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2016 China International Conference on Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 Technics of Locating Underground Cable Faults inside Conduits Gilbert Cheung, Yuan Tian, Tobias Neier Representative Office Hong Kong Baur GmbH Austria Abstract—A cable fault can be defined as any defect, inconsistency, weakness or non-homogeneity that affects the performance of a cable. All faults in underground cables are different and the success of a cable fault location depends to a great extent on practical aspects and the experience of the operator. To accomplish this, it is necessary to have personnel trained to test the cables successfully and to reduce their malfunctions. In many countries, many underground cables are laid inside PVC pipes or conduits. Pin-pointing cable faults would be much easier when the cables are buried underground directly. However, when the cables are laid inside a conduit, it becomes much more difficult to pin-point the exact fault location. There are certain ways to identify the cable fault location by applying different technics. This paper describes the use of different equipment and technics to locate underground cable faults inside conduits. Index Terms—Cable Fault Location (CFL), PVC Conduit, PVC Pipe, Pre-location, Pin-pointing. I. I NTRODUCTION Power supply networks are growing continuously and their reliability is getting more important than ever. The complexity of the whole network comprises numerous components that can fail and interrupt the power supply for the end user. For most of the worldwide operated low voltage and medium voltage distribution lines, underground cables have been used for many decades. Underground high voltage cables are used more and more because they are not influenced by weather conditions, heavy rain, storm, snow and ice as well as pollution. Even though the cable manufacturing technology is improving steadily, there are still many influences which may cause cables to fail during operation or test. Locating underground cable faults under direct buried cables has been quite successful during the past. However, for underground cables which are laid inside conduits, it can become much more difficult to locate cable faults. It is important to fully utilize the technics of CICED2016 Session 1 Paper No FP0731 locating underground cable faults inside conduits. II. PRE-LOCATION METHODS A. Multipule Impulse Method (SIM/MIM) The Multiple Impulse Method is the most advanced cable fault pre-location method available. Every cable fault that is either a high resistive or intermittent fault cannot be indicated by means of the Time Domain Reflectometer (TDR) method. The low voltage impulse sent out by the TDR is not reflected at the faulty position, as the fault impedance compared to the insulation impedance of the healthy part of the cable is not significantly lower. Based on this fact, the Multiple Impulse Method is supported by a single high voltage impulse that is generated by the coupled surge generator. Like this it is possible to change the high resistive fault temporarily into a short circuit (flash over, temporary low resistive fault condition) and therefore can be detected by a second TDR impulse (SIM) or multiple secondary Impulses (MIM). The low voltage TDR impulse is coupled to the high voltage output of the surge generator via the coupling unit. For many years, the Secondary Impulse Method was considered to be the most advanced method. Problems were figured out, as faults with difficult characteristic had to be located. Those influences like water in a joint, oil-reflow in oil filled cables, etc. either shorten the duration of the flash over or delayed the ignition time of the flash. All these effects are influences that make the timing for the triggering and release of the secondary impulse, to reach the fault exactly at the short time frame of arcing, very difficult. Manual trigger delay settings had to be varied and therefore requested the user’s skills significantly. The method of “trial and error” results in giving extra stress to the cable due to the continuous high voltage impulses sent out from the surge generator. Page /7 2016 China International Conference on Electricity Distribution (CICED 2016) Fig.2 Measurement graph Xi’an, 10-13 Aug, 2016 Fig.3 Example of CFL Unit (up to 4kV) - STG600 Fig.1 SIM/MIM graph sequence, display of automatically measured Multiple Sequence Multiple Impulse Method (MIM) is basically a much more advanced development of the Secondary Impulse Method (SIM). The big advantage of MIM is that it can monitor a wider timeframe of the fault condition which described earlier. Therefore, no manual trigger delay time adjustment is needed and no “trial and error” is required any more. B. Impulse Current Method (ICM) The previously mentioned cable fault pre-location methods based on a TDR impulse are generally affected by either damping of the signal in very long cables or reflections from the joints along the cable. The damping influences can be caused by corrosion of the cable sheath or any other influences in the joint. In other words, anything which has an influence to the length resistance of the cable can affect the damping. In very long cables the natural damping of the cable may cause the TDR impulse to be damped off before returning back to the Time Domain Reflectometer. Thus, the TDR (or SIM/MIM) method is not always successful. Impulse Current Method (ICM) can be applied in such conditions. When a surge generator releases a HV impulse, it causes a flashover at the fault point. This flashover/discharge causes a transient current wave travelling along the cable sheath between the surge generator and the flashover point. This current wave will repetitively travels back and forward from the fault point