Three-phase Dual Active Bridge Converter Research Articles

Practical Dead-Time Control Methodology of a Three-Phase Dual Active Bridge Converter for a DC Grid System

An effective dead-time control strategy for the three-phase dual active bridge (3P-DAB) converter of a distribution system is studied to reduce the switching losses of power switches and improve the under-light-load power conversion efficiency. Because of the advantages of a dual-active bridge converter, such as an inherent zero-voltage switching (ZVS) capability without any additional resonant tank and a seamless bi-directional power transition, this is an attractive topology for bi-directional application. The 3P-DAB converter is apt for high-power applications such as aircraft due to an interleaved structure, which can reduce conduction losses. However, the design of the dead time depends on engineering experience and empirical methods. In order to overcome the conventional practicality of the dead-time design method, the effective control of dead time is proposed based on the theoretical analysis. In this paper, the overall explanation of the 3P-DAB converter is shown with operation principles. In addition, the dead-time effect of the 3P-DAB converter is examined and the practical variable dead-time control strategy is studied. Finally, experimental results validate the proposed variable dead-time control strategy using a 25 kW prototype 3P-DAB converter.

Impact of Parameter Mismatch on Three-Phase Dual-Active-Bridge Converters

Three-phase dual active bridge converters (DAB3) are a widely used topology in battery charging applications thanks to their numerous advantages, such as bidirectional power flow, galvanic isolation, low output current ripple, and inherent soft-switching. In such applications, three single-phase transformers are commonly employed as the AC-link to simplify manufacturing and reduce costs. These transformers’ leakage inductance can be utilized instead of the external leakage inductance to achieve high power density. However, the assumption of uniformity in these inductances is not always accurate as they can vary significantly during fabrication. This study presents a comprehensive analysis of the impact of transformer leakage inductance variation, which can deviate by up to 24% from the desired value. The effects of this variation are investigated from different perspectives, including power transfer, soft-switching range, root-mean-square (RMS) current, and the temperature rise of the transformer winding. Although the power transfer and total copper loss of transformers are changed insignificantly even under highly mismatched leakage inductance, the currents and thermal distribution among phases are considerably impacted. Based on statistical probability, a maximum leakage inductance variation threshold of 10–15% compared to the desired value is recommended to ensure the maximum acceptable temperature rise among phases. Experimental results are presented to validate the analysis.

Three-Phase Single-Stage Bidirectional CCM Soft-Switching AC–DC Converter With Minimum Switch Count

In this article, a three-phase single-stage bidirectional ac–dc converter with low component count is proposed. The single-stage structure is configured by integrating a three-phase ac–dc converter and a three-phase dual active bridge converter. The power factor correction and bidirectional power control are performed by adjusting the modulation index of sinusoidal pulse width modulation (SPWM) and phase-shift angle between the primary and secondary bridges. The low-frequency components generated by SPWM are absorbed by fundamental blocking capacitors connected in series with transformer windings, resulting in true high-frequency isolation. The proposed converter can achieve soft-switching of all switching devices even in continuous conduction mode. A 110 Vac, 3 kW, 100 kHz prototype is implemented to validate the proposed concept and demonstrated 95.34% peak efficiency.

An Asymmetrical Phase-Shift Scheme of Three-Phase Dual Active Bridge With Minimum Current Root-Mean-Square Value Control

The three-phase dual active bridge (3ph-DAB) converter is a promising topology attracting increasing attention. However, because it is limited to symmetrical three-phase bridge control, the conventional single-phase-shift control method (SPS) remains the most common method for 3ph-DAB converter. The obvious shortcomings of SPS control include the soft-switching failure, massive circulating energy, and high root-mean-square (rms) current value at the light load, particularly with large voltage variation-ratio conditions. To overcome these drawbacks, this article proposes a novel asymmetrical phase shift (APS) control strategy. Unlike other optimized modulation schemes, this optimized strategy breaks the symmetrical control rules of three-phase bridges by introducing variable phase-shift angles among three-phase legs. The novel APS control strategy can substantially reduce the current's rms value and extend the soft-switching operation range due to these additional phase-shift angles. Further, the analytical solutions for the optimal phase-shift angles are derived by solving the global optimal condition equations. Analytical solutions, compared with complex numerical solutions derived by other optimized schemes, can be easily adopted in digital implementation and perform well under changing parameter conditions. Finally, experimental results verify the outstanding performance of the proposed modulation strategy.

