Alternative Methods for Testing the Pole-to-ground Insulation of Thyristor Valves in Uhvdc Systems

In the design of DC system, more than one MVU tower may be used between pole-to-neutral voltages. This is especially common for UHVDC systems. The insulation design of these MVU’s need to be verified by dielectric type tests as specified by international standards. However, the application of several MVU’s in series makes the test set-up complicated. One straightforward solution is to install all valve towers under the pole to neutral voltage for the dielectric tests. This multiple-tower solution brings with it the complexities and risks. This test set-up puts also high demands on the space and mechanical strength of the laboratory. From project engineering point of view, to assemble before the dielectric type tests and then to disassemble such a large test object is a labour and time consuming task. An alternative is to install only one valve tower with external voltage sharing components. Reduced complexity and risk are the main advantages to use the one-tower set-up. Such a one-tower set-up is a flexible solution with regard to the voltage at the terminals of valve towers. It has also advantages over multiple-tower set-up, with respect to requirement on laboratory conditions, labour and time consumptions. In this paper, the alternative test set-up’s for the MVU dielectric tests are presented.