Power Plant Electrical System

The electrical equipment included in a fuel cell installation has two main purposes. The first function is the adaption of the fuel cell output to suit the requirements at the point of delivery. The second function is the powering of all of the system auxiliaries and controls. The adaption of the direct current produced by the Santa Clara fuel cells into the three phase alternating current required by the utility is accomplished by solid state inverters and transformers with an efficiency approaching 98%. The switching of the current produces harmonics that are only acceptable at vety low levels. The present installation uses gate turn off thyristors (GTO's), typical of an industrial drive, with pulse width modulated switching to minimize the lower order harmonics. There are four inverters arranged in pairs. The two inverters in each pair are connected to phase shifted transformers so each pair is electrically independent Hie harmonics generated are well known and can be identified as (integral multiples of two times the number of phases)_± 1 (e.g. a simple three phase bridge produces (nx6)_£ 1 or no.'s 5,7,11,13,17,19 etc.) The magnitude is inversely proportional to the harmonic number. Advantage is taken of these relationships by using two inverters operated with a 30° phase shift to produce a lower magnitude by eliminating (theoretically) the 5th and 7th harmonics. When the total power to be inverted exceeds the capacity of two bridges, consideration can be given to another phase shift to further reduce the harmonics level. The transformers are of "rectifier" construction, with allowance for the harmonics that must be handled. They are also given special attention to minimize the radiated noise for the benefit of the plant neighbors. Control is arranged to regulate the real power output by controlling both the fuel rate and the electrical output. It is also able to regulate the imaginary power in magnitude and direction either for + VARs or for voltage control. Protection is provided so that utility disturbance cannot damage the fuel cell installation, by blocking the inverter firing for short periods. Protection is provided for the utility by conventional isolation in case of an overcurrent malfunction. The power conversion equipment was supplied by ABB Industrial Systems, Inc.

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