Impact on small-signal dynamics of using circulating currents instead of AC-currents to control the DC voltage in MMC HVDC terminals

The traditional approach for controlling the dc-voltage in Voltage Source Converter (VSC) HVDC terminals is to act on the reference for the active current or active power on the ac-side. For a Modular Multilevel Converter (MMC) with explicit control of the internally stored energy, this implies that the total energy sum must be controlled by acting on the dc-components of the circulating currents. However, the internal energy storage of an MMC acts as a buffer between the transient dynamics on the ac- and dc-sides. Thus, the dynamic response of the dc-voltage will depend on the closed loop dynamics of the internal energy control. Different system characteristics can be obtained if the reference signals from the dc-voltage control and the sum energy control are interchanged. As a result, the dc-voltage controller can provide the reference value for the dc-components of the circulating current, while the sum energy controller will provide the ac-side active current reference. In this paper, it will be demonstrated by time domain simulations and eigenvalue analysis that dc-voltage control by acting on the circulating current reference introduces a decoupling between the dynamics of the ac- and dc-side interfaces. This decoupling will also make the system dynamics less sensitive with respect to the operating conditions, which enables improved dynamic performances and less strict tuning requirements for the dc-voltage and sum energy controllers.