Cascaded H-bridge Rectifier Research Articles

Operation method for input‐series‐output‐parallel AC–DC–DC converter

In this study, the proposed configuration of input-series-output-parallel (ISOP) AC–DC–DC converter consists of a cascaded H-bridge (CHB) rectifier in the AC/DC stage and a parallel DC–DC converter in the DC/DC stage. This study reveals the relationship between input voltage sharing (IVS) and output current sharing (OCS) among the constituent modules. The factors causing DC-link voltage unbalance among series units in CHB rectifier are studied in detail. With the proposed operation method, the power balance including IVS, DC-link voltage balance and OCS can be achieved. In addition, the converter can always operate under a unity power factor with low-distortion grid current. The experimental results are presented to demonstrate the validity and features of the proposed operation method.

A Modular Bi-Directional Power Electronic Transformer

This paper presents a topology for a modular power electronic transformer (PET) and a control scheme. The proposed PET consists of a cascaded H-Bridge rectifier on the primary side, a high-frequency DC/DC conversion cell in the center, and a cascaded H-Bridge inverter on the secondary side. It is practical to use PETs in power systems to reduce the cost, weight and size. A detailed analysis of the structure is carried out by using equivalent circuit. An algorithm to control the voltages of each capacitor and to maintain the power flow in the PET is established. The merits are analyzed and verified in theory, including the bi-directional power flow, variable voltage/frequency and high power factor on the primary side. The experimental results validated the propose structure and algorithm.

DC link voltage balancing approach for cascaded H‐bridge active rectifier based on selective harmonic elimination‐pulse width modulation

This study presents an improved voltage balancing strategy which can be employed on multilevel cascaded H-bridge (CHB) rectifiers using low switching frequency. The proposed method balances the voltage of all DC links in a CHB rectifier to a predefined value, without increasing the number of switching transitions. The redundancy states which are available in the CHB rectifier to produce a voltage level are used to balance the DC links in each transition. Moreover, using the well-known selective harmonic elimination-pulse width modulation method, all current harmonic components up to 29th harmonic are eliminated in the three-phase CHB rectifier, whereas the average switching frequency of each switch is limited to 150 Hz. The effectiveness and validity of the proposed strategy is verified by several simulations and experiments that are carried out on a 7-level CHB rectifier.

A voltage balancing strategy with extended operating region for cascaded H-bridge converters

In this paper, a generalized switching technique for cascaded H -bridge (CHB) converters is proposed. To take advantage of both low- and high-frequency modulation techniques, a hybrid modulation method is utilized. The proposed technique employs the low-frequency switching for voltage balancing of the dc-link capacitors, and the high-frequency sinusoidal pulse-width modulation switching for shaping the input ac current. Using the proposed technique, the stable operating region of CHB converters is extended. The proposed technique is capable of maintaining the dc voltages balanced even under critical operating conditions in which the load of one H-bridge cell is removed. The validity and effectiveness of the proposed method is confirmed by several simulation and experimental tests on a seven-level CHB rectifier.

Stability and Voltage Balance Control of a Modular Converter With Multiwinding High-Frequency Transformer

In this paper, a novel modular cascaded multi-level converter with multiwinding high-frequency (MWHF) transformer is proposed for medium- or high-voltage applications. In the proposed converter, a cascaded H-bridge rectifier (CHBR) is connected directly with the input ac source while the traditional step-down transformer is no longer necessary. Then, an isolated dc-dc converter composed by a group of H-bridge converters and a MWHF transformer with high power density is used to isolate the dc buses produced by the CHBR. The equivalent circuit and mathematical model of the MWHF transformer and the high frequency (HF) converter are obtained firstly in this paper. Then, the voltage stability under unbalanced loads is analyzed and the naturally balance ability is verified. To accelerate the dc bus voltage balancing process, a voltage balance control algorithm based on energy exchange between different transformer windings is proposed that is realized by accomplishing the phase shift adjustment of the terminal voltages on different windings. Simulations and experiments are implemented to verify the performance of the proposed converter and voltage balance algorithm.

A new switching strategy for transformer‐less back‐to‐back cascaded H‐bridge multilevel converter

In this study, a new switching strategy is introduced for transformer-less back-to-back cascaded H-bridge (CHB) converters. The main purpose of this novel switching algorithm is to eliminate the need for an isolation stage between the line side and the load side H-bridge cells. In other words, using this strategy, the CHB rectifier and the CHB inverter can be connected directly to each other in a back-to-back structure. The CHB rectifier regulates the intermediate DC bus voltages to the reference value, whereas the CHB inverter generates a controllable AC voltage from the intermediate DC links. The low-frequency selective harmonic elimination technique is applied to eliminate specific harmonics from the output voltage and to minimise the switching loss. The validity and effectiveness of the proposed method are verified by simulations and experiments on a five-level back-to-back converter.

An Enumeration-Based Model Predictive Control Strategy for the Cascaded H-Bridge Multilevel Rectifier

In this paper, a model predictive control strategy is adapted to the cascaded H-bridge (CHB) multilevel rectifier. The proposed control scheme aims to keep the sinusoidal input current in phase with the supply voltage and to achieve independent voltage regulation of the H-bridge cells. To do so, the switches are directly manipulated without the need of a modulator. Furthermore, since all the possible switching combinations are taken into account, the controller exhibits favorable performance not only under nominal conditions but also under asymmetrical voltage potentials and unbalanced loads. Finally, a short horizon is employed in order to ensure robustness; this way, the required computational effort remains reasonable, making it possible to implement the algorithm in a real-time system. Experimental results obtained from a two-cell CHB rectifier are presented in order to demonstrate the performance of the proposed approach.

A Redundancy‐based scheme for balancing DC‐link voltages in cascaded H‐bridge rectifiers

A new voltage balancing algorithm is introduced for cascaded H-bridge rectifiers. This kind of converter is made of a series connection of H-bridge cells with separate dc-links. The proposed balancing algorithm will regulate the voltage of all dc-links to the desired reference value, even if the attached loads are very different. The input current is also programmed to be sinusoidal and has low distortion, while the switching frequency is kept low at 1 kHz. In addition, the input power factor can be controlled. The simulation and experimental results are presented to confirm the validity of the proposed algorithm.

A PI-based Control Scheme for Primary Cascaded H-Bridge Rectifier in Transformerless Traction Converters

Cascaded H-Bridge rectifier (CHBR) is a more attractive solution in traction application for its transformerless structure. Because of the currents of different cells are exactly the same one, it is a challenge job to regulate the voltages of cells with only one current controller. In this paper, a PI-based control scheme is presented to deal with the voltages balance issue in CHBR. To satisfy the demand of rectifier such as unity power factor and regulated output DC voltages, the proposed control scheme consists of two parts. One is for shaping the grid current waveform and regulating the sum of DC-link voltages of all the cells; the other one is for balancing DC-link voltages. The latter is more concerned in this paper and is discussed in detail especially. Simulations and experiments are carried on. The results verified the feasibility and effectiveness of the proposed scheme.

Transformerless Cascaded AC-DC-AC Converter for Multiphase Propulsion Drive Application

A transformerless converter suitable for multiphase drive application is presented in this paper. The topology employs a cascaded H-bridge rectifier as the interface between the grid and multi inverters which drive the multiphase motor. Compared with the conventional structure, the new topology eliminates the input transformer and also has the advantages such as four quadrant operation, simple configuration, low cost, high efficiency, and so on. The control strategies for the grid-side cascade H-bridge rectifier and the motor-side inverter are studied accordingly. Based on the multi-rotational reference frame, modular control scheme is developed to regulate the multiphase drive system. Simulation results show the proper operation of the proposed topology and the corresponding control strategy.

A modular power electronic transformer based on a cascaded H-bridge multilevel converter

In this paper a modular power electronic transformer (PET) for feeding critical loads is presented. The PE-based transformer is a multi-cellular step-down converter that can directly connect to medium voltage levels on the primary side and provide a low voltage, highly stable interface for consumer applications. The presented structure consists of three stages: a cascaded H-bridge (CHB) rectifier, an isolation stage, and an output stage. The CHB rectifier serves as an active rectifier to ensure that the input current is sinusoidal, and it converts the high AC input voltage to low DC voltages. The isolated DC/DC converters are then connected to the DC links and provide galvanic isolation between the HV and LV sides. Finally, a three-phase inverter generates the AC output with the desired amplitude and frequency. This paper introduces a new control strategy to maintain DC voltage balance among the CHB converter cells, even if the attached loads are different. The effects of voltage offsets and device mismatches on the equal load-current sharing are investigated, and an active load-current sharing method is presented to balance the load power among the parallel-output cells. The validity of the proposed controllers and the PET performance are verified by simulation and experimental results.

A Modular Strategy for Control and Voltage Balancing of Cascaded H-Bridge Rectifiers

In this paper, a new strategy for voltage balancing of distinct dc buses in cascaded H-bridge rectifiers is presented. This method ensures that the dc bus capacitor voltages converge to the reference value, even when the loads attached to them are extracting different amounts of power. The proposed method can be used for an arbitrary number of series H-bridges, different voltage levels, and different power levels in unidirectional or bidirectional rectifiers. To reduce the current harmonics and distortion, the input current is programmed to be sinusoidal and in phase with the input voltage; however, it is possible to adjust the input power factor to control both the active and reactive powers. In the proposed approach, both the low frequency (stepped modulation) and high frequency [pulse-width modulation (PWM)] switching methods are utilized to improve the performance of the rectifier. Using theoretical analysis, the acceptable load power limits for a rectifier with N-H-bridge cells are derived. The validity of the proposed method is verified by simulation and experimental results.