Three-phase Cascaded H-bridge Research Articles

Modified RPWM Strategy Based on Level-Shifted Random Carrier and Power Balance to Reduce the PWM Voltage Noise in Three-Phase CHB Inverters

Aimed at the pulse width modulation (PWM) voltage noise and power imbalance in three-phase cascaded H-Bridge (CHB) inverters, a modified random PWM (RPWM) strategy, named the power-balanced RPWM (PB-RPWM) strategy, is proposed in this paper. The PB-RPWM strategy mainly includes three steps: (1) random pulse signals are generated by compared modulation wave with level-shifted random carriers; (2) the random pulse signals are circularly distributed between CHB units by a logic operation method, and then the driving pulse signals of switching devices are produced; (3) the driving pulse signals are used to control the inverter. Under the PB-RPWM strategy, the spectra of the line voltage become uniform and continuous, that is, the PWM voltage noise of the line voltage can be effectively reduced. The output voltage of a single H-bridge unit can contain three basic voltages within 3/2 TA, that is, the power balance between CHB units can be realized. When compared with conventional non-random and random PWM strategies, the PB-RPWM strategy has a lower PWM voltage noise and smaller total harmonic distortion (THD). When compared with the level-shifted PWM (LS-PWM) strategy, the PB-RPWM strategy has a balanced power performance. The effectiveness and feasibility of the PB-RPWM strategy is verified by abundant simulations and experiments.

Fault Diagosis of CHB Inverter Using Machine Learning

Renewable energy has gained popularity due to depleting natural resources and escalating fossil fuel and nuclear pollution. Power electronic engineers design grid-connected power conversion systems. MLIs provide more power and solutions. Cascaded H-Bridge (CHB) MLIs start with two or more 3L single-phase H-bridge inverters. Each H-bridge may produce three separate voltage levels. Combining the separated dc voltage sources produces a stepped output voltage with a step size equal to the magnitude of the connected sources. The present work develops a method for detecting and resolving switch failures in a three-phase CHB inverter, ensuring system dependability and allowing for system redundancy. The recommended approach uses Wavelet transform to extract features, then Decision Tree classifier to detect and characterise defects. Increased classification accuracy shows the DT-based fault diagnosis system's efficiency in identifying inverter switch faults.

Voltage Vector Redundancy Exploitation for Battery Balancing in Three-Phase CHB-Based Modular Energy Storage Systems

This article presents a flexible controller algorithm for bidirectional modular cascaded H-bridge (CHB)-based battery storage systems. The primary objectives are to maintain high-quality power on the ac side while balancing dc-side battery voltages under charging or discharging operations. The proposed algorithm is based on a finite-set model-predictive current control (FSMPC) technique, which performs in-phase and phase voltage equalization in two complementary balancing terms by taking advantage of intrinsic voltage vector redundancy characteristic of three-phase CHB converters. Additionally, the proposed modifications to the FSMPC provide lower demand on computational resources of the controller through offline reordering of vectors and adjacent vector introduction, which also brings the least differential-mode <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv/dt$</tex-math></inline-formula> as an added value. Effects of battery voltage variation in charge/discharge cycles and voltage drop caused by nonideal semiconductors are considered to ensure the most accurate performance during steady-state, transient, and common-mode voltage reduction operation. The proposed approach is an efficient easy-to-implement multiparameter controller, which handles all the tasks needless to any additional control loops. Simulations and experimental prototyping results validate the feasibility of the battery voltage balancer terms to be utilized in battery storage applications with no downside impact on key power exchange functions.

A Multiport AC–AC–DC Converter for Soft Normally Open Point

The soft normally open point (SNOP) is a critical apparatus in AC distribution grid to increase the distributed generation (DG) penetration. The existing converters for SNOP are limited by either the utilization of abundant switching devices and passive components or the voltage coupling between two AC systems. Therefore, a multiport AC–AC–DC converter for SNOP is proposed in this brief, where two cascaded H-bridge (CHB) converters with a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> series resonant circuit in parallel connection with each phase-leg are adopted in back-to-back (BTB) structure. The two three-phase CHB converters are connected by three medium-frequency transformers (MFTs). Moreover, the mixed-frequency modulation method and voltage decoupling principle are introduced. Therefore, less switching devices and passive components are used for the multiport AC–AC–DC converter. In addition, the voltages between two AC systems are completely decoupled, and the electrical isolation is ensured. The operating principle, control strategy and parameter design of multiport AC–AC–DC converter are introduced in detail in this brief. Finally, the experimental results are provided to verify the feasibility of proposed multiport AC–AC–DC converter.

