Nowadays, HVDC systems have become a common solution for long distance power transmission. On the other hand, gas-insulated switchgear (GIS) is a proven technology in high voltage AC systems mostly driven by space saving and immunity to extreme weather conditions.
The combination of these properties makes it attractive for offshore HVDC application. Up to now, only a few HVDC GIS are in operation in Japan. Consequently, little service experience and limited information about the long-term capability of this type of technology is available.
International standards describing the requirements, methods and test procedures of HVDC GIS have not been developed yet. Additionally, due to lack of operational experience with HVDC GIS, a long-term test to prove the dielectric performance of a HVDC GIS prototype under service conditions is requested by the utilities.
PROMOTioN is a European Union project which seeks to enable meshed HVDC offshore grids based on cost-effective and reliable technological innovation in combination with a sound political, financial and legal regulatory framework. One of the aims of PROMOTioN is to demonstrate technology that overcomes hurdles, nowadays considered an obstacle for the deployment of meshed HVDC offshore grids. One of such demonstrators is the long-duration dielectric testing and performance evaluation of HVDC GIS under thermal load, a common task of ABB, DNV GL, Delft University of the Netherlands, Supergrid Institute of France, SHE Transmission of Scotland and TenneT TSO.
To define dielectric tests for HVDC GIS, the distinguishing characteristics of HVDC GIS have to be identified. In fact, a DC electric field is different from an AC field within a GIS. While in AC systems an electric field pattern based upon capacitance is established directly upon voltage application, in DC systems long transition periods are required to reach a steady state field whilst charge accumulation phenomena occur in the insulating materials and on their surfaces. In addition, DC electric field patterns depend strongly on temperature.
The difference of the dynamics of electric field between AC and DC GIS determines the context for defining dielectric test requirements of DC GIS. DNV GL takes the role of completing test requirements, methods and test procedures of the HVDC GIS based on the results of CIGRE WG D1/B3.57 and will perform a long-term prototype installation test on a 320 kV HVDC GIS in KEMA Laboratories’ HVDC test facility. A comprehensive document on test requirement will be made available to the PROMOTioN project (see https://www.promotion-offshore.net/) by WP 15 (Work Package). In a later stage, it will be combined with test results to contribute to pre-standardization of HVDC GIS, as for example has already been started in IEC AHG 37 of IEC TC 17.
The prototype installation test is proposed to demonstrate the long-term performance of the complete HVDC GIS and should normally be carried out after the type test has been carried out. The purpose of this test is to verify the reliable dielectric performance of an HVDC GIS under service conditions. Due to long periods that are required for obtaining steady state dielectric field and charge accumulation, a long duration test is recommended for a reliable representation of in-service dielectric stresses.
The recommended test will be performed on a complete 320 kV HVDC GIS, designed and assembled by ABB in KEMA’s HVDC laboratory. This GIS contains all the standard components, including fast acting earthing switches, earthing switches, disconnectors, a RC divider, a current sensor, two bushings, multiple PD sensors and (for the sake of this test) embedded heating transformers. A duration of more than one year is foreseen for the performance of such a test. It includes dielectric pre-tests, heating cycle voltage test and subsequent dielectric tests. This demonstrator will be subjected to load cycles based on SHE Transmission’s load pattern and intended to draw a full picture of the long-term performance of HVDC GIS. During this project, each partner has its own tasks covering various aspects of performance such as novel online PD monitoring and evaluation (Delft University), the impact of various SF6 gas alternatives (Supergrid Institute), temperature monitoring, discharge detection sensing, gas pressure/density monitoring etc.
Successful completion of HVDC gas-insulated switchgear (GIS) test prototype installation
PROMOTioN - New EU project to boost the development of meshed HVDC offshore grids in Europe
PROMOTioN: Progress on Meshed HVDC Offshore Transmission Networks