TAB Based Multiport Converter with Optimized Transformer RMS Current and Improved ZVS Range for DC Microgrid Applications

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.

The integration of energy storage elements to solar photovoltaic (PV) systems provide a wide range of operational flexibilities. Traditional approaches use separate converter for PV and energy storage element, i.e. battery which cause significant conversion losses. This paper presents an isolated high frequency link multi-port converter based on triple active bridge (TAB) topology for PV integrated battery systems in residential applications. The conventional dual active bridge (DAB) is extended to establish the proposed high frequency link TAB topology. Limited soft-switching capability and high circulating current are common problems in this style of converters when there is a mismatch in the dc-link voltages. This is pronounced at lower powers with the single phase shift (SPS) modulation technique. To overcome these problems, this paper proposes an improved modulation and control method based on a quintuple phase shift modulation (QPSM) strategy. The QPSM modulation method produces 3-level modulated square wave bridge voltages. A mathematical model of the proposed high frequency linked multi-port converter with QPSM modulation is developed based on a harmonic analysis approach. The simulation and experimental results show that the QPSM scheme applied to the proposed multi-port converter provides better operation and improves the efficiency compared to the traditional SPS modulation scheme.

Multiport converters are used in interfacing of distributed energy sources with grid/load. Isolated converters are needed in applications where converter gain is high and there is a requirement of isolation. Dual transformer asymmetric triple active bridge offers the advantage of reduced circulating current. However, the operating range is low for variation in load and source voltage. In this paper duty ratio modulation technique is proposed to regulate the load voltage and control the power flow in both the directions. As a result of the new gating scheme, the converter switches operate with ZVS, irrespective of variations in load power and source voltage. The converter is designed to ensure high switch utilization. The control technique is validatedthrough simulation of a 1kW three port DC-DC converter. It was observerd that the load voltage was regulated for wide range of variation in load power and source port voltages. The single input dual output mode was also verified.

Integrating Energy Storage (ES) with intermittent Renewable Energy Sources (RES) is critical for wide-spread adoption of RES. Such systems offer several system-level advantages such as participation in demand response programs, demand charge reduction, arbitrage to maximize time-of-use rates, and increase resilience. For efficient and cost-effective integration of ES with RES, a Multi-Port Converter (MPC) is highly desirable. An isolated Triple Active Bridge (TAB) converter for Solar Photovoltaic (PV) and battery (ES) integration is discussed in this paper. Multi-objective optimization of this topology, for interfacing 10 kVA PV-ES system with 240 V AC grid, is conducted with the objectives to reduce losses and overall volume of the MPC. Finite Element Analysis (FEA) is performed to validate the outcome of the analytical modeling based optimization procedure. A control scheme to manage power flow between AC grid, PV, and ES is designed, and several case scenarios are simulated to demonstrate advantages of the proposed approach in enabling high RES penetration.

Analysis of zero-voltage switching (ZVS) range is critical to the design and control of bidirectional dc-dc converters. This paper discusses the derivation of the accurate ZVS boundary for bidirectional dual-bridge series resonant converter (DBSRC) based on state-plane analysis. First, the closed-form solution of ZVS boundary for pure phase shift modulation is derived. Next, the analysis is extended to the case of phase shift plus pulse width modulation, and an implicit function expressing the ZVS boundary of all switches is deduced in closed form. Through this function, the hard switching and ZVS regions under any voltage conversion ratio and resonant-to-switching frequency ratio can be easily derived numerically using mathematical software. Compared with the fundamental harmonic approximation (FHA) method, the ZVS boundary obtained in this paper shows higher accuracy. Furthermore, the generality of the state-plane approach maintains its validity also with small resonant-to-switching frequency ratios, in which the converter behavior becomes that of a dual active bridge topology with a large dc-blocking capacitor. The analysis is validated via computer simulations and verified on an 800 W experimental DBSRC prototype.

