Seven-level Cascaded H-bridge Research Articles

A New Active Front-End Control for Regenerative Cascaded H-Bridge Motor Drives With Filter- Less Interfacing Capability

This paper proposes a new active-front-end (AFE) control method for regenerative Cascaded H-bridge (CHB) motor drives. Conventional CHB converters have dominated the market for the medium-voltage industrial motor drives. However, conventional CHB converters cannot provide regenerative capability. To enable regeneration; diode-front-ends (DFEs) in power cells are replaced with IGBT-based PWM AFEs. Despite the appealing dynamic performance of PWM AFEs, their integration to the CHB converters is not optimal. First, they introduce more semiconductor losses due to the high frequency switching. Second, they introduce switching harmonics that are not cancelled by phase-shifting transformers. Therefore, they require additional harmonic filtering solutions to comply with grid harmonic standards. To resolve these challenges, this paper proposes a new AFE control for regenerative CHB converters based on fundamental frequency switching (FFE) to reduce switching frequency and thus power losses. The operation of FFEs at nominal and sag voltage conditions is presented, in addition to the capability of filter-less interfacing to the transformer. Finally, the performance of the proposed control is validated experimentally on a seven-level regenerative CHB drive.

Control of Regenerative CHB Motor Drives at Fundamental Switching Frequency

To handle the high-power regeneration power in coal and mine industry applications, the regenerative version of the cascaded H-bridge (CHB) drives is made possible by replacing the diode-front-end rectifier with a three-phase IGBT-based active-front-end (AFE) rectifier in each power cell. However, the IGBT switching devices introduce extra switching losses to the system that must be properly handled due to the thermal constraints in high power medium voltage drive applications. In this article, a novel fundamental switching frequency control strategy is proposed for the AFEs in the regenerative CHB drive. With the proposed control strategy, the main harmonic contents generated by the AFEs are well-reshaped at a low-order harmonics range which can be further eliminated by the existing front-end phase-shifting transformer. This allows accomplishing the meeting of the IEEE std 519-2014 at the fundamental switching frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sw</sub> = 60. Moreover, with the proposed switching pattern, IGBTs can be turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> at zero current and therefore it can reduce switching losses simultaneously. The feasibility of the proposed control strategy is verified by experimental results on a seven-level regenerative CHB drive, which can be extended to regenerative CHB drives with any voltage levels.

A New Low Switching Frequency Control of Regenerative CHB Drive With Low-Voltage Ride-Through Capability

In conventional medium-voltage high-power cascaded H-bridge (CHB) drive, a three-phase rectifier is adopted in each power cell to provide isolated dc-bus voltage for the output H-bridge. This limits the application of the conventional CHB drive, where regeneration capability is desired. The regenerative CHB drive can be made possible by replacing the diode rectifier with a three-phase IGBT-based active-front-end (AFE) rectifier in each power cell. However, due to thermal constraints in the high-power medium-voltage drives, the first challenge of the regenerative CHB drive is to deal with the extra switching losses introduced by the IGBT devices. Another challenge of the grid-tied regenerative CHB drive is to handle the low-voltage sags in the power grid without triggering unnecessary downtime in transients. In this article, a novel low-switching frequency control strategy is proposed for the AFEs in the regenerative CHB drive with low-voltage ride-through (LVRT) capability. The main harmonic contents generated by the AFEs with the proposed control strategy can be eliminated by the existing phase-shifting transformer in the CHB drives. This allows meeting with IEEE STD 519-2014 requirement with a 60 Hz switching frequency to minimize the extra introduced switching losses in steady state. Moreover, LVRT capability is integrated into the proposed control scheme with reduced current sensor count. Accompanied with videos, the experiments on a seven-level regenerative CHB drive are implemented to validate the feasibility of the proposed control scheme, which can be extended to regenerative CHB drives with any voltage levels.

