PC-SPARE is a program that estimates the expected lifetime costs associated with the failure of a major critical component in an electric power plant. Typically, such a component has a small probability of failure, but, should it fail, the unit must be derated (or shutdown) for an extended time while the component is repaired (or replaced). During this repair time, replacement power must be purchased. The problem addressed by PC-SPARE is to determine the economic feasibility of several utilities, each with multiple generating units, together purchasing a spare component that could then be used as a temporary replacement should a plant component fail. In this way, the time a generating unit must be derated due to a component failure would be greatly decreased.

The program calculates the total costs associated with the critical component over the lifetime of a spare-component pool. These costs are computed for each member utility and for each generating unit in the pool. Specifically, the present-worth lifetime costs are computed for two different management policies for treating the possibility of failure of a large critical component. These policies, denoted by Plan A and Plan B, may be summarized as follows:

• PLAN A: Make no preparations beforehand; after the component fails, repair it or order and install a replacement.
• PLAN B: Purchase and stockpile a replacement now; use it as a temporary substitute at any unit that needs it, and then return it to the pool when a permanent re- placement is acquired for the affected unit.

The expected present-worth costs associated with both plans are computed for each utility and each unit over a specified time period (the pool lifetime). The costs considered by PC-SPARE include

• cost of repairing or replacing a failed component
• cost of replacement energy while a unit is derated
• salvage value (if any) of a failed component
• cost of purchasing the spare component and setting up the pool
• cost of using the spare component as a temporary substitute
• salvage value of the spare at the end of the pool
• operation and maintenance costs for each component
• costs of operating and maintaining the spare-component pool
• cost of reserve margins needed for each unit

In calculating all these costs, the program is capable of including a wealth of detail should the analyst so desire. For instance, units may have different lifetimes and even enter the pool after its formation. Various costs can escalate at different rates throughout the pool life, and generating units can have variable capacity and availability factors. Finally, the failure model for the components allows for the spare component to be unavailable, having been claimed by another unit that has experienced a prior failure of the critical component.

These, and other program features, make PC-SPARE considerably more versatile that earlier codes and models. From the total present-worth of all the costs associated with both plans, a benefit/cost ratio for creating a spare-component pool is computed for each unit protected by the pool and for each member utility. Should this ratio be greater than unity, then the formation of a spare component pool is economically justified.

The details and assumptions of the many economic models used by PC-SPARE are discussed in the report An Economic Model for Evaluating a Shared Spare Component by J.K. Shultis, Report 172, Engineering Experiment Station, Kansas State Univ., Sept. 1985.