Fixed Frequency Sliding Mode Control for Power Electronic Converters
In the modern era of industrialization, power electronic converters have gained key importance due to their wide range of applications such as; use in arc furnaces, induction heating systems, variable speed drives of motors, power supplies of various electronic systems, fluorescent lamps, elevators and cooling systems. Furthermore, the low-cost proliferation of power electronic devices due to the technological developments in the semiconductor industry has made them an attractive choice for integrating renewable energy resources. They are the essential and integral part of the systems providing solutions to issues like voltage sag, swell, frequency regulation, current sharing among different renewable energy resources, power factor correction and voltage regulation in different types of micro-grids.
In all the above-mentioned applications, the robustness of the system under different load and line conditions depend upon the control algorithm of the power electronic converters. This is why the research in the field of power electronics has been focused on the control of power converters during the last decade. Among different types of control techniques, Sliding Mode Control (SMC) is renowned for its disturbance rejection and parameter invariant nature which makes it an irresistible choice for controlling such systems. However due to physical constraints of the switching components, ideal condition of infinite frequency cannot be satisfied. This causes the system to operate at finite but time varying switching frequency which is a serious issue for power electronic converters as they consist of passive components whose size and value depends upon the switching frequency. Moreover, the time varying switching frequency degrades the power quality and causes electromagnetic interference (EMI) issues in case of grid connected power converters
In the first phase of this research, a fixed frequency sliding mode controller utilizing an integral sliding manifold is designed to improve the dynamic response of the power converter regulating the grid voltage, in the presence of unknown load condition and uncertain network parameter of the micro-grid. A specialpurpose test rig is designed to experimentally validate the results in comparison with conventional PI controllers.
In the second phase of the research, the power quality issues regarding DC microgrids are addressed. Power quality issues have been widely discussed regarding AC power systems but discussion regarding DC power quality in micro-grids is rare in research community because in past power transmission was carried out using AC systems only. However, in the recent years DC grids and transmission has emerged as an attractive solution to integrate renewable energy resources. Hence, to enhance the power quality, a fixed frequency SMC is proposed with harmonic cancellation and chattering reduction, using a PI type sliding manifold. Experimental results are presented to validate the theoretical derivations using the test rig.
Finally, a new method based upon filter extraction of the equivalent control is proposed to fix the switching frequency in power converters controlled by SMC. The technique is implemented on a boost converter and the results are presented in comparison with PWM-SMC. Rigorous mathematical analysis is provided along with experimental results to validate the proposed technique.