Power System Design and Implementation Based on Parallel DC-DC Converters: A Case Study for Fuel Cell Application

DC-DC converters are usually utilized for fuel cell power system interface to regulate the output voltage according to the load characteristics. In some applications, the converters are needed to be installed in parallel structure to increase the power system reliability and in some cases, adapt the system electric power level. In this paper, a test setup is designed and implemented to utilize two parallel DC-DC power converters with asynchronous control systems. The test setup is developed to be used for a PEM fuel cell power system and the converters' operating points are selected considering the fuel cell voltage-current curve. The fuel cell operating point is simulated by a lead-acid battery pack as the input source. To simulate a hybrid power system, another battery pack is connected to the converters output. The converters input source and output battery pack are hybridized to supply the load. The experimental results confirmed the acceptable current sharing between the two converters by the implemented setup despite their asynchronous control systems.Considering the growing use of high power level systems based on renewable energy sources, utilizing parallel DC-DC converters system is a solution to increase the output power level and to provide reliable power systems[۱-۲]. Due to the benefits over a single stand-alone unit in reliability, reconfigurability, cost, efficiency, fault tolerance, power processing capability, redundancy and modular architecture, parallel DC-DC converters systems have been popularly applied in many distributed power systems [۳-۵]. In this paper, a setup is developed and experimentally tested to utilize two parallel DC-DC converters to simulate and evaluate their behavior in the fuel cell power applications.Considering the fuel cell voltage-current curve, the nominal operating voltage point is about ۱۲ V. Therefore a lead-acid battery pack including ۵ parallel ۱۲ V/۲۰۰ Ah batteries are used to simulate the fuel cell at the nominal operating point. Two commercial ۱۲ V/۲۴ V DC-DC converters are used to boost the input voltage to ۲۴ V. The output bus connects the converters output to the output battery pack and load. The output battery pack includes two lead-acid ۱۲ V/۲۰۰ Ah batteries connected in series. The input and output battery packs, output DC bus and the load are shown in figure ۱-a. As mentioned before, the converters are implemented with asynchronous control systems, therefore, in order to control the load sharing between the converters and also to prevent the current circulation current between them, two diodes are used in the passes connecting each converter output to the bus. The configuration is presented in figure ۱-b. The parallel converters are shown in figure ۲.Figure ۱: a) The input and output batteries and electronics load, b) The parallel converters connection a) Measuring the upper converter output current b) Measuring the lower converter output currentFigure ۲: The parallel convertersAs can be seen in figure ۲, the current sharing between the converters was measured under differentloading conditions during the test.Considering the asynchronous control systems of the converters and the predictable unbalanced loadsharing, the maximum input power during the test was ۳۶۰۰ W which is lower than the total rating powerof the two converters. Thus, the converters probable over-load conditions can be easily controlled duringthe test. The converters were selected for a ۳۰۰۰ W PEM fuel cell power system and the test goal was toinvestigate the possibility of installing two parallel converters to pass the rated power with acceptablecurrent sharing. The test results are presented in figure ۳. The converters' input and output currents,voltages and powers are presented in figure ۳-a, figure ۳-b and figure ۳-c, respectively.Figure ۳: The parallel converters input (in blue) and output (in red) a) currents b) voltages c) powersConsidering figure ۳-a, the total converter output current is about ۱۳۱.۵ A and the converters output poweris about ۳۰۹۵ W. As can be seen in figure ۲, at this loading level, the upper converter current is ۴۹.۳ Aand the lower converter current is ۸۲.۲ A. Therefore, the upper converter passes about ۳۷.۵% and thelower converter passes about ۶۲.۵% of the total output current. Considering the converters' asynchronouscontrol systems, the current difference between the two converters were predictable, but the load sharingdifference is acceptable for the ۳۰۰۰ W fuel cell power system

کلیدواژه ها:

DC-DC converter ، Parallel ، Fuel cell ، Current sharing