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This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup. [ABSTRACT FROM AUTHOR]

Reliability of power systems can be improved by the use of voltage source inverters (VSIs) with fast dynamic control, which can handle uncertainties induced by renewable energy resources and nonlinear loads. To ensure fast-dynamic response and simplified controller design, various model predictive control (MPC) methods are investigated in this paper. In the conventional MPC design for three-level VSIs, the cost function requires an additional term to ensure the balance of the neutral-point voltage (NP-V), which however can considerably impact the main goal of the MPC. To address this issue, an enhanced MPC using virtual space vectors is proposed, which allows the use of a significantly reduced weighting factor for the term in the cost function for NP-V balancing, while retaining fast-dynamic response. Additionally, the proposed MPC leads to reduced harmonic distortion in the VSI output voltage and current. Both simulation and controller hardware-in-the-loop studies are performed to demonstrate the effectiveness of the proposed enhanced MPC in an operating microgrid under both steady-state and fast-transient conditions. [ABSTRACT FROM AUTHOR]

Output filters of the grid inverter usually have a large footprint in the total system. In order to increase the power density of the grid inverter, a passive integrated structure for the output filter with flexible multilayer foils is presented. Compared with a discrete output filter, the volume of the integrated filter is reduced by at least 20%. In this paper, the design of the integrated output filter is discussed. Then, a distributed electromagnetic component model of the integrated filter is used to verify the proposed design. Finally, a prototype of the integrated output filter is built and used in a grid inverter. Experimental results are included and compared with the results derived from the discrete output filter to prove the validity of this integrated filter design. [ABSTRACT FROM AUTHOR]

There is increasing interest in the use of transformer-less systems for grid-connected photovoltaic applications. Compared to transformer-coupled solutions, transformer-less systems offer a typical efficiency increase of 1–2%, reduced system size and weight, and a reduction in cost. However, the removal of the transformer has technical implications. In addition to the loss of galvanic isolation, dc current injection into the grid is a potential risk. While desirable, complete mitigation of dc current injection via convention current control methods is known to be particularly challenging. For this reason, this paper proposes an active dc suppression method, in which the dc current injection is accurately determined by extracting the line-frequency component from inverter dc-link current measurements, and then, mitigated with an active closed-loop controller. Experimental results from a laboratory grid-connected inverter system are presented to demonstrate the high performance of the proposed technique. [ABSTRACT FROM AUTHOR]

Transformer-less grid-connected PV inverters are an attractive technology choice due to their high-efficiency performance and relatively small volume. However, in the absence of galvanic isolation, dc current can potentially flow into the grid and adversely affect the grid performance. In addition, as the sinusoidal ac output is significantly larger than the dc injection component, accurate dc current sensing at the inverter output remains a technical challenge. As a result, this paper proposes a two-stage dc voltage detection technique that employs an isolated RC attenuation circuit and a software dc-component filtering algorithm. The principle of the proposed technique can be applied in many switch-mode power converter topologies. The simple and cost-effective combination facilitates dc component extraction within the accuracy of 1 mV; moreover, this can be further used as digital dc feedback for active current control purposes. Experimental results from a laboratory prototype demonstrate the high performance of the proposed dc extraction and suppression approach. [ABSTRACT FROM AUTHOR]

This paper proposes a new diode-clamped multilevel inverter that can effectively balance the voltages of dc capacitors. For the proposed inverter, the inductors are inserted between the diodes and the main switches. Also, the voltage balance of the dc capacitors can be guaranteed by controlling the opening time of the corresponding main switches in particular modes. Thus, the parallel circuits on the dc side in the conventional inverters are canceled. Consequently, the proposed topology requires fewer switches, diodes, and capacitors, although it adds the compensation currents to the corresponding main switches. Both the simulation and experimental results demonstrate the suitability and applicability of the proposed topology. [ABSTRACT FROM AUTHOR]

In this paper, a new topology of two-stage cascaded switched-diode (CSD) multilevel inverter is proposed for medium-voltage renewable energy integration. First, it aims to reduce the number of switches along with its gate drivers. Thus, the installation space and cost of a multilevel inverter are reduced. The spike removal switch added in the first stage of the inverter provides a flowing path for the reverse load current, and as a result, high voltage spikes occurring at the base of the stepped output voltage based upon conventional CSD multilevel inverter topologies are removed. Moreover, to resolve the problems related to dc source fluctuations of multilevel inverter used for renewable energy integration, the clock phase-shifting (CPS) one-cycle control (OCC) is developed to control the two-stage CSD multilevel inverter. By shifting the clock pulse phase of every cascaded unit, the staircase-like output voltage waveforms are obtained and a strong suppression ability against fluctuations in dc sources is achieved. Simulation and experimental results are discussed to verify the feasibility and performances of the two-stage CSD multilevel inverter controlled by the CPS OCC method. [ABSTRACT FROM AUTHOR]

The LCL grid-connected inverter has the ability to attenuate the high-frequency current harmonics. However, the inherent resonance of the LCL filter affects the system stability significantly. To damp the resonance effect, the dual-loop current control can be used to stabilize the system. The grid current plus capacitor current feedback system is widely used for its better transient response and high robustness against the grid impedance variations. The weighted average current (WAC) feedback scheme is capable to provide a wider bandwidth at higher frequencies but show poor stability characteristics under weak grid scenarios. To overcome this shortcoming, the improved WAC damping method is proposed with an additional capacitor current feedback loop. In this paper, a systematic parameter design guideline for the optimal selection of the current loop proportional resonant controller and the additional capacitor current feedback coefficient are presented for the improved WAC feedback control strategy. The satisfactory range of the system control parameters can be obtained under different delay conditions to meet the system performance specifications. The improved WAC method enhances system robustness under weak grid scenarios, and the stability and robustness have been enhanced under control delay. Finally, the experimental results are presented to validate the effectiveness of the proposed improved WAC control strategy and the parameter design method. [ABSTRACT FROM PUBLISHER]

For power conversion systems interfaced to four-wire supplies, four-leg converters have become a standard solution. A four-leg converter allows good compensation of zero-sequence harmonics and full utilisation of the dc-link voltage. These are very important features when unbalanced and/or nonlinear loads are connected to the system. This paper proposes a three-dimensional space vector modulation (SVM) algorithm and provides a comprehensive analysis of the algorithm implemented on a three-level, four-leg neutral-point-clamped converter. The algorithm allows a simple definition of the different switching patterns and enables balancing of the dc-link capacitor voltages using the redundancies of the converter states. A resonant controller is selected as the control strategy to validate the proposed SVM algorithm in a 6 kW experimental rig. [ABSTRACT FROM AUTHOR]