A Gen-3 10-kV SiC MOSFET-Based Medium-Voltage Three-Phase Dual Active Bridge Converter Enabling a Mobile Utility Support Equipment Solid State Transformer

The emergence of medium-voltage silicon carbide (SiC) power semiconductor devices, in ranges of 10&#x2013;15 kV, has led to the development of simple two-level converter systems for medium-voltage applications. A medium-voltage mobile utility support equipment-based three-phase solid state transformer (MUSE-SST) system, based on Gen3 10 kV SiC MOSFETs, is developed to interconnect a three-phase 4160 V/60 Hz grid to a three-phase 480 V/60 Hz grid to provide a shore-to-ship power interface for naval vessels. The MUSE-SST system consists of three power conversion stages, namely, MVac/MVdc stage (MV: active front-end converter), MVdc/LVdc stage (dual active bridge converter), and LVdc/LVac stage (LV: active front-end converter). The galvanic isolation is introduced in the MVdc/LVdc stage using MV/LV high-frequency transformers (HFTs). This article demonstrates the operation of the three-phase Y&#x2013;<inline-formula> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> connected dual active bridge converter used in the MVdc/LVdc stage of the MUSE-SST system. Equations for phase currents, power flow, and zero-voltage switching (ZVS) boundaries are derived for all possible modes for the three-phase Y&#x2013;<inline-formula> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> configuration. A detailed parasitic simulation model is derived by measuring and experimentally verifying the parasitic elements of the HFT. A brief discussion regarding the design considerations required for the hardware development of the medium- and low-voltage sides of the three-phase dual active bridge converter is also provided. Successful tests demonstrating the operation and feasibility of the medium-voltage dual active bridge converter, at medium-voltage levels (7.2 kV dc-link voltage), are shown. The results indicate that these devices can accelerate the growth and deployment of the medium-voltage SiC-based converter for isolated and bidirectional medium- to low-voltage dc systems.

Open Transistors Fault-Tolerant Schemes of Three-Phase Dual Active Bridge DC-DC Converters

This work proposes fault-tolerant schemes for open-transistor faults applied to three-phase dual active bridge converters (TPDABC), using different transformers: star-star, delta-delta, and delta-star. The proposal consists of operating the TPDABC as a single-phase dual active bridge converter, modifying the modulation strategy. This strategy avoids the need for additional components, which allows the converter to continue operating after an event of open transistor failure. The expressions of the average power when the converter operates in the fault-tolerant modes are determined. The analysis determines that the maximum power that can be transferred using the transformer delta-star is larger than the power obtained for star-star and delta-delta connections. Simulation results allow the proposed schemes to be validated.

Closed-Form Asymmetrical Duty-Cycle Control to Extend the Soft-Switching Range of Three-Phase Dual-Active-Bridge Converters

The three-phase dual-active-bridge (DAB) converter is a promising topology for high-power dc–dc conversion in dc grids and industrial applications. However, the conventional single-phase-shift modulation shows a limited soft-switching range and large reactive currents under light-load operation especially with large voltage variations. This article proposes an asymmetrical duty-cycle control method for the three-phase DAB to extend the soft-switching range and reduce reactive currents by realizing three-phase triangular- and trapezoidal-current-mode operation. Closed-form solutions of the proposed modulation schemes are derived that enable a simple online calculation of the control parameters. The proposed modulation schemes can substantially reduce losses of both power semiconductor devices and the medium-frequency transformer. Thus, the three-phase DAB converter can realize high efficiency over a wide operation range, especially under light-load conditions. Considering the practical implementation, a compensation method is presented to correct the distorted current waveforms caused by the dead time and ensure the desired soft-switching operation. Besides, the power loss imbalance between top and bottom switches induced by the asymmetrical duty cycle is addressed by an active thermal balancing strategy. The effectiveness of the proposed method is validated by simulation results and experiments on a downscaled converter prototype.

