Unsymmetrical short-circuit analysis for distribution system considering loads Akhilesh Mathur ⇑ , Vinay Pant, Biswarup Das Department of Electrical Engineering, Indian Institute of Technology, Roorkee, India article info Article history: Received 8 December 2014 Received in revised form 21 January 2015 Accepted 1 February 2015 Available online 20 February 2015 Keywords: Short-circuit analysis Distribution system Bus admittance matrix Unsymmetrical faults abstract Short circuit analysis is an essential tool for determining the short-circuit-current rating of the protective devices and different substation equipments to be installed in a distribution system as well as for co-ordination of the protective devices. Usually, the effect of load is neglected during short-circuit analysis, which introduces inaccuracy in the calculated values of short-circuit current. To address this issue, this paper proposes an efficient and accurate short-circuit fault analysis method for balanced and unbalanced distribution system considering the effect of loads. Comparison of the results obtained by the proposed method with those obtained by time-domain simulation using PSCAD/EMTDC establish- es the accuracy of the proposed method. Ó 2015 Elsevier Ltd. All rights reserved. Introduction During normal operating conditions, the currents through the elements of a power system are well within their specified values. On occurrence of faults, the currents far in excess of normal values usually start flowing through network elements. These excessively high currents, if not interrupted or limited, can cause serious dam- age to the equipments. It has been observed that 80% of service failures occur due to the faults in distribution systems [1]. The occurrence of fault affects system’s reliability, security, and power quality. The system must be protected against flow of heavy short- circuit currents by disconnecting the faulty section of system by means of appropriately rated circuit breakers and other protective equipments. Different types of faults in power system are single line-to-ground (SLG), line-to-line (LL), double-line-to-ground (LLG), and triple line-to-ground (LLLG) faults. The process of evaluating the system voltages and currents under various types of short-circuits is called fault analysis [2]. Fault analysis is necessary to improve the customer service reliability and security. Due to the integration of distributed gen- eration (DG) and changes in distribution network topologies owing to network reconfiguration, short-circuit currents may change from time to time. Therefore, a suitable fault analysis method is required for calculating the new settings of the protective elements (reclosers, sectionalizer switches, fuses etc.). Also short-circuit calculations are required to determine the short-circuit ratings of new switchgear and substation equipment to be installed in the system. Fault analysis can also be helpful in estimating the size of the additional reactors or fault current limiters which may be required to be inserted in the system to limit the short-circuit currents to a safe value which is below the withstand capacity of the installed circuit-breakers. Conventional fault analysis methods are symmetrical-compo- nent-based [2,3]. In [4], an error analysis of the symmetrical com- ponent based fault analysis methods for distribution system was carried out and it was shown that the maximum error can be as high as 8.53%. This was attributed to the unbalanced nature of dis- tribution system which can have single and two phase lines, unbal- anced loads and untransposed feeders leading to unbalanced impedance matrix [4]. As distribution systems are generally unbal- anced in nature; therefore fault analysis methods based on phase variable approach [5–18] give better results. In [5,6], a method based on triangular factorization of the admittance matrix is pro- posed to simulate different types of fault. A three-phase impedance matrix based fault analysis method for unbalanced radial distribu- tion systems was introduced in [7,8]. A method based on phase coordinate representation of power system component was pre- sented in [9] for short-circuit calculations of unbalanced distribu- tion systems. Hybrid compensation methods [10–14] have been developed for fault analysis in which compensating currents have been provided for the loops in the system, faults and distributed generators (DGs). In [15–18], another short-circuit analysis method for radial and weakly meshed distribution network is pro- posed, which is based on two relationship matrices namely, [BIBC] and [BCBV]. The [BIBC] matrix represents the relationship between http://dx.doi.org/10.1016/j.ijepes.2015.02.003 0142-0615/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: mathurkanu@gmail.com (A. Mathur). Electrical Power and Energy Systems 70 (2015) 27–38 Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes