ARCING HIGH IMPEDANCE FAULT DETECTION USING REAL CODED GENETIC ALGORITHM

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1 ARCING HIGH IMPEDANCE FAULT DETECTION USING REAL CODED GENETIC ALGORITHM Naser Zaanan Jan Sykulski A. K. Al-Othan School of Electronics & School of Electronics & Coputer Science Dept. Electrical Engineering Coputer Science University of Southapton University of Southapton College of Technological Studies U.K. U.K. Kuwait ABSTRACT Safety and reliability are two of the ost iportant aspects of electric power supply systes. Sensitivity and robustness to detect and isolate faults can influence the safety and reliability of such systes. Overcurrent relays are generally used to protect the high voltage feeders in distribution systes. Downed conductors, tree branches touching conductors, and failing insulators often cause high-ipedance faults in overhead distribution systes. The levels of currents of these faults are often uch saller than detection thresholds of traditional ground fault detection devices, thus reliable detection of these high ipedance faults is a real challenge. With odern signal processing techniques, special hardware and software can be used to significantly iprove the reliability of detection of certain types of faults. This paper presents a new ethod for detecting High Ipedance Faults (HIF) in distribution systes using real coded genetic algorith (RCGA) to analyse the haronics and phase angles of the fault current signals. The ethod is used to discriinate HIFs by identifying specific events that happen when a HIF occurs. KEY WORDS Downed power line, arcing high ipedance fault, transient analysis, haronics, real coded genetic algorith. 1. Introduction Detection of downed power lines is a long-standing proble to electric utilities. High ipedance faults result in very low currents which are often not detectable by conventional overcurrnet relays. A HIF occurs, for exaple, when a conductor breaks and falls on a nonconducting surface such as asphalts road, sand, grass or a tree lib producing a very sall current. These faults are difficult to detect when the ipedance at the point of fault is high enough to liit the fault current to the region unprotected by conventional overcurrent devices (Fig. 1). When no solid return path for the current is available, the fault exhibits arcing phenoena; these faults are then referred to as high ipedance arcing faults. HIFs are a dangerous phenoenon since risks of electric shocks are posed to the public and fire hazard also exist. It is estiated that ajority of electrically caused fires are due to arc type, hot neutral interittent faults [1]. Therefore, the principal otivation in high ipedance fault detection is not just syste protection, but to iprove safety. The threshold of overcurrnet relays ust be set at a relatively high current level to prevent tripping by inrush currents thereby causing unnecessary service interruption. Most detection schees involve the adjustent of the existing overcurrent protection to be ore sensitive by lowering its setting. Such schee have failed to operate in 32% of high ipedance faults and lead to several unexpected service interruptions [1]. Fig. 1. Relation of high ipedance fault current to overcurrent device settings In the past two decades any techniques have been proposed to iprove the detection of HIFs in power distribution systes, and recently the utilities have intensified research progras searching for ore efficient protection against this type of a fault. Soe of the techniques used to deal with this proble are echanical ethods where various echanical devices are used to provide a low ipedance fault by catching the fallen conductor [2]. Others have used electrical ethods and techniques in the tie doain such as the ratio ground relay [3], proportional relay algorith [4], the sart relay based on the tie doain feature extraction and the arc detection ethod [1]. There were suggestions to solve this proble using the frequency doain of the electrical signal and several papers have been published based on haronic coponents using Fourier transfor, such as analysing the inter haronic coponent, and high frequency spectra. Others have used ethods based on Kalan filtering and fractal theory [5-10]. Neural network schees have also been tried for the detection of

