Branch current asymmetric bus protection CT disconnection discrimination method and system
By comparing the magnitude of the bus protection differential current and virtual current, and dynamically adjusting the discrimination parameters in combination with the degree of branch current imbalance, the problem of poor CT disconnection discrimination effect in the case of branch current asymmetry in the prior art is solved, and the accuracy and reliability of bus protection are improved.
Patent Information
- Application Number
- CN202510197288.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-21
- Publication Date
- 2025-05-27
AI Technical Summary
The existing busbar protection CT line break judgment method is poor in the case of branch current asymmetry, which may lead to incorrect busbar protection and affect the reliability of power supply.
By comparing the magnitude of the bus protection differential current and the virtual current, and dynamically adjusting the discriminant parameters of the branch current imbalance, the accurate judgment of the broken branch is achieved.
The accuracy and reliability of busbar protection CT disconnection judgment is improved, and the broken branch can be effectively identified under the asymmetric branch current of the branch to avoid malfunctioning busbar protection.
Smart Images
Figure CN120044433A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a method and system for discriminating CT disconnection in bus protection with asymmetric branch currents, belonging to the technical field of bus protection. Background Art
[0002] Bus protection generally adopts the principle of ratio-restrained current differential protection, and its correct operation depends on the correct transmission and acquisition of branch currents on the bus. If a secondary circuit disconnection occurs in the current transformer (CT) during operation, it may cause incorrect operation of the bus differential protection.
[0003] With the development of relay protection technology, the discrimination logic for CT disconnection in buses has become increasingly perfect, and standard technical specifications have been formed in the domestic relay protection industry. However, the current discrimination methods for CT disconnection in the industry mainly rely on "differential current imbalance" + "zero-sequence current in a single branch". Such criteria can effectively identify the disconnection branch for the condition where the three-phase currents of each branch are symmetric, and then take measures to prevent misoperation of the protection.
[0004] However, in actual engineering applications, due to the incomplete consistency of the three-phase circuit impedances of each branch, current asymmetry, that is, the phenomenon of unbalanced three-phase currents, is relatively common. Especially in Figure 1 ring wiring modes such as the 2 / 3 wiring shown, it is easier to have unbalanced three-phase currents in multiple branches. However, most of the existing CT disconnection discriminations rely on branch current imbalance for line selection. When the degree of asymmetry of the three-phase currents of the operating branches on the bus is relatively large, the existing CT disconnection criteria will misjudge, and in severe cases, it will lead to incorrect operation of the bus protection, affecting the reliability of power supply.
[0005] Therefore, it is necessary to study a method for identifying CT disconnection in bus protection with asymmetric branch currents to effectively identify the disconnection branch and avoid incorrect operation of the bus protection. Summary of the Invention
[0006] The purpose of the present invention is to provide a method and system for discriminating CT disconnection in bus protection with asymmetric branch currents to solve the problem of poor discrimination effect of existing CT disconnection discrimination for the condition of asymmetric branch currents.
[0007] To achieve the above purpose, the solution of the present invention includes:
[0008] A method for discriminating CT disconnection in bus protection with asymmetric branch currents according to the present invention includes the following steps:
[0009] When It is determined that phase S of branch X is disconnected;
[0010] Wherein, is the magnitude of the differential current of phase S of the bus protection at the current moment, is the magnitude of the sum of the in-phase bus protection differential current and the virtual current of branch X at the current moment. Phase S is any one of the three phases. Branch X refers to the branch on the bus that satisfies the condition that the current of any one phase is less than I set1 , and the currents of the remaining two phases are both greater than I set1 , I r-NT -I r >I set1 and the operating branch, and the value of K is as follows:
[0011]
[0012] K X <K 1 , K = K 1
[0013] K X >K 2 , K = K 2
[0014] K 2 <K X <K 1 , K = K X
[0015] Among them, I set1 is the no-current threshold value, I set2 is less than the CT disconnection setting value, I r-NT is the braking current of NT, I r is the braking current at the current moment, is the magnitude of the bus protection differential current at the current moment, K X is the three-phase current unbalance coefficient of NT in branch X, K 1 is the lower limit value of the unbalance degree, K 2 is the upper limit value of the unbalance degree, I X is the magnitude of any one phase current of NT in branch X, I max is the maximum phase magnitude of NT, I min is the minimum phase magnitude of NT. NT is the moment of stable operation of the system.
