Multilevel Cascade Converter Research Articles

Design of Control System Used for Cells Cascaded Multilevel Converter Based on TMS320F2407

Because of its advantages, such as low input and output harmonics, high input power factor and efficiency, cells cascaded multi-level converter is widely used in the field of variable voltage and variable frequency control. Through the analysis of the multi-level converter’s structure and its PWM (Pulse Wavelength Modulation) control theory, a new structure of control system based on TMS320F2407 DSP was proposed. The control system structure and components of the central system and subsystem are given in this paper. SPWM control was achieved and multi-level PWM inverter output waveforms were produced. The results show that this type of multi-level inverter has superior performance.

Coupling Effect Reduction of a Voltage-Balancing Controller in Single-Phase Cascaded Multilevel Converters

This paper presents a new voltage-balancing controller for cascaded multilevel converters, especially for single-phase cascaded multilevel converters. It proposes a control algorithm that devotes itself not only to balancing the floating dc capacitors but also to eliminating the coupling effect between the voltage-balancing controller and the original system controller (controller without additional voltage-balancing controllers). Specifically, the average model in the d-q coordinate frame is derived and the control law is established. Then, the coupling effect between the voltage-balancing controller and the original system controller is identified and a new expression for duty cycle modification is proposed thus to eliminate the effect. Furthermore, this paper gives the design considerations of the pro- posed method, including the derivation of key transfer functions and effective voltage-balancing area, for the completeness of the discussion. Moreover, the reference generation techniques of the voltage-balancing controller are also discussed. This paper investigates the voltage imbalance in the soft-start process caused by an unsuitable reference, and presents a simple modified reference generation solution. Finally, both simulation and experimental results verify the performance of the proposed control system.

Multi-DSP and -FPGA-Based Fully Digital Control System for Cascaded Multilevel Converters Used in FACTS Applications

In this paper, a fully digital controller based on multiple digital signal processor (DSP) and field-programmable gate array (FPGA) boards has been proposed for parallel-operated cascaded multilevel converters (CMC) used in flexible AC transmission system (FACTS) applications. The proposed system is composed of a DSP-based master controller in combination with a multiple number of slave DSP boards, FPGA boards, microcontrollers, a programmable logic controller (PLC), an industrial computer, and their peripherals in interaction. Intercommunication of these digital controllers is achieved mainly through fiber-optic links, via synchronous serial data link wherever a high-speed, full duplex communication is needed, and via asynchronous serial communication interface wherever relatively slow communication speed is required. The proposed fully-digital control system has been implemented on a sample 11-level CMC-based 154-kV, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$+/-$</tex></formula> 50-MVAr transmission type static synchronous compensator (T-STATCOM). Field test results have shown that the proposed fully digital control system provides good transient response and steady-state characteristics for the oveall system including protection and monitoring functions.

Design and Implementation of a 154-kV $\pm$50-Mvar Transmission STATCOM Based on 21-Level Cascaded Multilevel Converter

In this research work, the design and implementation of a 154-kV <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\pm$</tex> </formula> 50-Mvar transmission static synchronous compensator (T-STATCOM) have been carried out primarily for the purposes of reactive power compensation and terminal voltage regulation and secondarily for power system stability. The implemented T-STATCOM consists of five 10.5-kV <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\pm$</tex></formula> 12-Mvar cascaded multilevel converter (CMC) modules operating in parallel. The power stage of each CMC is composed of five series-connected H-bridges (HBs) in each phase, thus resulting in 21-level line-to-line voltages. Due to modularity and flexibility of implemented HBs, each CMC module has reached a power density of 250 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\hbox{kvar/m}^{3}$</tex></formula> , thus making the mobility of the system implementable. DC-link capacitor voltages of HBs are perfectly balanced by means of the modified selective swapping algorithm proposed. The field tests carried out at full load in the 154-kV transformer substation where T-STATCOM is installed have shown that the steady-state and transient responses of the system are quite satisfactory.

Estratégia de Modulação para Conversores Multiníveis em Cascata Simétricos Sob Condições de Faltas

Este artigo propõe uma nova estratégia de modulação para conversores multiníveis em cascata simétricos, adequada para a operação com ou sem faltas nas células de potência que constituem o conversor. Inicialmente, é derivado o conjunto de todas as possíveis tensões de modo comum para que o conversor opere na região linear, considerando o estado de cada uma de suas células. É demonstrado que, através da seleção adequada da tensão de modo comum, toda a capacidade de síntese de tensão do conversor pode ser utilizada. A adição desta tensão de modo comum às fases do conversor compensa a perda de uma ou mais células, evitando as distorções resultantes e sintetizando tensões de linha de saída equilibradas. A estratégia proposta foi aplicada a um conversor multinível em cascata simétrico com duas células por fase, sendo apresentados resultados experimentais e de simulação que comprovam a eficácia do método proposto, validando as análises teóricas.

Research on cascade multilevel inverter with single DC source by using three-phase transformers

One of the most significant recent advances in power electronics is the multilevel inverter. Using this concept the power conversion is performed with enhanced power quality. In this scenario cascade H-bridge multilevel inverter (CHBMLI) is an exceptional one and it has many inherent benefits like: (1) its modular structure, (2) it can be easily implemented through the series connection of identical H-bridges, (3) generate near sinusoidal voltages with only fundamental frequency switching and finally and (4) no electromagnetic interference or common-mode voltages. This flexibility has resulted CHBMLI topology in various applications like medium-voltage industrial drives, electric vehicles and the grid connection of photovoltaic-cell generation systems. But one of the major limitations of the cascade multilevel converters is requirement of isolated dc voltage sources for each H-bridge, this increases the converter cost and reduces the reliability of the system. This drawback is the key motivation for the present work. The present paper investigates different cascade multilevel inverter (CMI) based topologies with reduced dc sources and finally the proposed CMI with single dc source by employing low frequency transformers is presented. Proposed topology significantly reduces size when compared with conventional topologies and increases the reliability. To verify the performance of proposed architecture, prototype experiments are carried out and adequate results are presented.

Cascaded Control of Multilevel Converter based STATCOM for Power System Compensation of Load Variation

The static synchronous compensator (STATCOM) is used in power system network for improving the voltage of a particular bus and compensate the reactive power.It can be connected to particular bus as compensating device to improve the voltage profile and reactive power compensation. In this paper, a multi function controller is proposed and discussed. The control concept is based on a linearization of the d-q components with cascaded controller methods. The fundamental parameters are controlled with using of proportional and integral controller. In closed loop method seven level cascaded multilevel converter (CMC) is proposed to ensure the stable operation for damping of power system oscillations and load variation.

Generalized Carrier-Based Modulation Strategy for Cascaded Multilevel Converters Operating Under Fault Conditions

This paper proposes a simple modulation approach for symmetrical and asymmetrical cascaded multilevel converters, suitable for operation under normal and fault conditions. Initially, the set of all possible converter common-mode voltages for linear operation is derived, considering the status of each converter cell. It is demonstrated that the methods previously described in the literature belong to the derived set. By properly selecting the common-mode voltage, the entire converter voltage synthesis capability can be achieved under any fault condition. The proposed modulation approach has been applied for both symmetrical and asymmetrical cascaded multilevel converters. Simulation and experimental results show the effectiveness of the proposed modulation approach and validate the theoretical analysis.

A Multi-functional Utility Interface of BIPV Systems Based on Cascade Multilevel Converter

Building integrated photovoltaic (BIPV) systems bring forward many requirements to grid-tied inverters in these aspects such as generating efficiency, shading effects, harmonic & reactive current compensation and quick economical payback, etc.. This paper presents a multi-functional utility interface of BIPV systems based on cascade multilevel converter. It has two operating modes: one is operated as grid-tied inverter at normal insolation level and the other is operated as active power filter (APF) at very low insolation level or at nighttime. Compared with traditional inverters, cascade multilevel converter is much more efficient and suitable for utility interface of BIPV system because it has the merits of high efficiency, minimum switching frequency, low switching stress and EMI, approximate sinusoidal waveform voltage output with low total harmonic distortion (THD), modular structure and good dynamic stability. Simulation and experimental results have verified its feasibility and advantage.

