A PLL-Free Grid-Forming Control With Decoupled Functionalities for High-Power Transmission System Applications

This paper proposes a grid-forming control strategy with a generic implementation approach to meet the transmission system requirements asking for the massive integration of power electronic devices into the power systems. In this context, several grid-forming controls have been proposed in the literature either with or without a Phase-Locked Loop (PLL). The PLL-based techniques allow decoupling the different control functionalities (i.e. inertia emulation, frequency support, active power setpoint tracking in steady-state) while the PLL-free schemes, which aim to avoid the PLL drawbacks, create a compulsory coupling between the control functionalities. The proposed grid-forming control in this paper is able to decouple the control functionalities without any dedicated PLL, which makes it more advantageous compared to what have been already proposed in the literature. Since the power converters are exposed to the small and large grid events, the presented control has been tested in both situations. For the small grid events, a simplified small-signal model is developed to assess the active power and frequency dynamics. In case of large grid events, a current limitation algorithm is included to the control in order to protect the power converter. To deal with the transient stability issues linked to the current limitation and enhance the converter performance during the post-fault, a method based on adaptive inertia constant is proposed. To validate the overall approach, time-domain simulations (in Matlab-Simulink) and experimentations are performed.