[0016] Furthermore, branch X is the branch among all the operating branches on the bus that also satisfies ;
[0017] In this inequality, is the magnitude of the change in the branch current of phase A or B or C of a certain branch among all the operating branches on the bus, is the same phase as the magnitude of the bus protection differential current at the current moment.
[0018] Further, the system stable operation moment refers to the 3rd cycle or more than the 3rd cycle before the current moment.
[0019] Further, K 1 = 2.
[0020] Further, K 2 = 10.
[0021] Further, I set2 is half of the CT disconnection setting value.
[0022] Further, I set1 ≥ 0.05 A.
[0023] A bus protection CT disconnection discrimination system for branch current asymmetry of the present invention includes a processor, and the processor is used to execute a computer program to implement the steps of the bus protection CT disconnection discrimination method for branch current asymmetry as described above.
[0024] Advantages of the present invention:
[0025] The present invention is a pioneering invention, providing a bus protection CT disconnection discrimination method for branch current asymmetry. This method conducts CT disconnection discrimination on operating branches where there is branch current asymmetry on the bus and the changes in braking current and the magnitudes of differential current are both greater than the corresponding threshold values. Specifically, it discriminates the disconnection branch by comparing the magnitudes of the virtual current and differential current of the aforementioned operating branches. Among them, the proportional relationship between the differential current and the magnitude of the current of the aforementioned operating branches can be dynamically adjusted according to the degree of current asymmetry during normal operation, making the bus protection CT disconnection discrimination effect under the condition of branch current asymmetry more accurate and reliable. Description of the Drawings
[0026] Figure 1 is a schematic diagram of the existing two-thirds bus connection;
[0027] Figure 2 is a flowchart of bus protection CT disconnection discrimination for branch current asymmetry. Detailed Embodiments
[0028] To solve the problems in the background technology, the present invention provides a bus protection CT disconnection recognition method for branch current asymmetry. This method conducts CT disconnection discrimination on operating branches where there is branch current asymmetry on the bus and the changes in braking current and the magnitudes of differential current are both greater than the corresponding threshold values. Specifically, it discriminates the disconnection branch by comparing the magnitudes of the virtual current and differential current of the aforementioned operating branches to effectively identify the disconnection branch and avoid incorrect operation of the bus protection.
[0029] To make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.
[0030] An embodiment of a bus protection CT disconnection discrimination system with asymmetric branch currents:
[0031] A bus protection CT disconnection discrimination system with asymmetric branch currents includes a processor, which is used to execute a computer program to implement the steps of a bus protection CT disconnection discrimination method with asymmetric branch currents.
[0032] Among them, a bus protection CT disconnection discrimination method with asymmetric branch currents discriminates CT disconnection for the operating branches on the bus where there are asymmetric branch currents and the changes in braking current and the magnitudes of differential currents are both greater than the corresponding threshold values. Specifically, the disconnection branch is discriminated by comparing the magnitudes of the virtual current and differential current of the aforementioned operating branches.
[0033] Among them, when the current of any phase of the operating branch on the bus is less than I set1 and the currents of the remaining two phases are both greater than I set1 , it is considered that there are asymmetric branch currents in the operating branch.
[0034] The method specifically includes the following steps:
[0035] When It is determined that the S phase of branch X is disconnected;
[0036] Among them, is the magnitude of the differential current of the S phase of the bus protection at the current moment, is the magnitude of the sum of the differential current of the bus protection in the same phase and the virtual current of branch X at the current moment. The S phase is any one of the three phases, and branch X refers to the operating branch on the bus that satisfies that the current of any phase is less than I set1 , the currents of the remaining two phases are both greater than I set1 , I r-NT -I r >I set1 and The value of K is as follows:
[0037]
[0038] K X <K 1 , K = K 1
[0039] K X >K 2 , K = K 2
[0040] K 2 <K X<K 1 , K = K X
[0041] Among them, I set1 is the no-flow threshold value, I set2 is less than the CT disconnection setting value, I r-NT is the braking current of NT, I r is the braking current at the current moment, is the magnitude of the bus protection differential current at the current moment, K X is the three-phase current unbalance coefficient of NT in branch X, K 1 is the lower limit value of the unbalance degree, K 2 is the upper limit value of the unbalance degree, I X is the amplitude of any phase current of NT in branch X, I max is the maximum phase amplitude of NT, I min is the minimum phase amplitude of NT, NT is the moment when the system operates stably;
[0042] The calculation formula for the virtual current of phase S in branch X is as follows:
[0043]
[0044] Among them, is the virtual current of phase A in branch X, is the virtual current of phase B in branch X, is the virtual current of phase C in branch X.
