Modeling of High-frequency Electromagnetic Oscillation for DC Fault in MMC-HVDC Systems

DC short-circuit faults pose a hazard to the operation of a modular multilevel converter (MMC)-based high voltage direct current (HVDC) system, necessitating reliable fault clearing solutions with rapid reaction. However, because the parasitic capacitances of the main equipment oscillate with the lumped inductances of the HVDC system, strong electromagnetic oscillations with multiple frequencies occur during clearance transients. These oscillations will disturb the HVDC system's protection and control systems. Therefore, this paper focuses on the modeling of these oscillations. First, an equivalent circuit for the MMC-based HVDC system is proposed, taking into account the parasitic capacitances of the system's major components, such as DC reactors, connecting cables, and DC circuit breakers (DCCBs). Second, four distinct oscillation stages are postulated based on action coordination of MMCs and DCCBs, and the associated analytical equations for the oscillation frequencies are derived. Third, a 200 kV MMC-based DC converter station is subjected to an 6ms/6kA pole-to-pole (PTP) short-circuit test. Electromagnetic oscillations have a frequency range of several kHz to several hundreds of kHz. The measured waveforms correspond well with simulated results, including the parasitic characteristics. Additionally, the relative errors between the simulated and measured frequencies are less than 5%.