Vahid Dargahi

Flying Capacitors Reduction in an Improved Double Flying Capacitor Multicell Converter Controlled by a Modified Modulation Method

This paper proposes an improved configuration of double flying capacitor multicell (DFCM) converter. The main advantages of the proposed converter, compared to the conventional DFCM converter, are the doubling of the number of output voltage levels and improvement of the output voltage frequency spectrum. This progress is achieved by adding only two low-power switches and one dc voltage source, whose voltage rating is a small fraction of the main dc-link voltage rating, to the conventional configuration of the DFCM converter. However, the number and voltage rating of high-frequency switches and capacitors and the number of high-frequency switchings during a full cycle are kept constant. The doubling of the number of output voltage levels in the proposed converter makes it possible to decrease the number of cells, the number of flying capacitors, as well as their voltage rating and the amount of stored energy in flying capacitors. Moreover, a modulation method based on phase-shifted carrier pulsewidth modulation is proposed to control the new converter. Simulation and measured experimental results are presented to illustrate the performance of the proposed configuration and its control strategy.

A New Family of Modular Multilevel Converter Based on Modified Flying-Capacitor Multicell Converters

Modular multilevel converters (MMCs) are one of the next-generation multilevel converters intended for medium/high-voltage high-power market. This paper initially studies a modified topology for flying-capacitor multicell converters (FCMCs) as a modular submultilevel module. The main advantage of the modified FCMC, in comparison with the conventional one, is that the number and voltage rating of the required dc voltage sources are halved. Afterward, the MMC that comprises the series connection of the modified FCMCs used as submultilevel modules is proposed. Simulation results and experimental measurements taken from the four-cell-five-level laboratory prototype system of the modified FCMC as a modular submultilevel module are presented in order to validate its performance and advantages. Moreover, simulation results and experimental measurements of three cascaded two-cell-three-level modules (ultimately seven-level proposed MMC) and four cascaded two-cell-three-level modules (ultimately nine-level proposed MMC) are presented in order to validate its viability, merits and the proposed control strategy.

Analytical Determination of Conduction and Switching Power Losses in Flying-Capacitor-Based Active Neutral-Point-Clamped Multilevel Converter

Multilevel converters are mainly used in medium-voltage high-power applications. Active neutral-point-clamped (ANPC) flying capacitor multicell (FCM) converter is a well-known type of multilevel converters which is commercially available in high-power medium-voltage motor drive market. Since power loss investigation can be very advantageous in the design phase of multilevel converters, this paper presents an analytical approach to calculate and investigate the conduction and switching power loss in ANPC-FCM converter. First, the RMS and average currents of insulated-gate bipolar transistors (IGBTs) and antiparallel diodes are analytically calculated by considering the associated duty cycle of each IGBT and diode, converter modulation index, load current, and load power factor. Numerical results of the derived closed-form equations to calculate the RMS and average currents of IGBTs/diodes are compared with simulation results and experimental measurements. Numerical results match the simulation results and experimental measurements which validates the derived closed-form equations. Afterward, the obtained equations for RMS and average current computations are utilized to calculate the conduction power losses in a 12.1-MVA 6.6-kV nine-level (line-to-line) ANPC-FCM multilevel converter. For this purpose, a 4.5-kV 1.2-kA IGBT module from ABB is considered as a power switch and its parameters are employed in analytical computations and simulation of the ANPC-FCM multilevel converter for conduction power loss determination. Moreover, closed-form equations are derived for analytical determination of switching power losses for ANPC-FCM converter using Kapteyn (Fourier-Bessel) series. Based on the derived closed-form equations for conduction loss and switching loss calculation, a method is presented to determine the junction temperature in IGBTs and diodes for ANPC-FCM converter.

New Multilevel Converter Based on Cascade Connection of Double Flying Capacitor Multicell Converters and Its Improved Modulation Technique

This paper proposes a new multilevel converter based on the cascade connection of double flying capacitor multicell (DFCM) converters, as multilevel modules, to decrease the voltage diversity of the flying capacitors. Furthermore, a new switching pattern based on the phase-shifted pulse-width modulation technique is proposed to reduce the voltage ripple across the flying capacitors. Moreover, the proposed modulation technique reduces the rms value of the current flowing through flying capacitors. This results in an increase in the life time of flying capacitors and a decrease in the capacitance of the flying capacitors, to keep the same amount of the ripple, meaning a reduction in the physical size of the converter. In addition, this paper presents an analytical approach to calculate the average and rms currents of the insulated gate bipolar transistors (IGBTs)/diodes in the DFCM converter in a closed-form expression. The derived closed-form equations to calculate the average and rms currents of the IGBTs/diodes are utilized to investigate the conduction power losses in a DFCM converter and the proposed multilevel converter. Numerical results of the derived closed-form equations match the simulation results well, which validates the derived equations. Furthermore, simulation results and experimental measurements of the proposed multilevel power converter, configured by cascading two two-cell five-level DFCM converters, are presented to validate the performance of the proposed converter as well as the suggested modulation technique.

