Cable Fault Locating and

VLF AC HipotTraining

David M. BoyerHigh Voltage, Inc.Copake, NY. USA

www.hvinc.com

MODEL CDS-3632U CABLE FAULT LOCATOR

DESIGNED FOR NETWORK SYSTEMS, OIL INSULATED CABLES, & LONG

RUNS OF 35 KV RATED INSULATION

Network Systems demand a powerful Thumper. The CDS-3632U offers high

energy outputs to make best use of above ground sensing instruments to locate a fault.

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COMMONLY USED TERMS

Thumper - a device that develops a HV pulse output Capacitive Discharge Fault Locator - a thumper Joules - the energy in a stored capacitor = ½ CV2

Burn - a technique to deliver high current to a fault TDR - Time Domain Reflectometry HV Coupler - allows connection of the TDR to the thumper Arc Reflection - a TDR method that utilizes the HV Arc of the fault to reflect a TDR pulse Current Impulse or Surge- a thumper technique that measures the thumper pulse reflection from the fault or end of the

cable (standing waveform) URD - Underground Residential Distribution

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CAPACITOR DISCHARGE SYSTEM“THUMPER”

Modes of Operation

Capacitor Discharge – Pulse for Arc Reflection &Continuous for Pinpointing

Cable Hipot/Burn: Hipoting and/or “Burning” faults to reduce arc over-voltage: thump at a lower voltage

Arc Reflection & Current Impulse Fault Pre-location TDR Only– Time Domain Reflectometry

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CAPACITOR DISCHARGE SYSTEM“THUMPER”

Basic Tenets Of Fault Locating

Find The Fault - WHILE CAUSING THE LEAST DAMAGE AND IN THE SHORTEST TIME POSSIBLE

Thump the Fault - WHILE USING THE LOWEST POSSIBLE VOLTAGE VIA CONSTANT ENERGY TAPS

Thump the Fault - WHILE USING THE HIGHEST POSSIBLE ENERGY VIA CONSTANT ENERGY TAPS

Find the Fault - WHILE MINIMIZING THE # OF THUMPS VIA USE OF TDR AND LISTENING DEVICE

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0-9/18/36 kV Thump Output 0-3200 Joules at each output Hipot/Burn with 280 mA Arc Reflection & Current Impulse

fault prelocaton with TDR

CDS-3632U

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Thumper Hook Up

Single point ground

Follow established grounding rules

Return wire should be as close as possible to neutralof cable under test

Local ground

Follow established grounding rules

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Control Panel CDS-3632UHipot/Burn &Cap Discharge mode.

Radar mode only when using TDR. Direct when continuous thumping

Select Output Tap based on fault breakdown voltage. Use lowest setting possible at full output.

Single Pulse to get radar trace.Continuous when thumping to listen.

POWER CONNECTIONS

Mains Connections

Input Power: 120V @ 25A

HV Output & Return

Ground Stud for #2 AWG

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THUMPER TO TDR CONNECTIONS

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10

Thumper To TDR ConnectionsAUX POWER 120V/3A to power TDR

SIGNAL connection to TDR bnc for TDR & Arc Reflection CIM connection to CIM bnc for Current Impulse Method TRIGGER: not used - for other TDRs

1669CI TDR Rear Connections

Thumper right side near top/rear

TDR CONNECTIONS

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TDR Connections on TDR Rear

120V power input from thumper TDR input from Signal connection

for TDR and Arc Reflection CIM input for Current Impulse

Method of locating RS232 to laptop for trace storage Ground stud to ground TDR

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Hipot/Burn ModeBasic Theory of Operation

1. Hipot Mode used to verify cable fault and learn fault break down voltage.

2. If Break down voltage is slightly about tap voltage – “Burn” it.

3. In “Burn” mode, up to 280 mA is available to burn fault, making it arc at a lower voltage.

4. Can then use lower voltage tap to thump to minimize cable damage.

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Capacitor Discharge ModeBasic Theory of Operation

1. Internal capacitors are charged up to desired voltage level

2. Charged caps are connected to cable

3. Stored energy is discharged at the fault

4. Arc used to reflect radar signal (Pulse Mode) for pre-location and for listening or magnetic sensing (Continuous Mode) for precise fault location

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CONTROLLED or CONSTANTENERGY THUMPER

Thumpers should have several outputvoltage taps all at the full joule rating.Permits thumping at lower voltages

but with full energy. Use Burn mode to reduce arc over voltage.

