Auxiliary Droop Frequency and Voltage Control for Grid-Forming Applied to Unbalanced Current Compensation

The virtual synchronous machine (VSM) has been a potential solution to control the grid-forming converters to integrate renewable energies (RES) into the grid. It is responsible for emulating the kinetic energy and static/dynamic properties of conventional synchronous generators to solve the stability problems due to decrease inertia in the systems with high levels of RES. This has been increasing in the distribution grids where is common the single-phase loads and the current unbalance, which causes stability problems as the intensified frequency oscillations and the frequency deviation in the system. The VSM control is common in converters present in the balance system. However, the VSM control implementation in converters present in the unbalanced system is not a straightforward task due as the unbalances intensify the power oscillations to cause the VSM to have abnormal behavior. Nevertheless, the frequency and voltage droops controls are commonly used in the converters present in the unbalanced system, but they are used as grid-following converters. Therefore, this paper proposes an auxiliary droop frequency and voltage control (AFV) scheme to integrate into a virtual synchronous machine (VSM-AFV) in order to improve a four-wire power system’s stability during an unbalanced system. This approach is based on droop frequency and voltage control, and its law control is obtained from a system’s active power and reactive power flow exchange equation. Changes in the active power of the VSM based on unbalances presented in the distribution grid are considered to show the performance of the VSM-AFV. Results show that the VSM-AFV controller presents lower ROCOF, steady-state errors in output reactive power, and output current of a four-leg three-phase voltage source converter (4-LVSC) when compared to the VSM approach.