Bidirectional AC-DC Research Articles

Two-Stage Isolated Bidirectional DC–AC Converters With Three-Port Converters and Two DC Buses

A novel two-stage structure with two dc buses is proposed for isolated bidirectional dc–ac power conversion. A variable dc-link voltage plus a constant dc-link voltage operation is adopted for the proposed two-stage structure. One of the dc-link voltages is regulated adaptively according to the variation of dc-side voltage, so that the dc–dc power stage can always work at its optimized point. New families of three-port (TP) bidirectional dc–dc converters and TP bidirectional inverter/rectifier (TP-BIR) are proposed to implement the proposed two-stage dc–ac power conversion structure. A bidirectional dc–ac converter composed of a TP-CLLC resonant converter and a T-type TP-BIR is taken as an example to verify the effectiveness of the proposed structure. Operation principles of both the TP-CLLC resonant converter and the T-type TP-BIR are analyzed in detail. Modulation and control strategies are proposed for the TP-BIR to achieve voltage regulation and power distribution between the two dc buses. A 1.5-kW prototype is built and tested to verify the feasibility and advantages of the proposed solution.

Improved Modulation Strategy for Single-Phase Single-Stage Isolated AC–DC Converter Considering Power Reversion Zone

In this article, an improved modulation strategy for the single-phase single-stage isolated bidirectional ac–dc converter is proposed. The modulation of ac side cycloconverter is based on the unipolar idea to reduce the switching quantity and the ac side current ripple and provide the bidirectional current flow path. The dc side H-bridge converter is driven by the pulsewidth modulation signals cooperatively to speed up the transformer current and thus to eliminate the voltage spike. Furthermore, the power reversion zone (PRZ) of the converter is verified. The action of the proposed modulation strategy in PRZ is analyzed for the first time. It reveals that the reverse current overshoot is caused in ac-to-dc direction and the voltage spike is caused in dc-to-ac direction. An online switching scheme of the proposed modulation strategies in two power directions is proposed to solve these problems. The detailed experimental results verify the correctness and reliability of the proposed modulation strategy.

A Matrix-based Isolated Bidirectional AC-DC Converter with <i>LCL</i> type Input Filter for Energy Storage Applications

A Matrix-based Isolated Bidirectional AC-DC Converter with <i>LCL</i> type Input Filter for Energy Storage Applications

High‐efficiency three‐phase bidirectional dc–ac converter for energy storage systems

This study presents a high-efficiency three-phase bidirectional dc–ac converter for use in energy storage systems (ESSs). The proposed converter comprises a modified three-level T-type converter (M3LT2C) and a three-level bidirectional dc–dc converter. The M3LT2C comprises two T-type cells to interface with a three-phase grid. By directly connecting the S-phase of the grid to a neutral point on the dc link, the active and passive components are required for only two grid phases, which reduces power losses, system volume, and production cost. A direct nominal voltage compensation control algorithm enables the converter to obtain high grid power quality. Also, the three-level bidirectional dc–dc converter enables low switching and conduction losses with reduced voltage stress across the switches. In addition, a simple dc-link voltage-balancing algorithm enables the balancing of two split dc-link capacitor voltages. To confirm the theoretical analysis of the proposed converter and assess its validity, a 10-kW prototype was implemented and tested.

Critical-Mode-Based Soft-Switching Modulation for High-Frequency Three-Phase Bidirectional AC–DC Converters

In this paper, a novel critical-conduction-mode-based modulation is proposed for three-phase bidirectional ac–dc converters. With this modulation, the switching frequency variation range shrinks, zero-voltage-switch soft switching is achieved, and the switching-related loss is reduced, which is especially beneficial for systems operating above hundreds of kHz high switching frequency with wide-band-gap power semiconductor devices to achieve both high power density and high efficiency. A 25 kW silicon carbide based high-frequency three-phase bidirectional ac–dc converter prototype is designed to achieve a power density of 127 ${\text{W/in}}^{3}$ , which is at least five times higher than commercial products. All the control functions are digitally implemented with one low-cost microcontroller, and the aforementioned benefits are experimentally verified on this prototype under both inverter mode and rectifier mode operations. With the proposed soft-switching modulation, the tested peak efficiency is close to 99.0% for this prototype even at above 300 kHz high switching frequency operation.

An Interleaved Bi-Directional AC–DC Converter With Reduced Switches and Reactive Power Control

In this brief, a new interleaved ac–dc converter with a reduced number of semiconductor components for high power energy storage applications is proposed. The interleaved structure reduces the ac side current ripple as well as increases the overall current rating. Moreover, the proposed switching pattern reduces switching and the developed controller provides reactive power control during both power flow directions. The proposed structure is simulated and analyzed using MATLAB/Simulink software, and a 3.5 kW prototype of the system has been implemented in the lab. The results confirm the performance of the proposed topology. There is a seamless transition between operation with different power factors and the current ripple is significantly reduced.

Distributed coordination control of hybrid energy resources for power sharing in coupled hybrid DC/AC microgrid using paralleled IFCs/ILCs

This study proposes flexible controllers for the interlinking converter (ILC) and interfacing converters (IFCs) used in coupled hybrid AC/DC microgrids (HMGs). Proposed controllers are specifically designed for the multiple stacked bidirectional DC–AC ILCs/IFCs based microgrid outlays, to omit the droop power flow and system stability issues. The ILC and IFC grid supportive converter controllers focus on the wide-spread AC/DC bus parameters control for both DC and AC bus voltage regulation and superfluous power sharing while operating in the grid forming and feeding modes. Proposed controllers minimise the need for the controller parameter tuning as opposed to the conventional controllers used in zonal HMG systems. This makes the system stable for a much wider operating conditions as opposed to the widely used higher-order PLL integrated PQ and dq 0 control algorithms. The proposed HMG also integrates the centralised battery energy stack through bidirectional dual active bridge DC–DC converter for achieving the high-power transfer efficiency and omitting the isolation issues between medium-voltage and low-voltage DC buses. The HMG system performance is evaluated using the simulation studies for various strategical operational modes. Further, the proposed controllers have also been tested individually on experimental platform.

Battery Energy Storage for Seamless Transitions of Wind Generator in Standalone Microgrid

This paper presents a control scheme for a standalone microgrid having a nondispatchable fixed-pitch variable-speed wind generator (WG) and a battery energy storage (BES). The WG is directly interfaced at the point of common coupling (PCC) and the BES is interfaced at the PCC via a bidirectional dc–ac converter. The unique aspect of the control strategy is that the BES not only regulates the PCC voltage and frequency by active–reactive power compensations, but also ensures proper synchronization and disconnection of the WG during wind velocity variations. A simple control scheme, with only one interfacing converter and one voltage feedback loop, provides an uninterruptible electrical power supply to critical loads during various operating modes and enables seamless transition of the WG. Performance of the proposed system under all operating modes and transitions is tested through modeling and simulation in MATLAB/Simulink, and validated on laboratory test bench through implementation of the control algorithm on a DSP platform.

Review of power electronics in vehicle-to-grid systems

Abstract Vehicle-to-Grid (V2G) is a promising technology that allows the batteries of idle or parked electric vehicles (EVs) to operate as distributed resources, which can store or release energy at appropriate times, resulting in a bidirectional exchange of power between the ac grid and the dc EV batteries. This bidirectional exchange of power is realized using bidirectional power electronic converters that connect the grid with the EV battery. Most research on bidirectional converters for V2G applications focuses on using two dedicated power conversion stages – a bidirectional ac-dc conversion stage that helps in power factor correction, followed by a bidirectional dc-dc conversion stage that provides voltage matching. However, a single bidirectional ac-dc conversion stage can also facilitate V2G and grid-to vehicle (G2V) active power transfers. This paper reviews and compares the various bidirectional ac-dc and dc-dc converter topologies that facilitate V2G and G2V active power flows. Moreover, the paper discusses the various classes of charger/discharger systems reported for V2G applications, like on-board/off-board, integrated/non-integrated and conductive/inductive, and a comparative statement is made based on certain proposed criteria. Further, the current trends in the application of wide band-gap devices in high power-dense V2G capable converters and integration of renewable energy sources into EV charging/discharging infrastructures have also been discussed.