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Thump at the lowest possible voltage

to minimize cable damage. Noise of

3200 joules at 9kV = noise of 3200J at 36kV.

Modern thumpers have multiple output voltage taps to deliver energy at reduced voltages.

Output Voltage

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HVI UNITS HAVE THREE VOLTAGE RANGES @ FULL JOULES

Model CDS-3632U: 0 - 9/18/36 kV @ 3200J

Thumping at lower voltages yet at full joules ispossible and preferred, minimizing cable damage

while finding the fault.

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Joules (watt seconds) = ½CV2

Full joule output is achieved at full voltage output. At half voltage output, only ¼ of the

joules are delivered to the fault.

A 25 kV thumper set to a 15 kV output (60%)

will deliver only 36% (.602) of full energy, making fault locating difficult.

uF rating of caps

Voltage across caps

HERE’S WHY

THUMP WITH LOW VOLTAGE BUT HIGH ENERGY

HIGHER VOLTAGES DON’T HELP TO FIND FAULT

The CDS-3632U has three full energy output voltages

0 – 9 kV @ 3200 Joules

0 – 18kV @ 3200 Joules all taps = same energy

0 – 36kV @ 3200 Joules

The noise and electro magnetic pulse are the same intensity at 9 kV as at 36 kV. Do not thump with higher voltage

than necessary to deliver arc – it doesn’t help.

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WHICH OUTPUT TAP TO USE?

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Apply voltage via Hipot mode to learn fault break down voltage. Set the thumper to a voltage ~25% higher. Turn up voltage to max and PULSE (thump) output. If cable arced, voltage meter will collapse and current meter will spike up as internal capacitor is again charged. If voltmeter and current meter do nothing, then no cable arc. Switch to next highest output.

OPERATING PROCEDURE CONSULT OPERATORS MANUAL

1. Connect Input, Output, & TDR if used

2. Turn On MAIN POWER

3. Select COUPLER MODE

4. DISCHARGE MODE switch in center off

5. Turn OUTPUT ADJUST knob to zero

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6. Select OUTPUT TAP – start low

7. Select DISCHARGE TIMING

8. Press START

9. Turn up voltage – OUTPUT ADJUST

10. Once at voltage, SINGLE PULSE or CONTINUOUS thump

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TDR – Time Domain Reflectometry Arc Reflection Current Impulse

A brief look at tdr/radar. See TDR training materials for more details.

RADAR/TDR

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Arc Reflection

Thumper discharges into cable.

Radar signal reflects off arc and

indicates distance to fault

Permits pre-location to within .5 - 1% of cable length

Cable endFault location

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PRE-THUMP TRACEWe first take a tdr trace of the cable before thumping.

end cursorstart cursor

Cable end

Radar waiting for thump

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PRE & POST THUMP TRACESWe pulse cable to capture fault using arc reflection. Compare the two traces.

Where curves separate is the fault location, or some change in cable resistance.

Pre-thump cable end

Thump Trace - Fault location

Same to this point

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Cable end open

Cable end shorted to ground

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Where curves separate is an “event” causing a change in impedance – probably a fault.

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Sometimes just a slight change occurs, but there is still a change. Something is happening at this location. Thump several times to confirm location and verify change

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Time Domain Reflectometry

When TDR box is used alone without thumper, or through thumper but without arc reflection, to show

“picture” of what’s underground.

TDR produces a “picture” of the cable. It measures distances, find opens or shorts, find transformers,

splices, and shows other accessories.

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Time Domain Reflectometry

Signal from TDR travels cable route and indicates changes in impedance: opens, shorts, splices, transition joints, transformer connections, etc.

High resistance splice

splice

Cable ends

splice

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TDR trace of three phases

Second splice of phase C is faulty, shows high resistance

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TDR Splice Trace

4 splices

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TDR Trace

Four transformers shown & cable end open and shorted

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The TDR offered by HVI is very easy to use

Turn it on and it’s ready for thump

Full feature mode available for those

familiar with TDR use. See other training materials for more on TDR use.

