## 7 Conclusions  
Three functionality tests related to PD diagnosis (a) PD recognition test, (b) PD clustering test, and (c) PD location test have been introduced. For the qualification of the diagnostic functionalities corresponding to PD recognition, a reference database of event trains related to single insulation defects and representative pulsating noises is required. This reference database of PD event trains should be created by means of round-robin tests, in which, different expert PD technicians must participate. For AC systems, phase-resolved PD patterns can be used to select the reference PD event trains, and for DC systems different graphs and PD patterns should be used to select the reference event trains: (a) Apparent charge of individual PD pulses versus onset times, (b) accumulated absolute charge versus time, (c) Monotonous Histogram of PD pulse charge values, (d) Histogram at distributed PD charge levels, (e) PD pattern \(\Delta\)q\({}_{n}\)\(-\)\(\Delta\)q\({}_{n-1}\), and (f) PD pattern \(\Delta\)t\({}_{n}\)\(-\)\(\Delta\)t\({}_{n-1}\). A qualification procedure with its requirements to approve PD recognition tools, PD clustering tools, and PD location tools has been developed, applicable for different grid sub-systems (Cable, AIS, GIS, and converters). This procedure has been implemented in a synthetic PD Simulator that plays PD pulse trains of representative insulation defects and noise signals for qualification of PD analyser functionalities. A round-robin test in which five research institutions participated (two metrological laboratories and three universities) have proven the robustness of the qualification procedure using the developed synthetic PD emulator. Applying this qualification procedure two PD methods for the detection and prevention of insulation defects have been approved, one for HVAC and the other for HVDC grids. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & & & **Location Error (m) or (\%)** & & & \\ \hline
**Distance to T1 (m)** & **L1** & **L2** & **L3** & **L4** & **L5** & **Max** \\ \hline C & 24 m & \(-1.0\) m & \(1.0\) m & \(-0.7\) m & +1.1 m & \(-1.0\) m & +1.1 m \\ A & 92 m & \(-0.43\%\) & \(0.00\%\) & \(0.19\%\) & \(1.63\%\) & \(0.05\%\) & \(1.63\%\) \\ B & 117 m & \(0.51\%\) & \(0.00\%\) & \(0.06\%\) & \(1.20\%\) & \(0.27\%\) & \(1.20\%\) \\ Max ABS (a) (m) & \(1.0\) m & \(1.0\) m & \(0.7\) m & \(1.1\) m & \(1.0\) m & \(1.1\) m \\ Max ABS (c) (\%) & \(0.51\%\) & \(0.00\%\) & \(0.20\%\) & \(1.60\%\) & \(0.30\%\) & \(1.63\%\) \\ \hline \end{tabular}
\end{table} 
Table 12: Location error. **Author Contributions:** [PERSON], [PERSON] and [PERSON] have defined the performance and functionality procedure, the acceptance tests, and the requirements for the qualification of a PD analyser. F.A. has supported them. [PERSON], A.-P.E., [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] reviewed the proposed qualification procedure. [PERSON] developed the implementation of the synthetic PD emulator that was used in the round-robin tests. [PERSON], A.R.M., C.M., [PERSON] and A.S. developed and supplied the different PD recognition and PD clustering AI tools for the qualification. [PERSON], [PERSON], [PERSON], A.C. and J.K. carried out the round-robin diagnosis tests. [PERSON], A.C. and F.A. developed PD pulse trains representative of converters under overvoltage and heating stress. [PERSON] carried out the analysis of the round-robin results and performed the paper writing. All the authors of the article participated in the discussion of the results and in the revision of the article, a special mention to [PERSON], [PERSON] and [PERSON] has participated in the editing and layout of this paper. All authors have read and agreed to the published version of the manuscript. **Funding:** This project 19 ENG02 FutureEnergy has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme.