Evaluation of the variations of short-circuit capacities along a feeder due to distribution system-type upgrading Tsai-Hsiang Chen a , Wei-Tzer Huang b, * a Department of Electrical Engineering, National Taiwan University of Science and Technology, No. 43, Keelung Road, Section 4, Taipei 106, Taiwan, ROC b Department of Electrical Engineering, Chienkuo Technology University, No. 1, Chieh Shou N. Road, Changhua 500, Taiwan, ROC article info Article history: Received 18 March 2007 Received in revised form 28 September 2008 Accepted 18 October 2008 Keywords: Distribution system Primary feeder Normally closed loop Radial Short-circuit capacity Short-circuit MVA method abstract The variations of short-circuit capacities (SCC’s) along a feeder while a primary network is upgraded from original radial to normally closed loop arrangement is essential for examining the protection system of the network after upgraded. The increments of SCC’s along a feeder may cause the existing protection devices installed at the original network and its customers’ distribution systems becoming inadequate. Hence, a simple, systematic and straightforward evaluation method is required, especially in the planning stage. To meet these requirements the short-circuit MVA method was adopted in this paper. Based on the adopted method, the variations of SCC’s along a primary feeder before and after the network upgraded were represented by simple formulas and illustrated by figures that make the variations more easily be recognized. The results not only are useful for engineers of utilities to better realizing the impact on the SCC’s along feeder during the planning stage of network upgrading, but also confirm that the short-circuit MVA method is quite suitable for evaluating the variations of SCC’s along a feeder. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. 1. Introduction Nowadays, the high-tech, high value-added and energy–effi- cient industries had been developed rapidly in Taiwan. The suffi- cient, reliable and high quality power supply is, therefore, essential. New network arrangements should be considered be- cause the two major primary network arrangements usually ap- plied in Taiwan power company (Taipower), radial and normally open loop, are not qualified for serving these kinds of critical cus- tomers. Hence, a more reliable arrangement, the normally closed loop, was adopted and constructed to serve these sensitive loads about five year ago. At present, in Taipower’s distribution systems, the radial arrangement is usually employed by the overhead feed- ers distributed in rural areas, and the normally open loop arrange- ment is utilized by the underground cables constructed in urban and suburban areas. According to the latest power outage statistics done by Taipower, most of the power outages that customers have experienced were due to faults occurring in distribution systems [1–3]. Furthermore, the statistics of the frequency and duration of customer outages in the Taipei city district of Taipower shows that the customer outages were mainly owing to faults occurring at the primary feeder. This major cause of customer outage ac- counted for more than forty percent of the total customer outages [4]. Accordingly, if we can ensure no service will be interrupted when a single fault occurs at the primary feeders, then the service reliability can be improved considerably. This is the major goal of Taipower at this time. In general, a normally closed loop is designed so that no cus- tomers connected to the loop will be out of service when a fault oc- curs at the main feeder of the loop. Up to now, not only Taipower, but also, many other utilities in the world, such as Florida power company, Hong Kong electric company, and Singapore Power, have adopted normally closed loops to serve their critical customers [5,6]. Taipower is now planning to upgrade its distribution systems from original radials to normally closed loop arrangements exten- sively, especially in the science-based industrial parks, metropoli- tan areas and specific districts where higher service quality is required. This plan will noticeably improve the continuity of power supply and overall service quality. Nevertheless, the impacts on the existing radial distribution systems and their customers while upgraded to normally closed loops should be evaluated in a detailed manner, especially during the planning and design stages. The variations of SCC’s along the primary feeders should be evaluated first because their increment may result in the interruption capacities (ICs) of the existing pro- tection devices becoming inadequate, and bring about a serious problem as well. It should be noted that the increment of SCC’s along the feeders due to the configuration upgrading from original radial to normally closed loop is mainly depended on the arrange- 0142-0615/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijepes.2008.10.009 * Corresponding author. Tel./fax: +886 4 7111111. E-mail addresses: thchen@mail.ntust.edu.tw (T.-H. Chen), vichuang@ctu.edu.tw (W.-T. Huang). Electrical Power and Energy Systems 31 (2009) 50–58 Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes