Transformer diagnosis test result treatment method based on residual magnetism influence

Abstract

The invention discloses a transformer diagnosis test result treatment method based on residual magnetism influence, and the method comprises the following steps: 1, RF polarity measurement; and 2, applying a direct-current voltage with proper polarity to gradually demagnetize the transformer iron core, and continuously eliminating residual magnetism by applying the proper direct-current voltage, so that the iron core RF can effectively meet the transformer diagnosis test requirement, thereby improving the effectiveness of the transformer test result.

Description

Technical Field

[0001] The invention belongs to the field of transformer testing of power transmission and distribution network equipment, and more specifically to the technical field of a method for managing transformer diagnostic test results. Background Art

[0002] The transformer is one of the most important and expensive equipment in a high-voltage substation. The remanent flux (RF) of the transformer core will cause a large surge current after the transformer is energized, which may even exceed the short-circuit current, causing increased mechanical stress on the winding and malfunction of the protection relay. It will also cause temporary overvoltage in the weak current system, which will have an adverse effect on the results of the transformer diagnostic test and lead to misjudgment of the actual status of the transformer.

[0003] Transformer circuit breaker (CB) instant closing control is a practical method to reduce the excitation inrush current when the transformer is connected to the system at the moment of voltage peak. Since severe RF may cause high inrush current of the transformer connected at the moment of voltage peak, the CB instant closing control method should be strengthened. For this purpose, the CB should be closed with opposite polarity when the core RF is equal to the flux generated from the moment of CB closing to the peak of applied voltage. This method requires accurate RF estimation, which is difficult to measure and will cause significant errors when its value, especially polarity, is changed after transformer diagnostic tests.

[0004] To achieve the goal of eliminating RF, an AC voltage of rated frequency and amplitude is applied to the transformer, and the voltage amplitude is gradually reduced. For each decreasing voltage step, the core induced flux and its associated RF will decrease. Although this method is effective in eliminating RF, it requires a controllable voltage source, which cannot be easily provided in the field. Therefore, an alternative method is derived to use a square wave voltage source with adjustable amplitude / frequency. In this case, a DC voltage is applied to the transformer windings to make the induced flux exceed the corresponding inflection point value, passing through the windings at a relatively constant current. Then, the DC voltage polarity is reversed and connected to the same winding, but before reaching the previous current, the transformer should be disconnected from the voltage source. This process involves interrupting the DC voltage source of opposite polarity at a current less than the previous step, and should be repeated to reach a current less than a predefined value. This method is usually implemented by applying a voltage of constant amplitude and gradually increasing frequency, or constant frequency and gradually decreasing amplitude. In order to use this method, a relatively large power supply is required, and it can be quite time-consuming to completely reverse the flux at each step.

[0005] Reference [Li Jingli, He Pengwei, Qiu Zaisen, et al. A review of the research on residual magnetism measurement methods for power transformer cores [J]. High Voltage Electrical Appliances, 2018, 54(7): 98-105.] The advantages and disadvantages of several existing residual magnetism measurement methods for power transformers were summarized, and the future development direction of residual magnetism measurement methods was proposed. Reference [Li Yong, Jin Mingliang, Li Haitao, et al. Research on residual magnetism measurement methods for power transformers [J]. Power System Protection and Control, 2019, 47(15): 102-107.] Based on the first peak value of the excitation surge current after the transformer is no-load closed, the residual magnetism at this time is derived by combining the DC excitation peak curve and the peak magnetization curve, and the accuracy of the method is verified by physical simulation experiments. References [Rong Yi, Yao Xiaofei, Sun Liqiong, et al. Effect of vacuum circuit breaker current cutoff on residual magnetism of transformer core [J]. Ningxia Electric Power, 2019 (3): 7-11.] For three-phase three-column and three-phase five-column core structures, the residual magnetism distribution of the three phases of the transformer under different opening angles and with or without CB current cutoff is analyzed. References [De Leon F, Farazmand A, Jazebi S, et al. Elimination of Residual Flux in Transformers by the Application of an Alternating Polarity DC Voltage Source [J]. IEEE Transactions on Power Delivery, 2015, 30 (4): 1727-1734.] By applying a DC voltage with a specific amplitude and positive polarity, the maximum magnetic flux +B is induced in the transformer core. max , use the same voltage with opposite polarity to energize the transformer and measure the flux reversal as –B max The transformer diagnostic test results can be negatively impacted by reapplying the positive polarity voltage source to the transformer for a time interval of 1 / 2 the time interval, which results in a rapid reduction in RF. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a method for managing transformer diagnostic test results based on residual magnetism, by applying a suitable DC voltage to continuously eliminate residual magnetism, so that the core RF can effectively meet the transformer diagnostic test requirements, thereby improving the effectiveness of the transformer test results.

