Maximum Boost Control of Diode-assisted Buckboost Voltage Source Inverter with Minimum
Abstract
Diode-assisted buck-boost voltage source inverter (VSI) achieves high voltage gain by introducing a switch-capacitor based high step-up dc-dc circuit between the dc source and inverter bridge. As for the unique structure, various pulse width modulation (PWM) strategies are developed with regard to the chopped intermediate dc-link voltage. In order to maximize voltage gain and increase efficiency, this paper proposes a novel PWM strategy. It regulates the average value of intermediate dc-link voltage in one switching time period (T ) the same as the instantaneous maximum value of three-phase line voltage by controlling the front boost circuit. Then, the equivalent switching frequency of power devices in the inverter bridge can be reduced to 1/3f s f s =1/T s s . The operating principle and closed-loop controller design are analyzed and verified by simulations and experiments. Compared with existing PWM strategies, the new control strategy demonstrates less power device requirement and higher efficiency in high voltage gain applications. It is a more competitive topology for wide range dc/ac voltage regulation in renewable energy applications. Furthermore, with new control strategy, the dc-side inductor current and capacitor voltage contains six-time the line frequency ripples. To overcome the undesired influence of low frequency ripples, it is also suitable for 400-800Hz medium frequency aircraft and vessel power supply system.
PROPOSED SYSTEM:
In view of additional power conversion stage increasing cost and lowering efficiency, a family of Z-source inverter introduces a unique impedance network between the dc source and the inverter bridge. It achieves the desired output voltage that is larger than the available dc source voltage by adopting shoot-through (ST) operation mode. Z-source inverter provides a potential cheap and single-stage power conversion. However, the ST state limits the modulation index and accompanies large ST current. Literature makes comparison between traditional VSI and Z-source inverter based on electric vehicle driver system. The results reveal that Z-source inverter demonstrates low cost and high efficiency under relatively low voltage boost ratio range. Although both of them can boost output voltage to any desired value without upper limitation in theory, the degradation of efficiency and increasing requirement of switching devices are prominent under high voltage gain. Literature proposed diode-assisted buck-boost VSI and related modulation strategy.
EXISTING SYSTEM:
Fuel cells and light-weight battery power supply systems are promising in future hybrid electric vehicle, more-electric aircraft and vessel. However, the obvious characteristic of these dc sources is low voltage supply with wide range voltage drop. Power electronic interface has to regulate the amplitude and frequency to obtain required high ac utility voltage. These applications raise stringent requirements for power converters such as low cost, high efficiency and wide range voltage buck-boost regulation ability. Traditional voltage source inverter (VSI) can only perform buck voltage regulation. Thus, various novel and improved dc-ac topologies with buck-boost capability as well as the related control methods have been proposed to solve the issues.
CONCLUSION
The paper starts by analyzing the modulation principle of three-phase VSI and then proposes a new PWM strategy to achieve the instantaneous maximum utilization of intermediate dc-link voltage, as well as to reduce the switching frequency of power devices in diode-assisted buck-boost VSI. Simulation and experiment results are included to verify theoretical analysis. Compared with existing modulation strategies in, diode-assisted buck-boost with maximum boost control strategy reduces the voltage stress of switches and demonstrates the optimal efficiency. Therefore, it is a more promising and competitive topology for wide range voltage regulation in renewable energy applications. Furthermore, with the maximum boost control strategy, the dc-side inductor current and capacitor voltage contains six-times line frequency ripples. Therefore, it is also suitable for relatively high output line frequency, including 400-800Hz medium frequency aircraft and vessel power supply system.
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