Large Scale Synthetic Laboratory Imitation of Transient Voltage Stresses of MMC-HVDC Links: Design Aspects on “Very Slow Front Overvoltages”

Inhalt:
The occurring overvoltages in High Voltage Direct Current (HVDC) systems utilizing Modular Multilevel Converters (MMC) are dependent on a multitude of influences and may significantly differ from normatively predefined overvoltage categories. Within recent scientific contributions it is identified that one type of those novel insulation stresses exceeds the time parameters which are assigned to the normatively predefined category of “slow front overvoltages”. As a consequence, the synthetic laboratory imitation of those novel types of overvoltages gains in importance. Therefore, this paper addresses laboratory testing concepts for this novel type of impulse, e.g. “very slow front overvoltages”, utilizing mainly typical available high voltage standard test equipment. Starting from the general prerequisites and design aspects for impulse testing with applicable test voltages ranging up to ±3.2 MV for lightning impulses (LI) and −2.7/+1.75 MV for switching impulses (SI) the required extensions for superimposed testing are addressed. Furthermore, the generation of (superimposed) impulse voltages with peak values up to 600 kV and the novel impulse characteristics with time to peak values ranging from 250 mus to above 5 ms and time to half values of approximately 50 ms and 100 ms are presented.