A New Topology of Cascaded Multilevel Converters With Reduced Number of Components for High-Voltage Applications

In this paper, a new topology of a cascaded multilevel converter is proposed. The proposed topology is based on a cascaded connection of single-phase submultilevel converter units and full-bridge converters. Compared to the conventional multilevel converter, the number of dc voltage sources, switches, installation area, and converter cost is significantly reduced as the number of voltage steps increases. In order to calculate the magnitudes of the required dc voltage sources, three methods are proposed. Then, the structure of the proposed topology is optimized in order to utilize a minimum number of switches and dc voltage sources, and produce a high number of output voltage steps. The operation and performance of the proposed multilevel converter is verified by simulation results and compared with experimental results of a single-phase 49-level converter, too.

A 6.6‐kV transformerless battery energy storage system based on a cascade multilevel PWM converter: Experimental verification by a 200‐V, 10‐kW, 3.6‐kWh laboratory model

AbstractRenewable energy sources such as wind turbine generators and photovoltaics are intermittent in nature, thus resulting in fluctuating electric power. Leveling of their electric power is indispensable to realize large‐scale renewable energy sources and to avoid severe power quality problems when they are interconnected into the grid. A battery energy storage system (BESS), combining batteries with a power converter and digital control, should be installed in the vicinity of an intermittent energy source. The fluctuating power is compensated by appropriately controlling active power stored in, or released from, the battery. This paper describes a 6.6‐kV transformerless battery energy storage system based on a cascade multilevel PWM (pulse‐width‐modulation) converter, with focus on a control method for active power and SOC (state‐of‐charge) balancing. A down‐scaled 200‐V, 10‐kW, 3.6‐kWh (13‐MJ) laboratory model using nine NiMH (Nickel Metal Hydride) battery packs was designed, constructed, and tested to verify the viability and effectiveness of the 6.6‐kV system. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(1): 43–54, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21127

ANALYSIS OF A GENERALIZED SYMMETRICAL MULTILEVEL INVERTER

The cascade multilevel converter requires the least number of main components among traditional multilevel converter. In this paper, comparison of main components between proposed and traditional converter is shown. In addition, a generalized symmetrical multilevel inverter is proposed. The proposed multilevel inverter can generate DC voltage levels similar to other topologies with less number of components. The proposed topology results in reduction of installation area and cost and has simplicity of control system. This converter has been used in an AC drive system. The operation and performance of the proposed generalized multilevel converter has been verified by the simulation results using SIMULINK/MATLAB. The simulation results show its capability in voltage generation and speed control.

New multilevel inverter with reduction of switches and gate driver

This paper presents a novel topology for symmetrical cascade multilevel converter. The proposed circuit consists of series connected sub multilevel converters units and it can generate DC voltage levels similar to other topologies. The proposed topology results in reduction of switches number, losses, installation area and converter cost. This converter has been used in a Dynamic Voltage Restorer (DVR). Simulation results carried out by MATLAB/SIMULINK show the voltage injection capability of converter and the efficiency of its controller in compensating voltage sag and swell.

A Cascade Multilevel Converter Topology With Reduced Number of Switches

This paper introduces a new multilevel converter topology that has many steps with fewer power electronic switches. The proposed circuit consists of series-connected submultilevel converters blocks. The optimal structures of this topology are investigated for various objectives, such as minimum number of switches and capacitors, and minimum standing voltage on switches for producing maximum output voltage steps. A new algorithm for determination of dc voltage sourcespsila magnitudes has also been presented. The proposed topology results in reduction of the number of switches, losses, installation area, and converter cost. The operation and performance of the proposed multilevel converter has been verified by the simulation and experimental results of a single-phase 53-level multilevel converter.

Frequency-Domain Analytical Model for a Cascaded Multilevel STATCOM

This paper develops a frequency-dependent analytical representation for a multilevel cascaded converter, with fundamental frequency switching. The gain and phase angle are studied with the firing angle as a parameter, where the converter is subjected to oscillatory excitation at a range of frequencies. This paper presents analytical formulae for a single cell and for a generic cascade multilevel converter where the number of cells is a parameter. Also, the formula for fundamental frequency studies is separately developed. The equivalent capacitance concept is introduced and its convergence to a constant value for high frequencies is demonstrated. The analytical formulae are tested against detailed digital simulation and good matching is reported. The formulae developed in this study will be of particular importance in studying interactions between a multilevel STATCOM and the host ac grid (similar to harmonic resonance) and in studying converter transient responses.

