How to achieve partial discharge in a transformer

　　Power transformers are very important equipment in power systems. Partial discharge measurement is very effective in judging the insulation status of transformers, and it has been used as one of the important detection methods to measure the quality of power transformers.

　　High-voltage power transformers mainly use oil-paper barrier insulation, which is composed of electrical paper layers and insulating oil. Due to the complex structure of large transformers, the insulation is very uneven. When improper design causes internal defects such as excessive local field strength, poor craftsmanship, or external factors, partial discharge will inevitably occur in the transformer, which will develop gradually, and then cause damage to the transformer. The internal partial discharge of power transformer mainly occurs in the following situations:

　　(1) Breakdown of the oil passage in the oil-paper barrier insulation in the middle of the transformer winding;

　　(2) Breakdown of the oil passage at the winding end;

　　(3) Breakdown of the oil gap close to the insulated wire and electrical paper (lead insulation, lap insulation, phase insulation);

　　(4) Breakdown of the longitudinal insulating oil passage between the coils (between turns and cakes);

　　(5) Branch discharge of insulating cardboard enclosures, etc.;

　　(6) Creepage of other solid insulation;

　　(7) Discharge of other metal foreign bodies infiltrated in the insulation, etc.

　　Therefore, for the transformers that have been shipped from the factory, partial discharge tests must be carried out in the following situations:

　　(1) Perform a partial discharge test before the new transformer is put into operation, and check whether there is any insulation damage during the transportation and installation of the transformer after it leaves the factory.

　　(2) Carry out partial discharge test on the transformer after overhaul or modification to judge the insulation condition after repair.

　　(3) Make further qualitative diagnosis of transformers suspected of having insulation faults during operation, such as discharge faults in gas chromatographic analysis of oil, and other abnormal conditions involving insulation.

　　The neutral point support method of Chongqing transformer partial discharge test usually adopts the phase-by-phase test method, and the test power supply generally adopts 100~150Hz double frequency power generator set. When there is no frequency multiplier power supply on site, the power frequency phase-by-phase support and pressurization method can also be used for the test, because the insulation structure of large transformers is more complicated, and the phase-by-phase pressurization method can also help determine the fault location. The pressurization method can adopt low-voltage side pressurization, and the test voltage can be obtained by induction on the high-voltage side. When the frequency multiplier power supply is used to pressurize, the main insulation and longitudinal insulation can be assessed at the same time. However, if the power frequency power supply is used for the test, the test voltage can only be increased to 1.1~1.2 times the rated voltage due to the limitation of overexcitation.

　　When the electrical measurement method finds that the transformer has a value exceeding the standard or a larger individual pulse, the electrical measurement method can be used for multi-terminal calibration and multi-point measurement to roughly determine the discharge location. First, use the split-phase test to determine which phase of the transformer the discharge is in, and then connect the detection impedance in series to the transformer's high-voltage, medium-voltage, and neutral point bushings and the iron core grounding point, and connect a coupling capacitor on the low-voltage side ( 1000-6000pF), the detection impedance is serially connected, as shown in Figure 2 and Figure 3. Therefore, when the transformer is tested for a certain phase, there can be 4-5 measuring points. Inject the standard calibration square waves with high pressure to ground, low pressure to ground, medium voltage to ground, and iron core to ground respectively. Correspondingly, the response coefficient of a certain injection point square wave is measured at each measuring point, and the correction of each point is recorded coefficient. After the calibration is completed, the pressure is applied for measurement, and the measured values of each measurement point are calculated with a certain injected correction coefficient. If the results calculated by the parameters of a certain point are close to each measurement point, the discharge position is near the correction point. For example, there is a fault discharge pulse at the A-phase high-voltage terminal. When the high-voltage terminal is used for calibration, the correction response coefficient measured at the high-voltage measuring point is K11, the neutral point is K21, and the coefficient is K41 at the core side. The low voltage side is measured as K31, see Table 1, and then the calculated value of each point during the measurement is calculated by K11 for high pressure, K21 for neutral point, K31 for iron core, and K41 for low pressure. The 4 results calculated from this should be similar.

　　Partial discharge measurement of large transformers, due to poor field equipment conditions and large interference, brings certain difficulties to accurate testing. Therefore, how to conduct the test according to the actual conditions of the site and what anti-interference measures are adopted are more important issues in the test.

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