TABLE OF CONTENTS The second hierarchical level (HLII) is frequently referred to as a composite (generation and transmission) system or a bulk power (or transmission) system. The purpose of assessing the reliability of power systems at this level is to estimate the ability of the system to perform its function of moving the energy provided by the generation system to the BSPs. The distribution networks are not considered at this level.
Assessment of composite system reliability is very complex [12 16] since it must consider the integrated reliability effects of generation and transmission. These two entities cannot be analysed separately at this level. To do so could create misleading results and conclusions. This does not mean they have to be owned by the same company, but it is essential that one body has the role of co-ordinating the planning and operation. In some countries, one company is responsible for both planning and operation, whilst in others the responsibilities are divided between a network owner and an independent system operator (ISO). This depends on how the system ownership is structured.
The function of a composite system is to produce electrical energy at the generation sources and then to move this energy to the BSPs. The ability to generate sufficient energy to satisfy the demands at the BSPs, and to transport it without violating the system operational constraints, can be measured by one or more reliability indices. In HLI planning studies, the primary concern in the reliability assessment is one...
