Seung-Ryul Moon

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

A Feasibility Study on a New Doubly Salient Permanent Magnet Linear Synchronous Machine

Permanent magnet linear synchronous motors (PMLSMs) have been successfully adopted in a wide variety of industry applications due to improved servo-capability and outstanding dynamic characteristics. However, the conventional PMLSMs need a large amount of permanent magnet (PM) material in the stator resulting in significant cost rise. This paper proposes and analyzes a new doubly salient PM linear synchronous machine (DSPMLSM). The proposed configuration consists of consequent pole stator to reduce PM material and modular mover structure to reduce force ripples. Four different models are chosen and compared by two-dimensional finite element analysis (FEA). The analysis reveals that the proposed DSPMLSM uses only a half amount of PMs, while retaining almost the same characteristics as the conventional PMLSMs.

Design of an Axial Flux Permanent Magnet Generator for a Portable Hand Crank Generating System

This paper deals with a design of an axial flux permanent magnet (AFPM) generator which is a key element of a compact size portable and manual generator system composed of a gear system, a generator, a rectifier, and a battery charger. The design objects of the AFPM generator are its compact size, light weight, and high efficiency, and the three design objects are the main criteria to select the value of design variables. A soft magnetic composite core is used instead of a silicon steel core in the AFPM generator to achieve a compact size. In this paper, the overall design process to meet the design goals and the design results are presented with experiment results.

Multiphase Isolated DC-DC Converters for Low-Voltage High-Power Fuel Cell Applications

Multiphase isolated dc-dc converters allow current sharing among phases and high-frequency ripple cancellation and are very desirable for low-voltage high-power fuel cell applications. The three-phase dc-dc converters particularly are getting more attention because they share the same circuit structure as the conventional three-phase dc-ac inverters, and they have been reported to have high-efficiency operation. This paper is to compare the similarities and differences between two soft-switched three-phase dc-dc converters: one with three phase legs and the other with six phase legs. This paper will describe basic operating principle of these two multiphase dc-dc converters and compare their performance for a low-voltage, high-power fuel cell application. Two 5-kW converters have been built and tested with a fuel cell simulator. Experimental switching waveforms and efficiency profiles are shown to support the described basic principles. Major features and differences between these two converters will be discussed.

Current Loop Control with Admittance Compensation for a Single-Phase Grid-Tie Fuel Cell Power Conditioning System

Current loop transfer function of a single-phase grid-tie inverter has been systematically derived with representations of conventional transfer function format and admittance terms for the sake of controller design and feed-forward compensation. A 5 kW grid-tie fuel cell power conditioning system (PCS) example is used in paper to show current loop controller design and admittance compensation technique. A second order lead-lag compensator is proposed to avoid low stability margin while maintaining sufficient gain at the fundamental frequency. The proposed current loop controller and admittance compensation have been simulated, and the same parameters have been used for a DSP-based PCS inverter controller. Simulation results indicate that without the admittance path compensation, the duty cycle of the current loop controller is largely offset by the admittance path. At light load settings, the power flow may be erratically fed back to the inverter that can cause dc bus over voltage and subsequently a catastrophe failure. With admittance path compensation and the designed lead-lag compensator, the output power shows a steady-state offset that coincidently is required by the fuel cell. Experimental results well matched the mathematical design and simulation results.

Comparative Study of Stator Core Composition in Transverse Flux Rotary Machine

This paper deals with the comparison of magnetic characteristics in transverse flux rotary machine according to different stator core composition with the same rotor. Three different stator designs are considered in the analysis according to the material composition of inner and outer stator cores. Electromotive force (EMF), inductance, torque, and core losses are calculated by threedimensional finite element analysis. Calculated and measured results of back-EMF according to the analysis models in dependency on speed are presented.

Closed-loop regenerative efficiency testing with electric vehicle bidirectional DC-DC converter

Bidirectional dc-dc converters have been widely used in hybrid electric vehicles (HEV), fuel cell vehicles (FCV), and electric vehicles (EV). In this paper, a 60 kW, three-phase, bidirectional dc-dc converter prototype for electric vehicle is presented along with experimental results. One important evaluation criterion for any converter is efficiency. Due to the high-power level and the bidirectional characteristic of EV bidirectional dc-dc converter, high power sources and compatible electric loads for both high and low side terminals are needed for efficiency testing; however, such equipments may not be easily available. In order to eliminate the need for high power source and loading equipments and to achieve high accuracy efficiency measurement, a regenerative efficiency testing method is employed. Also, the regenerative efficiency measurement is performed under closed-loop condition that yields consistent and repeatable efficiency test results.

Dynamic Characteristic Analysis Considering Core Losses in Transverse Flux Linear Machine With Solid Cores

This paper deals with a method for dynamic characteristic analysis considering core losses in transverse flux linear machines (TFLMs) with solid cores. This paper focuses on how to calculate the core losses of solid cores and how to apply the core losses to the dynamic simulation. The magnetic field characteristics, which are used for the core loss calculation and dynamic simulation, are calculated by using a 3-D equivalent magnetic circuit network analysis method. The accuracy of the proposed core loss calculation and dynamic simulation methods are examined by comparing the input currents of the dynamic simulation model with the measured input current of two TFLM prototypes.