Power Cable Fault Location Techniques by Rodolfo R Penalosa

Power Cable Fault Location Techniques - Overview &

Test Instrument Applications

By Engr. Rodolfo R. Penalosa, PEE, PECE, APEC Engr. President, Westco Electrical & Equipment Corp.

Former: Chairman, Board of Electrical Engineering, PRC Former: Chairman, Technical Panel for Engineering &

Technology, CHED

Power Products, (c) SebaKMT 2007, all rights reserved 1

POWER CABLE FAULT LOCATION

•  External damage •  Poor workmanship •  Earth subsidence •  Overload •  Sheath corrosion •  Lightning strikes & over-voltage events •  Manufacturing faults (Check test sheet) •  Aging •  Damage caused by termites or rodents

POWER CABLE FAULT LOCATION

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• Types of faults

• Ground • Sheath

•  L1 •  L2 •  L3

• Ohmic resistance Faults • core – core (shunt) • core – sheath (shunt) • sheath – ground • core – ground • breaks in core or sheath • ohmic series faults • flashover faults • penetration of moistre

•  Most failures appear between 2.5… 5 kV.

Two main types of fault

• 1. The series or open circuit conductor fault where the fault resistance is in series with the line • 2. The Shunt or Short insulation fault where the fault path is from conductor to ground or other conductors.

Choose the right approach based on the fault characteristic:

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• Basics of Reflection Measurement Technology • Overview

• Zfp > 10 Z Zfp< 10 Z

Zfs> Z 1

10

Fault Qualification

Prelocation

Pulse Echo Measurement

• (Reflectometer, TDR)

Short-term conversion - Arc reflection ARM®, ARM Plus®, Decay Plus®, KLV - Impulse Current in connection with SWG Surge Gen. - Voltage Decay in connection with HPG HV DC-tester Sheath Fault

• Pinpointing

Permanent Conversion

Burning 1.

3.

2. 4.

5.

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• Steps of Fault Locating - (1)

• TESTING

• PRELOCATION

• Objective: You identify cable defects and types of faults (e.g. damage of insulation or faulty joints

• Objective: You determine the distance to the fault

• no • fault

• fault • identified

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• Objective: You pinpoint the location of the cable fault.

• TRACE LOCATION

• PINPOINTING

• CABLE

• Objective: You determine the cable path and depth.

• Objective: You select the target cable.

• Steps of Fault Locating - (2)

• ? • IDENTIFICATION

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• TESTING

• PRELOCATION

• Insulation resistance

• Arc Reflection Measure- ment (ARM Plus®, ARM®, Decay Plus®)

• Surge generator • Power seperation filter

• Reflectometer (TDR) also with • PC-database

• VLF-system • DC-voltage tester • AC-voltage tester

• Megger • Voltage Testing

• Reflection measurement

• Arc burning • Decay-method • Impulse current method

• Burn instrument • Power separation filter • HV DC-test instrument • capacitive voltage coupler • Surge generator • inductive current coupler • Sheath fault location unit • Bridge measurement

• Voltage drop method

• Methods • Instruments • Task

• Steps of Fault Locating • Methods and Instruments (1)

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• Accoustic method • Distance method • Step voltage method • Audio frequency method • Minimum turbidity method

• Surge generator • Mains surge switch • Digiphone • Sheath fault location unit • Cable locator • Cable locator

• Cable identification set • Intensity test • Polarity test

• CABLE TRACING

• PINPOINTING

• Cable and line locator • Null (Minimum)-method • Peak (Maximum)-methode • SuperMax, SignalSelect

• ?

• Steps of Fault Locating • Methods and Instruments (2)

• Methods • Instruments • Task

• CABLE IDENTIFICATION

FAULT BURNING

•  The conversion of high resistive unstable faults into low resistive stable faults is achieved by passing current through the fault using a powerful “cable burner set”.

