Cascaded H-bridge Converter Research Articles

Seamless Fault-Tolerant Control for Cascaded H-bridge Converters Based Battery Energy Storage System

This article presents a phase-shifted pulsewidth-modulation-based fault-tolerant approach for the cascaded H-bridge-based battery energy storage system to ride through the single switch fault. When the fault occurs, the faulty cell is bypassed first. Afterwards, the missing voltage levels of the faulty cell are compensated by the remaining healthy cells to maintain the line-to-line voltages balanced. The compensation of the missing voltage levels presented is achieved by simple logic operation without modifying three-phase voltage references and phase-shifted angles of carriers, which enables seamless operation and enhances the system stability. In addition, the detailed active power control strategy used for state-of-charge balancing of the battery units is proposed and discussed. The validity and effectiveness of the proposed method are verified by simulation and experimental results.

A Fault Compensation Scheme for Cascaded H-Bridge Inverter With Reduced Common Mode Voltage

In this article, a common mode voltage (CMV) injection method is proposed to tolerate cell failure for cascaded H-bridge (CHB) inverters. The main purpose of CMV injection is to balance the unequal line-to-line voltages of the inverter without overmodulation when the faulty cells are bypassed. As any injected CMV can interfere with load and inverter proper operation, the proposed fault-tolerant method aims to reduce the peak value and the fundamental harmonic of the injected CMV to minimize its impact on the load and inverter functionality. Also, the proposed method provides features such as simplicity, maximum possible line-to-line voltage, fast real-time calculations, and supporting all faulty cases. The proposed method is evaluated by simulation and experimental results obtained from an 11-level CHB converter.

Discrete-Time Sliding Mode Current Control for a Seven-Level Cascade H-Bridge Converter

This paper deals with the implementation and performance analysis of discrete-time sliding mode (DTSM) current control applied to a seven-level cascade H-bridge converter to track three-phase reference currents for a reactive load. The converter output voltages are synthesized using a modulation scheme based on phase-shifted carrier modulation. Simulation and experimental tests have been added to demonstrate the performance of the proposed controller. At the same time, the effectiveness of the DTSM is verified under transient and steady-state conditions, respectively, by measuring the total harmonic distortion and the mean square error.

HERIC-Based Cascaded H-Bridge Inverter for Leakage Current Suppression in PV Systems

Leakage current can flow through the transformerless photovoltaic (PV) systems because of no galvanic isolation. This undesirable current results in more losses, electromagnetic interferences, and total harmonic distortion. This article presents a hardware-based method to suppress the leakage current in transformerless cascaded H-bridge (CHB) based PV systems. The proposed scheme uses high-efficient and reliable inverter concept in CHB converters for the first time. Using this idea and proper utilization of ac-side filters leads to leakage current suppression in individual cells and, thus, the whole system. In contrast to the most existing solutions, the proposed technique can be easily extended to CHB inverters with an arbitrary number of voltage levels. Moreover, this solution is applicable in the system's conditions with unequal parasitic capacitances and unequal dc-link voltages of H-bridge cells. The validity of the proposed solution is verified by several simulations in the PSCAD/EMTDC environment. Then, the experimental results on a five-level CHB inverter are presented to confirm the theoretical and simulation results.

Maximum Power per Ampere Modulation for Cascaded H-Bridge Converters

This article shows how to exploit the degree of freedom represented by the common-mode voltage, which is inherent in three-phase Cascaded H-bridge (CHB) converters, to minimize the total rms current value of the distributed dc sources. An optimal common-mode voltage injection law is derived, constituting a maximum power per ampere (MPPA) modulation strategy with respect to the currents of the dc sources. The potential benefits introduced by the proposed algorithm are analyzed in the context of battery-fed CHB converters and validated experimentally with a three-time-constant Randles model of a battery cell. The MPPA strategy is compared with traditional common-mode voltage injection methods, and battery loss reduction is demonstrated. The obtained ES is dependent on the converter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> and power factor. Experimental tests on a 36-cell full-bridge CHB converter validate the simulation and numerical derivation. To demonstrate the efficacy of the proposed method in practical applications, a 3 MW energy storage system (ESS) and a 110 kW battery electric vehicle (BEV) undergoing standard drive cycles are presented as case studies. Compared to traditional modulation strategies, the MPPA strategy reduces the battery losses by up to 10.9% in the ESS and 27.5% in the BEV application.

