Evaluation of the voltage withstand capability of electrical equipment insulation.

A technical means to test and evaluate the insulation withstand voltage capability of electrical equipment. Insulation structures need to be used to isolate the live parts of all electrical equipment from the grounded parts, or from other non-equipotential live bodies, 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 (unit is 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 to the insulation structure will cause the entire equipment to lose its insulation performance. Therefore, the overall insulation capability of the equipment can generally only be expressed by the test voltage (unit: kV) it can withstand. The insulation withstand test voltage can indicate the voltage level that the equipment can withstand, but it is not equivalent to the actual insulation strength of the equipment. The specific requirement for power system insulation coordination is to coordinate and formulate the insulation withstand test voltage of various electrical equipment to indicate the insulation level requirements of the equipment. The insulation withstand voltage test is a destructive test (see insulation test). Therefore, for some key equipment in operation that lacks spare parts or needs a long time to repair, you should carefully consider whether to conduct the insulation withstand voltage test.

When various electrical equipment in the power system are running, in addition to withstanding AC or DC working voltage, they will also suffer from various overvoltages. These overvoltages are not only high in amplitude, but also have waveforms and durations that are very different from the working voltage. Their effects on insulation and the mechanisms that may cause insulation breakdown are also different. Therefore, it is necessary to use the corresponding test voltage to conduct the withstand voltage test of electrical equipment. The insulation withstand voltage tests specified in Chinese standards for AC power systems include: ① short-time (1 minute) power frequency withstand voltage test; ② long-term power frequency withstand voltage test; ③ DC withstand voltage test; ④ operating shock wave withstand voltage test; ⑤Lightning shock wave withstand voltage test. It also stipulates that the insulation performance of 3 to 220kv electrical equipment under power frequency operating voltage, temporary overvoltage and operating overvoltage is generally tested by a short-time power frequency withstand voltage test, and the operating impact test is not required. For electrical equipment of 330 to 500kv, the operating impact test is required to check the insulation performance under operating overvoltage. The long-term power frequency withstand voltage test is a test conducted for the condition of internal insulation degradation and external insulation contamination of electrical equipment.

Insulation withstand voltage test standards have specific regulations in each country. Chinese standards (GB311.1-83) stipulate the baseline insulation level of 3-500kv power transmission and transformation equipment; 3-500kv power transmission and transformation equipment lightning impulse withstand voltage, one-minute power frequency withstand voltage; and 330-500kv power transmission and transformation equipment Impulse withstand voltage for electrical equipment operation. The electrical equipment manufacturing department and the power system operation department should comply with the standards when selecting the items and test voltage values for the withstand voltage test.

Power frequency withstand voltage test

Used to test and evaluate the ability of electrical equipment insulation to withstand power frequency voltage. The test voltage should be sinusoidal and the frequency should be the same as the power system frequency. It is usually specified that a one-minute withstand voltage test is used to test the short-term voltage withstand capability of the insulation, and a long-term withstand voltage test is used to test the progressive deterioration inside the 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 in a dry surface state, a voltage withstand test in an artificially simulated atmospheric environment (such as a wet or dirty state) is also required.

AC sinusoidal voltage can be expressed in terms of peak value or effective value. The ratio of peak value to effective value is square root two. The waveform and frequency of the test voltage actually applied during the test will inevitably deviate from the standard regulations. Chinese standards (GB311.3-83) stipulate that the frequency range of the test voltage should be 45 to 55Hz, and the waveform of the test voltage should be close to a sine wave. The conditions are that the positive and negative half-waves should be exactly the same, and the peak value and the effective value should be the same. The ratio is equal to ±0.07. Generally, the so-called test voltage value refers to the effective value, which is divided by its peak value.

The power supply used for 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. Its rated output voltage should meet the test requirements and leave room for leeway; the output voltage of the test transformer should be stable enough to not cause the output to change due to the voltage drop of the pre-discharge current on the internal resistance of the power supply. The voltage fluctuates significantly to avoid measurement difficulties or even affect the discharge process. Therefore, the test power supply must have sufficient capacity and the internal impedance should be as small as possible. Generally, the requirements for the capacity of the test transformer are determined by how much short-circuit current it can output under the test voltage. For example, for the test of small samples of solid, liquid or combination insulation in the dry state, the short-circuit current of the equipment is required to be 0.1A; for the test of self-restoring insulation (insulators, isolating switches, etc.) in the dry state, the short-circuit current of the equipment is required Not less than 0.1A; for external insulation artificial rain tests, the short-circuit current of the equipment is required to be not less than 0.5A; for tests of specimens with larger dimensions, the short-circuit current of the equipment is required to be 1A. Generally speaking, test transformers with lower rated voltages mostly adopt the 0.1A system, which allows 0.1A to continuously flow through the high-voltage coil of the transformer. For example, the capacity of a 50kV test transformer is set to 5kVA, and the capacity of a 100kV test transformer is 10kVA. Test transformers with higher rated voltages usually adopt the 1A system, which allows 1A to continuously flow through the high-voltage coil of the transformer. For example, the capacity of the 250kV test transformer is 250kVA, and the capacity of the 500kV test transformer is 500kVA. Because of the overall dimensions of the higher voltage test equipment, Larger, the equivalent capacitance of the equipment is also larger, and 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 technical and economic difficulties during manufacturing. The highest voltage of a single test transformer in China is 750kV, and there are very few single test transformers in the world with a voltage exceeding 750kV. In order to meet the needs of AC voltage testing of ultra-high voltage and ultra-high voltage 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 a 2250kV test voltage. This is called a series test transformer. When the transformers are connected in series, the internal impedance increases very quickly and greatly exceeds the algebraic sum of the impedances of several transformers. Therefore, the number of transformers connected in series is often limited to 3. The test transformers can also be connected in parallel to increase the output current, or connected in a △ or Y shape for three-phase operation.

