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Reliability of transformer partial discharge test

time:2021/1/14   source:华天电力  reading:617 time

Partial discharge refers to the non-penetrating discharge between the electrodes of the insulating medium in high-voltage electrical appliances under the action of high electric field strength. In the transformer insulation system, the weak insulation in the local area will be excited by partial discharge. The partial discharge is not enough to penetrate the two electrodes to form a discharge channel, which is usually called breakdown. If the amount of partial discharge is reduced It is controlled below a certain discharge level and will not cause damage to the insulation, so the partial discharge test is a test for nondestructive detection of insulation characteristics.

After a certain partial discharge test voltage and a partial discharge pre-excitation voltage that is greater than the partial discharge test voltage and simulates the overvoltage during operation, measure the discharge capacity of the partial discharge television during the subsequent partial discharge test voltage duration, such as partial discharge television When the discharge capacity is less than the standard value, it is considered that the transformer can pass the partial discharge test. This test is stricter than the traditional short-time power frequency withstand voltage test, because the short-term power frequency withstand voltage test is based on whether there is breakdown in the insulation structure As a criterion for passing the test.

Partial Discharge Detector.png

The partial discharge test can detect the weak part of the insulation, and the detection of the partial discharge amount during operation can detect the potential weak part of the insulation. The short-term power frequency withstand voltage test can only detect whether the insulation structure can withstand the effects of various overvoltages or test voltages. It can either withstand or fail to withstand the potential for weak insulation. Therefore, local Discharge test is a relatively ideal insulation test item, and it is a test item that is being promoted and applied. Any transformer that can pass the partial discharge test has relatively high reliability in operation, so the partial discharge characteristics and testing should be carried out Research, make the transformer meet the requirement of low partial discharge level, some test transformers should also reach the level of no partial discharge.

In oil-paper-insulated transformers, the partial discharges that occur on the internal charged electrodes, the surface of the solid insulating parts (the interface between the oil and the insulating material) or the inside, and the inside of the transformer oil are collectively called partial discharges, which occur when they are surrounded by gas The partial discharge in the surface of the electrode or the nearby gas is called corona. The allowable partial discharge level of the transformer does not include the allowable partial discharge level generated by the corona of the bushing in the air, but refers to the partial discharge generated inside the fuel tank量level.

For a three-phase transformer, the partial discharge level of each phase can be measured in phases. For the partial discharge of each phase, it includes the partial discharge delivered by other windings to the measured winding. The high voltage, medium voltage and low voltage windings of each phase have their own partial discharge capacity. The partial discharge of each phase high-voltage winding (or medium-voltage or low-voltage winding) may come from line end bushings, mid-point bushings, on-load tap changer or non-excitation tap changer, lead wire, winding, various grounding Parts, insulation, transformer oil, etc., but the places that are prone to partial discharge are the air gap, the air gap inside the insulation, and the air bubbles in the transformer oil.

When a voltage is applied to the transformer, the electric field strength in the insulating medium is inversely proportional to the dielectric constant. For example, paper contains an air gap. The dielectric constant of the paper is higher than that of the air gap, so the air gap has to withstand higher The electric field strength of the air gap is low, and the air gap in the paper is the weak point of the insulation, which generates partial discharge.

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