S&C Metal-Enclosed Switchgear Connects Wind Turbine Generators to Utility Grid.

Load growth in one western state has been two to three times the national average for the past eight years. Additional power is clearly needed. But public demand dictates that the new power be generated in a non-polluting manner. Wind power seems to be a good prospect.

But the state’s major utility was initially concerned about the economics of wind generation. The cost per kilowatt would be considerably higher than for coal-fired generation, although this gap would narrow significantly with the installation of more wind turbine generators. To make the project feasible, the utility commission agreed to higher rates for customers who voluntarily choose “green power.”

The utility then proceeded with plans for a pilot wind farm producing a guaranteed output of 20 MW. If successful, the plant will be expanded to 60 megawatts. Metal-enclosed switchgear will be utilized to feed the power to the utility grid.

What did they do?

The pilot farm was designed somewhat oversized to offset variations in wind velocity (which affect the output of the wind turbine generators) and to allow a few generators to be taken out of service at a given time for maintenance.

Forty-three 700-kW wind turbine generators were specified, providing a total design output of 30 MW. Each generator includes a 690-volt to 25-kV step-up transformer.

Two new six-bay lineups of S&C Metal-Enclosed Switchgear were also specified. The utility was very satisfied with other S&C equipment they had purchased and knew of S&C’s experience in cogeneration applications.

As shown in the single-line diagram, the system has six wind-turbine generator groups, each consisting of six to eight generators. Power from each group is metered and fed to one of the feeder bays in the switchgear lineups. From the switchgear, power is fed to a 25-kV to 115-kV step-up transformer connected to the transmission line.

Switching for each wind-turbine generator group is provided by a 400-ampere Mini-Rupter® Switch, power-operated by a Type MS-2 Switch Operator. S&C SM-4 Power Fuses clear cable faults between the switchgear and generator groups, isolating them from the utility grid. Fused voltage transformers provide control power for the switch operators as well as voltage monitoring.

Current transformers provide sensing for a directional overcurrent protection arrangement, which utilizes Schweitzer 351 Relays. Power in this system normally flows from the wind turbine generators to the utility grid. In the event of power flow from the utility grid to the wind turbine generators, the 351 Relay will send a trip signal to the switch operator of the affected Mini-Rupter Switch, for current below 400 amperes. For current above 400 amperes, no trip signal will be sent; such current is likely the result of a cable fault and, as discussed above, will be cleared by the SM-4 Power Fuses. Station service power is provided through another manually operated fused Mini-Rupter Switch in each lineup.

Results

The pilot wind farm has performed very well, meeting all expectations.

S&C has also provided other equipment for the application, including 115-kV Line-Rupters™, Series 2000 Circuit-Switchers, and SMD-2B Power Fuses, as well as numerous Loadbuster Disconnects®.