High-voltage direct current (HVDC) technology are an economic and technically necessary solution to transmit electric power over long distances, to enable the grid access of offshore wind farms or to realize back-to-back connectionsb etween different countries. To ensure a high reliability of the HVDC assets like cables, converter transformers and gas-insulated systems over the whole life cycle, they are tested with respect to all possible service conditions. This includes a dielectric test with composite voltages which are realized by a superposition of a DC voltage and an impulse voltage.
IEC 60060-1 specifies a general test circuit to generate composite voltages. A device under test (DUT) is connected via two blocking and coupling elements to two voltage sources. These blocking elements protect one voltage source from the voltage of the other and simultaneously couple the voltages to the DUT. The choice of the blocking elements affects the voltage waveform and the reproducibility of the dielectric test. This work contributes to the understanding of the impact of different blocking elements on the composite voltage waveform, consisting of DC voltage and switching impulse (SI) voltages.
The test setup includes a capacitive DUT connected via blocking elements to one impulse voltage generator and one DC voltage supply. The blocking element on the impulse voltage generator side is either a blocking capacitor with different capacitances or a spark gap in various configurations. As blocking element on the side of the DC voltage supply is a water resistor with different resistances is used. All measured voltages are recorded time synchronously and evaluated according to IEC 60060-1. In this process, the three parameters of the standardised switching impulse voltage waveform: peak time TP, time to half-value T2 and the peak value UP, are determined from the composite voltage.
Derived from the measurement results the impact of different blocking elements on the composite voltage waveform is evaluated. Recommendations for the test setup and the test execution are given.