A technical method for testing and evaluating the ability of electrical equipment insulation to withstand voltage. All electrical equipment between the live part and grounding part, or with other non-equipotential between the charged body, need to use insulation structure to make them isolated from each other, to ensure the normal operation of 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 generator, transformer, etc., is composed of a variety of materials, and the structure shape is also very complex. Any local damage to the insulation structure will cause the loss of insulation performance of the whole equipment. Therefore, the overall insulation capacity of the equipment can only be expressed by how high the test voltage can be tolerated (unit: kV). The insulation withstand voltage test voltage indicates the voltage level that the equipment can withstand, but it is not equivalent to the actual insulation strength of the equipment. The specific requirements of power system insulation coordination are to coordinate and develop various electrical equipment insulation withstand voltage test voltage, to show the insulation level of the equipment requirements. Insulation withstand voltage test is a destructive test (see insulation test), therefore, some of the lack of spare parts or repair time is long in the operation of key equipment, should be carefully considered whether to carry out insulation withstand voltage test.
All kinds of electrical equipment in the power system in operation, in addition to the ac or DC operating voltage, but also suffer from a variety of overvoltage. These overvoltages not only have high amplitudes, but also have waveforms and durations that are very different from the operating voltage, and have different effects on insulation and mechanisms that may cause insulation breakdown. Therefore, it is necessary to use the corresponding test voltage to carry out the voltage withstand test of electrical equipment. China's national standards for AC power system insulation withstand voltage test are: ① short time (1 minute) power frequency withstand voltage test; ② Long time power frequency withstand voltage test; Dc withstand voltage test; ④ Operation shock wave pressure test; ⑤ Lightning shock wave pressure test. In addition, the insulation performance of 3 ~ 220kV electrical equipment under power frequency operating voltage, temporary overvoltage and operating overvoltage is generally tested by short-term power frequency voltage withstand test, without operating impact test. For 330-500kV electrical equipment, the insulation performance under operating overvoltage should be tested by operating impact test. The long time power frequency withstand voltage test is a kind of test and inspection for the internal insulation deterioration and external insulation pollution of electrical equipment.
Insulation withstand voltage test standards, in each country have specific provisions. The National Standard of China (GB311.1-83) stipulates the benchmark insulation level of 3-500kV transmission and transformation equipment; 3 ~ 500kV transmission and transformation equipment lightning shock withstand voltage, one minute power frequency withstand voltage; And 330-500kV transmission and transformation equipment operating shock tolerance voltage. The electrical equipment manufacturing department and the power system operation department should comply with the provisions of the national standard when selecting the items and test voltage of the voltage test.
Power frequency withstand voltage test
Used for testing and evaluating the ability of insulation of electrical equipment to withstand power frequency voltage. The test voltage shall be sinusoidal and the frequency shall be the same as that of the power system. One minute withstand voltage test is usually prescribed to test the short-time withstand voltage of insulation, and the long-term withstand voltage test is used to test the progressive deterioration of insulation, such as the damage of partial discharge and the thermal damage caused by dielectric loss and 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 dry state of the surface, the withstand voltage test under the artificial simulated atmospheric environment (such as wet and dirty state) should be done.
Ac sinusoidal voltage can be expressed as a peak or RMS value. The ratio of peak value to effective value is 匇. The waveform and frequency of the actual test voltage applied in the test inevitably deviates from the standard. The Chinese national standard (GB31.3-83) 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 the peak value to the effective value is equal to ±0.07. Generally the so-called test voltage value refers to the effective value, is its peak divided by.
The power supply used in the test consists of a high voltage test transformer and a voltage regulating device. The principle of the test transformer is the same as that of the general power transformer, and its rated output voltage should meet the test requirements and leave room; The output voltage of the test transformer should be stable enough, so that the output voltage does not fluctuate significantly due to the voltage drop of the pre-discharge current on the internal resistance of the power supply, so as not to measure difficulties or even affect the discharge process. Therefore, the test power supply should have sufficient capacity, internal impedance should be as small as possible. In general, how much short-circuit current can be output under the test voltage is required to determine the capacity of the test transformer. For example, for solid, liquid or A combination of insulating small samples in the dry state of the test, the equipment short circuit current is required to be 0.1a; For the test of self-recovery insulation (insulators, disconnecting switches, etc.) in dry state, the equipment short circuit current is required not less than 0.1a; External insulation artificial rain test, the equipment short circuit current is not less than 0.5A; For the larger size of the test, the equipment short circuit current is required to be 1A. Generally speaking, the lower rated voltage test transformer to take 0.1a system, that is, to allow the continuous flow of 0.1a transformer high voltage coil, such as 50kV test transformer capacity is 5kVA,100kV test transformer capacity is 10kVA. The test transformer with higher rated voltage is more 1A system, that is, the continuous flow of 1A is allowed in the transformer high voltage coil, such as 250kV test transformer capacity of 250kVA, 500kV test transformer capacity of 500kVA, because the size of the higher voltage test equipment is larger, the equivalent capacitance of the equipment is larger, The test power supply is required to provide additional load current. The rated voltage of a single test transformer is too high, which will cause some technical and economic difficulties in manufacturing. The highest voltage of a single test transformer in China is 750kV, and the voltage of a single test transformer in the world is more than 750kV. In order to meet the needs of ULTRA-high voltage and ultra-high voltage power equipment AC voltage test, usually several test transformers are connected in series to obtain high voltage, such as three 750kV test transformers are connected in series to obtain 2250kV test voltage, which is called series test transformer. The internal impedance of transformer serial connection increases rapidly, greatly exceeding the algebraic sum of several impedance, so the number of serial connection is often limited to 3. The test transformer can also be connected to improve the output current, or connected to the △ or Y type for three-phase operation.