Three-Port-Converter-Based Single-Phase Bidirectional AC–DC Converter With Reduced Power Processing Stages and Improved Overall Efficiency

Single-phase bidirectional ac-dc converters based on a three-port converter (TPC) are proposed for energy storage applications. Three power interfacing ports, i.e., a dc-bus port, a dc input port, and an ac port, are provided by the proposed converter. A battery, whose voltage is lower than the peak amplitude of the ac voltage, is connected to the dc input port of the TPC, with which energy exchange between the battery and grid can be achieved directly. Therefore, power processed by the down-stream dc-dc converter is reduced, which can help to reduce the power losses induced by the down-stream dc-dc converter. In addition, switching losses of the TPC can be reduced as well due to multilevel characteristic. As a result, the overall efficiency of the entire bidirectional ac-dc converter is improved significantly. Topologies, operation principles, modulation, and control of the proposed TPC-based bidirectional ac-dc converter are analyzed in detail. A 2-kW prototype is built and tested to verify the effectiveness and advantages of the proposed solution.

An Overview of Bidirectional AC-DC Grid Connected Converter Topologies for Low Voltage Battery Integration

<p>Battery energy storage systems are becoming more and more popular solution in the household applications, especially, in combination with renewable energy sources. The bidirectional AC-DC power electronic converter have great impact to the overall efficiency, size, mass and reliability of the storage system. This paper reviews the literature that deals with high efficiency converter technologies for connecting low voltage battery energy storage to an AC distribution grid. Due to low voltage of the battery isolated bidirectional AC-DC converter or a dedicated topology of the non isolated converter is required. Review on single stage, two stage power converters and integrated solutions are done in the paper.</p>

Multivector Model Predictive Power Control of Three-Phase Rectifiers With Reduced Power Ripples Under Nonideal Grid Conditions

Model predictive power control (MPPC) is a promising control scheme for bidirectional ac–dc rectifiers. However, the control performance of conventional MPPC (C-MPPC) deteriorates under nonideal grid conditions. Furthermore, the one-switching-vector-per-control-interval characteristic of C-MPPC leads to high ripples of control variables. To improve the steady-state performance of rectifiers under nonideal grid voltage conditions, a multivector MPPC (MV-MPPC) scheme is proposed. The proposed method presents a constant switching frequency and better steady-state control performance without increasing its sampling frequency. By selecting two active vectors and one zero vector, the range of optimal vector for active and reactive power regulation can be extended from fixed phase and magnitude to arbitrary phase and magnitude. Incorporated with a second-order generalized integrator for obtaining the quadrature component of grid voltage, the proposed method is applicable to the grid conditions where low-order harmonics exist. The preliminary calculation is adopted to avoid repetitive computation of the predicted values in the implementation of MV-MPPC, which reduces the calculation burden of the proposed scheme. A thorough experimental evaluation of the proposed scheme with the C-MPPC and duty-optimal MPPC has been conducted to validate the superiority of the proposed MV-MPPC solution.

Modelling of a Two-Stage Bidirectional AC-DC Converter using Wavelet Modulation

<span lang="EN-US">In this paper, a Wavelet modulated isolated two-stage three-phase bidirectional AC-DC converter is proposed for electric vehicle (EV) charging systems. Half-bridge resonant CLLC converter is proposed due to its high efficiency, wide gain range, galvanic isolation and bidirectional power flow. Wavelet modulation technique is used for three-phase six leg AC-DC converter due to its benefits of high DC component and lower harmonic contents. The proposed two-stage converter is developed and simulated in MATLAB Simulink environment. The contribution of this paper is on the implementation and performance analysis of Wavelet modulation in bidirectional AC-DC converters. The results show that Wavelet modulation is suitable to be implemented for the proposed bidirectional converter. The performance of the proposed converter delivers very low output voltage ripple and total harmonic distortion output current of less than 10% which is within the expected results.</span>

