Generalized formulation of multilevel selective harmonic elimination PWM: Case I-Non-Equal DC Sources

The paper presents optimal solutions for eliminating harmonics from the output waveform of a multilevel staircase pulse-width modulation (PWM) method with non-equal dc sources. Therefore, the degrees of freedom for specifying the cost function increased without physical changes as compared to the conventional stepped waveform. The paper discusses an efficient hybrid real coded genetic algorithm (HRCGA) that reduces significantly the computational burden resulting in fast convergence. An objective function describing a measure of effectiveness of eliminating selected order of harmonics while controlling the fundamental for any number of levels and for any number of switching angels is derived. It is confirmed that multiple independent sets of solutions exist and the ones that offer better harmonic performance are identified. Different operating points including five- and seven-level inverters are investigated and simulated. Selected experimental results are reported to verify and validate the effectiveness of the proposed method

A Seven-level defined selective harmonic elimination PWM strategy

Selective harmonic elimination pulse-width modulation (SHE-PWM) techniques offer an optimized control approach for a given converter and are therefore suitable for the low switching frequency high-power applications. Optimization techniques can be successfully used to obtain the solutions of the equations defining the SHE-PWM waveform. In this paper, a seven-level multilevel strategy (MSHE-PWM) defined on the line-to-neutral basis and based on a ratio of a variable number of angles distributed over three levels to be able to calculate the transition points is reported. The technique provides eighteen switching transitions for every quarter period in the standard modulation index range. In the overmodulation region, this can be changed in order to increase the gain of the modulator which in turn results in a compromised bandwidth. The switching angles as a function of the modulation index are reported for the standard as well as the overmodulation range. Selected simulation results are presented to verify the effectiveness and feasibility of the proposed method

On abolishing symmetry requirements in the formulation of a five-level selective harmonic elimination pulse-width modulation technique

Selective harmonic elimination pulse width modulation (SHE-PWM) techniques offer a tight control of the harmonic spectrum of a given voltage waveform generated by a power electronic converter along with a low number of switching transitions. These optimal switching transitions can be calculated through Fourier theory, and for a number of years quarter-wave and half-wave symmetries have been assumed when formulating the problem. It was shown recently that symmetry requirements can be relaxed as a constraint. This changes the way the problem is formulated, and different solutions can be found without a compromise. This letter reports solutions to the switching transitions of a five-level SHE-PWM when both the quarter- and half-wave symmetry are abolished. Only the region of high-modulation indices is reported since the low-modulation indices region requires a unipolar waveform to be realized. Selected simulation and experimental results are reported to show the effectiveness of the proposed method

Five-level selective harmonic elimination PWM strategies and multicarrier phase-shifted sinusoidal PWM: A comparison

The multicarrier phase-shifted sinusoidal pulse-width modulation (MPS-SPWM) technique is well-known for its important advantage of offering an increased overall bandwidth as the number of carriers multiplied with their equal frequency directly controls the location of the dominant harmonics. In this paper, a five-level (line-to-neutral) multilevel selective harmonic elimination PWM (MSHE-PWM) strategy based on an equal number of switching transitions when compared against the previously mentioned technique is proposed. It is assumed that the four triangular carriers of the MPS-SPWM method have nine per unit frequency resulting in seventeen switching transitions for every quarter period. Requesting the same number of transitions from the MSHE-PWM allows the control of sixteen non-triplen harmonics. It is confirmed that the proposed MSHE-PWM offers significantly higher converter bandwidth along with higher modulation operating range. Selected results are presented to confirm the effectiveness of the proposed technique

Evaluation of DC-link decoupling using electrolytic or polypropylene film capacitors in three-phase grid-connected photovoltaic inverters

The life expectancy and long term reliability of grid-connected three-phase photovoltaic (PV) inverters can be increased by replacing the conventional electrolytic film capacitors by metallized polypropylene film capacitors. This paper presents a detailed evaluation of a three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor-one. The minimum dc bus capacitance leads to larger voltage ripples. However, such ripples were found to be within acceptable limits to run the inverter satisfactorily. Simulation results are presented for a 15-kW grid-connected inverter at nominal voltage of 700V dc and experimental results are provided for a 3.0-kW system at a nominal voltage of 400V dc, built in the laboratory.Peer ReviewedPostprint (published version

Model Predictive Control for Distributed Microgrid Battery Energy Storage Systems

© 2017 IEEE. This brief proposes a new convex model predictive control (MPC) strategy for dynamic optimal power flow between battery energy storage (ES) systems distributed in an ac microgrid. The proposed control strategy uses a new problem formulation, based on a linear d-q reference frame voltage-current model and linearized power flow approximations. This allows the optimal power flows to be solved as a convex optimization problem, for which fast and robust solvers exist. The proposed method does not assume that real and reactive power flows are decoupled, allowing line losses, voltage constraints, and converter current constraints to be addressed. In addition, nonlinear variations in the charge and discharge efficiencies of lithium ion batteries are analyzed and included in the control strategy. Real-time digital simulations were carried out for an islanded microgrid based on the IEEE 13 bus prototypical feeder, with distributed battery ES systems and intermittent photovoltaic generation. It is shown that the proposed control strategy approaches the performance of a strategy based on nonconvex optimization, while reducing the required computation time by a factor of 1000, making it suitable for a real-time MPC implementation

Voltage balancing method for a seven-level stacked multicell converter using minimum-switching transitions

This paper presents a voltage balancing method for stacked multicell converters based on phase disposition pulse-width modulation. This method is based on minimizing a cost function to determine the optimum redundant state for capacitor voltage balance for each particular voltage level. The robustness of the proposed voltage balancing method is verified against static and dynamic unbalanced load conditions. Furthermore, a significant reduction in the switching frequencies of the power devices is achieved by using sawtooth carriers instead of standard triangular carriers without affecting the voltage balancing capability.Peer ReviewedPostprint (published version

Optimum state voltage balancing method for stacked multicell converters

This paper presents a voltage balancing method for stacked multicell converters based on phase disposition pulse-width modulation. This method is based on minimizing a cost function to determine the optimum redundant state for capacitor voltage balance for each particular voltage level. The robustness of the proposed voltage balancing method is verified against static and dynamic unbalanced load conditions. Furthermore, a significant reduction in the switching frequencies of the power devices is achieved by using sawtooth carriers instead of standard triangular carriers without affecting the voltage balancing capability.Peer ReviewedPostprint (published version

Harmonic elimination control of a five-level DC-AC cascaded H-bridge hybrid inverter

A five-level hybrid cascaded inverter operating under selective harmonic elimination (SHE) pulse-width modulation (PWM) control is discussed in this paper. The topology is a cascaded connection of a conventional three-phase, two-level inverter and an H-bridge module for each phase with a single DC-source. The topology boosts the output voltage within limits and with no additional DC-DC converters. However, such boosting feature depends on the control of the floating capacitor voltage and the load power factor. The regulation of the floating capacitor for the given modulation strategy is also analyzed. Experimental results taken from a single-phase laboratory prototype are presented to confirm the operational characteristics of the converter