67 results about "Lightning impulse voltage" patented technology

The invention provides a method for determining insulation and lightning impulse withstanding properties of a same-tower double-loop power transmission line. The method comprises the following steps: determining the same-tower double-loop power transmission line to be determined, acquiring statistical parameters of lightning activities in an area in which the power transmission line is located and structural parameters of each pole tower and determining a lightning strike calculation cycle number Nl; sampling random parameters during a lightning strike process and determining lightning current amplitude, transient power frequency voltage and determination result that the lightning strike is shielding failure or back striking; judging whether tripping occurs in the pole tower after the certain lightning strike or not and carrying out statistics on the tripping times during the Nl lightning strikes according to the lightning current amplitude, the transient power frequency voltage and the shielding failure or back striking determination result, as well as the structural parameters of the pole tower, and calculating the lightning strike tripping rate; and determining the insulation and the lightning impulse withstanding properties of the power transmission line according to the lightning strike tripping rate and a target value. The determination method provided by the invention is small in error, high in precision and specific; and by adopting the determination method, design, operation and improvement of lightning protection measures of the same-tower double-loop power transmission line of an electric power department can be scientifically guided.

The invention discloses a controllable metal oxide arrester action characteristic testing device and a method therefor. The method comprises: the sphere gap spacing in an impulse voltage generator isregulated, impulse voltage with the peak value higher than the action threshold value of a controllable metal oxide arrester sample is applied on the controllable metal oxide arrester sample for a plurality of times, and the voltage value of the controllable metal oxide arrester sample at the action moment can be obtained by recording the voltage value of the sample at the action moment. The action characteristic testing method of the controllable arrester has good system equivalence under the conditions of power frequency, operation and lightning impulse voltage, so as to comprehensively testthe action characteristics of the controllable arrester in an actual system under various voltages.

The invention discloses a voltage withstand test analogue simulation method for ultra-high-voltage GIS standard lightning impulse voltage. The voltage withstand test analogue simulation method comprises the steps that equivalent impedance of each element is calculated according to the structural size of transformer substation GIS equipment and an electric connecting wire, and wave impedance parameters and wave speed parameters of a breaker, a disconnecting switch and a bus in a simulation model are calculated according to the geometrical sizes of all the elements; according to the geometrical sizes of all the elements and the model parameters, the simulation model is established by the utilization of ATP-EMTP software; a wiring mode of the simulation model and standard lightning impulse voltage waveform parameters are determined; impact simulation is conducted on the same wiring mode by the adoption of standard lightning impulse voltage waves of different wave heads; actual safety voltage withstand test parameters of the GIS equipment standard lightning pulse voltages are determined according to simulation results. The analogue simulation method can overcome the defects that in the prior art, the reliability is low, the safety is poor and economic looses are large, and has the advantages of being high in reliability, good in safety and small in economic loss.

The invention provides an alternating current, direct current and impulse joint superposition test method for extra-high voltage transmission level equipment used in a high-altitude area. In the method, an alternating current power supply device (1), a direct current power supply device (2) and an impulse power supply device (3) are jointly superposed and applied to areas with the altitude of between 1,800 and 2,200m; and under an alternating current, direct current and impulse joint superposition voltage testing work condition, the alternating current power supply device outputs a power frequency voltage of between 0 and -900kV, the direct current power supply device outputs a direct current voltage of between 0 and + / -1,200kV, and the impulse power supply device outputs the highest 3.5MV standard lightning impulse voltage or 2.5MV standard operation impulse voltage, so that the requirement for jointly superposed voltage test of super high voltage power transmission level electrical equipment can be met. The method is an effective alternating current, direct current and impulse joint superposition test method for extra-high voltage transmission level equipment used in the high-altitude area, and has the unique advantages that the power supply can output high electrical parameter level, tested equipment can be subjected to superposition tests and joint tests, equivalent external insulation in high altitude areas of the test is checked strictly and the like.

The invention discloses a method for calculating instantaneous electric field distribution of a transformer under lightning impulse voltage. The method comprises the following steps: establishing a finite element analysis model of a power transformer; grouping the transformer windings, and performing simulation calculation on the electric field and the magnetic field of the transformer windings toobtain equivalent capacitance and equivalent inductance distribution parameters of each part of windings; establishing an equivalent circuit model of the complete winding of the transformer in circuit simulation analysis software, and solving the voltage waveform of each part of winding in the whole process that the transformer is subjected to lightning impulse voltage; and reloading the voltagewaveform into the finite element simulation model, and calculating the transient electric field to obtain the instantaneous electric field distribution condition of the transformer in the whole process of receiving the lightning impulse voltage. Complex electromagnetic transient coupling is replaced by field circuit indirect coupling, the instantaneous electric field distribution condition of thetransformer in the whole process of suffering from lightning impulse voltage is calculated, the analysis and calculation efficiency is improved, the design period and cost of the transformer are reduced, the electrical performance is improved, and good practicability and economical efficiency are achieved.

The present invention relates to an impulse withstand voltage testing method of UPS system, wherein the impulse withstand voltage testing method comprises the following steps: loading an impulse voltage on a tested part, connecting and earthing the other circuits and the exposed conductive part, exerting short-time impulse voltage of three positive and three negative lightning waves with each slot time not less than 5s. The tested UPS system is qualified if no insulation breakdown or flashover phenomena of device occur in the testing course and no disruptive discharge occurs after testing. As the adopted technical settling plan totally settles the problem of over pressure resistance of each electrified loop in the tested UPS system and remarkably increases the efficiency for testing the pressure resistance of each electrified loop in the UPS system thereby more effectively guaranteeing the safety performance of UPS system.

The invention belongs to the technical field of low-temperature and superconducting application and particularly relates to a low-temperature insulating structure of a gas-cooled lead. The structure provided by the invention comprises a vacuum flange and a three-layer insulating structure which is arranged on the outer surface of the gas-cooled lead, wherein the three-layer insulating structure comprises a lead inner protective layer, a low-temperature insulating layer and an outer protective layer which are sequentially arranged from interior to exterior; and the vacuum flange is connected with the gas-cooled lead which is arranged at the inner core of the three-layer insulating structure through bolts. The low-temperature insulating structure provided by the invention can stand a 5min power frequency voltage test of a more than 35kV voltage class, a thunder impulse voltage test, a partial discharge test, a 15min alternating current withstand voltage test, a direct current withstand voltage test and the like, has the advantages of excellent electric property parameters, reasonable electric field and magnetic field distribution, low heat leakage and integral vacuum tightness withstand voltage properties with a stainless steel liquid nitrogen container, and can be used for improving the stability of a system and greatly lowering the investment cost.

The invention provides a mandrel material lightning impulse voltage withstanding test device, which has the advantages that electrodes and material are convenient to replace, and centring of the electrodes is good. The device comprises a sample table, a test bed cabinet, a cross beam and an oil tank coaxially fixed at the upper part of the test bed cabinet respectively, and a power mechanism arranged at the top part of the cross beam, wherein a T-shaped ground wire connection device is inserted and sealed in the bottom plate of the oil tank, the top contact end is arranged inside the oil tank, and the extending end is connected with the ground wire; the sample table comprises a support and an upper electrode and a lower electrode coaxially arranged on the support respectively; the upper electrode is fixed on an upper electrode connection rod in sliding connection with the support; the lower electrode connection end is provided with a spring; the spring sleeves a lower electrode connection rod piece fixedly connected with the support; the surface where the lower electrode connection piece is contacted with the top contact end of the ground wire connection device is provided with a groove; the groove is matched with the top contact end; the depth of the groove is smaller than the height of the top contact end; and the power output end of the power mechanism is connected with the sample table for controlling rising and falling of the sample table in the oil tank.

The invention discloses a simulation test platform for lightning-caused breaking of an insulated wire in a 10kV distribution network. The simulation test platform includes an impulse voltage unit, a simulation tower wire unit, a power frequency current source unit and a data acquisition unit, wherein the impulse voltage unit is connected to an insulated pillar on a tower at one side through the insulated wire; the impulse voltage unit is used to provide a simulation lightning impulse voltage to the insulated wire on the tower; the power frequency current source unit is connected to an insulated pillar on a tower at the other side through a third insulated wire; the power frequency current source unit is configured to provide a set power frequency current to the insulated wire on the towerafter the impulse voltage unit generates the simulation lightning impulse voltage and after the first preset time period; and the data acquisition unit is used to collect lightning overvoltage and thewire current on the insulated wire. Through the technical solution in the simulation test platform for lightning-caused breaking of an insulated wire in a 10kV distribution network, after simulationlightning impulse voltage is broken down and insulated, a continuous power frequency current is provided for the generated arc to improve the authenticity of the lightning-caused breaking test.

Provided is a fault diagnosis method of transformer thunder impacts based on big data. The method comprises steps of collecting historical data of a transformer thunder impact voltage withstanding test and carrying out rapid Fourier transform on head end voltage data and neutral point current data measured in the test; establishing a fault standard database, and carrying out standardization processing so as to obtain transfer function data; carrying out standardization processing on the new thunder impact voltage withstanding test data to obtain transfer function data, and by comparing the transfer function data under 50% thunder impact voltage and under full voltage, judging whether a failure occurs; selecting related data of the same type of transformers in a fault database, and solving a correlation index rho between the transfer function data of the new test and the transfer function data in the fault database; and selecting the proximal fault information according to sequenced fault lists, setting a deviation value eta% and by taking a fault corresponding position as a center, selecting a winding segment of the eta% of the total turn number to be a test range for carrying out fault positioning diagnosis. According to the invention, feature classification is performed on massive data with fault waveforms, so the rough range of the faults can be determined; fault positioning time can be greatly reduced; and fault diagnosis is allowed to be quite quick and effective.

The invention discloses a substation site lightning impulse and oscillation lightning impulse voltage test device which includes an impulse voltage generator, a wave front resistor, a wave front inductor, a wave tail resistor, a change-over switch and an impulse voltage measurement and control system. The high-voltage end of the impulse voltage generator is connected with one end of the wave tailresistor and one end of the wave front resistor. The other end of the wave tail resistor is grounded. The other end of the wave front resistor is connected with one end of the wave front inductor andone end of the change-over switch. The other end of the wave front inductor is connected with the other end of the change-over switch to form an output end. The impulse voltage measurement and controlsystem is used for controlling the charging and triggering of the impulse voltage generator. Both a lightning impulse voltage and an oscillation lightning impulse voltage can be generated. Interactive generation of a lightning impulse voltage and an oscillation lightning impulse voltage can be realized by switching the elements of the test circuit only. The operation is convenient, and the cost of equipment is low.

The invention discloses a method for analogue simulation of an ultrahigh-voltage GIS (gas insulated switchgear) oscillation type lightning surge voltage withstand test. The method comprises the following steps: according to the structural sizes of GIS devices of an actual transformer substation and electric wiring, calculating equivalent impedances of all components, and according to the geometric sizes of the components, calculating the wave impedance parameters and the wave velocity parameters of a breaker, a disconnector and a bus in a simulation model; according to the geometric sizes of the components and the parameters of the simulation model, establishing the simulation model by utilizing the ATP-EMTP software; determining the mode of connection of the simulation model and the waveform parameter of the oscillation type lightning surge voltage; adopting an oscillation type lightning surge voltage wave to perform impact simulation to the same mode of connection; determining the parameters of the actual GIS device oscillation type lightning surge voltage withstand test according to the simulation result. With adoption of the method disclosed by the invention, the defects in the prior art that the reliability is low, the security is poor, the economic benefits are poor and the like can be overcome, so as to realize the advantages of high reliability, excellent security and excellent economic benefits.

The invention relates to a gapless lightning impulse voltage generator. The gapless lightning impulse voltage generator is characterized in that the gapless lightning impulse voltage generator comprises a supply transformer T with the main edge connected with a charging source. One end of the auxiliary edge of the supply transformer T is connected with the ground, the other end of the auxiliary edge of the supply transformer T is connected with an emitting electrode of an insulated gate bipolar transistor VT0, a collector electrode of the insulated gate bipolar transistor VT0 is connected with a protective resistor r in series and then connected to an emitting electrode of an insulated gate bipolar transistor VT, a collector electrode of the insulated gate bipolar transistor VT is connected to a positive electrode of a charging capacitor C through a discharge resistor rt, a negative electrode of the charging capacitor C is connected to the emitting electrode of the insulated gate bipolar transistor VT, the positive electrode of the charging capacitor C is connected to one end of a high-voltage experimental article ZX through a wavefront resistor rf, the other end of the high-voltage experimental article ZX is connected with the ground, the supply transformer T is connected to the emitting electrode of the insulated gate bipolar transistor VT0, and a charging resistor R is connected to the collector electrode of the insulated gate bipolar transistor VT0. The gapless lightning impulse voltage generator has the advantages of being free of mistaken movement, easy to operate and control, high in work frequency, small in drive circuit power and the like.

The invention discloses a method for determining single lightning stroke protection effectiveness of an overhead line insulator parallel gap. Currently a method for determining whether an single lightning stroke event parallel gap performs effective protection on an insulator is a manual tower climbing line inspection manner, thereby searching whether a lightning burning trace exists on the surface of the insulator. The method has defects of relatively high manpower consumption and low efficiency. According to the method of the invention, a lightning current amplitude of a single lightning stroke event is acquired through a lightning positioning system, and furthermore a lightning impact voltage threshold which is bear by the insulator and the parallel gap is calculated. According to a protection effectiveness rule of the parallel gap in different lightning impact voltage thresholds, whether protection of the parallel gap to a disc type suspended insulator in the lightning stroke event is effective is determined. According to the method of the invention, protection effectiveness of the parallel gap is determined according to calculation, thereby realizing no manpower consumption and higher efficiency.

The invention provides a method for measuring and calculating transformer invading lightning voltage waveforms with a transformer bushing. The method comprises the following steps: S1, according to the impulse withstand voltage test national standard, exerting lightning impact voltage waves on the transformer bushing same as a measured transformer in type in a testing room, and measuring waveforms i(t) of the current flowing through an end screen ground connection wire of the transformer bushing and lightning impact waveforms u(t) of the voltage exerted on the transformer bushing, S2, obtaining the transfer impedance of the transformer bushing, S3, recording the waveforms il(t) of the current measured by a current sensor and flowing through the end screen ground connection wire of the transformer bushing, and S4, calculating to obtain U1(jomega) according to the formula U1(jomega)=Z(jomega)*I1(jomega), and obtaining the invading lightning voltage waveforms u1(t) of the measured transformer according to the obtained U1(jomega). According to the method for measuring and calculating the transformer invading lightning voltage waveforms with the transformer bushing, the method that a self high-voltage bushing of the transformer is inversely pushed into the invading lightning voltage is used, the operation is simple, the method is easy to implement, the real amplitude and the real waveforms of the invading lightning voltage can be obtained, the accurate lightning prevention parameters are provided for the transformer, and the lightning prevention level of the transformer is improved.

The invention discloses a lightning stroke impulse voltage test system. The lightning stroke impulse voltage test system includes a test room and a control room, wherein the test room is provided withan impulse voltage generator, a direct current charging source, a capacitive voltage divider, a multilevel chopping device, an impulse voltage divider, and an impulse signal measuring system; and thecontrol room is provided with a control table, the direct current charging source is connected with the impulse voltage generator, the multilevel chopping device is connected with the impulse voltagegenerator through a cable, and the control table is connected with the impulse voltage generator through an optical fiber and is connected with the capacitive voltage divider through a cable. The lightning stroke impulse voltage test system has the advantages of being wide in practicability, high in safety, high in interference resistance and convenient to install, and the lightning stroke impulse voltage test system can be applied to all high-voltage laboratories in factories, research institutes and universities and colleges.

The invention discloses an automatic lightning impulse voltage test system. The system comprises an impulse voltage divider, a chopping device, an impulse voltage generator, a charging power supply, an impulse voltage controller and an impulse measurement and analysis system. The chopping device and the impulse voltage generator are electrically connected to the charging power supply through a control cable, the charging power supply and the impulse voltage controller are connected through an optical fiber, the impulse voltage controller and the impulse measurement and analysis system are connected through an RS232 communication cable, and the impulse measurement and analysis system and the impulse voltage divider are connected through a measuring cable. The impulse voltage controller is used to control charging, triggering and grounding of the impulse voltage generator. The impulse measurement and analysis system is used to obtain an impulse signal generated by a tested product duringa lightning impulse test process, and analyze and process the impulse signal so as to form an analysis report. In the invention, fully-automatic control, measurement and analysis of the lightning impulse voltage test can be realized, and the test efficiency and the accuracy of a test result are increased.

The invention provides a phase-shifting transformer lightning impulse winding overvoltage calculation method, which comprises the steps of (1) generating an equivalent wiring schematic diagram of a phase-shifting transformer according to a wiring mode of the phase-shifting transformer; (2), establishing a preliminary equivalent circuit model of each winding of the phase-shifting transformer; (3) calculating each capacitance and inductance value in the preliminary equivalent circuit model; (4), determining a final equivalent circuit model of the phase-shifting transformer; (5) applying lightning waves with corresponding amplitudes to related positions in the final equivalent circuit model; and (6) solving the circuit model by using circuit simulation analysis software to obtain a change curve of the voltage of each part of the winding in the phase-shifting transformer along with time and voltage distribution conditions in the winding at different moments. According to the method, the insulation strength of the high-voltage winding of the phase-shifting transformer between line cakes under the lightning impulse voltage can be rapidly and accurately calculated, the insulation strengthof the phase-shifting transformer can be well analyzed by utilizing the calculation result, the manufacturing design of the insulation structure of the phase-shifting transformer is guided, and the detection test level is improved.

The invention discloses an experimental device for measuring lightning impulse voltage endurance of insulating liquid. The experimental device comprises a fixed seat, a high-voltage electrode component, a grounding electrode component and a test channel, the fixed seat comprises an upper insulating cover plate and a lower insulating cover plate, the upper insulating cover plate is provided with afirst assembly through hole, the lower insulating cover plate is provided with a second assembly through hole, the high-voltage electrode component comprises a pin electrode and a first conducting rod, the first conducting rod comprises a first mounting end and a first terminal, the pin electrode is arranged at the first mounting end, the grounding electrode component comprises a ball electrode and a second conducting rod, the second conducting rod comprises a second mounting end and a second terminal, the ball electrode is arranged at the second mounting end, and the pin electrode and the ball electrode are positioned below the liquid level of the insulating liquid to be measured. As the experimental device is simple in structure and light in weight, experimental operation steps can be simplified, cost can be reduced, split cleaning and maintenance after experiments are facilitated, and the experimental device is convenient to assemble and disassemble.

The embodiment of the invention provides a 110kV three-phase dry-type transformer, and the 110kV three-phase dry-type transformer comprises three connected single-phase dry-type transformers, whereineach single-phase transformer comprises a wire inlet side high-voltage coil and a wire outlet side high-voltage coil; the wire inlet side high-voltage coil is provided with a wire inlet end and a wireoutlet end, and the wire outlet side high-voltage coil is provided with a wire inlet end and a wire outlet end; the wire outlet end of the wire inlet side high-voltage coil is connected with the wireinlet end of the wire outlet side high-voltage coil, and the wire inlet side high-voltage coil is in series connection with the wire outlet side high-voltage coil. According to the invention, the wire inlet ends are arranged at the middles of the high-voltage coils at all sides, so the lightning impulse voltage resistance of the wire inlet ends of the high-voltage coils at all sides is improved,the lightning impulse voltage resistance of the whole three-phase dry type transformer is further improved, and the requirement for the voltage resistance of insulation of the three-phase dry type transformer under the 110kV voltage level is met.

The invention discloses a lightning protection device in multilevel clearance series connection. The lightning protection device in multilevel clearance series connection includes a high-voltage wireinlet end, a grounding end, a metal oxide arrester (MOA), N voltage partition capacitors, N voltage partition inductors in series connection, N-1 loop voltage partition inductors and N discharging gaps at equal intervals. Since a multilevel clearance series connection structure is adopted, the lightning protection device can be resistant to high working-frequency voltage, and a breakdown voltage value of the lightning protection device under the working-frequency voltage is extremely higher than a breakdown voltage value under lightning impulse voltage. After clearances in multilevel series connection are broken down, the metal oxide arrester starts to work, wherein the metal oxide arrester (MOA) has the characteristics of generating no continuous current under work frequency. The lightning protection device has the advantages that not only is the breakdown voltage value of the lightning protection device under the working-frequency voltage extremely higher than the breakdown voltage value under the lightning impulse voltage, but also the lightning protection device is high in work reliability, free of continuous current under the working frequency and long in service life.

The present invention relates to a double exponential fitting system and method for the lightning impulse voltage waveform by using a genetic algorithm. The system comprises a first module for data processing and storage of a recording curve of the lightning impulse waveform, a second module for data processing and storage of the fit waveform, a third module for the double exponential function, afourth module for setting genetic algorithm parameters, and a fifth module for iterative processing and displaying. Compared with the prior art, the technical scheme of the present invention has the advantages of simpleness, stability, practicality and rapidness.

The invention relates to a grounding network fault diagnosis method based on a transfer function curve. According to a transfer function change between a state when the grounding network is normal and a state when a corrosion or a break-point fault occurs at the inner part, a lightning impulse signal is used as an exciting signal for measuring a current signal under an impulse excitation and obtaining the transfer function curve of the grounding network. Furthermore grounding network fault diagnosis is performed according to the change of the transfer function curve. The grounding network fault diagnosis method comprises the following steps: 1) inputting the lightening impulse voltage signal U from the signal input line of the grounding network; 2) measuring an impulse current output signal I from the output end of the grounding network; 3) performing fast Fourier transform on the two signals for obtaining frequency domain signals F1(U) and F2(I); 4) comparing the frequency domain signals for obtaining the transfer function curve of the grounding network; and 5) performing fault diagnosis on the grounding network according to the change of the transfer function curve. Compared with the prior art, the grounding network fault diagnosis method based on the transfer function curve has advantages of high diagnosis precision, convenient operation, etc.

The invention discloses an alternate-current disc suspension type bell-shaped ceramic compound insulator special for 420KN. The insulator comprises a steel cap, a ceramic disc, a groove and a steel pin. The diameter of the insulator is of 400mm, the creepage distance is of 550mm, and the ceramic disc and the groove are coated with vulcanized silicone rubber with the thickness of 6mm. Since the 6mm external insulating component is formed on the surface of an insulator body in a vulcanized manner, the steep wave resistance of the insulator is improved greatly, and the ceramic compound insulator is capable of passing a test of 2.85 times of negative lightning impulse voltage by the amplitude method; according to an artificial contamination test, the insulator has fine contamination resistance under IV stage contamination, and the contamination flashover resistance of the insulator is at least 1.8 times of that of a disc suspension type bell-shaped ceramic insulator on the conditions of consistent salt deposit density and dust deposit density.

The invention relates to a lightning waveform parameter time domain statistical method based on an optical integrated electric field sensor, and belongs to the technical field of waveform analysis. According to the method, the three-phase independent lightning overvoltage waveform is separated from the original waveform through the method of filtering, overvoltage extraction and decoupling based on the measurement data of the optical integrated electric field sensor, and statistical analysis of the measured waveform parameters is performed in the time domain according to the definition of thewavefront time, the half peak time and the overvoltage multiple of the lightning impulse voltage standard waveform in the IEC. The method is based on the optical integrated electric field sensor and has the characteristics of being convenient to install and convenient to measure.

The invention relates to a test method for verifying the reliability of a transition joint. Currently, there is no effective and accurate test method for the transition joint. The method comprises the following steps: installing a test loop and a simulation loop; determining a temperature control cable; carrying out a load cycle test of the test loop; carrying out a DC overlapping operation impulse voltage test of the test loop; carrying out a DC overlapping lightning impulse voltage test; and carrying out inspection after testing. According to the technical scheme of the invention, through the temperature monitoring of the simulation loop, the method achieves the accurate control of the wire temperature of the test loop, and improves the accuracy of the testing of the reliability of the transition joint. Moreover, the temperature control cable is determined, thereby facilitating the improvement of the testing safety, and enabling the test to be carried out smoothly.

The invention discloses an oscillation type lightning impulse test system and method based on transformer self-sensing. An oscillation type lightning impulse voltage generator is adopted to apply an impulse signal to a high-voltage winding of a to-be-tested power transformer. The voltage waveform of the high-voltage side is measured through a capacitive voltage divider. The secondary end screen ofa high-voltage side tested-phase sleeve is used as a self-sensing capacitive voltage divider of the to-be-tested power transformer. A leakage current signal of the sleeve and a partial discharge current signal in the sleeve can be transmitted out. The iron core of the to-be-tested power transformer serves as a transformer self-sensing sensor. An ultrahigh-frequency signal generated by internal discharge is transmitted to an intelligent expert diagnosis system through a grounding wire to judge whether the to-be-detected power transformer has defects or not. The technical problems of large equipment size, low voltage generation efficiency and inconvenience in field debugging of a generator for generating a double-index impulse voltage due to the fact that a double-index impulse voltage is adopted in an impulse withstand voltage test of an existing power transformer are solved.