Utilization of a Reduced Switch-Count Topology in Regenerative Cascaded H-Bridge (CHB) Medium-Voltage Drives

This article employs a reduced switch-count topology in regenerative cascaded H-bridge (CHB) motor drives. Conventional regenerative CHB drive employs six-switch voltage source converters in the front end of each power cell to capture and utilize the regenerated energy. This solution is accompanied by a high number of switches, gate drives, and control circuits, especially when the number of levels increases. This causes high switching power losses, and increase in the cost, size, and weight of the overall system. This article utilizes a four-switch three-phase inverter active front end to reduce the switch count of the CHB power cell. The utilization of the four-switch inverter creates several challenges in terms of voltage imbalance on the capacitors and harmonics on the grid side. First, these challenges in medium-voltage operation are analyzed and addressed. Then, a new phase alternation connection method and two carrier phase-shifting techniques are proposed in this article to address the input current harmonics and comply with the grid connection standards. The performance of the proposed configuration and control techniques is analyzed theoretically and with simulation studies. The feasibility of the proposed configuration is validated experimentally on a scaled-down seven-level regenerative CHB drive.

A New Approach to Input Filter Design for Regenerative Cascaded H-Bridge Drives

The diode-front-end (DFE) cascaded H-bridge (CHB) inverter has prevailed in the nonregenerative industry drive domain for high power medium voltage applications due to its modularity, scalability, and fault-tolerant capability. The regenerative version of the CHB drives is made possible by adding the extra active-front-end (AFE) rectifier in each power cell, such as a three-phase Pulse width modulation (PWM) rectifier. However, AFE introduces the switching harmonics which requires to be attenuated by designing a grid interfaced filter to comply with the harmonic standard IEEE 519-2014. High-order filters like <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filters are not preferred due to complex inherent resonances in a multiparallel AFE system. A large number of capacitive components in the system rouse multiple resonances, which decreases system reliability and modularity. This article proposes an active filtering strategy based on the optimal asymmetric carrier-shifting method for regenerative CHB drives with only <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> filters. The required filter size is reduced significantly while complying with IEEE 519-2014 standard and thus improves the overall size, cost, and efficiency. The proposed filtering strategy is validated experimentally using a seven-level regenerative CHB drive.

A Generalized Single-Carrier PWM Scheme for Multilevel Converters

This article proposes a novel and generalized single-carrier (SC) pulsewidth modulation (PWM) scheme suitable for multilevel converters. Compared with the existing SC-PWM schemes, the proposed one presents a simpler approach to replicate the behavior of the phase disposition PWM method at any operating point. Besides having an easier digital implementation, the proposed SC-PWM scheme results in great control flexibility to optimize the operation of multilevel converters. Based on the rationales of the proposed SC-PWM scheme, an optimized switching strategy for a cascaded H-bridge (CHB) multilevel inverter architecture is developed. The proposed optimized switching strategy imposes appropriate switching sequences with a minimum number of power device commutations per cycle, leading to both half-wave and quarter-wave symmetries at the individual H-bridge cell level, which subsequently improves the converter harmonic performances and reduces the overall switching losses. The validity and effectiveness of both proposed modulation and switching strategies are demonstrated through the intensive simulation and experimental test results provided both in time and frequency domains. The experimental results are obtained on a three-phase seven-level CHB inverter for an 18-kW laboratory prototype.

DC-Link Voltage Ripple Control of Regenerative CHB Drives for Capacitance Reduction

The diode-front-end cascaded H-bridge (CHB) inverters have prevailed in the nonregenerative industry drive domain for high-power medium-voltage applications. The regenerative version of the CHB drives is made possible by adding the extra active-front-end rectifier in each power cell, such as a three-phase PWM rectifier. However, due to the instantaneous power unbalance, the dc-link capacitors of the regenerative power cell need to be overdesigned to maintain a stable low ripple dc-link voltage. To reduce the dc-link capacitance, this article proposes a novel closed-loop voltage ripple controller for the regenerative CHB drive without adding extra sensors. In the proposed method, dc-link voltage ripple amplitude and phase angle are accurately detected with a high-performance adaptive filter. Moreover, a latent instability issue is discussed and is avoided in the proposed controller. The performance of the proposed control strategy is validated experimentally on a seven-level regenerative CHB drive.

Quantitative Comparison and Analysis of Different Power Routing Methods for Single-Phase Cascaded H-Bridge Photovoltaic Grid-Connected Inverter

The output power of each photovoltaic (PV) module is different in the single-phase cascaded H-bridge (CHB) PV grid-connected inverter due to irradiance intensity, ambient temperature, and aging degree of PV modules. When the PV modules are operating at their respective maximum power points, the corresponding dc-side voltages of the H-bridge units drift due to uneven power distribution, which will cause the maximum power point tracking failure. As a result, the grid current will deteriorate and even the system will become unstable. The power routing control methods based on fundamental voltage reconstruction, third harmonic injection and hybrid pulse width modulation can realize unequal power distribution between H-bridges to ensure the system stable operation. This article summarizes the abovementioned typical power routing control methods, derives the expressions of power routing ranges of different methods, then comprehensively and deeply compares the power routing capabilities of various methods qualitatively and quantitatively. The simulation results of a single-phase seven-level CHB PV grid-connected inverter demonstrate the validity of the theoretical analyses.

A Fault-Tolerant Hybrid Cascaded H-Bridge Multilevel Inverter

The cascaded H-bridge (CHB) inverter is one of the most attractive multilevel topologies for renewable energy applications. Due to the fact that CHB inverters employ a large number of components, they suffer from a higher probability of fault, which reduces the system reliability. A fault-tolerant operation for a CHB inverter is described in this article. New features ensure reliable and robust operation of the converter in the event of a fault. The proposed strategy uses an additional cross-coupled CHB (X-CHB) unit in companion with the existing CHB units to support the output voltage and ensure continuity of operation in the event of an open/short-circuit fault. The operation of the proposed X-CHB inverter is described in detail. Simulation and experimental verification of the proposed concept are demonstrated using a seven-level CHB. Both simulation and experimental results confirm the fault-tolerant operation of the X-CHB for a battery energy storage system in case of switch faults.

A New Fault-Tolerant Technique Based on Nonsymmetrical Selective Harmonic Elimination for Cascaded H-Bridge Motor Drives

This article proposes a new fault-tolerant technique to increase the maximum balanced line-to-line output voltage of the cascaded H-bridge (CHB) motor drives. The CHB converters have been widely used for medium-voltage motor drives due to their scalability and reliability features. A significant indicator of the reliability is the maximum balanced line-to-line voltage amplitude under fault conditions. This article adopts a nonsymmetrical selective harmonic elimination (SHE) formulation to further extend the output voltage range with a good harmonic profile under fault conditions. The dc current component can be regulated for the dynamic braking operation. Based on the nonsymmetrical SHE formulation, the fault-tolerant problem that achieves the maximum output voltage range and good harmonic profile is converted to an optimization problem, which can be solved by the proposed optimization framework. By properly selecting the output voltage waveforms, the entire converter voltage capability can be achieved under fault conditions with a good harmonic profile. The performance of the proposed method is evaluated experimentally on a seven-level CHB motor drive.

Incremental Passivity Control in Multilevel Cascaded H-Bridge Converters

A low-capacitance static compensator (LC-StatCom) based on a cascaded H-bridge (CHB) multilevel converter can be an efficient and low-cost solution for reactive power compensation in power grids. However, the reduced capacitance, and, therefore, faster capacitor voltage dynamics, makes it challenging to control it. Classical cascade control schemes, in which the bandwidths associated with the inner current loop and the outer capacitor voltage loop need to be separated enough, are the most reported approaches. Nevertheless, for the LC-StatCom case, the voltage loop can be almost as fast as that one of the current loop. For this reason, this article proposes a control approach that does not separate voltage and current dynamics. Concretely, we propose a multi-input linear time-variant control law, based on incremental passivity theory for CHB multilevel converters. The proposed control guarantees stability despite saturation in the control signal. In addition, it provides capacitor voltage balance without the need to use a specific balancing stage. This article, first, reviews the average model of the converter and, then, analyses the steady-state dynamic behavior to determine the desired and coherent reference signals. Then, it applies an incremental passivity control approach to drive the LC-StatCom state variables to the desired references. After introducing the proposed control law, experimental results on a seven-level 1-kVA CHB LC-StatCom are shown to demonstrate its excellent performance in steady state and transients.

Selective harmonics elimination for three-phase seven-level CHB inverter using backtracking search algorithm

This paper introduces backtracking search algorithm (BSA) to solve the nonlinear transcendental equations to obtain triggering angles of a three phase seven-level cascaded H-bridge (CHB) inverter and also eliminates 5th and 7th order harmonics while satisfying the requested fundamental component. A comparative study has been made in terms of execution time, the statistical probability of achieving global minima, and a rank of convergence rate to establish the supremacy of the BSA optimisation technique over other recognised methods such as genetic algorithm, BEE algorithm, and particle swarm optimisation techniques. Simulation and experimental results confirm the effectiveness and accuracy for obtaining precise switching instants that fruitfully satisfy the required output voltage quality.

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.

Selective harmonics elimination for three-phase seven-level CHB inverter using backtracking search algorithm

This paper introduces backtracking search algorithm (BSA) to solve the nonlinear transcendental equations to obtain triggering angles of a three phase seven-level cascaded H-bridge (CHB) inverter and also eliminates 5th and 7th order harmonics while satisfying the requested fundamental component. A comparative study has been made in terms of execution time, the statistical probability of achieving global minima, and a rank of convergence rate to establish the supremacy of the BSA optimisation technique over other recognised methods such as genetic algorithm, BEE algorithm, and particle swarm optimisation techniques. Simulation and experimental results confirm the effectiveness and accuracy for obtaining precise switching instants that fruitfully satisfy the required output voltage quality.

Improvement of power utilisation capability for a three‐phase seven‐level CHB inverter using an improved selective harmonic elimination–PWM scheme by sharing a desired proportion of power among the H‐bridge cells

An improved selective harmonic elimination-pulse width modulation (SHE-PWM) scheme has been proposed for a three-phase seven-level cascaded H-bridge (CHB) inverter that enhances the power utilisation capability of the inverter by sharing the desired amount of power among the H-bridge cells and also eliminates both 5th- and 7th-order harmonic components from the inverter output voltage maintaining the desired fundamental voltage component for a wide range of modulation index. Compared to conventional SHE-PWM, this scheme introduces two additional switching in the first cell, while the second and the third cells are switched at advanced switching angles. The differential search algorithm (DSA) technique has been applied to solve the proposed SHE-PWM scheme, and it exhibited comparatively better performance than the algorithm based on genetic algorithm, BEE algorithm and particle swarm optimisation. The effectiveness of the proposed scheme has been verified by both simulation and experimental study on a seven-level CHB inverter. Finally, the proposed scheme has been applied to the closed-loop constant V/f control of the induction motor drive application. It has been established that the proposed scheme is independent of the load power factor angle, and it improved the power conversion efficiency of the conventional SHE-PWM scheme.

Reduced number of auxiliary H‐bridge power cells for post‐fault operation of three phase cascaded H‐bridge inverter

This work offers a fault-tolerant strategy that uses a combination of a hardware-based and a control-based fault-tolerant methods for multilevel (ML) cascaded H-bridge (CHB) converters. The possibility of a switch failure in an ML converter is higher than a conventional inverter due to the increased number of power electronics switches. Although the event of a single switch failure is more likely, nonetheless, the fault tolerant operation of a ML converter with two or more faulty switches needs to be addressed as well. This article first offers a hardware topology that can bypass the faulty H-bridge cell and replace it with an auxiliary cell, in the event of a single switch failure. Then, it enhances the hardware topology with a modulation-based fault-tolerant method for the event of more than one defective switch. Using this strategy, the CHB converter will be able to operate with maximum achievable output voltage in the post-fault condition. Experimental results are presented for a seven-level CHB converter for the validation of the theoretical outcomes.

Investigation of Switching Sequences on a Generalized SVPWM Algorithm for Multilevel Inverters

This paper investigates the various switching sequences on a generalized 60[Formula: see text] distributed coordinate system-based space vector pulse width modulation (SVPWM) algorithm for multilevel inverters. The main focus of this work is to improve the inverter output voltage profile by taking advantage of the redundancy inverter switching states. With the help of SVPWM algorithm, the three nearest vectors have to find out the synthesis of the [Formula: see text] location and then by applying the various optimum switching sequences, the inverter produces less harmonic content in the output voltage compared to the conventional switching sequence. All the switching sequence designs are developed with the help of minimum change detector (MCD) by using switching redundancies in order to reduce inverter switching loss. Simulation results to analyze various sequences on the general SVPWM algorithm are presented in the seven-level cascaded H-bridge (CHB) inverter. To validate the results, hardware results are presented on the seven-level CHB inverter.

Power Quality Improvement of High Voltage DC Link using Modified Shunt Active Power Filter

A new control strategy for active power filters is proposed, modeled and implemented in order to improve the power quality of a line commutated converter High voltage DC link. The ability of reactive power and harmonics reductions are generally met by using passive and active power filters. In this paper, modified active power filter with a modified harmonics pulse width modulation algorithm is used to minimize the source harmonics and force the AC supply current to be in the same phase with AC voltage source at both sending and receiving sides of a line commutated converter high voltage DC link. Therefore, it is considered as power factor corrector and harmonics eliminator with random variations in the load current. The modified harmonics pulse width modulation algorithm is applicable for active power filter based on a three-phase five-level and seven-level cascaded H-bridge voltage source inverter. Simulation results show that the suggested modified multilevel active power filters improve total harmonics distortion of both voltage and current with almost unity effective power factor at both AC sides of high voltage DC link. Therefore, modified active power filter is an effective tool for power quality improvement and preferable for line commutated converter high voltage DC link at different load conditions.

Optimal Selective Harmonic Mitigation Technique on Variable DC Link Cascaded H-Bridge Converter to Meet Power Quality Standards

In this paper, power quality standards such as IEC 61000-3-6, IEC 61000-2-12, EN 50160, and CIGRE WG 36-05 are fulfilled for single- and three-phase medium-voltage applications using selective harmonic mitigation-pulsewidth modulation in a cascaded H-bridge (CHB) converter. Furthermore, the ER G5/4 power quality standard, which has the strictest grid codes at the medium-voltage level, will also be met in the whole range of the modulation indices. In order to achieve this goal, a symmetrical CHB with variable dc links are employed, while the converter has a low number of switching transitions. In other words, all of the dc link voltages have the same voltage, which is dependent on the modulation index. Finally, in order to validate the effectiveness and the applicability of the proposed method, it is verified by simulations and experiments for a seven-level CHB converter.

A DC-Side Sensorless Cascaded H-Bridge Multilevel Converter-Based Photovoltaic System

In this paper, a cascaded H-Bridge (CHB)-multilevel converter (MC)-based photovoltaic (PV) system with no voltage or current sensors at the dc-side is proposed. Eliminating the dc-side sensors simplifies the hardware, leading to lower cost and higher reliability of the PV system. A novel scheme estimating the capacitors’ voltages from the ac output voltage of the inverter is developed. The scheme allows replacing all dc-side voltage sensors by a single-voltage sensor at the ac-side of the converter. Furthermore, the dc current sensors, conventionally required for the maximum power point tracking (MPPT), are also eliminated. Instead, the outputs from the capacitors’ voltage control systems are utilized for the MPPT. The effectiveness of the proposed dc-side sensorless system is experimentally demonstrated on a 2-kW single-phase seven-level CHB converter-based PV system.