Laboratory scale HVDC GIS test setup for DC/LI flashover studies

Abstract

The increasing adoption of High Voltage Direct Current (HVDC) transmission in recent years has been driven by the growth of long-distance bulk power transfer. This expansion is increasing the deployment of DC Gas-Insulated Switchgear (GIS) and Gas-Insulated Lines (GIL), which use pressurized SF6 for greater compactness than existing air-based insulation systems. DC operation, however, introduces charge accumulation on spacers, resulting in greater electrical stress on the insulation. The effect of the accumulated charges becomes particularly important during Lightning Impulse (LI) overvoltages and may cause spacer flashovers. Although this phenomenon has been studied for SF6, regulatory pressure is now driving a transition to alternative gases for which this behavior remains poorly understood.

The outcome of the thesis is a fully manufactured test cell with a camera arrangement to capture flashover across the insulator surface. A reduced-scale bowl plate electrode geometry is designed using Finite Element Method (FEM) simulations using an ion flow model to reproduce the quasi-homogeneous field distribution and surface charge accumulation behavior of HVDC GIS disc spacers, and the design is validated for operation at pressures up to 3 bar with technical air. A pressurized floating enclosure was also designed, dimensioned, and manufactured to safely contain alternative gases at a maximum operational stress of 30 kV DC with a superimposed ±150 kV LI. A composite voltage test circuit was then designed from available laboratory equipment and validated in LTspice.

Verification in the lab revealed repeatability issues with the spacer electrode contact, which were resolved by redesigning the spacer electrode to use a screw arrangement. Surface charge measurements showed poor agreement with the initial simulation, with additional ionization sources in the test setup not being accounted for in the model. Comparisons with external data, however, showed good agreement with measured surface potentials. The composite test circuit using a spark gap and a coupling capacitor has been tested in the TU Delft HV lab, and the test experience is presented. Further work must be pursued on charge measurements under DC application in technical air, and validation of the test circuit with higher applied voltages and LI breakdown tests with pressurized gas. With this remaining work completed, the cell can be used to systematically measure and visualize spacer flashover under composite DC/LI voltages in alternative gases.