Comparative Analysis Of Underground Cable Fault Distance Locator

ABSTRACT

Cable fault location is the process of locating periodic faults, such as insulation faults in underground cables, and is an application of electrical measurement systems.

Underground distribution cables are usually characterized with different technical difficulties regarding detecting and locating their faults. Different factors participate into these difficulties including their remarkable charging currents, cable construction and variations of their equivalency resulted from the variety of bonding and grounding methodologies. In this paper, a new fault location algorithm is proposed for underground cables in particular. The algorithm is able to precisely calculate the fault distance depending on modifying the basic apparent impedance approach to cope with the aforementioned characteristics of cable segments. In order to evaluate the performance of the proposed algorithm, different investigation tests are performed depending on a typical 11 kV underground distribution feeder in the Egyptian distribution system. All applied test cases are prepared with MATLAB-Simulink using the SimPower toolbox with accurate representation of power system elements utilizing distributed parameter line models. Moreover, the performance of the proposed algorithm is compared with the original apparent impedance approach using the same simulation platform. The applied test results cover a wide variety of fault conditions including fault resistance and loading circumstances. The results corroborate the efficacy of the proposed algorithm for locating such faults in underground distribution systems.

CHAPTER ONE

1.0                                                        INTRODUCTION

Cable fault location is the process of locating periodic faults, such as insulation faults in underground cables, and is an application of electrical measurement systems. In this process, mobile shock discharge generators are among the devices used.

This work relates generally to fault-location systems for determining the distance to a fault point and more particularly, it relates to a fault distance locator for underground cable circuits and a method for the same for calculating more accurately the location of a cable fault from a source-connected monitoring location in an efficient and effective manner.

As is generally known to those in the electric utility industry, buried underground cables utilized in Nigeria are typically formed with a center conductor surrounded by an outer polymeric insulation and a concentric neutral disposed over the polymeric insulation. These underground cables are employed to serve for the distribution or transmission of electrical voltage in the medium range between 11 KV and 35 KV. Faults sometimes develop, such as when the cable is punctured creating a short circuit between the conductor and the concentric neutral, which require the repair or replacement of the cable or a portion thereof. In order to facilitate the correction of the fault, it is desirable to know the exact location of the fault.

To this end, electric utilities have constructed access points which are generally provided at pedestals or towers that are located at spaced apart positions along the underground cables. Typically, the cable lengths are approximately 1 mile or longer with the access points being disposed about 500 feet apart for underground residential distribution (URD) or underground commercial distribution (UCD) circuits. At these various access points, there are provided cable circuit switches and faulted circuit indicators (FCI) which are located inside a transformer and switchgear box.

When a cable failure occurs, a fuse or circuit breaker or other circuit interrupting or protective device will be tripped so as to cause a circuit interruption. A linecrew will be sent to inspect the FCI in the transformer and switchgear box at each switch location to determine the last FCI unit (tripped) to indicate the passage of a fault current. In this manner, the faulted cable section can be located. With this information, the fault cable section can then be “switched out” or isolated so as to become a new open-loop point. Consequently, full restoration of service is provided during the time when the faulted cable section is being repaired. This procedure just described is commonly followed by virtually all of the electric utilities in the United States and involves about a 2 to 4 hour period of time to be accomplished.

1.2                                                 AIMS AND OBJECTIVES

fault distance locator for an underground cable for calculating accurately the location of a cable fault. The fault distance locator includes means for monitoring the cable current to produce a fault-current occurrence signal upon the occurrence of a cable fault and means for monitoring the cable voltage to produce a fault-voltage occurrence signal upon the occurrence of the cable fault. The main objective of this work is to analyze and identified the best method of fault locator for underground cable.

1.3                                         SIGNIFICANCE OF THE PROJECT

This work on this topic has the capability of determining the fault location within five feet or less of the faulted cable location. This is accomplished by the use of a pulse generator for injecting a series of chirped pulse streams into the cable shortly after the occurrence of the cable fault. The delay times between the pulses sent on the faulted cable and the reflected pulse signals are used to calculate the distance to the cable fault.

1.3                                                 SCOPE OF THE PROJECT

The variety of techniques described in this article included time domain reflectrometry, arc reflection, surge pulse, capacitive discharge (thumper), and fault reduction (burning). However, all of these methods require the use of costly special equipment in order to determine the distance to the fault or otherwise pinpoint the fault location with reasonable accuracy. For example, there is a commercially available prior art reflection system using a memory radar which is manufactured by VON Corporation of Birmingham, Ala. This prior art system is typically quite heavy and is mounted on a special van or truck and must be operated by specially-trained crewmen on site after the testing has been performed on the de-energized cable.

There is also known in the prior art of a digital fault locator which calculates the reactance of a faulty line, with a microprocessor, using the one-terminal voltage and current data of the transmission line.

This paper also stated generally that one of the methods for measuring the distance to a fault point on a transmission line is to set off pulses when a fault occurs and the pulse return time from the fault point is utilized to determine the location of the fault point.

There also exists a number of prior art patents that have been granted which are directed to fault locators of the type for determining the location of high resistance ground faults by using impedance based calculations. In other words, the voltage and the current are measured at both ends of the faulted line and some algorithm would make the calculation of the distance to the fault based upon the measurements. However, none of the prior art fault locators considered the effect the arc voltage had on the calculations, which could be very high, in the case of overhead lines, thereby preventing the pinpointing of the fault location with a high degree of accuracy.

There is disclosed an apparatus for determining the distance between a supervisory station in an A.C. system and a fault affecting the system. The apparatus includes means for measuring the voltage when the current traverses the zero datum and means for determining the derivative of the current at that instance. An output device determines from this information the inductive reactance under the fault condition to provide an indication of the distance to the fault.

1.4                                           LIMITATION OF THE PROJECT

This work  has several major disadvantages. For example, each of the transformer and switchgear boxes must be located, which may be covered or hidden by shrubs, bushes or other debris, and the locked doors thereof must be opened (which may be rusted) in order to check the status of the FCI units therein. This is a very time-consuming and laborious task. Further, prior to the fault locating step the cable is to be de-energized, tested for potential, and grounded so as to remove the charge due to the cable capacitance. Thus, this method requires additional time and labor expense which is a slow and tedious process.

After the steps of determining of an existence of a cable fault and determining the approximate location of such fault, linemen or repairmen later return in several days to find the exact and actual location of the fault so that the appropriate repairs can be made in order to restore the circuits to their previous normal operation.