During the AC test voltage, the commonly used partial discharge measurement procedure is as follows:

(1) Sample pretreatment

Before the test, the sample should be pretreated according to the relevant regulations:

1. Keep the surface of the test product clean and dry to prevent local squares caused by moisture or pollution on the insulating surface.

2. In the absence of special requirements, the sample should be at ambient temperature during the test.

3. After the previous mechanical, thermal or electrical action, the test product should be left for a period of time before the test, so as to reduce the influence of the above factors on the test results.

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(2) Check the partial discharge level of the test circuit itself

Do not connect the test product first, but only apply voltage to the test circuit. If no partial discharge occurs under the test voltage slightly higher than the test product, the test circuit is qualified; if the partial discharge interference level exceeds or approaches the maximum allowable discharge capacity of the test product 50% of the value, the source of interference must be identified and measures taken to reduce the level of interference.

(3) Calibration of the test loop

The instrument in the test circuit should be calibrated routinely before pressurization to determine the scale coefficient of the test circuit when the test product is connected. This coefficient is affected by the characteristics of the circuit and the capacitance of the test product.

Under the calibrated circuit sensitivity, observe whether there is a large interference when the high-voltage power supply is not connected or after the high-voltage power supply is connected, and if so, try to eliminate it.

(4) Determination of partial discharge inception voltage and extinguishment voltage

Remove the calibration device and keep other wiring unchanged. When the waveform of the test voltage meets the requirements, the voltage is added from the voltage far below the expected partial discharge inception voltage, and the voltage is raised at a specified speed until the discharge capacity reaches a specified value. The voltage at this time is the partial discharge inception voltage. Then the voltage is increased by 10%, and then the voltage is lowered until the discharge capacity is equal to the above-mentioned specified value, and the corresponding voltage is the extinguishment of partial discharge. When measuring, the applied voltage is not allowed to exceed the rated withstand voltage of the test object. In addition, repeated application of voltages close to it may damage the test object.

(5) Measure the partial discharge under the specified test voltage

It can be seen from the above that the parameters that characterize partial discharge are all measured at a specific voltage, which may be much higher than the partial discharge inception voltage. Sometimes it is stipulated to measure the discharge capacity under several test voltages, and sometimes it is stipulated to maintain a certain time under a certain test voltage and perform multiple measurements to observe the development trend of partial discharge. While measuring the discharge volume, it can also measure the number of discharges, average discharge current and other partial discharge parameters.

1. Measurement without pre-applied voltage

During the test, the voltage on the sample is gradually increased from a lower value to the specified value, and kept for a certain period of time before measuring the partial discharge, then lowering the voltage and cutting off the power supply. Partial discharges are sometimes measured during voltage ramp-up, ramp-down, or throughout the test period at the specified voltage.

2. Measurement with pre-applied voltage

During the test, the voltage is gradually increased from a lower value, and after exceeding the specified partial discharge test voltage, it rises to the pre-applied voltage, maintains it for a certain period of time, then drops to the test voltage value, maintains the specified period of time, and then measures at a given time interval partial discharge. During the entire period of voltage application, attention should be paid to the variation of the partial discharge quantity.