Current Stress Optimization Research Articles

Current stress optimization of nonagonal MMC for FFTS‐based offshore wind power application

SummaryNonagonal Modular Multilevel Converter (MMC) is a new type of three‐port AC‐AC converter suited for applications in FFTS‐based offshore wind power. It can accommodate three AC ports with only nine branches, greatly reducing the cost and size of energy conversion. However, the multiplexing structure of nonagonal MMC inevitably increases the peak branch current resulting in high semiconductor ratings, which weakens its advantage in cost and size. Since the control degrees of freedom are reduced, traditional current attenuation methods are not applicable. To address this issue, in this paper, a parameter adjustment method is proposed for nonagonal MMC to reduce the current stress. By establishing the mathematical model of nonagonal MMC, phase angle difference between ports of nonagonal MMC is proved to be a key influencing factor of peak branch current. By adjusting phase angle difference, the average current stress of nonagonal MMC can be reduced by as great as 66.31%. Finally, the correctness of the conclusions of this paper is verified by RT‐LAB results.

Current stress optimization control of isolated AC-DC matrix converter

The Isolated AC/DC matrix converter realizes bi-directional energy transmission through phase shift control. The existing control schemes do not coordinate the modulation coefficient and phase shift ratio well, which will cause large current stress to the converter, resulting in low efficiency. To solve this problem, this paper analyses the working mechanism of IAMC under dual phase shift control. It constructs a mathematical model under dual phase shift control to calculate the current stress. An optimal control method for the current stress of isolated AC/DC matrix converter is proposed. The current stress is suppressed by coordinating modulation coefficient and internal and external phase shift ratio. Finally, a simulation platform is built to verify the proposed method.

Efficiency improvement of dual active bridge converter using simple graphical optimization method

SummaryAiming to the problems of high circulating current and low efficiency caused by the traditional strategy of dual active bridge (DAB) converter under light load, in this paper, simple graphical optimization method (GSO) is proposed to improve the efficiency of dual active bridge converter. Through the contour plot of transmission power and current stress, optimal efficiency point can be obtained by tangent of contour map under condition of constant power, and heavily calculation or complex iterations of existing conventional control strategies are avoided. Moreover, analysis of proposed graphical current stress optimization is addressed, and controller design of proposed method is given and described. Efficiency comparison between traditional modulation strategy and GSO is given to show the advantage of proposed mothed. An experimental prototype is built to verify the analysis.

Asymmetric Trapezoidal Wave Modulation of Modular Multilevel Resonant DC/DC Converter for Current Stress Optimization

A modular multilevel resonant dc/dc converter (MMRDC) possesses high power and high voltage rating, which is adopted for high step-ratio interconnection in dc grid applications. However, the current stress and reactive power of MMRDC are high with the conventional quasi-square-wave modulation scheme when in a wide voltage range and light load conditions. Hence, this article proposes an optimized asymmetric trapezoidal wave (ATW) modulation scheme that reduces current stress over wide load and voltage ranges by separately modulating the duty cycles of string output voltage rising and falling processes. The time-domain mathematical model of MMRDC with the ATW modulation scheme is established. Then, based on the soft switching characteristic and reactive current analysis, the optimized operating point of MMRDC can be derived while considering low current stress and zero voltage switching constraint of low voltage side full-bridge switches. On this basis, the numerical solutions of optimized control variables are obtained, and a control block diagram is presented for the practical implementation of the optimized ATW modulation scheme. Finally, a 4kW MMRDC prototype is constructed, and the experimental results verify the correctness and effectiveness of the analysis and the optimized ATW modulation scheme.

