## 2 PD Pattern Recognition Based on MCNN  
### PD Data Acquisition of Transformer  
According to the common types of insulation defects in transformer operation, four typical PD models are made. These PD models are used to simulate the transformer tip discharge, surface discharge, air gap discharge and suspended discharge. In the experiment, a PD detection system based on ultra-high frequency (UHF) sensors is used. The sampling rate of the system is 4 GHz, and the bandwidth is 1 GHz. During the experiment, the voltage is stepped up and (UHF) sensors are used to collect PD data from the four PD models. The collected PD data is used to build a sample database of PRPD maps for subsequent feature extraction and pattern recognition. The specific test steps are listed as follows:1. A PD insulation defect model is selected and connected to the partial discharge test circuit. 2. The background noise of the laboratory is determined. Before the voltage is applied, the signal detected by the signal acquisition system is the background noise of the PD test environment. 3. The inception voltage of the PD is determined. The voltage of the regulating transformer is increased uniformly and slowly in steps. When the PD signal is detected by the monitoring instrument, the corresponding voltage is recorded as the initial voltage U\({}_{1}\). 4. The PD data is generated and collected at different voltage supplies. The voltage is continuously increased, and when it is greater than 1.5 times U\({}_{1}\), a stable and distinct PD signal series is recorded in the monitoring system. The voltage is then still increased to approximately twice U\({}_{1}\). The PD data generated during this process is collected by the UHF sensors. The sensors can record the partial discharge signals of different intensities generated by the partial discharge model at different voltage levels. 5. Different types of insulation defect models are replaced. After completing the PD test of the current PD insulation defect model, the voltage is slowly lowered and the power is turned off. Different types of insulation defect models are replaced. After completing the PD test for the current PD insulation defect model, the voltage is slowly reduced and the power is turned off. The discharge bar is used to discharge the test circuit, and the insulation defect model is replaced with another one. Then, the experiment steps (2) to (5) are then repeated until all four types of PD data are collected. The PRPD spectra obtained from the PD test are shown in Figure 1, in which each spectrum contains 50 power frequency cycles of discharge data. In the PRPD spectra, by observing the vertical and horizontal coordinates of a pulse point, we can directly obtain information about the phase and amplitude of the pulse. Figure 1: PRPD data of different PD types. (**a**) tip discharge, (**b**) surface discharge, (**c**) air gap discharge, and (**d**) suspended discharge. In the experimental process of the tip discharge model, with the increase of applied voltage, a cluster of PD signals appear at the positions with the phase of 270\({}^{\circ}\). This is because in 50 Hz AC, the AC voltage peaks at 270\({}^{\circ}\) in phase. The electrons are most active and more easily repelled at the moment when the voltage is at a negative peak, which coincides with the easy excitation of a negative polarity tip. With the further increase of voltage, the second cluster discharge signal appeared at the 90\({}^{\circ}\) phase position. This is because the voltage reaches a positive peak at 90\({}^{\circ}\) phase. When the voltage applied on the conductor is positive polarity, more energy is required for electron excitation, so the overall discharge amplitude at this time is lower than the first cluster signal at position 270\({}^{\circ}\). Furthermore, the distribution phase width is narrower than that at position 270\({}^{\circ}\). Suspended discharge is produced by suspended potential.