TDR Use Made Easy

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LISTENING, OR PINPOINTING, DEVICES

Detects both Acoustical and Magnetic pulses

Model SDAD

from Aquatronics, Inc.

Designed to work on URD and Network Systems

Not Purchased by Metro North

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“Pinpointing” Device

Used above ground to locate fault. For network systems, identifies between which two manholes is

fault located.Measures electro-magnetic impulse

coming from thump, which is why high joules of discharge

energy is needed.

See separate training video.

X35 from TEC Power

Not Purchased by Metro North

MODEL VLF-6022CM

AC VLF Hipot

Theory

Operation

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This Can Be Prevented

In-service failures cause great damage to faulted cables and adjacent cables. Not so if failed under a VLF test.

Major Cable Components

Jacket (Recommended)

Metallic Shield/Neutral

Insulation Shield

Insulation

Conductor or Strand Shield

Conductor

Good Cable = Uniform Electric Field

When both shields are:

• smooth

• intact

Then, electric field lines are uniform, with a controlled electrical stress distribution.

Protrusion “Empty”Void

High Permittivity Contaminant

HighStress

HighStress High

Stress

HigherStress

Basic Stress Enhancements

No Defect

Voltage

Conversion of Water to Electrical Trees

Acts as a stress enhancement or protrusion (non-conducting)

Water tree increases local electric field

Water tree also creates local mechanical stresses

If electrical and mechanical stresses high enough electrical tree initiates

Electrical tree completes the failure path – rapid growthElectrical tree growing

from water tree

Water & Electrical Trees, Installation Damage, Splice Material or Workmanship Defects, Post Repair Verification, etc. VLF It!

We are testing to cause defects to fail during the test rather than in-service.

Cause failure, locate the fault, make the repair, and be left with a good cable.

If a cable can’t hold 2 – 3Vo, find out now and make repair.

WHAT IS VLF?

A VLF instrument is just an AC hipot but with an output frequency lower than 50/60 Hz.

Very Low Frequency: 0.1 Hz and lower.By decreasing the frequency, it is possible to

test miles of cable with a small and affordable unit.

Models range from 0.1 – 0.01 Hz.

VLF Explained

Xc = 1 2 x pi x f x C

The lower the frequency, the higher Xc (capacitive reactance).

The higher Xc (or resistance across the power supply output),

the lower the current/power needed to apply a desired voltage. At 0.1 Hz, it takes 600 times less power to test a cable, or any other high

capacitance load, than at 60 Hz. At 0.01 Hz, 6000 times higher capacitive loads can be tested than at 60 Hz.

60 Hz vs. 0.1 Hz

At 60 Hz. a 1 μF cable has an Xc of 2.65 kOhms.

At 22 kV, it requires 8.3 amps of current to test.

Total power supply rating must be 183 kVA.

At 0.1 Hz, the Xc is 1.59 megohms.

At 22 kV, the current needed is 14 mA.

Total supply power needed is .304 kVA.

(22 kV is the typical test voltage for 15 kV cable)

50 kVAC @ 3 kVA

Can test ~ 50’ of cable

60 Hertz

40 kVAC @ 1.2 kVA

Can test ~ 6 miles of cable

0.1 – 0.02 Hertz

60 Hz. vs. 0.1 Hz.

WHY TEST WITH AC VOLTAGE?• DC voltage testing damages insulation.

• DC voltage testing is often ineffective.

• Cables are designed to carry AC voltage.

• They are factory tested with AC voltage.

• Cables operate under AC voltage stress.

• Cables should be tested with AC voltage.

VLF rapidly grows defects to failure

VLF is non-destructive to good insulation

VLF is destructive to defective insulation and accessories. VLF is the best splice checker

VLF with Tan Delta offers an excellent non destructive diagnostic test.

Won’t There Be Near Fault Defects Left After Testing?

Very few if test is performed correctly.

Far less than when DC testing

Far less than if no testing is performed

At 3Vo for 30 minutes, >95% of defects will clear. At 60 minutes nearly 100%

No better odds from any other method

Growth rate at 0.1-Hzsinusoidal test voltage

(mm/h) 2.3

10.9-12.658.3-64.2

336

Test voltage factor(V/Vo )

 2345

XLPE Tree Growth RatePer IEEE 400-2001

A 15kV 133% cable has an insulation thickness of 5.9 mm.

In a 30 minute test, nearly all defects will grow to failure.

There is no better way than an AC stress test to verify if a cable can reliably hold service voltage.

Cables often see 2x normal voltage in service. If a cable holds 3x normal voltage during a

VLF test, you can be sure of its integrity.

VLF testing is especially useful for testing after installation or repair.

IEEE VLF Std 400.2/D3 (11/03)

Recommends test voltage of 3V0

(V0 equals line-to-ground voltage).

Test duration is 15 – 60 minutes. Recommendation is for 30 minutes. Most

defects are exposed in first 10 minutes, longer testing is optional to expose minor cable faults.

System Voltage

phase to phase

kVrms

5

15

25

35

Installation

phase to ground

kVrms/kVpeak

9/12

18/25

27/38

39/55

Acceptance

phase to ground

kVrms/kVpeak

10/14

20/28

31/44

44/62

Maintenance

phase to ground

kVrms/kVpeak

7/10

16/22

23/33

33/47

---------------------- 0.1 Hz Test Voltage --------------------

Suggested IEEE Field Test Voltages

For Shielded Power Cable Systems

Using Sine Wave Output VLF

Test voltages are generally 2.5 – 3 time the line-to-ground system voltage.

The above per IEEE400.2/D3 dated 11/03.

The World View Of VLF

DC not recommended by cable companies

on cables >5 years & in moist environments

IEEE 2 – 3Vo for 15-60 minutes

30+ minutes @ 3Vo recommended.

Germany 3Vo for 60 minutes

Japan 3Vo for 15 minutes

Malaysia 3Vo for 60 minutes

Over 75 countries have purchased the HVI VLF

VLF AC Hipot

Model VLF-6022CM

0 - 60 kV Peak

0.1 Hz @ 1.1 μf Load0.05 Hz @ 2.2 μf Load0.02 Hz @ 5.5 μf Load

Controls: 55 lbs/25 kgHV Tank: 100 lbs/45 kg

Cables Included

HV outputfrom tank

Phase jumpers Interconnectwith grounds

Scope bnc

2 test leads for capacitance

measurement

Ground hookResistor

Cable connectorshook or clamp

VLF-4022CM

VLF-4022CM

How to VLF Test

VLF testing is easier than DC testing. Isolate cable ends like with DC testing, although no

cleaning and bagging is necessary. Connect VLF HV lead to conductor – ground to shield. Apply HV to cable. There are no leakage currents to read. Test is go/no-go Wait the desired test time. If cable holds, test is over. De-energize. If cable fails, make repairs and repeat test, or replace. Most models can test all three phases at once, saving time.

Operating Instructions

1. Select Frequency

2. Turn to mA

3. Rotate to zero

4. Press Main Power

5. Press HV On

6. Rotate to raise voltage

What is Cable Failure Indication?

Thermal overload on panel will trip

Voltmeter will indicate breakdown

Diagnostic Cable Test MethodsPartial Discharge

Tan Delta

63

Simplified Cable Model and Phasor DrawingTan Delta = IR/ IC - measured in radians

With perfect insulation, a cable is a near perfect capacitor, with a 90° phase shift between voltage and current. Less than 90° indicates insulation degradation. Cables can be rated good, marginal, or bad.

The tangent of this angle is calculated

δ

I

V

IC

IR

IR IC

= tangent of δ

C R

Cable insulation Cable Cross Section

64

Tan Delta vs.Voltage for New and Aged XLPE Cables

0

0.01

0.02

0.03

0.04

0.05

0.06

0 2.5 5 7.5 10

Lo

ss

An

gle

(Ta

n D

elt

a)

VLF Voltage (kV rms)

New and Aged 15 kV XLPE Cable (Nov 2000)

Aged

New Cable

New cable linear tan delta #s

versus voltage

Aged cable non-linear tan

delta #s versus voltage

+-

m ale m ale

H igh Voltage Supply

e.g.VLF System ,Transform er,R esonant Test System ,…

H V F ilter IC M flex

fem alefem ale

PD Set Up

PD and TD Field Test

PD INFO

Questions?

Thank You

David Boyer - High Voltage, Inc.

www.hvinc.com