{"references": ["C. W. Taylor, Power System Voltage Stability (McGraw-Hill, Inc.,\nNewYork, America, 1994).", "P. W. Sauer, M. A. Pai, Power system steady-state stability and the loadflow\nJacobian, IEEE Transactions on Power Systems, vol. 5 n. 4,\nNovember 1990, pp. 1374 - 1383.", "N. Yorino, H. Sasaki, Y. Masuda, Y. Tamura, M. Kitagawa and A.\nOshimo, An investigation of voltage instability problems, IEEE\nTransactions on Power Systems, vol. 7 n. 2, May 1992, pp. 600 - 611.", "P. -A. L\u00f6f, G. Andersson, D. J. Hill, Voltage stability indices for\nstressed power systems, IEEE Transactions on Power Systems, vol. 8 n.\n1, February 1993, pp. 326 - 335.", "G. K. Morison, B. Gao, P. Kundur, Voltage stability analysis using static\nand dynamic approaches, IEEE Transactions on Power Systems, vol. 8\nn. 3, August 1993, pp. 1159 - 1171.", "B. Lee, V. Ajjparapu, Invariant subspace parametric sensitivity (ISPS) of\nstructure-preserving power system models, IEEE Transactions on Power\nSystems, vol. 11 n. 2, May 1996, pp. 845 - 850.", "M. Chakravorty, and D. Das, Voltage stability analysis of radial\ndistribution networks, International Journal of Electrical Power &\nEnergy Systems, vol. 23 n. 2, February 2001, pp. 129 - 135.", "G. Verbic, F. Gubina, A new concept of protection against voltage\ncollapse based on local phasors, IEEE Transactions on Power Delivery,\nvol. 19 n. 2, April 2004, pp. 576 - 581.", "F. Gubina, B. Strmcnik, Voltage collapse proximity index determination\nusing voltage phasors approach, IEEE Transactions on Power Systems,\nvol. 10 n. 2, May 1995, pp. 788 - 794.\n[10] S. Satyanarayana, T. Ramana, S. Sivanagaraju, G. K. Rao and P.V.\nPrasad, A Novel Approach to Evaluate the Maximum Loadability of\nDistribution systems with Voltage dependent load models, ELEKTRIKA,\nvol. 12 n. 1, 2010, pp. 19 - 24.\n[11] M. Moghavemmi, F. M. Omar, Technique for contingency monitoring\nand voltage collapse prediction, IEE Proceedings Generation\nTransmission Distribution, vol. 145 n. 6, November 1998, pp. 634 - 640.\n[12] V. Balamourougan, T. S. Sidhu, M. S. Sachdev, Technique for online\nprediction of voltage collapse, IEE Proceedings Generation\nTransmission Distribution, vol. 151 n. 4, July 2004, pp. 453 - 460.\n[13] K. Vu, M. M. Begovic, D. Novosel, and M. M. Saha, Use of local\nmeasurements to estimate voltage-stability margin, IEEE Transactions\non Power Systems, vol. 14 n. 3, August 1999, pp. 1029 - 1035.\n[14] M. Begovic, B. Milosevic, D. Novosel, A novel method for voltage\ninstability protection, 35th Hawaii International Conference on System\nSciences~HICSS-35~,January 7-10, 2002, Big Island, HI, USA.\n[15] I. Smon, G. Verbic, F. Gubina, Local voltage-stability index using\nTellegen-s theorem, IEEE Transactions on Power Systems, vol. 21 n. 3,\nAugust 2006, pp. 1267 - 1275.\n[16] K. Nagaraju, T. Ramana, S. Sivanagaraju and P.V. Prasad, A Novel\nLoad Flow Method for Radial Distribution System, International\nJournal of Power and Energy Systems, vol. 31 n. 1, 2011, pp. 73 - 81.\n[17] Power system engineering committee, Bibliography on load models for\npower flow and dynamic performance simulation, IEEE Transactions on\nPower Systems, vol. 10 n. 1, February 1995, pp.523 - 538."]}

This paper presents a simple method for estimation of additional load as a factor of the existing load that may be drawn before reaching the point of line maximum loadability of radial distribution system (RDS) with different realistic load models at different substation voltages. The proposed method involves a simple line loadability index (LLI) that gives a measure of the proximity of the present state of a line in the distribution system. The LLI can use to assess voltage instability and the line loading margin. The proposed method also compares with the existing method of maximum loadability index [10]. The simulation results show that the LLI can identify not only the weakest line/branch causing system instability but also the system voltage collapse point when it is near one. This feature enables us to set an index threshold to monitor and predict system stability on-line so that a proper action can be taken to prevent the system from collapse. To demonstrate the validity of the proposed algorithm, computer simulations are carried out on two bus and 69 bus RDS.