Time-Domain-Based Superposition Analysis for Triple Active Bridge and Its Application for ZVS and Current Stress Optimization

With the advantages of simple topology, voltage matching, electrical isolation, and bi-directional energy control, Triple Active Bridge (TAB) has become an active exploration and attempt for the flexible interconnection of DC ports. The high-frequency chain current (HFCC) calculation is the crucial step of zero voltage switching (ZVS) regions and current stresses, providing the basis for the subsequent performance optimization of TAB. Due to the number of switches increasing, TAB´s degrees of freedom (the phase shift angles and duty cycles) have multiplied compared with its two-port topology. The existing current calculation methods of TAB are challenging to balance accuracy and simplicity. Currently, the simple and accurate HFCC model considering all the TAB variables can hardly be found in the literature. This paper proposes a unified and novel solution for TAB HFCC calculation based on the superposition theorem, Thevenin theorem, and time domain method. It can get the uniform formula of the HFCC when all the control variables change. Based on the formula derived, this paper further presents the corresponding ZVS and current stress analysis of TAB. A multi-objective optimal control method is proposed, demonstrating efficiency improvement. Moreover, the trade-offs between the control simplicity and optimization effect are analyzed based on the proposed HFCC model. The modulation pattern with the least algorithmic complexity for efficiency maximization is found. The validity and accuracy of the proposed method have been verified experimentally.

A Bidirectional Fast EV Charger for Wide Voltage Range Using Three-Level DAB Based on Current and Voltage Stress Optimization

Converter cost is the crucial concern while designing off-board electric vehicle (EV) chargers for high-voltage charging applications using advanced high breakover voltage power semiconductor switches. To solve this problem, multilevel topology is used with the typical dual active bridge (DAB) for the development of high-voltage and high-power EV charging. It offers low cost, compact, less complex control, highly efficient, and reliable development. This article presents the design and development of bidirectional off-board fast EV charger with voltage and current stress optimization for wide voltage range. It is designed with two-stage power conversion. Here, active front-end converter is connected to three phase grid and controlled through synchronous reference frame (SRF) d-q control approach for active rectification. The large range of power and voltage variation in off-board EV charger design makes it challenging to develop a controller that allows the converter to operate inside the zero voltage switching (ZVS) operating region. In this context, a detailed circuit analysis of dc–dc stage with dual phase shift (DPS) control under different operating conditions is presented to identify the coordinates of safe operating region with reduced voltage stress. To select the converter control parameters with optimum current stress in conjunction with reduced voltage stress, particle-swarm-optimization technique is applied to the identified operating regions. This provides a unique selection criterion for both control parameters over wide voltage conversion ratio. A coordinated control is designed based on the identified boundaries for easy implementation and less computation burden on microcontroller units (MCUs). A 7.2-kW system is designed and simulated on MATLAB/Simulink. A 3.3-kW laboratory prototype is designed to validate the identified operating points with wide battery voltage range under varying source and loading scenarios.

Current stress optimization control strategy of the buck topology with the variable frequency/amplitude mode

The Buck circuit is a common topology in DC-DC converter. The existing control strategies have the advantages of short modulation time and high dynamic performance, but they also have the disadvantages of poor active modulation of the working mode of the converter and large current stress. In response to the above problems, an improved frequency/amplitude modulation control strategy is proposed in this paper. The relationship between the output power and the peak inductor current in three modes, namely, Discontinuous Conduction Mode Critical Conduction Mode (CRM) and Continuous Conduction Mode is derived. The precise control strategy of the inductor peak current is introduced based on the traditional voltage loop control strategy to realize the free control of the system working mode and reduce the current stress during the working process. Finally, the experimental platforms of 100V/12V and 220/80V are established respectively. The experimental results show that the flexible switching circuit operating mode in the full load range can be realized under the new control method. In the 100/12V experimental platform, the current stress of the switch tube is optimized by at least 10.7%, and the average system efficiency is improved by 2.21%, with the same main circuit parameters.

Model Predictive Power Control for Bidirectional Series Resonant Isolated DC–DC Converters With Steady-State and Dynamic Performance Optimization

With more and more attention on dual bridge series resonant isolated dc–dc converters in power electronic transformer, various phase-shift optimization methods have been widely studied. In this article, a model predictive power control with a triple phase shift scheme is proposed to improve both the steady-state and dynamic performance of the adopted converter. In addition, the proposed method can realize the dynamic response improvement without the output current sensor, compared with the existing methods. With the current stress optimization model, the minimum current trajectory control can be achieved. The proposed model predictive power control can greatly improve dynamic performance compared with the traditional PI control and is not sensitive to the circuit parameters. The design procedure of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> resonant tank is given to ensure minimum current stress operation. Finally, comparative experimental tests in a 400 W hardware prototype are carried out to verify the effectiveness of the proposed method in both steady-state and dynamic performance optimization.

Current Stress Optimization of Dual Active Bridge Converter in Two-Stage Single-Phase Inverter System With Second Harmonic Current Shaping

With consideration of double line-frequency instantaneous power in two-stage single-phase inverter system, a second harmonic current shaping (SHCS) method is proposed in this paper to minimize the current stress of front-end dual-active-bridge (DAB) DC/DC converter. The current stress of DAB is optimized by analytical expressions in time domain, so that both the low-frequency fluctuations and inherent high-frequency characteristics of DAB current are considered. As a result, the current stress of DAB can be minimized. The analytical expressions of optimized double line-frequency output current of DAB are derived. An implementation method is presented to combine the traditional single voltage closed-loop (SVC) control method with the feedforward compensation of only double line-frequency output current of DAB. Therefore, the stability and dynamic performance of SVC are not affected by SHCS method. The parameters design of SHCS method is the same as that of SVC method. The minimum current stress and fast dynamic response can be achieved by SHCS method without any compromise. The performance evaluation indicates that SHCS method is superior to traditional methods in terms of optimization effects and implementation method. The experimental results verify the good performance of SHCS method both in steady-state and dynamic state.

Sensor-Reduction Control for Dual Active Bridge Converter Under Dual-Phase-Shift Modulation

This paper proposes a sensor-reduction control in order to control the output voltage of the dual active bridge (DAB) converter under dual-phase-shift (DPS) modulation. The deadbeat control-based Lagrange multiplier method (LMM) is adopted to achieve current stress optimization (CSO). A first-order extended state observer (ESO) is established to observe the load current based on the basis of the dynamic equation of the output capacitor. Compared to the existing ESO for the DAB converter, the proposed observer is able to reject disturbances even when output capacitor mismatches occur, resulting in better observer performance. Besides, the proposed method exhibits good steady-state performance without using any compensating controller. Consequently, the proposed method enables current sensorless control and significantly decreases the number of control parameters, resulting in increased simplicity and cost-saving. The simulation and experimental results obtained with a 300 W prototype demonstrate that the proposed method outperforms the existing methods under changing load current and input voltage conditions.

Current Stress Optimization for Double-Sided CLLLC Topology-Based IPT System With Constant Output Current Tolerating Pad Misalignments

In the inductive power transfer (IPT) system, a hybrid topology with the anti-misalignment ability and load-independent output is usually used to improve the stability of the system and simplify the control scheme. However, the power distribution of the hybrid topology is uneven with misalignment, resulting in high current stresses in partial coils. In order to minimize the current stresses, a parameter design method with double-sided capacitor-inductor-inductor-inductor-capacitor compensation topology is proposed to optimize the coil currents without the effect on the output fluctuation. Finally, a 3.5-kW laboratory-scale prototype was built to validate the proposed method. The results show that the proposed parameter design method can minimize the maximum current stress significantly, and the hybrid IPT system has good performances of misalignment tolerance and load-independent output current.

Research on Marine DAB bidirectional DC converter based on DPS

This paper takes the application of bidirectional DC-DC converter in Marine hybrid power system as the background, the problems of high reflow power and current stress in dual active bridge converter under traditional phase shift control are studied in this paper. Based on the dual phase shift control strategy, the power transmission optimization algorithm model is analyzed and derived, and a control method for optimizing mode switching is proposed. The optimized control algorithm is validated by Matlab/Simulink simulation experiment. The results show that the reflow power and current stress optimization control algorithm can effectively reduce the reflow power and the current stress of the converter, so as to improve the operation performance of DAB converter in high-power ship hybrid power system.

