Thursday, August 25, 2016

Fault locating methods for High and low tension cables

In electrical systems its usual that cable may get faulty there are following methods of checking the type of fault in cable:-
1.      Using Meggar for LT Cables:-
Meggar is most commonly used method for checking the fault in cables. If Meggar is used at 500V/ 1000V DC then cable shows infinite value If cable is Ok.  if value lies above 100 Mohm then cable is also acceptable it indicates only that there cable is having some moisture which will be dried out when cable is put on load but if value falls below this value upto 50 Mohm then it will be used where there were light loads.

If there is value is near to zero than cable should not be used as if cable insulation level between two leads is zero this means cable is short circuited. If Cable insulation with respect their leads is ok but between lead and earth is zero than this indicates cable get short circuited with armored.
In Meggar we can check resistance value also in this 2 leads one is placed on resistance and other is placed on common. 500/1000V is applied to two terminals of cable and resistance value is measured.

2.      Using Hi-Pot For testing High tension cables
Hi-Pot test is used for testing High tension cables in this test DC voltage is applied to cable for 5-15 min’s. According to IEEE-400 hi-pot voltage for a 15-kV class cable is 56 kV for an acceptance test and 46 kV for a maintenance test.

There are fault locators which are used to identify the location of fault in a cable as Most of HT cables are laid underground so it becomes very much necessary to find out the location of fault as in this case cable replacement will not be easy and also it will leads to lot of cost of replacement of whole cable.

1.           Divide rule:-
This method is very old one but still used in LT system, in this method cable is cut
down into 2 pieces and check for insulation resistance one piece of cable will give perfect value other will give low values so higher valued cable will be used and other faulty part will be again tested used divide rule.

    2.  High voltage test:-

This is another old method, in this method high voltage is applied to faulty cable this will leads to loud noise which you can hear above ground. In method very high voltages upto the level of 25-32 KV needed to be applied to cable so that noise can be hear able.   This will also leads to heating of cable as high currents generated during that process. Thus it will cause degradation of cables.

     3. Time domain reflectometry 

This is new technology for detection of faults in under ground HT cables. In Time domain reflectometry  there is a device which sends a low-energy signal through the cable if cable is perfect than cable returns that signal in a known time and in a known profile. 

There following limitations of TDR:-
(i)         It does not Provide accurate location of  pinpoint faults. This method is accurate upto 1% of testing range.
(ii)        It cannot see faults-to-ground with resistances much greater than 200 ohms. So, in the case of a "draining fault" rather than a short or near-short, TDR is blind.

     4. High-voltage radar methods:-

There are following types of this method:-

(i)         Arc reflection
(ii)        Surge pulse reflection
(iii)       Voltage pulse reflection.
Lets discuss them one by one :-

(i)      ARC Reflection Method:-
This method consists of  TDR with a filter and thumper (High voltage).  In this method filter is used to limit current and voltage due to surge on cable which means minimal stress to the cable. This method provides an approximate distance to the fault

(ii)     Surge pulse reflection method
This method uses a current coupler and a storage oscilloscope with a thumper. The advantage of this method is its superior ability to ionize difficult and distant faults. Its disadvantages are that its high output surge can damage the cable, and interpreting the trace requires more skill than with the other methods.

(iii)    Voltage pulse reflection method
This method uses a voltage coupler and an analyzer with a dielectric test set or proof tester. This method provides a way to find faults that occur at voltages above the maximum thumper voltage of 25kV.