[0007] The technical solutions adopted by the present invention to solve the above technical problems are as follows:

[0008] The method for managing transformer diagnostic test results based on residual magnetism is as follows:

[0009] Step 1, RF polarity determination;

[0010] Step 2: Apply a DC voltage of appropriate polarity to gradually demagnetize the transformer core.

[0011] Preferably, step 1, RF polarity determination should be implemented based on the following strategy:

[0012] (1) Apply AC voltage to the transformer and measure the initial no-load current I m,1 , according to experimental studies, if the applied voltage is reduced, the measured no-load current is more sensitive to RF. On the other hand, the voltage should be high enough to produce an easily measurable no-load current, so the voltage amplitude and frequency are determined in such a way that the magnetic flux induced in the transformer core is about 0.0025 of the rated magnetic flux. Since the generation of square wave voltage is simpler than that of sinusoidal voltage and its inverter does not require an output low-pass filter, a square wave is selected. In addition, to meet these conditions, the frequency f of the square wave voltage is sq The calculation formula is shown in formula (1):

[0013]

[0014] Where: V n is the rated voltage of the transformer winding; f n is the rated frequency of the transformer winding; V sq is the square wave voltage amplitude, set to 12V,

[0015] (2) Apply a positive DC voltage to the winding for T1 second and measure the no-load current I at the end of the kth time. m,k , where T1 is the time interval corresponding to the change of the induced magnetic flux to 0.05 times the rated magnetic flux. Since the rated magnetic flux is related to the rated voltage T1 and the rated frequency f n is related, and the magnetic flux generated by the DC voltage corresponds to V DC ×T1, so the calculation formula of duration T1 is as shown in formula (2):

[0016]

[0017] Where: V DC The voltage amplitude for demagnetization is set to 12V. For example, in a 230 / 63kV, 50Hz, YNd transformer, if demagnetization is performed from the high voltage side, T1 = 2.5s,

[0018] (3) The previous step should be repeated twice, i.e. k = 2 and 3, and the no-load current I m,k , if the RF polarity is positive, the following conditions need to be met, as shown in formula (3):

[0019] I m,k+1 >I m,k k=1,2 (3)

[0020] Where: I m,k+1 is the no-load current at the end of the k+1th time,

[0021] If the RF polarity is negative, the following conditions need to be met, as shown in equation (4):

[0022] I m,k+1 <I m,k k=1,2 (4)

[0023] Otherwise, step (2) of the first step should be repeated again until one of the conditions of formula (3) or formula (4) is satisfied.

[0024] Preferably, in step 2, a DC voltage of appropriate polarity is applied to gradually demagnetize the transformer core, and the following steps should be taken:

[0025] (1) A DC voltage V with appropriate polarity should be applied to the transformer winding. DC , duration T1, if it satisfies equation (3) or equation (4) respectively, the polarity is negative or positive,

[0026] (2) It is necessary to investigate whether RF is sufficiently reduced. To this end, the V AC Signal to measure no-load current I m,k ,

[0027] (3) When condition I is met m,k+1 m,k When the core is demagnetized, the steps (1) and (2) of the second step are repeated. At the same time, condition I m,k+1 m,k Failure to meet this indicates that the RF polarity has become opposite relative to the initial polarity. In this case, repeating step (1) of step 2 will result in an increase in RF. Therefore, the DC polarity should be changed again to reduce RF as described in step (4) of step 2.

[0028] (4) After completing the previous step, the maximum RF can be equal to V DC The flux generated during 2×T1, which is 10% of the rated flux, has a polarity opposite to the initial polarity.

