Guorong Zhu

Mitigation of Low-Frequency Current Ripple in Fuel-Cell Inverter Systems Through Waveform Control

Fuel-cell power systems comprising single-phase dc/ac inverters draw low-frequency ac ripple currents at twice the output frequency from the fuel cell. Such a 100/120 Hz ripple current may create instability in the fuel-cell system, lower its efficiency, and shorten the lifetime of a fuel cell stack. This paper presents a waveform control method that can mitigate such a low-frequency ripple current being drawn from the fuel cell while the fuel-cell system delivers ac power to the load through a differential inverter. This is possible because with the proposed solution, the pulsation component (cause of ac ripple current) of the output ac power will be supplied mainly by the two output capacitors of the differential inverter while the average dc output power is supplied by the fuel cell. Theoretical analysis, simulation, and experimental results are provided to explain the operation and showcase the performance of the approach. Results validate that the proposed solution can achieve significant mitigation of the current ripple as well as high-quality output voltage without extra hardware. Application of the solution is targeted at systems where current ripple mitigation is required, such as for the purpose of eliminating electrolytic capacitor in photovoltaic and LED systems.

Direct AC/DC Rectifier With Mitigated Low-Frequency Ripple Through Inductor-Current Waveform Control

In a rectification system with unity power factor, the input power consists of a dc and a double-line frequency power component. Traditionally, an electrolytic capacitor (E-Cap) is used to buffer the double-line frequency power such that the dc output presents a small voltage ripple. The use of E-Cap significantly limits the lifetime of the rectifier system. In this paper, a differential ac/dc rectifier based on the use of an inductor-current waveform control methodology is proposed such that a single-stage direct ac/dc rectification without the need of an E-Cap for buffering the double-line frequency power, and a front-stage diode rectifier circuit can be achieved. The feasibility of the proposal has been practically confirmed in an experimental prototype.

Development of a maximum-power-point tracking algorithm for direct methanol fuel cell and its realization in a fuel cell/supercapacitor hybrid energy system

Direct methanol fuel cells (DMFC) have been widely researched for applications in portable electronics due to their use of liquid fuel for easy storage and transportation compared to gaseous hydrogen. However, DMFCs performance is strongly affected by methanol crossover that significantly degrades the fuel conversion efficiency at low output power, and is characterized by an increasing efficiency at increasing output power. The maximum efficiency point (MEP) is inherently difficult to track due to the commonly unknown methanol crossover rate, but since it is typically located very close to the maximum power point (MPP), an alternative tracking approach based on the MPP is proposed. In this paper, a fuel-cell-oriented MPP tracking (MPPT) algorithm based on resistance matching is developed, implemented, and tested in the context of a DMFC/supercapacitor hybrid power system. To account for the generally slow fuel cell dynamics, the DMFC is constantly tracked at the MPP while any surplus or deficit power is absorbed or delivered by the supercapacitor bank. The detailed formulation of the algorithm and the power flow design and realization are also discussed.

An AC side-active power decoupling modular for single phase power converter

The conversion between DC and AC power will typically introduce a low-frequency pulsation power in the DC side of single phase power converter which may create instability, lowers its efficiency, and reduces the reliability. In this paper, a novel Power Decoupling Modular (PDM) with 2 series differential decoupling capacitors at the ac side is proposed. Compared with the DC PDM, the proposed modular holds smaller filter inductor resulting from reusing the ac inductor as the freewheel inductor, less voltage stress of the capacitor voltage because of full use of the capacitance and shortest flowing path of the pulsation power due to supply the pulsation power directly by the series capacitors at the ac side. Theoretical analysis and results are provided to explain the operation and showcase the performance of the modular. Moreover, the power flow involved in this circuit and associated controller design are detailed in the paper. Experimental results validate that the proposed solution without effect on the original system can achieve significant mitigation of the pulsation power as well as high quality output voltage.

Direct AC/DC Rectifier with Mitigated Low-Frequency Ripple Through Waveform Control

In a rectification system with unity power factor, the input power consists of a DC and a double-line frequency power component. Traditionally, an electrolytic capacitor (E-Cap) is used to buffer the double-line frequency power such that the DC output presents a small voltage ripple. The use of E-Cap significantly limits the lifetime of the rectifier system. In this paper, a differential AC/DC rectifier based on the use of an inductor-current waveform control methodology is proposed. The proposed configuration achieves single-stage direct AC/DC rectification without the needs of a front-stage diode rectifier circuit, an input EMI filter, and an E-Cap for buffering the double-line frequency power. The feasibility of the proposal has been practically confirmed in an experimental prototype.

Dynamic characteristics of boost inverter with waveform control

The input current of single-phase inverter typically has an AC ripple component at twice the output frequency. The low-frequency current ripple can cause a reduction in both the operating lifetime of its DC source and the energy conversion efficiency of the system. In this paper1, a proposed waveform control method which can eliminate such a ripple current in boost inverter system, is discussed. The characteristics of the waveform control method in boost inverter under input voltage or wide range load variations are studied. It is validated that by including an output current feedback loop into the waveform control method, the reference voltage of the boost inverter capacitors can be instantaneously adjusted to match the new load, thereby achieving ripple mitigation for a wide load range. With the control and feedback mechanism, there is minimal level of 2ω component at the DC bus during steady state, and the transient response is rapid with negligible effect on the output voltage. Analysis, simulation and experimental results are presented to support the investigation.

Cost assessment of three power decoupling methods in a single-phase power converter with a reliability-oriented design procedure

Electrolytic Capacitors (E-Cap) as the passive energy buffer in single-phase converter are often assumed to be the reliability bottleneck of power electronic system. Various Active Power Decoupling (APD) methods have been proposed intending to improve the reliability of the DC-link E-Caps qualitatively, making great effort to diverting the instantaneous pulsation power into extra reliable storage components. However, it is still an open question, which method is the most cost-effective one for a specific application with a given lifetime requirement. In this paper, two of the representative APD methods and the classical passive DC-link design method are evaluated from the reliability and cost perspective. The reliability-oriented design procedure is applied to size the chip area of active switching devices and the passive components to fulfill a specific lifetime target. Component cost models are applied to obtain the overall cost of each DC-link design methods. The cost comparisons are performed with a lifetime target of 10 years and 35 years. It reveals that different conclusions can be drawn with different lifetime targets in terms of cost-effectiveness.

Balancing Control Strategy for Li-Ion Batteries String Based on Dynamic Balanced Point

Li-ion battery is becoming the optimal choice for EVs power supply. In order to keep the Li-ion battery away from charging damage and prolong the batterys life, we present and evaluate here a special control strategy based on dynamic balanced point along with a non-dissipative equalizer. The main focus of the proposed control strategy is to insure that individual cell of a battery pack will be rapidly, efficiently and simultaneously balanced by adjusting equalizing current dynamically. In this paper, a model of four series connected Li-ion batteries pack has been established to evaluate proposed control strategy. The superior performance is shown by the simulation.

Waveform control of fuel-cell inverter systems

Fuel-cell power systems comprising single-phase DC/AC inverters draw low-frequency AC ripple currents at twice the output frequency from the fuel cell. Such a 100/120 Hz ripple current may create instability in the fuel cell system, lowers its efficiency, and shortens the lifetime of fuel cell stack. This paper1 presents a waveform control method that can mitigate such a low-frequency ripple current from being drawn from the fuel cell while the fuel-cell system delivers AC power to the load through a differential inverter. Theoretical analysis, simulation, and experimental results are provided to explain the operation and showcase the performance of the approach. Results validate that the proposed solution can achieve significant mitigation of the current ripple as well as high quality output voltage without the need for extra hardware.

Lifetime estimation of DC-link capacitors in a single-phase converter with an integrated active power decoupling module

In single-phase inverters, DC-link capacitors are installed at the DC-link to buffer the ripple power between the AC side and DC side. Active decoupling methods introduce additional circuits at the DC side or AC side to partially or fully supply the ripple power. So that the demanded DC-link capacitor capacitance can be decreased. However, few research is about the effect of DC side and AC side decoupling on the DC-link capacitor reliability considering its electro-thermal stresses. This paper presents a quantitative analysis on the lifetime of capacitors with power decoupling circuits at the DC side and AC side, respectively. The ripple current spectrum of the capacitors is obtained by double Fourier analysis of a H-bridge inverter with natural sampling PWM modulation. A study case is demonstrated by a 2,000 W H-bridge inverter with 400 V DC-link voltage.