Fault Location in Distribution System with Load ...

2014 International Conference on Power System Technology (POWERCON 2014)

Chengdu, 20-22 Oct. 2014

Fault Location in Distribution System with Load Uncertainty Analysis Lihan He, Zhilin Wu, Zhihan Xu, and Ilia Voloh  Abstract — Accurate fault location in distribution network of power system is an essential technology, yet facing more challenges than that in transmission systems. This study develops novel methods for feeder fault location using current/voltage sensors sparsely deployed in the network, in addition to the voltages and currents measured in the substation. Depending on the available sensor locations relative to the fault and the lateral conditions, the algorithm searches every possible path and calculates the fault distance and fault resistance by reducing the circuit to one of the two scenarios. Like all impedance based methods for feeder fault location, the proposed method relies on load values. However, the load values are difficult to obtain and time varying. In this paper, two load estimation methods, based on optimization and regression, respectively, are developed. The impact of load uncertainty on the proposed method is also studied. Simulation experiments show that the proposed fault location method is robust to load uncertainty and achieves high accuracy in various network and fault conditions. Index Terms—fault location, distribution network, feeder, load estimation, and sensor.

I. INTRODUCTION

A

CCURATE fault location is an essential technology that enables utility companies in maintaining service reliability to reduce SAIDI (System Average Interruption Duration Index) [1]. When a fault occurs, particularly for a permanent fault, immediate actions have to be taken to reduce the impacted areas and customers. In this condition, fault location must be accurately detected and then the fault can be removed from the power system. At presence, automatic fault location is mostly performed in transmission lines, where the lines are long without laterals. In such applications, a few major technical solutions have been developed. One solution is the impedance based method [2][3], Manuscript received April 30, 2014. Lihan He is with GE Global Research, Shanghai, 201203, China (phone: +86-21-38771259; fax: +86-21-50806339; e-mail: [email protected]). Zhilin Wu is with GE Global Research, Shanghai, 201203, China (email: [email protected]). Zhihan Xu is with GE Digital Energy, Markham, ON, L6C 0M1, Canada (e-mail: [email protected]). Ilia Voloh is with GE Digital Energy, Markham, ON, L6C 0M1, Canada (e-mail: [email protected]).

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which utilizes fault current and voltage to calculate the apparent impedance of the line and hence determine the fault location. This method is low cost, but relatively low accuracy. Another major solution is the traveling wave based method [4]. This method can be passive or active. The passive approach measures the time delay between the fault signal arrival and its echo from the fault point. The active approach measures the time delay between the injected signal and the echo signal from the fault point. With known electricity propagation speed, the fault distance can be calculated from the echo time. In practice, the traveling wave based method is often performed along with manual line check (the active approach) to find the shorted line position. There are growing needs for fault location in distribution systems [5][6], as the distribution systems directly connect to the end customers. Although the fault location methods in transmission lines are widely used and are relatively mature, the methods face huge challenges in distribution systems. In practice, fault location in distribution systems still heavily relies on manual line check. The accuracies in research are mostly around kilometer [7]. One major challenge is that the distribution systems have a large number of laterals. In such a scenario, the impedance based method lacks the fundamental assumption of uniform line configuration, thus the method is invalid. Furthermore, in the studies of fault location in distribution system, generally it is assumed that the loads in the system are known [8] so that circuit analysis can be performed. However, the large number of loads in the complex distribution system can only be estimated within a range [9]. Moreover, loads may show different dynamics during fault [10], which makes it even more difficult to estimate the load values in impedance based fault location algorithm. For traveling wave based method, echoes from multiple laterals confound the fault echo detection and make it difficult to locate the fault. In addition, waveform propagation speeds are different with various line configurations. With multiple laterals in the complex system, the sensing information is highly limited. In the distribution systems, there is usually no sensing device except at substation. For the application of fault location, a fault recorder is available at the substation to collect information of voltage and current. Yet, this limited information at a single point can hardly help

