The so-called pulse current method is due to charge exchange during partial discharge, so that the pulsating voltage appears at both ends of the sample, and the pulse current appears in the loop connected to the sample. Therefore, the partial discharge can be obtained on the detection impedance in the loop. Signal to measure.
First, the test circuit
There are three basic circuits for measuring partial discharge by electrical method, as shown in Figure 2-19, where (a), (b) is the direct method measurement loop, and (c) is the balance method measurement loop.
The measured impedance Zm in Figure 2-19 is a component of a four-terminal network. It can be a single component of resistor R or inductor L. It can also be an RC and RL circuit in which the resistors and capacitors are connected in parallel or the resistors and inductors are connected in parallel. The resistors, inductors, and capacitors can also be used to form an RLC tuning loop. The frequency characteristics of the tuning loop should match the operating frequency of the measuring instrument. The measured impedance should have a frequency response that prevents the test power supply frequency from entering the instrument. Connect the connection impedance to the amplifier unit in the measuring instrument, usually a single shielded coaxial cable. The high voltage filter is a blocking impedance that reduces interference from the power supply and also increases the minimum measurable level of the measurement loop.
    The three test loops in Figure 2-19 can generally be selected according to the following basic principles:
    (1) Under the test voltage, if the power frequency capacitor current of the sample exceeds the allowable value of the measured impedance Zm, or the grounding part of the sample is fixedly grounded, the test circuit shown in Figure 2-19 (a) can be used.
    (2) Under the test voltage, the power frequency capacitor current of the sample conforms to the allowable value of the measured impedance Zm, and the test circuit shown in Figure 2-19 (b) can be used.
    (3) Under test voltage, Figure 2-19 (a), (b) When the test circuit has an excessive disturbance signal, the test circuit shown in Figure 2-19 (c) can be used.
    The corresponding measuring instrument can be selected according to the level of environmental disturbance. When the interference is strong, a narrowband instrument can be selected; when the interference is weak, a wideband instrument can be selected.
    The partial discharge measuring instrument is an instrument that measures the amount of apparent discharge, and is indicated by an oscilloscope and a digital discharge amount (PC) meter or a digital display. Each time the test loop is changed, the apparent discharge must be calibrated once. Calibration is available in both direct and indirect calibrations.
Second, the measurement procedure of the PD test
    (1) Sample preparation:
    1) The test sample should be kept clean and dry to prevent partial discharge due to moisture absorption or contamination on the insulating surface.
    2) During the test, the test sample should be at ambient temperature.
    3) After the last mechanical, thermal or electrical action, it should be allowed to stand for a while before testing to reduce the impact of the above factors on this test.
    (2) Check the level of partial discharge of the test circuit itself. Do not pick up the test product and pressurize the test circuit. If no partial discharge occurs at a test voltage slightly higher than the test voltage, the circuit is qualified; if the partial discharge interference level exceeds or approaches 50% of the maximum allowable value of the test discharge, the cause must be found and excluded.
    (3) Test loop calibration. The test loop should be calibrated prior to pressurization to determine the scale factor of the test loop when the test sample is accessed. Under the calibrated loop sensitivity, see if there is any large interference before and after the high voltage power is turned on, and if so, it should be excluded.
    (4) Determine the partial discharge starting voltage and the extinguishing voltage. After the calibration device is removed and boosted at a predetermined speed until the discharge amount reaches a certain value, the voltage is a partial discharge starting voltage. After that, the voltage is further increased by 10%, and then the voltage is reduced until the discharge amount is equal to the specified value, which is the partial discharge extinguishing voltage.
    (5) Measure the amount of partial discharge at the specified test voltage. The amount of discharge (PC) at the corresponding specified voltage is measured as required by the Pre-Regulation and is in accordance with the regulations.
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