Energy Droop Control for MMC based Multiterminal HVDC Systems

This paper provides a comprehensive approach for modelling and development of a novel control method for Modular Multilevel Converter (MMC) based Multiterminal (MT) High Voltage Direct Current (HVDC) systems called Energy Droop Control (EDC). The fundamental control scheme for the design of the Energy Droop Control is based on the Voltage Droop Control (VDC). The VDC provides a droop characteristic that modifies the setpoint of the active power of each terminal of a MT HVDC system, if a deviation of the DC voltage as a consequence of an instantaneous active power disequilibrium is detected. The EDC is developed to react to the change of the energy stored inside the converter. Thus, it is obtainable to keep the DC voltage constant and to improve the stability of the MT system, since the active power disequilibrium is detected and eliminated before it affects the DC side of the HVDC system. Furthermore, a design specification for the droop constants of the VDC and EDC considering the power limits of the converters is developed. A simulation of a four terminal HVDC system using the EDC shows that the EDC is a suitable control approach as the power disequilibrium is eliminated and the DC voltage remains unaffected.