Dynamic Addition of Common Mode Voltage in Post Fault Operation of Cascaded H-Bridge Converter Fed Induction Motor Drive

In this article, a postfault control technique is proposed for a rotor field-oriented induction motor (RFO-IM) drive fed from a cascaded H-bridge multilevel (CHB-ML) converter. The control strategy is to vary the magnitude and phase shift of the injected fundamental common mode voltage (FCMV) according to the motor’s operating condition. The addition of the proposed FCMV with the reference phase voltages, not only ensures maximum line voltage in the postfault operating condition but also shares equal power among the remaining healthy cells of the converter. Moreover, in the postfault operation, a detailed analysis is performed on both motor and converter, and an integrated control is presented to maintain the command speed and load torque of the RFO-IM. This includes the mathematical equation explaining the new operating point of the motor for different fault configurations, traversal of operating point, and the variation in power factor of each leg of the converter in postfault conditions. From the above analysis, the dynamic FCMV is calculated from the new operating point of the stator current components in the current <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -plane and the fault configuration of the CHB-ML converter. The proposed control technique is validated using simulation and experimental results conducted on a 3-hp IM.

Unbalanced Active Power Distribution of Cascaded Multilevel Converter-Based Battery Energy Storage Systems

In a battery energy storage system (BESS) based on the cascaded H-bridge (CHB) converter or modular multilevel converter (MMC), the active power distribution among the submodules (SMs) can be highly unbalanced, especially when batteries with different power ratings are used. Meanwhile, there are certain limits for the unbalanced power distribution among the SMs. It is a challenge to determine whether the active power distribution is viable for cascaded multilevel converters with unequal capacitor voltages because the complexity grows exponentially as the number of SMs increases. This article proposes a generalized analytic solution to this problem, which reduces the complexity significantly and is readily applicable to converters with large number of SMs. The validity of this method is proved theoretically and verified with simulation and experimental results on both the MMC and CHB converter.

Multiport AC-AC-DC Converter for SNOP With One Medium-Frequency Transformer

With the access of increased renewable energy sources, the conventional AC distribution network is hard to flexibly adjust the line voltage and power flow. Therefore, the soft normally open point (SNOP) is applied to increase the flexibility of AC distribution network. The existing converters for SNOP use many submodules (SMs) and passive components, which have the poor economy. On this basis, a multiport AC-AC-DC converter for SNOP with one medium-frequency transformer (MFT) is proposed in this paper. The proposed multiport AC-AC-DC converter is based on the back-to-back (BTB) cascaded H-bridge (CHB) converter structure, and uses several LC resonant circuits and one MFT to replace the multiple DC-DC converters in the existing BTB CHB converters. Therefore, many SMs and passive components can be saved. Moreover, the voltages at multiple AC and DC ports of proposed multiport AC-AC-DC converter are completely decoupled, and the multiple AC and DC systems can be adjusted independently. The detailed circuit structure, control principle and design procedure of multiport AC-AC-DC converter are introduced in this paper. Finally, the simulation and experimental results verify the effectiveness of multiport AC-AC-DC converter.

A Constrained Intersubmodule State-of-Charge Balancing Method for Battery Energy Storage Systems Based on the Cascaded H-Bridge Converter

In the operation of battery energy storage systems (BESSs) based on the cascaded H-bridge (CHB) converters, it is desirable to balance the state of charge (SoC) among the submodules (SMs) within each phase arm. However, there are constraints on the active power distribution among the SMs that intend to balance the SoC. For instance, each SM in a practical system is designed with a specific power rating and can be damaged if it is exceeded. Therefore, a novel rule-based method is proposed in this article to achieve a fast SoC balancing, while respecting the power constraints of the SMs. The proposed method is general and readily applicable to hybrid BESSs, where the SMs are integrated with heterogeneous energy storage. Experimental results are obtained to verify the effectiveness of the proposed algorithm and to compare its performance with respect to existing methods.

Power Line Communication Method for the Simultaneous Transmission of Power and Digital Data by Cascaded H-Bridge Converters

Power line communication (PLC) enables the interconnectivity and interoperability of distributed power production units in smart grids and microgrids by using existing cabling to monitor and control the overall power system. In this article, a novel PLC method for cascaded H-bridge converters is proposed, which is based on the use of the circuits of the individual power modules for the transmission of both power and digital information. Compared to the existing PLC approaches, the method proposed in this article combines the advantages of applicability to cascaded H-bridge structures with either a dc or an ac output, avoidance of using an additional power amplifier or coupling circuit for the transmission of information and independence from the implementation scale and the type of the power-supplied load. The design of both the proposed data transmitter and receiver units on field-programmable gate array devices is presented in the article. The proposed PLC system has been applied to an experimental prototype of a two-cell cascaded H-bridge inverter. The experimental results verified the successful transmission of telemetry data through the power converter circuit and their retrieval by a receiver attached in the power loop, along with the simultaneous transmission of power to the load.

A Simple Hardware-Based Fault-Tolerant Method for Cascaded H-Bridge Converters

A Simple Hardware-Based Fault-Tolerant Method for Cascaded H-Bridge Converters

Integration of PV Panels and EV Chargers on the Modular Multilevel Converter Based SST

This article develops a solid-state transformer (SST), which integrates PV panels and EV chargers together through the open-dc modular multilevel converter (MMC). Different from the well-known MMC, which has a common dc link, the discussed MMC is open-circuited at the dc link so that it should be analyzed from the three-phase viewpoint. The developed SST performs like two parallel-connected cascaded converters, and they are operated with opposite power flow directions. Compared with the conventional SST solution using two separate cascaded H-bridge (CHB) converters, the developed SST has relatively low component count so that it is expected to be of superiority in cost, footprint, and even power density. In addition, the usage of zero sequence voltage in the CHB converter can be replaced by circulating currents in the open-dc MMC so as to greatly reduce the risk of overmodulation. After the thorough power flow analysis on the open-dc MMC and the corresponding controller design, the developed SST is capable of freely working at any operating points with PV panels and EV chargers. Moreover, the developed SST takes the unbalanced grid conditions into account, which is another key contribution of this article. Finally, the balanced and unbalanced operations of the developed SST are both confirmed by the laboratory test results, and furthermore, the dynamic performance of the developed SST considering the time-variant characteristics of PV panels and EV chargers is also demonstrated through the PLECS simulation platform.

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 distributed VSG control method for a battery energy storage system with a cascaded H-bridge in a grid-connected mode

With the high penetration of renewable energy, new challenges, such as power fluctuation suppression and inertial support capability, have arisen in the power sector. Battery energy storage systems play an essential role in renewable energy integration. In this paper, a distributed virtual synchronous generator (VSG) control method for a battery energy storage system (BESS) with a cascaded H-bridge converter in a grid-connected mode is proposed. The VSG is developed without communication dependence, and state-of-charge (SOC) balancing control is achieved using the distributed average algorithm. Owing to the low varying speed of SOC, the bandwidth of the distributed communication networks is extremely slow, which decreases the cost. Therefore, the proposed method can simultaneously provide inertial support and accurate SOC balancing. The stability is also proved using root locus analysis. Finally, simulations under different conditions are carried out to verify the effectiveness of the proposed method.

Online Diagnosis and Ride-Through Operation for Cascaded H-Bridge Converter Based STATCOM With a Single Open-Circuit IGBT

To address the most commonly seen single insulated gate bipolar transistor (IGBT) open-circuit fault problems in cascaded H-bridge (CHB) converter based static synchronous compensator, a simple online fault diagnosis method and the corresponding ride-through operation approach are proposed. The method can quickly determine the exact location of the open-circuit IGBT through three steps, via analyzing the relationships between the dc bias in the three-phase output current of CHB converter, the transient dc rail voltage variations with respect to the averaged dc rail voltage, and the response of dc rail voltage and output current when the modulation approach of the faulty power module is adjusted. Afterwards, a fault ride-through approach is adopted to seamlessly transfer the modulation approach of faulty H-bridge module and it behaves as a half-bridge to provide a half of the rated output ac voltage. Meanwhile, the corresponding dc offset voltage from this faulty module is compensated by producing the same dc voltage in the healthy phase legs to avoid the dc bias in the three-phase output current. It has been demonstrated that the proposed fault diagnosis approach and the fault ride-through approach are effective in various adverse situations including grid voltage sudden disturbances and the load power sudden changes. Comprehensive simulated and experimental results have been provided to validate the effectiveness of the proposed approach.