•  A DC voltage from a regulated short circuit proof current source (“the Burner”) high enough to cause a flashover at the point of fault, is applied to the cable

•  The passing current heats up the insulation material causing carbonisation and eventually the formation of a conductive carbon bridge

•  The current fed into the fault is critical Ø Bringing “unstable” fault into “stable” fault

•  Burning should not be carried out thoughtlessly as it can jeopardise the condition for successful pinpointing using surge generators

Some Fault Burning Surge Generators:

-> Apply HV to carbonize the fault

Bridge Methods

-> For outer sheath faults

Bridge Methods Drawback

•  Requires access to both ends of the cable •  Requires the faulty conductor to be continuous. •  Requires at least on healthy conductor. •  All connections must be of low resistance. •  All lead lengths must be accounted for (in case of different

cable sizes and auxiliary leads the equivalent length per section needs to be calculated)

•  More time consuming than other terminal methods. Expert •  Subject to interference from stray and induced voltages.

Advantages of Bridge Methods are: •  High accuracy •  Applicable for situations were TDR techniques can not be

employed such as sheath faults

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• MVG 5

• MMG-5

• Connection for Insulation Fault Locating • with Bridge MVG 5

• Cable with Faulty Conductor

• Reference Conductor • (in same cable)

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Time domain Reflectometry (TDR) or Pulse Echo Test

Modern type test instruments:

Time domain Reflectometry (TDR) or Pulse Echo

•  The pulse echo unit sends very short pulses periodically into the cable

•  At the point of impedance change due to an irregularity in the structure of the cable such as an open end or a fault the pulses are reflected and return to the source

•  This time delay of the transmitted and reflected pulse is proportional to the fault distance

Time domain Reflectometry (TDR) or Pulse Echo

•  Knowing the pulse speed of propagation for a cable (V) and the time it takes for a pulse signal to travel from the signal generator to the fault and back (tx) the distance to the fault can be calculated by multiplying tx with V/2.

•  The propagation velocity depends mainly on the type of dielectric (eg Paper, XLPE, EPR, PVC) used and the physical construction of the cable. This factor varies between different types of cable.

•  The propagation factor refers to a measure of the speed at which a signal travels down a cable with respect to the speed of light in vacuum

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• Propagation Time and Distance

v

t x

l x

Propagation time of the impulse to the end of the conductor and back

Length of conductor

Propagation velocity of the electric impulse

2 v half of

propagation velocity

l = 2 x v

· t x

• L1 • L2 • L3 • Schirm

TDR Transmission Elements per circuit length

•  The ohmic resistance R of the conductor. •  The inductance L of the cable •  The capacitance C between the wires or between the wires and the screen. •  The conductance G of the insulating material between the wires

C ... G ... L ... R ...

parallel capacitance parallel conductance series inductance series resistance

23

Electric Line Model / Impedance

• G • C Z = R + j w · L G + j w C

•  General formula for calculating the characteristic impedance of the cable

•  Simplified formula for high frequencies: (e. g. for pulses)

•  The ohmic fault resistance is determined by •  The characteristic impedance of the cable is determined by

• . G R and

C L and

L C Z small large small

• thick conductor • thin insulation

L C Z large small large

• thin conductor • thick insulation

Z = L C

TDR Transmission Surge Impedance

If a section in the cable has an impedance Zx that differs from the surge impedance Zo of the cable, e.g. at the end of the cable, parts of pulse energy are reflected as soon as the pulse reaches this spot. The ratio between the reflected part and the part of the pulse that is travelling further can be described by means of the reflection coefficient

r


Open circuit

r = ZX - ZL

ZX + ZL =

∞ - ZL

∞ + ZL =

∞

∞ = 1


Short circuit

r = ZX - ZL

ZX + ZL =

0 - ZL

0 + ZL =

- ZL

ZL = - 1


Short circuit


High Resistance Fault

TDR Surge Impedance Joints

•  In a straight joint a positive deflection is due to the increase in inductance and the decrease in capacitance as the cores separate inside the joint. When the cores come again together towards the end of the joint Zo decreases causing a negative deflection as seen on the next trace

TDR Surge Impedance T-Joints

•  Each tee has a reflection coefficient of 33% •  This often means that the fault echo can not be

recognised on the screen.