In order to perform power frequency withstand voltage tests on samples 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. There will be difficulties in realizing this kind of power supply device. Some departments have adopted power frequency high-voltage series resonance test equipment (see AC high-voltage series resonance test equipment).

Lightning impulse withstand voltage test

The ability of electrical equipment insulation to withstand lightning impulse voltage is tested by artificially simulating lightning current waveforms and peak values. According to the actual measurement results of lightning discharge, it is believed that the lightning waveform is a unipolar bi-exponential curve with a wave head that is several microseconds long and a wave tail that is tens of microseconds long. Most lightning is negative polarity. The standards of various countries around the world have calibrated the standard lightning shock wave as: apparent wave front time T1=1.2μs, also known as wave head time; apparent half-wave peak time T2=50μs, also known as wave tail time (see figure). The allowable deviation between the voltage peak value and waveform generated by the actual test device and the standard wave is: peak value, ±3%; wave head time, ±30%; half-wave peak time, ±20%; the standard lightning waveform is usually expressed as 1.2 /50μs.

The lightning impulse test voltage is generated by an impulse voltage generator. The transformation of the multiple capacitors of the impulse voltage generator from parallel to series is achieved through many ignition ball gaps, that is, multiple capacitors are connected in series when the ignition ball gaps are controlled to discharge. The speed of the voltage rise on the device under test and the speed of the voltage drop after the peak value 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 the standard impulse voltage wave are obtained by changing the resistance values of the wave head resistor and wave tail resistor. By changing the polarity and amplitude of the rectified power supply output voltage, the required polarity and peak value of the impulse voltage wave can be obtained. From this, impulse voltage generators ranging from hundreds of thousands of volts to several million volts or even tens of millions of volts can be realized. The higher voltage of the impulse voltage generator designed and installed by China is 6000kV.

Lightning impulse voltage test

Content includes 4 items. ①Impact withstand voltage test: It is usually used for non-self-restoring insulation, such as the insulation of transformers, reactors, etc. The purpose is to test whether these devices can withstand the voltage specified by the insulation grade. ② 50% impact flashover test: Usually self-restoring insulation such as insulators, air gaps, etc. are used as objects. The purpose is to determine the voltage value U with a flashover probability of 50%. With the standard deviation between this voltage value and the flashover value, other flashover probabilities can also be determined, such as a 5% flashover voltage value. U is generally regarded as the withstand voltage. ③Breakdown test: The purpose is to determine the actual strength of the insulation. Mainly carried out in electrical equipment manufacturing plants. ④Voltage-time curve test (Volt-second curve test): The voltage-time curve shows the relationship between the applied voltage to insulation damage (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 transformers and protective equipment such as arresters.

In addition to testing with the full wave of lightning impulses, sometimes electrical equipment with windings such as transformers and reactors also needs to be tested with truncated waves with a truncation time of 2 to 5 μs. Truncation can occur at the beginning or end of the wave. The generation and measurement of this truncated wave and the determination of the degree of damage caused to the equipment are all relatively complex and difficult. Due to its fast process and high amplitude, lightning impulse voltage test has high technical requirements for testing and measurement. Detailed test procedures, methods and standards are often stipulated for reference and implementation when conducting tests.

