Voltage source converters are presented as the key devices for the future massive integration of distributed renewable energy resources in the network. This paper 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.Index Terms-Four-leg voltage source converter (4LVSC), voltage control, grid-forming, full state feedback law, nonlinear unbalanced system.

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The ever-growing penetration of Converter-Interfaced Distributed Renewable Energy Sources (CI-DRES) and the gradual decommission of synchronous generators (SGs) has posed several challenges related to the stability and robustness of the electric power systems. Since the main interface of the CI-DRES with the grid are the Voltage Source Converters (VSCs), there has been a major shift in the VSC control philosophy, where the absence of SGs is compensated by the Ancillary Services (ASs) provided by a new generation of CI-DRES. In order to make these ASs feasible and emulate the dynamic behaviour of a SG, the presence and advanced control of energy storage systems (ESS) together with the CI-DRES is essential, so that the CI-DRES/ESS has a dynamic behaviour similar to SG. In this paper, a new energy management control system is proposed for an ultracapacitor (UC) connected to the DC-bus of a CI-DRES. The aim is to control the DC bus voltage using the UC and, simultaneously, maintain the UC voltage within the limits while a given AS is provided. The control strategy is validated experimentally using a prototype with results revealing a reliable and stable operation.

Voltage source converters have become the main enabler for the integration of distributed energy resources in microgrids. In the case of islanded operation, these devices normally set the amplitude and frequency of the network voltage by means of a cascade controller composed of an outer voltage control loop and an inner current control loop. Several strategies to compute the gains of both control loops have been proposed in the literature in order to obtain a fast and decoupled response of the voltages at the point of common coupling. This paper proposes an alternative and simple methodology based on the introduction of a virtual conductance in the classic cascade control. This strategy allows to design each control loop independently, obtaining a closed-loop response of a first-order system. In this way, the gains of each control loop are easily derived from the parameters of the LC coupling filter and the desired closed-loop time constants. Furthermore, a state observer is included in the controller to estimate the inductor current of the LC filter in order to reduce the number of required measurements. A laboratory testbed is used to validate and compare the proposed controller. The experimental results demonstrate the effectiveness of the proposal both in steady-state and transient regimes.

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