TNAG-0635-FCO40-783-Supplies-of-electricity-for-Hong-Kong-1977 — Page 96

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The first consideration involves evaluation of

overall plant availability. During the summer months, when generating capacity is most needed, the average availability has been about 87%.

Looking into the future, as CLP install larger sets,. which have been proved less reliable because of high steam pressure and temperature, the overall availability is likely to decrease. On experience elsewhere with big sets, CLP should not expect the average overall summer availability to be much better than 85% when the next phase of generating plant is developed.

This is to say that, unless a reserve capacity equal to 15% of the capacity installed is provided, the system is unlikely to be able to satisfy the demand in an average day.

The second consideration refers to the dynamic

aspects of system reserve, which should be available at

all times and would be called for in case of a sudden loss of a generating unit. This reserve can be made up, in relevant proportion, by 'spinning' spare, hot standby, and gas turbine standby. In order to cater for the sudden loss of the biggest generating unit, this reserve should equal the size of the biggest unit.

The reserve capacity is normally the generating capacity above the yearly system peak demand. However, a system peak occurs only for a period of about 15-20 minutes, and, after that, the load rapidly falls to about 96% of the peak. It makes more sense, therefore, to calculate reserve capacity to cover 96% of the estimated peak.