0 the introduction
Power capacitor long-term work under the high field strength, often due to various reasons such as inadequate impregnated in the process of producing cause partial discharge, lead to reduce the service life, serious accident happens. The traditional partial discharge measurement by electrical measuring method, measuring sensitivity caused by large capacitor capacity decreased, even unable to detect partial discharge signals. Sound measurement method is not sensitive to electromagnetic interference, partial discharge acoustic emission detection device developed by the author, partial discharge measurement by acoustic emission signal can be converted to facilitate storage and processing of digital signal, and then according to the characteristics of waveform analysis, spectral analysis, phi - q - n 3 d chromatogram fingerprint analysis and artificial neural network method to judge the fault types, fault can be located as the basis of design process and improve the capacitor. 
1 general structure
The detection device overall structure as shown in figure 1. Acoustic emission signals generated by partial discharge by post in the condenser, the four walls of acoustic emission sensors collect the will
It into electrical signals, after the preamplifier unit, signal preprocessing unit, high-speed adc to partial discharge acoustic emission analog signals into digital signals, using the computer for processing, analysis and display. The whole system of coordination and management control and processing by the CPU. 

Unit 2 sensor and preamplifier
Sensor and preamplifier unit capacitor internal partial discharge produced by acoustic emission signal is converted to electrical signals, and in the field will be treated as a weak signal amplification. The principle of the part as shown in figure 2. 

Acoustic emission sensors are the main components of piezoelectric crystal. Power capacitor in order to deeply research the different mode of acoustic emission signals produced by partial discharge frequency characteristic, chooses 02, 0 ~ 1 MHz broadband acoustic band emission sensors. In order to reduce the influence of interference in the process of weak signal transmission, and the converted electric signals in situ amplification processing, preamplifier, 0 02 ~ 1 MHz frequency band, gain is 40 dB. Between the preamplifier and power supply and signal conversion circuit coaxial cable provides 24 V constant current source, transmit signals at the same time, and then by the signal is transmitted in coaxial cable measuring device main body instrument box in the signal preprocessing unit.
Three pretreatment unit
Signal preprocessing unit designed for flexibility to choose the band-pass filter, used to extract the frequency signal component and remove some of the low frequency or high frequency of periodic disturbance. Further amplification signal after filtering processing to meet the requirements of A/D sampling card input amplitude. Its principle as shown in figure 3. 

Made from a mix of low-pass and high-pass filter, all of the 4 order butterworth unit. The lower limit and upper limit cut-off frequency can be controlled by the program, easy to switch. After filtering signal after follower impedance matching, program-controlled multiples attenuation and fixed amplification, to ensure the consistency of each frequency band under magnification. Driven by the buffer circuit output, make its have certain ability to load drivers, so that directly connected to the input impedance of 50 Ω A/D sampling card.
4 in adc and signal processing and display unit
Synchronous device adopts four road work of high-speed data acquisition card d/a conversion, sampling frequency up to 10 MHz, convert digits to 12, the data length 512 k per channel data points. Sampling and test voltage synchronization, so as to obtain the discharge phase information. Display unit can display waveform of discharge waveform, specify the data and related statistics.
5 trigger unit
Outside the power frequency voltage and computer together to create A trigger signal is added to the sample card to make A/D conversion and applied power frequency test voltage synchronization. The principle is shown in figure 4. 

6 simulation test
The device in the laboratory test with air bubbles in the insulation of the model to simulate discharge (see figure 5). Took some measures to make the acoustic emission signal attenuation soon, in order to use phi - q - n spectra method such as analysis, reach the purpose of improving pulse resolution. 

Figure 6 is the detector, a typical waveform were collected figure 7 for its spectrum. In the experiment, the applied voltage is 9 V, 500 by partial discharge instrument measured discharge pulse on the sensitivity correction after corresponding bureau's capacity for 9 PCS, and background noise suppressed under 1 pC. If don't use the anti-interference measures, such as active filtering signals submerged in noise, can't measure, and the noise of the bureau's capacity up to 100 PCS. In practice, by setting the appropriate filter band and the digital anti-interference measures, can be detected from the figure 6 shows, this kind of model of acoustic emission signal attenuation fast, duration of < 10 mu s, based on the partial discharge information, three-dimensional spectra fingerprints analysis to extract the discharge. Figure 6 the time domain waveform feature extracting, can the firing pattern identification. From figure 7 shows, characteristic spectrum peak appears in the vicinity of 100 kHz, in three other frequency range is higher spectral peak, all of these can be used to discharge pattern recognition. 

7 "theory
A. the test device is suitable for the laboratory and on-site partial discharge detection of power capacitor.
B. the device can capture the test product defects caused by acoustic emission signals generated by partial discharge, record the full waveform. And can be carried out on the recorded data waveform feature extraction, spectrum analysis and 3 d chromatogram fingerprint extraction, used for further analysis and research.