Method for AC Induction Withstand Voltage Test of Single-Phase Autotransformer Line Ends

Abstract

The present invention relates to a method for performing an AC induction withstand voltage test on the line ends of a single-phase autotransformer. The method uses an intermediate transformer to replace the supporting transformer in a traditional test method. A medium-frequency generator, a compensating reactor, an intermediate transformer T1, and an intermediate transformer T2 are connected in sequence, and the intermediate transformer T2 is connected to the single-phase autotransformer being tested. The supporting transformer is omitted in the test system of the present invention, making the design of the test system simpler. The test is performed using an intermediate transformer T2, eliminating the phenomenon of opposite polarities and support vectors between the intermediate transformer and the supporting transformer, and eliminating the phase angle difference caused by different parameters such as capacity and impedance between the intermediate transformer and the supporting transformer. The excitation method is different, so that the input capacitance does not change much when the voltage is corrected, eliminating the risk of generator self-excitation. After the voltage correction is completed, the voltage divider is removed, and the capacitance rise factor will change. Compared with the traditional method, voltage correction is more accurate.

Description

Technical Field

[0001] The invention belongs to the technical field of transformer testing, and in particular relates to a new method for line-end AC induction withstand voltage testing of a single-phase autotransformer. Background Art

[0002] The entire power grid system is divided into four stages: power generation, transmission, distribution, and consumption. Power transformers are used in the first three stages, making them crucial electrical equipment in the power system. The safe operation of transformers is crucial to the safe operation of the power grid. To prevent and detect insulation defects within electrical equipment, major primary equipment such as transformers undergo factory testing before leaving the factory. The line-end AC induction withstand voltage test is crucial for transformer insulation testing and is the most effective and direct method for determining the insulation strength of power equipment.

[0003] The traditional test method for the line-end AC induced withstand voltage test of a single-phase autotransformer requires an intermediate transformer and a supporting transformer. Although the intermediate transformer and the supporting transformer are powered by the same power supply, the angle difference, capacitance rise, etc. generated are different. Therefore, the head-end voltage correction must be performed when conducting the line-end induced voltage test. The capacity, impedance and other parameters of the intermediate transformer and the supporting transformer are different, and there is a high possibility of insufficient reactive power compensation, which increases the risk of self-excitation of the generator. After the voltage divider is removed during the test, the capacitance rise becomes smaller, resulting in a large voltage correction deviation, and the test voltage is prone to deviation. During the test, the polarity of the supporting transformer may be opposite to that of the intermediate transformer. If the medium voltage to neutral point voltage is too high during the test, there may be a risk of inter-turn insulation damage. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a method for AC induction withstand voltage test of a single-phase autotransformer line end. The technical solution adopted by the present invention is as follows:

[0005] A method for performing an AC induced withstand voltage test on the line ends of a single-phase autotransformer is disclosed. An intermediate transformer is used to replace the supporting transformer in the traditional test method. A medium-frequency generator, a compensating reactor, an intermediate transformer T1, and an intermediate transformer T2 are connected in sequence. The intermediate transformer T2 is then connected to the single-phase autotransformer under test.

[0006] Beneficial effects of the present invention:

[0007] 1. The support transformer is omitted in the test system, making the design of the test system simpler.

[0008] 2. The test is carried out through an intermediate transformer T2, which eliminates the phenomenon of opposite polarity and support vector between the intermediate transformer and the supporting transformer, and eliminates the phase angle difference caused by different parameters such as capacity and impedance between the intermediate transformer and the supporting transformer.

[0009] 3. The traditional method needs to monitor the voltage of multiple points at the same time to check the polarity problem. Because of the angle difference and the capacity rise, it is difficult to detect that the polarity of the supporting transformer is opposite to that of the intermediate transformer. The voltage correction of the present invention only needs to correct the medium voltage. Because the intermediate transformer T2 is a three-phase transformer, the voltage U after the connection is A-O and voltage U O-BC The angle is guaranteed to be 180 degrees, eliminating any angular discrepancies. Only the medium voltage is calibrated, measuring the capacitance rise factor of intermediate transformer T2, eliminating the need for two voltage dividers. Fewer voltage dividers and a different excitation method minimize changes in input capacitance during voltage calibration, eliminating the risk of generator self-excitation. The voltage divider is used to measure voltage, while the calibration voltage is used to measure voltage.

[0010] 4. After the voltage calibration is completed, the voltage divider is removed and the capacitance rise coefficient will change. Therefore, the voltage calibration is more accurate than the traditional method. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings described below are some specific embodiments of the present invention. For those of ordinary skill in the art, without inventive effort, other drawings within the scope of protection of this application can be derived from these drawings.

[0012] Figure 1 This is the electrical principle diagram of the original single-phase autotransformer line-end AC induction withstand voltage test;

[0013] Figure 2 Schematic diagram of the electrical principle of a new single-phase autotransformer line-end AC induction withstand voltage test according to an embodiment of the present invention. DETAILED DESCRIPTION

[0014] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, but not all of the embodiments.

[0015] like Figure 1 Figure 2 shows the electrical schematic diagram of a conventional single-phase autotransformer line-end AC induction withstand voltage test. The test system includes the single-phase autotransformer under test, a supporting transformer, an 18,000 kVA intermediate transformer, and a compensating reactor. A generator provides power to the entire test system.

[0016] When conducting line-end AC induced voltage withstand tests on single-phase autotransformers, a supporting transformer is generally used to provide voltage support at the neutral point of the corresponding winding in order to achieve the test objectives, ensure that the transformer's head-end voltage reaches the test voltage, and ensure that the inter-turn voltage is within the transformer's inter-turn insulation tolerance. The end-end support voltage is calculated based on the test product's parameters, and the supporting transformer's turns ratio is selected. An intermediate transformer is selected to provide low-voltage excitation for the test product. Since both the supporting transformer and the intermediate transformer are powered by the same generator, the turns ratio of the intermediate transformer to the supporting transformer must be selected based on the test product's parameters to ensure that the test voltage is achieved. Two independent transformers are used to apply pressure to the test product, and appropriate turns ratios must be selected for the test product and the supporting transformer to achieve the test results. However, the intermediate transformer and the supporting transformer have different parameters such as capacity and impedance. Although powered by the same generator, the resulting angle difference and capacitance rise are different. Therefore, head-end voltage correction is required during line-end induced voltage testing. When performing voltage correction, the medium voltage, low voltage excitation voltage, and neutral point support voltage of the test object need to be monitored simultaneously. Since the low voltage excitation input capacitance is large, when using a voltage divider to measure the voltages of each part, the input capacitance will increase significantly. When measuring all three voltages simultaneously, the medium voltage and neutral point will produce a larger capacitance rise after the voltage divider is connected. This makes reactive power compensation calculation more difficult, there is a high possibility of insufficient reactive power compensation, and increases the risk of generator self-excitation. After removing the voltage divider during the test, the capacitance rise decreases, resulting in a large voltage correction deviation and a deviation in the test voltage. During the test, the polarity of the supporting transformer may be opposite to that of the intermediate transformer. If the polarity is opposite and there is capacitance rise, it may be difficult to distinguish during voltage correction. The result is that the medium voltage voltage to ground of the test object is low, and when the medium voltage to neutral point voltage is high during the test, there may be a risk of inter-turn insulation damage.

[0017] like Figure 2 Figure 2 shows the electrical principle diagram of a novel single-phase autotransformer line-end AC induction withstand voltage test according to an embodiment of the present invention. A method for performing a single-phase autotransformer line-end AC induction withstand voltage test employs a three-phase intermediate transformer T2 to directly excite the medium voltage of the test product in a non-test phase support mode.

[0018] The specific steps include:

[0019] Step 1: Connect the medium-frequency generator, compensating reactor, intermediate transformer T1, and intermediate transformer T2 in sequence. Intermediate transformer T2 is connected to the single-phase autotransformer under test, omitting the support transformer used in traditional testing. A, X, Am, a, and x represent the test product's terminals. The medium-frequency generator outputs two wires, while the compensating reactor has two terminals. Simply connect the two terminals of the compensating reactor to the two output wires of the generator. The wiring diagram is detailed and self-explanatory. Connect the compensating reactor in parallel with the output of the medium-frequency generator. The medium-frequency generator serves as the test power source, supplying power to intermediate transformer T1.

[0020] During the final test in step 2, the voltage between the low-voltage side and AC side of intermediate transformer T2 can be monitored in real time. Therefore, before the test, a voltage divider must be connected to the test object's Am terminal for voltage calibration to measure the capacitance rise factor of intermediate transformer T2. Calibration is performed by simultaneously reading the Am and AC voltages. This is achieved using a single intermediate transformer, T2, to eliminate the phenomenon of opposite polarity and support vectors between the intermediate transformer and the supporting transformer. It also eliminates phase angle differences caused by differences in capacitance, impedance, and other parameters between the intermediate transformer and the supporting transformer.

