Cable Fault Location by Baur (Sources, Types & Pre-location Methods) - Part 1 of 3

By Chris Dodds on 22nd February, 2016

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Cable Fault Location by Baur (Sources, Types & Pre-location Methods) - Part 1 of 3

 

 

 

 

 

 

Cable Fault Location On Low, Medium & High Voltage Cable Networks by Baur

Baur Cable Fault LocatorsBaur cable fault locators, cable sheath testers and cable/phase identification systems cover the entire cable fault location process from pre-location to pinpointing - T&D are Baur Distributors, contact us with your requirements.

In the first of a series of Guest Blogs by Baur we learn in depth all about cable fault location.

Part 1: Introduction to Cable Fault Location by Baur (Sources, Types & Pre-location Methods)

  • About the process
  • Fault sources and types
  • Pre-location methods

Part 2: Cable Fault Location by Baur (Connecting Measurement Technology)

  • Connecting the most varying systems
  • Earthing
  • Safety aspects

Part 3: Cable Fault Location by Baur  (TDR Time Domain Reflectometry)

  • Method and prerequisites
  • Reflection factor
  • Tracing low-resistive faults

Cable Fault Location by Baur (Sources, Types & Pre-location Methods)

Faster cable fault location

The ratio of overhead lines in power distribution networks has gradually dropped. Even in the transmission network, underground cables are becoming more commonplace.

At the medium voltage level alone, about 30 million kilometres of MV cables are being used around the globe*. This corresponds to 750 times the length of the equator.

*Literature: Lehner, Gerstner, Buschjost, Haag, “Cable database”, 2011, BAUR Prüf und Messtechnik GmbH, A-6832 Sulz, 37-50

Many countries, e.g. the Netherlands, have almost no overhead lines anymore. In addition to aesthetics and higher acceptance by citizens, underground cables offer certain technical benefits.

Underground cables are protected from wind, weather and frost, and consequently remain unaffected by storms and snowfall. However, underground cables are not entirely without faults.

Insulation and cable accessories (joints and terminations) can lead to service failures. The more frequent causes for cable faults include external influence, especially during construction work. In such cases, the location and type of the cable fault are usually detected quickly.

However, external influence is responsible only for approx. 45% of all cable faults. For the rest, the underground hidden faults need to be identified and located as soon as possible - a task that smaller distribution network companies are confronted with regularly.

In big cities, cable fault location and rectification are still a fact of daily life, because about eight cable faults occur every year per 1000km of medium voltage cable.

Locating cable faults is complex.

Firstly, networks historically comprise all kinds of cables and cable accessories. Oil-insulated paper pulp cables and plastic-insulated cables (XLPE) exist in the same network.

Both the cables and the accessories are of varying ages, come from different manufacturers and are subject to different loads. Consequently, you will not find “typical” cable faults.

About eight cable faults occur every year per 1000km of medium voltage cable

Cable Fault Location by Baur (Sources, Types & Pre-location Methods)Cable Fault Location by Baur (Sources, Types & Pre-location Methods)Cable Fault Location by Baur (Sources, Types & Pre-location Methods)

Almost every LV, MV or HV cable fault is unique. This makes it all the more important to have a strategy that helps determine and locate faults in as short a time as possible. 

Besides the right measurement technology, having information about possible cable faults, their probability and the fault tracing methods are also helpful.

Whether aged insulation, moist cable joints or defective cable sheaths - with the right strategy, you will be able to trace the fault more quickly.

The result: less pressure, more satisfied power customers. And even controllers and traders will be happy with faster cable fault rectification, as it minimises failure times in the network statistics.

This in turn is important for defining the network usage charges and subsequently for the profitability of the entire company.

Well-planned is half won

Cable fault location is a complex task. Cable faults depend on cable type, network structure, voltage levels and ambient conditions.

Therefore, almost every cable fault is unique. So there is no “typical” cable fault. But there are probable and improbable faults, depending on the age, voltage levels, loading capacity, accessories and type of assembly.

Cable Fault Location by Baur (Sources, Types & Pre-location Methods)

Even the cable insulation type is important, because cable fault causes and fault traces in plastic insulated cables differ from those in cables with paper pulp or paper-oil insulation.

Regardless of the cable type - besides external influence, e.g. damage caused during earth works or earth displacements - the most frequent fault causes include:

  • Ageing or service life of the cables
  • Overvoltage
  • Thermal overload
  • Cable corrosion
  • Improper cable laying
  • Processing faults
  • Cable damage during transportation and storage

The most common types of faults are:

  • Short-circuits (earth faults)
  • Cable breaks
  • Intermittent cable faults (occurring temporarily)
  • Cable sheath faults

Evaporated core of a 132 V XLPE cable due to a cable fault

Evaporated core of a 132 kV XLPE cable due to a cable fault.