Operation impulse overvoltage test

By artificially simulating the power system operation impulse overvoltage waveform, the ability of the insulation of electrical equipment to withstand the operation impulse voltage is tested. There are many types of operating overvoltage waveforms and peaks in power systems, which are related to line parameters and system status. Generally, it is an attenuated oscillation wave with a frequency ranging from tens of Hz to several kilohertz. Its amplitude is related to the system voltage, which is usually expressed as several times of the phase voltage, up to 3 to 4 times of the phase voltage. Operation shock waves last longer than lightning shock waves and have different effects on the insulation of the power system. For power systems of 220kV and below, short-time power frequency withstand voltage tests can be used to approximately test the condition of 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, and short-time power frequency voltage tests can no longer be used to approximately replace operating impulse voltage tests. It can be seen from the test data that for air gaps above 2m, the nonlinearity of the operating discharge voltage is significant, that is, the withstand voltage increases slowly when the gap distance increases, and is even lower than the short-term power frequency discharge voltage. Therefore, the insulation must be tested by simulating the operating impulse voltage.

For long gaps, insulators and equipment external insulation, there are two test voltage waveforms to simulate operating overvoltage. ① Non-periodic exponential decay wave: similar to lightning shock wave, except that the wave head time and half-peak time are much longer than the lightning shock wavelength. The International Electrotechnical Commission recommends that the standard waveform of operating impulse voltage is 250/2500μs; when the standard waveform cannot meet the research requirements, 100/2500μs and 500/2500μs can be used. Non-periodic exponential decay waves can also be generated by impulse voltage generators. The principle of generating lightning shock waves is basically the same, except that the wave head resistance, wave tail resistance and charging resistance must be increased many times. A set of impulse voltage generators is commonly used in high-voltage laboratories, equipped with two sets of resistors, both for generating lightning impulse voltage and for generating operating impulse voltage. According to 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 Attenuated oscillation wave: The duration of the 01 half-wave is required to be 2000~3000μs, and the amplitude of the 02 half-wave should roughly reach 80% of the amplitude of the 01 half-wave. The attenuated oscillation wave is induced on the high-voltage side by using a capacitor to discharge the low-voltage side of the test transformer. This method is mostly used in on-site power transformer operating wave tests at substations, using the tested transformer itself to generate test waveforms to test its own voltage withstand capability.

The contents of the operating impulse overvoltage test include 5 items: ① operating impulse withstand voltage test; ② 50% operating impulse flashover test; ③ breakdown test; ④ voltage time curve test (volt-second curve test); ⑤ operating impulse voltage wave head Curve test. The first four tests are the same as the corresponding test requirements in the lightning impulse voltage test. Test No. 5 is required for operating shock discharge characteristics because the discharge voltage of a long air gap under the action of operating shock waves will change with the shock wave head. At a certain wave head length, such as 150μs, the discharge voltage is low, and this wave head is called the critical wave head. The critical wave length increases slightly with the gap length.

DC withstand voltage test

Use DC power to test the insulation performance of electrical equipment. The purpose is to: 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, use DC high voltage instead of AC high voltage to conduct voltage withstand tests on large-capacitance AC equipment.

The DC test voltage is generally generated by the AC power supply through a rectifier device, and is actually a unipolar pulsating voltage. There is a voltage maximum value U at the wave peak, and a voltage minimum value U at the wave trough. The so-called DC test voltage value refers to the arithmetic mean value of this pulsating voltage, that is, obviously we do not want the pulsation to be too large, so the pulsation coefficient S of the DC test voltage is stipulated not to exceed 3%, that is, the DC voltage is divided into positive and negative polarities. Different polarities have different mechanisms of action on various insulations. One polarity must be specified in the test. Generally, a polarity that severely tests the insulation performance is used for the test.

Usually a single-stage half-wave or full-wave rectifier circuit is used to generate high DC voltage. Due to the limitation of the rated voltage of the capacitor and the high-voltage silicon stack, this circuit can generally output 200~300kV. If higher DC voltage is required, the 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, where n represents the number of series connections. The voltage drop and ripple value of the output voltage of this device are functions of the number of series, load current and AC mains frequency. If there are too many series and the current is too large, the voltage drop and pulsation will reach intolerable levels. This cascade DC voltage generating device can output a voltage of about 2000-3000kV and an output current of only tens of milliamperes. When doing artificial environment tests, the pre-discharge current can reach several hundred milliamps, or even 1 amp. At this time, a thyristor voltage stabilizing device should be added to improve the quality of the output voltage. It is required that when the duration is 500ms and the amplitude is 500mA When the pre-discharge current pulse flows through once per second, the voltage drop caused does not exceed 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 the insulation withstand voltage test is also performed. Tests have shown that when the frequency is in the range of 0.1 to 50Hz, the voltage distribution inside the multi-layer medium is basically distributed according to the capacitance. Therefore, the voltage withstand test using 0.1Hz ultra-low frequency can be equivalent to the power frequency withstand voltage test, which avoids the use of large voltage withstand voltage. The difficulty of capacity AC withstand voltage test equipment can also reflect the insulation condition of the equipment under test. At present, ultra-low frequency withstand voltage tests are carried out on the end insulation of motors, which are considered to be more effective than power frequency withstand voltage tests.

Weshine Electric Manufacturing Co., Ltd.