Sung-Yeul Park

An Improved Zero-Voltage Switching Inverter Using Two Coupled Magnetics in One Resonant Pole

A novel soft-switching inverter using two small coupled magnetics in one resonant pole is proposed to ensure the main switches operating at zero-voltage switching from zero load to full load and the auxiliary switches at zero-current switching with load adaptability and small current stress. Since independent coupled magnetics structure avoids the unwanted magnetizing current freewheeling loop, the size of the coupled magnetics can be minimized with low magnetizing inductance, and the saturable inductor can be eliminated. Detailed circuit operation is described, and voltage-second balance condition of the magnetics is expressed mathematically. A 4-kW hardware prototype has been designed, fabricated, and tested to verify the validity of the novel circuit and the improved performance of the proposed soft-switching inverter. Experimental results show an excellent agreement with analytical results. Since the measured efficiency from 20% to 100% load consistently shows above 97.8% and peaks at 98.2%, the proposed inverter is very attractive for high-efficiency applications where energy saving is a major concern.

A Wide-Range Active and Reactive Power Flow Controller for a Solid Oxide Fuel Cell Power Conditioning System

A wide-range active and reactive power flow controller is designed to operate the inverter in pure leading, pure lagging, and the mix with active and reactive power conditions. The key to achieving lagging power flow control is to ensure sufficiently high enough DC bus voltage to avoid duty cycle saturation. The key to achieving precision power flow control for a wide range of power level is to adopt the quasi-proportional resonant controller for the current loop and the admittance compensator to cancel the grid-voltage-induced negative power flow. In this paper, the current loop transfer function has been systematically derived for the controller design purpose. Phasor analysis was adopted to explain the need of dc bus voltage requirement. A 5-kVA grid-tie fuel cell inverter was used as the platform to show current loop controller design and admittance compensation. The proposed controller has been simulated, and the same parameters have been used for a DSP-based controller. Both simulation and hardware experimental results agree well with the theoretical analysis.

A Seamless Control Strategy of a Distributed Generation Inverter for the Critical Load Safety Under Strict Grid Disturbances

This paper is to introduce a seamless grid interconnection control strategy for the renewable energy distributed generations such as photovoltaic, wind generator, and fuel cell systems. The proposed control method consists of a voltage controller in stand alone mode, a current controller in grid connected mode, and a hybrid voltage controller in a transition mode, which is to minimize the grid overvoltage. The hybrid voltage controller is added to the Q-axis current control loop in order to improve the response time under light load condition or to protect the critical load in SA operation during the grid fault conditions. The proposed control strategy reduces the impact of the renewable energy and the critical load under the grid fault or disturbance conditions. In addition, the smooth operation of the inverter will also enhance the stability and reliability of the utility grid. Real time digital simulator based hardware in the loop tests and simulation results show that distributed generation inverter can achieve seamless mode change under grid fault conditions.

Design and Control for LCL-Based Inverters with Both Grid-Tie and Standalone Parallel Operations

The inductor-capacitor-inductor (LCL) filter allows higher noise attenuation and universal output in which a power conditioning system or an inverter can operate in both grid-tie and standalone modes. In this paper, the LCL filter design considerations including sensor position selection and component selections are discussed for single-phase paralleled inverters operating in both grid-tie and standalone modes. For grid-tie mode operation, each inverter is operating under a single current loop with proportional-resonant controller and admittance path compensation to reduce the steady-state error by providing a high gain at the fundamental frequency. For standalone mode operation, one of the inverters is implemented with a dual-loop controller to regulate the output voltage while the rest inverters operate in single current-loop controller with communication channels in between to ensure the uniformity of current sharing. Both the simulation and experimental results verify that the designed controllers are capable of paralleling inverter operation in grid-tie and standalone modes by adapting to different controller settings while keeping the same hardware setup.

Admittance Compensation in Current Loop Control for a Grid-Tie LCL Fuel Cell Inverter

Current loop transfer function of a single-phase grid-tie inverter has been systematically derived with representations of conventional transfer function format using admittance terms for controller design and loop compensation. The power circuit adopts the LCL type filter to allow universal output that can be operated in both standalone and grid-tie modes. The proposed admittance compensation along with a quasi-proportional-resonant controller is designed to achieve high gain at the fundamental frequency while maintaining enough stability margins. The entire current loop controller and admittance compensation have been simulated and tested with a 5-kW fuel cell prototype. Without the admittance path compensation, simulation results indicate that the system cannot start up smoothly and the zero current command cannot be tracked very well. At first simulation cycle, the power flow erratically fed back to the inverter that may cause catastrophe failure. With admittance path compensation, the time-domain current steady-state error can be easily reduced with the loop gain design in frequency-domain. Simulation and experimental results show that the inverter is capable of both standalone and grid-tie connection mode operations and smooth power flow control even with zero current command.

