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Z-SOURCE INVERTER FOR FUEL CELL VEHICLES Adriana FLORESCU, Alexandru VASILE, Constantin RADOI, Dan Alexandru STOICHESCU University POLITEHNICA of Bucharest Faculty of Electronics, Telecommunications and Information Technology Department of Applied Electronics and Information Engineering Topic: C Presentation: P Abstract - DC-DC converters for fuel cell power system are usually bi-directional and have complicated circuits, needing many devices and switching configurations. This article presents the most advanced and new converter suitable for many applications (including FCHEVs applications), including both the DC/DC converter and DC/AC inverter. It proves that high efficiencies, low costs and future developments in fuel cell power systems can and will be achieved with simple, ingenious, widespread converters. Keywords – Fuel cell (FC), PWM inverter, boosted PWM inverter, Z-source inverter, hibrid electrical vehicle (HEV). Motivation Fuel cells are usually classified by the electrolyte employed in the cell. An exception to this classification is the DMFC (Direct Methanol Fuel Cell) that is a fuel cell in which methanol is directly fed to the anode. A second grouping is by their operating temperature: there are low temperature fuel cells – the Alkaline Fuel Cell (AFC), the Polymer Electrolyte Fuel Cell (PEMFC – that presents today the most interest of all types of fuell cells), the Direct Methanol Fuel Cell (DMFC), the Phosphoric Acid Fuel Cell (PAFC) - and high-temperature fuel cells that operates at 600-1000°C – the Molten Carbonate Fuel Cell (MCFC) and the Solid Oxide Fuel Cell (SOFC – also used in many applications). Fig. 1 shows the basic fuel cell power systems for all the main applications that include the fuel cell - who needs to be controlled in flow rate, pressure, humidity, temperature etc -, the power converters (DC-DC converters and/or DC-AC converters) that adapt the fuel cell to the load and the energy storage, such as a battery or an supercapacitor, required by fuel cell’s dynamic limitations. Fig. 2 represents the block diagram of a fuel cell power system for mobile applications such as hybrid vehicular applications. Besides the DC/DC converter and the traction inverter, the secondary battery guarantees the load leveling, assuring braking energy recovery and good performances in the transient operations. It also supplies with energy the air compressor, the hydrogen circulation pump and the cooling pump for inverter/motor etc. Fig.6 represents the block diagram of a fuel cell power system for portable applications such as laptops, cell phones or PDAs. It is similar to the block diagram in fig.4 excepting the inverter that is missing, taking account of the DC load. Isolation may be or may be not needed. Fig.1. Basic fuel cell power systems Fig.2. Block diagram of fuel cell power system for mobile applications Fig.3. Fuel cell power system with Z-source inverter supplied by a bidirectional DC/DC converter Results By replacing one of the capacitors in the Z-source network (LC) with a battery as shown in Fig. 1, the Z-source inverter can be used in FCHEVs. Traditional PWM inverter always requires an extra DC/DC converter to interface the battery in FCHEVs. The Z-source inverter eliminates the DC/DC converter and utilizes instead an exclusive Z-source (LC) network to link the main inverter circuit to the FC (or any dc power source). The comparison efficiency results in fig.4 [7] shows that the Z-source inverter has lower average switching device power in low boost ratio range (1–2) in which most fuel cells reside. In cases when a low voltage fuel cell is used and a boost ratio much higher than 2 is needed, the dc–dc boosted PWM inverter is the best configuration. Z-source inverter also provides higher efficiencies in most operation ranges. From the above comparison, the Z-source inverter provides the highest efficiency in most regions of the power range of the inverter itself. Fig. 4. Calculated efficiency of inverters Fig. 5. Inverter efficiency calculated using Mitsubishi average loss simulation software References [1] V. C. Regep, E. Mamut, “Stand For The Experimental Study Of Pem Fuel Cells”, Rom. Journ. Phys., Vol. 51, Nos. 1–2, P. 41–48, Bucharest, 2006 [2] C. Liu, A. Ridenour, J.S. Lai, “Modeling and Control of a Novel Six-Leg Three-Phase High-Power Converter for Low Voltage Fuel Cell Applications”, IEEE Transactions on Power Electronics, Vol. 21, No. 5, pp. 1292-1300, Sept. 2006