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• Reflection Trace in a Branched Network (idealised)

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• Cable without fault

• Short circuit

• Parallel resistance

• Series resistance

• Break / Interruption

• Typical Reflection Images (measured)

• R • F

• R • F

• R • F

• Joint / Splice

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• m • m • m • m • m/µs • m/µs

Rparallel = 0 Ohm Rparallel = 10 Ohm Rparallel = 50 Ohm

Rparallel = 0 Ohm Rparallel = 100 Ohm Rparallel = 250 Ohm

• Typical Reflection Images (shunt)

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• Conductor-to-Sheath-Fault

• L1

• L3 • Good phase

• Faulty phase

• Comparison

• Difference

• E

• E

• L1→ E

• L1→ E • L3→ E

• L3→ E • /

• -

• L1 • L2 • L3 • Schirm

Transient Methods

•  Open circuit and low resistance shorts can easily be located using TDR methods but high resistance, flashing or intermittent faults can’t be detected

Transient Methods Arc Reflection

•  In the ARC refection method a TDR is coupled through a “power separation filter” to a high-voltage surge generator. The filter protects the pulse reflection instrument from the applied high voltage while allowing the low energy TDR pulse to pass

•  The power separation filter is used to shape the surge wave, extent the arcing duration and decouple the TDR from the surge generator

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• SWG

• M 219 and"• LSG 300

• Reflectometer

• Faulty cable

• with Arc

• without arc

• Voltage level • with ARM®:

• Up to max. 32 kV with • stand-alone units

• Up to max. 50 kV in a • test van (depending on • equipment)

• Arc Reflection Measurement (ARM®) • Passive coupling

• Without arc

• With arc

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• SWG

• Reflekto-meter

• Faulty cable

• Voltage level • with ARM®:

• Up to max. 32 kV with • stand-alone units

• Up to max. 50 kV in a • test van (depending on • equipment)

• Arc Reflection Measurement (ARM®) • Passive coupling

• M 219 and"• LSG 300

• Without arc

• With arc

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Reflectometer Figure Active ARM® Measurement

•  Initially a TDR trace without using a surge generator is stored. Then a TDR trace when the arc from the surge generator converts the fault into a low resistance fault, is also stored. Both traces are displayed on the screen. •  Advanced Options (Arc-Plus) are available for test vans)

Transient Methods Arc Reflection

•  Faults sometimes exceed the maximum output voltage of the surge generator. (Typically 30-kV)

•  Then arc reflection can not be used. •  Decay or Impulse current methods in combinations with commonly available

DC test sets can still be used to pre-locate faults.

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Reflecto- meter

• Faulty cable

Uncoupling"Unit

DC -"Tester

• Travelling wave display"

Decay Method -> Test Set-Up

• The cable is charged up with a HV DC test until flashover at the fault occurs. • A transient is generated which travels back and forth between the fault and the injection point at the start of the cable. A capacitive divider connected to the start of the cable measures this signal utilising a transient recorder

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Reflectometer Figure Decay

Cycle measurement:

lx = C2-C1

2

• C1 • C2

- lA

lA ... HV connection cable

The wavelength of resultant trace is constant and related to the fault distance, which is calculated L(fault) = t * V/4 where V is the speed of propagation for the cable

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• Reflectometer Figure Decay

Transient Methods Impulse Current

•  The impulse current method utilises the current transients, which occur when a cable breaks down on application of HV DC or a HV pulse

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Current Pulse Method -> Direct fault ignition

Lx

tx+t tx tx • Parasitic • Reflections

t .... Ignition delay

• Fault ignition

• The distance to the fault is determined from the time interval between successive reflections of the breakdown pulse •  Measurements can not be taken from the start of the travelling wave since breakdown is delayed by “ionisation delay”

2004-04-02 50

Lx

Current Pulse Method -> Fault ignition by an inward propagating wave

tx tx tL1

Parasitic Reflections

Parasitic Reflections

L1

• Fault ignition

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Fault Loaction using

Cursors

• Current Pulse Method

Jan08 53

•  Decay •  good for HV-fault locating at

higher voltages (Centrix up to 80 kV, Classic up to 130 kV)

•  faulty cable has to be “chargeable”, failing with a flash-over. Leakage current faults cannot be located

•  subtract connecting cable •  set TDR range to 5 or 10

times cable length

•  ARM •  most common HV-fault

locating method •  most details visible (joints,

cable end, ...) •  up to SWG-voltage

(typically 32 kV) •  connecting cable

automatically subtracted •  set TDR range to cable

length

• Comparison of basic HV-Prelocating Methods

•  ICE Impulse Current •  good for long lead cables and

faults in wet joints •  up to SWG-voltage

(typically 32 kV) •  measure length of one period •  subtract connecting cable •  set TDR range to 5 or 10 times

cable length •  don‘t consider first period

(includes ignition delay time) •  measured length may be

7 to 15 % too long due to varying v/2, depending on pulse ignition and shape

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Cable Location

Audio Frequency twist, minimal turbidity

Accoustic accoustical field, distance

Final Cable test

• no

Step Voltage Method DC or audio frequency

Repair

R < 100 R > 100

Pinpointing in Power Cables - Overview

Pinpointing Methods Surge Generator

•  Test in locality indicated by pre-location to confirm precise location of fault

Pinpointing Methods Surge Generator

•  Surge Energy in Joules=½ C*V2

•  Surge Voltage & Surge capacitance determine the Energy released

Pinpointing Methods Surge Generator Time Delay Technique

•  The magnetic signal arrives at the microphone

•  A timer measurement is started. •  When the acoustic signal arrives

at the microphone the timer is stopped and the time difference between magnetic and acoustic signal is displayed.

•  Above the cable fault the difference reaches a minimum

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• Pinpointing with Speeed-of-Sound Measurement • Distance method !

• lowest fault distance = • minimum number"

• start signal, triggered • by magnetic field

• stop signal, • triggered by sound

•  SWG

Pinpointing Methods Surge Generator Sound Volume depends on: §  Energy of the surge generator

§  The more joules the louder.... § The higher the voltage per range settings

§  Type of the faults §  low or high resistance faults, water etc. (no zero ohm faults)

§  Type of the soil §  For example sand or other loose reduce the loudness of the sound §  local area 1 – 3 m are high frequencies well audible §  long distance area 3 – 5 m are low frequencies better

•  Contact sensor to the soil §  Plate, tripod, spike

Pinpointing Methods Audio Frequency Methods

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• Surge coil • Fault

• Fault Location with Audio frequency • Core – Core Twisted field

Various other Audio Frequency methods are available to indentify cables, pinpoint low restive faults or to locate joints

Pinpointing Methods Sheath Faults

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• Sheath Fault Pinpointing with DC-Step Voltage

• V • 0

• + • - • V • 0

• + • - • V • 0

• + •  MMG 5

• right"• towards fault

• Direction and Intensity of Instrument Indication

• Fault Resistance • of Earth Contact

• !

•  ESG 80-2 • left towards"

• fault

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100 %

60 %

- 20 %

- 20 % 10 %

10 %

10 %

10 %

+ 40 %

Cable Selection before cutting

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Surgeflex 15 or 25 (1150 J)

Surgeflex 32 (1750 J)

Series of Mobile Fault Locating Systems

SFX 40 (1000 / 2000 J)

Mobile Systems

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Cable Vans

Thank you!


Power Cable Fault Location Techniques by Rodolfo R Penalosa

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