Exploded 132kV cable joint after cable fault

Exploded 132 kV joint after cable fault.

With the right strategy, you will be able to trace the cable fault more quickly.

A short-circuit is a low-resistive connection of minimum two conductors. In the case of an earth fault (also called short circuit to earth), there is a low-resistive connection to the earth potential.

It can occur in a defunct network or in an operational isolated network, as well as due to an earth fault in an earthed network. One talks of double earth faults when two conductors have earth faults at different locations.

In this case, a short-circuit current is formed in the relevant conductors and in the earth impedance. Cable breaks are mostly the consequence of mechanical damage or earth movements.

Intermittent cable faults are faults that occur only temporarily and/or under specific load conditions. Another cause is the drying out of oil-insulated cables. In plastic insulated cables, ageing or electrical trees lead to such temporary faults.

Cable sheath faults often exhibit a lag in their reaction, because leaked moisture causes delayed insulation damage.

Restricting faults step by step

A cable fault can be traced in two steps. During pre-location, the fault is determined precisely on the meter.

The subsequent pin-pointing is used to determine the fault precisely so as to minimise earthwork as much as possible and thus reduce repair time.

Next comes cable identification, as it is necessary to identify the defective cable in a bundle of multiple cables at the fault location. This is especially important if the fault is not visible from outside.

Safety first

Before beginning the work on site, it is necessary to ensure that the relevant cable route is disconnected (currentless).

Complying with the respective safety measures, such as cleaning the seals, suitable earthing, maintaining safety distances, etc. is absolutely necessary.

Starting well informed

Before a pre-location, you should already have information on the cable route: cable type, position, date of installation, repair history, cable joint types used, subsequent branches and a lot more information is useful while locating faults. In particular, it is also helpful to identify the position of cable joints.

Information about sagging (due to water collection), crossings, etc. also helps. Obviously, the type of error message is important. Mechanical damage is often reported while looking for causes - fault analysis and location in such cases are relatively simple.

However, mechanical external influence is responsible for only half the faults. So with most faults, the analysis is more complex.

Before beginning pre-location, you should already have information on the cable route.

Cable fault pre-location

Cable fault pre-location

Earthworks

Earthworks

Identification of the faulty cable line

Identification of the faulty cable line

Proceeding in an organised manner

Cable type, cable length and network structure are verified on site. Initial measurements are used to determine the insulation resistances of all phases of the cable.

This allows you to distinguish between low-resistive, high-resistive or intermittent faults and the faulty phase is identified. Finally, you need to choose the pre-location method.

The following methods that we will present to you during the series are often applied:

Method Applicable for These Fault Characteristics
Impulse Reflection Method (Time Domain Reflectometry):
Determination of cable end, cable section, localisation of a cable joint by means of impedance change
Low-resistive fault, cable breaks
BAUR SIM/MIM (Secondary Impulse Method / Multiple Impulse Method):
Over 90% of faults can be positively prelocated with this method alone.
High-resistive cable faults, intermittent faults
Impulse Current Method (ICM) High-resistive faults, intermittent faults (in long cables)
Decay Method Intermittent cable faults with high breakdown voltage
Burning (Burn Down Method - to change the fault characteristics) High-resistive faults, wet cable joints
Bridge Measurement High-resistive faults, low resistive faults
Sheath faults

 

Before we present the individual methods in detail, we would like to first focus on the basics of each fault location: the preparatory connection and earthing measures.

They ensure safety, and are also essential for successful fault location for the purpose of reliable measurement results. Over 90% of faults can be positively located with the SIM/MIM method alone. 

 

Thorne & Derrick Specialist Electrical Distributor

Established since 1985, T&D distribute the most extensive range of LV, MV & HV Cable Jointing, Terminating, Fault Location & Installation Equipment - contact us today for a competitive quotation.

Key Products : MV Joints & Terminations, Cable Fault Locators, Cable Cleats, Duct Seals, Cable Transits, Underground Cable Protection, Jointing Tools, Feeder Pillars, Cable Duct, Earthing & Lightning Protection, Electrical Safety, Cable Glands, Arc Flash Protection & Fusegear.

Distributors for : Baur, 3M, Pfisterer, Nexans Euromold, Elastimold, Catu Electrical, Roxtec, Emtelle, Centriforce, Lucy Zodion, Alroc, Hivotec, Cembre, Prysmian, Ellis Patents, ABB & Furse.

Cable Joints Terminations Tools Cleats

 

 

  • Further reading :

Video Tutorial Masterclass - PD Partial Discharge & Electrical Cable Breakdown

Condition Assessment of HV Cables - Guest Blog by EA Technology

"Reducing Failure Rates and Better Management of Underground Cable Networks"

Portable Partial Discharge Diagnostics - Baur PD-TaD 60 PD Diagnostics System

 

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