Differential protection method suitable for autotransformer

Abstract

The invention belongs to the related technical field of power system relay protection, and discloses a differential protection method suitable for an autotransformer. According to the method, the difference value of the results is calculated by using the leakage reactance and voltage drop modes of the low-voltage side winding of the autotransformer, so that a protection criterion is formed, and the method can reliably act when an internal fault occurs in the autotransformer and reliably does not act when an external fault occurs or magnetizing inrush current occurs. Compared with traditional current differential protection, the self-coupling transformer excitation surge current protection method can effectively deal with the working condition that excitation surge current occurs in the self-coupling transformer, so that the situation of incorrect protection action is avoided, and extra braking criteria are not needed.

Description

Technical Field

[0001] This invention belongs to the technical field of power system relay protection, and more specifically, relates to a differential protection method suitable for autotransformers. Background Technology

[0002] Traditional differential protection for autotransformers uses current differential protection. When a fault occurs inside or outside the transformer's protection zone, a significant unbalanced current is generated in the differential circuit, triggering the protection to operate. However, when the transformer core saturates and inrush current occurs, a significant unbalanced current is also generated in the differential circuit, potentially causing maloperation of the protection. How to effectively distinguish between internal faults in autotransformers and inrush currents is a key challenge that needs to be addressed.

[0003] In existing technologies, additional inrush current identification criteria are typically required to distinguish between internal faults and magnetizing inrush current. A common inrush current identification method is second harmonic restraint, which determines magnetizing inrush current when the ratio of the second harmonic to the fundamental frequency in the differential current is greater than the set value, and then locks out the protection. However, in some scenarios, the magnetizing inrush current criterion may act incorrectly. For example, when an internal fault occurs accompanied by saturation, the fault current may also contain a large second harmonic, causing the second harmonic restraint criterion to erroneously lock out the protection and delay its operation. Summary of the Invention

[0004] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a differential protection method suitable for autotransformers, which aims to solve the technical problem that traditional differential protection of autotransformers is prone to maloperation under inrush current conditions.

[0005] To achieve the above objectives, the present invention provides a differential protection method suitable for autotransformers, comprising the following steps: Step 1: Using voltage and current transformers installed on the high-voltage and low-voltage sides of the autotransformer, the three-phase voltage signal u at the high-voltage port of the autotransformer is acquired in real time. ha (t), u hb (t), u hc (t), three-phase current signal i ha (t), i hb (t), i hc (t), and the three-phase voltage signal u at the low-voltage port. la (t), u lb (t), u lc (t), three-phase current signal i la (t), i lb (t), i lc (t) yields a continuously updated sequence of voltage and current sample values.

[0006] Step 2: Denote the winding between the high-voltage and low-voltage ports of the autotransformer as winding 1, with N1 turns, and calculate the voltage u across winding 1. 1a (t), u 1b (t), u 1c (t), the algorithm is as follows:

[0007] Step 3: Denote the winding between the low-voltage port and the neutral point of the autotransformer as winding 2, with N2 turns, and calculate the current i flowing through winding 2. 2a (t), i 2b (t), i 2c (t), the algorithm is as follows:

[0008] Step 4: Calculate the induced electromotive force e of winding 1 based on the voltage across winding 1 obtained in step (2). 1a (t), e 1b (t), e 1c (t), the algorithm is as follows:

[0009] Among them, R σ1 For the leakage resistance of winding 1, L σ1 This is the leakage inductance of winding 1.

[0010] Step 5: Based on the induced electromotive force of winding 1 obtained in step (4), convert it to winding 2 and calculate the leakage reactance voltage drop of winding 2. The leakage reactance voltage drop calculated by this method is denoted as leakage reactance voltage drop 1, and the symbol is u. σ1a (t), u σ1b (t), u σ1c (t), the algorithm is as follows:

[0011] Step 6: Based on the current flowing through winding 2 obtained in step (3), calculate the leakage reactance voltage drop of winding 2. The leakage reactance voltage drop calculated by this method is denoted as leakage reactance voltage drop 2, and the symbol is u. σ2a (t), u σ2b (t), u σ2c (t), the algorithm is as follows:

[0012] Among them, R σ2 For the leakage resistance of winding 2, L σ2 This is the leakage inductance of winding 2.

[0013] Step 7: Based on the leakage withstand voltage drop 1 and leakage withstand voltage drop 2 obtained in steps (5) and (6), calculate the difference Δu between leakage withstand voltage drop 1 and leakage withstand voltage drop 2 in real time. σa (t), Δu σb (t), Δu σc (t), the algorithm is as follows:

[0014] Step 8: Compare the difference between the leakage withstand voltage drop calculated in step (7) and the set value to form the action criterion. The algorithm is as follows:

[0015] Where u set The setting value is not less than the maximum steady-state error of the calculated voltage difference when a three-phase metallic short circuit or inrush current occurs outside the protection zone, while ensuring sufficient sensitivity during faults within the protection zone. If one of the three phases satisfies the condition that the absolute value of the calculated voltage difference is greater than the setting value at S points out of R consecutive sampling points, it can be judged as a fault within the transformer zone. The protection criterion is always applied, equivalent to the judgment result of the current sampling point and the judgment results of the previous R-1 sampling points. If S points meet the action criteria, it can be judged as a protection action at the current moment. The values ​​of R and S are related to the sampling rate. The value of S should not be too large to avoid affecting the protection action speed. The value of RS should also be ensured to be not less than the number of sampling points where the absolute value of the calculated voltage difference near the zero crossing point is less than the setting value during faults within the protection zone.

[0016] The present invention also provides an electronic device, comprising: a computer-readable storage medium and a processor; The computer-readable storage medium is used to store executable instructions; The processor is used to read executable instructions stored in the computer-readable storage medium and execute the above-described method.

[0017] The present invention also provides a computer-readable storage medium storing computer instructions for causing a processor to perform the above-described method.

[0018] The present invention also provides a computer program product, including a computer program or instructions that, when executed by a processor, implement the above-described method.

[0019] In summary, compared with existing technologies, the technical solutions conceived in this invention utilize the difference between the leakage reactance voltage drop of the low-voltage winding of the autotransformer calculated by two methods to form a protection criterion. This allows for reliable operation when a fault occurs within the autotransformer's fault zone, and reliable non-operation when a fault occurs outside the fault zone or when inrush current occurs. Compared with traditional current differential protection, this invention can effectively handle the condition of inrush current in the autotransformer, thereby avoiding incorrect protection operation and eliminating the need for additional braking criteria. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a typical test scenario system structure according to an embodiment of the present invention, including airdrop operation (a) and in-area fault (b).

[0021] Figure 2 This refers to the calculated difference (b) between the three-phase voltage and current on the high-voltage side and the three-phase voltage and current on the low-voltage side of the autotransformer (a) and the leakage reactance voltage drop during the operation of the autotransformer under no-load conditions in one embodiment of the present invention.

[0022] Figure 3 In one embodiment of the present invention, when a fault occurs in the autotransformer, the calculated difference between the three-phase voltage and current on the high-voltage side and the three-phase voltage and current on the low-voltage side of the autotransformer (a), and the leakage reactance voltage drop (b) is obtained. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0024] This invention discloses a differential protection method suitable for autotransformers, comprising the following steps: Step 1: Using voltage and current transformers installed on the high-voltage and low-voltage sides of the autotransformer, the three-phase voltage signal u at the high-voltage port of the autotransformer is acquired in real time. ha (t), u hb (t), u hc (t), three-phase current signal i ha (t), i hb (t), i hc (t), and the three-phase voltage signal u at the low-voltage port. la (t), u lb (t), u lc (t), three-phase current signal i la (t), i lb (t), ilc (t) yields a continuously updated sequence of voltage and current sample values.

[0025] Step 2: Denote the winding between the high-voltage and low-voltage ports of the autotransformer as winding 1, with N1 turns, and calculate the voltage u across winding 1. 1a (t), u 1b (t), u 1c (t), the algorithm is as follows:

[0026] Step 3: Denote the winding between the low-voltage port and the neutral point of the autotransformer as winding 2, with N2 turns, and calculate the current i flowing through winding 2. 2a (t), i 2b (t), i 2c (t), the algorithm is as follows:

[0027] Step 4: Calculate the induced electromotive force e of winding 1 based on the voltage across winding 1 obtained in step (2). 1a (t), e 1b (t), e 1c (t), the algorithm is as follows:

[0028] Among them, R σ1 For the leakage resistance of winding 1, L σ1 This is the leakage inductance of winding 1.

[0029] Step 5: Based on the induced electromotive force of winding 1 obtained in step (4), convert it to winding 2 and calculate the leakage reactance voltage drop of winding 2. The leakage reactance voltage drop calculated by this method is denoted as leakage reactance voltage drop 1, and the symbol is u. σ1a (t), u σ1b (t), u σ1c (t), the algorithm is as follows:

[0030] Step 6: Based on the current flowing through winding 2 obtained in step (3), calculate the leakage reactance voltage drop of winding 2. The leakage reactance voltage drop calculated by this method is denoted as leakage reactance voltage drop 2, and the symbol is u. σ2a (t), u σ2b (t), u σ2c (t), the algorithm is as follows:

[0031] Among them, R σ2 For the leakage resistance of winding 2, L σ2 This is the leakage inductance of winding 2.

[0032] Step 7: Based on the leakage withstand voltage drop 1 and leakage withstand voltage drop 2 obtained in steps (5) and (6), calculate the difference Δu between leakage withstand voltage drop 1 and leakage withstand voltage drop 2 in real time. σa (t), Δu σb (t), Δu σc (t), the algorithm is as follows:

[0033] Step 8: Compare the difference between the leakage withstand voltage drop calculated in step (7) and the set value to form the action criterion. The algorithm is as follows:

[0034] Where u set The setting value is not less than the maximum steady-state error of the calculated voltage difference when a three-phase metallic short circuit or inrush current occurs outside the protection zone, while ensuring sufficient sensitivity during faults within the protection zone. If one of the three phases satisfies the condition that the absolute value of the calculated voltage difference is greater than the setting value at S points out of R consecutive sampling points, it can be judged as a fault within the transformer zone. The protection criterion is always applied, equivalent to the judgment result of the current sampling point and the judgment results of the previous R-1 sampling points. If S points meet the action criteria, it can be judged as a protection action at the current moment. The values ​​of R and S are related to the sampling rate. The value of S should not be too large to avoid affecting the protection action speed. The value of RS should also be ensured to be not less than the number of sampling points where the absolute value of the calculated voltage difference near the zero crossing point is less than the setting value during faults within the protection zone.

[0035] Example 1 Figure 1 This is a schematic diagram of a typical test scenario system structure, including airdrop operation (a) and in-area fault (b). Figure 1 The test was conducted using the typical test scenario shown, with the setting value set to 11kV, R to 40, and S to 12.

[0036] In this embodiment, the transformer is set to operate under no-load conditions, and the three-phase voltage and current on the high and low voltage sides are as follows: Figure 2 As shown in (a), the inrush current generated by the unloaded transformer is calculated as follows: Figure 2 As shown in (b), the absolute value of the difference is less than the set value, and the protection reliably does not operate. In this embodiment, a phase-to-phase fault is set in the low-voltage port area of ​​the transformer, and the three-phase voltages and currents on the high- and low-voltage sides are as follows: Figure 3 As shown in (a) above, the difference in leakage withstand voltage drop calculation is as follows: Figure 3 As shown in (b), the peak value of the phase difference between A and B reaches 500kV, which exceeds the setting value, and the protection is activated.

[0037] Example 2 The present invention also relates to an electronic device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the method described above.

[0038] The electronic device can be a desktop computer, laptop, handheld computer, or cloud server, etc. The processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The memory can be used to store computer programs and / or modules. The processor performs various functions of the electronic device by running or executing the computer programs and / or modules stored in the memory, and by accessing data stored in the memory.

[0039] Example 3 The present invention also relates to a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described above.

[0040] Specifically, the memory may include high-speed random access memory, as well as non-volatile memory, such as hard disks, RAM, plug-in hard disks, smart media cards (SMC), secure digital (SD) cards, flash cards, at least one disk storage device, flash memory device, or other volatile solid-state storage devices.