New Flying-Capacitor-Based Multilevel Converter With Optimized Number of Switches and Capacitors for Renewable Energy Integration

The flying-capacitor-based multilevel converter is one of the well-known breeds of the multilevel power converters. This paper proposes a new flying-capacitor-based multilevel converter to minimize the number of flying capacitors (FCs) and power switches. The advantage of the proposed FC-based multilevel converter in comparison with the conventional flying-capacitor multicell converter is that it needs fewer FCs. Also, in comparison with the stacked multicell converter, the proposed multilevel converter requires fewer semiconductor switches. In order to balance the voltage of the FCs in proposed multilevel converter, a new active voltage balancing method which is fully implemented using logic-form equations is presented. The proposed voltage balancing method measures output current and FC voltages to generate switching states to produce the required output voltage level, as well as balance the FCs voltages at their reference values. The output voltage of the proposed multilevel converter controlled with suggested active voltage balancing method can be modulated with any pulse-width-modulation (PWM) method, such as phase-shifted-carrier PWM or level-shifted-carrier PWM. Simulation results and experimental measurements of proposed FC-based multilevel converter are presented to verify the performance of the proposed converter, and its novel switching and modulation strategy, which is based on the active voltage balancing method.

A New Breed of Optimized Symmetrical and Asymmetrical Cascaded Multilevel Power Converters

Multilevel voltage source power converters are the state-of-the-art and key elements for medium-voltage (MV) high-power applications. The cascaded multicell (CM) topologies reach higher output voltage and power levels, and also retain higher reliability due to their modular and fault-tolerant features. This paper initially proposes an optimized topology for symmetrical CM (SCM) multilevel converters. The superiority of the proposed SCM, as compared with the conventional CM converter structure, is that the number of required high-frequency power switches is reduced. Next, a new topology of an asymmetrical CM (ACM) converter, which is formed based on the proposed optimized modules of an SCM converter, is suggested. The advantage of the proposed new ACM converter in comparison with the traditional ACM topology is that the variety of the dc links with different voltage ratings reduces, which makes the proposed topology more modular. The simulation results and the experimental measurements taken from the laboratory prototypes are presented for the proposed converters in order to validate the effectiveness and the advantages of these converters as well as their control strategy.

Detailed and comprehensive mathematical modeling of flying-capacitor stacked multicell multilevel converters

Purpose – This study aims to propose a mathematical model for stacked multicell converters (SMCs), to be exploited in the analytic determination of natural voltage balancing dynamics of the flying-capacitor (FC) stacked multicell multilevel converters, i.e. investigations of the start-up behavior, dynamic response, and natural voltage balancing phenomenon. Design/methodology/approach – The crux of the proposed strategy is based on the closed-form analytic solution derivation for the switching functions used in the switching of the SMCs operated under phase disposition (PD) and phase shifted carrier (PSC) pulse width modulation (PD-PSC-PWM) technique. Hence, the suggested approach develops an analytic solution for the Fourier series and associated Fourier coefficients pertinent to the switching functions of the SMCs by obtaining the switching instants of the PD-PSC-PWM modulator in terms of Kapteyn series when the frequency of the triangular carrier waveform (fc) and that of the sinusoidal reference wavefo...

Capacitors voltage balancing modeling in three phase flying capacitor converters with booster

This paper provides a mathematical model for voltage natural balancing process in three phase capacitor-clamped multicell converters. The analysis leads to state-space model of the converters. State-space representation of converter can be utilized to investigate the start-up and steady states of internal flying capacitors voltages. To provide verification, numerical solutions for three phase capacitor-clamped multicell converters analytic model are presented.

Analytical determination of conduction losses for modified flying capacitor multicell converters

Multilevel converters are mostly applied for medium-voltage high-power applications. Flying-capacitor-based multilevel converters such as flying capacitor multicell (FCM) and modified FCM (MFCM) are promising breeds of multilevel converters. Considering the advantages of MFCM converter over conventional FCM converter, and noting that conduction power loss investigation can be very advantageous in the design phase of multilevel converters, this paper presents an analytical approach to calculate and analyze conduction power losses in MFCM converters. First, the rms and average currents of the insulated-gate bipolar transistors (IGBTs) and anti-parallel diodes are analytically calculated by considering the associated duty cycle of each IGBT/diode in terms of the converter modulation index, load current, and load power factor. Numerical results of the derived closed-form equations to calculate the rms and average currents of IGBTs/diodes are compared with simulation results. All the simulation and analytic results agree well with each other which validate the derived closed-form equations. Afterwards, the obtained equations for rms and average current computations are utilized to calculate the conduction power losses in a 12.4MVA 3.3kV 9-level (line-to-line) MFCM converter. A 2.5kV 1.5kA IGBT module from ABB is considered as the power switch in the performed study for MFCM converter.

Analytical determination of conduction power losses for active neutral-point-clamped multilevel converter

Active neutral-point-clamped (ANPC) converter is a well-known type of multilevel converter which is commercially available in high-power medium-voltage drive market. Since conduction loss investigation is advantageous in design phase of converters, this paper presents an analytical approach to calculate and investigate conduction power loss in ANPC converter. First, rms and average currents of IGBTs and anti-parallel diodes are analytically calculated by considering the associated duty cycle of each IGBT and diode, converter modulation index, load current, and load power factor. Numerical results of the derived analytical equations to calculate rms and average current of IGBT/diode are compared against simulation results and experimental measurements. All simulation, analytic, and experimental results agree well with each other which substantiate derived closed-form equations. Afterwards, obtained equations for rms and average current computations are utilized to calculate conduction power losses in a 12.4MVA 3.3kV 9-level (line-to-line) ANPC converter analytically. For this purpose, a 2.5kV 1.5kA IGBT module from ABB is considered as a power switch.