VLF and Tan Delta test:

Advantages and Limitations

for MV Cables
Javier Ruiz Leiva
Cable Fault Location, Test and Diagnostic
Area Sales Manager

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 Moderator

n Michael Fleischer
• Digital Marketing Specialist

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 Q&A

n Send us your questions and

comments during the
presentation

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 Today’s Presenter and Panelists

Presenters
n Javier Ruiz Leiva
• Cable Sales Manager

Panelists
n Marshal Bird
• Regional Sales Manager

n Charles Nybeck
• Applications Engineer

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 n Basics
• MV cables construction
Agenda
• Failures mechanism in MV cables
• Cable Lifetime
n Cable Maintenance
n VLF Test
• What is the VLF test?
• Parameters
• Goals
• Standards
• VLF Advantages and Limitations
n Tan Delta Test
• What is the TD test?
• Parameters
• Assessment criteria
• How to interpret the results
n Tan Delta Advantages and Limitations
n Tan Delta on New Cables
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 Basics
Medium Voltage Cables
n Cable Insulation
(15kV 25kV 35kV)

Inner Outer
Conductor Semicon Semicon Jacket / Sheath

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Shield
Main Insulation (neutral concentric)
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 Basics
Medium Voltage Cables

n Cable construction
Single Phase Shielded MV Three Core
3 Conductors, each one with its individual shield

Belted Cable
3 conductors with common shield

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 Basics
n Cable Insulation Medium Voltage Cables

EPR XLPE PILC

8 Solid Dielectrics or Plastic Cables Laminated Dielectric

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 Basics
Failures mechanism in MV cables

Termination Insulation Joint Weak Spot

local issue global issue local issue local issue

Manufacture
Water Tree
defects
Insulation Ageing

Workmanship Workmanship
Cellulose Small bending Radius
9 degradation
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 Basics
VLF, PD Test & TD Test
VLF: Severe installation and/or manufacture issues
PD: Incipient installation and/or manufacture issues
Tan Delta: age insulation issues

Termination Insulation Joint Weak Spot

local issue global issue local issue local issue

Manufacture
defects
Workmanship
Water Tree Small bending Radius

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Workmanship

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 Basics
Local Problems

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 Basics
Global Problems - Cable Insulation Ageing Process

All MV power cables have a high life expectancy;

n During the whole service life they are subjected to thermal, electrical,
mechanical and environmental stresses;

n These stresses will change the morphological properties of the

insulation, so will age/degrade the insulation material

n Result is a decrease of the breakdown-strength of the insulation;

n Decrease of the insulation breakdown-strength will finally lead to cable

failures;
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 Basics
Global Problems - Cable Insulation Ageing Process (XLPE)

Water trees are the most important ageing phenomenon for polymer cables
and could lead into early cable faults!

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 Basics
Global Problems - Cable Insulation Ageing Process (XLPE)
n From water tree till failure
1) Ingress of water by diffusion or sheath-faults;
2) The generation and growth of water trees (process takes at least 5 years);
3) Water trees will change the electrical field -> higher field stress in the
insulation part underneath the water tree;
4) An overvoltage can create an electrical tree;
5) Electrical tree will rapidly grow until a breakdown (hours till maximum a couple of days);

A water tree is a chemical

degradation of the insulation An electrical tree
due the ingress of water, is a progressive
electrical stress and time carbonization of
the insulation

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 Basics
Global Problems - Cable Insulation Ageing Process (PILC)

The most important ageing phenomenon for PILC cables is the degradation
of cellulose, what basically means that moisture comes free from the paper
insulation because of thermal and chemical processes.

