## 2 Design and Implementation of On-Line PD Monitoring System
### Module for Conditioning and Analog-to-Digital Signal Processing of PD Pulses  
The schematic diagram of electronic modules for conditioning and analog-to-digital processing of PD pulses is presented in Figure 20. Conditioning of the UHF signals is realized in an electronic circuit consisting of two low noise RF amplifiers basing on a chipset SPF5043Z of maximal amplification of \(G=18\) dB at 900 MHz, tunable bandpass filter, and a system of peak detector based on multistage demodulating logarithmic amplifier type AD8313. Application of a peak detector circuit (also called frequency converter or envelope detector) is a common practice in the case of the registration of UHF pulses because it allows to significantly reduce the sampling frequency [99; 100]. Due to that, it is possible to process the PD pulses and determine their parameters in real-time. Unfortunately, this is done at the expense of losing information on time-domain parameters (e.g., rise and fall time) of individual pulses. The AE signals conditioning module consists of a low-noise preamplifier (with 40 dB gain) based on high-speed instrumentation amplifier AD8421 BRZ, active bandpass filter (20-500 kHz) based on Sallen-Key architecture and voltage follower (based on AD813 ARZ op-amp) to buffer the output. Since the PD pulses registered with the use of sensitive HFCT sensor may reach the amplitude of even a few volts, it was decided to give up on the dedicated conditioning system and to process the  
Figure 19: Comparison of typical partial discharge time waveforms and power spectral density (PSD) characteristics registered for (**a**) PD in oil, and (**b**) surface PD with the use of selected prototype and commercial (RFCT-4, HFCT 140/100) HFCT sensors. raw HF signals. At the input of the analog-to-digital signal processing module, there is A/D converter type AD9266 BCPZ-40 with 40 MS/s sampling rate and 16-bit resolution. To each PD sensor, a separate A/D converter was allocated. Figure 21 shows exemplary time waveforms of conditioned AE and UHF pulses and raw HF pulses registered during laboratory tests of the signal conditioning modules and A/D signal processing module. During the tests, in the oil-filled transformer tank, the surface discharges on a pressboard sample were generated. Their apparent charge was in the range between 400 and 5300 pC. As one can see in Figure 21a, the application of the peak detector circuit at a relatively low sampling frequency equal to 40 MS/s allows to effectively detect the UHF pulses. A comparative analysis of the results obtained with the HF method demonstrated that the UHF module detected all PD pulses and it confirmed the high sensitivity of the ISO-002-HF sensor. Despite the resignation from the HF signal conditioning module, pulses registered with the high-frequency current transformer had on average 1.8 times higher amplitude than the strongly amplified UHF pulses. The placement of the HFCT sensor on the ground wire of the electrode system, nearby the PD source, certainly had a high influence on obtaining such a good result. In the case of the AE method, the pulses of comparable to the UHF amplitude were registered. In Figure 21b the visible time differences (\(T=272\)\(\upmu\)s) in UHF and AE signal arrival result from the different velocity of EM wave propagation (approx. 2/3 speed of light in vacuum) and acoustic wave in  
Figure 21: Exemplary time waveforms registered during laboratory tests of the PD signal conditioning modules: (**a**) conditioned UHF PD pulses and raw HF PD pulses, (**b**) conditioned single UHF PD pulse and conditioned acoustic emission (AE) PD pulses. Figure 20: Schematic diagram of PD signal conditioning modules and A/D signal processing module and 3D drawing showing the mechanics of these modules: A = signal conditioning boards, B = analog-to-digital signal processing board, C = internal power supply module. oil (1400-1500 m/s).