supply customers with an average power demand of Pd the power plant installed capacity for the overall system can be represented as: For electric utility systems consisting of numerous generating units with transmission and distribution equipment, the system reliability is measured in terms of an annual loss-of-load probability (LOLP). Typically, the LOLP is about a tenth of a day per year. Once the operating characteristics of a utility system are known, it is possible to relate the LOLP to the system reserve, /?s, by a parameter, M, as defined in [6]. Then, as a new generating unit is added to supply a growing load demand, the effective capability of the new unit can be calculated. The effective capability of the added unit in kilowatts is the difference between the kilowatt rating of the unit and the increase in system reserve kilowatts that is necessary to maintain the LOLP at the desired value. Once the system parameter, M, is known, the effective capability of the added generating unit can be related to its rating and its annual forced outage rate. The annual forced outage rate of a generating unit is: If ro = 0.02 or 2%, the generating unit will be out of service for 0.02 (8760) = 175 hours per year, and the unit reliability is (1—ro)=0.98. Garver, in Ref. [6], defines the system parameter M as the load demand increase that will give a LOLP increase e times greater than before, where e = 2.718, the base of the system of natural logarithms. Based on experience, M can be estimated as the sum of each generating unit's rating times its forced outage rate or: Note that given the above equation, a unit with a zero forced outage rate will not affect the system parameter M. The larger the added unit or the larger its forced outage rate, the greater its effect on the system parameter M. Therefore, the first large generating unit that is added to an existing system may not have a large percentage of load carrying capability, but it will have a significant effect on the parameter M and prepare the system to accept subsequent large generating unit additions. Then from Ref. [6], the equation for the increase in system reserve as a function of Af, Us, and r0 can be written as:
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