Distributed energy resources (DER) such as solar photovoltaic (PV) interfaces with the utility grid by high-efficiency power electronic converters. This equipment is sometimes underutilized in terms of its power capacity; thus, the remaining capacity can be used to provide support to the distribution system. In this sense, this work assesses the performance of the three-level T-type quasi-impedance source inverter (3L-T-type qZSI) injecting not only active power to the grid, but also providing support to it by injecting reactive power and acting as an active power filter (APF) simultaneously. The global control strategy of the grid-connected inverter is derived from the instantaneous $i_{d}$ - $i_{q}$ power theory. The combination of an improved deadbeat current controller and a level-shifted carrier-based pulse-width-modulation (PWM) technique, using both the upper shoot-through (UST) and the lower shoot-through (LST) alternating states, allows a successful tracking of the current references. Both the dc-link voltage control and the neutral-point balancing are properly achieved by taking the advantage of the shoot-through (ST) states and the redundant states simultaneously. These control actions are implemented by simple proportional-integral (PI) controllers. The simulation and experimental results demonstrate the above-mentioned functionalities and verify the stability and good dynamic response of the grid-connected 3L-T-type qZSI.

The modern concept of a universal converter is intended as a power converter suitable for application in both dc or ac grids using the same external connectors. This novel family was recently proposed to allow an easier integration of renewable energy sources and energy storage systems, interfacing with dc/ac grids and/or loads with a minimum redundancy of power switches and passive elements. This kind of solution and applications are expected to be a reality in the nearest decade, as ac and dc low voltage distribution networks will coexist. Nevertheless, the initial solutions proposed as universal converter were focused on ac single-phase power conversion systems. In this sense, this article proposes and describes a new member of the universal converter family suitable for dc to dc and dc to ac power conversion (both in three-phase three-wire and in three-phase four-wire). The proposed power topology is derived as a modular extension from the single buck-boost bidirectional cell. Its main operation modes (buck and boost) are discussed, and a pulse-width-modulation technique is developed to generate the corresponding switching patterns. The proposed solution is successfully validated in open-loop mode both in simulation and experimentally with a laboratory prototype. The measured efficiency of the power converter was above 97% in the dc to ac mode and around 99% in the dc to dc mode.

Distributed energy resources (DER) such as solar photovoltaics (PV) are required to achieve a high performance and an efficient use of generated renewable power in grid-integrated applications, both in normal and fault operating conditions. In this paper, a grid-following multi-functional control strategy for a three-phase three-level T-type quasi-impedance source inverter (3L-T-type qZSI) is studied through simulation and experimental tests. The grid-following functionality is achieved by a PI-based dq current controller for the active and the reactive power tracking. A PI-based dc controller is used for the dc-link voltage control by taking advantage of the quasi-impedance source network ability for input voltage boosting. The dc-ac power conversion is accomplished through a space vector pulse-width modulation (SVPWM) with inner capacitors voltages balancing capability and minimum common-mode voltage generation. Besides, a low voltage ride-through strategy was implemented to fulfill the fault requirements imposed by the Spanish standard. The simulation and experimental results demonstrate the above-mentioned functionalities and validate the stability and good dynamic response of the grid-connected 3L-T-type qZSI. Thus, this work supposes a novel contribution due to the few works previously reported in the literature concerning the performance of this relatively recent inverter topology in grid-tied applications.

Hardware-in-the-loop (HIL) testing methods by means of HIL units or real-time simulators (RTS), are becoming broadly accepted for validating of control systems in many technical areas. This new approach brings advantages such as the acceleration of the industrial and the research development cycle and offers a risk-free and flexible environment for the testing and the evaluation stages before the control deployment in the physical system. Nevertheless, the realization of such methodology can be very challenging in some large or complex plants. In this scenario, a proper simulation of the physical system, that is, the development of its accurate digital twin, demands a suitable and reliable sectioning of the different components into different parts running in parallel into different HIL units. This is the case of a nanogrid (nG) involving some prosumers with distributed energy resources (RES), and their corresponding multiple power electronic converters. These power electronic converters operate at high switching frequencies, which require a very low simulation time step, as well as a high number of input and output signals in closed-loop operation. This work analyzes in detail how to realize a digital twin of a two-prosumer residential nG including a photovoltaic (PV)-battery assisted three-level T-type (3L T-Type) inverter and managed by a hierarchical grid-forming (GFM) control structure by means of RTS based on the RT Box 1 by Plexim. The analysis allows to test the proposed GFM controls and the control hardware in steady and transient conditions, validating their accurate performance.

The recently proposed three-phase dual-purpose dc-ac/dc power converter (PC) as the new member of the universal converter family shows high potential for facilitating the integration of renewable energy sources (RESs) and energy storage systems (ESS) in any type of grid (dc or ac), aiming at high performance and versatility with minimal redundancy or modification. The proposed dual-purpose PC requires flexible, adaptive, reliable and scalable control strategies that align with the goal of providing a standardized solution for the power conversion stages in the coexisting ac and dc microgrids (MGs). In this paper, a model-free deep reinforcement learning (DRL)-based current control methodology is proposed for that dual-purpose PC. The non-linear and model-free nature of the proposed approach reduces the system sensitivity to parameter variations, increasing the system robustness and reliability; and can potentially reduce the development and operational costs since a detailed modeling and an extensive parameter tuning are avoided. Two DRL agents were trained by means of the twin delayed deep deterministic (TD3) policy gradient algorithm for both dc-ac and dc-dc modes of operation. Simulation results are provided to demonstrate the performance of the proposed control strategy.