978-1-4799-5141-3/14/$31.00 ©2014 IEEE Dynamic Thermal Rating and Allowable Operating Time under Transient Conditions PUROBI PATOWARY Reliability Engineering Centre, IIT Kharagpur. Kharagpur, West Bengal, India. reenaiit@gmail.com NEERAJ K GOYAL Reliability Engineering Centre, IIT Kharagpur. Kharagpur, West Bengal, India. ngoyal@hijli.iitkgp.ernet.in Abstract— In electrical systems, one of the main factors that limit the transport of energy is the temperature of the conductor. Knowing the transmission system, and the critical sections both geographical and climatic, the temperature of the line as well as the transient thermal rating can be calculated. A plot of the allowable safe operation time under transient conditions allows increase in transmission capacity without compromising operational security. The estimate of probabilistic thermal risk can afford significant advantage making economic dispatch during favorable conditions without violating the maximum temperature of the transmission lines. This information will be useful in operational decision making and ultimately help to improve the reliability of the power system. Keywords— Risk Assessment, Cumulants, Moments, Thermal Rating, Probabilistic, Reliability. I. INTRODUCTION Increasing consumption of electric energy coupled with financial constraints and environmental restrictions on setting up of new plants has created highly stressed operating conditions and increased vulnerability in networks. This has necessitated the operation of existing systems of transmission close to the limits of their transport capacity without risking the integrity of its physical components, while maintaining reliable operation. Regulations require that the transmission lines must be operated safely at all times. The most significant restrictions on the line are the steady state and transient thermal rating of the conductor. The current carrying capacity of a conductor is limited by the conductor’s maximum design temperature which determines the maximum sag and rate of annealing. High conductor temperatures can cause deeper conductor sag and may result in clearance violations while annealing of the conductors can weaken the conductor and cause its irreversible damage. Increasing the current carrying capacity of bare overhead conductors has long been of interest to engineers. In [1] a probabilistic method to assess the thermal capacity based on risk is presented. In [2] an approach to assess the cumulative risk associated with overload security for the purpose of midterm power system planning is presented. In [3] authors use the sequential Monte Carlo simulation to assess composite power system reliability and suggest two indices associated with transient stability. An attempt to demonstrate the influence the thermal limits of transmission lines and how these can be exploited for economic generation dispatch is described in [5]. In [6] authors verify through experiments the conductor temperature calculations based on the weather model in [4] and calculate the time to thermal overload. In [7] the authors suggest that a dynamic thermal rating system can be used to upgrade the static rating of existing lines to provide a significant increase of transmission capacity in comparison to the traditional approach. Historically, transmission lines have had static or fixed ratings that limit the amount of power that can be transferred on the electric grid. Static ratings usually consider a worst case scenario of ambient conditions for calculating the steady state ampacity: high ambient temperature, low wind speed and high solar radiation. The rating is used by the system operator to ensure that the transmission line conductors do not sag below specific limits and come in contact with trees or other objects thus affecting reliability and safety. It was long felt that this rating is conservative and resulted in under-utilization of conductors with increasing energy consumptions and growing financial and environmental restrictions, especially after deregulation. Increase of transmission capacity during favorable ambient conditions would make a significant