2 HIFs [11, 12], as well as a wavelet transfor used for digital signal processing of HIF signals [13, 14]. This paper describes a novel digital technique suitable for detecting high ipedance faults. A Real Coded Genetic Algorith (RCGA) has been eployed to analyse the haronics and phase angles of high ipedance fault current signals. The ethod is used to discriinate HIFs based on specific events that happen during the occurrence of a HIF. 2. Arc current Nature An arc is defined as a luinous electrical discharge flowing through a gas between two electrodes. In the case of an arcing HIF, when an energized conductor touches the ground, the electric contact is not solid. Due to the existence of air between the ground and the conductor, the high potential difference across a short distance excites the appearance of an arc. Many authors have worked on the theory and dynaics of voltages and currents in an electric arc, ost such studies are experientally based. In [15] and later in [16] a odel explaining the phenoenon using a spark gap was proposed. This air gap will not conduct till the applied voltage reaches the breakdown point. Then the current flows and reaches a axiu when the applied voltage equals the arc voltage. After that, the arc current decreases and becoes zero, i.e. the arc is extinguished. When extinction occurs, the arc requires a potential, known as restrike voltage, to reignite. This reignition will have the opposite polarity. This explains the typical voltage-current wavefor of an arc shown in Fig. 2. Many electric odels have been proposed describing arc behaviour as reviewed by [17]. accurately odelled so far. Soe previous researchers have reached a consensus that HIFs are nonlinear and asyetric, and that odelling should include rando and dynaic qualities of arcing. Eanuel et al [19] suggested two dc sources connected antiparallel with two diodes to siulate zero periods of arcing and asyetry. Yu et al [20] used cobinations of nonlinear resistors, while Wai et al [21] introduced a sophisticated TACS switch controlling the open/closed loop of a HIF to introduce nonlinearity and asyetry. In this paper, a ore dynaic and rando HIF odel is applied. It cobines ost of the advantages of the previous odels proposed while reaining siple and universal; it was first put forward by the authors in [22]. The high ipedance fault odel proposed in [22] is shown in Fig. 3 and includes two DC sources, V p and V n, which represent the arcing voltage of air in soil and/or between trees and the distribution line; two resistances, R p and R n, between diodes, which represent the resistance of trees and/or the earth resistance; and since ost observed arcs occur in highly inductive circuits [23] two inductances, L p and L n, added to the circuit. The effect of the inductances leads to the nonlinearity loop in the V-I curve and the desired asyetrical shape for the HIF current. When the line voltage is greater than the positive DC voltage V p, the fault current starts flowing towards the ground. The fault current reverses backward fro the ground when the line voltage is less than the negative DC voltage V n. In the case when the line voltage is in between V p and V n, the line voltage is counterbalanced by V p or V n so that no fault current flows. As a direct result of the presented odel the typical high ipedance fault current and V-I curves were produced and are shown in Figures 4 and 5. Dn Vp Ln Rn Vn Lp Vp Rp Fig. 3. A two diode fault odel for a HIF containing R n, R p, L n, L p Fig. 2. Electric arc voltage and current shapes In the context of downed conductors, Russell [18] conducted staged HIF tests studying dependencies of arc current agnitude on potential difference, gap distance, features of the grounding surface and environental conditions of the grounding point. A high degree of rando behaviour was observed due to ipurities near the grounding point, heat fro the arc that is intense enough to fuse substances and the evolution of different paths for current flow on surfaces. Current, A High ipedance fault current 2.1 HIF odel High ipedance fault is a difficult case to odel because ost HIF phenoena involve arcing, which has not been Tie in sec Fig. 4. Current curve for HIF 36

3 Volt, V 1.5 x V-I Curve Current, A Fig. 5. Voltage-current characteristic curve. 3. The haronic odel A signal can be defined as a function that carries inforation, usually about a state or a procedure of a physical syste. However, signals can be represented in several ways. Matheatically, a periodic and distorted signal can be suitably represented in ters of its fundaental frequency and haronic coponents, expressed as a su of sinusoidal wavefors referred to as the Fourier series. Each frequency is an integer ultiple of the fundaental syste frequency. In order to obtain an approxiation of such waves, atheatical odels are eployed. Consider a current wavefor with haronic coponents, written as [24, 25] I ( t) = { Αο + Αficos( ωfit+ φfi)} cos( ωοt + φο ) (1) i = 1 where: Α ο is the fundaental current aplitude, ω ο is the power frequency, Α fi is the aplitude of the flicker current, ω fi is the flicker current frequency, is the nuber of flicker odels, φ fi is the phase angle of flicker current. With only one flicker frequency this can be written as: I ( t) = Αο cos( ωοt + φο ) + Α1cos( ωf 1t + φf 1) cos( ωοt + φο )(2) Given that ωο is known, the proble is now to find the optiu values for Α ο, φ ο, Α 1, ω f 1, φ 1 using RCGA. 4. Genetic Algorith A genetic algorith is a coputational odel that solves optiization probles by iitating genetic processes and the theory of evolution [26-28]. Solutions fro a population are used to for a new population. This is otivated by the hope that the new population will be better than the old one. Solutions that will for new solutions are selected according to their fitness: the ore suitable they are, the ore chances they have to reproduce. This is repeated until soe condition (for f exaple a nuber of generations or an iproveent in the best solution) is satisfied. In the traditional GA, all the variables of interest ust first be encoded as binary digits (genes) foring a string (chroosoe). To iniize a function f ( x1, x2,..., xk) using GA, first, each x i is coded as a binary or floating-point string of length. Thus x1 = [ ] x2 = [ ]... xk = [ ] where { x 1, x,..., x 2 k } is called a chroosoe and xi are genes. Then three standard genetic operations, i.e. reproduction, crossover, and utation are perfored to produce a new generation [26-28]. Such procedures are repeated until the pre-specified nuber of generations is achieved, or the required accuracy is reached. Other coding types have been considered, such as Real Coded Genetic Algoriths (RCGAs), which see particularly natural when tackling optiization probles of paraeters with variables in both continuous and discontinuous doains. In the Real Coded GAs, a chroosoe is coded as a finite-length string of real nubers corresponding to the design variables. The RCGA is rigorous, precise, and efficient, because the floating point representation is conceptually close to the real design space. In addition, the string length reduces to the nuber of design variables. A coparative study conducted in [29] concluded that the Real Coded GAs outperfor binary-coded GAs in optiization probles. 4.1 Fitness Function A fitness function (FF) is one of the key eleents of GAs as it deterines whether a given potential solution will contribute its eleents to a future generation through the reproduction process. The FF should be able to provide a good easure of the quality of the solution and should differentiate between the perforance of different strings. In this study the fitness function is set to iniize the axiu individual error. The evaluation function is the function responsible for the deterination of the fitness of each individual. Its objective is to evaluate the estiation error (e). The coded paraeters are copared to the easured value in each tie step I(t) to calculate the average error (e). We use the evaluation function as the su of quadratic errors. The error at each tie step can be calculated as Ii ( actual) Ii( calculated ) = ei for i = 1,2,... (3) The quadratic error is calculated as Fsu = e i i=1 2 (4) 37

