Cable Testing:
On-site Test
Dr. Asawin Rajakrom
11 July 2019
1
Presentation Outline
Defects in cable and cable accessories
Detection of defects
Cable testing concept
Tests with different voltages
Standards for on-site cable testing
Defects in Cable & Accessories
1.
2.
3.
4.
5.
6.
7.
8.
9.
Protrusions on the semi
conductive layers
Stripping void between the
insulation and semi
conductive layers,
conductor and insulation
shield
Voids
Moisture
Impurities (Metal, Amber)
Water tree from conductor
shield
Water tree form insulation
shield
Electrical tree caused by
partial discharge
Bow-tie tree
Defects in Cable & Accessories
Defects in Cable & Accessories
Water Tree
➢
Water trees are formed only in the presence of a liquid.
➢
The applied voltage must be greater than zero.
➢
The site at which propagation begins can be inclusions,
micro-voids, irregularities, or ionic contaminants which
have seeped from the extruded shields into the
insulation.
➢
As partial discharges are not necessary for the inception
and propagation of water trees.
➢
Propagation time through the insulation wall is long
(years).
Water Tree
Water Tree
Four distinct phases of the treeing
process:
1. Moisture permeation
2. Initiation of water trees
3. Propagation of water trees
4. Conversion to electrical trees
leading to failure
Water Tree in Polymeric Insulation
Bow- tie- tree growing from an included particle
inside the insulation of a 30 kV- PE- cable
Water Tree in Polymeric Insulation
Puncture
Punctureout
outofofa aventedvented-tree
tree
Water Tree and Electrical Tree
electrical field
water
disturbance field
time
outer semiconducting layer
conditions for
development of
"water trees"
"bow-tie trees"
insulation
inner semiconducting layer
conductor
"vented trees"
Electrical Tree
Electrical trees are formed only
under the influence of partial
discharges in the insulation wall
or at the shielding layers.
These discharges may occur at
voids, inclusions or asperities.
The applied voltage must be
above the corona inception level
for electrical trees to form.
Water Tree and Electrical Tree
Stress enhancement areas, sites of possible electrical trees (ET),
created by a water tree (WT).
Water Tree and Electrical Tree
Vented water tree (WT) and electrical tree (ET) growing into each
other from opposite screens
Water Tree and Electrical Tree
Electrical trees (ET) are linking two contaminants and another ET is
emanating from the conductor screen.
Water Tree and Electrical Tree
Comparison of Water Treeing - Electrical Treeing
Water Treeing (WT)
Electrical Treeing (ET)
Already seen at small field strength
(e.g. < 1kV/mm)
At high local field stress
Extremely slow tree growth (e.g. over
6-10 years)
Very fast tree growth in PE or XLPE
insulation
No partial discharge recognizable
Accompanied by partial discharge
No visible channels
Long channel structures (visible trees)
Not visible without special coloring
procedure
Clear indication of an electrical
breakdown
How to Avoid Water Trees
• Insulation material should be clean.
• Shield material should be clean.
• The insulation should be extruded tightly over
the conductor shield.
• Shield/insulation interfaces should be smooth
and clean.
• The curing process should minimize formation
of microvoids.
• There should be no water in the strands.
XLPE Process to Avoid Water
Trees
Dry cured
Triple extrusion
Super clean XLPE compound
Super smooth semi conductive
compound
• Contamination free raw material
handling system
• Computerized process control
• Inline dimension control
•
•
•
•
Space Charge
DC hipot output negatively charge up void. These trapped space charges
remain after test. When AC is reapplied, there’s a high difference of potential
across very little insulation. Leads to electrical trees – cable fails.
Partial Discharge in Cables
• A localized electrical breakdown in the electrical insulation
system under voltage stress that only partially bridged the
insulation between conductors.
• A consequence of local breakdown either as a result of
– An electric field enhancement within or on the surface of the
insulation, or
– A region of low breakdown field
• Occurred at:
–
–
–
–
Voids or cavities within the insulation or at interfaces
Interfacial cavities in cable and accessory interfaces
High-resistance insulation shield or broken neutral
Electrical trees initiated from protrusions, voids, or water trees
Partial Discharge in Cables
Several kinds of cable defect
1.