Effect of Capacitor Voltage Ripples on Submodule Active Power Control Limits of Cascaded Multilevel Converters

In the operation of cascaded H-bridge converters and modular multilevel converters with energy storage or renewable power resources, unbalanced active power distribution among the submodules (SMs) is unavoidable. Depending on the operating conditions, there are certain upper and lower limits on the active power that can be processed by a single SM or a subset of SMs. The control system needs to restrict the SM power references to these limits, hence, accurate knowledge of the power limits is important. In existing methods to derive the power limits, the SM capacitor voltages are assumed to have negligible ripples, whereas in practice the ripples can be considerable. This article analyzes the effect of capacitor voltage ripples on the SM active power control limits and highlights the importance of considering the ripple effect. A methodology is proposed to accurately incorporate capacitor voltage ripples in the derivation of SM active power control limits. Simulation and experimental results are provided to evaluate the effectiveness of the proposed methodology.

A Novel Online Inductance Identification of Traction Network and Adaptive Control Method for Cascaded H-Bridge Converter

The grid-tied cascaded H-bridge (CHB) converter may become unstable, especially for the traction onboard application where the grid impedance may gradually vary as the train moves. In order to overcome this weakness, a small-signal stability-based adaptive control strategy is proposed in this article, where the parameter of the current-inner-loop controller is adaptively regulated according to the online grid inductance identification (GII) result. In the proposed control structure, there is only one control parameter needed to be regulated online, which is based on the estimated value of the grid inductance. In this article, the grid inductance is estimated by injecting the specific harmonic current. With the corresponding harmonic current regulator, stable and sustained harmonic current can be injected, which is beneficial to mitigate the fierce fluctuation of the GII result. The GII algorithm is made up of three parts, including the analog low-pass filter, the sliding discrete Fourier transform, and the digital composite filter, which can suppress the switching noise, the low-frequency grid harmonics, and the grid frequency deviation. With the proposed adaptive control strategy, the CHB can ensure small-signal stability wherever it is connected. All these theoretical analyses are verified by the experiment.

Model-Based Predictive Control with Graph Theory Approach Applied to Multilevel Back-to-Back Cascaded H-Bridge Converters

The multilevel back-to-back cascaded H-bridge converter (CHB-B2B) presents a significantly reduced components per level in comparison to other classical back-to-back multilevel topologies. However, this advantage cannot be fulfilled because of the several internal short circuits presented in the CHB-B2B when a conventional PWM modulation is applied. To solve this issue, a powerful math tool known as graph theory emerges as a solution for defining the converter switching matrix to be used with an appropriate control strategy, such as the model-based predictive control (MPC). Therefore, this research paper proposes a MPC with the graph theory approach applied to CHB-B2B which capable of not only eliminating the short circuit stages, but also exploring all the switching states remaining without losing the converter controllability and power quality. To demonstrate the proposed strategy applicability, the MPC with graph theory approach is tested in four different types of SST configurations, input-parallel output-parallel (IPOP), input-parallel output series (IPOS), input-series output-parallel (ISOP), and input-series output series (ISOS), attending distribution grids with different voltage and power levels. Real-time experimental results obtained in a hardware-in-the-loop (HIL) platform demonstrate the proposed strategy’s effectiveness, such as DC-link voltages regulation, multilevel voltage synthesis, and currents with reduced harmonic content.

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.

Optimization-Based Capacitor Balancing Method with Customizable Switching Reduction for CHB Converters

This paper presents a method for switching reduction in cascaded H-bridge converters. Given the wide applicability of this topology, it would be especially desirable to increase its efficiency with switching losses reduction techniques. Since this type of converter requires voltage balancing methods, several modulation methods consider the possibility of combining the balancing and switching reduction goals together. In this paper, a previously disclosed optimization-based balance method was modified further to consider the switching losses in its objective function. Each commutation was penalized in proportion to the phase current and the module voltage, thus avoiding commutations that would produce the most losses but tolerating low-losses commutations. The structure of the original method was maintained so that the algorithm could be applied with minimal change. The results show that it is possible to reduce the switching up to 14% without any noticeable drawback and up to 22% at the cost of a greater DC-link ripple. It is also possible to selectively reduce the effective switching frequency of only some modules, making it significantly low. This extends the adaptability of the converter, possibly allowing hybrid converters with modules of different transistor technologies.