A new power converter for fuel cells with high system efficiency

This paper presents a new high-efficiency grid-connected single-phase converter for fuel cells. It consists of a two-stage power conversion topology. Since the fuel cell operates with a low voltage in a wide voltage range (25?V–45?V) this voltage must be transformed to around 350–400?V in order to be able to invert this dc power into ac power to the grid. The proposed converter consists of an isolated dc–dc converter cascaded with a single-phase H-bridge inverter. The dc–dc converter is a current-fed push-pull converter. The inverter is controlled as a standard single-phase power factor controller with resistor emulation at the output. Experimental results of converter efficiency, grid performance and fuel cell dynamic response are shown for a 1?kW prototype. The proposed converter exhibits a high efficiency in a wide power range (higher than 92%) and the inverter operates with a near-unity power factor and a low current THD.     

Long-Lifetime Power Inverter for Photovoltaic AC Modules

This paper presents a power inverter tailored for low-power photovoltaic (PV) systems. The inverter features high reliability, thanks to a circuit topology that obviates aluminum electrolytic capacitors from the circuit. Moreover, all components, including logic and control, have been designed to exhibit high reliability at high temperatures. Three conversion stages form the power topology. First, a full bridge connected to a high-frequency transformer and a full-bridge rectifier amplifies the voltage of the PV panel to approximately 475 V. This stage is controlled by using a phase-shift pulsewidth-modulation controller that permits zero-voltage switching, thereby minimizing losses. Second, a buck converter is connected in series with the rectifier and is controlled by using current mode in order to shape the current injection into a rectified sine wave. Last, a full bridge is operated at line frequency to unfold the current injection. The amplification stage has a proportional compensator that maintains the voltage at the PV terminals constant. The current injection stage has a proportional-derivative compensator that controls the amplitude of the grid current so that the dc-link average voltage is maintained constant. Experimental results show that the peak efficiency of the system is 89%, and the total current harmonic distortion is below 5%. Finally, analyses show a designed lifetime of approximately ten years.     

风力发电系统变流器的直接功率控制策略

以永磁直驱型风力发电系统为研究对象,针对其变流器结构和控制策略进行了研究。通过选择最优双PWM“背靠背”变流拓扑结构,并采用直接功率控制策略进一步提高了风力发电系统的并网性能。建立了输出功率为10 kW的并网系统仿真模型,验证控制策略的正确性。结果表明,基于直接功率控制策略的“背靠背”变流拓扑具有结构合理、控制策略新颖的优点,在保证直流侧电压稳定的同时,电网电流谐波畸变率低、波形良好,能够实现单位功率因数并网,满足并网要求。     

Reduced Switch Count DC-Link AC–AC Five-Leg Converter

This paper investigates a reduced switch count dc-link ac–ac five-leg converter for three-phase power conversion. The converter provides both an input rectifier and an output inverter by sharing a leg in order to reduce the number of power switches. Scalar and vector pulsewidth modulation techniques are presented and the concepts of local and global apportioning factors are introduced. A control technique that aims at maximizing the utilization of the dc-link voltage is proposed. A hysteresis current controller that allows operating with one leg being shared by the load and grid sides is developed. In addition, several relevant characteristics of the converter are addressed, such as voltage capability, harmonic distortion, shared-leg and capacitor currents, and power rating. The features of such a converter are compared to those of the six-leg and four-leg converters. Selected experimental results are presented.     

A Novel Soft-Commutating Isolated Boost Full-Bridge ZVS-PWM DC–DC Converter for Bidirectional High Power Applications

A soft-commutating method and control scheme for an isolated boost full bridge converter is proposed in this paper to implement dual operation of the well-known soft-switching full bridge dc/dc buck converter for bidirectional high power applications. It provides a unique commutation logic to minimize a mismatch between current in the current-fed inductor and current in the leakage inductance of the transformer when commutation takes place, significantly reducing the power rating for a voltage clamping snubber and enabling use of a simple passive clamped snubber. To minimize the mismatch, the method and control scheme utilizes the resonant tank and freewheeling path in the existing full bridge inverter at the voltage-fed side to preset the current in the leakage inductance of the transformer in a resonant manner. Zero-voltage-switching is also achieved for all the switches at the voltage-fed side inverter in boost mode operation. The proposed soft-commutating method is verified through boost mode operation of a 3-kW bidirectional isolated full bridge dc/dc converter developed for fuel cell electric vehicle applications. The tested result verified the isolated boost converter can operate at an input voltage of 8.5–15V and an output voltage of 250–420V with a peak efficiency of 93% and an average efficiency of 88% at 55-kHz switching frequency with 72$^circ$C automotive coolant.     

新型共直流母线高压变频器

针对电机叠频试验容易产生较大的有功功率低频波动,在弱电网条件下对电网造成污染导致电网上其他设备无法正常工作,提出了一种新型共直流母线的高压变频器。其整流侧采用移相变压器加二极管整流方案,通过电容串联建立高压直流母线,直流母线储存能量使其不回馈至电网,特别适用于弱电网下的电机叠频试验;逆变侧采用MMC逆变器(Modular Multilevel Converter),通过平衡控制策略达到模块电压平衡,环流抑制的效果。最后应用PSCAD/EMTDC搭建系统的仿真模型,证实了方案的可行性。     

A comparison of load commutated inverter systems for inductionheating and melting applications

A comparative analysis of a current source inverter and a voltage source inverter suitable for induction heating and melting applications is presented. Both power supplies considered operate on the principle of load commutation. The comparison is based on criteria such as input power factor, component ratings, maximum and minimum operating frequencies, operation under varying load conditions, inverter starting capability, and system and control simplicity. The voltage source series resonant inverter is found to offer the best overall performance with respect to converter utilization     

A current source PWM inverter with actively commutated SCRs

Conventional SCR based current source inverters suffer from poor waveform quality due to six step switching. Pulse width modulated current source inverters typically require gate turn off devices with reverse voltage blocking capability which have limited their application. In this paper, a new pulse width modulated current source inverter topology using one gate turn off switch and six SCRs is presented. The converter uses active commutation to realize pulse width modulation in a conventional SCR based current source inverter. Modulation techniques for the proposed inverter, simulation and experimental results are described in the paper. This topology is suitable for high performance, high power applications     

130 kHz 7.5 kW current source inverters using static inductiontransistors for induction heating applications

A control scheme of a high-frequency high-power current source inverters using static induction transistors is described which aims at the suppression of the surge voltage and reduction of the switching loss during the commutation of current. The inverter is operated at a leading power factor, which requires the phase angle of the output current to be adjusted to each specific load point by the controller. The stable operation is verified by the experiments under the commutation inductance 1.8 μH, i.e., 18% reactance (130 kHz, 250 V, 30 A, base). As a result, the inverter efficiency is estimated to be 97%, excluding the loss of the drive circuit and the control circuit     

Current Source Inverter Based Grid Connected Hybrid PV-Wind Power Generation Unit

ABSTRACT

This paper presents a current source inverter (CSI)-based hybrid power generation system, which uses wind turbine and photovoltaic cells (PVs). A permanent magnet synchronous generator (PMSG) is connected to the CSI using a diode rectifier and a buck converter that is used to control the speed of the rotor. Another buck converter is used to control the maximum power point tracking of PVs. The operation of proposed system is studied under normal and grid voltage dip conditions. According to new grid codes, most power generating units are supposed to remain connected to the grid during voltage sag conditions and inject reactive current to grid as defined by grid codes. The CSI has fault current limiting capability that makes it appropriate to use in grid-connected applications and during voltage sag conditions in particular. The proposed system tracks the maximum power point of wind turbine and PVs under normal mode and injects required reactive current to the grid during voltage drop. However, incorporation of CSI with the inherent behaviour of wind turbine and PVs causes fault current to be within the tolerable range for power electronic devices. Simulations are carried out by using PSCAD/EMTDC software to verify the proposed system.     

Input-output harmonics of line-current-modulated high-frequencylink DC to utility power converters

Two types of commutation schemes (called Type I and Type II) are proposed and analyzed for the line-connected inverter used in line-current-modulated high-frequency DC link to utility interface power conditioning systems. In the Type I scheme the high-frequency inverter is switched off well in advance of the zero crossings of line voltage, allowing the line current to go to zero and enabling the line-connected inverter to turn off. A simple lossless forced commutation circuit is used in the Type II scheme. The effects of these schemes on line-side and input DC-side current harmonics are studied in the case of single-phase systems, neglecting the effect of controller action. The performance indices are evaluated. The results obtained from prototype experimental systems are used to verify the theory presented. The analysis presented for single-phase systems is extended to three-phase systems     

High-frequency link inverter for fuel cells based on multiple-carrier PWM

Fuel-cell inverter applications typically have a relatively low voltage input, and require a battery bus for energy buffering. Circuit topology issues are examined based on these needs. The need for high step-up ratios, current control, low ripple, and battery storage leads to a current-sourced link converter as perhaps the best choice of conversion topology. High-frequency ac link conversion offers a possible way to reduce the number of power stages, in the form of a cycloconverter, known from previous work. It is shown that the control complexity in this converter can be addressed by adapting pulse-width modulation (PWM) techniques. Here, a multicarrier PWM approach is introduced as a convenient way to implement a high-frequency link inverter. The approach is a direct extension of conventional PWM, and supports square-wave cycloconversion methods that have appeared in prior literature. Simulation and experimental results are developed for a low-voltage ac link inverter, leading to a 48-V fuel cell input design.     

A Novel High-Frequency Transformer-Linked Soft-Switching Half-Bridge DC–DC Converter With Constant-Frequency Asymmetrical PWM Scheme

This paper presents a novel soft-switching half-bridge dc–dc converter with high-frequency link. The newly proposed soft-switching dc–dc converter consists of a single-ended half-bridge inverter controlled by an asymmetrical pulsewidth-modulation scheme and a center-tapped diode rectifier. In order to attain the wide range of soft commutation under constant switching frequency, the single active edge-resonant snubber cell composed of a lossless inductor and a switched capacitor is employed for the half-bridge inverter leg, providing and assisting zero-current-switching operations in the switching power devices. The practical effectiveness of the proposed soft-switching dc–dc converter is demonstrated by the experimental results from an 800 W–55 kHz prototype. In addition, the feasibility of the dc–dc converter topology is proved from the viewpoints of the high efficiency and high power density.      

Analysis and design of a high-frequency link DC to utilityinterface using square-wave output resonant inverter

A line-current-modulated high-frequency (HF) link DC to utility interface power conditioning system using a square-wave output resonant inverter is proposed. The proposed system consists of a square-wave output HF resonant inverter, a rectifier, and a line-connected inverter (LCI). The system is modeled using the constant current model for the HF inverter and the step model for the DC link current. The operating modes, analysis, design, and experiment results are presented. It has been shown that systems operating with less than 5% line current harmonic distortion are realizable when the type II commutation scheme is used for the LCI. A typical application of the system presented is in interfacing photovoltaic arrays to utility lines     

Operation of the LCC-type parallel resonant converter as a lowharmonic rectifier

A single-phase high-frequency transformer isolated single-stage AC-to-DC controlled rectifier with low line current harmonic distortion using a variable-frequency controlled LCC-type (or series-parallel) resonant power converter (SPRC) is presented. A simple analysis and design procedure is used for designing the converter for low line current harmonic distortion and high power factor operation. The converter performance characteristics have been verified with SPICE3 simulations (without active control) and experimental prototype SPRC (rated at 150 W, with and without active control) for variation in load as well as line voltage. When operated with active current shaping, this converter operates in zero-voltage-switching mode for the complete range, maintaining power factor close to unity with low line current distortion and low peak current compared to the parallel resonant converter     

Resonant-boost-input three-phase power factor corrector

This paper presents a novel three-phase power factor corrector (PFC) circuit which uses two power switches working in zero-voltage-switching (ZVS) condition. The two switches along with a high-frequency inductor constitute a high-frequency current source which is responsible for the energy transfer in the circuit. The input current is partly continuous and partly discontinuous. The total harmonic distortion (THD) in the input current has a low value of 4.5%, and the output DC voltage is very close to the peak line voltage. The operation of the converter is explained by identifying the different switching modes, and the simulation and experimental waveforms are presented     

Low order harmonic cancellation in a grid connected multiple inverter system via current control parameter randomization

In grid connected multiple inverter systems, it is normal to synchronize the output current of each inverter to the common network voltage. Any current controller deficiencies, which result in low order harmonics, are also synchronized to the common network voltage. As a result the harmonics produced by individual converters show a high degree of correlation and tend to be additive. Each controller can be tuned to achieve a different harmonic profile so that harmonic cancellation can take place in the overall system, thus reducing the net current total harmonic distortion level. However, inter-inverter communication is required. This paper presents experimental results demonstrating an alternative approach, which is to arrange for the tuning within each inverter to be adjusted automatically with a random component. This results in a harmonic output spectrum that varies with time, but is uncorrelated with the harmonic spectrum of any other inverter in the system. The net harmonics from all the inverters undergo a degree of cancellation and the overall system yields a net improvement in power quality.     

Dead-Time Elimination of PWM-Controlled Inverter/Converter Without Separate Power Sources for Current Polarity Detection Circuit

This paper will present a dead-time elimination scheme for a pulsewidth-modulation (PWM)-controlled inverter/converter. The presented dead-time elimination scheme does not require separated power supplies for freewheeling-current detection of high- and low-side power devices. The presented scheme includes the freewheeling-current polarity detection circuit and the PWM control generator without dead time. It will be shown that the presented scheme eliminates the dead time of PWM control for inverter/converter and therefore dramatically improves output voltage loss and current distortion. Experimental results derived from a field-programmable-gate-array-based PWM-controlled inverter are shown to demonstrate the effectiveness.      

基于Cuk变换器的无刷直流电机转矩脉动抑制仿真

为解决无刷直流电机在换相过程中转矩脉动大的问题,提出了在三相逆变桥前端加入前级Cuk变换器和开关选择电路。通过对单一逆变器输入电压进行控制来抑制转矩脉动的方法。该方法使得瞬间换相区间电流上升相与电流下降相的电流斜率平衡,从而有效减小了换相期间的转矩脉动。通过仿真验证了控制策略的有效性。     

The bang-bang hysteresis current waveshaping control technique used to implement a high power factor power supply

This work reports the operation and development of a high power factor power supply that operates at high switching frequency. An optimum power factor correction is obtained using an ac-dc boost converter associated to a nondissipative snubber as a pre-regulator circuit, which presents reduced commutation losses. The same nondissipative snubber is associated to a Forward converter and then used as a dc-dc stage. The proposed switched mode power supply presents high power factor (0.998), high efficiency (91%), low harmonic content (current and voltage total harmonic distortion rates equal to 2.84% and 2.83%, respectively), and also satisfactory regulation. The converter has been theoretically analyzed, designed, simulated and implemented, where experimental results show that soft commutation in all switches is achieved.