Some parameters and their relationships of partial discharge

1. The starting voltage and extinguishing voltage of discharge, when the applied voltage of the test object rises from zero and partial discharge is not visible, the minimum voltage at which the partial discharge exceeds a specified value observed from the test device during the boosting process is called the starting voltage of partial discharge; When the applied voltage on the test sample gradually decreases from a higher value exceeding the starting voltage of partial discharge, the highest voltage at which the partial discharge amount is less than a specified value is the partial discharge extinction voltage.

2. The relationship between the actual discharge quantity qc and the apparent discharge quantity. The actual discharge quantity is the charge that moves inside the medium during partial discharge, which cannot be measured. The apparent discharge quantity is a decrease of △ Uc in the voltage on the bubble for each partial discharge, and an increase in external voltage must supply an electric quantity qa. This qa can be measured using specialized instruments and calibrated using the formula qa=uc, Usually, the apparent discharge capacity qa is smaller than the actual discharge capacity qe, which is conducive to the judgment of test results. The calibration of the apparent discharge capacity is to determine the conversion coefficient of the test circuit. The principle of calibration is to inject a square wave with amplitude uo into both ends of the test object through a small capacitor Co in series, and the charge it charges is Q=UoCo (PC).

Inject the known charge Qo directly into the test sample. This is when the measurement system of the instrument responds with L ', remove the standard square wave generator, and perform a voltage test. When there is internal discharge, the measurement system responds with L, and there is a conversion coefficient k=L/L'. Therefore, the discharge amount of the test sample Q=UoCok (PC).

The partial discharge test of power transformers is also considered a destructive test when the internal discharge is large. Therefore, before the test, it is required to test the insulation to ensure that the surface of the test object, especially the bushing, is clean and dry. After oil injection, it should be allowed to stand still for sufficient 48 hours. Generally, impedance is measured by series connection of the terminal of the capacitive bushing of a transformer to ground; The partial discharge test power supply is the highest voltage Um, usually using an intermediate frequency power supply of 150-200Hz.

The experimental steps and pressurization time are as follows: first, increase the voltage to the measured voltage u2, u2=1.5um/√ 3, read and record the discharge amount, hold at u2 for 5 minutes, then increase the voltage to the pre applied voltage u1, u1=um, hold for 5 seconds, read and record the discharge amount. After 5 seconds, the voltage drops again to u2. At this point, u2 will persist for 30 minutes and then return to zero. Record the initial discharge voltage and extinction voltage values during the voltage rise to u2 and decrease to u2.

During the entire test process, if there is no breakdown of the test object, and the discharge Q measured at all measuring terminals is continuously maintained within the allowable value without any obvious growth trend, it is considered that the partial discharge is qualified.