The Study On Grid-connected Inverters For Fuel Cell Generations

Fuel cell generation now is regarded as one of the most competitive energy sources for the distributed power generation systems (DG) due to its advantages of high efficiency, low emissions, and high power density. The grid-connected inverter is the main issue of the fuel cell DG because of its impact to the power quality, reliability and safety. Heretofore, many problems still need further studies, and the some new requirements are raised by the fuel cell DG. Among these, the power quality to grid and the power reliability to local loads are two most important aspects. These two aspects are used to study separately; seldom researches have been made on considering the system structures and functions. The current control strategies for the grid-connected inverters with LCL-filters were degraded by the resonance frequency without ideal solutions for long. On the other hand, the operating modes transfer is a new subject of ongoing researches.Focusing on the requirements for fuel cell DG, this paper has discussed the operating modes and the inverter structures for the power systems. A suitable system structure and the power managements have been presented. The system models, control strategies, and filter design methods for standalone or grid-connected operating modes are also given here.As to the switching frequency depression for the PWM inverter, the characteristics of the LCL-filter and impacts on inverter system control are analyzed in detail. For the current control of the grid-connected inverter, the problem of low bandwidth and gain resulted from the LCL-filter resonance frequency by traditional current feedback control has demonstrated. Novel current feedback control methods which appear as current weighted average and split capacitor method have been presented. The new control methods can create two zeros to contact the effects of the two poles in LCL-filter. Consequently, the V-I transfer function of the grid-connected inverter system with the LCL-filter is degraded from a third-order function to a first-order one, therefore the closed-loop current feedback control system can be optimized easily for minimum steady-state error and current harmonic distortions, as well as the system stability.For operating mode transfer control, this paper proposes four new transfer control algorithms based on voltage amplitude regulation, voltage phase regulation, instantaneous voltage regulation and current regulation for the fuel cell inverters, to implement forced current commutation between the grid and the inverter during the transition from grid-connected mode to stand-alone mode. The algorithms and their transition performances have been described and discussed. Simulations of the transfer control algorithms were carried out in PSpice to verify the methods.Finally, the inverter design and the presented control strategies have been experimentally verified on a 5kW DSP controlled fuel cell inverter.