Determination of AC test currents for thermo-electric laboratory stresses on gas-insulated HVDC systems

A particular challenge in the design of HVDC equipment is the accumulation of electrical charge carriers at gas-solid interfaces and in the bulk of the solid insulation material, which can cause locally increased electrical field stress. During operation, a current flows through the conductor, leading to an inhomogeneous temperature distribution between conductor and enclosure. Thus, the solid epoxy insulators are experiencing the whole temperature gradient. As the electric field distribution at DC voltage is mainly determined by the conductivities of the applied insulating media and the conductivity of epoxy resin is strongly temperature-dependent, the temperature distribution has a major impact on the electric field distribution and with this on the dielectric performance of the insulation system. Hence, testing of gas-insulated HVDC equipment always requires the consideration of high current and high DC voltage at the same time. An AC test current is typically used to generate the temperature gradient in the insulating system. However, the commercial operation of the HVDC equipment with DC current must be reflected, when thermo-electric tests in the laboratory are performed with AC current. This paper presents temperature-rise tests with different AC and DC test currents up to 5000 A, applied to commercial gas-insulated HVDC equipment. Based on the test data, different approaches for the determination of representative test currents for thermo-electric tests are discussed. In conclusion, a representative AC test current for the given HVDC equipment is proposed, which considers all major effects for thermo-electric testing. To reduce the practical efforts for future laboratory tests, the calculation approach for AC test currents of HVDC bushings (IEC/IEEE 65700-19-03) is evaluated with regard to a possible application to the tested gas-insulated HVDC equipment. The result is compared with the measured data. The paper shows the application of this standard to gas-insulated systems and proposes further possible optimizations.