In order to do the work frequency withstand voltage test for the products with large electrostatic capacity, such as capacitor, cable and large capacity generator, the power supply device is required to be both high voltage and large capacity. The implementation of such a power supply can be difficult, and some departments have used power frequency high voltage series resonance test equipment (see AC high voltage series resonance test equipment).
Lightning shock withstand voltage test through artificial simulation of lightning current waveform and peak value to test electrical equipment insulation withstand lightning shock voltage. According to the measured results of lightning flash discharge, it is considered that the lightning wave shape is a unipolar double exponential curve with wave head length of several microseconds and wave tail length of tens of microseconds. Most thunder is of negative polarity. The national standards of the world all calibrate the standard lightning shock wave as: apparent wave head time T1=1.2μs, also known as wave head time; The apparent half-wave peak time T2=50μs, also known as wave tail time (see figure). The allowable deviation between the peak voltage and the waveform and the standard wave generated by the actual test device is: peak voltage, ±3%; Wave head time, ±30%; Half wave peak time, ±20%; The standard lightning wave is usually represented as 1.2/50μs.
Lightning impulse test voltage is generated by impulse voltage generator. The multiple capacitors of the impulse voltage generator are connected in series through many ignition ball gaps, that is, the ignition ball gap is controlled to make it discharge in series. The rate of voltage rise on the test device and the rate of voltage drop after peak value can be adjusted by the resistance value in the capacitor circuit. The resistance affecting the wave head is called the wave head resistance, and the resistance affecting the wave tail is called the wave tail resistance. By changing the resistance values of wave head resistance and wave tail resistance, the predetermined wave head time and half-wave peak time of standard impulse voltage wave are obtained. The polarity and peak value of impulse voltage wave can be obtained by changing the polarity and amplitude of output voltage of rectifier power supply. From this can be achieved from hundreds of thousands of volts to millions of volts, even tens of millions of volts of impulse voltage generator. The highest voltage of the impulse voltage generator designed and installed in China is 6000kV.
Lightning impulse voltage test
The content includes four items. ① Impact voltage resistance test: usually used for non-self-recovery insulation, such as transformer, reactor insulation, the purpose is to test whether these equipment can withstand the voltage specified by the insulation level. ②50% impact flashover test: usually to self-recovery insulation, such as insulator, air gap, etc., as the object, the purpose is to determine the voltage value U of flashover probability of 50%. With the standard deviation between this voltage value and the flashover value, other flashover probability can be determined, such as 5% flashover voltage value, generally U as the tolerated voltage. ③ Breakdown test: the purpose is to determine the actual strength of insulation. Mainly in electrical equipment manufacturers. (4) voltage time curve test (volt second curve test) : voltage time curve is the relationship between the application of voltage to insulation damage (or porcelain insulation flashover) and time. The v-T curve can provide a basis for considering the insulation coordination between transformer and lightning arrester.
In addition to the full wave of lightning impact test, sometimes transformer, reactor and other winding electrical equipment, but also do truncation time of 2 ~ 5μs truncation wave test. Truncation can occur at the wave head or at the wave tail. The generation and measurement of the truncation wave and the determination of the damage degree to the equipment are complicated and difficult. Lightning impulse voltage test due to the process of fast, high amplitude, high technical requirements for the test and measurement, often provide detailed test procedures, methods and standards, refer to the implementation of the test.
Operation impulse overvoltage test
The ability of insulation of electrical equipment to withstand operating impulse voltage was tested by simulating operating impulse voltage waveform of power system manually. There are many waveforms and peak types of operating overvoltage in power system, which are related to line parameters and system state. In general, it is a attenuated oscillation wave, with a frequency from tens of Hertz to several kilohertz, and its amplitude is related to the system voltage, usually expressed by several times of the phase voltage, up to 3 to 4 times of the maximum phase voltage. The operation shock wave has longer action time than the lightning shock wave and has different influence on the insulation of power system. For power systems up to and including 220kV, short-time power frequency withstand voltage test can be used approximately to equivalent test equipment insulation under operating overvoltage. For ultra-high voltage and ultra-high voltage systems and equipment of 330kV and above, operating overvoltage has a greater impact on insulation, so short-time power-frequency voltage test can no longer be used to approximately replace operating impulse voltage test. According to the test data, for the air gap above 2m, the nonlinearity of operating discharge voltage is significant, that is, the tolerance voltage increases slowly when the gap distance increases, even lower than the short-time power-frequency discharge voltage. So it is necessary to simulate the operation of impulse voltage insulation test.
