phase short-circuit fault in the CT secondary circuit are as follows: Figure 3 As shown, Figure 2 The image above shows the actual circuit diagram of phase A experiencing a ground fault. Figure 2 Below is the equivalent circuit diagram corresponding to the actual circuit diagram of phase A experiencing a ground fault. Figure 3 The diagram above shows the actual circuit diagram of a phase-to-phase short circuit fault occurring between phases B and C. Figure 3 Below is the equivalent circuit diagram corresponding to the actual circuit diagram of a phase-to-phase short circuit fault occurring between phases B and C; Figure 2 and Figure 3 middle, I A I B I C These are the currents on the secondary side of phases A, B, and C of the CT, respectively; R LA R LB R LC The equivalent resistance of the three-phase current loop inside the protection device; I a I b I c R represents the actual three-phase current flowing into the protection device. f For short-circuit transition resistance, I f The current flowing through the transition resistor is the current that, when a short circuit occurs in the secondary circuit of the CT, is due to the short-circuit transition resistor R. f Due to the presence of [something], a portion of the current will flow during the short circuit, i.e., current shunt I. fThis leads to the actual current I flowing into the protection device. a I b I c Changes occur, with the short-circuited branch generating a negative sequence current, and the phase current of the short-circuited branch also decreases, thus creating a differential current in the short-circuited phase. This method analyzes and finds that the CT secondary circuit exhibits negative sequence current regardless of whether it's a phase-to-ground short circuit or a two-phase-to-phase short circuit. Furthermore, when a short circuit occurs, the phase current decreases, generating an unbalanced differential current. Therefore, this method combines the characteristics of differential current, braking current, negative sequence current of the branch current, and phase current as criteria for determining whether a short circuit has occurred in the CT secondary circuit. It judges based on these characteristics sequentially, and when all criteria are met, a short circuit fault occurs, enabling a more reliable, accurate, and comprehensive identification of branches in the CT secondary circuit that are prone to short circuits.

[0034] The logic diagram of the method for detecting short circuits in the secondary circuit of a CT is shown below. Figure 4 As shown below, in conjunction with Figure 4 This method will be explained in detail as follows:

[0035] S1: If a differential current I in a certain phase of the busbar is detected d Greater than the differential current setting (also known as the CT abnormal lockout value or Figure 4 CT short-circuit blocking threshold I bsset If the phase is abnormal, then the abnormal phase is considered an anomalous phase (abnormal phase is...). Figure 4 If the X phase is in the middle, continue with S2; otherwise, repeat S1.

[0036] In this embodiment, before executing S1, the current of all operating branches of each phase bus is collected, and the differential current of each phase is determined based on the current of all operating branches of each phase bus.

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

[0038]

[0039] In the formula, I d Let j be the differential current of that phase, j be the branch number, and n be the number of branches in that phase. Let be the vector current of the j-th branch. In other embodiments, the differential current of each phase can be obtained using other existing techniques.

[0040] The differential current setting is the CT abnormal blocking value. This setting is determined based on the actual power system imbalance. Specifically, the differential current setting is greater than the maximum unbalanced current flowing through the busbar protection during normal operation. In this embodiment, the differential current setting is 1.2 times the maximum unbalanced current flowing through the busbar protection during normal operation. If the differential current of a certain phase of the busbar is greater than the differential current setting, it indicates that a secondary short circuit, open circuit, primary system fault, or other fault causing power system imbalance may occur in that phase. Step S1 can filter out phases that may experience secondary short circuits, open circuits, primary system faults, or other faults causing power system imbalance.

[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 less than N cycles ago is met, then proceed to S3; otherwise, repeat S1.

[0042] The braking current is determined based on the absolute value of the current in all operating branches of that phase.

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

[0044]

[0045] In the formula, I r is the braking current of that phase; j is the branch number; n is the number of branches in that phase. Let be the vector current of the j-th branch.

[0046] Specifically, the method for determining whether the braking current of the abnormal phase in the current sampling period meets the condition corresponding to the braking current of the abnormal phase less than N periods ago is 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 setting, 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] Wherein, 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 setting is determined according to the actual imbalance of the power system. Specifically, the differential current setting is greater than the braking current change value when the actual power system is unbalanced (the braking current change value when the power system is unbalanced compared to the braking current change value when the system is stable). The 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 period and the braking current of the abnormal phase N periods ago (in this embodiment, N is 2) is greater than the braking current setpoint:

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

[0051] In the formula, k is the current current sampling period corresponding to the current moment, T is the period length, and I... set1 For the braking current setting, I r[k] I is the braking current of the abnormal phase in the current sampling period. r[k-2T] The current is the braking current two 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 a first set multiple of the braking current N (in this embodiment, N is 2) periods ago:

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

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

[0055] If the braking current of the abnormal phase in the current sampling period is less than the braking current of the previous set number of periods, it is determined that the condition is not met.

[0056] If the braking current of the abnormal phase in the current sampling period meets the condition corresponding to the braking current of the abnormal phase less than N periods ago, it indicates that the braking current in that phase has decreased. Since the braking current often increases when the CT experiences a primary system fault and decreases when the CT experiences a secondary short circuit, phases whose braking current meets the condition corresponding to the braking current of the abnormal phase less than N periods ago are selected. This further increases the probability of the CT experiencing a short circuit and reduces the probability of the CT experiencing a primary system fault. Step S2 can improve the accuracy of the determination.

[0057] S3: If at the current moment, only one branch of the operating branches of the abnormal phase has a negative sequence current greater than the negative sequence current setting, then that branch is considered an abnormal branch (i.e., Figure 4 If the branch m is in S4, then proceed to S4; otherwise, repeat S1.

