International Journal of Scientific & Engineering Research, Volume 3, Issue 6, June-2012 1 ISSN 2229-5518 IJSER © 2012 http://www.ijser.org Available Transfer Capability Calculations with Contingencies for Andhra Pradesh State Grid Chava Sunil Kumar, Dr. P. S Subramanyan, Dr. J. Amarnath Abstract— Contingency analysis and risk management are important tasks for the safe operation of electrical energy network. Potential harmful disturbances that occur during the steady state operation of a power system are known as contingencies. Contingency analysis is carried out by using repeated load flow solutions for each of a list of potential component failures. In this paper is work is carried out by se- lecting the contingencies according to the line loading for single transmission line outage and identified the severe most contingency based on transmission line loading. This process has to be executed for all the possible contingencies for tie lines and limiting lines, and repeated every time when the structure changes significantly and Available Transfer Capability (ATC) is calculated between the areas f or each con- tingency. The results are analyzed and discussed on 124-bus real life Indian utility system of Andhra Pradesh State Grid. Index Terms— Contingency, ATC, Outage, MVA rating, Blackout. ——————————  —————————— 1 INTRODUCTION NE of the most important factors in the operation of a power system is the desire to maintain system security. System security involves practices designed to keep the system operating when components fail. For example, a gene- rating unit may have to be taken off-line because of auxiliary equipment failure. By maintaining proper amounts of spin- ning reverse, the remaining units on the system can make up the deficit without too low a frequency drop or need to shed any load. Similarly, a transmission line may be damaged by a storm and taken out by automatic relaying. If, in committing and dispatching generation, proper regard for transmission flows is maintained, the remaining transmission lines can take the increased loading and still remain within limit. Because the specific times at which initiating events that cause compo- nents to fail are unpredictable, the system must be operated at all times in such a way that the system will not be left in a dangerous condition should any credible initiating event oc- cur. Since power system equipment is designed to be operated within certain limits, most pieces of equipment are protected by automatic devices that can cause equipment to be switched out of the system if these limits are violated. If any event oc- curs on a system that leaves it opening with limits violated, the event may be followed by a series of cascading failures continues, the entire system or large parts of it may complete- ly collapse. This is usually referred to as a system blackout [1]. An example of the type of event sequence that can cause a blackout might start with a single line being opened due to an insulation failure; the remaining transmission circuits in the system will take up the flow that was flowing on the now- opened line. If one of the remaining lines is now heavily loaded, it may open due to relay action, thereby causing even more load on the remaining lines. This type of process is often termed a cascading outage. Most power systems are operated such that any single initial failure event will not leave other components heavily overloaded, specifically to avoid cascad- ing failures. Contingency Analysis (CA), as a part of static security analysis, is critical in many routine power system and power market analysis, such as ATC evaluation, security assessment and transaction arrangement. A typical CA has models, single element outage (one –transmission line, one generator outage, etc.), multiple element-outages (two-transmission line outage, one –transmission line and one generator outage, etc.) and sequential outage (one outage after another) [3]. In the case of loss of one component, this corresponds to the N − 1 criterion, i.e. the system should be able to support the load when one of the N basic transmission system components (transmission lines, generators or transformers) is out of operation. The ap- plication of the criterion can also be extended for the case of loss of combinations of these basic components. When applied to the loss of two components, it leads to the N – 2 criterion. Since a contingency can take place at any instant of operation, the system design should be such that the system is able to deal with the worst-case scenario, i.e. the peak load. Limit checking is done for each contingency to determine whether the system is secure [4]. With the global towards the deregulation in the power sys- tem industry, the volume and complexity of the CA results in the operation and the system studies have been increasing. Not only has deregulation resulted in much larger system model sizes, but also CA is computed more frequently in the restructured power markets to monitor the states of the sys- tem under ȃwhat ifȄ situations in order to accommodate the maximum number of power transfers. The net impact of these changes is a need for more effective CA results are required to help with the comprehension of the essential security informa- tion, information which could be buried in the enormous and complex CA data sets [7], [8]. A 124-bus real life Indian Utility system is considered to find the variation of ATC between the areas for transmission line outage of tie-lines. Contingency ranking is taken accord- ing to the percentage loading of the lines. O