Georgios Konstantinou

A Review of Multilevel Selective Harmonic Elimination PWM: Formulations, Solving Algorithms, Implementation and Applications

Selective harmonic elimination pulse width modulation (SHE-PWM) offers tight control of the harmonic spectrum of a given voltage and/or current waveform generated by a power electronics converter. Owing to its formulation and focus on elimination of low-order harmonics, it is highly beneficial for high-power converters operating with low switching frequencies. Over the last decade, the application of SHE-PWM has been extended to include multilevel converters. This paper provides a comprehensive review of the SHE-PWM modulation technique, aimed at its application to multilevel converters. This review focuses on various aspects of multilevel SHE-PWM, including different problem formulations, solving algorithms, and implementation in various multilevel converter topologies. An overview of current and future applications of multilevel SHE-PWM is also provided.

Performance evaluation of half-bridge cascaded multilevel converters operated with multicarrier sinusoidal PWM techniques

High-power high-voltage power electronics systems with fully-controlled semiconductors can benefit from the development of modular solutions based on voltage-sourced building blocks. This paper offers a performance comparison of various multicarrier sinusoidal pulse width modulation (PWM) techniques for the control of the modular multilevel converter (MMC) based on the half-bridge capacitor cell. It is found that the PWM method can significantly affect the ripple of the capacitor free DC-bus, hence the selection of the carrier frequency and the multi-carrier PWM method is an important design question for the MMC topology. Simulation results for a three-phase rectifier-inverter system generated with the PSCAD/EMTDC software package are provided to support the theoretical considerations.

Circulating Current Injection Methods Based on Instantaneous Information for the Modular Multilevel Converter

This paper studies different circulating current references for the modular multilevel converter. The circulating current references are obtained from the instantaneous values of the output current and modulation signal of the phase leg. Therefore, the determination of the amplitude and phase of the output current is not needed, which is a significant improvement compared to other methods such as those based on injecting specific harmonics in the circulating currents. Among the different methods studied in this paper, a new method is introduced, which is able to reduce the capacitor voltage ripples compared to the other methods. A closed-loop control is also proposed which is able to track the circulating current references. With the discussed methods, the average values of the capacitor voltages are maintained at their reference while the voltage ripples are kept low. Experimental results are presented to demonstrate the effectiveness of the proposed and discussed methods.

Power Balance of Cascaded H-Bridge Multilevel Converters for Large-Scale Photovoltaic Integration

Multilevel cascaded H-bridge converters are promising candidates for large-scale photovoltaic power plants. They allow direct connection to medium-voltage distribution networks without the presence of bulky line frequency power transformers. Owing to the stochastically variable nature of irradiance level, ambient temperature, and other factors, power levels in the three phases are expected to be unequal. The power imbalance condition creates unexpected problems with this topology, which was initially designed to operate under balanced power conditions. To deal with this issue, the paper proposes three novel zero-sequence injection methods as an expansion to the conventional zero-sequence injection method. Results obtained from simulations and a 430-V 8-kW three-phase seven-level cascaded H-bridge prototype are presented to verify the effectiveness and feasibility of the proposed methods.

Active Redundant Submodule Configuration in Modular Multilevel Converters

The modular multilevel converter (MMC) is based on the cascaded connection of identical submodules (SMs) enabling additional redundancies. This paper proposes the configuration of the MMC topology with redundant SMs and demonstrates the effects of active redundancies. The proposed configuration decreases the switching frequency per SM while reducing the SM capacitor voltage ripples. An analytical model for determining the SM capacitor voltage ripple and voltage dynamics is derived. The results from the analytical model are compared with the switching model for a 21-level MMC with five redundant (25 in total) SMs per arm. Experimental results based on a single-phase laboratory prototype with five SMs per arm and a single redundant cell further illustrate the operation and verify the derived mathematical model and simulation results.

Cascaded H-bridge multilevel converter for large-scale PV grid-integration with isolated DC-DC stage

The integration of large-scale PV systems to the grid is a growing trend in modern power systems. The cascaded H-bridge (CHB) converter is a suitable candidate for the grid interconnection due to its modular characteristics, high-quality output waveforms and capability of connecting to medium-voltage grids. However, the CHB converter requires isolated DC sources. In order to avoid the leakage currents caused by the high potential differences across the parasitic capacitance of the PV panels to ground, an isolated DC-DC conversion stage is required when the CHB topology is used. The objective of this paper is to compare two PV system configurations based on the CHB multilevel converter using two isolated DC-DC converter topologies, namely the boost-half-bridge (BHB) and the flyback, for their performance on providing isolation and achieving individual MPPT at the DC-DC power conversion stage of large-scale PV power systems. Simulation results from a 263 kW PV system based on a seven-level CHB converter with the two aforementioned isolated DC-DC converters are provided for comparison and evaluation with different input PV voltages.

Operation of a modular multilevel converter with selective harmonic elimination PWM

The modular multilevel converter (MMC) is based on the cascaded interconnection of half-bridge switching sub-modules and features modular characteristics that allow its expandability while providing high quality voltage and current output waveforms and removing the need for filtering. The switching frequency of each of the individual converter sub modules being maintained low and the balancing of the capacitor voltages are important requirements for the operation of the converter. Two different modulation approaches of the converter are discussed. Selected simulation and experimental results taken from a low-power laboratory prototype are presented.

A Modified Voltage Balancing Algorithm for the Modular Multilevel Converter: Evaluation for Staircase and Phase-Disposition PWM

This paper introduces a low complexity implementation of the voltage balancing algorithm aiming to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). The proposed algorithm features a relatively simple implementation without any conditional execution requirements and is easily expandable regardless of the number of submodules (SMs). Two modulation techniques are evaluated, namely the staircase modulation and the phase-disposition pulse width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 SMs per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype.

Power Balance Optimization of Cascaded H-Bridge Multilevel Converters for Large-Scale Photovoltaic Integration

Multilevel-cascaded H-bridge converters are promising candidates for next generation photovoltaic power converters. They feature reduced switching losses and higher conversion efficiency with modular structure; characteristics vital for large-scale photovoltaic power plants. However, the stochastically-variable nature of irradiance levels and ambient temperatures affects the normal operation of this topology, because power levels in the three phases can be unequal. The existing zero sequence injection method can deal with the power imbalance problem, but it is limited in its application. The paper proposes a zero sequence injection method to optimize the converter power balance, extending the converter operation with severe power imbalance. Based on the proposed optimal method, a simplified optimal zero sequence injection method requiring less calculation effort is derived and compared with the optimal method. Simulation and experimental results validate the effectiveness and feasibility of the proposed methods.

Hybrid Seven-Level Cascaded Active Neutral-Point-Clamped-Based Multilevel Converter Under SHE-PWM

This paper presents the hybrid seven-level cascaded active neutral-point-clamped (ANPC)-based multilevel converter. The converter topology is the cascaded connection of a three-level ANPC converter and an H-bridge per phase. The voltage of the H-bridge is actively maintained to the required level through selection of the switching states of the converter. The topology is operated under selective harmonic elimination pulsewidth modulation (SHE-PWM), maintaining the switching frequency of the converter to a minimum. The operating principles, voltage balancing methods, and limitations of the converter are analyzed together with extensive simulation results of the topology. Experimental results from a low-power laboratory prototype are presented that verify the operation of the hybrid converter under SHE-PWM.