Undergroundcables have been intensively employed in power distribution systems owing totheir numerous advantages such as; less vulnerability to damage by weatherconditions and lightning, underground connections, aesthetic requirements, ecofriendliness, low maintenance and low costs for shorter distances. Despitetheir numerous advantages, identification of fault location in underground cablesis a very complicated issue and poses a serious challenge to the reliability,stability and security of an underground distribution network. A timely andaccurate identification of a faulted segment on the transmission network isvery critical to reduce circuit interruption time during a fault. In thisresearch work we present two methods which will be very useful to identify theexact distance of fault of underground system from base station. One of themethods is Murray loop method and other one is Ohm’s Law Method.
Murray loopmethod uses the whetstone bridge to calculate exact distance of fault locationfrom base station. Whereas in Ohm’s law method, when any fault occurs, voltagedrop will vary depending on the length of faultin cable, since the current varies. Both the methods use voltage convertor, microcontrollerand potentiometer to find the fault location under Line to ground, Line to line,LLL faults. Hardware implementation of both methods will be carried out todetermine the effectiveness of both the methods for fault detection 1. Introduction: Most of the transmission linesare laid using overhead line method but transmission line by underground methodalso finds its use and application over a large area. In areas like hospitalsor colleges, underground cable is widely preferred to ensure safety.
Underground cable installations are costly as compared to overhead cable butare more reliable and also the life of underground cable are more as comparedto overhead lines.Although underground cables areunaffected by adverse conditions like a storm, rainfall and the chances forfault in underground cables are less than that of overhead cables but when thefault happens at undergrounds cables its detection becomes difficult. So itbecomes essential to calculate the distance of fault for an efficient way toemploy underground cable method 1. Locating a faulted segment of undergroundcable system requires broader aspects of consideration and analysis 2. Unlikeoverhead lines, underground cables have the characteristics of smallerinductance but larger capacitance.
The analysis becomes complicated when varioustypes of underground cables are used 3. TYPES OF FAULT:Fault in a cable can beclassified as: A.) Open circuit faultB.) Short circuit fault Open circuit fault:This type of fault is caused bybreaking in conducting path etc. Such fault happens when one or more phaseconductor wire break. The value of current in such fault becomes zero and theload side gets isolated from the Generation side. This fault is less harmful asno current flows when short circuit fault occurs.Short circuit fault:When conductors of differentphases get connected with each other than such fault comes under short circuitfault.
In this type of fault the value of current increases so it becomesharmful at the load ends. There are basically 2 types ofshort circuit fault:-i. Symmetrical Faultii. Unsymmetrical Fault Symmetrical Fault:The 3-phase fault is called asymmetrical fault. In this, all 3-phases are short-circuited. In this fault thephase angles are unchanged but the magnitude of the current can vary.Unsymmetrical Fault:In this fault magnitude of thecurrent is not equal and also not displaced by 120-degree angle. The differentphases are short-circuited with each other.
Two methods presented in thisresearch work can resolve this problem related to underground system. Both ofthese methods are very fast and accurate in finding the faultLocation therefore can be provedvery useful. These methods are explained in detail: 1- MURRAYLOOP METHOD Murray loop test is an effectiveway to locate fault in an underground cable. This test can be carried out foreither a short circuit fault or an earth fault in an underground cable. The keyto this method is that generally resistance of the fault affects the resultsonly when its magnitude is very high. This test is based on Wheatstone bridgeand includes Murray loop test as well as Varley loop test. To find out fault inthe cable a Wheatstone bridge is made in it and resistances are compared todetermine fault location.
The faulty cable is connected with the un faultycable through a wire having very low resistance as low resistance does notaffect total resistance of the entire cable and permits loop current tocirculate through the Wheatstone bridge without loss 4. 2- OHM’SLAW METHOD Ohm’s law method is relatively avery simple method used to locate a short circuit fault. Depending upon lengthof the power cable current varies when at feeder end when DC voltage is appliedthrough a series resistor. The voltage drop in the series resistor variesaccordingly and fault location is determined using voltage drop across theresistor.
