Compact gas-insulated metal-enclosed systems for HVDC applications (HVDC GIS) have been developed for HVDC projects in need of space-saving installations, such as offshore platforms, cable transition stations or multi-terminal switching stations. Successfully type-tested HVDC GIS are on the market up to rated DC voltages of ±550 kV. However, until today, only little commercial operating experience is available. Hence, this contribution shares the experience with HVDC GIS application during commissioning and early operation phase for two dedicated installations in Germany – one ±320 kV installation in the North Sea at the offshore converter platform DolWin kappa of the 900 MW DolWin6 grid connection system and one ±400 kV installation in an onshore cable transition station of the 600 MW Kontek interconnector between Denmark and Germany.
Although the IEC standardization is still under development within TC17 SC17C WG42, both installations were successfully routine tested and commissioned based on the CIGRE recommendations of the technical brochure no. 842 about “Dielectric testing of gas-insulated HVDC systems”. Routine tests and on-site tests were utilizing a high alternating voltage to rely on the vast available experience of ultra-high frequency partial discharge (PD) measurement under alternating voltage. For both installations, partial discharge signals were detected during on-site testing. In the offshore project, the source of the detected partial discharge signals was identified as external disturbance introduced by the test environment and eliminated. Thus, the HVDC GIS installation was PD free at any time. In the onshore project, the partial discharge source was identified as contamination in one of the HVDC GIS enclosures and was removed by cleaning. In both projects, the repeated high-voltage AC test with partial discharge measurement was successfully passed without any further indication of internal or external partial discharges. Extensive experience with AC GIS installations has shown that the absence of detectable failures during on-site dielectric testing leads to the best achievable condition for commercial operation.
Additionally for the offshore project, a high-voltage test with direct voltage was performed as a special part of the commissioning. During the one-hour application of the direct test voltage, no special incidents have occurred, and the test was successfully passed. However, no partial discharge measurement was accompanying this DC test, since only little experience with partial discharge detection under direct voltage in real HVDC GIS systems exists and no commonly accepted and obvious evaluation methods are available.
To gain more experience with DC partial discharge detection in commercial HVDC GIS systems, a PD measurement under DC operating voltage with the HVDC converter system in operation was performed during the offshore commissioning phase of the converter platform DolWin kappa. For this, a commercially available on-site PD measurement device was used. Per pole two internal UHF sensors were available in the gas-insulated system and two external antennas were used to investigate the noise in the converter reactor room, where the HVDC GIS is located. Especially the time resolution of the utilized measurement device was identified as a major limit of result evaluation. Investigation results indicate that the measured signals are caused by external PD or noise. This is supported by the incident-free operation so far. Nevertheless, the results underline that a sufficiently high resolution in the time domain of the measuring device is the key for a meaningful partial discharge measurement analysis under direct voltage.
Finally, the two further successful commissioned HVDC GIS installations are enabling the sustainable integration of renewable energy into the existing transmission system. More installations, especially on offshore converter platforms in Germany, United Kingdom and United States are already in sight. Beyond that, the next step of increasing the sustainability of HVDC GIS installation is under development by using synthetic air as insulating gas with a global warming potential of zero.