Dynamic Voltage Stiffness Control Technique for a Virtual Oscillator-Based Grid- Forming Controller

Voltage stiffness is a vital parameter for grid-forming inverters. With a very high voltage stiffness, an inverter picks up all the reactive power demands of the point of common coupling. In contrast, with a very low voltage stiffness, the inverter does not participate in voltage regulation at all. Hence, any grid-forming controller should be able to control voltage stiffness dynamically. Virtual oscillator-based controllers (VOCs), the latest and most advanced grid-forming controllers, provide all the steady-state functionalities of the conventional droop controllers and, in addition, time-domain synchronization with a connected network. However, existing VOCs cannot control voltage stiffness dynamically. As a result, limiting reactive power output during a higher voltage sag, especially when connected to a weak grid, is challenging for VOCs. The only existing solution for a VOC in such conditions is to enter into current control mode, which leads to ineffective synchronization, output power oscillation, and, finally, inefficient grid-forming operation. Integrating a dynamic voltage stiffness controller inside a VOC is not as straightforward as a droop controller. Because, unlike phasor-based droop controllers, VOCs have time-domain implementation. In this article, we introduce virtual impedance-based dynamic voltage stiffness control technique for VOCs with systematic design procedures and rigorous stability analysis.