[0029] Preferably, to further reduce RF, a new step should be started in which the voltage V DC Apply to the transformer with a time interval of T2 = T1 / 10. At the same time, steps (2) and (3) in step 2 should be repeated, but T2 and V of opposite polarity should be used. DC At the end of this step, when condition I m,k+1 m,k ​​​When not satisfied, the core RF will be limited to less than 1% of the inflection point flux, which is much less than the predefined threshold of 3% to ensure that the RF does not have any adverse effect on the accurate interpretation of the diagnostic test results. It should be noted that the entire process of the proposed method will be performed automatically, including the periodic application of DC and AC voltages.

[0030] Preferably, three-phase transformer considerations, the usual practice to eliminate RF on a three-phase transformer is to perform a demagnetization procedure on the phase with the highest no-load current, leaving the other phases open-circuited.

[0031] In most cases, experience has shown that this procedure is sufficient to demagnetize the entire core. When the core of one phase is demagnetized, the remanence of other phases will also be effectively reduced.

[0032] It is important to note that this article discusses a method for completely demagnetizing a single-phase transformer (e.g., a current transformer, a voltage transformer, or a three-phase transformer with three independent cores) or a branch of a three-phase transformer. Complete demagnetization of the entire core of a three-phase transformer will be considered as a future work that needs to be addressed.

[0033] The technical effects of the present invention are:

[0034] 1. When the residual magnetism of the transformer increases, the measured no-load current will increase, and the slope of the hysteresis loop obtained by applying an AC voltage with an amplitude much smaller than the rated value will also decrease;

[0035] 2. The core RF can be limited to 0.18% of the inflection point flux by this method. The numerical analysis and field test results show that compared with the widely used traditional method, the proposed method takes less time and energy to demagnetize the transformer, which can greatly reduce the size and weight of the portable RF elimination device.

[0036] 3. This method can effectively eliminate the negative impact of RF on transformer diagnostic test results and can be well used for various transformer types such as power transformers, distribution transformers and instrument transformers. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a flow chart of the core RF elimination method of the present invention;

[0038] Figure 2 In the present invention, (a) is the simulation result of transformer core induced magnetic flux;

[0039] (b) is the simulation result of the effective value of the transformer no-load current;

[0040] Figure 3 It is the core flux generated by using -VDC and AC voltage in the present invention;

[0041] Figure 4 is the effective value of the no-load current during the transition interval between steps (3) and (4) of the second step of the present invention;

[0042] Figure 5 (a) is the simulation result of the effective value of the no-load current at the end of step (4) of the second step in the present invention;

[0043] (b) is the core flux simulation result at the end of step (4);

[0044] Figure 6 It is the circuit diagram of the conventional degaussing method in the present invention;

[0045] Figure 7 It is the sawtooth flux of the iron core after the conventional demagnetization in the present invention;

[0046] Figure 8 It is the principle circuit diagram of the RF elimination device in the present invention. DETAILED DESCRIPTION

[0047] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments:

[0048] Based on the simulation of the studied 230 / 63kV three-phase transformer, the proposed RF elimination method is studied. From equation (3), it can be seen that when V DC =12V is applied to the winding terminals with a rated voltage of 230 / 63kV, T1=2.5s, T2=250ms are calculated. The YNd transformer studied is demagnetized by applying DC and AC voltages to only one phase, and the other phases are open. The proposed method is implemented in a simulation environment as follows Figure 1 The magnetic flux density of the transformer core studied is shown in Figure 2 As shown in (a), starting from 1.05T, as the unknown RF, the effective value of the transformer no-load current at different steps is as follows Figure 2 (b) as shown.

[0049] First, the RF polarity should be determined by applying a square wave AC voltage VAC and measuring the no-load current, which is I m,1 =2.69mA. Then add a DC voltage V to the same winding DC = +12V, duration is T1. Then, use V AC To measure the current I m,2 , its value is 2.91mA. Figure 2 (b), these steps are continued 2 times, and the result is I m,3 =3.17mA. From equation (4), we can see that the increase of no-load current measured in two consecutive steps indicates the existence of positive RF in the core. Therefore, in order to reduce RF, a negative polarity DC voltage –V DCBy applying such a voltage at each time interval T1, the core flux gradually decreases, as Figure 3 In addition, applying a square wave voltage during the time interval ΔT3 results in a sawtooth waveform flux, during which the no-load current is measured and the current amplitude is decreasing, as shown in Figure 2 As shown in (b), in addition, the time delay ΔT4 between the application of AC and DC voltage is equal to 8 ms.

