Improving Performance of Isolated Fuel Cell Power Conditioners through Series Capacitive Compensation

In isolated fuel cell power conditioners, the dc output voltage of the fuel cell is first converted to a high-frequency train of positive and negative pulses of equal, but adjustable, widths. The resulting quasi square-wave voltage is then input to a high-frequency step-up isolation transformer, whose secondary voltage is rectified to a regulated dc voltage. This dc voltage, which is supported by batteries and/or ultracapacitors, is finally converted to the desired dc and ac voltages. The equivalent series inductance of the isolation transformer limits the power transfer capability of the system. As a result, the expensive installed capacity of the fuel cell is poorly utilized. Furthermore, the efficiency of power conditioner will be low, as at low power levels, losses are comparable to the total power. In this paper, it is proposed to use a series capacitor, on the secondary-side of the high-frequency transformer, to nullify the equivalent series inductive reactance of the isolation transformer at the switching frequency. It is shown through simulation that this results in considerable improvement in the power transfer capability and efficiency of the system.