## 1 Introduction
### Reference PD Pulses and Pulsating Noise Trains
#### 2.1.2. PD Pulse Trains in GIS, Cable Systems, and AIS under HVDC Stress  
The insulation defects used in the recognition test under HVDC stress were also made using PD pulse trains generated in test cells applying DC voltage in positive and negative polarity. For each insulation defect, six test cells were designed, built, and tested, three for \begin{table}
\begin{tabular}{l l l l l l l l l l l l} \hline \hline \multirow{2}{*}{**Defect**} & \multirow{2}{*}{**Polarity**} & \multirow{2}{*}{**N\({}^{*}\) Trains**} & \multicolumn{3}{c}{**Figures per DP Pulse Train**} & \multicolumn{3}{c}{**0-50 s**} & \multicolumn{3}{c}{**50-150 s**} \\ \cline{3-11}  & & & **m (Pulses)** & & **q (pC)** & & **q (nC)** & & **1 (pA)** & **m (Pulses)** & **n (p/s)** & **1 (pA)** & **m (Pulses)** & **n (p/s)** \\ \hline Cavity & (+) & 538 & 11.1 & 75.2 & 0.8 & 15.3 & 10.2 & 0.2 & 0.95 & 1.3 & 0.0126 \\  & (\(-\)) & 569 & 8.8 & 72.0 & 0.6 & 11.6 & 8.0 & 0.2 & 0.78 & 1.1 & 0.0108 \\ \hline Floating & (+) & 371 & 3.4 & 899.5 & 3.1 & 59.9 & 3.3 & 0.1 & 0.90 & 0.1 & 0.0010 \\  & (\(-\)) & 195 & 3.2 & 1108.5 & 3.6 & 70.6 & 3.2 & 0.1 & 0.27 & 0.0 & 0.0002 \\ \hline Corona & (+) & 657 & 110,718.5 & 7115.6 & 787,829.1 & 5,252,194.1 & 36,906.2 & 738.1 & 5,252,194.1 & 73,812.3 & 738.1 \\  & (\(-\)) & 609 & 1,611,509.5 & 272.1 & 438,558.0 & 2,923,720.0 & 537,169.8 & 10,743.4 & 2,923,720.0 & 1,074,339.6 & 10,743.4 \\ \hline Surface & (+) & 427 & 93.4 & 560.9 & 52.4 & 349.3 & 31.1 & 0.6 & 349.3 & 62.3 & 0.6 \\  & (\(-\)) & 343 & 124.0 & 312.5 & 38.7 & 258.2 & 41.3 & 0.8 & 258.2 & 82.6 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main parameters of PD pulse trains of insulation defects under HVDC stress. positive polarity and another three for negative polarity. Insulation defects in SF\({}_{6}\) under HVDC stress with positive and negative polarity voltage were also generated by means of test cells with SF\({}_{6}\) (see Figure 1). A general database of around 200 PD event trains of each insulation defect related to GIS, Cable systems, AIS, and converters must be available to perform PD recognition tests and PD clustering tests. For example, PD event trains of each representative insulation defect related to Cable systems and AIS are shown in Table 2 (cavity, surface on air, floating potential, and corona). Of the total PD pulse trains generated with the test cells, 95% were used for training AI recognition tools. The remaining 5% (at least 10 pulse trains for each PD defect) were reserved for the PD recognition test. Each pulse train corresponds to measurements recorded during a time interval of 150 s. For surface and corona defects, this test time interval was divided into five 30 s sub-intervals due to a large number of PD pulses recorded. A direct voltage with a constant value u(t) = Uo = 30 kV was used for corona and surface DC tests and a growing direct voltage from 0 volts to 30 kV with an exponential growing (\(\tau\) = 16.6 s), according to formula (1) was applied for cavity and floating potential DC tests emulating a voltage change. Applying this formula is intended to achieve a voltage stabilisation of 99.75% Uo at 100 s and 99.99% Uo at 150 s. These time intervals enable the consideration of the slow dynamic changes of interest in the DC voltage. With an exponential voltage that increases with \(\tau\)= 16.6 s it is possible to achieve these stabilisation times.