Yu-Kai Chen

3C strategy for inverters in parallel operation achieving an equal current distribution

A circular chain control (3C) strategy for inverters in parallel operation is presented in the paper. In the proposed inverter system, all the modules have the same circuit configuration, and each module includes an inner current loop and an outer voltage loop control. A proportional-integral controller is adopted as the inner current loop controller to expedite the dynamic response, while an H/sup /spl infin// robust controller is adopted to reach the robustness of the multimodule inverter system and to reduce possible interactive effects among inverters. With the 3C strategy, the modules are in circular chain connection and each module has an inner current loop control to track the inductor current of its previous module, achieving an equal current distribution. Simulation results of two-module and a three-module inverter systems with different kinds of loads and with modular discrepancy have demonstrated the feasibility of the proposed control scheme. Hardware measurements are also presented to verify the theoretical discussion.

Analysis and design of an isolated single-stage converter achieving power-factor correction and fast regulation

This paper presents the analysis and design of an isolated single-stage converter achieving high-power-factor correction and fast regulation. By using the technique suggested by Wu et al., a buck-boost converter and a flyback converter can be integrated to form the discussed converter. The buck-boost semistage working in the discontinuous conduction mode (DCM) functions as a power-factor corrector, and the flyback semistage operating in the DCM is a voltage regulator which is controlled, theoretically, to be independent of load variation. An approximated small-signal model of the converter operating in the DCM is developed. Design of a peak-current feedback loop with an optimal proportional integral controller is also presented. A prototype is implemented to verify that the analysis and design are effective and feasible.

Current Weighting Distribution Control Strategy for Multi-Inverter Systems to Achieve Current Sharing

A current-weighting-distribution-control (CWDC) strategy for multi-inverter systems to achieve current sharing is presented in this paper. With a CWDC strategy, the inverters connected in parallel are allowed to have different power ratings and can achieve a weighted output current distribution by adding only simple circuits to each inverter. In such systems, each inverter has an outer voltage loop controller to govern system stability, an inner current loop controller to expedite dynamic response, and a weighting current controller to achieve current distribution and to reduce possible interactive effects among inverters. Experimental results from a two-inverter system and a three-inverter system have demonstrated the feasibility of the proposed strategy in weighting current distribution and fast regulation during a step-load change or hot-swap operation

ACSS for paralleled multi-inverter systems with DSP-based robust controls

An averaged current-sharing strategy (ACSS) for paralleled multi-inverter systems with digital signal processor (DSP)-based robust controls is presented. With an ACSS, the inverters are in parallel operation and each inverter has a voltage robust controller to achieve system stability and robustness, and a current robust controller to track the averaged inductor current of the inverters to achieve equal current distribution. In the proposed system, the current-sharing control loop is independent of the voltage control loop. Therefore, equal current distribution among the inverters, fast response, and tight regulation can be achieved. Additionally, the ACSS in each inverter can be readily implemented with two operational amplifiers. Simulation results and hardware measurements or a single-inverter system and a two-inverter system, and simulation results of a three-inverter system with linear and nonlinear loads have demonstrated the feasibility of the proposed control scheme in equal current distribution and fast regulation.

A systematic and unified approach to modeling PWM DC/DC converters based on the graft scheme

A systematic and unified approach to modeling PWM DC/DC converters based on the graft scheme is presented in this paper. With the graft scheme, the typical PWM switch-mode converters, such as buck-boost, Cuk, Zeta and Sepic, can be generated from the two basic converters, buck and boost. The small signal models of these converters can be, therefore, derived by combining those of the buck and boost. Using the proposed approach can help to yield highly related dynamic models of the converters in a family, and, in addition, physical insights between the converters can be readily identified. This has made the proposed modeling method valuable and viable.

Parallel-Inverter System With Failure Isolation and Hot-Swap Features

This paper presents the analysis, design, and implementation of a parallel-inverter system with failure isolation and hot-swap features. The system is controlled with a system control unit to achieve output voltage regulation, inverter synchronization, and the aforementioned features. By incorporating a current- weighting-distribution control, the system allows inverters with identical or different power ratings. The failure isolation feature is achieved with a scanning circuit, a resistor-capacitor filter, and transistors, which are also used in achieving the hot-swap one. In the proposed system, power capacity can be readily expanded, and inverter-failure disturbance can be correspondingly reduced. Experimental results that are obtained from a six-inverter system have illustrated the discussed features and significantly demonstrated its feasibility, improving system reliability, and stability.

A fuzzy logic controlled single-stage converter for PV powered lighting system applications

This paper presents a fuzzy logic controlled single-stage power converter (SSC) for photovoltaic powered lighting system (PPLS) applications. The SSC is the integration of a bidirectional buck-boost charger/discharger and a class-D series resonant parallel loaded inverter. The designed fuzzy logic controller (FLC) can control both the charging and discharging current, and improve its dynamic and steady state performance. Furthermore, a maximum power point tracking (MPPT) scheme based on a perturb-and-observe method is also realized to effectively draw power from photovoltaic arrays. Both of the FLC and the MPPT are implemented on a single-chip microprocessor. Simulated and experimental results obtained from the proposed circuit with an FLC have verified the adaptivity, robustness and feasibility.

Integration and Operation of a Single-Phase Bidirectional Inverter With Two Buck/Boost MPPTs for DC-Distribution Applications

This study is focused on integration and operation of a single-phase bidirectional inverter with two buck/boost maximum power point trackers (MPPTs) for dc-distribution applications. In a dc-distribution system, a bidirectional inverter is required to control the power flow between dc bus and ac grid, and to regulate the dc bus to a certain range of voltages. A droop regulation mechanism according to the inverter inductor current levels to reduce capacitor size, balance power flow, and accommodate load variation is proposed. Since the photovoltaic (PV) array voltage can vary from 0 to 600xa0V, especially with thin-film PV panels, the MPPT topology is formed with buck and boost converters to operate at the dc-bus voltage around 380xa0V, reducing the voltage stress of its followed inverter. Additionally, the controller can online check the input configuration of the two MPPTs, equally distribute the PV-array output current to the two MPPTs in parallel operation, and switch control laws to smooth out mode transition. A comparison between the conventional boost MPPT and the proposed buck/boost MPPT integrated with a PV inverter is also presented. Experimental results obtained from a 5-kW system have verified the discussion and feasibility.

Modeling of single-stage converters with high power factor and fast regulation

This paper presents an approach to systematically model single-stage DC/DC converters operated in discontinuous conduction mode (DCM) based on the graft scheme. With the graft scheme, the active switches which are with a common node and operating in unison can be integrated to form a single stage converter (SSC). The small-signal models of the SSC can, therefore, be derived by combining those of its originally separate converters. Using the proposed approach can help yield highly related dynamic models of the converters in a family and, in addition, physical insights between the converters can be readily identified. Moreover, the expressions of the small-signal models for the SSCs operated in DCM can be extended to those in continuous-conduction-mode operation. These have made the proposed modeling method valuable and viable. Experimental measurements have demonstrated that the small-signal model of an SSC derived with the proposed approach is relatively accurate.

A structural approach to synthesizing soft switching PWM converters

This paper presents a structural approach to synthesizing soft switching PWM converters based on the concept of basic converter units (BCUs). With a proper reconfiguration, several families of passive and active soft switching PWM converters can be synthesized from either buck BCU or boost BCU plus certain linear networks. These typical soft switching converters include the well known topologies of buck, buck-boost, Zeta, boost, Cuk and Sepic. It has been shown that analysis of the converters can be conveniently performed from the derived general configurations, reducing the complexity significantly. Therefore, using the structural approach not only can explore more physical insights into the converters in a family, but can reveal more relationships among these soft switching converters over conventional approaches. These merits have made the proposed approach become uniquely valuable.