In recent years, photovoltaic systems designers have been started to pay attention in transformerless multilevel inverter topologies. This is due to they possess several advantages, such as, high efficiency, good performance, multiple voltage output levels and small size. In order to meet the safety requirement for transformerless grid-tied photovoltaic inverters, the leakage current has to be tackled carefully. To accomplish this requirement, common mode voltage has to be maintained in a constant value to avoid the current flow through the stray capacitances that appear between the photovoltaic array and the ground path. Due to the wide variety of available inverter topologies, in this paper a comparative analysis of efficiency between three neutral point clamped topologies is realized, besides modulations strategies are proposed to keep the common mode voltage constant to avoid leakage ground currents. Numerical results are performed in order to validate the analysis and modulation.

This paper studies the benefits of the virtual ground (VG) connection for single-phase transformerless grid-tied inverters that is used to mitigate the leakage-ground currents (LGC) issue. This work also proposes common-mode and differential-mode models of single-phase inverters connected to the grid through an LCL output filter, that are used as a theoretical support. It is shown that LGC are simply common-mode currents (CMC) circulating through the ground path, which represents a contribution in the development of an approach to find solutions in general scenarios. In most of the cases, these currents are produced by the existence of common-mode voltages (CMV), which find a way through the ground-path established after the stray capacitances in the terminals of, for instance, photovoltaic (PV) panels. To prove the advantages of the proposed interconnection and for analysis, the single-phase full-bridge version of a two-level conventional inverter, as well as an NPC inverter are considered for model derivation and experimental validation.

In this paper, a solution to mitigate the leakage-ground currents in a grid-tied H-bridge neutral-point clamped inverter is presented. This solution consists in splitting the LC filter capacitor and connecting each capacitor to a DC-link terminal. The proposal is referred to as the DC-link-tied LC output filter. Out of this hardware modification, it is not necessary to increase the number of switches nor design a new modulation scheme. Furthermore, a formal analysis is provided based on the common-mode model describing the role of this particular connection of the output LC filter on the minimization of the leakage-ground current. In addition, a closed loop control strategy has been implemented to allow testing the prototype connected to the electrical grid. The mitigation of the leakage-ground currents has been evaluated under this condition as well. To validate the proposed circuit modification, numerical simulations and experimental tests have been performed in a 1 kW laboratory prototype of the H-bridge neutral-point clamped inverter with the DC-link-tied LC filter solution.

This work presents the design of a control law based on the average model of a shunt active power filter considering an H-bridge neutral point clamped topology and its experimental validation. Therefore, the proposed controller is formed by three control loops, namely current (inner), regulation (outer), and balance control loops. The current loop aims to compensate both the displacement power factor and the harmonic distortion produced by nonlinear loads connected to the point of common coupling. To deal with harmonic current distortion, the current loop involves an adaptive mechanism based on a bank of resonant filters tuned at odd harmonics of the fundamental grid frequency. The regulation and balance loops are aimed to maintain the voltage of the capacitors forming the DC-link at a desired constant level. For this, proportional-integral controllers are designed. The design of all three loops considers the average model of the system. The performance of the proposed multi-loop control law is evaluated through numerical results and real-time experimental implementation, both considering a 2 kW academic benchmark with a constant switching frequency of 7 kHz. In order to provide harmonic distortion, a nonlinear load based on an uncontrolled diode bridge rectifier is considered. Additionally, step-load changes from 0.5 kW to 1 kW are considered for the nonlinear load. As a result, a suitable current tracking, voltage regulation, and balance are observed despite parametric uncertainties, load variations, and harmonic distortion. As a consequence, in steady state, simulation results indicate that the compensated grid current THD is 1.75%; meanwhile, the nonlinear load current THD is 52.5%. Experimental results indicate that the compensated grid current THD is 2.32%; meanwhile, the nonlinear load current THD is 53.8%.

This paper presents a control method to balance, in average, the DC-link capacitor voltages while injecting power to the grid using an h-bridge neutral-point clamped five-level inverter grid-tied through an LCL filter. The method exploits the structure of a particular model representation of the system comprising a differential-mode part and a common-mode part, which turn out to be decoupled from one another. These decoupled representation enormously facilitates the control design as it can be performed as two independent processes as well. The first design process involves the differential-mode model part and focuses on the grid current injection, while the second design process focuses on the balance of the DC-link capacitors voltages, and is based on the common-mode model part. Out of this, two control components are derived, namely differential-mode and common-mode control components, which are later recombined to form the two control signals, one per inverter branch, required as references in a conventional modulation algorithm to generate the switching sequence. Numerical and experimental results are performed to evaluate the performance of the proposed controller for the h-bridge neutral-point clamped inverter subject to those interconnections.