[0045] To improve the discrimination efficiency and the accuracy of discrimination effect, branch X is the branch among all the operating branches on the bus that also satisfies ; in this inequality, is the magnitude of the branch current change of phase A or phase B or phase C of a certain branch among all the operating branches on the bus, is the same phase as the magnitude of the bus protection differential current at the current moment.
[0046] That is, branch X refers to the operating branch on the bus that satisfies that any one phase current is less than I set1 , the remaining two phase currents are both greater than I set1 , I r-NT - I r > I set1 , and .
[0047] Considering that the CT disconnection discrimination of the bus protection is carried out before the fault occurs, that is, when the system is operating stably. Since the protection logic of the bus protection needs to use the data of the previous 2 cycles at the current moment, the discrimination should use the data when the system is operating stably. Therefore, the stable operation moment of the system refers to the 3rd cycle before the current moment or the stable operation moment of the system refers to more than the 3rd cycle before the current moment.
[0048] Considering that the influence of a lower degree of imbalance can be ignored, according to experience, generally make K 1 = 2. Of course, appropriate adjustments can be made according to the actual situation.
[0049] Considering that the influence of a higher degree of imbalance has exceeded the control, according to experience, generally make K 2 = 10. Of course, appropriate adjustments can be made according to the actual situation.
[0050] Specifically, I set2 is half of the CT disconnection setting value.
[0051] Specifically, I set1 ≥ 0.05A.
[0052] The present invention starts the CT disconnection discrimination logic according to the change of the "sum current of the bus (the sum of the absolute values of the currents of all connected elements of the bus, that is, the braking current)" and the magnitude of the differential current of the bus, and then comprehensively discriminates the magnitude relationship between the differential current and the virtual current of the suspected disconnection branch to lock the disconnection branch. Among them, the proportional relationship between the differential current and the current of the suspected disconnection branch can be dynamically adjusted according to the degree of current asymmetry (unbalanced three-phase load) during normal operation, so as to improve the current imbalance of each branch during system operation, that is, it has the ability to discriminate CT disconnection when CT disconnection occurs under the zero-sequence current condition.
[0053] Such as Figure 2 shown, the specific steps include:
[0054] Step 1: Collect the A / B / C three-phase currents of all operating branches on the bus, and circularly store the current data, and record the data duration as 100 ms; Considering the storage space and usage requirements, the 100 ms here is the previous 100 ms of the current moment, which can provide the data for subsequent calculations. Of course, the duration of recording the data can also be adjusted according to actual needs.
[0055] Step 2: Real-time judge the magnitude of the branch current of each operating branch. If only one-phase current of a certain branch is less than the no-current threshold value I set1 , and the other two phases are both greater than I set1 , then calculate the degree of imbalance of the three-phase current of this branch 3 cycles (60 ms) ago, and the imbalance coefficient of the three-phase current of this branch 3 cycles (60 ms) ago For example: The current of phase A of branch n is less than Iset1 The current of phase B and phase C is greater than I set1 , then I X is the amplitude of the current of phase A three cycles ago, and I max is the maximum phase amplitude three cycles ago, and I min is the minimum phase amplitude three cycles ago.
[0056] Among them, no current is not judged. Considering that the influence of the abnormality when the current is small on the operation of the whole device is small enough to be ignored, the no-current threshold value I set1 is set. Only when the threshold value is reached is the unbalance degree judged, that is, only when a disconnection occurs after exceeding this threshold value will it affect the operation of the device.
[0057] Among them, when the system is operating stably, CT disconnection is judged, and once a fault occurs, it is no longer judged. Considering that the judgment of the bus protection logic will use the data of the previous 2 cycles, and 2-cycle data is used to judge the fault, so using the data of the 3rd cycle or the 4th cycle or more cycles before the current moment to judge must be to judge CT disconnection during the stable process of the normal operation of the system, rather than judging CT disconnection during a fault.
[0058] Step 3: Collect the currents of all operating branches on the bus and calculate the differential current by phase The current change of each branch and the current restraint current at present and the restraint current three cycles ago Among them is the current of each branch at the current moment, is the current of each branch three cycles ago.
