One of the major thermal power plants from Northern India was living with the problem of an undetected cable fault for over two years on their LT Cable. Since this cable was used for Street lighting and had a backup line which used to power up the street lights, this faulty cable went into the background. The officials did their best to clear this fault, for which they took the help of local vendors, but all 3 vendors failed to detect the fault and clear the line. These local vendors used the conventional set-up of the Surge Generator & Pin-Pointer to locate the fault but failed to pinpoint the fault. Determination of the type of fault with proper diagnosis and measurement of fault distance before pinpointing plays an important role. It avoids the unnecessary stress the cable has to go through because of High Voltage surging or thumping on the Cable Under Test (CUT).
After two years of fault occurrence, SCOPE T&M supplied a Surge Generator, Prelocator, Audio Frequency Generator (Route Tracer), and Pinpointer to the thermal power plant. SCOPE Team was challenged to locate the above-mentioned long pending fault during the commissioning of the products. SCOPE Team came out with flying colours.
In this blog, we are going to discuss the details of cable fault analysis and the correct approach for critical faults applied by SCOPEs trained representatives.
Here are the cable details and initial assessment done before the test.
Cable Under Test (CUT) details
| Cable Insulation Type | PVC |
| Cable Rating | 1.1 kV |
| Cable Length | Approx 100m |
| Other Information | 4 core, 95 mm2 |
Diagnosis from End 1 (R, Y, B: Three phases, N-Neutral, A-Armour)
| Sequence | Continuity | Ohmmeter | IR Tester = 5kV | TDR Results | |
| R | A | Ok | Infinite | 600GΩ | 120m Open, 89m Partial Open |
| Y | A | Ok | Infinite | 1kΩ | 120m Open, 89m Partial Short |
| B | A | Ok | Infinite | 510GΩ | 120m Open, 89m Partial Open |
| N | A | Ok | Infinite | 2kΩ | 120m Open, 89m Partial Short |
| R | N | Short | 0 Ω | 0 kΩ | 89m Short |
| Y | N | Short | 0 Ω | 0 kΩ | 89m Short |
| B | N | Short | 0 Ω | 0 kΩ | 89m Short |
| R | Y | Short | 0 Ω | 0 kΩ | 89m Short |
| Y | B | Short | 0 Ω | 0 kΩ | 89m Short |
| B | R | Short | 0 Ω | 0 kΩ | 89m Short |
During initial assessment, it was observed that LT Cable had a short circuit fault. Please refer to our previous blog on the steps involved in the Short Circuit Fault Location.
Now we will discuss the actual fault location steps carried out by the SCOPE Team and their respective results.
- Prelocation Summary (R-N) – Refer earlier blog for detail understanding
| VOP (V/2) | 79m/µs | Total Cable length | 120m |
| Moisture | Not Found | T Branch | Not Found |
| Open Ckt Fault | Not Found | Short Ckt | 89m |
| Joint Location | Not Found | ICM Reading | No Reflection |
| SIM/ARM Reading | No Reflection | Voltage Decay Reading | No Reflection |
Actual Cable Length = Distance L shown on Prelocator – Prelocator’s Test Lead Length
= 91.68 – 2.00
= 89.68m
- Route Tracing – Earlier blog link
The audio Frequency Generator was connected between R-N Pair and the cable route was successfully located till 90 – 92 meters approx. with the help of Pinpointer Set using 1kHz frequency. After that audio signals were lost. As this pair has a short circuit fault at 89.63m, audio signals were lost after it. This fault-suspected zone was marked.
Cable Depth measured approx. 3 ft using the Triangulation Method.
- Pinpointing of the Fault – Refer earlier blog for detail understanding
Since the R phase to Armour pair had a higher resistance fault than other pairs, we applied a 4kV surge via a 32kV Surge Generator, and the breakdown of the fault was detected. The fault sound was detected with the help of a Pinpointer Set and pinpointed the exact fault point at approx. 90m from End 1 within the fault suspected zone.
Conclusion
After digging at the pinpointed location we found that all the phases are burned and melted. Due to the excessive surging by different testing service providers for longer durations without Prelocation and selection of proper pinpointing method, resulted in the burning and carbon formation at fault point. Refer to the below image.
Also, exclusion of the Diagnosis, fault distance measurement, Route Tracing steps and inappropriate approach for Cable Fault Location resulted in the failure of Cable Fault Location by testing service providers.
This test report shows that a complete diagnosis is necessary and important to decide the proper cable fault methods to reduce fault location time and unnecessary HV Stress on the cable.
Also this blog proves that the Dead Short circuit fault can be pinpointed with the help of Audio Frequency Method (Route Tracing).
For more information on SCOPE cable fault locator capabilities, please visit https://www.scopetnm.com/test-and-measurements. We also invite you to e-mail us at marketing@scopetnm.com for any queries, product quotations or services.
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