to the beginning of the cable. The interval of this wave/pulse can then be determined as the fault distance. For the coupling unit, an inductive coupler (SK1D) is connected to the sheath of the Surge Generator (SSG) cable. The Time Domain Reflectometer (IRG 2000 or IRG 3000) can then be set to automatic adjustments/settings and display the graphs on the screen. Since the pulse width of the transient current wave is very wide, ICM method is more suitable in long cables. CICED2016 Session 1 Paper No FP0731 Fig.4 Example of Cable Fault Location Unit (up to 32kV) - Syscompact 2000 with IRG2000 portable and vehicle mountable version C. Bridge Method Cable faults happening between two defined cores can usually be pre-located using the reflectometry method. Certain cable structures allow cable faults to happen between a core to outer sheath and then to soil. Unshielded cables such as high voltage DC cables used in railway supply, low voltage cables, signal cables or so called pilot cables, faults mainly happen between the core and the surrounding soil. As the cable ground or metal screen cannot be accessed anymore, the theory of reflectometry is no more useful. The reflectomerty method can only travel when there are two parallel conductive paths. Cable sheath fault usually means a defect in the outer protective PVC insulation. Cable sheath faults do not directly influence the electrical performance of a shielded cable, but it will have a negative effect in long term operation of the cable. The damages of the outer sheath enable water from the surrounding soil to penetrate into cable. Corrosion of the cable sheath as well as development of water trees will lead to sooner cable breakdowns. Therefore, according to IEC 60229, protective oversheaths have to be tested and fault repair has to be done to ensure the long term performance of the cable. These kinds of cable faults can only be pre-located by using a measuring bridge. Bridge methods are basically used for pre-location of low resistive faults. By using a high voltage source that is integrated in the latest generation of measuring bridge instruments, even high resistive faults can be pre-located. Page /7 2016 China International Conference on Electricity Distribution (CICED 2016) Fig.5 Measuring circuit according to Murray (left) and Glaser (right) Xi’an, 10-13 Aug, 2016 Electromagnetic Trigger and Acoustic Trigger. The two signals have different propagation velocity. Further the distance to the fault influences the difference in triggering the acoustic trigger and electromagnetic trigger. The digital receiver automatically converts the measured time (propagation time) from the fault location and indicates the distance with a digital meter. According to the meter indication, where the distance indication is the nearest, the fault position can be found. By detecting both electromagnetic and acoustic signal, the exact final location of the fault position can be determined. This special feature increases the performance compared to other ‘acoustic only’ pick-up sets. Furthermore, the electromagnetic indication can also assist in doing simple cable tracing. Fig.6 Example of 10kV Cable and Cable Sheath Testing and Fault Location System - Shirla III. PIN-POINTING METHODS A. Acoustic Method For cables laid inside conduits, which have similar characteristics with direct buried cables, pin-pointing high resistive and intermittent faults are often difficult. Acoustic method can be used to pin-point the exact fault location. As signal source, a surge generator is used in repetitive pulsing mode. High energy pulses from a surge generator (SSG) create a voltage pulse to travel along the cable. Flashover happens at the point of the fault. These repetitive noises/sounds are detected from the surface of the ground by using a ground microphone, receiver and headphone. The closer the distance from the fault to the microphone, the higher amplitude it will detect from the flashover noise. If it is placed straight on top of the fault position, the highest level of flashover noise can be detected. Using a higher energy surge generator would have a better effect on creating higher flashover noise. The flashover noise may vary depending on how deep the cable conduits are buried, but the flashover noise is usually audible even when the cable fault is inside the conduit. The acoustic fault location set comprising the receiver (UL30) and the ground microphone (BM30) contain the special feature of digital propagation time – distance measurement. Firstly, the ground microphone measures the electromagnetic signal that can be recorded all along the cable where the HV impulses are travelling. As this signal is available all along the cable trace towards the fault, it can further be used to make sure that the “cable trace” is followed. The maximum signal can confirm that the position of the ground microphone is now directly above the cable. Secondly, the ground microphone detects the flashover noise coming from the fault via the ground when it gets closer to the fault location. Therefore, every flashover activates two different triggers, CICED2016 Session 1 Paper No FP0731 Fig.7 Schematic connection and shape of acoustic signal – acoustic fault Fig.8 Electromagnetic signal along the whole cable, acoustic signal at point of flashover Fig.9 Example of signal pick up set BM30 and UL30, UL30 digital receiver indicates electromagnetic and acoustic signals and the distance to the fault location (e.g. 6.9 meters). B. Step Voltage Method Using a surge generator would not be able to create a flashover at the fault point when the cable faults have direct contact to the soil or ground. When there is no acoustic signal or no flashover noise is audible, cable fault pin-pointing using the acoustic method is not Page /7 2016 China International