682 Utility Interconnection

A fuel cell can be designed and installed in a number of different modes. The various modes of operation include parallel operation with the utility grid to supply a user, direct connection to the utility grid, backup power to a normally grid-connected load, and connection to a dedicated load. The most commonplace installations to date are those of the first type. In all of the modes of operation listed above, except the last case, some interaction with the utility grid is required. Interconnection with the utility grid has many advantages including reliability improvement, increase in load factor, and reductions in electricity demand. Connection with the utility grid, however, requires that the power conditioning equipment provide:

• Synchronization with the grid,

• Reactive power supply adjustable between 0.8 lagging and 1.0 power factor without impact to power output

• System fault protection

• Suppression of ripple voltage feedback

• Suppression of harmonics to within IEEE 519 limits

• High efficiency

• High reliability

• Stable operation

Current inverter and power conditioning equipment is available to meet all of the above requirements with some need for cost reduction, which could be accomplished through volume manufacturing (Penner, 1995). However, a few technical challenges still exist with regard to the inverter system's capability to supply current transients associated with motor starts, the operation of overcurrent devices to clear equipment or cable faults, or other inrush currents (Hirschenhofer et al., 1998).

Whether a fuel cell is connected to a dedicated load or to the utility grid, the response of the fuel cell to system disturbances or load swings must be considered. The fleet of ONSI PC-25TM 200 kW power plants, the only commercial fuel cell fleet today, has demonstrated power conditioning equipment responses that should be characteristic of most current systems. These system responses are as follows (King and Ishikawa, 1996; Hirschen-hofer et al., 1998):

1. No transient overload capacity beyond the power rating of the fuel cell

2. Load ramp rate of 0 to 100% in one cycle when operated independent of the grid

3. Load ramp rate of 10 kW/second when grid connected

4. Load ramp rate of 80 kW/second when operated independent of the grid and following the initial ramp up to full power

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