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

Abstract

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 ((Formula presented.)) and reactive power ((Formula presented.)), and any VSG technique therefore requires (Formula presented.) decoupling in order to operate effectively. This article proposes a new (Formula presented.) decoupling technique based on the transformation of the (Formula presented.) 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 (Formula presented.) decoupling. Then, to achieve full (Formula presented.) decoupling, the authors propose further to modulate the radius of the translated (Formula presented.) power circle; this is achieved using a series resistance-capacitance–inductance ((Formula presented.)) 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 (Formula presented.) 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.