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A Predictive Current Control Technique on Fuel Cell Based Distributed Generation in a Standalone AC Power Supply 892 1. Introduction Previous empirical research provides contradictory and inconclusive evidence on the value relevance of comprehensive income disclosures promulgated in different countries. Thus, present study using comprehensive income of Iranian firm's data shed more lights on the issue. Fuel cells are devices capable of converting chemical energy into heat and dc electrical energy by means of the oxidation of a fuel, usually hydrogen (Padulles et al, 2000). Distributed generation (DG) technologies can provide energy solutions to some customers that are more cost-effective, more environmentally friendly, or provide higher power quality or reliability than conventional solutions. The voltage of fuel cell stack decreases largely as the load current increase, and the voltage increases as the temperature increase at the same current. Thus DGS should be interfaced with Power Electronic systems such as DC to DC or/and DC to AC power converters to obtain a sinusoidal AC output voltage with fixed frequency from variable or high-frequency AC voltage sources or DC voltage sources (Haiping et al, 2005). So the DC-DC converter plays a key role in making the fuel cell DC power available for stand-alone applications. To boost low output DC voltage of the fuel cell to high DC voltage, a forward DC to DC boost converter, a push-pull DC to DC boost converter or an isolated full-bridge DC to DC power converter can be selected. In addition, various topologies such as the H-bridge series resonant buck and boost converters have been presented in (Blaabjerg et al, 2004). Anderson et al, (2002) present a current fed push-pull converter. This topology decreases the conduction losses in the switches due to the low fuel cell voltage. Nergaard et al, (2002) suggest an interleaved front-end boost converter. This topology considerably reduces the current ripple flowing into the fuel cell. A dual loop control strategy (current and voltage loop) has been used for the interleaved converter. Jung et al, (2005;2005) introduce a Zsource converter. This is a new concept in which a shoot-through vector directly steps up the DC source voltage without using a boost DC-DC converter. The boost voltage rate depends on the total duration of the shoot-through zero vectors over one switching period Jung et al, (2005). Also, In reference (Wang and Nehrir, 2006) a non-isolated boost converter with a conventional PI controller has been used for the converter control. Akkinapragada (2007) had been employed a non-isolated buckboost DC-DC converter with a closed loop PWM (pulse width modulation) control strategy as described in Reference (Mohan et al, ). The reference (Duran-Gomez et al, 2006) had been presented an approach to convert the generated dc output voltage of a PV cell array into a higher regulated dc voltage. The approach employs a series-combined connected boost and buck boost dc-dc converter for power conditioning of the dc voltage provided by a photo-voltaic array. Chandrasekaran and Gokdere (2004) introduce a novel composite integrated magnetic (IM) core structure, which minimizes inductor current ripple. Their subject is a compact, IM core structure for a three-phase interleaved, DC-DC boost converter that feeds off the fuel cell output and can provide a programmable, regulated, high voltage DC bus for the distributed power system. In recent years, the higher efficiency and the more advanced power conversion, energy utilization equipment, a variety of circuit topologies of the soft-switching DC-DC power converter are urgently required. The reference (Ogura et al, 2003) has introduced a boost type ZVS-PWM chopperfed DC-DC power converter with a single active auxiliary resonant snubber in the load side for the power interface of solar photovoltaic and fuel cell power conditioners. The character of this boost type ZVS-PWM chopper-fed DC-DC power converter is feasible. Also, a unidirectional isolated full-bridge DC to DC power converter can be used to boost low fuel cell voltage (Bendre et al, 2003; Aydemir, 2002; Kim et al, 1997; Jain et al, 2002; Brunoro and Vieira, 1999; Jeon and Cho, 2001). In addition, a bidirectional full-bridge DC to DC power converter can be used for stepping up low battery voltage or stepping down high-voltage-side DC link according to battery discharge or recharge mode (Peng et al, 2004; Jiang and Dougal, 2003). Among the presented power converters, two phase-shifted full-bridge DC to DC converters, which are one of the most attractive topologies for high power generation are adopted as described in (Jung, 2005; Keyhani and Jung, 2004) with a unidirectional full-bridge DC to DC boost converter for the fuel cell and a bidirectional full-bridge DC to DC boost/buck converter for
893 K. G. Firouzjah, H. Eshaghtabar, A. Sheikholeslami and S. Lesan battery. This research develops a circuit model and controllers of fuel cell based distributed generation systems (DGS) in a standalone AC power supply. Dynamic model of the fuel cell is considered. To boost low output DC voltage of the fuel cell to high DC voltage and compensate for its slow response during the transient, two full-bridge DC to DC converters are adopted and their controllers are designed. Furthermore, (Suzuoka et al, 2003) presents a novel circuit topology of a voltage source type zero voltage soft-switching full bridge inverter DC-DC power converter with an isolated highfrequency transformer link this power converter incorporates zero current soft-switching (ZCS) mode phrase-shifted PWM active power switches in series with diode into two diode arms of full-bridge rectifier or center-tapped rectifier in transformer secondary-side. In spite of desirable performance of the mentioned methods, a novel method based on predictive current control has been proposed to control the fuel cell and backup battery on the presented structure in (Jung, 2005; Keyhani and Jung, 2004). The proposed method produces the reference current by using of the DC link capacitor voltage. In other word, the presented algorithm calculates the required current of load and capacitor to regulate and control DC output at a desired voltage level. Thereupon the method determines the portion of fuel cell and backup battery to feed load and capacitor. In this research, the predictive current control strategy as described in References (Rodriguez, 2007; Rodriguez, 2004; Wen and Zheng, 2006; Premrudeepreechacharn and Poapornsawan, 2000) has been employed. 2. System Structure The adopted structure of fuel cell, backup battery and the interfaced converters are based on the reference (Jung, 2005; Keyhani and Jung, 2004) as described in Fig. 1. In this system, a low voltage DC output of fuel cell is used along with unidirectional boost converter to prevent reliability deterioration by stacking a number of series cells. A low DC voltage battery for backup is connected in parallel with the high side DC link through a bidirectional buck/boost converter because difficulties in battery management can be significantly reduced. Furthermore, an isolated full-bridge DC to DC power converter is adopted to boost low output DC voltage of fuel cell because its topology is suitable for high power applications. To boost low output DC voltage of the fuel cell to high DC voltage, a forward, a push-pull or an isolated full-bridge DC to DC power converter is selected. Among these power converters, two phase-shifted full-bridge DC to DC converters are consist of a unidirectional full-bridge DC to DC boost converter for fuel cell and a bidirectional full-bridge DC to DC boost/buck converter for the battery. In Fig. 1, the unidirectional power converter system for the fuel cell consists of a fuel cell, an input filter (L1, C1), a full-bridge power converter (F1 to F4), a high frequency transformer (N1:N2), a bridge-diode (DF1 to DF4), and an output filter (L2, C2), while the bidirectional power converter system for the battery consists of a battery, a static switch (SB), two full-bridge power converters (B1 to B4, B11 to B44), and a high frequency transformer (n1:n2) (Jung, 2005).
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