Analysis and Suppression of Active Power Backflow of Three-Phase Common DC-Bus Cascaded H-Bridge PV Grid-Connected Inverter During LVRT

Featured with the expandable modular structure, three-phase isolated cascaded H-bridge (CHB) inverters are capable of directly connecting to medium voltage power grid without bulky and heavy line-frequency transformer, which has outstanding advantages applied in large-scale photovoltaic (PV) power plants. However, different from traditional PV inverters, three-phase CHB topology is essentially composed of three single-phase CHB inverters. Affected by negative-sequence grid voltage, the active powers of some phases may inversely flow from ac grid to PV inverter during low voltage ride through (LVRT), resulting in that dc-bus voltages of all H-bridges in these phases continue to rise, and that system will shut down due to over-voltage protection. Aiming at this issue, this article studies the mechanism of active power backflow during LVRT, and deduces the quantitative relationship between active current and reactive current to be necessarily injected under different types of voltage faults and different degrees of voltage sags. In addition, combined with the relevant standard of LVRT, the feasibility of suppressing active power backflow by positive-sequence active power current injection is analyzed in detail. The validity of the proposed method is verified by simulation and experimental results.

Module Power Balance Control Strategy for Three-Phase Cascaded H-Bridge PV Inverter Under Unbalanced Grid Voltage Condition

In the field of high-voltage and high-power photovoltaic (PV) power generation, three-phase cascaded H-bridge (CHB) inverter has been attracting more and more attention because of its easy modularization and high output voltage. Compared with other structures, the three-phase CHB inverter based on common dc-bus structure has more advantages, which is capable of avoiding interphase and intermodule power imbalance problems. When the grid voltage is balanced, the total active power transmitted by each phase of the three-phase CHB inverter is the same, and because the number of modules in each phase is designed to be equal, the active power transmitted by all modules is the same. However, if the grid voltage is unbalanced or some modules fail, the module power in different phases may be unequal, which makes heat dissipation requirements and service life of each module significantly different. In this article, a module power balance control strategy is proposed, it ensures that all modules still transmit equal powers when the grid voltage is unbalanced, or some modules fail. Simulation and experimental results all verify the effectiveness of the proposed strategy.

Research on Power Equalization of Three-Phase Cascaded H-Bridge Photovoltaic Inverter Based on the Combination of Hybrid Modulation Strategy and Zero-Sequence Injection Methods

Due to the nonuniform solar irradiance, unequal ambient temperatures, or inconsistent degradation of photovoltaic (PV) modules in three-phase cascaded H-bridge (CHB) PV inverter, the unbalanced output power among PV modules will lead to the imbalanced power between phases and bridges, resulting in unbalanced or even distorted grid current between three phases, which cannot meet the requirements of grid codes. Concerning this issue, this article proposes a novel power equalization method based on the combination of hybrid modulation strategy and zero-sequence injection methods, which deals with interbridge power imbalance by hybrid modulation and interphase power imbalance by zero-sequence injection methods. By utilizing the proposed method, three-phase-balanced grid currents with low total harmonic distortion are able to be achieved even when the interbridge and the interphase power are seriously unbalanced. In addition, each H-bridge cell in three-phase CHB inverter uses square-wave modulation, which maximizes the dc-side voltage utilization of each H-bridge cell, thus extending its power balance region. Experimental results achieved by a laboratory prototype of a three-phase seven-level CHB inverter demonstrate both feasibility and validity of the proposed method.

On PWM Strategies and Current THD for Single- and Three-Phase Cascade H-Bridge Inverters with Non-Equal DC Sources

Cascade H-bridge (CHB) inverter is an attractive choice for integration of DC sources of different nature, e.g., for distributed generation with energy storage, photovoltaic generation, etc. In general, non-equal DC voltage sources can affect the total harmonic distortion (THD) of the CHB by introducing undesirable low-frequency subharmonics. This paper investigates different level-shifted (LS) and phase-shifted (PS) pulse width modulation (PWM) strategies for single- and three-phase cascade H-bridge inverters with non-equal DC sources from the load current THD minimization perspective. The best current quality is provided by LS PWM, as reported in the literature. The paper provides a simple time domain explanation of LS PWM superiority. However, PS PWM may be a preferable choice for practical applications due to fair power and loss sharing across individual H-bridges. The paper explains how to obtain the best current quality by PS PWM carriers’ order arrangement (DC sources switching sequence selection).

