By hvhipot 
The principle of the partial discharge tester is the pulse current method principle, that is, when a partial discharge is generated, an instantaneous voltage change Δ u is generated at both ends of the sample Cx. At this time, if the electrical Ck is coupled to a detection impedance Zd, the loop will be A pulse current I is generated, and the pulse voltage information generated by the detection impedance of the pulse current is detected, amplified and displayed, and then some basic parameters of partial discharge (mainly discharge q) can be measured.
When the authority’s radiometer shows the discharge waveform chart based on the test results, it needs to analyze the discharge situation of the test product according to some characteristics of the waveform. To analyze the waveform of the partial discharge tester, you can refer to the following points.
(1) The simulation test of the preliminary spectrum analysis in the laboratory is in good agreement with the actual measurement of the on-site high-voltage equipment
1) The spectra of different types of discharge (including interference) are different.
2) The internal discharge wave process is related to the loop parameters, but the waveforms reflected by different discharges are different.
(2) The discharge waveform is related to the discharge type and discharge amplitude.
1) There is no rapid discharge process through the electrode or gap, and the spectral characteristics are not much different.
2) Discharges that penetrate between the gaps or discharge a large amount of discharge have a longer wave process, heavy low-frequency components, and low-frequency oscillation wave tails. The main frequency component of the spectrum is 40~80kHz, and other high frequency components account for a small proportion.
3) Discharges in the air, such as corona, bubble discharge, and other discharges with small amplitude, are steep, and the rising edge is about 0.5μs. Spectrum analysis shows that there are abundant high-frequency components.
4) When the discharge is concentrated in the field strength, the discharge volume is small, the discharge basically has no oscillation, and the high-frequency component of the discharge accounts for a large proportion, and it has a larger component in the 600kHz fashion. When the discharge is a discharge that penetrates the electrode, oscillation usually occurs. The discharge process in oil is longer, up to 200μs, while the discharge process in air is less than 100μs.
Therefore, when using a partial discharge tester, for faulty large-scale discharges, the characteristics of the discharge can be comprehensively judged by combining waveform changes and frequency characteristics.
(3) Transformer type test product has inductance
The discharge signal will change after passing through the inductance, and the high frequency components will be weakened. The frequency spectrum characteristics of the signal measured at different points of the transformer for the same discharge depends on the discharge location. If the electrical distance between the discharge point and the two measurement points is close, the difference is different. The measured signals are consistent. With this feature, the type and location of the discharge can be judged according to the changes in the waveform and frequency characteristics at different points.
(4) The impact of high and low voltage transformers
Since the attenuation between the high and low voltage of the transformer can be up to 10 times, if the discharge is close to the low-voltage side or the neutral end, the signal measured on the high-voltage side of the 5000pC discharge will be less than 500pC, which may mistakenly regard the transformer as a fault-free generation.
At this time, waveform analysis can be used to determine whether the equipment has faulty discharge. When the fault discharge is critical, the discharge does not occur every cycle, and sometimes it will not discharge for several seconds.
Frequency characteristic analysis is performed on the entire wave process of each discharge. Therefore, when a certain frequency component is much larger than other frequency components, especially when the frequency component below 100kHz is large, it is not easy to see other frequency components in the amplitude spectrum. Therefore, it is necessary to further analyze the wave head (high frequency part) of each wave process separately, and it is also very valuable to observe the change of its frequency.