Digital Control of an Interleaving Operated Buck-Boost Synchronous Converter Used in a Low-Cost Testing System for an Automotive Powertrain

Based on the standardization in the automotive industry, systems require extensive testing, which represents significant costs regarding personnel and equipment. The testing systems must be built in such a way that a bidirectional power flow is possible between the power source and the tested system. Additionally, applied testing systems have to possess high disturbance immunity. Classical current programmed control performed using an analogue approach suffers from low disturbance rejection during switching operation. The digital control of DC–DC converter can solve this problem with the use of digital integration in a measurement chain. The integrals of values are obtained by using a Voltage Control Oscillator (VCO) and appropriate counters. Digital control of an interleaving operated bidirectional buck-boost synchronous converter can be applied in the testing system for automotive powertrains. The voltage and current measurements with the application of an integral-measurement principle act as low-pass filters, which remove the disturbances from the measured values. The digital implementation of a compensation ramp (current mode control) and method for choice of control parameters are described. All the tasks for measurements, as well as current and voltage control, were implemented within the FPGA (Field Programmable Gate Array). The presented converter can operate as a close to ideal voltage or current source, and satisfies the requirements of testing electric motor drive-trains with bidirectional DC–AC converters that are applied in automotive applications. The proposed system was verified by simulation and experiments.

Performance comparison of DC and AC controllers for a two-stage power converter in energy storage application

This paper presents a procedure for modelling and controlling a single-phase, two-stage, bidirectional DC–AC converter for application of energy storage system (ESS) in autonomous microgrids, in addition to experimental verification of its uninterruptible power supply (UPS) functionality. Average theory and frequency analysis are utilized to model the power converters and design the controllers in a generic fashion. Performance comparison of different controllers (type-2 and type-3 Venable compensators, Proportional Integral, Resonant-PI and multiple-RPI) is realised through simulations and tests. To tune the controllers, parameter values of crossover frequency and phase margin are adjusted by comparing simulation responses of the converters. A 5-kW ESS is evaluated through simulations during normal operation (grid-connected or stand-alone mode), as well as under transition events such as disconnection and recovery of the main grid. Besides that, a 100-W system was implemented, in which grid-connected mode was evaluated by measuring the power factor (about 0.998 in simulation and 0.993 in test at rated power), while stand-alone mode was evaluated by the total harmonic distortion of the output voltage (about 1.57% in simulation and 2.35% in test at rated power). The presented procedure can also be used to design higher power systems.

Development and Application of a Fuzzy Control System for a Lead-Acid Battery Bank Connected to a DC Microgrid

This study presents the development and application of a fuzzy control system (FCS) for the control of the charge and discharge process for a bank of batteries connected to a DC microgrid (DC-MG). The DC-MG runs on a maximum power of 1 kW with a 190 V DC bus using two photovoltaic systems of 0.6 kW each, a 1 kW bidirectional DC-AC converter to interconnect the DC-MG with the grid, a bank of 115 Ah to 120 V lead-acid batteries, and a general management system used to define the operating status of the FCS. This FCS uses a multiplexed fuzzy controller, normalizing the controller’s inputs and outputs in each operating status. The design of the fuzzy controller is based on a Mamdani inference system with AND-type fuzzy rules. The input and output variables have two trapezoidal membership functions and three triangular membership functions. LabVIEW and the NI myRIO-1900 embedded design device were used to implement the FCS. Results show the stability of the DC bus of the microgrid when the bank of batteries is in the charging and discharging process, with the bus stabilized in a range of 190 V ± 5%, thus demonstrating short response times to perturbations considering the microgrid’s response dynamics.

