Jih-Sheng Lai

A multilevel voltage-source inverter with separate DC sources for static VAr generation

A new multilevel voltage-source inverter with separate DC sources is proposed for high-voltage, high power applications, such as flexible AC transmission systems (FACTS) including static VAr generation (SVG), power line conditioning, series compensation, phase shifting, voltage balancing, fuel cell and photovoltaic utility systems interfacing, etc. The new M-level inverter consists of (M-1)/2 single phase full bridges in which each bridge has its own separate DC source. This inverter can generate almost sinusoidal waveform voltage with only one time switching per cycle as the number of levels increases. It can solve the problems of conventional transformer-based multipulse inverters and the problems of the multilevel diode-clamped inverter and the multilevel living capacitor inverter. To demonstrate the superiority of the new inverter, a SVG system using the new inverter topology is discussed through analysis, simulation and experiment.

Generalized instantaneous reactive power theory for three-phase power systems

A generalized theory of instantaneous reactive power for three-phase power systems is proposed in this paper. This theory gives a generalized definition of instantaneous reactive power, which is valid for sinusoidal or nonsinusoidal, balanced or unbalanced, three- phase power systems with or without zero-sequence currents and/or voltages. The properties and physical meanings of the newly defined instantaneous reactive power are discussed in detail. With this new reactive power theory, it is very easy to calculate and decompose all components, such as fundamental active/reactive power and current, harmonic current, etc. Reactive power and/or harmonic compensation systems for a three-phase distorted power system with and without zero-sequence components in the source voltage and/or load current are then used as examples to demonstrate the measurement, decomposition, and compensation of reactive power and harmonics.

Design of Parallel Inverters for Smooth Mode Transfer Microgrid Applications

In this paper, a microgrid architecture comprised of parallel inverters, critical loads, and CAN communication is studied. The controller designs in both grid-tie and islanding modes for the designed MG system are described. How to distribute the currents among paralleled inverters with CAN bus as communication interface is addressed. The mode transfer tests are conducted with an inverter-simulated grid to define the proper transfer procedures. Experimental results show that the inverter can work properly in different basic microgrid operation modes. With current sharing scheme, the output current is equally shared among different inverters without noticeable circulating current. Both the simulation and experimental results of mode transfer show that the inverter can switch between grid-tie and islanding modes smoothly, which indicates that the proposed mode transfer procedure is capable of minimizing the transients during mode transfer.

Low Frequency Current Ripple Reduction Technique With Active Control in a Fuel Cell Power System With Inverter Load

A fuel cell power system that contains a single-phase dc-ac inverter tends to draw an ac ripple current at twice the output frequency. Such a ripple current may shorten fuel cell life span and worsen the fuel efficiency due to the hysteresis effect. The most obvious impact is it tends to reduce the fuel cell output capacity because the fuel cell controller trips under instantaneous over-current condition. In this paper, the ripple current propagation path is analyzed, and its linearized ac model is derived. The equivalent circuit model and ripple current reduction with passive energy storage component are simulated and verified with experiments. An advanced active control technique is then proposed to incorporate a current control loop in the dc-dc converter for ripple reduction. The proposed active ripple reduction method has been verified with computer simulation and hardware experiment with a proton exchange membrane type fuel cell using a multiphase dc/dc converter along with a full-bridge dc-ac inverter. Test results with open loop, single voltage loop, and the proposed active current-loop control are provided for comparison

High-Power Density Design of a Soft-Switching High-Power Bidirectional dc–dc Converter

A bidirectional dc-dc converter typically consists of a buck and a boost converters. In order to have high-power density, the converter can be designed to operate in discontinuous conducting mode (DCM) such that the passive inductor can be minimized. The DCM operation associated current ripple can be alleviated by interleaving multiphase currents. However, DCM operation tends to increase turnoff loss because of a high peak current and its associated parasitic ringing due to the oscillation between the inductor and the device output capacitance. Thus, the efficiency is suffered with the conventional DCM operation. Although to reduce the turnoff loss a lossless capacitor snubber can be added across the switch, the energy stored in the capacitor needs to be discharged before device is turned on. This paper adopts a gate signal complimentary control scheme to turn on the nonactive switch and to divert the current into the antiparalleled diode of the active switch so that the main switch can be turned on under zero-voltage condition. This diverted current also eliminates the parasitic ringing in inductor current. For capacitor value selection, there is a tradeoff between turnon and turnoff losses. This paper suggests the optimization of capacitance selection through a series of hardware experiments to ensure the overall power loss minimization under complimentary DCM operating condition. According to the suggested design optimization, a 100-kW hardware prototype is constructed and tested. The experimental results are provided to verify the proposed design approach.

