Prelocation of Cable Faults using Time Domain Reflectometer Method

In a three-part series on underground cable fault location, we discussed various aspects of cable fault location – from types and causes of cable faults, methodology for identifying cable faults and finally, we illustrated the step-wise process for detecting faults in underground cables.  We also introduced the Time Domain Reflectometer (TDR) method for identifying underground cable faults and discussed a critical aspect of this method; i.e., the need to ensure that the current velocity is applied. In this blog post, we will expand more on the TDR method, building on the knowledge shared in our previous blogs and thereby showcasing how the location of underground cable faults can be accurately identified.

Reviewing the TDR Method

 Time Domain Reflectometer (TDR) is used to detect low resistance, short circuit, ingress of moisture and open circuit faults. It can also be applied to test cable length and joints. The method works on the basic principle of radar. A low voltage, high-frequency pulse from the prelocator is injected into the cable under test. If the cable has constant impedance and is properly terminated, all the energy will be absorbed. If the pulse faces an impedance discontinuity/mismatch due to a fault present on the cable, part or all the pulse energy will be reflected back to the instrument. The reflected pulse will have positive or negative amplitude depending on whether the fault impedance is greater than the characteristic impedance of the cable. The transmitted pulse and reflected pulse are plotted against the time on the display, like an oscilloscope. The time taken by a pulse having a particular velocity of propagation for returning to the source end is the measure of the fault distance and the shape of the reflected pulse will help to understand the nature of the fault.

The magnitude of the reflection at the fault point is calculated with the help of the reflection coefficient “ρ”.

ρ= (Z_f – Z_c) / (Z_f + Z_c)

Where,

Z_f is fault impedance and Z_c is the characteristic impedance of the cable.

The value of ρ ranges from +1 (Open Circuit Fault) to -1 (Short Circuit Fault).

It should be noted that the velocity of propagation (V) is constant for a particular cable and depends on the cable dielectric/insulation material, insulation thickness, conductor size, conductor resistance manufacturing process, etc. and this ‘V’ needs to be configured in prelocator for distance measurement. We request the reader to review our blog on the importance of velocity of propagation in TDR method for more details

The instant of the pulse transmitted, till the time received from pulse reflected back, the time period Δt stands for the time period trip, forth and back from testing point to fault point.

The fault distance is supposed to be L_f, the Velocity of propagation (VOP) is V, and the fault distance will be:

L_f =  (V * Δt)/2

Limitations of TDR Test

While the TDR test method is a good way to accurately pinpoint the location of the underground cable fault, there are some inherent limitations of this method. These are:

• The TDR Test can be performed on a cable with two mechanically parallel conductors only
• Insulation failure/puncture/pinhole type of faults cannot be located with low voltage TDR
• TDR test cannot locate sheath to ground faults on the cable
• For proper interpretation of results, a history of the Cable system is required
• For accurate results of fault distance, the proper velocity of propagation setting is required

Step-wise connections to be done for the TDR test

• Step – 1: identify the wanted cable terminations at both ends. Discharge all the cores for soft discharge followed by hard discharge with a tested discharge rod
• Step – 2: remove cable ends from terminals and bring them in the air, suitably away from each other
• Step – 3: ensure the Armour / Sheath of the Cable is grounded from both ends
• Step – 4: check short circuit fault using continuity test, with the help of multimeter, by keeping both ends open
• Step – 5: short all cores with Armour / Sheath of the cable at the far end and check open circuit fault using continuity test, with the help of multimeter at test end
• Step – 6: connect prelocator to the healthy core of the cable under test with help of the TDR method, determine the total cable length (Refer to the table of TDR Graphs for connections)
• Step – 7: connect the prelocator to the faulty core (open circuit or short circuit fault), to determine the fault distance (Please refer to the table below on TDR graphs for connections)

Operations

• Step – 1: After successful connections, select the TDR mode in prelocator instrument
• Step – 2: set ‘Velocity of Propagation’ in the parameters setting as per the cable under test
• Step – 3: select the proper settings of pulse amplitude, pulse width, amplification/gain, output impedance (balance) etc.
• Step – 4: start the test, wait till the reflections are displayed on the screen and stop the test
• Step – 5: perform cursor measurements and analyse the resulting waveform (Please refer to the table below on TDR graphs for connections). If necessary, save the waveform into the device’s internal storage
• Note down the observations in Cable Log Sheet

Conclusion

In our blog introducing the concept of underground cable fault location, we noted the criticality of underground cables in the electricity grid. Long outages on account of cable faults can cause major revenue loss to electricity utilities and cause major inconvenience to customers. The TDR method is a good way to accurately determine the location of an underground cable fault.

For more information on SCOPE cable fault locating capabilities, please visit https://www.scopetnm.com/test-and-measurements or write to us at marketing@scopetnm.com. We also invite your attention to a set of webinars delivered by SCOPE product experts on underground cable fault locations, available here and here.