A High-Efficiency 5-kW Soft-Switched Power Conditioning System for Low-Voltage Solid Oxide Fuel Cells

This paper presents a high-efficiency power conditioning system design that employs a soft-switched dc-dc converter and a soft-switched dc-ac inverter for a low-voltage 26-V, 5-kW solid oxide fuel cell. The dc-dc converter converts fuel cell voltage to a 400-V dc bus voltage using a 3-phase 6-leg phase-shift modulated converter to achieve higher than 96% peak efficiency with zero-voltage zero-current switching. The dc-ac inverter then produces 120/240V ac using a CoolMOS based soft-switching inverter to achieve 98% peak efficiency. The inverter output is universal for both standalone and grid-tie modes operation. The standalone load comes out of two sets of LC filter with a neutral line for equal voltage splits. For grid-tie mode, a circuit breaker needs to turn on, and an additional inductor is included for grid current ripple reduction. Test results indicated that peak efficiency of 94% is achieved for the two-stage power conversion PCS. In addition to high efficiency design, this paper also introduces advanced controls including fuel cell current ripple reduction using a current loop in the dc-dc converter and ac output current steady-state error reduction using an admittance compensation technique in the dc-ac inverter.

Impact of SOFC fuel Cell Source Impedance on Low Frequency AC Ripple

A fuel cell is a non-ideal voltage source with a wide varying source impedance, depending on output power scale and operating conditions, such as operating temperature, fuel pressure, quality of fuel, etc. A stationary fuel cell power system that consists of a single-phase dc-ac inverter tends to draw an ac ripple current at twice the output frequency, and the peak-to-peak amplitude of the ac ripple at the fuel cell output depending largely on the source impedance. In this paper, the ripple current propagation path is analyzed, and its linearized ac model is presented. Relationship between the peak-to-peak amplitude of ac ripple and the fuel cell source impedance is studied using equivalent circuit model, and the model is verified via simulation and experimental results. To reduce ripple seen by the fuel cell, an advanced active ripple control technique is integrated into the dc-dc converter. The effect of the control technique on various source impedances is experimented, and the test results between open-loop and closed-loop control techniques are compared. The dynamic response of the entire fuel cell power system under load transient condition is tested to verify the stability of the controller. The results indicate that the controller originally designed for steady state ripple reduction ripple reduction remains stable during severe load transients. 1

Variable Timing Control for Wide Current Range Zero-Voltage Soft-Switching Inverters

This paper presents a variable timing controlled soft-switching inverter. The proposed timing control is based on a simple voltage sensing circuit that detects zero voltage crossing condition to determine the main switch turn-on time. The proposed technique can be applied to different types of zero-voltage switching type inverters. In this paper, the coupled-magnetic type soft-switching inverter is used as example to show the implementation method and its simulation and experimental results. A 5-kW hardware single-phase inverter prototype using CoolMOS as the main device and IGBT as the auxiliary device has been built and tested. Simulation results were well verified with experimental results. Efficiency was compared between the proposed variable timing and the conventional fixed-timing controlled inverters. Significant efficiency improvement was found at light-load conditions.

Seamless Boost Converter Control Under the Critical Boundary Condition for a Fuel Cell Power Conditioning System

The boost converter operates either in discontinuous conduction mode (DCM) or in continuous conduction mode (CCM). The operation mode is determined by the duty ratio, load, and parameters of the boost converter. The plant models in DCM and CCM are different in the frequency domain. Therefore, it will be difficult to design a controller with stable operation and fast transient response for both modes. Moreover, if the boost converter operates in CCM with the DCM control gain or vice versa, it will be unstable. In this paper, the proposed control strategy can make mode transitions between DCM and CCM seamlessly by adding a mode tracker, and then the boost converter can autonomously operate by selecting the appropriate control loop in both operation modes. The proposed controller still has a voltage control loop in DCM and current/voltage control loops in CCM. The proposed mode tracker will be explained with a frequency-domain analysis. In the case of a portable fuel cell, the boost converter is required to operate from very light load (DCM) to regular load (CCM) con- ditions. Because of the wide range operation of the portable fuel cell, the strategy of the proposed smooth mode transition will be suitable. In addition, smooth operation of the converter will also be beneficial to the reliability of the fuel cell stack. Furthermore, the proposed principle will be applicable to other mode transient mechanisms such as grid mode transitions, master-and-slave mode transitions, and so on. A 20-W boost converter prototype will be used to verify the performance of the proposed control scheme.

Dynamic Response Analysis of DC–DC Converter With Supercapacitor for Direct Borohydride Fuel Cell Power Conditioning System

The direct borohydride fuel cell (DBFC) is directly fed sodium borohydride as a fuel and hydrogen peroxide as an oxidant. The output voltage of the DBFC varies with respect to current demand. Therefore, it requires a dc-dc converter for a regulated output voltage. The dc-dc converter should be designed considering both the impact of the fuel cell and load disturbances to achieve wide range voltage regulation. This paper analyzes the impact of the DBFC impedance on the dc-dc converter. Based on the converters small signal model including the DBFC impedance, the boost converter was evaluated. Impacts of the voltage and current control loops on the boost converter were investigated. Improved response time and stability of the DBFC dc-dc converter were observed by adding a supercapacitor between the DBFC and the dc-dc converter. Finally, two converter controllers, a nonlinear feedforward controller and a state feedback controller are proposed for further improvement of the dc-dc converter response time. A prototype 20-W dc-dc converter was built and tested to show the response improvement of the proposed analysis and control scheme.