Cable Fault Location in LV, MV and HV Underground Cable Networks Practical Experience

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. During the last years, also high voltage lines have been developed to cables. To reduce the sensitivity of distribution networks to environmental influences underground high voltage cables are used more and more. They are not influenced by weather
conditions, heavy rain, storm, snow and ice as well as pollution. Even the technology used in cable factories is improving steadily certain influences may cause cables to fail during operation or test. Cables have been in use for over 80years. The number of different designs as well as the variety of cable types and accessories used in a cable network is large. The ability to determine all kind of different faults with widely different fault characteristics is turning on the suitable measuring equipment as well as on the operator's skills. The right combination enables to reduce the expensive time that is running during a cable outage to a minimum.


Content

1     Introduction ... 3
2     Cable types and their characteristics....................................................................... 3
3     Cable Faults... 7
4     Cable Fault Location Procedure .............................................................................. 7
4.1     Cable Analysis and Insulation Test .................................................................. 8
4.2     Cable Fault Types ............................................................................................ 9
4.3     Cable Connections HV and MV cables........................................................... 10
4.4     Cable Connections at LV cable networks ....................................................... 12
4.5     Grounding conditions...................................................................................... 13
4.5.1      Normal grounding conditions................................................................... 13
4.5.2      Ground conditions with high earth resistance ......................................... 14
5     Cable Fault Prelocation ......................................................................................... 15
5.1     Overview... 15
5.2     Impulse Reflection Method (TDR) .................................................................. 16
5.3     Multiple Impulse Method (SIM/MIM) ............................................................... 18
5.4     Impulse Current Method (ICM) ....................................................................... 24
5.5     Decay method................................................................................................. 27
5.6     Differential Impulse Current Method / Differential Decay Method .................. 29
5.7     Bridge Method ................................................................................................ 32
5.7.1      Principle of the Wheatstone circuit.......................................................... 33
5.7.2      Measuring circuit according to Murray .................................................... 33
5.7.3      Measuring circuit according to Glaser ..................................................... 36
5.8     Burn Down Technique .................................................................................... 38
5.9     Cable Fault Location Systems ........................................................................ 38
5.9.1      Fault location system for low voltage networks ....................................... 38
5.9.2      Fault location system for medium voltage networks ................................ 39
5.9.3      Fault location systems for medium voltage networks .............................. 39
6     Cable Route Tracing.............................................................................................. 40
6.1     Coupling of Audio Frequency Signal .............................................................. 40
6.2     Signal detection .............................................................................................. 41
6.3     Selection of Audio Frequency......................................................................... 42
7     Cable Fault Pin-Pointing........................................................................................ 43
7.1     Acoustic Fault Location .................................................................................. 43
7.1.1      Acoustic Fault Location in direct buried cables ....................................... 43
7.1.2      Pin-pointing of cable faults in pipe arrangements ................................... 44
7.2     Fault pin-pointing of low resistive cable faults ................................................ 46
7.2.1      Step voltage method................................................................................ 47
7.2.2      Twist method ........................................................................................... 48
8     Cable identification ................................................................................................ 50
9     Practical cable fault location examples.................................................................. 53
9.1     Cable fault location in HV cables with cross bonding joints............................ 53
9.2     Cable fault location in airport lighting arrangements ...................................... 55
9.3     Cable Fault location in High Voltage cables e.g. 115kV................................. 58


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