A Study on High Efficiency Bidirectional
Grid-tied Converters
Sung-Ho Lee Department of Electrical Engineering
Pohang University of Science and Technology
SOUTH KOREA
Thesis for degree of Doctor of Philosophy in the Department of Electrical Engineering.
ABSTRACT
This thesis presents three high efficiency grid-tied converter topologies and their
respective control algorithms. These converters are applicable to various industrial fields
such as energy storage systems (ESS), renewable energy systems, uninterruptible power
supply (UPS) systems, and electric vehicles (EV).
First, a high efficiency bidirectional grid-tied flyback converter that uses a single
power conversion technique and a control system for it are introduced. The proposed
converter consists of a bidirectional flyback dc-dc converter and an unfolding circuit.
Due to its switching control strategy, the proposed converter performs bidirectional
power conversion between the energy storage device and the grid through only single
step. From the model analysis, the inherent dynamic characteristics are figured out.
Based on the analysis, a control system is developed, which consists of a linear
feedback controller with a low pass filter, a repetitive controller, and a feed-forward
controller; this overcomes the constrains caused by a right-half-plane zero and a filter
resonance and makes the proposed converter achieve the desired control
performances and stability. In conclusion, the proposed converter can obtain high
efficiency using a single power conversion technique, and the developed control
system makes the proposed converter feasible.
Second, a high efficiency single-phase bidirectional inverter for a PV energy
system integrated with an energy storage system. The proposed single-phase inverter
is developed using the transformerless system configuration so that it provides high
power conversion efficiency, excellent power density, and low production cost.
Using its circuit structure and switching operation, the proposed inverter can
suppress the ground leakage current which is considered to be one of the most
important design parameters in a transformerless PV system. Therefore, the proposed
inverter gives the feasibility for the transforemrless system configuration in a PV
energy system integrated with an energy storage system and enhances the overall
performance of the bidirectional power converter system.
The third is a novel high efficiency three-phase bidirectional grid-tied converter with high
power density for high power applications. The proposed converter is composed of the threelevel
dc-dc converter and the modified three-phase T-type three-level inverter. The threelevel
converter circuit configuration gives smaller passive component size and lower switch
voltage stress compared to other two-level grid-tied converters. In addition, due to its novel
circuit structure, the proposed converter can use the active and passive components for only
two phases of the grid in the inverter stage and eliminate the leakage current problem. Thus,
because the proposed converter enables to not only decrease the number of components but
also make the transformerless configuration feasible, it provides higher efficiency and higher
power density compared to other high power grid-tied converters.
All proposed converters are analyzed theoretically, and implemented practically
to evaluate their performance. Finally, the experimental results show that the
proposed converters improve overall performance such as high power conversion
efficiency, power density, and production cost with satisfying the standards for the
grid regulations.
LINK
https://www.mediafire.com/file/z79z7ss8hl69w07/A_Study_on_High_Efficiency_Bidirectional_Grid_tied_Converters.pdf/file