AC/DC converters are widely used in various fields. To address the issues of large volume and limited power density associated with conventional two-stage converters, the Dual-Active-Bridge (DAB) based quasi-single-stage AC/DC converters (QSSDAB) have garnered significant attention. Benefiting from the wide voltage gain range of DAB converters, the QSSDAB topology requires only film capacitors to maintain the sinusoidal half-wave rectified by synchronous rectification. This configuration eliminates the need for electrolytic capacitors, thereby enabling volume reduction for enhanced power density. To further enhance the efficiency of QSSDAB, a wide range of Zero Voltage Switching (ZVS) is needed. However, when the AC voltage is low, the ZVS is hard to guarantee solely by the control strategy. In this paper, a wide range ZVS strategy that utilizes split inductors and magnetizing current compensation to broaden the ZVS range is proposed. First, in this paper, the topology and model of the converter are analyzed. Then, the operation modes of the DAB are explained together with the ZVS- required current. Herein, ZVS is realized through a combination of control strategy, and magnetizing current compensation, where the control strategy ensures that each switch achieves at least Zero Current Switching (ZCS), and the ZVS current is realized through magnetizing current compensation. Finally, a comprehensive simulation are performed to validate the proposed strategy.

Integration of the on-board charger (OBC) and auxiliary power module (APM) in electric vehicles (EVs) can reduce the overall weight, volume and cost of the on-board charging system, as well as increase efficiency. This paper proposes optimized modulation strategies for an integrated on-board charger (IOBC) based on triple-active-bridge (TAB) converter. The proposed multiport converter is capable of charging the high-voltage (HV) and low-voltage (LV) batteries simultaneously from the grid. In addition, the HV and LV batteries can also be charged individually from the grid without the need of additional mechanical switches or relays, due to the proposed 5-degrees-of-freedom (DOF) modulation scheme. Detailed loss analysis of TAB is presented for the first time, targeting a simple and accurate estimation of the converter losses. Furthermore, four optimization schemes are proposed aiming to improve the performance of the integrated charger, including reduction of the converter total loss and zero-voltage-switching (ZVS) turn-on for all converter devices. A constant-current/constant-voltage (CC/CV) charging scheme is considered for both HV and LV batteries and the results of the optimization schemes are evaluated using a 4.3 kW experimental prototype, while the measured converter efficiency with the proposed optimization schemes reaches 96.1% at nominal power levels.

This paper presents the small signal model and closed loop control design using average current control for an L-L type active-clamped zero-voltage switching (ZVS) current-fed isolated DC-DC converter. State-space averaging technique is used to derive the small signal model. The controller is designed for fixed-frequency duty cycle modulation and works for wide input voltage and load variation. A complete design procedure is presented. Frequency response curves are obtained at different input voltage and load conditions to verify the stability and design, and to evaluate the performance of the controller. Simulation results for step changes in input voltage and load are given to check the transient response of the converter.

This paper presents a technique to eliminate circulating and leakage power flow between source ports of Triple Active Bridge (TAB) isolated bidirectional dc-dc converter. When two different dc sources are used to supply load power simultaneously in TAB topology, interconnecting winding of transformer provides a leakage inductance path for the flow of circulating and leakage current. It happens due to unmatched voltage magnitude and phase difference of ac voltages impressed across winding. The ac voltages are produced by full bridge of corresponding dc source ports. This circulating and leakage current results in increased conduction and copper losses in switches and transformer respectively. High value of this circulating and leakage current can even damage the transformer core as well as Windings. Therefore, a blocking capacitor is employed to create infinite impedance between source ports and thus it open circuit current path to block circulating and leakage power. With proposed technique, the different source ports of TAB can operate with Wider range of phase delays and voltage magnitudes with negligible circulating and leakage power. Simulation results employing this technique has been shown which verify the theoretical concept. MATLAB/Simulink platform has been used for simulation.

