High-frequency Transformer Link Research Articles

Two-stage grid-connected inverter topology with high frequency link transformer for solar PV systems

This study introduces a new topology for a single-phase photovoltaic (PV) grid connection. This suggested topology comprises two cascaded stages linked by a high-frequency transformer. In the first stage, a new buck–boost inverter with one energy storage is implemented. The buck–boost inverter can convert the PV module’s output voltage to a high-frequency square wave (HFSWV) and can enhance maximum power point tracking (MPPT) even under large PV voltage variations. The high-frequency transformer gives galvanic isolation for the system, which decreases the leakage current and improves the system power quality. The second stage of the topology involves using a rectifier-inverter system to interface the produced HFSWV to the utility grid. The proposed system uses high switching frequency which increases the power density, reduces the grid filter size, and increases the system reliability. Buck–boost DC/AC inversion, MPPT and low grid current injection are implemented. The working principles of the proposed topology have been investigated, and the theoretical and experimental results are developed and analyzed.

Modulation and Control of a DC-AC Converter With High-Frequency Link Transformer for Grid-Connected Applications

Modulation and Control of a DC-AC Converter With High-Frequency Link Transformer for Grid-Connected Applications

A Dual-Active-Bridge With Half-Bridge Submodules DC Solid-State Transformer for DC Distribution Networks

High-frequency dc solid-state transformer (DCSST) plays an important role in DC bus connection, voltage conversion, electrical isolation, and bidirectional power transmission in dc distribution networks. In various DSSCT topologies, the dual-active-bridge (DAB) scheme becomes a typical solution for DCSST due to its various advantages. However, the DAB scheme still has drawbacks during practical applications. In this article, a DAB with half-bridge submodules (HSDAB) is proposed. By replacing the switches in front bridge arm with half-bridge submodules (SMs), the proposed HSDAB scheme maintains the advantages of conventional DAB and obtains bidirectional dc fault-handling capability, reducing the application of large dc capacitors and dc circuit breaker; therefore, the volume and construction cost of DCSST can be effectively reduced. Besides, a high-frequency-link (HFL) voltage regulation strategy for HSDAB is also proposed in this article. The HFL voltages can be adjusted to match the HFL transformer voltage ratio during the whole operating range, reducing the current stress, HFL reactive power, and power loss of DCSST. Moreover, the proposed HSDAB scheme can also be applied as a basic circuit to establish multiple modular architecture DCSST, flexibly connecting low-voltage dc, medium-voltage dc, and high-voltage dc buses in the dc distribution network. The topology, operation principle, control structure, corresponding bidirectional dc fault handling, and HFL voltage regulation strategy of the proposed HSDAB scheme are investigated. The experimental results verify the correctness and effectiveness of the analysis and proposed scheme.

A Multiple Modular Isolated DC/DC Converter With Bidirectional Fault Handling and Efficient Energy Conversion for DC Distribution Network

The isolated bidirectional dc/dc converter (IBdc) plays an important role in dc buses connection, voltage conversion, electrical isolation, and bidirectional power transmission for dc distribution networks. In this article, a multiple modular IBdc scheme based on hybrid dual-active-bridge (HDAB) for medium-voltage and high-voltage dc distribution network is proposed. In proposed HDAB, the switches in upper position of full-bridge are replaced by the half-bridge submodules, and the proposed HDAB maintains the advantages of conventional DAB and achieves bidirectional fault handling capability, avoiding the application of large dc-link capacitors and dc circuit breakers, thus reducing the volume and construction cost. Besides, a high-frequency-link (HFL) voltage regulation strategy is also proposed in this article, adjusting the HFL voltages in HDAB to match HFL transformer voltage ratio adaptively, thus improving the efficiency and applicability of IBdc under the fluctuation of power flow and dc bus voltage. The topology, operation principle, control structure, and corresponding bidirectional fault handling and HFL voltage regulation strategy of the proposed IBdc scheme are investigated. The experimental results verify correctness and effectiveness of proposed scheme and control strategy, promoting the adaptability and practicality for both IBdc and dc distribution network.

Adaptive Modulation Strategy for Modular Multilevel High-Frequency DC Transformer in DC Distribution Networks

The modular multilevel high-frequency DC transformer (MDCT) plays an important role in the bus connection, voltage conversion, power transmission and electrical isolation in the DC distribution networks. During practical operation, the mismatch between high-frequency-link (HFL) voltages and HFL transformer voltage ratio caused by the variation of DC bus voltages occurs, resulting in the increased reactive power, larger current stress and decreased efficiency in MDCT. To solve this problem, an adaptive modulation strategy (AMS) is proposed in this paper. The proposed AMS adjusts HFL voltages by varying the number of inserted sub-modules adaptively, ensuring the HFL voltages match HFL transformer voltage ratio. Compared with the conventional phase-shift strategies, the proposed strategy obtains higher transmission power capacity and HFL power factor, and it maintains the ideal efficiency of MDCT under light load situation. Besides, the proposed strategy avoids the optimal phase-shift angle calculation containing numerous parameters, reducing computing burden and improving practicality for MDCT. Moreover, the hybrid-wave modulation in proposed strategy makes HFL voltages compose of quasi-square-wave and square-wave, addressing the $\text{d}v/\text{d}t$ stress problem and ensuring the higher DC voltage utilization and power transmission capability simultaneously. The mathematical model, operation principle and control architecture of proposed AMS are investigated, and the experimental results in a MDCT prototype verify the correctness and effectiveness of the analysis and proposed strategy.

Boost Inverter Topology with High-Frequency Link Transformer for PV Grid-Tied Applications

Boost Inverter Topology with High-Frequency Link Transformer for PV Grid-Tied Applications

A high-frequency-link voltage matching phase-shift control strategy for high-frequency modular multilevel DC transformer in MVDC distribution networks

High-frequency modular multilevel DC transformer (MMDCT) plays an important role in the bus connection, voltage conversion and power transmission in MVDC distribution networks. However, the variation of power flow in distribution network leads to the high-frequency-link (HFL) voltages in MMDCT mismatch the HFL transformer voltage ratio, resulting in the increase of circulating power and current stress, and the decrease of power factor and efficiency in M2DCT. To solve this problem, a HFL voltage matching phase-shift control (VMPC) strategy for MMDCT is proposed in this paper. Different with the traditional phase-shift control (TPC), the proposed VMPC adjusts the HFL voltages by varying the number of inserted sub-modules (SMs) in real-time, ensuring the HFL voltages match the HFL transformer voltage ratio in the whole operating range, resulting in the improvement of efficiency and performance. The detailed operation principle, control architecture and corresponding capacitor voltage balance algorithm of VMPC are investigated. On this basis, the improvements on the current stress, power factor and efficiency of MMDCT under the proposed VMPC are also analyzed. The experiment results verify the correctness and effectiveness of the analysis and proposed strategy.

Optimized Control Strategy of Modular Multilevel DC Transformer for High-Frequency-Link Voltages Matching in the Whole Operation Range

High-frequency modular multilevel dc transformer (MDCT) plays an important role in the voltage conversion and power transmission for medium-voltage direct-current (MVdc) power networks application. For MDCT, when high-frequency-link (HFL) voltage amplitudes do not match the HFL transformer ratio, the increase in HFL reactive power and the decrease in efficiency become a bottleneck for its application. To solve this problem, a modified MDCT (M2DCT) is analyzed, and a HFL voltage optimized control (HVOC) for M2DCT is proposed in this paper. Different from the conventional phase shift strategies, the proposed HVOC adjusts the HFL voltage amplitudes by varying the pulsewidth of modulation wave for power switches in submodule (SM), ensuring HFL voltage amplitudes match the HFL transformer ratio in the whole operation range, reducing the HFL reactive power, and improving the efficiency of M2DCT. In addition, the soft switching is achieved in the whole operation range under HVOC. The detailed control method, architecture, SM voltages balancing algorithm, and switching characteristics of HVOC are discussed, and the influences on power transmission capacity, HFL power factor, operating efficiency of M2DCT under HVOC, and conventional HFL phase shift controls are analyzed comparatively. The experimental results verify the correctness and effectiveness of the analysis and proposed strategy.

Hybrid‐wave modulation for modular multilevel high‐frequency DC converter in DC distribution application

Compared with AC distribution networks, DC distribution networks reduce the utilisation of power converters, power loss, and construction cost. In DC distribution network, the high-frequency modular multilevel DC converter (MDCC), which connects the low-voltage DC link with medium- or high-voltage DC links, becomes a practical equipment to realise the bus connection, voltage conversion, and electrical isolation. Especially, high-frequency link (HFL) modulation plays an important role in converter performance. The square-wave (SW) modulation obtains better power transfer capability and higher DC voltage utilisation. However, the SW modulation leads to a large dv/dt stress upon HFL transformer. The sinusoidal, triangle and quasi-square-wave (QSW) modulations reduce dv/dt and harmonics significantly. However, the power transfer capability and DC voltage utilisation are also reduced. To address dv/dt stress problem, ensure high DC-voltage utilisation and maintain a strong power transfer capability simultaneously, a hybrid-wave modulation strategy, which composed of QSW and SW modulation, is proposed for MDCC. The experimental results verify the analysis and proposed scheme.

PWM Switching Technique for Three-Phase Bidirectional Grid-Tie DC–AC–AC Converter With High-Frequency Isolation

Galvanic isolation based high-frequency transformers have become very attractive in bidirectional dc/ac converters. This paper presents a new pulse width modulation (PWM) switching technique for controlling a bidirectional isolated dc–ac–ac converter employing a high-frequency link transformer. 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. The primary-side is an H-bridge converter used to convert the dc voltage to a high-frequency square-wave single-phase voltage. The secondary side is a matrix converter used to convert the grid three-phase voltage to a high-frequency single-phase waveform. The dc–ac–ac converter utilizes a high-frequency link transformer for the galvanic isolation between the H-bridge and matrix converters as alternative for the bulky line-frequency transformers (50/60 Hz). The bidirectional power flow is controlled by the phase shift angle between the primary and secondary voltages of the high-frequency transformer. The mathematical model and the circuit operational modes are presented along with the voltage controllable limit. The feasibility of the proposed PWM switching technique and the accuracy of the mathematical model are demonstrated experimentally by using a 200 V/1 kW laboratory prototype system.

New Start-Up Scheme for HF Transformer Link Photovoltaic Inverter

Safe start-up procedure is critical for the reliable operation of power converters. The challenges in start-up schemes of high-frequency transformer link inverters involve precharge of the capacitors without significant inrush currents and overvoltages. The precharge is normally followed by enabling of the closed-loop control. The overvoltage or inrush current problems in conventional start-up schemes available in the literature are identified in this paper. A new start-up scheme is proposed which achieves the objectives of limiting inrush current and start-up overvoltages. This method does not require any additional dedicated precharge circuitry. The proposed scheme is validated using simulation and experimental results which agree with the theoretical prediction.

Decoupling capacitor minimization in HF-link single-phase cycloconverter based microinverter

This paper presents a technique to reduce the value of the decoupling capacitor in a single-phase PV microinverter system, allowing its implementation through long-lifetime high reliability film capacitors. Single-phase system inherently requires the handling of energy variation at twice the output frequency through an energy buffer, referred to as decoupling capacitor; such buffer traditionally consists of an electrolytic type capacitor which presents limited lifetime, reducing system reliability. The presented technique makes use of the impedance reflection effect in the high-frequency link transformer through an additional ripple-power converter, an adequate commutation scheme is proposed for the switches of the additional converter to manage the energy variation; without adding extra circuitry for modulation and control of the added converter. An analysis and estimation of losses in the proposed system are presented showing that the inclusion of the ripple-port converter, with proper switching devices selection, in comparison with conventional electrolytic decoupling solution allows to extract the available input power and does not significantly affects the efficiency of the system.

Analysis and Design of a Bidirectional Cycloconverter-Type High Frequency Link Inverter with Natural Commutated Phase Angle Control

In this paper a cycloconverter-type high frequency transformer link inverter with a reduced switch count is analyzed and designed. The proposed topology consists of an H-bridge inverter at the transformer’s primary side and a cycloconverter with three bidirectional switches at the secondary. All of the switches of the cycloconverter operate in non-resonant zero voltage and zero current switching modes. To overcome a high voltage surge problem resulting from the transformer leakage inductance, phase angle control based on natural commutation is employed. The effectiveness of the proposed inverter is verified by constructing s 750W prototype. Experimentally, the inverter is able to supply a near sinusoidal output voltage with a total harmonic distortion of less than 1%. For comparison, a PSpice simulation of the inverter is also carried out. It was found that the experimental results are in very close agreement with the simulation.

Bidirectional high-frequency link inverter with regenerative snubber and deadbeat control

This article presents a bidirectional high frequency transformer link inverter with reduced power switches. On the secondary side of the transformer, two switches are used for rectification and four switches are used for the unfolding stage. The latter are switched at low frequency (50 Hz). Furthermore, a regenerative snubber is incorporated to reduce the voltage surge due to the leakage inductance of the transformer's secondary. The proposed inverter is controlled by a deadbeat controller, which consists of the inner current loop, outer voltage loop and a feed forward controller. Additional disturbance decoupling networks are employed to improve the system robustness towards load variations. A 1-kVA prototype inverter is constructed and the deadbeat control algorithm is experimentally verified. The experimental results show that the inverter has high efficiency (86–94%) with low steady-state output voltage total harmonics distortion (1.5%).

Practical Evaluations of a Novel Asymmetrical ZCS-PWM DC-DC Converter with High-Frequency Transformer-Link

This paper presents a novel Zero Current Switching (ZCS)-PWM controlled half-bridge DC-DC converter with High-Frequency (HF)-link. The newly-proposed soft switching DC-DC converter consists of a multi-resonant half-bridge HF-isolated inverter controlled by an asymmetrical PWM scheme and a center-tapped diode rectifier with a choke input filter. In order to attain the wide range of soft-commutation under the condition of constant switching frequency, the single Active Edge-Resonant Cell (AERC) that is composed of a lossless inductor and a switched resonant capacitor is originally employed for the half-bridge inverter arm, providing and assisting ZCS operations in the switching power devices. The ZCS-PWMDC-DC converter treated here is evaluated in experiments using the 800 W-55 kHz prototype. The practical effectiveness of the proposed soft switching DC-DC converter is actually demonstrated and discussed with the experimental results under open loop and closed loop configurations. Finally, the feasibility of the DC-DC converter topology is discussed from the view points of the high efficiency and high power density.

High-Frequency Flyback Transformer Linked PWM Power Conditioner with An Active Switched Capacitor Snubber

A single active capacitor snubber-assisted soft-switching sinewave pulse modulation utility-interactive power conditioner with a three-winding flyback high frequency transformer link and a bidirectional active power switch in its secondary side has been proposed With the aid of the switched-capacitor quasi-resonant snubber cell, the high frequency switching devices in the primary side of the proposed DC-to-AC sinewave power inverter can be turned-off with ZVS commutation. In addition to this, the proposed power conditioner in the DCM can effectively take the advantages of ZCS turn-on commutation. Its output port is connected directly to the utility AC power source grid At the end, the prototype of the proposed HF-UPC is built and tested in experiment. Its power conversion conditioning and processing circuit with a high frequency flyback transformer link is verified and the output sinewave current is qualified in accordance with the power quality guidelines of the utility AC interactive power systems.

High-frequency link symmetrical active edge resonant snubbers-assisted ZCS-PWM DC–DC converter

A novel symmetrical zero current switching (ZCS)-pulse width modulation (PWM) cells-assisted high-frequency transformer link DC–DC converter using insulated gate bipolar transistor (IGBT) modules is presented. The proposed soft switching scheme is based on the switched-capacitor and inductive snubber in the high-voltage side inverter, assisted by active switching of MOSFET synchronous rectifier in the secondary-side low-voltage converter stage. By introducing the ZCS-PWM snubber cells, soft switching commutation which is less sensitive to the current level through the IGBTs can be achieved under the wide output power ranges. The converter circuit topology and the ZCS snubber cell operation are examined and evaluated with simulation results, and the feasibility of the converter topology is verified by experiments using a 1.0 kW–25 kHz prototype system.

A Novel type of High-Frequency Transformer Linked Soft-Switching PWM DC-DC Power Converter for Large Current Applications

This paper presents a new circuit topology of DC busline switch and snubbing capacitor-assisted full-bridge soft-switching PWM inverter type DC-DC power converter with a high frequency link for low voltage large current applications as DC feeding systems, telecommunication power plants, automotive DC bus converters, plasma generator, electro plating plants, fuel cell interfaced power conditioner and arc welding power supplies. The proposed power converter circuit is based upon a voltage source-fed H type full-bridge high frequency PWM inverter with a high frequency transformer link. The conventional type high frequency inverter circuit is modified by adding a single power semiconductor switching device in series with DC rail and snubbing lossless capacitor in parallel with the inverter bridge legs. All the active power switches in the full-bridge inverter arms and DC busline can achieve ZVS/ZVT turn-off and ZCS turn-on commutation operation. Therefore, the total switching losses at turn-off and turn-on switching transitions of these power semiconductor devices can be reduced even in the high switching frequency bands ranging from 20 kHz to 100 kHz. The switching frequency of this DC-DC power converter using IGBT power modules is selected to be 60 kHz. It is proved experimentally by the power loss analysis that the more the switching frequency increases, the more the proposed DC-DC converter can achieve high performance, lighter in weight, lower power losses and miniaturization in size as compared to the conventional hard switching one. The principle of operation, operation modes, practical and inherent effectiveness of this novel DC-DC power converter topology is proved for a low voltage and large current DC-DC power supplies of arc welder applications in industry.

Utility AC interfaced soft-switching sinewave PWM power conditioner with two-switch flyback high-frequency transformer

High-frequency switching pulse modulation utility-interfaced power conditioners (HF-UPC) with a high-frequency transformer link have attracted special interest for renewable and sustainable energy utilisation. These conditioners are small and cost-effective for meeting the requirements of small-scale residential power applications. However, switching power losses due to high-frequency switching operations are unavoidable. Efforts to reduce the power losses by implementing additional snubber components can complicate the controlling methods of the system. To solve these problems, a flyback transformer linked HF-UPC is proposed. The high-frequency flyback transformer is introduced into the power conditioner system instead of the typical forward transformer to simplify the circuit configuration and related controlling methods. With the aid of the resonant snubber circuits, the high-frequency switching devices in the primary side of the proposed DC-to-AC power conditioner can be turned off with zero-voltage soft-switching (ZVS). In addition, the proposed power conditioner system takes advantage of operating the currents by the discontinuous conduction mode (DCM) to realise turn-on commutation of zero-current soft-switching schemes. The output ports of the HF-UPC treated here are directly connected to the utility grid single-phase AC power source. A prototype of the proposed soft-commutation HF-UPC has been built and tested in experiments. Its power conversion processing is verified and its output current is qualified in accordance with the guidelines of the power utility companies.

High-Efficient High Frequency Linked Zero Voltage Soft Switching PWM DC/DC Power Converter with ON/OFF-Assisted Synchronous Rectifier Scheme

Advanced development of the high frequency pulse modulated DC/DC power converters are indispensable in order to achieve smaller size, higher performances of the power supplies. In resent years the problems of the electromagnetic noise generation caused by the hard-switching pulse width modulation (PWM)—based high frequency operation of the switching power devices have been significant, including switching losses and switching surges. As result a variety of soft-switching power conversion circuit technologies have been attracted special interests for reducing switching power losses and minimizing electromagnetic interference. The soft-switching pulse modulation of DC/DC power circuit topologies can be divided into continuous current mode resonant type and quasi-resonant voltage mode type. The soft-switching DC/DC converters, a continuous current mode zero current soft-switching (ZCS) DC/DC converter with pulse frequency modulation (PFM) has been already put into a practical use. In this sort of power converters soft-switching commutation can be achieved over wide load variation ranges and high stability can be also performed, but high current peak stress through power semiconductor devices arises, however, and it causes relatively large increase of conduction power losses in the power semiconductor devices, high frequency transformers as well as rectifier diodes. As a result, the problem of a high efficiency for DC/DC power converter could not still unsolved.Thereupon, in this paper, quasi-resonant voltage mode based soft-switching PWM DC/DC power converter with a high frequency transformer link is presented, which has on/off assisted synchronous rectifier in its secondary side. The PWM operation of this converter is described in comparison with conventional one. Due to using power MOSFET as on/off assisted synchronous rectifier in the secondary side of the high frequency isolated transformer, it is possible to achieve stable zero voltage soft-switching (ZVS) conditions from no load to the rated load for minimum requirement of a magnetizing current. As result of a high value of the magnetizing inductance design, the power converter actual efficiency of this converter can be designed so as to above 97% in experiment. Moreover, results of a switching losses analysis are discussed from an experimental point of views in this paper. The effectiveness of the power converter treated hare is proved from a practical point of view by using 32kHz-2.5kW breadboard setup.