[0050] like Figure 4 As shown, when the simulation time is about 86 seconds, condition I m,k+1 m,k is not satisfied, so step (4) should be started. It should be noted that Figure 4 The no-load current shown in , i.e. at the end of step (3), is much smaller than Figure 2 The no-load current is shown, i.e. at the beginning of step (1), which demonstrates the significant effect of the proposed demagnetization method.

[0051] As a final step, to fully reduce RF, use a +V DC The measured no-load current amplitude drops until 95 seconds, as Figure 5 As shown in (a), since the current measured during the time interval ΔT3 starts to increase at t=96.5 seconds, the algorithm is completed. Figure 5 (b) shows that the core RF at the end of step (4) is equal to 0.003T, which is about 0.18% of the inflection point flux density Bkp=1.7T.

[0052] Comparative Analysis

[0053] The conventional method reported in the literature [IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus Part 1: Oil Filled Power Transformers, Regulators, and Reactors, IEEE Std 62, 1995.] was implemented and its results were compared with the present method. In this method, a 12V DC voltage source with a 4Ω series resistor was connected to a DC / AC converter, such as Figure 6 As shown in Figure 1. As the first step, a voltage of +12V is applied to the transformer, saturating its core and bringing the no-load current close to about 3A. Then, the voltage polarity is changed, with an interval considered to be 0.65 times that of the previous step, and this is continued several times until RF is sufficiently reduced. This method produces a sawtooth flux in the core of the transformer under study, with a decreasing amplitude, as shown in Figure 1. Figure 7 At the same time, the final RF is limited to 0.0146T, which is 0.86% of the inflection point flux.​

[0054] By comparing this method with the traditional method, the following conclusions can be drawn:

[0055] (1) The demagnetization intervals based on the proposed method and the traditional method are approximately 97 seconds and 232 seconds, respectively. Therefore, the proposed method can effectively reduce the time required for transformer shutdown;

[0056] (2) The energy required for transformer demagnetization by the proposed method is 7.5 kJ, while the conventional method consumes 34 kJ. Therefore, the proposed method requires less energy resources, which is important for portable devices;

[0057] (3) Low computational burden, especially the rapid degaussing process reduces the rating of the electronic switches used and the cooling system is simpler. This reduces the size and weight of the portable RF elimination device;

[0058] (4) Both methods can effectively eliminate the residual magnetism of the core, that is, far below the required RF.

[0059] Field testing

[0060] The circuit diagram of the portable device for RF elimination implemented in this method is as follows Figure 8 As shown, the device consists of the following parts: a DC voltage source (a set of rechargeable 12V, 3Ah batteries), a full-bridge inverter (48V, 4A MOSFET switches) and TR1 and TR2 thyristors, where the full-bridge inverter is to generate a square wave AC voltage with the required frequency and a DC voltage with the required duration and polarity. In other words, the full-bridge inverter replaces three independent voltage sources to generate +V DC , -V DC and V AC , to reduce the weight and volume of RF elimination equipment. TR1 and TR2 thyristors are to prevent high voltages caused by the sudden interruption of transformer magnetizing current by MOSFET switching, according to V DC The polarity of the voltage, one of the thyristors should fire 1ms before turning off the MOSFET switch and remain on for 3ms.

[0061] In order to generate the maximum possible RF, a 12V DC voltage is applied between the U and V terminals of the transformer, which correspond to the middle branch of the core, resulting in a severe saturation of the core. In this case, all other windings on the primary and secondary sides are open-circuited. Then, the no-load current is measured from the same terminals using a 12V AC voltage and its value is equal to 110.91μA. If the core is completely demagnetized using a sinusoidal voltage source with a gradually decreasing amplitude, the measured no-load current is 22.05μA. To evaluate the proposed method, the transformer core is again severely saturated and the no-load current measured after demagnetization is equal to 22.20μA. Alternatively, if the transformer is demagnetized according to the method reported in the literature

[16] , the resulting no-load current will be 22.12μA. Therefore, the experimental study shows that the proposed method can demagnetize the transformer core fairly well by using a small portable device.

