Economical Evaluation of N–1 Safe Criterion and Load Shedding Index in Transmission System Expansion Planning

Transmission system reliability is an important issue in transmission network expansion planning (TNEP). Most of papers and research about TNEP employed N–1 safe criterion to formulate the transmission reliability, while a few of them have considered load shedding (LS) index to meet the required reliability of consumers for electric demand. Thus, an economic evaluation is necessary to clear which of these reliability criteria is more appropriate for transmission expansion planning studies. This paper tries to compare these reliability indices to each other and show that which of them are more economic and better for expansion planning of a transmission system. The aim is to compromise between transmission lines and substations expansion cost, network losses, and system reliability considering maintenance and repair expenses as well as replacement cost of timeworn lines. To formulate the network reliability, first deterministic reliability criterion of N–1 was employed and embedded in the problem formulation and results were discussed, then N–1 criterion was replaced by probabilistic reliability index of load shedding in TNEP formulation and the results were compared with results of first model. Both of TNEP models are tested on well known as Garver’s 6-bus network and IEEE reliability 24-bus test system (IEEE RTS).