Three-stage control architecture for cascaded H-Bridge inverters in large-scale PV systems – Real time simulation validation

In large-scale PV power stations, Cascaded H-Bridge (CHB) inverter based PV power conditioning systems are recommended over a conventional Two-Level Inverter based systems since CHB operates at medium voltage levels and provides better power quality. An insulated-gate bipolar transistor (IGBT) based H-bridge along with the auxiliaries such as DC link capacitors, breakers, contactors, bypass switch, voltage and current transducers is the fundamental power module of a CHB inverter. In this paper, the procedure for selection of components for Basic building block is presented. Due to a higher number of H-Bridge modules in a large-scale system, it is difficult to control the system with a single controller card. In this work, a control architecture for three-phase CHB based PV power conditioning systems is proposed in which the controls are distributed into three different stages. With the proposed control architecture, independent Maximum power point tracking (MPPT) controls of each PV array is carried out at module level itself. Carrying out MPPT controls at module level helps in improving the computational speed and in maintaining modularity. Hardware requirements of individual processor cards also minimized with the proposed control architecture. In this work, functionalities of each controller card namely module level control card, phase-level control card and master controller cards are explained in detail. Detailed interfacing and signal exchange between H-Bridge modules and the other controller cards are also presented. Controller-in-loop simulations are carried out with the help of Real-Time Simulator to validate the functionalities of each controller card. Real-Time simulation results are presented to verify the operation of the system with the proposed control architecture. Performance and dynamic response of MPPT controls for sudden changes in irradiance inputs on PV arrays are studied. Operation of the system during unequal irradiance inputs on the PV arrays is also analyzed. Current sharing between PV Inverter and grid to feed a fixed load for different values of irradiance inputs is explained through the presented results.

A Discontinuous Space Vector PWM Algorithm in abc Reference Frame for Multilevel Three-Phase Cascaded H-Bridge Voltage Source Inverters

Previous space vector pulse-width modulation (SVPWM) methods for a multilevel three-phase voltage source inverter (VSI) have the problems including the computational complexity to identify the location of the reference voltage vector, the memory requirements of switching states and corresponding duty ratios, or no generalized switching pulse durations of switches. These problems are related to the level number, and they become critical at a high level number. In this paper, a discontinuous SVPWM algorithm in the abc reference frame is first proposed for a multilevel three-phase cascaded H-bridge (CHB) VSI. The L -level three-phase SVPWM problem is solved based on the two-level three-phase SVPWM one. Thus, the computational complexity to determine the nearest three switching states and corresponding duty ratios is independent of the level number. Furthermore, the proposed algorithm provides a systematic approach to calculate the switching pulse durations for a multilevel three-phase CHB VSI, and the approach can be extended to different multilevel three-phase VSIs. The proposed algorithm has no memory requirement and low computational complexity. It is verified by the simulation and experimental results for a five-level three-phase CHB VSI.

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.

Fault-Tolerant Control of Cascaded H-Bridge Converters Using Double Zero-Sequence Voltage Injection and DC Voltage Optimization

Cascaded H-Bridge (CHB) converters can be directly connected to medium-voltage grids without using transformers and they possess the advantages of large capacity and low harmonics. They are significant tools for providing grid connections in large-capacity renewable energy systems. However, the reliability of a grid-connected CHB converter can be seriously influenced by the number of power switching devices that exist in the structure. This paper proposes a fault-tolerant control strategy based on double zero-sequence voltage injection and DC voltage optimization to improve the reliability of star-connected CHB converters after one or more power units have been bypassed. By injecting double zero-sequence voltages into each phase cluster, the DC voltages of the healthy units can be rapidly balanced after the faulty units are bypassed. In addition, optimizing the DC voltage increases the number of faulty units that can be tolerated and improves the reliability of the converter. Simulations and experimental results are shown for a seven-level three-phase CHB converter to validate the efficiency and feasibility of this strategy.

Sample Time Staggered Space Vector Modulation for Medium Voltage Cascade Seventeen-level Static Synchronous Compensator

Abstract An implementation method of applying the sample time staggered space vector modulation technique (STS-SVM) to three-phase medium voltage cascade H-bridge static synchronous compensator (STATCOM) was proposed. Proposing an equivalent topology relation between two-level three-phase bridge and three-phase cascade H-bridge based on combined transformer, according to which, using the three left and three right bridge legs of each stage three-phase H-bridge as two adjacent three-phase full bridge to SVM modulate, thus realized the implementation of using STS-SVM technique to the cascade H-bridge STATCOM. Simulation results show that the method has the advantages of high DC voltage utilization, easy-to-digital implementation, low switching losses, and so on, which is easy to use in medium voltage cascade H-bridge STATCOM. The practical application proved that the medium voltage cascade seventeen-level STATCOM based on STS-SVM, which is of fast dynamic response, could realize comprehensive compensation of harmonics and passive power precisely and efficiently at lower switch frequency in actual power grid.