Analysis and Design of Fixed-Frequency Controlled LCL-T Type DC-DC Soft-Switching Power Converter for Renewable Energy Applications

Abstract

Electrical power is one of the important requirements for sustainable development of any nation. A wider gap is being created between the power supply and the ever increasing power demand. The available conventional energy sources are either insufficient or cannot sustain for long to meet the current power demand as they are depleting in nature. Renewable energy sources (RESs) have been the most attractive alternate sources of energy for meeting the ever increasing power demand. Power generation from renewable energy sources depend on atmospheric conditions and hence the power produced is highly fluctuating in nature. To convert this fluctuating power into usable constant power, a power conditioning system is essential. DC-DC converter is one of the important components of the power conditioning system. This research is to find a suitable DCDC resonant power converter topology that can be used in solar power generation applications and investigate on its performance. Therefore, in this work, the literature survey on resonant converter topologies, power controlling methods, and analysis methods are presented. Fixed-frequency control makes the design of magnetic components and filters simple for effective filtering. Therefore, in this study, two fixedfrequency control schemes have been proposed. The first fixed-frequency control scheme is phase-shifted gating (PSG) control and the second is modified gating signal (MGS) control. The proposed PSG and MGS control schemes are experimentally validated and the choice between schemes is made by comparing the performance of the converter. It is found that both the gating schemes are effective in regulating the output voltage for variable input voltage and loading conditions. However, the efficiency of the converter is found to be higher with MGS due to the fact that only one switch loses ZVS as compared to two with the PSG when operated with maximum input voltage. Also, the variation in pulse-width angle (δ) required to regulate the output voltage is small in MGS as compared to that with PSG. The complete behavior of the resonant converter at different intervals of the operation can be predicted by analysing the circuit in steady-state iii and transient state. Two steady-state analysis methods have been proposed in this work. Firstly, fundamental harmonic approximation (FHA) method, and second, Fourier series (FS) method. The proposed steadystate analysis methods are experimentally validated. The performance of the LCL-T converter designed by using the FHA and FS analysis methods is compared. Fourier series method gives efficient results since it considers n-harmonic components of voltages and currents as compared to the fundamental harmonic approximation (FHA) method where, only fundamental component is considered. In order to understand the complete behavior of the converter for fluctuations in the input, load, and control parameters, small-signal modeling of the converter is essential. Therefore, an extended describing function (EDF) method available in the literature is used in this work for small signal modeling of the converter. It is convenient to derive all small-signal transfer functions and improve the accuracy by using the EDF method since it combines both the time-domain and frequency-domain analyses.