Abstract:
Distributed hybrid power systems consist of AC and DC sub-systems connected to various load types, where the distributed generation (DG) resources can be either AC or DC. These resources are normally interfaced to the grid through power electronic converters. A single phase grid tied bi-directional AC-DC converter is presented in this research work. The bi-directional AC-DC converter works as the interface between the DC link and the AC grid. It meets the requirements of bidirectional power flow and to ensure high power factor and low THD as well as regulate the DC link voltage. It operates in two modes: the rectifier mode and the inverter mode. This research work is mainly focused on improvement and optimization of the converter in rectifier mode where it converts the AC line voltage into constant DC voltage which is used for energy storage systems and other DC loads.
The goals of the research work are to design and control a bi-directional AC-DC converter which has improved total harmonic distortion (THD) of the current and nearly unity power factor. The previously used AC-DC converters designed with diodes and/or thyristors have a great influence over the grid in terms of current harmonics and power factor degradation. The existing designs of bi-directional AC-DC converter use larger passive components like line inductor and output filter capacitor. The PI control and current hysteresis control (CHC) methods are used for the control of such converters. The CHC method has a great impact on converter performance in terms of switching losses and load dependency. The conventional PI control scheme has poor current tracking and non-zero steady state errors at line frequency.
A novel proportional-resonant (PR) control scheme is presented in the research work. A comparison of the conventional diode bridge AC-DC converter and the proposed bi-directional converter is made. Also, comparison is made between PI control and novel PR control for their effects on the converter in steady state and load changing conditions. The PR controller tracks reference currents with a very small steady state error and fast response to changes of grid voltage, load value and frequency which contributes to energy conversion and bidirectional flow of electricity.
The converter design and proposed control system is validated and verified in MATLAB/SIMULINK and practical prototype. The simulation and practical results proved the converter design and proportional resonant control structure for their capabilities of very low THD and power factor.