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solving the problem. It is natural to think about adding more sensing devices to collect information from every lateral. Yet, the cost would be unacceptable due to the large number of laterals in the distribution system. Even though the cost for a single sensor may be low, the cost for a full network deployment is huge. In our recent studies [11][12], novel and automatic fault location methods are developed to utilize current/voltage sensors in the distribution systems. With the help of only a few sensors sparsely deployed in a complex distribution system, fault location can be determined with the proposed algorithms. Unlike many other studies, which assume metal grounding or known fault resistance, the proposed method treats the fault resistance as an unknown parameter and solves it together with fault distance. As a result, the method is relatively insensitive to the fault resistance. Load values are required in all impedance based fault location algorithms. In this study, two load estimation methods are further developed to estimate the load values at multiple nodes. One method is optimization based method. It achieves accurate load estimation, but needs more computational time. The other method is based on linear regression of available current measurements in the feeder. Compared to the first method, this method achieves a coarse estimation with accuracy of about 10%, but is much faster. A careful sensitivity analysis of proposed fault location algorithm to load uncertainty is performed, and the result indicates that even though the second load estimation method is applied with some estimation uncertainty (error), the proposed algorithm can still provide high fault location accuracy for different levels of load and long lines. As single phase line to ground fault counts for about 90% of all faults, this type of fault is targeted in this study. The proposed method can be easily extended to other types of fault, such as line to line fault, etc.

Chengdu, 20-22 Oct. 2014

component, harmonics, and noise. The steady state amplitude and phase can be obtained by Fourier Transform to extract the component of grid frequency. B. IEEE 34-node Test Feeder The IEEE 34 node test feeder is applied to verify the proposed fault location method. This test feeder is a real network served as one of the standard feeder networks. Detailed parameters are available from IEEE for reference [14]. This test feeder covers the most important features, including 3-phase lines, single-phase lines, transformer, regulators, balanced and unbalanced loads. Figure 1 shows the test feeder system topology with the sensor deployment. In addition to the measurements at substation 800, three sensors are deployed at nodes 812, 830, and 834, respectively. In our algorithm, a regulator is modeled as an ideal transformer with voltage ratio k and current ratio 1/k. The scalar k is usually a number close to 1. Theoretical analysis and simulation result both show that this approximation, instead of a detailed regulator model, has little impact on the fault location accuracy. The transformer can be modeled by a T model or a П model with symmetric component method for fault current/voltage analysis. However, in practice, fault current from the downstream of a transformer is difficult to be detected due to the current ratio k (especially for a single phase line to ground fault). Therefore, if necessary, the transformer with its downstream laterals is usually considered as a sub-feeder for further analysis.

II. SENSING AND NETWORK A. Sensors The proposed method requires sensors sparsely installed in the distribution system to provide current/voltage measurement. The basic requirements of the sensors are: 1) measure steady state amplitude and phase; 2) transmit the data to a centralized station with time stamp so that data from multiple sensors can be synchronized. There are low cost sensors in the market [13] meeting these requirements. In this study, all data from sensors are considered as already time aligned. The real measured short-circuit fault signals from sensors are usually transient current and voltage waveforms of a few cycles, consisting of grid AC component, decayed DC

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Fig. 1. IEEE 34-node test feeder with sensor deployment

III. ENHANCED FAULT LOCATION ALGORITHM The impedance based methods have already been used in transmission line fault location. However, it is quite difficult to borrow the same idea in distribution networks. This is because the distribution systems have many laterals, non-uniform lines and distributed loads, which increase the complexity of the systems, introduce the uncertainty, and make the assumptions used in transmission systems invalid. This section introduces the proposed algorithm with framework and theoretical analysis.

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A. Algorithm Framework In this study, it is assumed that only one fault occurs at a time in the network. It is widely accepted that the fault is usually resistive, thus without reactance part. Moreover, we focus on the low impedance fault, i.e., the fault resistance RF