[0059] Step 4: Discriminate by phase that the restraint current becomes smaller and the differential current becomes larger. If I r-3T -I r >I set1 and lock the K X value calculated in the second step. If K X <K 1 , take K = K 1 ; if K X >K 2 , take K = K 2 , if K 1 <K X <K 2 , take K = K X , and store the K value in a specific area. I set2 is smaller than the CT disconnection setting value, generally half of the setting value, and can work 3 - 5 milliseconds in advance.
[0060] Among them, the degree of imbalance should not be too large or too small. If it is too small, the influence can be ignored; if it is too large, it cannot be managed. Therefore, let K 1 = 2, K 2 = 10.
[0061] Step 5: Traverse each branch and judge its current change amount. If the magnitudes of the same phase and are basically the same (the criterion is ), then lock this branch as a suspected broken-line branch, defined as "Branch X".
[0062] Step 6: Calculate the virtual current of the suspected broken phase of the locked "Branch X". Taking the virtual current of Phase A as an example: Among them
[0063] Step 7: Compare the magnitude relationship between the differential current and the virtual current of "Branch X". Taking Phase A as an example, if then it is determined that Phase A of this branch is broken.
[0064] The method for discriminating CT breakage in bus protection of the present invention can quickly and reliably discriminate CT breakage. Its discrimination effect is not affected by the unbalance of branch currents, and can dynamically adjust the proportional parameters in the discrimination criterion according to the degree of branch current unbalance. This method has made great optimization and improvement for the CT breakage discrimination criterion under the condition of three-phase current unbalance of the side switch with two-thirds wiring, improving the overall reliability of bus protection and effectively ensuring the safe and stable operation of the power system.
[0065] An embodiment of a method for discriminating CT breakage in bus protection with asymmetric branch currents:
[0066] A method for discriminating CT breakage in bus protection with asymmetric branch currents has been described in detail in an embodiment of a system for discriminating CT breakage in bus protection with asymmetric branch currents, and will not be elaborated here.
Claims
1. A method for determining a busbar protection CT disconnection with asymmetric branch current, characterized in that: The steps include: when Determine if the S phase of the X branch is broken; in, is the magnitude of the S-phase differential current of the busbar protection at the current moment, It is the sum of the bus protection differential current and the X branch virtual current of the same phase at the current moment. The S phase is any phase among the three phases. The X branch refers to the bus that satisfies any phase current less than I set1 , the remaining two phase currents are both greater than I set1 ,I r-NT -I r >I set1 and The value of K is as follows: K X <K1,K=K1 K X >K2,K=K2 K2<K X <K1,K=K X Among them, I set1 is the no-flow threshold, I set2 Less than CT disconnection setting value, I r-NT is the braking current of NT, I r is the braking current at the current moment, is the magnitude of the busbar protection differential current at the current moment, K X is the three-phase current unbalance coefficient of the X branch NT, K1 is the lower limit of the unbalance degree, K2 is the upper limit of the unbalance degree, I X is the current amplitude of any phase of X branch NT, I max is the maximum phase amplitude of NT, I min is the minimum phase amplitude of NT, and NT is the moment when the system is in stable operation.
2. The method for determining a busbar protection CT disconnection with asymmetric branch current according to claim 1, characterized in that: The X branch is all the running branches on the bus that also meet the branch road; In this inequality, It is the magnitude of the branch current change of phase A, phase B or phase C of a branch among all the running branches on the bus. For The magnitude of the busbar protection differential current at the same phase at the current moment.
3. The method for determining a busbar protection CT disconnection with asymmetric branch current according to claim 1, characterized in that: The system stable operation time refers to the third cycle or more than the third cycle before the current time.
4. The method for determining a busbar protection CT disconnection with asymmetric branch current according to claim 1, characterized in that: K1＝2。 5. The method for determining a busbar protection CT disconnection with asymmetric branch current according to claim 1 or 4, characterized in that: K2＝10。 6. The method for determining a busbar protection CT disconnection with asymmetric branch current according to claim 1, characterized in that: I set2 It is half of the CT wire break setting value.
7. The method for determining a busbar protection CT disconnection with asymmetric branch current according to claim 1, characterized in that: I set1 ≥0.05A。 8. A busbar protection CT disconnection judgment system with asymmetric branch current, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the bus protection CT disconnection determination method with asymmetric branch current as described in any one of claims 1 to 7.