Analysis of feasibility of asymmetrical cascade multilevel frequency converters

The feasibility of cascade multilevel frequency converters with three inverters per phase at various dc power supply voltages that are multiples of the minimal value is considered. Analysis is performed with different voltage ratios based on the loading of power supplies and the quality of the converter output voltage and input current.

A Transformerless Energy Storage System Based on a Cascade Multilevel PWM Converter With Star Configuration

This paper describes a transformerless energy storage system based on a cascade multilevel pulsewidth modulation converter with star configuration. The system is intended for power leveling of renewable energy sources, as well as for improving power quality and reliability of a power distribution system. This paper pays attention to active-power control and voltage-balancing control that are indispensable for proper operation of the energy storage system. A 200-V 10-kW 8.8-kJ downscaled laboratory system is designed, constructed, and tested, replacing electric double-layer capacitors with large-capacity electrolytic capacitors. Experimental results obtained from the laboratory system verify the viability and effectiveness of the 6.6-kV energy storage system.

Advanced Control and Analysis of Cascaded Multilevel Converters Based on P-Q Compensation

This paper introduces new controls for the cascaded multilevel power converter. This converter is also sometimes referred to as a ldquohybrid converterrdquo since it splits high-voltage/low-frequency and low-voltage/pulsewidth-modulation (PWM)-frequency power production between ldquobulkrdquo and ldquoconditioningrdquo converters respectively. Cascaded multilevel converters achieve higher power quality with a given switch count when compared to traditional multilevel converters. This is a particularly favorable option for high power and high performance applications such as naval ship propulsion. This paper first presents a new control method for the topology using three-level bulk and conditioning inverters connected in series through a three-phase load. This control avoids PWM frequency switching in the bulk inverter. The conditioning inverter uses a capacitor source and its control is based on compensating the real and reactive (P-Q) power difference between the bulk inverter and the load. The new control explicitly commands power into the conditioning inverter so that its capacitor voltage remains constant. A unique space vector analysis of hybrid converter modulation is introduced to quantitatively determine operating limitations. The conclusion is then generalized for all types of controls of the hybrid multilevel converters (involving three-level converter cells). The proposed control methods and analytical conclusions are verified by simulation and laboratory measurements.

Evaluation of Cascade-Multilevel-Converter-Based STATCOM for Arc Furnace Flicker Mitigation

As an industry customer of electric power, an electrical arc furnace (EAF) is a major flicker source that causes major power quality problems. For a 40-MVA EAF in Tennessee, USA, a cascade-multilevel converter (CMC)-based STATic synchronous COMpensator (STATCOM) with high bandwidth is proposed for EAF flicker mitigation. In this paper, flicker mitigation techniques by using a CMC-based STATCOM are presented and verified through a transient network analyzer (TNA) system. The required STATCOM capacity is first studied through a generalized steady-state analysis. Second, the STATCOM control strategy for flicker mitigation is introduced, and simulation results are given. Finally, a TNA system of the STATCOM and an EAF system are designed and implemented. Experimental results from the TNA test show that the proposed CMC-based STATCOM and its controller can efficiently and rapidly mitigate the EAF flicker

Reduction of dc voltage sources and switches in asymmetrical multilevel converters using a novel topology

This paper presents a novel topology for asymmetrical cascade multilevel converter. The proposed circuit consists of series connected sub-multilevel converters blocks and it can generate more dc voltage levels than other topologies. The proposed topology results in reduction of dc sources and switches number, losses, installation area and converter cost. This converter has been used in a DVR, which has not any coupling transformer. The DVR which is based on the proposed converter has been modeled and the simulation results show its capability in solving power quality problems. The operation and performance of the proposed multilevel converter has been verified by the measurement results of a single-phase 147-level multilevel converter.