Aiming at fulfilling the desirable features of compact structure, high efficiency, high reliability, and multifunctionality for the solid state transformers (SSTs), a single-cell matrix-type SST as well as its control method is proposed as the interface between the medium-voltage (MV) and the low-voltage (LV) levels for direct MV AC to LV AC conversion. To achieve zero voltage switching (ZVS) at both the MV and LV sides, a modulation scheme to construct the special resonant currents during the dead time by actively controlling the MV and LV side gate signals is implemented. Furthermore, the ZVS conditions of the MV and LV side bridges are analyzed, and system parameters are properly designed to ensure full range ZVS operation. A 1-kW SST prototype is developed to verify the functionality and effectiveness of the proposed methods. The experiments show that sinusoidal input and output, bidirectional power flow capability, and ZVS at both sides over wide input voltage and load ranges are achieved with the nominal efficiency of 96.1%. The experimental results match the theoretical analysis and the simulation results well.

This paper presents a study on a new full bridge series resonant converter (SRC) with wide zero voltage switching (ZVS) range, and higher output voltage. The high frequency transformer is connected in series with the LC series resonant tank. The tank inductance is therefore increased; all switches having the ability to turn on at ZVS, with lower switching frequency than the LC tank resonant frequency. Moreover, the step-up high frequency (HF) transformer design steps are introduced in order to increase the output voltage to overcome the gain limitation of the conventional SRC. Compared to the conventional SRC, the proposed converter has higher energy conversion, able to increase the ZVS range by 36%, and provide much higher output power. Finally, the a laboratory prototypes of the both converters with the same resonant tank parameters and input voltage are examined based on 1 and 2.2 kW power respectively, for veryfing the reliability of the performance and the operation principles of both converters.

The numbers of photovoltaic (PV) systems at residential premises are increasing worldwide. This thesis explores new power electronic solutions for premise energy management systems (PEMS) which incorporate a battery energy storage system (BESS) to manage the utilisation of solar and grid power as the load demand varies. A flexible, isolated three port converter (TPC) is proposed that allows unrestricted power flows between the dc connected PV systems and battery storage and the ac connected consumer loads and the external power system. The traditional PEMS solutions will seek to perform the power exchanges at a dc and/or ac bus and use multiple power converters to interface the PV and BESS components and to allow power flows between the dc and ac systems. A number of topologies exist but all require at least two separate power converters for an integrated PV-BESS that results in multistage power conversion steps for some power flows. Several three-port converter (TPC) topologies are reported in the present literature as possible improvements. However, these topologies often have important restrictions on the possible power flows. This thesis develops and demonstrates a high frequency linked three-port converter (HFL-TPC) topology for PEMS employing PV and BESS. A topological and analytical approach was taken and identified that an HFL-TPC topology could be developed by using three coupled active bridges based on dual active bridge (DAB) converters. The converter is symmetric and readily extensible to an arbitrary number of ports. All the ports are bidirectional and have the same power transfer capability. A key challenge for the HFL-TPC is developing appropriate modulation strategies. Several modulation and control strategies are reported for HFL-TPC topologies using a piecewise linear time domain approach. This approach is adequate for the dual active bridge but becomes much more complex if the number of ports increases. The practical implementation of the optimised switching variables on the digital signal processor (DSP) is very difficult to achieve. This thesis adapts the well-known power systems concepts related to real and reactive power flows between generators coupled by inductive lines to establish the key underlying principles that dictate the real and reactive flows in multiport converters. It is shown that the real and reactive flows are strongly dominated by fundamental frequency power flows. For a multiport converter, the fundamental frequency angle differences between the ports determine the real power flows while the magnitude differences largely determine reactive flows. Based on these power system concepts, the thesis develops an improved quintuple phase shift (QPS) modulation for the HFL-TPC. It achieves the required power transfers at minimum current and provides favourable switching operations to improve the efficiency. The proposed algorithm avoids the extensive computational burden associated with traditional optimisation processes which are barely tractable even for converters with a few ports. The thesis also develops the small signal models for the HFL-TPC and control decoupling networks to allow independent closed loop power control strategies. Within the literature, TPCs with a four-quadrant ac port are very rare. It is technically possible to modify one port of the proposed converter by replacing the conventional bridge converter with a matrix converter. The approach was not pursued far as this has significant practical difficulties. It is challenging to maintain soft switching. In a single phase system, the output voltage zero points make reactive power flows difficult to control. In this thesis, the ac port has been developed by adding an inverter stage. Single phase inverters have power fluctuation at the dc bus at twice the output frequency. The thesis presents methods that allow the ripple power to be directed to a nominated port. The operation of the HFL-TPC with the proposed QPS modulation and control strategies has been demonstrated by extensive simulation studies conducted in MATLAB/Simulink. A 60 V, 400 W laboratory scale prototype has been constructed and demonstrated at module level to verify the practical feasibility of the proposed optimised modulation strategies. Simulation and experimental results show that the proposed optimised QPS modulation method controls the reactive power, improves the efficiency, and maintains the zero voltage switching (ZVS) operation over a wide voltage and power range. The proposed converter is a flexible alternative that can facilitate the full range of power flows that might be expected in a PEMS system. It has general applicability across a range of industrial applications where power flows between multiple sources and loads need to be facilitated. The transfer of power systems concepts to the control of the converter modulation is a significant advance in the control of multi-port systems.