Corrosion on the outer sheath and the

generation of holes in the lead sheath
will originate an increase in moisture
content (external water ingress)

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 Basics
Cable Life
n Lifetime of a cable (Bathtub)
Probablity of failure

Aging breakdowns
Teething Troubles Unexpected Breakdowns

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Life Expectancy
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 Cable Maintenance

n Strategy to increase the cable reliability

INBETWEEN END
BEGINING Replacement of the
Periodic maintenance testing
Commissioning test of entire cable or sections
of service - aged cable circuit
newly installed cable of it
circuit

VLF Test VLF and Tan Delta

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PD Test and PD Tests 17
 What is VLF?
n Fundamentally an AC overvoltage test
• Also called AC Hi-Pot
• Performed at 0.1 Hz
n Like a pressure test
• Apply a voltage above operating
– Prescribed by IEEE 400.2
• Look for breakdown
n Pass/Fail
• Limited diagnostic information
– Can use diagnostics tests for more information (PD,
Tan Delta)
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 VLF Parameters
n Wave shapes
• Sinus
• Cosine Rectangular
Sinus wave shape Cosine rectangular wave shape

n Test Voltage
• The test voltage levels are between 2Uo and 3Uo where Uo is the rated
rms phase to ground voltage.
n Time
• The test time between 30 and 60 minutes – Acceptance vs. Maintenance

n Frequency
19 • 0.1Hz
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 Test Frequency Influence

10 kV VPE -Cable
1000
n The growth rate of Electrical trees
.1 Hz sin
.1 Hz cos rect
depend on the frequency = the test
100 duration depends on the frequency
Growth Rate mm/h

50 Hz sin
.01 Hz sin
10 W. Weißenberg, L. Goehlich, J. Scharschmitt:
Site tests of XLPE-insulated high-voltage cable systems U. Schichler: Erfassung von Teilentladungen an
with AC voltage, Elektrizitätswirtschaft, Polymerisierten Kabeln bei der Vor-Ort-Prüfung und im
Jg(96), 1997, Heft 9, S. 400 ff Netzbetrieb, Dissertation, Hannover 1996

E. Neudert, M. Sturm: Characterization of tree process R. Koch, E. Neudert, R. Porzel: Wachstum von TE-
in XLPE by PD Measurement at 50 Hz and very low
1 frequenzies, ICDI Budapest, 1997
Kanälen
in Kabelisolierungen bei unterschiedlichen Spannungs -
belastungen, HIGHVOLT - KOLLOQUIUM 1999
R. Bach, W. Kalkner, H. Oldehoff: Spannungsprüfungen
zur Beurteilung von Mittelspannungskabelanlagen,
Elektrizitätswirtschaft, Jg(92), 1993, Heft 17/18, S. 1068 ff

0.1
1 2 3 4
Test Voltage V / Vo RMS

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 VLF Parameters

VLF Test Voltages prescribed by the IEEE 400.2 - 2013

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 VLF Goals
n Improve reliability of the system, typically if a cable pass the VLF test
assuming the right test time and voltage were provided during the
test the cable should not fail in the next two or three years.

n Accelerate existing weak spots to failure

• During scheduled outage
• Under controlled test conditions
• Reduced energy levels – energy released!

n Limit collateral damage and restoration time

n Repair/Replace on planned/proactive basis

22 • Not unplanned and reactive
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 Advantages and Limitations of the VLF Test

n Advantages
• Simple withstand test, no expert required to operate or interpret the results.
• VLF has no adverse effects on cable life at prescribed levels.1)
• VLF test is effective as a withstand test to detect low and high resistive local defects in
cable systems, provided the proper test parameters are being used
• Tan Delta and Partial Discharge diagnostics are available using VLF as a source 2)

n Limitations
• Only gross workmanship defects are likely to be detected on new cable systems
• When testing cable systems with extensive insulation degradation, simple VLF withstand
testing alone may result in repeated failures, although this rarely occurs in practice.
• Cable systems must be taken out of service for testing
• The VLF test will not provide information about the aging conditions of the insulation

23 1) NEETRAC Project Numbers: 04-211/04-212/09-166

2) Tan Delta diagnostic is only possible with VLF Sine units 23

 Applicable Standards
North America only:
IEEE 400 Guide for Field Testing of
Shielded Power Cable Systems using Very Low Frequency

National standard in
Germany
DIN VDE 0276 part 620/621

European standard
Cenelec HD 620/621 S1

International standard
IEC 60060-3 High Voltage Test Techniques, part 3
Definitions and requirements for on-site tests

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 Summary

n 0.1 Hz VLF Testing is the state-of-the-art cable test method

• is effective to be used by construction crews as a substitute for the DC hipot test
before switching a circuit in (Operational Integrity)

n But doesn’t tell you the overall conditon of the cable:

• High losses
• Water trees Tan Delta Test
• How good or bad is the cable insulation

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 Tangent Delta
Don’t guess the age of your cables

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 What is the Tan Delta Test?