Conference on Electricity Distribution (CICED 2016) possible. This condition is mainly resulting from a completely burnt cable fault that is furthermore resistive to the surrounding soil. These kinds of cable faults can then be pinpointed by using the step voltage method. Faults in low voltage cables as well as pilot cables (signal lines) are often difficult to pin-point, this is because the high voltages that applied to the cables do not have sufficient surge energy to create a strong audible flashover. As these cables are mainly unshielded, cable faults in most cases appear to have a direct contact to the surrounding soil. The step voltage method becomes more suitable in such case. Another fault type showing similar conditions is the cable sheath fault. As there is no definite potential point, a fault at the outer protective PVC insulation of a XLPE cable cannot be located via the acoustic method. Accordingly, it is again difficult to create a strong flashover noise. The step voltage method enables the localization of the cable sheath faults. This method also enables to locate several sheath faults along a cable. In here, two potential earth probes are used together with the receiver. As soon as the earth probes are placed on the ground along the cable trace, the potentiometer in the receiver will point to the direction with higher potential which is the fault point. Follow the direction until the potentiometer points to zero. The fault position is then determined. Fig.10 Step voltage method, two earth probes connected to KMF1 Receiver i. Pin-pointing of cable sheath faults - for cables laid in Short PVC conduit Cable conduits usually contain water or humidity inside. Therefore sheath faults can be pre-located and pin-pointed. The leakage current exits from the nearby conduit joints and creates voltage funnels that can be detected and located. For pin-pointing, it is important to use low current or voltage setting of maximum 20-30mA or 1kV for pulse output. This enables to keep the signal only in the close area from the fault. Too high current setting can cause voltage funnels to appear at every conduit joint over a long CICED2016 Session 1 Paper No FP0731 Xi’an, 10-13 Aug, 2016 distance. Fig.11 Example of cable fault in short PVC conduit ii. Pin-pointing of cable sheath faults - for cables laid in Long PVC conduit Some cables and conduits are laid by using ‘Direct Horizontal Drilling’ method. In this case, the length of the conduit can range up to 100m or above. An additional tool will be needed to pin-point the cable fault. One of the popular ways is to use a conduit rod combined together with the step voltage method. Knowing the fact that cable conduits usually contain water or humidity inside, step voltage method can be used in a same way to find the point of leakage from the conduit joints. This particular section has to be excavated once the leakage point is found. Once excavated, it is necessary to cut out an opening from the PVC conduit. A voltage pulse generator can then be used to inject a sequence of voltage pulses (100-200V) into the cable sheath from one of the cable ends. For safety reasons, it is no more required to inject high voltage pulses and it is still necessary to wear a pair of high voltage protection gloves during this stage. A conduit rod should then be prepared according to the length of the conduit. The receiver cables and conduit rod should be tied together approximately every 1-2m apart. It has to be noted that the conduit rod and the first receiver cable are aligned in the same position and the tip of the receiver cables are exposed. The conduit rod can then be slid inside the PVC conduit. While the conduit rod is sliding in, it is required to read the potentiometer in the receiver. The conduit rod should be slid inside until the potentiometer points to zero. The fault position can then be determined. The conduit rod should be marked to make sure how much distance has been pushed into the conduit. The conduit rod can then be pulled out and the distance can be measured, this should be the distance until the cable fault location. This method should be very convenient to pin-point cable faults inside long PVC conduit. Page /7 2016 China International Conference on Electricity Distribution (CICED 2016) Xi’an, 10-13 Aug, 2016 REFERENCES [1] “Cable Fault Location in LV, MV and HV Underground Cable Networks Practical Experience, Author: Ing. Tobias Neier Version 2, 01-2013” BAUR GmbH, Austria. [2] “Shirla Application Guide - All in one - Fully Automatic Cable Sheath Testing and Fault Location System, Author: Ing. Tobias Neier Version 1, 04-2010” BAUR GmbH, Austria [3] IEC Standards IEC 60229, Edition 3.0 2007-10 “Electric cables – Tests on extruded oversheaths with a special protective function” Fig.12 Application of using Receiver & Conduit Rod in long PVC conduit Fig.13 Example of KMF1 Receiver (left), Conduit Rod (middle), Photo of cable fault in conduit (right) IV. CONCLUSION There are many ways of using different technics to locate cable faults inside conduits. By knowing and applying different cable fault location technics, it would become easier and faster for the electrical technicians to locate cable faults even when the cables are laid inside conduits. The important points are to understand the environmental conditions, for example soil conditions, weather conditions, road conditions, etc., as well as understanding the cable fault conditions. Furthermore, using the correct equipment can shorten the time to find the cable fault location. ACKNOWLEDGMENT The authors would like to give special thanks to Mr. Chew Chin Seang from CJ Hi-Tech Sdn Bhd Malaysia who supported us on providing useful experiences and technics on pin-pointing cable faults in different circumstances. CICED2016 Session 1 Paper No FP0731 Page /7