## 1 Introduction  
The presence of PDs in HV assets is a symptom of the existence of insulation defects. The electrical tree growth and phase-resolved PD (PRPD) patterns have been studied by different authors to prevent failures in the power grids [1; 2; 3]. Over time, these defects can lead to short circuits and corresponding blackouts in HV networks. The negative effects are loss of power supply, increase of direct and indirect costs (loss of profit), and eventual danger for human beings due to the explosions and fires involved. To avoid these negative consequences in HV grids, PD measurements are carried out by electrical companies. Utilities need to know the performance and functionality efficiency of commercial PD analysers to detect, recognise, and locate insulation defects that can provoke the aforementioned negative consequences. Despite the interest of electrical companies in qualifying the diagnostic tools of PD analysers, there is a lack of the standardised requirements needed. This article presents and validates, as a novelty, a procedure to qualify the diagnostic tools for PD analysers. On site, PD measurements under offline and online conditions have been increasing in the last few years as they represent a useful approach to detecting insulation defects. The IEC-TS 62478 [4] introduces PD measurements by applying electromagnetic and acoustic methods, but no requirements concerning PD analysers are stated on it. Performance tests related to PD quantities to be measured, minimum required sensitivity under standardised conditions and noise rejection capabilities are among the several tests that must be defined in the next IEC TS 62478 revision. Furthermore, the development of functionality tests related to diagnostic efficiency, in order to detect internal insulation defects and identify their location, is also very important to improve the present IEC-TS 62478. Functionality testing for the qualification of PD analysers [2] working in the HF range [3] according to IEC-TS 62478 [4] must focus on two basic concepts: their clustering capabilities to group pulsating signals from different insulation defects or pulsating noises, and their ability to recognise insulation defects. In practice, a simple way of PD clustering is to determine PD sources located in different sites along a HV grid, by applying the transient traveling wave theory [5; 6]. Therefore, the clustering capabilities of a PD analyser can be split into two different tests: the PD location test and the PD clustering test when various PD sources are located at the same site [7; 8]. This last clustering concept is called in this paper PD clustering test. Consequently, three tests have been designed: (1) PD recognition test, (2) PD clustering test, and 3) PD location test. The PD location test in HV grids can be split considering three different concepts: (3-1) Phase location, only applicable for three-phase AC systems, (3-2) PD location along a cable system and (3-3) PD location in different HV elements that are interconnected in the grid, for example, a cable system connected to a GIS. The PD location test along a grid can be performed using a conventional PD calibrator generating PD pulses with constant charge values, while the other two tests must be performed using representative pulse signals related to insulation defects and noise signals. Reference PD pulse trains related to HVAC and HVDC insulation faults in cable systems, GIS, and converters and different representative noises are described in Section 2. The purpose of the validation of this qualification procedure, promoted by a European project [9], is to verify whether the set of proposed tests is appropriate to detect and locate representative insulation defects that can appear in different subsystems of HVAC and HVDC grids. Gas Insulated Substations (GIS), Cable-Systems (Cable), Air Insulated Substations (AIS), and converters (IGBT, thyristors, etc.) installed in HVAC--HVDC grids have been considered.