A modified synchronverter for a weak grid with virtual power circles-based PQ decoupling scheme

The high penetration of renewable energy sources (RES) results in a low-inertia, weak power grid. To mitigate this and restore system inertia, it has been widely proposed to operate the inverters of RES units to mimic synchronous generators; this technology is known as a virtual synchronous generator (VSG). In weak grids there is, however, strong coupling between active power ($P$) and reactive power ($Q$), and any VSG technique therefore requires $PQ$ decoupling in order to operate effectively. This article proposes a new $PQ$ decoupling technique based on the transformation of the $PQ$ power circle of a VSG connected to a weak grid: first, the power circle is translated from its designed position to that of a conventional synchronous generator (SG) connected to a strong grid, achieving partial $PQ$ decoupling. Then, to achieve full $PQ$ decoupling, the authors propose further to modulate the radius of the translated $PQ$ power circle; this is achieved using a series resistance-capacitance–inductance ($RCL$) circuit which is virtually implemented in the VSG controller. The efficacy of the proposed scheme is validated using a modified synchronverter connected to the weak grid in representative loading scenarios. It is demonstrated that the technique achieves a decoupled $PQ$ control for the synchronverter connected in a weak grid. Moreover, the modified synchronverter is capable of supporting frequency and voltage regulation in the grid without inducing large transient grid currents during mild frequency and voltage variations.