FFT Based Fault Location Algorithm for Transmission Lines  1 and Müslüm Arkan 2 1,2 Inonu University, Engineering Faculty, Electrical & Electronics Eng. Dept., Malatya, Turkey 1 mehmet.mamis@inonu.edu.tr, 2 muslum.arkan@inonu.edu.tr Abstract To maintain continuity of electrical energy, fast clearance of a fault is needed, which requires accurate determination of the fault location on transmission lines. This paper proposes a method for fault location, which is based on time to frequency domain transformation of measured data after fault by using Fast Fourier Transform (FFT). The frequency of the first harmonic is utilized for determination of the fault location using travelling wave theory. The results show that the accuracy of the method is not affected by the fault resistance and phase angle of fault instant. 1. Introduction Reliability and continuity of electrical energy has gained more importance in last decades as power outages lead to loss of manpower and resources in industrial plants. The most important causes of disturbances in the power systems are unexpected faults, and short circuit faults are more common fault types, which are arisen due to lightning surges, usage of defective materials, improper operation, human errors, overloading and aging, and may cause loss of system stability, failure of transformers, generators and transmission lines. Fast clearing of faults is greatly significant and the first step for removing a fault in a short time is to estimate the fault location quickly and accurately. Advance in the computer technology allows development of new algorithms for determination of fault location and in recent years several methods have been proposed for identification of the fault location in power systems. The studies for determination of fault location may be classified into two categories; the methods which employ post-fault steady-state voltage and current phasors and the methods based on the transient wave theory. In some of the methods which power frequency phasors have been used fault distance is estimated from the information received from one end of the transmission line [1-6]. As fault location is estimated by impedance from measuring terminal to the fault point, unknown fault impedance affects the accuracy. To overcome this difficulty, two- or multi- ended fault location techniques have been used [7-11]. However, measurement from two end is difficult and synchronized sampling of the voltage and current data from two ends of the line is usually required. In the travelling wave based methods on the other hand, time-space analysis have been used for fault location. Short and open circuit faults on transmission lines cause sudden changes in the distribution of electric and magnetic energy which result travelling waves. In order to determine the fault distance, the analysis of wave time-position graphs are needed [12-17]. In recent years, many studies have been devoted to develop different methods based on the wavelet transform to determine the fault type and location of faults [14- 17]. Wavelet transform is a recently developed mathematical tool, which is used to capture the dynamic characteristics of unstable signals using short data windows. Depending on the direction in the protection of transmission lines, fault classification and fault distance identification using wavelet transform was carried out by separating the necessary information from the short circuit transient behavior. The most important limitation of the existing methods based on the wavelet transformation is the low degree of accuracy in the prediction faults points near the busbar in general. In this study, a new method based on FFT for determination of fault location using frequency domain data obtained by transformation of transient response measured on one terminal of transmission line is proposed. Short circuit current and voltage waveforms obtained by computer simulations are evaluated, and algorithms will be applied to transient current and voltage waves in one or two periods as inputs, fault location is determined. The organization of the paper is as follows: After this introductory section the state-space techniques are presented for transient voltage and current waveforms of faulty transmission line in Section 2. In the 3th section the theory of fault distance calculation using travelling wave theory of the distributed parameter transmission line is introduced. In Section 4 application results are summarized; the effect of fault resistance, the affect of phase angle and the effect of source inductance is investigated. 2. State-Space Modeling of Transmission Line with Short Circuit Fault Computer programs such as ATP or MicroTran are available for transient analysis of transmission lines. However, many simulations are needed for several cases and transformation of numerical data into Matlab environment is difficult. To overcome this difficulty, state-space method is used to analyze transmission line with short circuit fault. State equation of a linear time invariant system are written as () () () t t t   x Ax Bu  (1) Where x(t) is state vector, A and B are coefficient matrices with proper dimensions and u(t) is vector of inputs. The transmission line shown in Fig. 1 is simulated by series connection of lumped parameter L-sections and a single phase to ground short circuit at a point on transmission line with zero fault resistance is simulated by removing the conductance and capacitance at the specified node on lumped parameter model and v k becomes zero and it is not a state variable at all. Then the matrices , x A , and B can be written as follows: ELECO 2011 7th International Conference on Electrical and Electronics Engineering, 1-4 December, Bursa, TURKEY 71