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Evaluation of insulation withstand voltage capability of electrical equipment

Add time:2022-02-18 11:18:23   Number of views:68  

It is a technical means to inspect and evaluate the insulation withstand voltage capability of electrical equipment. Insulation structure shall be adopted between the live part and grounding part of all electrical equipment or with other non equipotential charged bodies to isolate them from each other, so as to ensure the normal operation of the equipment. The dielectric strength of a single insulating material is expressed as the average breakdown electric field strength along the thickness (in kV / cm). The insulation structure of electrical equipment, such as the insulation of generators and transformers, is composed of a variety of materials, and the structural shape is also extremely complex. Any local damage of insulation structure will make the whole equipment lose insulation performance. Therefore, the overall insulation capacity of the equipment can only be expressed by the test voltage that can withstand (unit: kV). The insulation withstand voltage test voltage can indicate the voltage level that the equipment can withstand, but this is not equivalent to the actual insulation strength of the equipment. The specific requirement of power system insulation coordination is to coordinate and formulate the insulation withstand voltage test voltage of various electrical equipment to indicate the insulation level requirements of the equipment. Insulation withstand voltage test is a destructive test (see insulation test). Therefore, for some key equipment in operation that lacks spare parts or has a long repair time, it should be carefully considered whether to conduct insulation withstand voltage test.

In addition to AC or DC working voltage, various electrical equipment in the power system will also suffer from various overvoltages. These overvoltages not only have high amplitude, but also have very different waveform and duration from the working voltage. The influence on insulation and the possible mechanism of insulation breakdown are also different. Therefore, it is necessary to use the corresponding test voltage for the withstand voltage test of electrical equipment. The insulation withstand voltage test specified in Chinese national standards for AC power system includes: ① short-time (1 minute) power frequency withstand voltage test; ② Long time power frequency withstand voltage test; ③ DC withstand voltage test; ④ Operating shock wave withstand voltage test; ⑤ Lightning impulse withstand voltage test. It is also stipulated that the insulation performance of 3 ~ 220kV electrical equipment under power frequency operating voltage, temporary overvoltage and operating overvoltage is generally inspected by short-time power frequency withstand voltage test, and the operation impact test is not required. For 330 ~ 500kV electrical equipment, it is necessary to test the insulation performance under operating overvoltage by operating impulse test. Long time power frequency withstand voltage test is a kind of test and inspection aimed at the deterioration of internal insulation and pollution of external insulation of electrical equipment.

Insulation and withstand voltage test standards are specified in various countries. The Chinese national standard (gb311.1-83) specifies the reference insulation level of 3 ~ 500kV transmission and transformation equipment; Lightning impulse withstand voltage and one minute power frequency withstand voltage of 3 ~ 500kV power transmission and transformation equipment; And the operating impulse withstand voltage of 330 ~ 500 kV power transmission and transformation equipment. The electrical equipment manufacturing department and power system operation department shall comply with the provisions of national standards when selecting the items and test voltage value of withstand voltage test.

Power frequency withstand voltage test

It is used to inspect and evaluate the ability of electrical equipment insulation to withstand power frequency voltage. The test voltage shall be sine wave and the frequency shall be the same as that of the power system. It is generally stipulated that one minute withstand voltage test is used to test the short-time withstand voltage capacity of insulation, and long-time withstand voltage test is used to test the progressive deterioration inside insulation, such as partial discharge damage, dielectric loss and thermal damage caused by leakage current. The external insulation of outdoor power equipment is affected by atmospheric environmental factors. In addition to the power frequency withstand voltage test under the surface dry state, the withstand voltage test under the artificial simulated atmospheric environment (such as wet and dirty state) should also be carried out.

AC sinusoidal voltage can be expressed as peak value or effective value. The ratio of peak value to effective value is 匇. The waveform and frequency of the test voltage actually applied in the test will inevitably deviate from the standard. The Chinese national standard (gb311.3-83) stipulates that the frequency range of the test voltage should be 45 ~ 55Hz, and the waveform of the test voltage should be close to the sine wave. The condition is that the positive and negative half waves should be exactly the same, and the ratio of peak value to effective value is equal to ± 0.07. The so-called peak voltage divided by the general test value is the effective value.

The power supply used in the test is composed of high-voltage test transformer and voltage regulating device. The principle of the test transformer is the same as that of the general power transformer, and its rated output voltage shall meet the test requirements and leave room; The output voltage of the test transformer shall be stable enough to avoid obvious fluctuation of the output voltage due to the voltage drop of the pre discharge current on the internal resistance of the power supply, so as to avoid difficult measurement and even affect the discharge process. Therefore, the test power supply should have sufficient capacity and the internal impedance should be as small as possible. Generally, the requirements for the capacity of the test transformer are specified by how much short-circuit current can be output under the test voltage. For example, for the test of small insulating samples of solid, liquid or a combination of both in the dry state, the short-circuit current of the equipment is required to be 0.1A; For the test of self recovery insulation (insulator, disconnector, etc.) in dry state, the short-circuit current of the equipment shall not be less than 0.1A; Conduct artificial rain test for external insulation, and the short-circuit current of the equipment shall not be less than 0.5A; For the test of test objects with large overall dimensions, the short-circuit current of the equipment is required to be 1a. Generally speaking, the test transformer with low rated voltage mostly adopts the 0.1A system, that is, the continuous flow of 0.1A in the high-voltage coil of the transformer is allowed. For example, the capacity of 50kV test transformer is 5KVA and that of 100kV test transformer is 10KVA. The test transformer with higher rated voltage mostly adopts 1A system, that is, 1a is allowed to flow continuously in the high-voltage coil of the transformer. For example, the capacity of 250kV test transformer is 250kVA and that of 500kV test transformer is 500KVA. Because the overall dimension of the tested equipment with higher voltage is large and the equivalent capacitance of the equipment is also large, the test power supply needs to provide more load current. The rated voltage of a single test transformer is too high, which will cause some difficulties in technology and economy. The high voltage of a single test transformer in China is 750kV, and there are few single test transformers with a voltage exceeding 750kV in the world. In order to meet the needs of AC voltage test of EHV and UHV power equipment, several test transformers are usually connected in series to obtain high voltage. For example, three 750kV test transformers are connected in series to obtain 2250kV test voltage, which is called series test transformer. When the transformer is connected in series, the internal impedance increases rapidly, which greatly exceeds the algebraic sum of several impedances. Therefore, the number of series connected transformers is often limited to 3. The test transformer can also be connected in parallel to increase the output current, or connected into △ or Y-type for three-phase operation.

In order to conduct power frequency withstand voltage test on test objects with large electrostatic capacitance, such as capacitors, cables and large capacity generators, the power supply device is required to be both high voltage and large capacity. It will be difficult to realize this power supply device. Some departments have adopted power frequency high-voltage series resonance test equipment (see AC high-voltage series resonance test device).

Lightning impulse withstand voltage test is to test the ability of electrical equipment insulation to withstand lightning impulse voltage by manually simulating lightning current waveform and peak value. According to the measured results of lightning flash discharge, it is considered that the lightning wave shape is a unipolar double exponential curve with a wave head length of several microseconds and a wave tail length of tens of microseconds. Most of them are negative. The national standards of all countries in the world calibrate the standard lightning shock wave as: apparent wave head time T1 = 1.2 μ s. Also known as wave head time; Apparent half wave peak time T2 = 50 μ s. Also known as wave tail time (see Figure). The allowable deviation between the voltage peak generated by the actual test device and the waveform and the standard wave is: peak, ± 3%; Wave head time, ± 30%; Half wave peak time, ± 20%; The standard lightning waveform is usually expressed as 1.2/50 μ s。

The lightning impulse test voltage is generated by the impulse voltage generator. The conversion of multiple capacitors of the impulse voltage generator from parallel to series is realized through many ignition ball gaps, that is, multiple capacitors are connected in series when the ignition ball gap is controlled to discharge. The rising speed of the voltage on the tested equipment and the speed of the voltage drop after the peak can be adjusted by the resistance value in the capacitor circuit. The resistance that affects the wave head is called the wave head resistance, and the resistance that affects the wave tail is called the wave tail resistance. During the test, the predetermined wave head time and half wave peak time of standard impulse voltage wave are obtained by changing the resistance values of wave head resistance and wave tail resistance. By changing the polarity and amplitude of the output voltage of the rectifier power supply, the polarity and peak value of the required impulse voltage wave can be obtained. Thus, an impulse voltage generator from hundreds of thousands of volts to millions of volts or even tens of millions of volts can be realized. The high voltage of impulse voltage generator designed and installed by China is 6000kv.

Lightning impulse voltage test

The content includes 4 items. ① Impulse withstand voltage test: it is usually used for non self recovery insulation, such as the insulation of transformers, reactors, etc. to test whether these equipment can withstand the voltage specified by the insulation grade. ② 50% impulse flashover test: usually take self-healing insulation such as insulator and air gap as the object, in order to determine the voltage value u with 50% flashover probability. With the standard deviation between this voltage value and flashover value, other flashover probabilities can be determined, such as 5% flashover voltage value, generally taking u as withstand voltage. ③ Breakdown test: the purpose is to determine the actual strength of insulation. Mainly in electrical equipment manufacturing plant. ④ Voltage time curve test (volt second curve test): the voltage time curve represents the relationship between the applied voltage to insulation failure (or porcelain insulation flashover) and time. The volt second curve (V-T curve) can provide a basis for considering the insulation coordination between protected equipment such as transformer and lightning arrester.

In addition to the full wave test of lightning impulse, sometimes for electrical equipment with windings such as transformer and reactor, the cut-off time is 2 ~ 5 μ S truncated wave test. Truncation can occur at the wave head or at the wave tail. The generation and measurement of this truncated wave and the determination of the damage degree to the equipment are complex and difficult. Due to the fast process and high amplitude of lightning impulse voltage test, the technical requirements for test and measurement are high. Detailed test procedures, methods and standards are often specified, which can be referred to and implemented during the test.

Operating impulse overvoltage test

The ability of electrical equipment insulation to withstand operating impulse voltage is tested by manually simulating the operating impulse overvoltage waveform of power system. There are many types of operating overvoltage waveforms and peaks in power system, which are related to line parameters and system state. In general, it is an attenuated oscillation wave with a frequency ranging from tens of Hz to thousands of Hz. Its amplitude is related to the system voltage. It is usually expressed as several times of the phase voltage, up to 3 ~ 4 times of the large phase voltage. The action time of operation shock wave is longer than that of lightning shock wave, and its influence on power system insulation is also different. For the power system of 220kV and below, the short-time power frequency withstand voltage test can be used to equivalently test the equipment insulation under operating overvoltage. For the EHV and UHV systems and equipment of 330kV and above, the operating overvoltage has a greater impact on the insulation, so the short-time power frequency voltage test can not be used to approximately replace the operating impulse voltage test. It can be seen from the test data that for the air gap above 2m, the nonlinearity of the operating discharge voltage is significant, that is, when the gap distance increases, the withstand voltage increases slowly, even lower than the short-term power frequency discharge voltage. Therefore, the insulation must be tested by simulating the operating impulse voltage.

For long gap, insulator and external insulation of equipment, there are two test voltage waveforms simulating operating overvoltage. ① Aperiodic exponential attenuation wave: similar to lightning impulse wave, but the wave head time and half peak time are much longer than the lightning impulse wavelength. The standard waveform of operating impulse voltage recommended by the International Electrotechnical Commission is 250 / 2500 μ s; When the standard waveform cannot meet the research requirements, 100 / 2500 can be selected μ S and 500 / 2500 μ s。 The aperiodic exponential attenuation wave can also be generated by the impulse voltage generator, which is basically the same as the principle of generating lightning impulse wave, except that the wave head resistance, wave tail resistance and charging resistance should be increased many times. A set of impulse voltage generator is commonly used in high-voltage laboratory, with two sets of resistors, which are used to generate lightning impulse voltage and operation impulse voltage. According to the regulations, the allowable deviation between the generated operating impulse voltage waveform and the standard waveform is: peak, ± 3%; Wave head, ± 20%; Half peak time, ± 60%. ② Attenuated oscillation wave: the duration of one half wave is required to be 2000 ~ 3000 μ s. The amplitude of the second half wave is about 80% of that of the second half wave. The attenuated oscillation wave is induced on the high voltage side by discharging the capacitor on the low voltage side of the test transformer. This method is mostly used in the operation wave test of power transformer on the site of substation, and the test waveform is generated by the tested transformer itself to test its withstand voltage ability.

The operation impulse overvoltage test includes 5 items: ① operation impulse withstand voltage test; ② 50% operation impact flashover test; ③ Breakdown test; ④ Voltage time curve test (V-s curve test); ⑤ Operating impulse voltage wave head curve test. The first four tests have the same requirements as the corresponding tests in the lightning impulse voltage test. The fifth test is required for the discharge characteristics of operating shock, because the discharge voltage of long air gap under the action of operating shock wave will change with the shock wave head. At a certain wave head length, such as 150 μ When the discharge voltage is low, it is called the critical wave head. The critical wave head length increases slightly with the increase of gap length.

DC withstand voltage test

The insulation performance of electrical equipment shall be tested by DC power supply. Its purpose is: ① to determine the ability of DC high-voltage electrical equipment to withstand DC voltage; ② Due to the limitation of AC test power capacity, DC high voltage is used to replace AC high voltage to carry out withstand voltage test on large capacitance AC equipment.

The DC test voltage is generally generated by the AC power supply through the rectifier, which is actually a unipolar pulsating voltage. There is a voltage maximum u at the crest and a voltage minimum u at the trough. The so-called DC test voltage value refers to the arithmetic mean value of the pulsating voltage, that is, it is obviously not expected to pulsation too much. Therefore, it is stipulated that the pulsation coefficient s of DC test voltage shall not exceed 3%, that is, the DC voltage is divided into positive and negative polarity. The action mechanism of different polarity on various insulation is different. A polarity must be specified in the test. Generally, the polarity with serious test on insulation performance is used for the test.

A single-stage half wave or full wave rectifier circuit is usually used to generate high DC voltage. Due to the limitation of rated voltage of capacitor and high-voltage silicon stack, this circuit can generally output 200 ~ 300kV. If higher DC voltage is required, cascade method can be adopted. The output voltage of the cascade DC voltage generator can be 2n times of the peak voltage of the power transformer, and N represents the series. The voltage drop and pulsation of the output voltage of this device are the functions of series number, load current and AC power frequency. If there are too many series stages and the current is too large, the voltage drop and pulsation will reach an unacceptable level. The cascade DC voltage generator can output voltage of about 2000 ~ 3000KV and output current of only tens of milliamps. During the artificial environment test, the pre discharge current can reach hundreds of Ma, or even 1 A. at this time, a thyristor voltage stabilizing device should be added to improve the quality of output voltage. It is required that when the pre discharge current pulse with duration of 500ms and amplitude of 500mA flows once per second, the voltage drop caused by it shall not exceed 5%.

In the insulation preventive test of power system equipment (see insulation test), DC high voltage is often used

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