4 5. Testing the algorith In this case a practical power syste is siulated to deonstrate the ability of the Real Coded Genetic Algorith to track haronics during noral and abnoral conditions in a power syste. The syste is siulated using Siulink and SiPowerSystes block set. The siulated syste is shown in Figure 3. A threephase, 50 Hz, 11 kv power syste transitting power fro a substation to an equivalent network through a 8 k transission line. The voltage source is siulated with a Siplified Si power systes voltage block. Universal transforer blocks are used to odel the transforers. The transission line is odelled as π sections, where it is split in four 2 k lines connected between buses. A three-phase load is located at each end of the π section through a 1000 to 1600kVA 11/0.433 kv transforer. The load ay be varied to siulate either balanced or unbalanced loading conditions. Voltages and currents are easured in a B1 block. In order to verify the proposed algorith and assess its transient perforance, a high ipedance fault was applied to the syste at the first section of the line. 5.1 Detection criteria Fig. 3. Syste of study A Real Coded Genetic Algorith was used to analyze the haronics of the current wavefor. A decision whether there is a HIF is based on the existence of the 3 rd and 5th haronics and the angle shift of the 3 rd haronic with respect to the fundaental current. After any siulation tests a threshold has been specified for the haronics and the angle shift to deterine a HIF. For the 3 rd and 5 th haronics a 1% and a 0.5% of the fundaental current were set as threshold values, respectively. For the 3 rd haronic angle, an angle shift threshold of 100 degrees with respect to fundaental current angle was set. 6. Testing and results As ay be seen in Table 1, before the HIF occurs, the load current is noral with no change in the phase angle and no haronic currents. After the HIF has been applied there is an increase of 13.7 aps in the load current; this can be interpreted by the protection relay as a load increase instead of a fault, siply because the HIF does not draw sufficient current for the relay to act. In a noral fault, the current in the fault will be uch greater than the relay setting, therefore the relay will react to the fault; in the HIF case this will not happen because the setting in the relay is uch less than the HIF current (see Fig. 1). During the existence of the HIF there is a noticeable change in the circuit haronics. The existence of the 3 rd and 5 th haronics, plus the slight increase of load current and the angle shift of the third haronics with respect to fundaental current, together confir the existence of the HIF, since these are the peculiar characteristics of a HIF. haronics and phase angle Before HIF After HIF 1 st Θ nd Θ rd Θ th Θ th Θ th Θ th Θ7 0 0 DC transient 0 0 Table 1. Current haronics and phase angles before and after high ipedance fault 7. Conclusion A new ethod for high ipedance fault detection is proposed. The proble is forulated as an estiation task and a Real Coded Genetic Algorith is used to solve this optiization proble. The ethod was successfully tested on tracking haronics and current angles associated with HIF. The very accurate results obtained show that the proposed approach can be used as a very reliable ethod of identifying high ipedance faults. References [1] A. M. Sharaf and S. I. Abu-Azab, "A sart relaying schee for high ipedance faults in distribution and utilization networks," Halifax, NS, Canada, [2] C. G. Wester, "High ipedance fault detection on distribution systes," presented at 1998 Rural Electric Power Conference Presented at 42nd Annual Conference, April 1998, St. Louis, MO, USA, [3] H. Ching-Lien, C. Hui-Yung, and C. Ming- Tong, "Algorith coparison for high ipedance fault detection based on staged fault test," IEEE Transactions on Power Delivery, vol. 3, pp ,

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