A method for monitoring open-phase of a main transformer

Abstract

The present application belongs to the technical field of power system relay protection, and specifically provides a main transformer open-phase monitoring method, which obtains high-voltage side three-phase current, low-voltage side bushing three-phase current and high-voltage side three-phase voltage of the main transformer; and judges the open-phase fault condition according to the data information of the main transformer, and when a certain phase reaches the preset open-phase fault condition, the corresponding phase of the main transformer has an open-phase fault. The method of the present application can comprehensively analyze whether the main transformer has an open-phase fault through the obtained data, i.e. the data obtained include not only the high-voltage side three-phase current, but also the low-voltage side bushing three-phase current and the high-voltage side three-phase voltage, so that the problem of difficult discrimination of the open-phase fault of the high-voltage side under the low-load operation condition of the main transformer can be solved, and the problem of unsensitive detection of the open-phase fault by the main protection can be solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power system relay protection, and particularly relates to a main transformer open-phase monitoring method. BACKGROUND

[0002] In December 2021, the C-phase bushing of the high-voltage side of the No. 3 main transformer of the 750kV Shahu transformer substation in Ningxia fractured, causing the transformer to operate in an open-phase mode. Under low-load conditions, there was no protective alarm for a long time. The existing main transformer protection device cannot effectively distinguish and quickly remove the main transformer open-phase fault. The monitoring and operating personnel lack accurate alarm information prompts. In the event of an open-phase fault in the low-load operation state of the main transformer, there may be a long period of time without alarm uploading. If the open-phase fault inside the main transformer bushing cannot be quickly sensed and necessary processing measures are not taken, there is a risk of the bushing falling and causing a main transformer near-zone fault, which may even cause permanent damage such as internal explosion, insulation breakdown, and winding deformation of the main transformer.

[0003] In recent years, there has been some research on open-phase faults of transmission lines in China, and some achievements have been made. For transformer open-phase protection, especially open-phase detection in the light-load or no-load state of the transformer, the Xuji Group Co., Ltd. has proposed an injection-type transformer line open-phase protection method and a transformer open-phase protection scheme based on an optical current transformer (OCT), and the Nanjing Nanrui Relay Protection Electrical Co., Ltd. has proposed a transformer open-phase protection scheme based on an optical current transformer (OCT). The basic principle of the injection-type transformer line open-phase protection is to inject a non-power frequency signal (small signal) from the transformer grounding neutral point into the system, detect the current and voltage of the injected signal, and use a comprehensive characteristic criterion to determine whether the transformer is operating in an open-phase mode according to the variation characteristics of the current and voltage. The transformer open-phase detection method based on the optical current transformer (OCT) measures the three-phase current on the high-voltage side of the transformer using the optical current transformer (OCT), and determines whether an open-phase fault has occurred according to the current.

[0004] However, the injection transformer line open-phase protection needs to be directly grounded at the neutral point of the high-voltage side of the transformer (the grounding mode of other sides is not limited), and can be a single transformer or a plurality of parallel running transformer system. The main transformer of the 750kV transformer substation is generally a split-phase autotransformer, and the neutral point grounding mode and the system zero sequence impedance are different from those of the main transformer of the nuclear power plant. Therefore, the injection transformer line open-phase protection principle is not suitable for the open-phase monitoring of the main transformer. The transformer open-phase detection method based on the optical current transformer OCT needs to make the OCT primary sensor into an optical cable and be sleeved on the main transformer high-voltage sleeve. The cost of one OCT optical current transformer is about 300,000 yuan, and a total of 3 are needed, which is relatively high in cost. The existing technology gives an alarm signal or trips after a long delay. If the open-phase fault in the main transformer sleeve cannot be quickly sensed and necessary processing means are taken, there is a risk of the sleeve falling and causing a main transformer near-zone fault, which may even cause permanent damage such as internal explosion, insulation breakdown and winding deformation of the main transformer. SUMMARY

[0005] The purpose of the present application is to provide a main transformer open-phase monitoring method to solve the problem of the existing technology that the substation open-phase fault detection is not sensitive.

[0006] To solve the above technical problems, the present application provides a main transformer open-phase monitoring method, comprising the following steps:

[0007] 1) obtaining data information of the main transformer; the data information includes high-voltage side three-phase current, low-voltage side sleeve three-phase current and high-voltage side three-phase voltage;

[0008] 2) judging the open-phase fault condition according to the obtained data information; the open-phase fault condition includes:

[0009] a) the high-voltage side phase current of a certain phase is less than the set multiple of the minimum load current setting value, and the ratio of the high-voltage side zero sequence current component to the high-voltage side positive sequence current component is greater than the zero sequence current asymmetry degree setting value or the ratio of the high-voltage side negative sequence current component to the high-voltage side positive sequence current component is greater than the negative sequence current asymmetry degree setting value;

[0010] b) the low-voltage side zero sequence current component is greater than the low-voltage side sleeve unbalanced zero sequence current threshold setting value;

[0011] c) the high-voltage side negative sequence voltage component is greater than the negative sequence voltage setting value, or the high-voltage side zero sequence voltage component is greater than the zero sequence voltage setting value, or the high-voltage side line voltage is less than the line voltage setting value;

[0012] 3) if a) and b) are satisfied and c) is not satisfied, the main transformer corresponding phase has an open-phase fault.

[0013] The beneficial effects are that the three-phase current of the high-voltage side of the main transformer is monitored in real time, the three-phase voltage of the high-voltage side of the main transformer and the three-phase current of the bushing of the low-voltage side of the main transformer are detected, and when the current and voltage criteria meet the open-phase condition, it is determined that the open-phase fault occurs. The method of the application obtains not only the three-phase current of the high-voltage side, but also the three-phase current of the bushing of the low-voltage side and the three-phase voltage of the high-voltage side, that is, the obtained data is more, and then through the obtained data, whether the open-phase fault of the main transformer occurs can be comprehensively analyzed, and which phase of the main transformer occurs the open-phase fault can be determined based on the method of the application, that is, the problem that the open-phase fault of the high-voltage side under the low-load operation condition of the main transformer is difficult to determine can be solved, and the problem that the main protection is not sensitive to the open-phase fault detection can be solved.

[0014] Further, the formula of the open-phase fault condition a) is:

[0015]

[0016] wherein, I Φ is the current of a certain phase of the A, B and C three-phase of the high-voltage side of the main transformer, I p is the minimum load current setting value, I h0 is the zero sequence current component of the high-voltage side, I h1 is the positive sequence current component of the high-voltage side, I h2 is the negative sequence current component of the high-voltage side, K 0set is the zero sequence current asymmetry degree setting value, K 2set is the negative sequence current asymmetry degree setting value.

[0017] Further, the formula of the open-phase fault condition b) is:

[0018] (I l0 >I 0set );

[0019] wherein, I 10 is the zero sequence current component of the low-voltage side, I 0set is the low-voltage side bushing unbalanced zero sequence current threshold setting value.

[0020] Further, the formula of the open-phase fault condition c) is:

[0021]

[0022] wherein, U0 is the zero sequence voltage component of the high-voltage side, U2 is the negative sequence voltage component of the high-voltage side, U 线max is the maximum value of the line voltage of the high-voltage side, U 2set is the negative sequence voltage setting value, U 0set is the zero sequence voltage setting value, U set is the line voltage setting value.

[0023] Further, in step 2), the open-phase fault condition is judged by an open-phase monitoring device; the open-phase monitoring device comprises an open-phase detection module at the high-voltage side of the main transformer, an auxiliary module for detecting disconnection of the secondary of the bushing current CT at the low-voltage side of the main transformer, and a non-fault detection module for the voltage at the high-voltage side.

[0024] The process of judging based on different modules realizes fine processing of the obtained data, and the process of judging based on sub-modules shortens the processing time due to the reduced data processed by each module, thereby speeding up the judgment, and the error probability of each module is reduced due to the reduced data processed, thereby making the processing result more reliable.

[0025] Further, in order to solve the problem of reliability of the measured data under the condition of low-load operation of the main transformer, in step 1), the three-phase current at the high-voltage side is obtained by a measurement-level CT.

[0026] Further, in order to ensure the accuracy of the obtained data, the measurement-level CT has an accuracy level of 0.5 or 0.2S.

[0027] Further, in step 1), the three-phase current at the low-voltage side of the bushing is obtained by measuring the internal current of the delta winding at the low-voltage side.

[0028] Further, the minimum load current setting value is in the range of 2% to 5% of the secondary rated current, the low-voltage side bushing unbalanced zero sequence current threshold setting value is in the range of 10mA to 40mA, the negative sequence voltage setting value is in the range of 2V to 6V, the zero sequence voltage setting value is in the range of 4V to 8V, and the line voltage setting value is in the range of 60% to 70% of the secondary rated voltage.

[0029] Further, in order to avoid various fault transient disturbances of the main transformer and the adjacent system, in step 3), after the open-phase fault of the main transformer occurs, the open-phase protection action of the main transformer is performed after a set delay time. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a wiring diagram of a typical application of an electrical main wiring and a main transformer open-phase monitoring device of the present application;

[0031] Figure 2 is a logic diagram for judging the open-phase fault of phase A of the present application;

[0032] Figure 3 is a logic diagram for judging the open-phase fault of phase B of the present application;

[0033] Figure 4 is a logic diagram for judging the open-phase fault of phase C of the present application. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0035] Example of a main transformer phase loss monitoring method:

[0036] This embodiment detects the three-phase current on the high-voltage side of the main transformer, the three-phase voltage on the high-voltage side of the main transformer, and the three-phase current in the bushings on the low-voltage side of the main transformer. When the detected data meets the phase loss criterion, a phase loss fault is determined to have occurred, and an alarm signal or tripping is issued after a delay. Figure 1 The diagram shows a typical electrical main wiring diagram used in this embodiment (taking a 750kV substation main transformer system as an example to illustrate the specific implementation of this method). The main transformer is a three-phase, three-winding autotransformer with a Yn / Yn / D-11 connection, and the high-voltage side is 750kV. The 3 / 2 connection is used, with the low-voltage side using a delta winding connection. The phase failure monitoring device receives analog quantities including the three-phase current on the high-voltage side of the main transformer, the three-phase voltage on the high-voltage side of the main transformer, and the three-phase current in the bushing on the low-voltage side of the main transformer. The three-phase current on the high-voltage side of the main transformer is obtained through a measuring-grade CT (current transformer). The accuracy class of the measuring-grade CT is 0.5 (or 0.2S in other implementations), for example, with a transformation ratio of 1200 / 2A. The measuring CT only needs to ensure high accuracy under normal current to ensure accurate measurement. Its accuracy class (i.e., accuracy grade) is usually 0.1, 0.2, 0.5, 1.0, 3.0, and 5.0. The larger the accuracy class value, the lower the accuracy. The accuracy class with "S" is a special CT, which requires sufficiently high accuracy within the 1% to 120% load range. Generally, it is measured at 5 load points, and the error should be less than the specified range. The three-phase current in the bushing on the low-voltage side of the main transformer is obtained through a protection-grade CT, for example, with a transformation ratio of 2000 / 1A. The specific steps for implementing phase loss monitoring on the high-voltage side of the main transformer in a substation are as follows:

[0037] 1) The phase failure monitoring device acquires the three-phase current (I) on the high-voltage side of the main transformer in real time. A I B and I C ), zero-sequence current component on the high-voltage side (I) h0 ), positive sequence current component on the high-voltage side (I) h1 ), High-voltage side negative sequence current component (I) h2 ), High-voltage side three-phase voltage (U A U B And U C ), High-voltage side three-phase line voltage (U AB U BC And U CA ), High-voltage side zero-sequence voltage component (U0), high-voltage side positive-sequence voltage component (U1), high-voltage side negative-sequence voltage component (U2), main transformer low-voltage side bushing three-phase current (ILA , I LB and I LC ) and low-voltage side zero-sequence current component (I 10 ).

[0038] 2) Determine the minimum load current setting value I p , I p The general value range is 2% to 5% of the secondary rated current. In this embodiment, the primary rated current of the transformer is 1132.1 A, the high-voltage side measurement CT ratio is 1200 / 2 A, that is, the secondary rated current is 1.887 A, and then 2% of the rated current is 37.74 mA, that is, the minimum load current setting value I p can be set to 40 mA. Also determine the zero-sequence current asymmetry setting value K 0set , the negative-sequence current asymmetry setting value K 2set , the low-voltage side bushing unbalanced zero-sequence current threshold setting value I 0set , the negative-sequence voltage setting value U 2set , the zero-sequence voltage setting value U 0set and the line voltage setting value U set ; in this embodiment, K 0set is 0.2, K 2set is 0.2, I 0set is generally in the range of 10 mA to 40 mA (10 mA in this embodiment), U 2set is generally in the range of 2 V to 6 V (4 V in this embodiment), U 0set is generally in the range of 4 V to 8 V (5 V in this embodiment), and U set is generally in the range of 60% to 70% of the secondary rated voltage (70 V in this embodiment).

[0039] 3) The phase break monitoring device performs phase break fault discrimination.

[0040] The phase break monitoring device includes a main transformer high-voltage side phase break detection module, a main transformer low-voltage side bushing current CT secondary disconnection auxiliary module, and a high-voltage side voltage non-fault detection module, and each module discriminates as follows:

[0041] The main transformer high-voltage side phase break detection module discriminates the formula:

[0042]

[0043] The main transformer low-voltage side bushing current CT secondary disconnection auxiliary module discriminates the formula:

[0044] (I l0 > I 0set ) (2)

[0045] High-voltage side voltage non-fault detection module discriminant formula:

[0046]

[0047] When formula (1), (2) is established and formula (3) is not established, then the corresponding phase open phase condition is met, and it is determined that the corresponding phase is open phase. Wherein I Φ is a certain phase current in the high-voltage side A, B, C three phases of the main transformer, U 线max is the maximum value of the line voltage at the high-voltage side (for example, when I Φ is the A-phase current, I A , then the corresponding U 线max is the maximum value of the line voltage U AB , U BC and U CA ).

[0048] 4) After determining the corresponding phase open phase, the open phase alarm or tripping signal is given after a set delay (as other real-time methods, the open phase alarm and tripping action can also be performed after a set delay), the set delay is set to avoid various fault transient disturbances of the main transformer and adjacent system, and the set delay value is in the range of 0s-100s (the delay value in the embodiment is set to 0.5s). When the real-time calculation value of the current and voltage of a certain phase does not meet the open phase criterion, the next phase open phase discrimination is entered, and the A, B, C three phases are discriminated in a cycle. The specific A-phase open phase fault discrimination logic diagram is shown in the accompanying Figure 2 , the B-phase open phase fault discrimination logic diagram is shown in the accompanying Figure 3 , and the C-phase open phase fault discrimination logic diagram is shown in the accompanying Figure 4 .

[0049] By using the method of the embodiment, the three-phase currents at the high-voltage side of the main transformer are monitored in real time through the application of the main transformer high-voltage side measurement CT, the three-phase voltages at the high-voltage side of the main transformer and the three-phase currents of the main transformer low-voltage side bushing are detected, when the current and voltage criterion meets the open phase criterion, it is determined that the open phase fault occurs, and the alarm signal or tripping is given after a delay to remind the operator to process in time. The method of the embodiment can solve the problem that the open phase fault at the high-voltage side of the main transformer under low load operation condition is difficult to discriminate, solve the problem that the main transformer protection is not sensitive to the open phase fault detection and the tripping is not timely, upgrade the function of the existing main transformer protection, and is beneficial to the safe and stable operation of the power grid.

[0050] The above is only a preferred embodiment of the application and is not used to limit the application, the patent protection range of the application is subject to the claims, and any equivalent structural changes made by using the content of the specification and drawings of the application shall also be included in the protection range of the application.

Claims

1. A method of monitoring for loss of phase in a main transformer, the method comprising: The method comprises the following steps: 1) obtaining data information of the main transformer; the data information comprises three-phase current on the high-voltage side, three-phase current of the bushing on the low-voltage side, and three-phase voltage on the high-voltage side; 2) judging a phase failure condition according to the obtained data information; the phase failure condition comprises: a) the phase current on the high-voltage side is less than 0.6 times of the minimum load current setting value, and the ratio of the zero-sequence current component on the high-voltage side to the positive-sequence current component on the high-voltage side is greater than the zero-sequence current asymmetry degree setting value or the ratio of the negative-sequence current component on the high-voltage side to the positive-sequence current component on the high-voltage side is greater than the negative-sequence current asymmetry degree setting value; the minimum load current setting value is determined according to the main transformer parameters, the measurement CT transformation ratio for obtaining the three-phase current on the high-voltage side, and the accurate level of the measurement CT; b) the zero-sequence current component on the low-voltage side is greater than the low-voltage bushing unbalanced zero-sequence current threshold setting value; c) the negative-sequence voltage component on the high-voltage side is greater than the negative-sequence voltage setting value, or the zero-sequence voltage component on the high-voltage side is greater than the zero-sequence voltage setting value, or the line voltage on the high-voltage side is less than the line voltage setting value; 3) if a) and b) are true and c) is not true, it is judged that the phase failure of the main transformer occurs.

2. The main transformer open-phase monitoring method of claim 1, wherein, If a phase does not satisfy a) and b) of the phase failure condition and c) is not true, the next phase is judged, so as to judge among the three phases A, B and C.

3. The main transformer open-phase monitoring method of claim 1, wherein, The minimum load current setting value ranges from 2% to 5% of the secondary rated current.

4. The main transformer open-phase monitoring method of claim 1, wherein, The zero-sequence current asymmetry degree setting value is 0.2, and the negative-sequence current asymmetry degree setting value is 0.

2.

5. The main transformer open-phase monitoring method of claim 1, wherein, In step 2), the phase failure condition is judged by a phase failure monitoring device; the phase failure monitoring device comprises a high-voltage side phase failure detection module of the main transformer, a bushing current CT secondary disconnection auxiliary module on the low-voltage side of the main transformer, and a high-voltage side voltage non-fault detection module.

6. The main transformer open-phase monitoring method of claim 1, wherein, The low-voltage bushing unbalanced zero-sequence current threshold setting value ranges from 10 mA to 40 mA.

7. The main transformer open-phase monitoring method of claim 1, wherein, The accurate level of the measurement CT is 0.5 level or 0.2S level.

8. The main transformer open-phase monitoring method of claim 1, wherein, In step 1), the three-phase current of the bushing on the low-voltage side is obtained by measuring the internal current of the delta winding on the low-voltage side through the protection CT.

9. The main transformer open-phase monitoring method of claim 1, wherein, The negative-sequence voltage setting value ranges from 2 V to 6 V, the zero-sequence voltage setting value ranges from 4 V to 8 V, and the line voltage setting value ranges from 60% to 70% of the secondary rated voltage.

10. The main transformer open-phase monitoring method of claim 1, wherein, In step 3), after the main transformer fails, the phase failure protection of the main transformer is performed after a set delay time, so as to avoid various fault transient disturbances of the main transformer and the adjacent system.

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