Voltage Control of Four-Leg VSC for Power System Applications With Nonlinear and Unbalanced Loads

Voltage source converters are presented as the key devices for the future massive integration of distributed renewable energy resources in the network. This article presents a novel approach to control a three-phase four-leg voltage source converter for grid-forming operation. The objective of the controller is to generate a balanced three-phase voltage with a given amplitude and frequency at the point of common coupling. The proposed control algorithm works over the stationary axes. It is based on full state feedback law in combination with a resonant control loop tuned at the fundamental frequency in order to guaranty zero steady-state error on the voltage. The main advantage of the controller with respect to the classical cascade controllers is that this strategy is not modified depending on the type of load connected at the point of common coupling. Moreover, a systematic methodology to compute the controller gains is presented by solving an linear quadratic regulator problem that considers an extended model. This method guarantees small signal stability and provides active damping to the system. A laboratory testbed with different type of loads is used to validate and compare the proposed algorithm with the classical one. The experimental results demonstrate the effectiveness of the proposal by achieving low levels of harmonic distortion and imbalances in steady-state as well as a fast transient response.