Improvement of the (n-1) criterion introducing a probabilistic failure-related reliability criterion

In the Hamburg Electricity Board (HEW), the generally recognised (n-1) principle has so far been used as the reliability criterion in the planning and operation of the transport and distribution system. There are a number of conditions for its use which it is not always possible to meet. Alternatives to the (n-1) principle are the probabilistic methods, which were developed for system planning. Their quantitative results are statistical, technical (failure-related, customer-related respectively) and economic parameters. But there are no binding and generally accepted criteria for reliability. One practicable solution consists of an extension of the relationship between the duration of the failure and the interrupted power to form a reliability criterion which is both failure-related and customer-related and which takes into account the frequency of supply interruptions. At HEW, provisional acceptance values for this criterion were derived from the actual failure events. The new criterion takes into account not only the preconditions of the (n-1) criterion-it can also be applied to cases which are not accessible to this criterion, and it specifies its qualitative statements in greater detail. In planning and operation, this opens up potential savings with a quantifiable reduction in the reliability of the supply-and in some cases a reduction which is negligibly low