Soft-Switched Modulation Techniques for an Isolated Bidirectional DC–AC

Two carrier-based unipolar-sinusoidal pulse width modulation (SPWM)-oriented modulation techniques for an isolated bidirectional dc–ac converter are proposed, compared, and validated in this paper. The dc–ac converter is composed of a full-bridge (FB) inverter cascaded with a cycloconverter through a high-frequency transformer. Both modulation techniques proposed in this paper can realize zero-voltage switching (ZVS) for the FB inverter and zero-current switching or ZVS for the cycloconverter in all load range, and are able to suppress the voltage spikes introduced by the transformer leakage inductance as well. In order to increase the converter efficiency and power density, we propose to utilize SiC MOSFETs for the converter. The first modulation technique enables the utilization of Si-SiC hybrid switches with no synchronous rectification (SR), for the purpose of lowering the converter cost. The second modulation technique requires all switches to be SiC MOSFETs, but with SR, which increases the converter efficiency. A 400-V dc to 240-V ac 1.2-kW prototype has been developed to validate the effectiveness and performance of the proposed carrier-based unipolar-SPWM-oriented modulation techniques.

Analysis and Implementation of Closed-Loop Control of Electrolytic Capacitor-Less Six-Pulse DC-Link Bidirectional Three-Phase Grid-Tied Inverter

Cascaded bidirectional dc–ac converters are commonly used in uninterruptible power supply applications and battery chargers for electric vehicles. Power conversion units for such applications employ a large electrolytic capacitor at high-voltage dc bus, which not only reduces the lifetime but also adds to the weight of the converter. In this paper, a novel bidirectional dual active bridge cascaded three-phase converter (DABCC) with six-pulse dc link is proposed. Also, a new closed-loop control scheme to implement the six-pulse modulation (SPM) technique in DABCC is proposed. The advantages of the proposed control scheme in DABCC are that the electrolytic capacitor is eliminated and is replaced with a low-value film capacitor resulting in increased reliability, compactness, and reduced cost. SPM technique also increases the dc bus utilization and decreases the inverter average switching frequency to 33% when compared with the conventional sine pulse-width modulation (SPWM). Harmonic components introduced in the pole voltages of the SPM-modulated DABCC due to inverter dead time are analyzed, and a resonant controller is designed to mitigate them. Also, root mean square current stress for high-voltage dc-link capacitor is studied, and it is shown that the capacitor requirement for SPM modulation is lower than the conventional SPWM. To validate the proposed control scheme, an 800-W proof-of-concept laboratory hardware prototype is fabricated and experimental results are demonstrated.

Improved Control Strategy for Bidirectional Single Phase AC-DC Converter in Hybrid AC/DC Microgrid

This paper presents the improved control strategy with optimal switching scheme to improve the bidirectional converter's power flow control capability and to allow for different modes of operation in hybrid AC/DC microgrid. Also, this improved control strategy eliminates the need of PI controller for independent reactive power control. With the improvements in digital controllers, it is possible to implement complex control circuitry easily. The performance of bidirectional converter with improved control strategy is verified first in simulation environment and then on lab scale experimental set-up with the control circuit is implemented using hardware in loop in the real-time digital simulation platform. The simulation and experimental results for all the cases show that the controller is highly capable for power transfer between AC and DC grid with very fast response and also control reactive power without additional PI controller. Also, it maintains the current total harmonic distortion (THD) within standard limits.

Soft-Switching PWM Technique for Grid-Tie Isolated Bidirectional DC–AC Converter With SiC Device

Soft-switching techniques have become very attractive in bidirectional grid-tie dc–ac converters utilizing high-frequency link transformers for galvanic isolation. This paper presents a new pulse-width modulation (PWM) switching technique for controlling a bidirectional isolated dc–ac–ac converter along with its soft-switching method. The proposed PWM technique has the ability to control the input dc current and to inject a sinusoidal three-phase current to the grid at unity power factor. In the first stage, an H-bridge converter is used to convert the dc voltage to a high-frequency square-wave single-phase voltage. In the second stage, a matrix converter is used to convert the high-frequency voltage waveform to conventional three-phase voltage synchronized with the grid. Therefore, a single-phase high-frequency transformer is used to link the H-bridge output voltage to the matrix converter input voltage. The proposed soft-switching technique is achieved by connecting shunt ceramic capacitors across all converter switches. The mathematical model and the circuit operation for soft-switching are presented along with the voltage controllable limits. The effectiveness of the proposed technique has been verified experimentally using a laboratory prototype.