Response of MMC-HVDC Systems to Geomagnetic Disturbances

This paper presents a detailed investigation of the performance of MMC-based HVDC systems in the event of a solar Geomagnetic Disturbance (GMD). Detailed models of an MMC and submodules are used to acquire the accurate behavior of the MMC during GMDs. Under such conditions, the current harmonics arising from the DC shift in the transformer flux are distributed between the MMC and grid, distorting the current and voltage waveforms. In this study, a model for the distribution of the current harmonics is developed. It is revealed that the distribution of the second harmonic current is affected not only by the MMC impedance but also by the output current and voltage of the converter. The simulation results in the EMTP show that with the constant power factor control strategy, the MMC does not respond to the increase in reactive power consumption by the transformer. Consequently, a solution approach is proposed to minimize the contribution of the grid to the reactive power demand increase. This control approach provides the saturated transformer with reactive power through the MMC, improving the grid voltage stability. The proficiency of the proposed solution is verified through time-domain simulations.