Tangent Delta test is a global method of testing cables to monitor aging and
deterioration of cable systems.

“There is a correlation between an increasing 0.1 Hz tangent delta and a

decreasing insulation breakdown voltage level at power frequency” (Extract
IEEE 400.2-2013)
n Based on VLF technology
• Add measurements

n Diagnostic test
• Results in qualitative numbers
• Results can be compared to IEEE guide

n Prescribed test method

27 • IEEE 400.2-2013
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 When to Use Tan Delta?

n Typically used on aged cables

• ≥5 years
• Detect ageing conditions

n As part of cable maintenance strategy

• Alert to critically aged cables
• Before in-service fault
• Up to 1-2 years notice

n As part of cable replacement project

• Wise use of available money

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 What Can it Detect?

n Provides an overall condition assessment

• Help target “worst” cables.
n Can detect
• Presence of water trees
• Contaminates in insulation
• Insulation moisture
• Degraded accessories
• Oil leakage in PILC cables
n Can assess effectiveness of repairs
• Before and after measurements

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 What Can it Not Detect?
n Can not locate discrete problems
• Only that there is or is not a problem

n What (specifically) the problem is

• Is it a corroded concentric or water trees?

n Problems in the jacket or lead sheath

n Poor workmanship

n Manufacture defects

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 Theoretical approach
n For simplistic purposes the circuit represents a cable with a perfect insulation

IT = IR + IC
IT
If the cable insulation is perfect then the resistor in my circuit is perfect, in
other words the value of the resistance trends to infinite…

…. This will mean that all the current in my circuit (cable) is a

IR capacitive current and then It is 90˚ shifted to the voltage applied
ꝏ Ic

R C U

IR=0
φ= 90° I
IR δ
IC

IR 1
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I C wRC 31
 Wave Shape, Frequency and Voltage

We Use a Sine Wave Shape

Test frequency? 0.1 Hz

Number of Tanδ values? 8 – 10 per voltage step

Stepwise increase in voltage

Which voltage levels? 1. Step 0.5Uo

2. Step 1.0Uo
3. Step 1.5Uo

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VLF Tan Delta, (c) SebaKMT, all rights reserved. 32

 Why we need to use a Sinusoidal wave shape?
U

I
I δ
I
R
C

We don’t have phase

reference between
Current and voltage

Cosine-Rectangular
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 Tan Delta Measurement

n Mean VLF –TD as per IEEE 400.2

Tanδ

10 values

0.5 1 1.5 U/Uo

i=N

å Tand i
TD = i =1
N
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 Tan Delta Measurement

n Differential VLF –TD (Tip up) as per IEEE 400.2

ΔTanδ or TipUp is a indication for

Tanδ the behaviour of the tanDelta with
increasing voltage

ΔTanδ

0.5 1 1.5 U/Uo

DTand = TD1.5Uo - TD0.5Uo

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 Tan Delta Measurement

n VLF –TD Time Stability (Standard Deviation) as per IEEE 400.2

Tanδ

STDEV is an indication about

the deviation of the tanDelta
values within a voltage step

0.5 1 1.5 U/Uo

STDEV low STDEV high STDEV high

å (Tand )
i=N
2
i - TD
STDEV = i =1
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N -1
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 Assessment criteria

n The measured values of the TD, DTD and Standard Deviation are
primarily influenced by the conditions of the cable system components

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 Assessment criteria
n No action required
• No indication of severe problem in the short term
• The cable system can be returned to service
• The cable system should be retested at some later date, maybe 4 years

n Further study advised

• Additional information is needed to make an assessment
• Comparing historical TD results of the tested cable system
• Additional diagnostics test (Monitored withstand test)
• Visual analysis of circuit components.

