There are unavoidable defects in the insulation of high voltage equipment (such as interspaced in solid insulation, bubbles in liquid insulation etc.). Partial discharge will occur when the electric field strength of the surface and the solid insulation of the air bubbles reaches a certain value. This discharge does not form a penetrating channel, but it produces heat and active gas (such as ozone, nitrogen oxide) local corrosion insulation, causing irreversible damage. If gradually expanded, the whole insulation breakdown or flashover. Partial discharge test can identify the characteristics of partial discharge, quantitative measurement of its discharge intensity, in order to find out the hidden dangers of product insulation quality and then take appropriate measures.
In this paper, there are frequently asked questions in the process of partial discharge test.
What is the main requirement for coupling capacitors when we do partial discharge of electrical equipment?
The main requirement of the coupling capacitor is that the coupling capacitor should not be partial discharge at the high voltage, otherwise it is impossible to distinguish whether the partial discharge is from the sample or from the coupling capacitor.
Why do we need to have a pre - applied voltage during partial discharge measurements?
In partial discharge of measuring equipment, including test standard in a short time with a high voltage value pre regulated voltage ratio, which is given to the actual operation of partial discharge is often due to excessive voltage excitation, voltage in advance is to artificially create a condition to simulate the overvoltage the actual operation, to observe the insulation level of partial discharge under specified conditions.
How to determine the initial voltage of partial discharge?
When the voltage rises slowly from a low value to a discharge value that has just begun to exceed a specified value, the voltage is called the initial voltage of partial discharge.
Method for determining extinction voltage of partial discharge
When the voltage applied to the test specimen is observed from the higher value of the partial discharge, the minimum voltage can be observed in the test circuit. In practice, the extinction voltage is the minimum voltage at which the partial discharge amplitude is equal to or less than a specified value.
Why the pulse square wave calibration should be carried out in partial discharge measurement?
Instrument for measuring partial discharge may not directly give the value of discharge depending on pulse numerical parameters. It must to simulate the partial discharge pulse through the known size standard square wave inject to test loop. The injection pulse can be generated by a square wave voltage generator with a magnitude of Uc, which is connected in series with a small known capacitance Cg, and the calibration pulse is equivalent to a discharge power of Qc, which is Qc=UcCg. The measured partial discharge amplitude are compared to pulse square wave in order to quantitatively determine the size of the partial discharge, which referred to square wave calibration.
Test procedure for measuring the voltage of partial discharge of transformer
The test voltage should be no more than 1/3 . The power supply is switched on, to boost voltage in advance,and keep 10s above, then non-stop to step-down voltage to measuring voltage, maintained above 1min, and then read the discharge quantity. Finally the test voltage drops below 1/3 measuring voltage, and cut off power supply.
This article is posted by Hannah.cao.(https://www.quora.com/profile/Hannah-Cao-4)
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