Integrated optimization Control Strategy of DAB Based on DPS and Adaptive Genetic Algorithm

In order to comprehensively consider the energy transmission efficiency of the isolated Dual-Active-Bridge (DAB) DC-DC converter, this paper proposes a control method that maximizes the power transmission efficiency of the whole converter and optimizes the stress of the switching device by using the dual-phase-shifting control mode under the premise of simultaneously considering current stress, back-flow power and power loss. Firstly, we compare and analyze the advantages and disadvantages of DPS, SPS and TPS, and finally determine the control method using DPS. Then analyze and compare the basic principles of the traditional current stress optimization target control method and the comprehensive efficiency optimal control method proposed in this paper. We compared the working mode analysis with the stability, current stress and application range of soft-switching. The adaptive genetic algorithm is used to optimize the two duty ratio degrees of freedom of the control strategy, and the optimal duty ratio corresponding to the objective function is obtained. Finally, the correctness of the control method and the superiority of the power transmission efficiency are proved by simulation and experimental comparison.

High-Frequency-Link Current Stress Optimization of Cascaded H-Bridge-Based Solid-State Transformer With Third-Order Harmonic Voltage Injection

Cascaded H-bridge (CHB) -based solid-state transformer (SST) consists of CHBs and dual active bridge (DAB) converters. Series-resonant DAB converters (SRCs) are widely used for its advantages of soft switching characteristic and simple control. However, the high-frequency-link (HFL) currents of SRCs contain double-line-frequency harmonic fluctuation in three-phase CHB-based SST, resulting in high current stress. In this article, the SRC is substituted with dc terminal dynamic equivalent circuit, and the analytical expression of the HFL current fluctuation is deduced. With consideration of third-order harmonic voltage injection (THVI), the HFL current fluctuation characteristics with this method are analyzed and the optimal amplitude and phase of the injected third-order harmonic voltage are derived, with which the HFL current stress can be minimized. Both the simulation and experiment results indicate that the HFL current stress of SRCs can achieve a reduction of about 20% with the optimized THVI method.

Transient DC Bias Elimination of Dual-Active-Bridge DC–DC Converter With Improved Triple-Phase-Shift Control

A transient dc-bias current due to the voltage-second imbalance of isolated bidirectional dual-active-bridge (DAB) converters for the disturbance in line or load may result in the transformer saturation and oscillations in both sides dc currents. This article focuses on the transient dc-bias current elimination by using an improved triple-phase-shift (ITPS) control for DAB converters. The inductor peak current stress optimization is adopted in the proposed ITPS to determine the steady-state phase-shift variables. Originated from the dc-bias current model of DAB converters with the TPS control, the transient phase-shift adjustment strategy can be determined, which has the ability to improve the inductor current changing slope and shorten the settling time. Both simulation and experiments for different conditions are provided to evaluate main dynamic indexes such as the transient period, dc-bias current, and inductor current stress for three different transition cases. The proposed ITPS is proved as a promising solution in eliminating the dc-bias current, minimizing the transient current stress.

A Simple Power Estimation With Triple Phase-Shift Control for the Output Parallel DAB DC–DC Converters in Power Electronic Traction Transformer for Railway Locomotive Application

Aimed at the input-independence and output-parallel dual-active-bridge (DAB) dc–dc converters applied to power electronic traction transformer (PETT), this paper proposes a simple power estimation scheme with triple phase-shift control (PES-TPS) to achieve the transmission power balance, dynamic performance improvement, and efficiency optimization, simultaneously. Through executing the current stress optimization (CSO) and introducing the power estimation based on operational parameters, the efficiency of converters can be further improved and the power balance among DAB cells can be achieved over the whole power range without adding extra sensors to sample the output/inductor current of each cell. In addition, the dynamic performance of the adopted converters can be improved by compensating power loss online. Especially, dynamic response of the output voltage can be enhanced under input voltage and load fluctuation conditions. Finally, a comprehensive experimental comparison of CSO with dual phase shift (CSO-DPS), CSO with TPS (CSO-TPS), and the proposed PES-TPS control is conducted in the DAB converters experimental prototype with three cells. The experimental results have confirmed the excellent performance of the proposed PES-TPS control and the correctness of the theoretical analysis.

Model Predictive Control With Power Self-Balancing of the Output Parallel DAB DC–DC Converters in Power Electronic Traction Transformer

This paper focuses on the output parallel dual-active-bridge (DAB) dc–dc converters in power electronic traction transformers. A model predictive control with current-stress-optimized (MPC-CSO) scheme based on dual-phase-shift (DPS) control is proposed to improve the dynamic performance, balance the transmission power, and realize the current-stress optimization. The dynamic behavior of output voltage in the next horizon is predicted accurately under the input voltage fluctuation and load disturbance conditions by developing the prediction model. In addition, the proposed MPC-CSO scheme can track the output voltage with the desired value directly with no overshoot during the start-up process. Combining the MPC and CSO scheme, the fast dynamic response and high-efficiency performance of DAB converters can be achieved simultaneously, and the transmission power of each DAB cell can be self-balanced. Finally, three control schemes consisting of traditional voltage closed-loop control with single phase shift, traditional CSO control with DPS, and MPC-CSO with DPS schemes are compared in a scale-down three DAB dc–dc converter cells experimental prototype by using TMS320F28335+FPGA_6SLX45 as core controller. Extensive experimental results have verified the excellent performance of the proposed MPC-CSO scheme and associated analysis in this paper.

Unified Triple-Phase-Shift Control to Minimize Current Stress and Achieve Full Soft-Switching of Isolated Bidirectional DC–DC Converter

This paper presents a comprehensive analysis of the current-stress optimization and soft-switching operation of the isolated bidirectional dual active bridge (DAB) dc–dc converter with the unified triple-phase-shift (UTPS) control. On this basis, the current-stress-optimized modulation scheme is proposed for DAB, which leads to the minimum current stress with the required transmission power and voltage conversion ratio in the whole load range. Moreover, the full soft-switching operation is achieved for the converter simultaneously. Distinct from the previous modulation schemes, the proposed optimized modulation scheme is deduced from a unified analysis of TPS where all effective switching modes are investigated. A novel algorithm based on the Karush–Kuhn–Tucker (KKT) conditions is originally proposed to derive closed-form solutions for the global optimal control parameters. This paper also presents a typical closed-loop control strategy for DAB with TPS control and detailed descriptions about the closed-loop operation. A laboratory prototype is applied, and the experimental results validate that the current-stress optimization and efficiency improvement are realized by applying the optimized modulation scheme. The experimental results also verify the effectiveness of the closed-loop control strategy for DAB with TPS control.

Minimum-Current-Stress Scheme of Dual Active Bridge DC-DC Converter With Unified-phase-shift Control

To reduce current stress and improve efficiency of dual active bridge (DAB) dc–dc converters, various control schemes have been proposed in recent decades. Most control schemes for directly minimizing power losses from power loss modeling analysis and optimization aspect of the adopted converter are too difficult and complicated to implement in real-time digital microcontrollers. Thus, this paper focuses on a simple solution to reduce current stress and improve the efficiency of the adopted DAB converter. However, traditional current-stress-optimized (CSO) schemes have some drawbacks, such as inductance dependency and an additional load-current sensor. In this paper, a simple CSO scheme with a unified phase-shift (UPS) control, which can be equivalent to the existing conventional phase-shift controls, is proposed for DAB dc–dc converters to realize current stress optimization. The simple CSO scheme can overcome those drawbacks of traditional CSO schemes, gain the minimum current stress, and improve efficiency. Then, a comparison of single-phase-shift (SPS) control, simple CSO scheme with dual-phase-shift (CSO-DPS) control, simple CSO scheme with extended-phase-shift (CSO-EPS) control, and simple CSO scheme with UPS (CSO-UPS) control is analyzed in detail. Finally, experimental results verify the excellent performance of the proposed CSO-UPS control scheme and the correctness of theoretical analysis.