HVPD Case Study | Offline TDR Testing of a 25kV Cable for Rapid Transit Organisation
Published 21 May 2021
-
Special thanks to Harriet Robertshaw at HVPD for the kind permission to republish
Medium Voltage Project
Following the visual identification of a fault in a 25kV cable, a Rapid Transit Organisation in the UK had a requirement to locate the presence of any further faults across its cable network, which was approximately 40 years old.
Offline Time Domain Reflectometry (TDR) testing was conducted across three sections of 25kV trackside PILC to pinpoint the locations of any further insulation degradation present.
Solution
- HVPD Pulse Generator
- Diagnostic Unit
Offline TDR testing was conducted from the known failure point of the cable, back to each cable sealing end termination using an HVPD Diagnostic Unit.
Offline TDR testing was also conducted on an additional cable section from an Overhead Line Equipment (OLE) switch. The cable was mapped using a Pulse Generator.
Background Theory
Offline TDR is performed by injecting a low voltage pulse onto the core of the cable under test. High changes of impedance within the cable will create reflections of the original injected pulse.
These areas of high impedance change can typically be attributed to either cable faults or joints and therefore analysis of these pulse reflections can indicate fault and joint locations. TDR can also be used as a tool to verify the total length of the cable if this is unknown.
The test is carried out by measuring the injected pulse return time (i.e. the time for an injected pulse to propagate from one cable end and back) in order to provide the locations of changes in impedances, such as cable joints and the end of the cable. TDR testing is utilised to provide accurate fault location data in cables.
Background Theory
Results
Initial results showed that there were no visible impedances relating to cable joints or faults in the cable section located from the original failure point back to sealing end terminations.
However further testing of the cable section leading back to an OLE switch demonstrated a large impedance change not associated with any known joint location. This impedance location was observed to be approximately 239 meters from the test location.
Following this observation, a visual inspection along the identified cable section route was performed. Upon lifting the cables covers at approximately 241 meters along the cable from the test point a visibly clear cable fault was identified.
TDR Trace
Inspection of Cable Fault Location
Conclusions and Recommendations
- Offline TDR testing revealed an unknown cable fault, which had the potential to cause further risk of an in-service cable failure
- The cable section with the fault was subsequently removed and cable repaired
- Further TDR testing is required on this cable and additional cables in the network to determine if any other points of degradation are present
About HVPD
HVPD supplies a comprehensive range of equipment to detect and monitor On-line PD for the condition assessment of in-service power cables, switchgear, motors and generators, and transformers rated at 3.3kV and above. Our PD testing and monitoring technology provide you with an early warning of developing insulation faults, helping you to avoid costly failures and resulting unplanned network outages.
To find out how we can test and monitor your cable network for PD and locate any faults, terminations or joints, get in touch today at [email protected] or call us on 0161 877 6142.
Thorne & Derrick
Specialist Electrical Distributor
Established since 1985, T&D International based in the UK distribute the most extensive range of LV, MV & HV Cable Jointing, Terminating, Pulling & Installation Equipment – contact us today for a competitive quotation.
Key Products : MV Joints & Terminations, Access Chambers, Cable Cleats, Duct Seals, Cable Transits, Jointing Tools, Feeder Pillars, Cable Duct, Earthing & Lightning Protection, Electrical Safety, Cable Glands, Arc Flash Clothing Protection & Fusegear.
Distributors for : 3M, Pfisterer CONNEX, Nexans Euromold, Elastimold, Catu, Roxtec, Emtelle, Centriforce, Lucy Zodion, Alroc, Cembre, Prysmian, Ellis Patents, ABB & Furse.
T&D Providers Of Jointer Training Courses By Pfisterer CONNEX & Nexans Euromold
