52

i here are three general ways localized on-site generation can be applied in relation to the utility grid:

• Isolated operation. As illustrated in Figure 28-1, a stand-alone system can serve designated facility loads independent of the utility. These systems typically require off-line backup generators, as well as on-line reserve capacity, to provide sufficient power during load swings and scheduled or unscheduled outages. High installation and operating costs result.

• Isolated operation with utility system backup. This arrangement, illustrated in Figure 28-2, includes two variations. With open transition transfer, the load is disconnected completely from one power source before being connected to the other. This causes a brief power loss and resultant power surge, or bump. With closed-transition transfer, the transfer switches may overlap or parallel between the two sources, requiring actual synchronization of the two power sources during a period of 100 milliseconds to 60 seconds, depending on the utility and their allowable configurations. While more costly than open-transition transfer systems, closed transition has the advantage of a bumpless transfer and it is not as expensive as a fully parallel system.

• Parallel operation with the utility system. As illustrated in Figures 28-3 and 28-4, paralleling controls are configured in one of two ways. They may track the facility's electric load and adjust the output of the generators accordingly, with excess required power being furnished from the utility. Or, they may operate the generators at full load and sell excess power to the utility or another

Utility Transformer

Generator C.B.

buyer. Parallel operation, which is the primary focus of this chapter, provides the greatest degree of flexibility and generally the best opportunity to optimize economic performance.

Utility Transformer

Utility Meter -®

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Building Load

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