High-frequency transformer isolated power conditioning system for fuel cells to utility interface




Rathore, Akshay Kumar

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This thesis presents interfacing of fuel cells to a single-phase utility line using a high-frequency transformer isolated power converter. This research contributes towards selecting a suitable utility interfacing scheme and then designing a power conditioning system along with its control for connecting fuel cells to a single-phase utility line that can achieve high efficiency and compact size. The power conditioning system, designed and built in the research laboratory is connected with the utility line and the experimental results are presented. Based on the literature available on photovoltaic (PV) array and fuel cell based utility interactive inverters with high-frequency transformer isolation, the interfacing schemes for connecting a DC source, in particular fuel cells, to a single-phase utility line are classified. Based on the fuel cell characteristics and properties, performance and the comparison of these utility interfacing schemes, a suitable scheme for the present application is selected. Because of low voltage fuel cells, the system takes higher current from the fuel cell and results in lower efficiency of the system. The inverter stage of the selected scheme deals with the higher voltage (lower current) and therefore, its efficiency is higher. In this sense, the efficiency of the whole system depends mainly on the efficiency of the front-end DC-DC converter. To realize a low cost, small size and light weight system, soft-switching is required. Various soft-switched DC-DC converter topologies are compared for the given specifications. Based on the soft-switching range, efficiency and other merits and demerits, a current-fed DC-DC converter configuration is selected. The performance of the selected topology is evaluated for the given specifications. Detailed analysis, a systematic design, simulation and the experimental results of the converter (200 W, operating at 100 kHz) are presented. To achieve soft-switching for wide variation in input voltage and load while maintaining high efficiency has been a challenge, especially for the low voltage higher input current applications. The variation in pressure/flow of the fuel input to the fuel cells causes the variation in fuel cell stack voltage and the available power supplied to the load/utility line. It causes the converter to enter into hard switching region at higher input voltage and light load. A wide range soft-switched active-clamped current-fed DC-DC converter has been proposed, analyzed and designed and the experimental results (200 W, operating at 100 kHz) are presented. The fuel-cell voltage varies with fuel pressure and causes the variation in the output voltage produced by the front-end DC-DC converter at the input of the next inverter stage and will affect the inverter operation. Therefore, the front-end DC-DC converter should be controlled to produce a constant voltage at the input of the inverter at varying fuel pressure. Small signal modeling and closed loop control design of the proposed wide range L-L type active-clamped current-fed DC-DC converter has been presented to adjust the duty cycle of the converter switches automatically with any variation in fuel pressure to regulate the output voltage of the converter at a specified constant value. To convert the DC voltage output of the front-end DC-DC converter into utility AC voltage at line frequency and feeding current into utility line with low THD and high line power factor, an average current controlled inverter is designed. The complete power conditioning unit is connected to the single-phase utility line (208 V RMS, 60 Hz) and experimental results are presented. The system shows stable operation at varying reference power level.



Fuel cells, DC-DC converter