Current optimal control method for three-phase dual active bridge converter in light load conditions

Three-phase dual active bridge converter has many performance advantages, and is widely used in electric vehicle charging, battery energy storage system, power electronic transformer, and other energy conversion occasions. However, in the traditional control method, it has the problem of low efficiency under light load conditions. In this paper, firstly, the power and current expressions of the converter under light load conditions are solved by time-domain analysis, and an optimal current control method under light load conditions is proposed. This control method can simultaneously realize the minimum inductance current stress and RMS. Finally, the effectiveness of this method is verified by experiments.

Practical Controller Design of Three-Phase Dual Active Bridge Converter for Low Voltage DC Distribution System

In a low voltage DC (LVDC) distribution system, isolated bi-directional DC-DC converters are key devices to control power flows. A three-phase dual-active-bridge (3P-DAB) converter is one of the suitable candidates due to inherent soft-switching capability, low conduction loss, and high-power density. However, the 3P-DAB converter requires a well-designed controller due to the influence of the equivalent series resistance (ESR) of an output filter capacitor, degrading the performance of the 3P-DAB converter in terms of high-frequency noise. Unfortunately, there is little research that considers the practical design methodology of the 3P-DAB converter’s controller because of its complexity. In this paper, the influence of the ESR on the 3P-DAB converter is presented. Additionally, the generalized average small-signal model (SSM) of the 3P-DAB converter including the ESR of the capacitive output filter is presented. Based on this model, an extended small-signal model and appropriate controller design guide, and performance comparison are presented based on the frequency domain analysis. Finally, experimental results verify the validity of the proposed controller using a 25 kW prototype 3P-DAB converter.

Current Stress Optimisation Strategy for Three-phase Dual Active Bridge Converter during the Start-Up Process

Three-phase dual active bridge (3p-DAB) converter is widely used in high-power bidirectional power transfer applications, due to its unique high power density and easy implementation of soft switching. In this paper, the start-up process of the 3p-DAB converter is studied. The cause of high current stress, especially in the case of heavy-load start-up is analysed, and a new start-up control strategy for the start-up process is proposed. The control strategy is helpful to reduce the current stress of the transformer and the switches by adjusting the duty-cycle and the phase shift angle coordinately without adding any other hardware circuits. Finally, based on a TMS320F28377D + XC6SLX25 controller, a 30kW 3p-DAB converter prototype with input voltage 650-750V and output voltage 400-500V is developed to verify the effectiveness of the proposed start-up strategy.

Second Harmonic Current Reduction for a Battery-Driven Grid Interface With Three-Phase Dual Active Bridge DC–DC Converter

Two-stage isolated battery-driven grid interface (IBDGI) with front-end three-phase dual active bridge (DAB) dc–dc converter is promising to be applied in the residential battery energy storage system for supporting photovoltaic (PV) integration, load shifting, and backup power purposes. However, the second harmonic current (SHC) caused by the pulsating power of the downstream single-phase inverter will increase the battery's degradation and the component stress of the front-end converter. In this paper, a reduction method is proposed for the IBDGI with three-phase DAB converter using a load current feedforward control. The proposed idea is to incorporate virtual impedance to the output impedance of the front-end converter. The easy-to-be-implemented method does not need to modify the original voltage closed-loop controller but suppress SHC while keeping good dynamic performance. As the first step, the small-signal model of the three-phase DAB converter is provided. The parameter sensitivity analysis is proposed to demonstrate that the SHC can be well-suppressed within $\pm \text{20}\%$ parameter error of the feedforward control. Finally, a 1 kW experiment setup is built to verify the performance of the proposed SHC reduction method.

Control Method for Avoiding Transformer Saturation in High-Power Three-Phase Dual-Active Bridge DC–DC Converters

The classic top to down structure of electrical infrastructure is changing due to the extensive spread of renewable energy sources which are connected to medium-voltage and low-voltage grids. Especially for medium-voltage direct-current grids, the three-phase dual-active bridge (DAB) dc–dc converter is one of the most promising converter topologies, as it features bidirectional power flow and galvanic isolation at highest efficiency. Unwanted saturation of the transformer of this converter can occur, particularly for multimegawatt converters with highly efficient, low-loss transformers. The saturation leads to additional core losses, elevated voltage stress at the windings, and increased magnetostriction of the transformer core, which can result in undesired acoustic emissions. The following article presents a feedback control and modulation method that controls the DAB converter such that saturation of the transformer is avoided. The chosen approach evaluates the star-point voltage of the transformer to determine the dc component of the magnetizing current, thereby avoiding the need for expensive high-precision current sensors. The control method is analytically derived and verified by simulation. Experiments carried out using a 5 MW medium-voltage three-phase DAB converter prototype verify the presented modulation method.

Open-Phase Fault-Tolerant Operation of the Three-Phase Dual Active Bridge Converter

The three-phase dual active bridge (3p-DAB) converter is widely considered in dc-grid applications. Because of the higher number of switches in the 3p-DAB, it can be argued that the reliability of the 3p-DAB is reduced when compared to other isolated-bidirectional dc-dc converter topologies. The previous work has shown that the 3p-DAB can be operated in a frozen leg fault-tolerant mode, i.e., with the two transistors of the same phase being opened by their gate driver internal protections. Because the free-wheeling diodes are left self-commutated, the analytical characterization of the converter for all voltage and loading conditions is not trivial. In this article, it is proposed to open the faulty phase such as it eliminates the interaction with the faulty-phase free-wheeling diodes. This allows the converter to fall in a characterizable operating mode for all voltage and loading conditions. The results further show that the open-phase operation provides advantages over the frozen leg operation in terms of current stress and power transfer capability. Experimental results on a small-scale closed-loop gallium nitride-based prototype as well as time-domain simulation results are provided to support the theoretical analyses.

Instantaneous Flux and Current Control for a Three-Phase Dual-Active Bridge DC–DC Converter

The three-phase dual-active bridge converter is a promising topology for the high-power high-frequency dc–dc conversion, due to the soft-switching capability and inherent galvanic isolation. With the state-of-the-art instantaneous current control (ICC) based on the single phase-shift (SPS) operation, a bidirectional power flow can be controlled dynamically between two dc ports without inducing overshoots in the transformer currents. However, a transient dc-bias flux may still occur in the transformer, which may cause the transformer core saturation in the dynamic operation such as start-up, power- off , and abrupt load changes. To analyze and address this issue, this paper presents a set of model-based methods to avoid inducing the dc-bias flux in various transient situations. The proposed double-side SPS operation could nullify the dc-bias flux in abrupt load changes and even instant power-flow reversals. Furthermore, the soft-magnetizing and soft-demagnetizing techniques are presented to eliminate the initial and residual dc-bias flux during start-up and power- off instantly. The proposed flux control method can be integrated with the ICC. Thus, the transformer magnetizing flux and the winding currents can be controlled simultaneously and instantaneously in transient situations. Simulations and experiments on a down-scaled hardware prototype validate the effectiveness of the proposed methods.

Large-signal modeling of three-phase dual active bridge converters for electromagnetic transient analysis in DC grids

The three-phase dual active bridge (3p-DAB) converter is widely considered in next-generation DC grid applications. As for traditional AC grids, the successful integration of power electronic converters in DC grids requires accurate time-domain system-level studies. As demonstrated in the existing literature, the development and efficient implementation of large-signal models of 3p-DAB converters are not trivial. In this paper, a generalized average model is developed, which enables system-level simulation of DC grids with 3p-DAB converters in electromagnetic transient type (EMT-type) programs. The proposed model is rigorously compared with alternative modeling techniques: ideal-model, switching-function and state-space averaging. It is concluded that the generalized average model provides an optimal solution when accuracy of transient response, reduction in computation time, and wideband response factors are considered.

