This paper outlines the technical approach adopted to meet the specifications laid down for the '2001 Future Energy Challenge (FEC)' organized by the Department of Energy and IEEE in August 2001. Abstract -In this paper, the development of a low cost fuel cell inverter system is detailed. The approach consists of a three-terminal push-pull DC-DC converter to boost the fuel cell voltage (48V) to f2OOVDC. A four switch (IGBT) inverter is employed to produce 120V/240V, 60Hz AC outputs. High performance, easy manufacturability, lower component count., safety and cost are addressed. Protection and diagnostic features form an important part of the design. Another highlight of the proposed design is the control strategy, which allows the inverter to adapt to the requirements of the load as well as the power source (fuel cell). A unique aspect of the design is the use of the TMS320LF2407 DSP to control the inverter. Two sets of lead-acid batteries are provided on the high voltage DC bus to supply sudden load demands. Efficient and smooth control of the power drawn from the fuel cell and the high voltage battery is achieved by controlling the front end DC-DC converter in current mode. The paper details extensive experimental results of the proposed design on DOE National Energy Technology Laboratory (NETL) Fuel Cell.

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Abstract-In microgrids that are predominantly resistive, real and reactive power can be controlled by implementation of voltage and frequency droop laws respectively. However, the variable frequency displayed by such a system complicates analysis such that design approaches rely on approximations and linearized models. In this work, we present a modified form of droop control where only the voltage versus real power relationship is upheld and the frequency is held constant. Since the frequency is not explicitly controlled and the reactive power is not measured, the controller can be simplified. In such a setting, the only assumption we make is that all inverters have access to a common time-reference. Because fixed frequency operation is enforced by design, a variety of analytical tools can be leveraged to formulate a comprehensive analytical framework which facilitates a precise design methodology. In particular, closed-form expressions on the output current phase differences are obtained which yield practical selection guidelines on the voltage-power droop gains such that reactive flows between inverters are kept small. As a corollary, it is demonstrated that there are no reactive power flows in the presence of purely resistive loads. For the particular case of a single inverter, an almost exact solution describing the nonlinear dynamics of the inverter output voltage, current, and power are derived. Accompanying simulation results validate the analytical results and demonstrate the feasibility of the proposed control approach.

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