Economic contribution of generating units to reliability: a step to a unified system approach

System reliability and operating cost are analyzed in the context of a search for a unified approach. An economic reliability needs to be identified on the basis of each generator contribution to it, in order to disaggregate their influence. With that purpose firm power is studied first as a static parameter and then as a dynamic function which can be evaluated in terms of its time evolution. Firm power time evolution is found to be a growing function of time whereas outage costs depend on outage duration. From both effects reliability presents a cost which is variable in time and provides a signal for improvements based on suitable location of power reserves. The economic contribution of each generator is obtained by considering the reductions in interruption cost for different times after a large disturbance has occurred. Test on a sample system show the significant outage cost reductions that sometimes are obtained by power reserves. It is concluded that the the units in a system contribute economically in different amounts, depending on generator speed to take load, on average availability, on amount of power reserve. The value is derived from outages costs that are possible to. avoid due to each generator presence and condition. With this criterion system reliability can be kept at a suitable level in agreement with user own evaluation of reliability requirements.