System Fault Location

Transcript

4/10/2008 System Fault Location This Topic Covers Troubleshooting Of Faults In Underground Cable. ETE503 UNDERGROUND CABLE 1 1. Faults  1.  Faults  y y y y If a fault occurs in a underground cable, it becomes essential that it is located as quickly and accurately as possible. Accuracy is necessary if excessive trenching work is to be avoided. The type of fault most likely to occur is single conductor fault-to-earth or to the protective metal sheath. In multi-core cables, the fault current is likely to give rise to excessive local heating at the fault, causing further breakdown of insulation and extending die fault to the remaining conductors. Open circuits may occur occasionally, which will usually be at the cable joints. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 2 1 4/10/2008 1. Faults (Cont.) 1.  Faults (Cont.) Various tests have been developed to locate either yp of fault and most of them involve the type application of a low dc test voltage. y However, it should be noted that in some cases the fault may only be apparent when a high dc voltage is applied since a low voltage will give a meaningless result. y In general, higher the test voltage applied, the more sensitive the test. y ETE503 UNDERGROUND CABLE 3 1. Faults (Cont.) 1.  Faults (Cont.) Transporting cable fault detector for on-site tests. The system easily fits into a normal field service vehicle. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 4 2 4/10/2008 2. Fault Identification y Prior to locating a fault, it is necessary to determine the nature of fault so as to make a better choice of the method to be used for fault location. location 1. 2. First, isolate die faulty cable and test each core of the cable for earth fault. One terminal of the insulation tester is earthed and every conductor of the cable is, in turn, touched with the other terminal. If the insulation resistance tester indicates zero resistance duringg any measurement, conductor-to-earth fault for that particular conductor is confirmed. Then check the insulation resistance between the conductors. In case it is a short-circuit fault, the insulation resistance tester will indicate zero resistance. ETE503 UNDERGROUND CABLE 5 2. Fault Identification (Cont.) 3. After this, short and earth the three conductors of the cable at one between individual conductors (at the other end) to check open circuit fault. 4. In case there is any fault, the insulation test of individual cores with sheath or armor and between the cores is essential. The test should also be done by reversing the polarity of the insulation resistance tester. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 6 3 4/10/2008 2. Fault Identification (Cont.) 5. In case of any difference in readings, the presence of moisture in the cable insulation is confirmed. 6. The moisture in the cable forms a voltage cell between the lead sheath and conductor because of the difference in the conductivity of these metals and the impregnating compound forms an organic acid when water enters it. ETE503 UNDERGROUND CABLE 7 2.1 Check Cable Fault By Means Of A Megger Megger test is conducted from each end of the cable. y Megger test from end B (next figure) on conductor to earth indicates infinite (insulation resistance) indicating a possible open conductor in that phase. y ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 8 4 4/10/2008 2.1 Check Cable Fault By Means Of A Megger (Cont.) (A)Checking by Megger - Megger reads zero - Short circuit detected (B)Checking by Megger - Megger reads infinity - Indicates healthy cable ETE503 UNDERGROUND CABLE 9 2.1 Check Cable Fault By Means Of A Megger (Cont.) Checking by Megger high resistance fault detected from both ends. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 10 5 4/10/2008 3. Fault 3.  Fault Localization Methods y y The following methods [6] are applied for fault location: ◦ Murray Loop Test ◦ Fall Of Potential Test ◦ Dc Charge And Discharge Test ◦ Induction Test ◦ Impulse Wave Echo Test ◦ Time T D Domain Reflectometry R fl Test T The first four tests are conventional and are used for underground or control cables and the fifth and sixth tests can be used for both insulated cables and overhead lines. ETE503 UNDERGROUND CABLE 11 3.1 Murray Loop Test y This method can be used for both low and high resistance faults in the following circumstanced: a) b) c) d) When there is a fault in either one or two conductors, the third conductor in the cable remaining unaffected. When all the three phases are faulty, provided a core of adjacent cable is used for measurement. When three conductors are faulted, if the contact resistance of the conductors at the fault differs from each other byy a factor more than 500. When the contact resistance does not exceed 5,000 ohms if working with a low voltage bridge and 1.5 mega ohms if working with a high voltage bridge. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 12 6 4/10/2008 3.1 Murray Loop Test (Cont.) The Murray loop test is the most common and accurate method of locatingg faults and should be made use of whenever circumstances permit. y It can precisely locate the fault if the fault current is more than 10 mA, e.g., if the battery voltage is 100 volts, then the fault resistance may be of the order of 10 kΩ. y The sensitivity depends on the detector used in the test circuit. y ETE503 UNDERGROUND CABLE 13 3.1 Murray Loop Test (Cont.) y y y y A high gain electronic dc amplifier can be used for a higher g degree g of sensitivityy of the instrument. In its simplest form, the faulty conductor is looped to a sound conductor of the same cross-sectional area and a slide wire or resistance box with two sets of coils are connected across the open ends of the loop. A galvanometer l t is i also l joined j i d across the th open end d off the loop and a battery or a dc hand generator supplies the current for the test. Balance is obtained by adjusting the slide or resistance. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 14 7 4/10/2008 3.1 Murray Loop Test (Cont.) Precondition:The actual cable resistance p per meter must be known. y A conducting jumper of low resistance jumper must be installed at the far end of the cable between the good conductor and the faulted conductor. y An unfaulted conductor must be available to complete the bridge connection. 15 ETE503 UNDERGROUND CABLE y 3.1 Murray Loop Test (Cont.) Locating A Ground Fault (Short Circuit) By The Murray-loop Test ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 16 8 4/10/2008 3.1 Murray Loop Test (Cont.) y At balance, y where RL is the total resistance of the loop (faulty conductor plus return conductor) and Rx is the resistance of the faulty conductor from the bridge terminal to the location of the ground fault. ETE503 UNDERGROUND CABLE 17 3.1 Murray Loop Test (Cont.) y Since the wire resistance is proportional to the length and the cross sectional area off the conductor, we can substitute length for resistance. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 18 9 4/10/2008 3.1 Murray Loop Test (Cont.) y In a multicore cable the return conductor l1 has the same length and the same cross section as the faulty core, so that l1=l2=l and therefore y where l is the length of the multicore cable from the bridge terminals to the point of termination. ETE503 UNDERGROUND CABLE 19 3.1 Murray Loop Test (Cont.) Procedure:y y y y Murray Loop Bridge is connected to the Good conductor (C) and faulted conductor (D) at one terminal of the cable. The loop p of the g good conductor and faulted conductor form the two arms of the bridge. The 4 arms of the bridge are (1) Resistance A (2) Resistance B (3) Good conductor of Length L and (4) Faulted conductor of length (L) with fault at distance X. The bridge is balanced when galvanometer G shows null point. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 20 10 4/10/2008 3.1 Murray Loop Test (Cont.) Procedure (Cont.) :y Balance is achieved by adjusting ratio of resistances B/A. y When the bridge is given by X = 2L[(A)/(A + B)] ◦ X is the unknown length resistance measurement terminal and the fault point. between the ◦ B, A, G testing bridge ◦ A Healthy conductor ◦ B Faulty conductor in the same cable. ◦ L Length of the faulted cable conductor being test ◦ X Distance of fault from the test terminal ETE503 UNDERGROUND CABLE 21 3.2 Fall 3.2  Fall of Potential Test In this test, the principle involved depends upon the measurement of the voltage drop on the cable conductor when a current is flowing through it. y The only essential instruments consist of an accumulator, rheostat, ammeter and low range moving coil voltmeter. y ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 22 11 4/10/2008 3.2 Fall 3.2  Fall of Potential Test (Cont.) The measurement gives the voltage drop up to the fault and by comparing the voltage measurements made from each end, the position of the fault can be readily calculated. y There are many different circuit arrangements, but the accuracy is not as high as that of the Murray Loop Test. y ETE503 UNDERGROUND CABLE 23 3.3 Charge and Discharge Tests This method is used for broken cores with high resistance to earth. y The usual method of measuring the charge capacity is to charge the cable under test to certain dc voltage for about 15 seconds and then discharge it through a moving coil galvanometer, the point to which the needle kicks being noted. 24 ETE503 UNDERGROUND CABLE y ETE503 UNDERGROUND CABLE 12 4/10/2008 3.3 Charge and Discharge Tests (Cont.) For the p purpose p of locatingg breaks,, it is usuallyy sufficient to measure the relative values of capacity from each end of the broken core. y To avoid false readings, it is necessary to earth all the broken cores at the far end except the one under test at the test point. on the next slide to verify your slide show settings. y ETE503 UNDERGROUND CABLE 25 3.3 Charge and Discharge Tests (Cont.) y If C1 aandd C2 aaree tthee obse observed ved capac capacities t es at each end and l the length of the cable, then the distance d of the fault from the end giving the reading C1 is: ⎛ C1 ⎞ ⎟⎟l d = ⎜⎜ ⎝ C1 + C2 ⎠ ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 26 13 4/10/2008 3.4 Induction Method y y y The induction method can be used for the location off faults-to-earth f lt t th in i the th case off a cable bl having h i no metallic sheath or armouring. It has the advantages of indicating the exact position of the fault without the necessity for any calculation or assumption regarding the cable resistance. resistance The cable is supplied with intermittent pulses of current derived from a dc source and an interrupter. ETE503 UNDERGROUND CABLE 27 3.4 Induction Method (Cont.) y The cable route is then explored with a search coil connected to a telephone p receiver, this coil takingg the form of about 200 turns of fine gauge wire wound to form a triangle of about 1 m side. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 28 14 4/10/2008 3.4 Induction Method (Cont.) The coil is held near the ground with its plane parallel to the run of the cable as shown in the figure. y Until the fault is reached, the cable will carry pulses of current and the intermittent nature of the magnetic field set-up set up will induce an intermittent emf in the coil producing a note which can be heard on the earphones. y ETE503 UNDERGROUND CABLE 29 3.4 Induction Method (Cont.) As soon as the fault is passed, the cable will carry no current and the note will cease. cease y This technique may also be used to trace faults in buried cables, the precise route of which is not recorded. y But as mentioned above, the presence of a metallic sheath will effectively screen the cable and nothing will be heard on the telephone. y ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 30 15 4/10/2008 3.5 Impulse Wave Echo Test This method is based on the principle that a pulse propagating along a cable will be reflected when it meets with an impedance mismatch. y This effect can be shown visually in a cathode ray tube. y The pulse propagation velocity is inversely proportional to the square root of the dielectric constant of the cable. y ETE503 UNDERGROUND CABLE 31 3.5 Impulse Wave Echo Test (Cont.) y FFor a cable bl off uniform if di l dielectric, i the h pulse reflected at the mismatch is displayed on a cathode ray tube at a time proportional p to the delayy directlyy p distance of the mismatch from the test; irrespective of the conductor size. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 32 16 4/10/2008 3.5 Impulse Wave Echo Test (Cont.) a) Open circuit fault (b) Short circuit fault Example of typical waveforms on oscilloscope in impulse wave echo test ETE503 UNDERGROUND CABLE 33 3.5 Impulse Wave Echo Test (Cont.) y The fault p position is ggiven byy t1 X = x route length t2 Where :- X = distance from test end t1 = pulse time to fault t2 = pulse time to far end of cable ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 34 17 4/10/2008 3.5 Impulse Wave Echo Test (Cont.) This is the qquickest and universallyy applicable pp method. Nowadays portable digital fault locators are available embodying the impulse wave echo technique. y It consists of a unit having a crystal controlled digital timing technique thus giving increased simplicity of operation and accuracy of more than 1-2 per cent of the range. y ETE503 UNDERGROUND CABLE 35 3.5 Impulse Wave Echo Test (Cont.) y y Fault distances are displayed in meters on 5 digit LCD, eliminating the need for zero or scale setting. Pulse travelling wave reflection/transmission is shown on oscilloscope screen of the meters. Oscillograms For Cable Faults ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 36 18 4/10/2008 3.5 Impulse Wave Echo Test (Cont.) Such instruments can locate the fault at any distance between one meter and 25 km, with the ability to operate either from mains or nickel cadmium battery. y This Thi equipment i can be b used d for f LT/HT power cables, and control cables and overhead lines. y ETE503 UNDERGROUND CABLE 37 3.6 Time Domain Reflectometry (TDR) Test The Impulse Wave Echo Test is a satisfactory method for single-cable runs. y TDR is most accurate for the automatic g cable fault location method for low-voltage distribution cable network involving multiple tee-offs. y ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 38 19 4/10/2008 3.6 Time Domain Reflectometry (TDR) Test (Cont.) Method:y y The TDR analysis begins with the propagation of a step or impulse of energy into a system and the subsequent observation of the energy reflected by the system. By analyzing the magnitude, duration and shape of the reflected waveform, the nature of the impedance variation in the transmission system can be determined. ETE503 UNDERGROUND CABLE 39 3.6 Time Domain Reflectometry (TDR) Test (Cont.) y Time-domain reflectometry, y basic equipment q p setup. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 40 20 4/10/2008 3.6 Time Domain Reflectometry (TDR) Test (Cont.) y Waveform Analysis y A reflection with the same polarity indicates a fault with OPEN (high impedance) tendencies. The reflection shown at the second cursor is a COMPLETE OPEN 41 ETE503 UNDERGROUND CABLE 3.6 Time Domain Reflectometry (TDR) Test (Cont.) y Waveform Analysis (Cont.) A reflection with the opposite polarity indicates a fault with short (low impedance) tendencies. The reflection shown at the second cursor is a DEAD SHORT. 42 ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 21 4/10/2008 3.6 Time Domain Reflectometry (TDR) Test (Cont.) y Waveform Analysis y (Cont.) ( ) The middle reflection at the second cursor is a PARTIAL OPEN followed by a COMPLETE OPEN (end of the cable). The more severe the fault, the larger the reflection will be. 43 ETE503 UNDERGROUND CABLE 3.6 Time Domain Reflectometry (TDR) Test (Cont.) y Waveform Analysis (Cont.) A high resistance joint or splice will produce an "S" shaped reflection. A high impedance upward reflection will be followed by a lower impedance downward reflection. ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 44 22 4/10/2008 3.6 Time Domain Reflectometry (TDR) Test (Cont.) y Waveform Analysis y (Cont.) ( ) A properly terminated cable will absorb the TDR signal resulting in no reflection. Faults prior to the termination may show up as reflections along the waveform. If a terminator causes a reflection, the termination may be bad. 45 ETE503 UNDERGROUND CABLE 3.7 HV Surge Tester Surges g of high g energy gy are applied pp to the fault at the set voltage and time interval for pin pointing the exact spot on the cable length. y These surges create noise and vibrations at the site. y The intensity of the noise and vibrations get attenuated during their travel to the ground surface. y ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 46 23 4/10/2008 3.7 HV Surge Tester y A ground microphone and a sensitive surge wave receiver carried on the route of the cable on the pre located area pin-point the exact spot of the minimum time. ETE503 UNDERGROUND CABLE 47 Thank You Question ??? ETE503 UNDERGROUND CABLE ETE503 UNDERGROUND CABLE 48 24