General Requirements
Measuring the power frequency discharge voltage is a mandatory item for both distribution-type (FS) lightning arresters and those with series gap metal oxide lightning arresters. For each lightning arrester, three power frequency discharge tests should be conducted, with an interval of no less than 1 minute between each test. The average value of the three discharge voltages is taken as the power frequency discharge voltage of the arrester.
For the operating FZ arrester, generally, a withstand voltage test under power frequency is not required. However, after disassembly and maintenance, or when necessary, the power frequency discharge voltage should be measured. The discharge voltage value should comply with the provisions of DL/T 596.
The wiring for the power frequency discharge test is the same as that for the general power frequency withstand voltage test. The waveform of the test voltage should be a sine wave. To eliminate the influence of high-order harmonics, if necessary, the power supply voltage of the voltage regulator or a filtering circuit should be added to the low-voltage side of the test transformer.
For metal oxide surge arresters with series gaps, a current meter should be connected in series at the lower end of the tested surge arrester to determine whether the gap has operated.
Selection of the protective R in the test circuit
The protection resistor R is used to limit the short-circuit current during the discharge of the lightning arrester. For FS-type lightning arresters without a parallel resistor, the value is generally 0.1 to 0.50. The protection resistor should not be set too large; otherwise, an arc cannot be established in the gap, causing the measured power frequency discharge voltage to be too high.
For ordinary valves with parallel resistors, they are lightning arresters. The power supply should be cut off within 0.5 seconds after the gap discharges. Therefore, an overcurrent instantaneous protection device should also be installed in the test circuit, and the industrial frequency current passing through the tested item should be limited to a range of 0.2A to 0.7A. Since the leakage current of the parallel resistor is relatively large, when approaching the discharge voltage, the voltage drop on the protection resistor is also large. At this time, a resistor with a lower resistance value can be selected, or the protection resistor can be omitted.
For the metal oxide surge arresters with series gap, since the resistance value of the valve plates is relatively high, the discharge current is relatively small. Therefore, the overcurrent trip relay should be adjusted to be more sensitive. Adjust the protection resistor to control the discharge current within the range of 0.05A to 0.2A. After the discharge, the power supply should be cut off within 0.2 seconds.
For FS-type lightning arresters without parallel resistors, the voltage rise rate should not be too fast (to avoid reading errors caused by the mechanical inertia of the instrument), and it is advisable to be 3 to 5 kV per second.
For lightning arresters with parallel resistors, when conducting a power frequency discharge test, the rising speed of voltage must be strictly controlled. This is because the heat capacity of the parallel resistor is small. When approaching the discharge point, if the rising time is too long, the parallel resistor may overheat and be damaged. Therefore, in GB7327, it is stipulated that during the power frequency discharge test, the time after the voltage exceeds the rated voltage (arc-extinguishing voltage) of the lightning arrester should be controlled to be within 2 seconds. Usually, the rising time from exceeding the arc-extinguishing voltage to the discharge of the lightning arrester should not exceed 0.2 seconds. To achieve this, the voltage regulating device can usually be modified to meet the requirements.
When testing a lightning arrester without a parallel resistor, the leakage current is very small before the gap breakdown. Under a sinusoidal voltage waveform, the discharge voltage of the lightning arrester can be calculated based on the reading of the low-voltage side voltmeter and the transformation ratio of the test transformer. The transformation ratio of the test transformer should be calibrated in advance, and the accuracy of the voltmeter should not be lower than 0.5 grade.
For lightning arresters with parallel resistors, the withstand voltage under power frequency should be directly measured at both ends of the tested lightning arrester. This can be done using a voltage transformer or a voltage divider with a resolution of 0.5 grade or higher, combined with an oscilloscope or other recording instrument. At the same time, the waveform of the discharge voltage can be observed. It should be noted that during the discharge, the power frequency voltage waveform will be superimposed with high-frequency oscillations. The amplitude of these oscillations may sometimes exceed the power frequency part. In this case, the instantaneous power frequency discharge voltage during the discharge should be taken as the discharge voltage. A damping resistor of several kiloohms can also be connected in series in the low-voltage circuit of the voltage divider to suppress the high-frequency oscillations. At this time, the voltage divider’s voltage division ratio needs to be recalibrated. An AC peak voltage meter should be used to measure the voltage, with an accuracy of no less than 1.0 grade. Attention should also be paid to eliminating the errors caused by the discharge high-frequency oscillations.
Post time: Mar-30-2026