Claims

1. Transformer diagnostic test result management method based on residual magnetism influence, Features Here’s how: Step 1, RF polarity determination; Step 2: Apply a DC voltage of appropriate polarity to gradually demagnetize the transformer core.

2. The method for managing transformer diagnostic test results based on residual magnetism influence according to claim 1, Features: Step 1, RF polarity determination should be implemented based on the following strategy: (1) Apply AC voltage to the transformer and measure the initial no-load current I m,1 , the frequency of the square wave voltage is f sq The calculation formula is shown in formula (1): Where: V n is the rated voltage of the transformer winding; f n is the rated frequency of the transformer winding; V sq is the square wave voltage amplitude, set to 12V, (2) Apply a positive DC voltage T to the winding. 1 seconds, and measure the no-load current I at the end of the kth time m,k , where T 1 It is the time interval corresponding to the change of induced magnetic flux to 0.05 times of rated magnetic flux. Since rated magnetic flux is related to rated voltage T 1 and rated frequency f n is related, and the magnetic flux generated by the DC voltage corresponds to V DC ×T 1 , so the duration T 1 The calculation formula is shown in formula (2): Where: V DC The voltage amplitude for demagnetization is set to 12V. For example, in a 230 / 63kV, 50Hz, YNd transformer, if demagnetization is performed from the high voltage side, T 1 =2.5s, (3) The previous step should be repeated twice, i.e. k = 2 and 3, and the no-load current I m,k , if the RF polarity is positive, the following conditions need to be met, as shown in formula (3): I m,k+1 >I m,k k=1,2 (3) Where: I m,k+1 is the no-load current at the end of the k+1th time, If the RF polarity is negative, the following conditions need to be met, as shown in equation (4): I m,k+1 <I m,k k=1,2 (4) Otherwise, step (2) of the first step should be repeated again until one of the conditions of formula (3) or formula (4) is satisfied.

3. The method for managing transformer diagnostic test results based on residual magnetism influence according to claim 2, Features: Step 2: Apply a DC voltage of appropriate polarity to gradually demagnetize the transformer core. The following steps should be taken: (1) A DC voltage V with appropriate polarity should be applied to the transformer winding. DC , duration T 1 , if they satisfy equation (3) or equation (4) respectively, the polarity is negative or positive, (2) It is necessary to investigate whether RF is sufficiently reduced. To this end, the V AC Signal to measure no-load current I m,k , (3) When condition I is met m,k+1 m,k When the core is demagnetized, the steps (1) and (2) of the second step are repeated. At the same time, condition I m,k+1 m,k Failure to meet this indicates that the RF polarity has become opposite relative to the initial polarity. In this case, repeating step (1) of step 2 will result in an increase in RF. Therefore, the DC polarity should be changed again to reduce RF as described in step (4) of step 2.​​ (4) After completing the previous step, the maximum RF can be equal to V DC In 2×T 1 The flux generated during this period, which is 10% of the rated flux, has a polarity opposite to the initial polarity.

4. The method for managing transformer diagnostic test results based on residual magnetism influence according to claim 3, Features: To further reduce RF, a new step should be started in which the voltage V DC Applied to the transformer for a time interval of T 2 =T 1 / 10, and at the same time, steps (2) and (3) in step 2 should be repeated, but T 2 and V of opposite polarity DC At the end of this step, when condition I m,k+1 m,k When not met, the core RF will be limited to less than 1% of the inflection point flux, which is much less than the predefined threshold of 3% to ensure that the RF does not have any adverse effect on the accurate interpretation of the diagnostic test results.​ 5. The method for managing transformer diagnostic test results based on residual magnetism influence according to claim 3, Features: The practice of eliminating RF on a three-phase transformer is to perform a demagnetization procedure on the phase with the highest no-load current, leaving the other phases open circuit.

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