The core objective of thisresearch work is to design and implement hardware modules employing both of theabove mentioned techniques for fault detection and study their effectiveness carryingout various performance tests. 2. Literature Review: Power system fault location andidentification of the different faults on a underground power cable system forquick & reliable operation of protection scheme. Fault location estimationis very important issue in power system in order to clear faults quickly 5.Different techniques commonly used for underground cable fault detection andrelevant work is summarized below: 6 proposes a novel method fordetecting and locating a multicycle incipient fault in a cable. The incipientfault is modeled as a self-clearing arcing fault. The distortion degree ofcalculated voltage is used to detect the occurrence of an incipient fault.
Thedegree of match between the measured and calculated waveforms is used to guidethe search for the fault distance. The accuracy is further improved by takinginto account the incipient fault angle as seen in the voltage waveform and thepower loss characteristics. A three step process is followedin 7 with creation of transmission system model using a Matlab/Simulink andfollowed by creation of faults in the system. In second step the Fourieranalyzed fault voltages and currents obtained from the SIMULINK model are fed to the training set of artificialneural network (ANN) in order to detect the type of fault.
In the last step, an independentsoftware OrCad is used to locate the fault distance from the either ends usingthe principle of time domain reflectometry in a simulated practical undergrounddistribution system. In 8 the feasibility ofapplying complex wavelet analysis to fault detection. We combine complexwavelets with continuous wavelet transform (CWT), and calculate the impedancefrom the voltage and current data in the wavelet domain. We then examine themagnitude and phase distributions of the impedance under various conditions. Wetest our analysis approach with measurement data from different types ofcables. The results show that the complex wavelet analysis based approach isable to provide unique signatures for distinguishing between the cables, thusvery promising for fault detection.
9 shows a fault detectionmethod by monitoring current of sheath. Using MATLAB to analysis the simulationdata, the results show that the method can effectively detect cable maininsulation breakdown. Using this can realize the online monitoring of powercable fault. 10 Presents a new method fordetection of incipient cable failures by using measured current and voltage atone end of the cable.
The incipient faults are detected using an innovationsignal calculated from the measured fault current via a Kalman filter. Uponchange detection by the innovation signal, the change is checked fordiscriminating possible incipient fault from other similar conditions. 11 Medium voltage undergroundcables may exhibit incipient, self-clearing arcing faults prior to failingpermanently. These events typically last one half- cycle and extinguish at thefirst natural zero crossing of the current. The magnitude of the half-cycleevent is primarily dependent on the location of the fault on the feeder, but isalso dependent on the point on the voltage waveform that the fault starts. 3. Statement of the Problem Identification of fault locationin underground cables is a daunting task and poses a serious challenge to thereliability of underground distribution network.
A timely and accurateidentification of a faulted segment on the transmission network is verycritical for reduction of circuit interruption time during a fault. Faultdetection using Ohm’s law method and Murray loop method is the most costeffective method to estimate the distance of the fault from base station. Inthis proposed research work it is intended to design prototypes to estimatefault location and present an in-depth analysis of their performance in aconventional underground distribution system.
4. ResearchMethodology In this research work we present two methods which will be very useful toidentify the exact distance of fault of underground system from base station.One of the methods is Murray loop method and other one is Ohm’s Law Method.Murray loop method uses theWhetstone bridge to calculate exact distance of fault location from basestation. Whereas in Ohm’s law method, when any fault occurs, voltage drop willvary depending on the length of fault in cable, since the current varies. Bothof these methods discussed make use of Microcontroller, Voltage converters andpotentiometer to locate the fault in an underground cable for LL, LG and LLLfault types. Hardware implementation of both methods will be carried out todetermine the effectiveness of both the methods for fault detection. Theproposed research work will be carried out in the following steps: 1- Adetailed analysis of fault detection methods will be presented2- Identificationof design specification3- Selectionof design components4- Hardwareimplementation5- Discussionbased on result