There are two test voltage waveforms simulating operating overvoltage for long gaps, insulators, and equipment external insulation. (1) Non-periodic exponential attenuation wave: similar to the lightning shock wave, but the wave head time and half peak time are much more than the lightning shock wavelength. The standard waveform of operating impulse voltage recommended by the International Electrotechnical Commission is 250/2500μs. When the standard waveform cannot meet the research requirements, 100/2500μs and 500/2500μs can be used. An aperiodic exponential decay wave can also be generated by an impulse voltage generator, in much the same way as lightning shock waves, except that the wave head resistance, wave tail resistance, and charge resistance are many times larger. High voltage laboratory commonly used a set of impulse voltage generator, with two sets of resistance, not only for lightning impulse voltage, but also for operating impulse voltage. According to the regulations, the allowable deviation between the generated operating impulse voltage waveform and the standard waveform is: peak value, ±3%; Wave head, ±20%; Half peak time, ±60%. ② Attenuation oscillation wave: the duration of the first half wave is required to be 2000-3000 μs, and the amplitude of the second half wave is approximately 80% of the amplitude of the first half wave. The attenuated oscillation wave is generated by the capacitor discharging at the low voltage side of the test transformer and induced at the high voltage side. This method is used to do power transformer operation wave test in substation site, using the tested transformer itself to generate test waveform, in order to test its own ability to withstand voltage.
The content of the operation impact overvoltage test includes 5 items: (1) the operation impact voltage resistance test; ②50% operation impact flashover test; ③ Breakdown test; ④ Voltage-time curve test (volt-second curve test); ⑤ Operation impulse voltage wave head curve test. The requirements of the first four tests are the same as those in the lightning impulse voltage test. The fifth test is required for the operation shock discharge characteristics because the discharge voltage of the long air gap under the operation shock wave varies with the shock wave head. At a certain wave head length, such as 150μs, the discharge voltage is the lowest, and this wave head is called the critical wave head. The critical wave head length increases slightly with the increase of the gap length.
Dc withstand voltage test
Test insulation performance of electrical equipment with DC power supply. Its purpose is: (1) determine the ability of dc high voltage electrical equipment to withstand DC voltage; (2) Due to the limitation of ac test power supply capacity, dc high voltage is used instead of AC high voltage to carry out voltage withstand test on large capacitor 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 maximum voltage U at the crest and a minimum voltage U at the trough. The so-called DC test voltage value refers to the arithmetic average value of the pulsating voltage, that is, it is obviously not expected to pulsate too much, so it is stipulated that the pulsating coefficient S of the DC test voltage should not exceed 3%, that is, the dc voltage is divided into positive and negative polarity. Different polarities have different mechanisms for different insulation. A polarity must be specified in the test, which is generally used to test the insulation performance.
A single - stage half - wave or full - wave rectifier circuit is usually used to generate high dc voltage. Due to the limitation of capacitor and rated voltage of high voltage silicon reactor, this circuit can output 200 ~ 300kV. If higher DC voltage is required, cascade method can be used. The output voltage of the cascade DC voltage generator can be 2N times the peak voltage of the power transformer, and N represents the series of the cascade. The voltage drop and pulsation of the output voltage of this device are functions of series, load current and ac power frequency. If the series series is too much, the current is too large, the voltage drop and pulsation will reach an inadmissible degree. This cascade DC voltage generator can output voltage about 2000 ~ 3000kV, output current only tens of milliamps. In the artificial environment test, the pre-discharge current can reach hundreds of milliampere, or even 1 ampere. At this time, the SCR voltage regulator should be added to improve the quality of the output voltage. When the duration is 500ms and the amplitude is 500mA, the pre-discharge current pulse flows once per second, the voltage drop caused by not more than 5%.
In the insulation preventive test of power system equipment (see insulation test), dc high voltage is often used to measure the leakage current and insulation resistance of cables, capacitors, etc., and insulation withstand voltage test is also done. The test shows that when the frequency is in the range of 0.1-50Hz, the voltage distribution in the multilayer media is basically distributed according to the capacitance, so the 0.1Hz ultra-low frequency voltage withstand test can be equivalent to the power frequency withstand voltage test, which not only avoids the difficulty of using large capacity AC withstand voltage test equipment, but also reflects the insulation condition of the tested equipment. At present, ultra-low frequency withstand voltage test for motor end insulation is considered more effective than power frequency withstand voltage test.
