In this paper, the principle of modularity is used to derive the different multilevel voltage and current source converter topologies. The paper is primarily focused on high-power applications and specifically on high-voltage dc systems. The derived converter cells are treated as building blocks and are contributing to the modularity of the system. By combining the different building blocks, i.e., the converter cells, a variety of voltage and current source modular multilevel converter topologies are derived and thoroughly discussed. Furthermore, by applying the modularity principle at the system level, various types of high-power converters are introduced. The modularity of the multilevel converters is studied in depth, and the challenges as well as the opportunities for high-power applications are illustrated.

Modular Multilevel Converters for HVDC Applications: Review on Converter Cells and Functionalities

This paper is the 2nd part of the survey titled “High Voltage Direct Current Transmission - A Review, Part I”. The main converter technologies and HVDC systems' components will be discussed in this complementary paper.

High voltage direct current transmission - A Review, Part II - Converter technologies

Connecting legs in parallel is a common way to increase the output current of a power converter. Interleaving the carriers used to modulate the reference signals for each leg leads to a reduction in the output current ripple without resorting to increasing the switching frequency. A whole set of shifted carriers is required if interleaved pulse-width modulators (PWMs) are used. Implementing this by means of a digital signal processor (DSP) means that the higher the number of carriers, the higher the number of DSP timing resources required. Provided that the latter are usually limited, this could be a drawback when increasing the number of interleaved carriers. This study presents the implementation of a PWM scheme where all modulators use the same carrier offering the same results as if a set of n interleaved carriers were used. Since the proposed algorithm takes maximum benefit from the PWM units available in a DSP, a higher number of legs connected in parallel can be controlled without adding any external processing hardware. A MATLAB/Simulink model has been set up for simulation purposes. Selected experimental results obtained from a three-leg single-phase inverter are reported, confirming the effectiveness of the proposed implementation.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-pel.2013.0545

Minimum signal modulation scheme based on a single carrier for interleaved operation of parallel phase legs in voltage source converters

This paper presents a voltage changer optimized to multiply or to divide the input voltage by an integer factor of 2, 3, 4, ..., with a very high efficiency. The paper first introduces a review of the quasi-continuous current transformer converter in a step by step fashion. The last step presents the full bidirectional zero voltage switching (ZVS) converter. The theoretical study demonstrates the way to achieve ZVS operation by only decreasing the value of the transformer magnetizing inductance. ZVS conditions (developed in the paper) lead to an interesting way to control the converter, and a discrete variable frequency control is proposed. Allowing triangular current in the transformer leads to ZVS in the expanse of higher root-mean-square (RMS) current. The evaluation of this current is also carried out in the theoretical study. An experimental implementation shows an efficiency higher than 97.5% and confirms the accuracy of the theoretical study.

Quasi-continuous current transformer converter and new associated ZVS structure

Solar PV is one of the demanding nonconventional sources for generation of electricity but it has its own limitation due to non-linear characteristics. Solar PV needs MPP tracker to extract maximum power form solar PV source. Furthermore, multilevel voltage source inverter (VSI) is used for converting dc voltage into closed to sinusoidal ac supply. In this paper, incremental conductance (IC) technique for tracking maximum solar power is proposed with five-level VSI for converting solar PV dc voltage into three phase ac output voltage. Moreover, a unipolar PWM technique is proposed for VSI to producing gating pulses for Insulated gate bipolar transistor (IGBT) used in the VSI. Multilevel inverter is used to reduce filtering requirements and to reduce amplitude of all harmonics in inverter output. The complete model is simulated in MATLAB/SIMULINK and extensive simulations are carried out to validate the proposed system.

Performance of isolated generating system with multilevel inverter using photovoltaic source

A PWM method that results in low harmonic distortion for a 5-level PWM inverter is discussed in this paper. Comparison with the conventional 5-level unipolar PWM technique reveals that a greater number of vectors are produced with the proposed strategy thus resulting in high performance converter with minimized filter requirements since the amplitude of all harmonics is reduced. The resulting line-to-line voltage waveform has 5-levels at high and 3-levels at low modulation indexes (variable-level). Harmonic analysis is presented along with simulated and experimental results to support the theoretical considerations.

http://ieeexplore.ieee.org/iel3/4796/13254/00601582.pdf

A low-distortion variable-level PWM technique

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A low-distortion variable-level PWM technique