[0058] Here, a branch is considered abnormal only when the negative sequence current of one and only one branch is greater than the negative sequence current setting. This is because generally, only one branch in a phase can short-circuit at any given time, and it is almost impossible for multiple branches to short-circuit simultaneously. Therefore, if two or more branches are detected to simultaneously satisfy the condition of having a negative sequence current greater than the negative sequence current setting, it is more likely that other faults (such as primary system faults) have occurred compared to the case where two or more branches actually satisfy the condition of having a negative sequence current greater than the negative sequence current setting. Therefore, the condition of having only one branch short-circuit is taken as the condition that a CT secondary circuit short circuit must be met. The case of two or more branches simultaneously satisfying the condition of having a negative sequence current greater than the negative sequence current setting is considered as a primary system fault, not a CT secondary circuit short circuit. Thus, the method of "treating the branch as an abnormal branch when the negative sequence current of one and only one branch is greater than the negative sequence current setting" can reduce the possibility of a CT primary system fault and further increase the possibility of a CT secondary circuit short circuit. Therefore, step S3 improves the accuracy of CT secondary circuit short circuit detection.

[0059] The magnitude of the negative sequence current is determined based on the magnitude of the three-phase current in each branch.

[0060] S4: If the abnormal phase current of the abnormal branch in the current sampling period meets the condition corresponding to the abnormal phase current of the abnormal branch less than M periods ago, then continue 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 as 2. In other embodiments, M can also be 1 or 3.

[0061] The specific method for determining whether the abnormal phase current of the abnormal branch in the current sampling period meets the condition corresponding to the abnormal phase current less than M periods ago is 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 ago (M is 2 in this embodiment), then:

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

[0064] In the formula, k is the current time, T is the period length, k2 is the second set multiple, and I j[k] I represents the abnormal phase current of the abnormal branch in the current sampling period. j[k-2T] This refers to the abnormal phase current of the abnormal branch two cycles ago;

[0065] If the abnormal phase current of the abnormal branch in the current sampling period is determined to meet the condition that it is less than the abnormal phase current of M periods ago, then it is determined that it is not met. In this embodiment, the value range of the second set multiple is [0.65, 0.85]. In other embodiments, the way to determine whether the abnormal phase current of the abnormal branch in the current sampling period meets the condition that it is less than the abnormal phase current of M periods ago can also be: determining whether the difference between the abnormal phase current of the abnormal branch in the current sampling period and the abnormal phase current of M periods ago is greater than the set abnormal phase current value (if it is greater, then it is met) or determining 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 of M periods ago (if it is less, then it is met); or it can also be other determination 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 of M periods ago.

[0066] S5: If the abnormal phase current of the abnormal branch is greater than the current threshold, it is determined that the abnormal branch has a short circuit. Otherwise, repeat S1. By executing step S5, the possibility that the decrease in the abnormal phase current of the branch is caused by the branch being open is eliminated.

[0067] Example of a short circuit detection system for CT secondary circuit

[0068] This embodiment provides a technical solution for a CT secondary circuit short-circuit detection system. The system includes a processor, which executes a computer program to implement the steps of the CT secondary circuit short-circuit detection method embodiment.

[0069] Since the specific implementation process and principle of the CT secondary circuit short circuit discrimination system in this embodiment have been described in detail in the embodiment of the CT secondary circuit short circuit discrimination method, they will not be repeated here.

Claims

1. A method for determining short circuits in the secondary circuit of a CT scanner, characterized in that, Includes the following steps: S1: If the differential current of a certain phase of the busbar is greater than the differential current setting, then that phase is regarded as an abnormal phase and S2 is continued; otherwise, S1 is repeated. S2: If it is determined whether the difference between the braking current of the abnormal phase in the current current sampling period and the braking current N periods ago is greater than the braking current setting, or if it is determined whether the braking current of the abnormal phase in the current 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 set value, or the braking current is less than the first set multiple of the braking current N cycles ago, then it is determined that the braking current of the abnormal phase in the current current sampling period meets the condition of being less than the braking current corresponding to the set number of cycles ago, and S3 continues. Otherwise, if the condition is not met, repeat step S1; S3: If at the current moment, only one branch of the abnormal phase has a negative sequence current greater than the negative sequence current setting, then the branch is considered an abnormal branch and S4 continues; otherwise, S1 is repeated. S4: If the abnormal phase current of the abnormal branch in the current current sampling period is less than the second set multiple of the phase current M cycles ago, then it is determined that the abnormal phase current of the abnormal branch in the current current sampling period meets the condition corresponding to the phase current M cycles ago, and S5 is continued; otherwise, it is determined that it does not meet the condition, and S1 is repeated. S5: If the abnormal phase current of the abnormal branch is greater than the current threshold, the abnormal branch is determined to be short-circuited; otherwise, repeat S1. N and M are greater than or equal to 1 and less than or equal to 3.

2. The method for determining short circuits in the CT secondary circuit according to claim 1, characterized in that, The magnitude of the negative sequence current is determined based on the magnitude of the three-phase current in each branch.

3. The method for determining short circuits in the CT secondary circuit according to claim 1, characterized in that, The first set multiple is set to a value range of [0.65, 0.85].

4. The method for determining short circuits in the CT secondary circuit according to claim 1, characterized in that, The second set multiple is set to a value range of [0.65, 0.85].

5. A short-circuit detection system for a CT secondary circuit, comprising a processor, characterized in that, The processor is used to execute a computer program to implement the steps of the method for determining short circuits in the CT secondary circuit as described in any one of claims 1-4.

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