n Action required
• Poor insulation condition
• The cable system should be consider for replacement or repair immediately
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 Voltage Dependency of the TD in XLPE & PILC cables

tan d . 10-3
7
reference cable (new)
6 slightly service-aged (1)
0.1 Hz dissipation factor

moderately service-aged (2)

5 moderately service-aged (3)
strongly service-aged (4)
4

0
0 0,5 1 1,5 2 2,5 3
U/U 0

Voltage Dependency of TD in XLPE insulation Voltage Dependency of TD in Paper insulation

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 How to Interpret the results

Tan Delta on good Cables

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 How to Interpret the results

Tan Delta on a good circuit but during a rain The same circuit during a sunny day

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 How to Interpret the results

Tan delta on a wet cable

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 Making decisions
n Tan Delta test on 5 kV rated XLPE cables
n Installed in 1990

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 Advantages and Limitations of the Tan Delta Test
n Advantages
• Non-destructive; very unlikely to fault the cable during or after test
• The measurement of the bulk properties of extruded insulation is an indicator of the
severity of water treeing.
• Cable system insulation condition can be graded among no action required, further
study advised, or action required
• TD tests provide an overall condition assessment on a given phase when compared
to adjacent phases.

n Limitations
• The frequency must be 0.1 Hz, if other frequency than 0.1Hz is used then we can not
use the tables of the IEEE.400.2 to evaluate the TD
• Limited length in the cables under test, around 3km.
• Must be a single type of cable insulation, different insulations mask defects.
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• With the exception of wet accessories the TD test cannot detect singular defects in
extruded cable insulation.
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 Tan Delta on New Cable Systems

n The use of the Tan Delta on new cable systems is carried out by some utilities to obtain a
“finger print” of the cable before its energization and to detect contamination, it is important
considered the next information for the tan delta test on new cables;

• The figures of merit from diagnostic tests on aged cable (Table 4 to Table 7) should not be applied to
new cable systems (Tables Annex G, IEEE 400.2)

• Some volatile elements in the cable insulation , essentially cross-linked byproducts and additives for
improving insulation properties disappear slowly during service, unfortunately, because of this, the
losses of new XLPE cable insulation can be even higher than those of service aged PE or XLPE 1)

• At the present many cable manufactures apply XLPE copolymers (blend of XLPE-H and copolymer,
e.g., ethylene - butyl acrylate) and these copolymers change the dielectric response 1)

1) Dielectric response measurements time and frequency domain of different

45 XLPE homo- and copolymer insulated medium voltages cables

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 Tan Delta on New Cable Systems

XLPE Copolymer

XLPE Homopolymer

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 Summary
n The Tan Delta is a scalar quantity (Dimensionless unit)

n Measure the angle

• Ratio of Resistive to Capacitive current

n As cables age
• Insulation resistance decreases
• Resistive current increases
• Angle increases

n Compare this angle to IEEE 400.2 – 2013 tables.

• Able to make an assessment

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 Megger VLF Units

VLF with Cosine Rectangular units VLF Sine units

VLF CR 28kV, 40kV and 60 VLF Sine 34kV, 45kV and 62kV

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 Doing the Tan Delta Test
n Once the test is done…

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 Survey and Contact Information

Contact Information
Presenter
Javier Ruiz Leiva
Javier.ruizleiva@megger.com

• USA and Mexico Sales

–USAsales@megger.com
– +1 800 723 2861

• Canada Sales
–CAsales@megger.com
Please help us improve by filling out
– +1 800 297 9688 the survey after the webinar ends
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 Questions?

At Megger, we understand that keeping the power on is essential for the success of your
business. That is why we are dedicated to creating, designing and manufacturing safe,
reliable, easy-to-use portable test equipment backed by world-leading support and expertise.

We can assist your acceptance, commissioning and maintenance testing for predictive,
diagnostic or routine purposes. By working closely with electrical utilities, standards bodies
and technical institutions, we contribute to the dependability and advancement of the
electrical supply industry.

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