Energy Management Power Converters in Hybrid Electric and Fuel Cell Vehicles

Using a bidirectional dc-dc converter along with low-voltage energy storage for the high-voltage dc bus and traction motor drives has been a prominent option for hybrid electric and fuel cell vehicles. This paper will describe the significance of energy management power converters and their circuit topology options for efficiency, size, and cost considerations. Whether isolated or nonisolated, soft switching techniques have been widely used in high-power bidirectional dc-dc converters. Through some design examples, the component selection and circuit design optimization are discussed, and their efficiency evaluation results are also given. Major difficulties of developing a high-power bidirectional dc-dc converter are found in lack of high-power passive components and lack of multiphase dc-dc controllers. More development work needs to be done in these areas

A high-efficiency grid-tie battery energy storage system

Lithium-ion-based battery energy storage system has started to become the most popular form of energy storage system for its high charge and discharge efficiency and high energy density. This paper proposes a high-efficiency grid-tie lithium-ion-battery-based energy storage system, which consists of a LiFePO4-battery-based energy storage and a high-efficiency bidirectional ac-dc converter. The battery management system estimates the state of charge and state of health of each battery cell and applies active charge equalization to balance the charge of all the cells in the pack. The bidirectional ac-dc converter works as the interface between the battery pack and the ac grid. A highly efficient opposed-current half-bridge-type inverter along with an admittance-compensated quasi-proportional resonant controller is adopted to ensure high power quality and precision power flow control. A 1-kW prototype has been designed and implemented to validate the proposed architecture and system performance.

Optimum harmonic reduction with a wide range of modulation indexes for multilevel converters

This paper proposes a novel modulation technique to be applied to multilevel voltage-source converters suitable for high-voltage power supplies and flexible AC transmission system devices. The proposed technique can generate output stepped waveforms with a wide range of modulation indexes and minimized total voltage harmonic distortion. The main power devices switch only once per cycle, as is suitable for high-power applications. In addition to meeting the minimum turn-on and turn-off time requirements for high-power semiconductor switches, the proposed technique excludes from the synthesized waveform any pulses that are either too narrow or too wide. By using a systematic method, only the polarities and the number of levels need to be determined for different modulation levels. To verify the theory and the simulation results, a cascaded converter-based hardware prototype, including an 8-b microcontroller as well as modularized power stage and gate driver circuits, is implemented. Experimental results indicate that the proposed technique is effective for the reduction of harmonics in multilevel converters, and both the theoretical and simulation results are well validated.

Design consideration for power factor correction boost converter operating at the boundary of continuous conduction mode and discontinuous conduction mode

A boost power converter operating at the boundary of continuous conduction mode (CCM) and discontinuous conduction mode (DCM) for power factor correction is described. Theoretically, the operation at the boundary of CCM and DCM is considered constant on-time for the boost switch. Due to finite switching frequency and capacitor filter effect, the switch turn-on time varies throughout the entire cycle. This variation of the switch on-time affects the average switching frequency and the circuit component selection criterion. The relationship between the circuit components and the switch on-time based on the fundamental principle of the power circuit operation is derived. A 44 W boost converter is designed and constructed accordingly. Experimental results show that the theoretical analysis along with practical design consideration can accurately predict the switch on-time and switching frequency.<<ETX>>

High Reliability and Efficiency Single-Phase Transformerless Inverter for Grid-Connected Photovoltaic Systems

This paper presents a high-reliability single-phase transformerless grid-connected inverter that utilizes superjunction MOSFETs to achieve high efficiency for photovoltaic applications. The proposed converter utilizes two split ac-coupled inductors that operate separately for positive and negative half grid cycles. This eliminates the shoot-through issue that is encountered by traditional voltage source inverters, leading to enhanced system reliability. Dead time is not required at both the high-frequency pulsewidth modulation switching commutation and the grid zero-crossing instants, improving the quality of the output ac-current and increasing the converter efficiency. The split structure of the proposed inverter does not lead itself to the reverse-recovery issues for the main power switches and as such superjunction MOSFETs can be utilized without any reliability or efficiency penalties. Since MOSFETs are utilized in the proposed converter high efficiency can be achieved even at light load operations achieving a high California energy commission (CEC) or European union efficiency of the converter system. It also has the ability to operate at higher switching frequencies while maintaining high efficiency. The higher operating frequencies with high efficiency enables reduced cooling requirements and results in system cost savings by shrinking passive components. With two additional ac-side switches conducting the currents during the freewheeling phases, the photovoltaic array is decoupled from the grid. This reduces the high-frequency common-mode voltage leading to minimized ground loop leakage current. The operation principle, common-mode characteristic and design considerations of the proposed transformerless inverter are illustrated. The total losses of the power semiconductor devices of several existing transformerless inverters which utilize MOSFETs as main switches are evaluated and compared. The experimental results with a 5 kW prototype circuit show 99.0% CEC efficiency and 99.3% peak efficiency with a 20 kHz switching frequency. The high reliability and efficiency of the proposed converter makes it very attractive for single-phase transformerless photovoltaic inverter applications.