7.2 HLI-generation capacity

During the planning phase of global generation, it was necessary to determine both how much capacity needed to be installed and when, so as to satisfy the expected demand at some point of time in the future and to provide sufficient reserve to perform corrective and preventive maintenance. This forms the HLI assessment. At HLI, the ability to move the energy to bulk supply points (BSPs) or end customers was not considered. Historically, the reserve capacity was set equal to a percentage of the expected load, or equal to one or more largest units, or a combination of both. These deterministic criteria could be, and in many cases were, replaced by probabilistic methods that responded to the stochastic factors influencing the reliability of the system.

A number of probabilistic criteria and indices are available for HLI studies [12 15]. These include loss of load probability (LOLP), loss of load expectation (LOLE), loss of energy expectation (LOEE), expected energy not supplied (EENS), expected unserved energy (EUE), energy index of reliability (EIR), energy index of unreliability (EIU) and system minutes (SM). A brief description of each is given in Table 7.1. It is important to recognise that these indices are not measures actually seen by the end customers but are really relative measures by which alternative planning scenarios can be objectively compared.