Frozen Leg Operation of a Three-Phase Dual Active Bridge Converter

Three-phase dual active bridge (DAB) topology is a potential alternative for high-power applications when a compact and efficient converter with a bidirectional power transfer capability is desired. In a constructed prototype, high-power SiC modules with dedicated drivers are utilized to achieve high-efficiency and compact size. Each module has two interconnected switches with anti-parallel diodes resembling a converter leg. It is observed that the driver halts the module operation as a result of protective actions such as overcurrent, gate undervoltage, or gate overvoltage. In this frozen leg mode, the module operates as a leg with two diodes until an external hardware signal resets the driver. The converter continues operation but with a reduced performance. Analysis, simulation, and verification of a three-phase DAB converter under a frozen leg operation are considered in this paper. The converter with a frozen leg has two different behaviors at light loads and heavy loads. Consequently, two different analysis methods are developed to solve converter operation in different load conditions. Results show that the power transfer capability is reduced, but this fault mode is nondestructive.

Optimized Modulation and Dynamic Control of a Three-Phase Dual Active Bridge Converter With Variable Duty Cycles

The three-phase dual active bridge (3p-DAB) converter is a promising topology for high power dc–dc conversion due to advantages of bidirectional power flow, inherent soft-switching capability, and reduced filter volume. This paper presents comprehensive analysis of the duty cycle control (DCC) for optimizing the performance of the 3p-DAB. Based on DCC, an optimized modulation strategy is proposed to minimize the conduction losses of the 3p-DAB in the whole load range. The proposed modulation strategy extends the soft-switching range of the 3p-DAB with large voltage variations simultaneously. It is established through loss analysis that the proposed modulation strategy boosts the efficiency of the 3p-DAB, especially at low loads. When the duty cycles change fast as a result of the abruptly changed transmission power, the transformer currents can become unbalanced, leading to the magnetic bias and oscillations in dc currents. This paper further proposes a fast transient current control (FTCC) method for the 3p-DAB with variable duty cycles. The FTCC enables the converter to transfer from one steady state to another within about one-third switching period, hence balancing the transformer currents rapidly and avoiding oscillations in dc currents. Finally, experimental results verify the outstanding performance of the proposed modulation strategy and FTCC method.

Hybrid Average Modeling of Three-Phase Dual Active Bridge Converters for Stability Analysis

The three-phase dual active bridge (3p-DAB) converter is widely addressed in emerging power systems applications such as solid-state transformer and dc microgrids. Its successful integration requires accurate modeling of its small-signal characteristics. Due to its dc–ac–dc structure, the DAB converter brings many challenges to small-signal modeling. The state-space averaging (SSA) has been the first proposed methodology to approximate the control-to-output and line-to-output transfer functions of the 3p-DAB. However, as shown in this paper, SSA is not precise for the stability analysis of 3p-DAB converters. A generalized state-space averaging (GSSA) model based on the dynamic phasor concept is developed in this paper for the Y-Δ 3p-DAB. A hybrid SSA and GSSA model representation is then proposed for the evaluation of all the converter transfer functions. The developed models are validated with detailed time-domain switch-level simulations in an electromagnetic transient-type (EMT-type) program. They are also used for the accelerated stability prediction in an EMT-type program.

Dead‐time effect analysis of a three‐phase dual‐active bridge DC/DC converter

The dead-time effect is observed in the three-phase dual-active bridge (DAB) DC/DC converter. The occurrence of the dead-time effect depends on the relationship of the switching frequency, the phase shift value, the dead-time value and the equivalent conversion ratio. The dead-time effect may have a significant impact on the converter performance when high switching frequency, wide input and output voltage range or wide operation power range are required. Therefore, comprehensive research of the dead-time effect is essential to improve the design of the three-phase DAB converter over a wide operation range. In this study, all the cases of the dead-time effect in the three-phase DAB converter are analysed in terms of the buck, boost, and matching states. The expressions of the transmission power, constraint conditions, and key time of the dead-time effect are derived for each state. The operation waveforms of the dead-time effect are also presented to better understand the dead-time effect. Finally, the analysis is verified by both simulation and experimental results.

Design Methodology of a Three-Phase Dual Active Bridge Converter for Low Voltage Direct Current Applications

Design Methodology of a Three-Phase Dual Active Bridge Converter for Low Voltage Direct Current Applications