[0041] Example 4 This invention provides a computer program product or computer program that includes computer instructions stored in a computer-readable storage medium. A processor of a computer device reads the computer instructions from the computer-readable storage medium and executes the computer instructions, causing the computer device to perform the steps of the method described in the above embodiments of this invention.

[0042] The technical features of the embodiments described above can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. It should be noted that the terms "in one embodiment," "for example," and "again" in this invention are intended to illustrate the invention and are not intended to limit the invention.

[0043] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A differential protection method suitable for a autotransformer, characterized in that, The method comprises the following steps: Real-time acquisition of three-phase voltage signals u from a high-voltage port of an autotransformer ha (t), u hb (t), u hc (t), three-phase current signals i ha (t), i hb (t), i hc (t), and three-phase voltage signals u from a low-voltage port la (t), u lb (t), u lc (t), three-phase current signals i la (t), i lb (t), i lc (t) The winding between the high voltage port and the low voltage port of the autotransformer is denoted as a first winding with a number of turns N1, the voltage u across the first winding is calculated as the difference between the three-phase voltage signals at the high voltage port and the low voltage port 1a (t), u 1b (t), u 1c (t); The winding between the low-voltage port of the autotransformer and the neutral point is denoted as a second winding, with the number of turns being N2, and the current i flowing through the second winding is calculated as the sum of the three-phase current signals of the high-voltage port and the low-voltage port 2a (t), i 2b (t), i 2c (t); According to the first winding voltage u 1a (t), u 1b (t), u 1c (t) to calculate the induced electromotive force e 1a (t), e 1b (t), e 1c (t); The induced electromotive force e 1a (t), e 1b (t), e 1c (t) is converted to the second winding, the leakage reactance voltage drop of the second winding is calculated, denoted as the first leakage reactance voltage drop u σ1a (t), u σ1b (t), u σ1c (t); According to the current i of the second winding 2a (t), i 2b (t), i 2c (t) calculates the leakage reactance voltage drop of the second winding, denoted as the second leakage reactance voltage drop u σ2a (t), u σ2b (t), u σ2c (t); According to the first leakage reactance voltage drop u σ1a (t), u σ1b (t), u σ1c (t) and the second leakage reactance voltage drop u σ2a (t), u σ2b (t), u σ2c (t) and the difference Δu σa (t), Δu σb (t), Δu σc (t) is compared with a setting value to form an action criterion, and if one of the three phases satisfies that, among continuous R sampling points, S points satisfy that the absolute value of the calculated voltage difference is greater than the setting value, the transformer internal fault can be determined, wherein R and S are integers.

2. The differential protection method of claim 1, wherein, said calculating the induced electromotive force e of the first winding 1a (t), e 1b (t), e 1c (t) is where R σ1 is the leakage resistance of the first winding, L σ1 is the leakage inductance of the first winding.

3. The differential protection method of claim 2, wherein, said calculating a leakage reactance voltage drop of the second winding, denoted first leakage reactance voltage drop u σ1a (t), u σ1b (t), u σ1c (t) is: 。 4. The differential protection method of claim 3, wherein, said calculating a leakage reactance voltage drop of the second winding, denoted second leakage reactance voltage drop u σ2a (t), u σ2b (t), u σ2c (t) is where R σ2 is the leakage resistance of the second winding, L σ2 is the leakage inductance of the second winding.

5. The differential protection method of claim 1, wherein, The range of the setting value is not less than the maximum steady error of the calculated voltage difference when a three-phase metallic short circuit or a surge current occurs outside the area.

6. The differential protection method of claim 1, wherein, The values of R and S are related to the sampling rate, and the value of R-S ensures that the absolute value of the calculated voltage difference near the zero-crossing point is less than the sampling point number of the setting value when an internal fault occurs. 7.An electronic device comprising a memory and a processor, the memory storing a computer program, wherein, The processor implements the steps of the method of any one of claims 1 to 6 when executing the computer program.

8. A computer-readable storage medium having stored thereon a computer program, characterized in that, The processor implements the steps of the method of any one of claims 1 to 6 when executing the computer program.

9. A computer program product comprising computer programs or instructions, characterized in that, The processor implements the steps of the method of any one of claims 1 to 6 when executing the computer program or instructions.

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