Multi-port converters have attracted special interest in applications such as self sustaining residential homes and buildings with multiple energy sources and energy storage. In this paper a constant switching frequency series resonant three-port bi-directional DC-DC converter is proposed. The converter uses a three-winding transformer with two of its windings connected to H-bridges through series resonant circuits. To control the output voltage and to share the power between the two sources, two control variables are proposed. One of them is the phase shift between the fundamental of the outputs of the H-bridges and the other between two legs in one of the bridges. Sinusoidal analysis is presented and equations for output voltage and input port power are derived as a function of the phase shifts. It is observed from the analysis that the power can be made bi-directional in either of the source ports by varying the phase shifts. Switching frequency is chosen such that the converter operates above resonant frequency and hence aid in zero voltage switching (ZVS). A design procedure to select converter parameters is explained. Simulation and experimental results of a prototype converter are presented.

This article presents a control strategy for the single-stage dual active bridge (DAB) ac-dc converters. To reduce the conduction loss and achieve zero voltage switching (ZVS), the global optimal working modes are reviewed. Based on the global optimal working modes, the input current of the DAB ac-dc converter is analyzed for the sinusoidal input voltage. The control algorithm of the DAB ac-dc converter is proposed to achieve the unit power factor with the minimum current stress and ZVS. The range of the working modes is analyzed to meet the wide load power. The working mode is varied with the line-frequency angle and load power, which guarantees that the DAB ac-dc converter works in the optimal working modes. The conduction loss in the converter with the variation of the series inductance and the turns ratio of the transformer is analyzed. The series inductance and the turns ratio of the transformer are designed to further improve the conduction loss. The proposed control strategy is applied to a 1 kW DAB ac-dc converter with 220 V/50 Hz input voltage and 200 V output voltage. The experimental results demonstrate the working mode, the soft switching performance, fast dynamic response, high efficiency, and high power quality.

This article is aimed at enlarging the zero-voltage switching (ZVS) range and reducing current stress for the dual-active-bridge (DAB) converter with dc blocking capacitors (DCBCs). A hybrid extended phase shift modulation strategy (HEPSMS) is proposed, which makes the converter switch between the full-bridge mode and the half-bridge mode to obtain multivoltage matching operation points. Thus, the root mean square (rms) current and the burden of achieving full load range ZVS are reduced. In addition, the voltage–second of the leakage inductor is controlled to achieve equivalent voltage matching under wide voltage output, thereby simplifying the control law and achieving full load range ZVS under a wide voltage range. The operation principle, ZVS range, and rms current are analyzed and compared with the conventional modulation method. Finally, a 1-kW experimental prototype with a 200-V input and a 100–400-V output was built to verify the feasibility and effectiveness of the proposed method.