Modular DC Test System

The demand to transport electric energy over large distances increases further. To fulfill this task, the rated DC voltages increased within the last decade. HVDC systems up to 800 kV are in operation and higher transmission voltages are under development. All HVDC equipment, e.g. HVDC valves, transmission lines (air or cables), HVDC transformers, bushings, insulators etc. have to be tested. To perform these development tests, type tests, routine tests and on-site tests, special HVDC and UHDC test systems are needed. This is only realizable with adequate test systems which have to operate at higher voltage levels than the rated voltages of the components to be tested. In doing this, DC test equipment are operating near at the physical and technical limits. This is because DC test equipment does not have to withstand pure DC voltages and currents. In case of a breakdown or flashover of the device under test, the DC test system has to withstand these high transient voltages, often followed by fast polarity changes. Therefore, the electrical field distribution, which mainly is controlled by a resistive field distribution (DC current plus space charges and surface charges), will be overlaid by a (fast) capacitive field distribution, which is in some cases opposite to the resistive field distribution. Area and space restrictions within test fields, the occurrence of fast transients (like in bushings filled with SF6), in combination with surface and space charges lead to very high and complex field stress of the DC test equipment. This implies strong electrical stress on the DC test system and the used materials. This paper will present some examples of special designed DC test equipment. Demanded DC test voltages reach 2000 kV or even more. Thus a realized DC test system with rated voltage of 2000 kV and with a DC current of 100 mA will be described. A concept how to protect the test object and the HV source, like DC-Generator and Impulse Generator used for combined voltage tests, will also be presented. In addition, the need of mobile test systems to perform on-site tests on cables and other HVDC components increases also. Main task here is to handle the field distribution under different environmental conditions and to realize a very compact and robust design. Finally, when testing long DC cables, the safe and reproducible discharging of these long cables is another issue. How to solve this task safe with adequate mobile DC test systems and discharging devices will be presented in this contribution also.