[0021] The voltage calibration only requires the medium voltage. Traditionally, the polarity of the support transformer must be consistent with that of the test product. If the generator is connected to the support transformer with opposite polarity to the test product, the support voltage will be reversed. To ensure correct polarity, the voltage at points X and Am on the test product must be monitored simultaneously. Calculations based on these two voltages verify correct polarity.

[0022] Step 3: Calculate the test voltage reached by the test sample based on the voltage correction result, which is the low-voltage ac voltage value of the intermediate transformer T2. Monitor the low-voltage ac voltage of the intermediate transformer T2 in real time according to the calculated value and apply pressure to meet the test requirements. After the voltage correction is completed, remove the voltage divider, and the capacitance rise coefficient will change, so the voltage correction is more accurate than the traditional method. The traditional method connects at least two voltage dividers. Since the voltage divider has a greater impact on the inlet capacitance of the test sample, the more voltage dividers are connected, the greater the impact. When the inlet capacitance is large, the capacitance rise coefficient of the correction voltage will increase. When the voltage divider is removed, the inlet capacitance decreases and the capacitance rise coefficient decreases accordingly. Therefore, the correction voltage deviation is large. The improved method of the embodiment of the present invention has fewer connected voltage dividers, and the inlet capacitance changes less after the voltage divider is removed, and the capacitance rise coefficient changes relatively less, so it is more accurate.

[0023] Comparison table between this application and existing experimental methods

[0024]

[0025] The embodiments of the present invention improve the line-end AC induction withstand voltage test method for a 500kV autotransformer, obtain and analyze test data, and demonstrate the advantages of omitting the support transformer in the line-end AC induction withstand voltage test. The method of the present invention for line-end AC induction withstand voltage testing of single-phase autotransformers can be applied to autotransformers of 400,000kVA / 500kV, 340,000kVA / 500kV, 500,000kVA / 750kV, and 700,000kVA / 750kV, achieving excellent test results. This method can meet the line-end induction voltage test requirements of various types of autotransformers.

[0026] Finally, it should be noted that the above embodiments are only specific implementation methods of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them, and the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any person skilled in the art can modify or easily think of changes to the technical solutions described in the above embodiments within the technical scope disclosed by the present invention, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. A method for AC induction withstand voltage test of a single-phase autotransformer line end, characterized in that: An intermediate transformer is used to replace the supporting transformer in the traditional test method. The medium-frequency generator, compensation reactor, intermediate transformer T1 and intermediate transformer T2 are connected in sequence. The intermediate transformer T2 is connected to the single-phase autotransformer under test. The specific steps include: Step 1: Connect the medium frequency generator, compensation reactor, intermediate transformer T1 and intermediate transformer T2 in sequence. Connect the intermediate transformer T2 to the single-phase autotransformer under test. Connect the compensation reactor in parallel to the outlet of the medium frequency generator. The medium frequency generator serves as a test power supply to supply power to the intermediate transformer T1. Step 2: During the final test, the voltage between the low-voltage side and AC of the intermediate transformer T2 is monitored in real time. Before the test, a voltage divider is connected to the Am end of the test product to perform voltage calibration to measure the capacitance rise factor of the intermediate transformer T2. The calibration method is to read the Am and AC voltages simultaneously. Step 3: Calculate the test voltage reached by the test product based on the voltage correction result, which is the low-voltage AC voltage value of the intermediate transformer T2. Monitor the low-voltage AC voltage of the intermediate transformer T2 in real time based on the calculated value and increase the voltage to meet the test requirements. In step 1, A, X, Am, a, and x are the terminals of the test product. The medium-frequency generator outputs two wires, and the compensation reactor has two terminals. Connect the two terminals of the compensation reactor to the two output wires of the generator respectively. In step 2, to ensure the correct polarity, it is necessary to monitor the voltages at points X and Am of the test product simultaneously, and perform calculations based on the voltages at the two points to verify whether the polarity is correct.

2. The method for AC induction withstand voltage test of a single-phase autotransformer line end according to claim 1, characterized in that: Applicable to: 400000kVA / 500kV, 340000kVA / 500kV, 500000kVA / 750kV and 700000kVA / 750kV autotransformer products.

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