2.
3.
interface gap formation
cavities
cavities eventually with conductive sharp points
Partial Discharge in Cables
Partial Discharge in Cables
Acoustic
Heat
PD
Chemical
reaction
EMW
Light
Partial Discharge in Cables
Voids may be formed in insulation
systems due to manufacturing or
installation defects, ageing, water tree
growth. Continued stress and
overvoltages can initiates PD in voids.
Heat and other forms of
energy released by PD cause
erosion of the internal surface
of void.
Continued erosion forms
channels that develop into
so-called electrical trees in
insulation.
Cable
Failure
Continued PD produced further
erosion until the electrical tree
bridges the insulation
Partial Discharge in Cables
Electric Stress Distribution With No Void
( 12.7 kV rms on the cable conductor )
Electric Stress Distribution With a Void
( 12.7 kV rms on the cable conductor )
PD in Insulation Cavity
Partial Discharge in Cables
PD Patterns
Partial Discharge in Cables
PD Patterns
PD Tests
What PD parameters are measured?
• PD inception voltage (PDIV)
• PD density
• PD extinction voltage (PDEV)
• Phase angle of PD pulse
• PD location
• Phase resolved PD plot
• PD magnitude
• PD magnitude vs. voltage plot
• PD repetition rate
Voltage Sources
Resonance Test System (AC)
Oscillating Wave Test
System (DAC)
Very Low Frequency
(VLF)
PD Tests
PD test methods:
• Offline test:
– An elevated AC voltage of about 1.5-2.0 U0 is applied to generate PD and then
a proprietary digital signal analysis platform is used to detect transient signals
that are generated at the discharge site and travel through the cable to the
detection equipment
– Test voltage source can be selected from VLF, resonance or damped AC voltage
tester
• Online test:
– Cable circuit is under normal operating voltage and loading condition
– Sensors – HFCT or CCV – are used to detect transient signals and digital signal
analysis platform is used to capture and record signals
• Tests shall be done in comparative manner, i.e. PD growth
PD Offline Test
Typical test setup for offline PD testing
PD Offline Test
Generation of Test Voltage (DAC)
HV Source
HV Solid State Switch
Inductor L
Test Object: power cable
Test Object: Power Cable
S
Cc
For short cable
an additional CC
is required.
Embedded PC
Process
Unit
200
MHzControl
AD Converter
Data Storage
HV Divider
HV Divider
PD Coupling Capacitor
PD Coupling Capacitor
adaptive PD detector
PD detector
PD Analysis
Dielectric losses
estimation
Source: SebaKMT
Dissipation Factor (Tan d)
Dissipation Factor (Tan d)
voltage
current
0
Dissipation Factor tan d
10
time/sec
=
true power
=
reactive power
U² / R
U². w C
=
1
w C. R
Dissipation Factor (Tan d)
Simplified Model of a Water Tree
wt
R2
C2
C1
R1
Dissipation Factor (Tan d)
Test sutup for Tan d
Dissipation Factor (Tan d)
How to test Tan d
➢ Non-destructive and integral procedure, and is hence useful for assessing the
entire tested cable route eg. gross defects
➢ 6 to 10 measurements are performed at 0.5U0, 1.0 U0, 1.5U0 and 2U0
➢ Tan delta mean value (MTD) of measurements in the individual voltage steps
Delivers information on water trees, i.e. damages caused by water in the
insulation of plastic-insulated cables. (These water trees can become electrical
trees where partial discharges and breakdowns may occur).
➢ Gives information on the thermal or chemical ageing behaviour of the cable
route.
➢
➢ Tan delta standard deviation (STD) of measurements in the individual
voltage steps
➢ Indicates of partial discharges (PD)
➢ Detect moist joints
➢ Mean value difference (DTD) in the various voltage steps
➢ Detect water trees
➢ Detect partial discharges
➢ Detect vaporisation effects (e.g. at terminations)
Cable Testing Concept
Field Distribution in Cables: AC vs DC
• AC Conditions:
– tan δ = ↓↓
– Ic >> Ir
– Capacitive field control
• DC conditions (without
consideration of space
charges):
– C becomes irrelevant
– Resistive field control
– Defects usually have lower
resistance and lower field
Insulation of MV, HV and EHV Cables
• Examples for Typical Cable Geometry
– MV Cable: 24 kV, insulation thickness 5.5 mm, mean
operational field strength at voltage peak 3.1 kV/mm
– HV Cable: 138 kV, insulation thickness 17.8 mm, mean
operational field strength at voltage peak 6.3 kV/mm
– EHV Cable: 400 kV, insulation thickness 26 mm, mean
operational field strength at voltage peak 12.6 kV/mm
Insulation of MV, HV and EHV Cables
Technical Consequences
➢ Field strength increases with cable voltage
➢ Recommended limit for test field strength: 27-30 kV/mm
➢ Allowed ratio between test voltages and operational voltages for MV 4
times and for HV 2 times higher than for EHV XLPE cables
➢ Test methods using higher overvoltages can be applied for MV cables
➢ Using these methods for HV and EHV cables test voltages would have
to be reduced, in turn reducing sensitivity of test
Continuous Alternating Voltage Test
IEC 60060-3 (2006) High-voltage test techniques
Part 3: Definitions and requirements for on-site testing
AC Resonance Testing
Series Resonant Circuit
AC Resonance Testing
ACTC = conventional test Transformers with
Compensating reactors
ACRL = Resonant circuits based on tuneable HV reactors
(inductance)
ACRF = Resonant circuit based on HV reactors with fixed
inductance tuned into resonance by variable Frequency
Circuits for AC test voltage generation
1– test object
2 – voltage divider/coupling capacitor
3 – test transformer
4 – compensating reactor
5 – regulating transformer
6 – switchgear,
7 – control and measuring unit
8 – tuneable reactor
9 – exciter transformer
10 – fixed reactor
AC Resonance Testing
Source: HIGHVOLT
AC Resonance Testing
Source: HIGHVOLT
AC Resonance Testing
Source: HIGHVOLT
1 – Power inverter (400 kVA)
2 – Power inverter (200 kVA)
3 – Fiber optic links
4 – Three identical exciter transformers
5 – Resonant reactors ((110 kV / 194 A)
6 – Voltage divider
7 – Test objects (Cables under test)
AC Resonance Testing
Source: HIGHVOLT
Damped AC System
IEC 60060-3 (2006) High-voltage test techniques
Part 3: Definitions and requirements for on-site testing
Damped AC System
Features
➢ Combination of Resonant and Pulse Technologies
➢ DAC based field test equipment capable of High Test Capacitance @
HV (highest test capacitance 13μF @ 350kV Peak)
➢ DAC wave shape to allow PD testing within power frequency range
(20-300Hz)
➢ DAC wave shape simultaneously displays PDIV and PDEV
➢ DAC wave shape visualizes & estimates Dielectric Losses (tan ∂)
➢ DAC represents very low “risk” of cable due to short HV pulse
exposure
Summary
➢ Cost effective method to PD test larger, longer & higher voltage cables
Damped AC System
Working Principle
Damped AC System
Block Diagram DAC Technology
Damped AC System
Damped AC System
Damped AC System
PDIV
can be detected simultaneously
PDEV
Dielectric Losses = tan∂ can be calculated
from attenuation of wave shape
Damped AC System
Damped AC System
VLF System
IEC 60060-3 (2006) High-voltage test techniques
Part 3: Definitions and requirements for on-site testing
VLF System
Features
➢ Makes use of the VLF technology to reduce “reactive” power
➢ Most effective technology for small test capacitance @ fairly modest
test voltages
➢ Only suitable wave shape to perform tan∂ measurements
➢ “Electrical Stress” dV/dt approx.500 x less compared to 50Hz
➢ Higher test capacitances are achieved by reducing the test frequency
from 0.1Hz to as low as 0.01Hz,
➢ Caution: Results not directly comparable between different
frequencies. Stress Level dV/dt very different & # of cycles different
Summary
➢ The only wave shape for VLF tan ∂ measurement,
➢ Cost effective for smaller & lower voltage type cables
VLF System
Working Principle
VLF System
The Growth Rate of Electrical trees
depends on the Frequency
All test voltage values in RMS
VLF System
Dissipation Factor tan ∂ of New and Serviced
aged XLPE Cables
VLF System
VLF System
Comparison of Different Tests
Comparison of Different Tests
Norms
AC Resonance
DAC
VLF
DC
IEC 60060-3 (2006)
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√
√
√
IEC 60502-2(2014)
√
√
√
IEC 60840 (2011)
√
IEC 62067 (2011)
√
√
IEC 60229 (2007)
IEEE 400 - 2012
√
√
√
√
IEEE400.1 - 2007
√
IEEE 400.2 - 2013
√
IEEE 400.4 - 2015
PD/TD possibility
√
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Portability
Power consumption
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IEC Standards
❑
IEC 60060-3 (2006)
➢ High-voltage test techniques
➢ Part 3: Definitions and requirements for on-site testing
❑ IEC 60502-2(2014)
➢ Power cables with extruded insulation and their accessories for rated voltages
from 1kV (Um =1.2kV) up to 30kV (Um =36kV)
➢ Part 2: Cables for rated voltages from 6kV (Um =7.2kV) up to 30kV (Um
=36kV)
❑ IEC 60840 (2011)
➢ Power cables with extruded insulation and their accessories for rated voltages
above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV)
➢ Test methods and requirements
❑ IEC 62067 (2011)
➢ Power cables with extruded insulation and their accessories for rated voltage
above 150 kV up to 500 kV
➢ Test methods and requirements
❑ IEC 60229 (2007)
➢ Electric cables – Tests on extruded oversheaths with a special protective
function
IEC 60060-3 (2006)
❑
Due to a variety of external factors not present in factory and
laboratory tests such as external electric and magnetic fields,
weather conditions, etc.
❑
Apply for electrical equipment with a highest voltage Um greater
than 1 kV
❑
On-site high-voltage tests are required:
➢
➢
➢
As withstand tests as part of a commissioning procedure on
equipment to demonstrate that transport from manufacturer to site,
and the erection on-site complies with manufacturer’s specification;
As withstand tests after on-site repair, to demonstrate that the
equipment has been successfully repaired, and is in a suitable
condition to return to service;
For diagnostic purposes, e.g. PD measurement, to demonstrate if the
insulation is still free from dangerous defects, and as an indication of
life expectation
IEC 60060-3 (2006)
❑
Provide information about:
➢
➢
➢
➢
➢
➢
❑
General
Definitions
Test voltage
Measurement of the test voltage
Tests and checks on measuring systems
Test procedure
Types of voltage to be performed on site:
➢
➢
➢
➢
➢
➢
Direct voltage;
Alternating voltage;
Lightning impulse voltage of aperiodic or oscillating shape;
Switching impulse voltage of aperiodic or oscillating shape.
Very low frequency voltage;
Damped alternating voltage.
IEC 60502-2(2014)
IEC 60502-2(2014)
IEC 60840 (2011)
IEC 60840 (2011)
IEC 62067 (2011)
IEC 62067 (2011)
IEC 60229 (2007)
IEEE Standards
❑
IEEE 400 - 2012 (Omnibus)
➢
❑
IEEE400.1 - 2007
➢
❑
IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low
Frequency (VLF) (Less Than 1 Hz).
IEEE 400.3 - 2006
➢
❑
IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable
Systems Rated 5 kV and Above with High Direct Current Voltage.
IEEE 400.2 - 2013
➢
❑
IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded
Power Cable Systems Rated 5 kV and Above.
IEEE Guide for PARTIAL Discharge Testing of Shielded Power Cable Systems
in a Field Environment.
IEEE 400.4 - 2015
➢
IEEE Guide for Field Testing of Shielded Power Cable Systems Rated 5 kV and
Above with Damped Alternating Current (DAC) Voltage.
IEEE 400 - 2012
❑
Suggested steps for field testing and evaluation of shielded power cable
systems
a)
Identify testing objectives.
b)
Identify cable systems to be tested.
c)
Review specifications and operating conditions of cable and cable system
components to be tested.
d)
Select and apply suitable field tests.
e)
Record information or documentation for analysis.
f)
Perform recommended corrective actions on cable system.
IEEE 400 - 2012
❑
❑
Tests: For the purpose of this guide, several test categories are
considered:
From the application point of view, there are three categories of tests:
➢
Installation test:
➢
➢
➢
➢
Acceptance test:
➢
➢
➢
➢
A field test conducted after cable installation but before the application of joints or
terminations.
Intended to detect shipping, storage, or cable installation damage with the
advantage of testing cable sections only.
Care should be taken to have a proper interface for tests at cable ends to avoid
excessive leakage or a possible flashover. Temporary terminations are generally
required.
A field test made after cable system installation, including terminations and joints,
but before the cable system is placed into normal service.
To demonstrate that the transportation, handling and installation have not
damaged the cable system components;
To identify poor workmanship as well as to demonstrate that the equipment has
been successfully repaired after an on-site repair of new components and
significant defects in the insulation have been eliminated.
Maintenance test:
➢
➢
➢
➢
A field test made during the operating life of a cable system.
To assess the present condition of in-service cable systems
Test data serve as reference for future evaluation and be used for trending to
enhance diagnostics.
Test data on other cables similar in design and service conditions can be used to
establish decision criteria
IEEE 400 - 2012
❑
From the technical point of view, there are five broad sets of tests:
➢
Diagnostic test: A field test made during the operating life of a cable
system to assess the condition of the cable system and, in some cases, locate
degraded regions that can result in a failure.
➢
Non-monitored or simple withstand test: A diagnostic test in which a
voltage of a predetermined magnitude is applied for a predetermined time
duration. If the test object survives the test it is deemed to have passed the
test.
➢
Monitored withstand test: A diagnostic test in which a voltage of a
predetermined magnitude is applied for a certain time period. During the test,
other properties of the test object are monitored to help determine its
condition and also evaluate if test duration needs to be extended or may be
reduced.
➢
Offline testing: The cable system under test is disconnected from the
service power source and energized from a separate field test power supply.
➢
Online testing: The cable system under test is energized by its normal
service power source, usually at 50 Hz or 60 Hz. This type of test enables
temporary or permanent monitoring.
IEEE 400 - 2012
❑
Field testing methods:
a)
Voltage withstand
b)
Dielectric response
❖
❖
❖
❖
❖
c)
Dissipation factor (tan delta)
Leakage current
Recovery voltage
Polarization/Depolarization current
Dielectric spectroscopy
Partial discharge
❖
❖
Electrical measurement
Acoustic measurement
d)
Time-domain reflectometry
e)
Thermal infrared imaging
IEEE 400.1 - 2007
IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable
Systems Rated 5 kV and Above with High Direct Current Voltage
❑
Point standard for testing laminated insulated cables with HVDC
❑
Includes testing procedures
❑
Provides guidelines for test voltages
❑
Methods of evaluation
❑
➢
Current-time relationship
➢
Resistance values
Not recommend for service aged solid dielectric cables
IEEE 400.2 - 2013
IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very
Low Frequency (VLF) (Less Than 1 Hz) Point standard for testing laminated
insulated cables with HVDC
❑
Point standard for VLF withstand tests
❑
Presents rational for VLF versus DC
❑
Test parameters for tan delta
❑
Test values in appendix
IEEE 400.3 - 2006
IEEE Guide for Partial Discharge Testing of Shielded Power Cable Systems in
a Field Environment Point standard for testing laminated insulated cables with HVDC
❑
Background information on partial discharge detection and location
❑
Interpretive guidance provided
❑
Technology has improved sensitivity of measurements
❑
Very good and very bad cables identified
❑
Remaining life cannot be predicted with great accuracy
IEEE 400.4 - 2015
IEEE Guide for Field Testing of Shielded Power Cable Systems Rated 5 kV
and Above with Damped Alternating Current (DAC) Voltage
❑
Provides for use of damped alternating current voltages for field testing
❑
Guidelines for evaluation of test results
❑
DAC applications advanced diagnostic testing
❑
Most common use partial discharge and dissipation factor
MEA Standards for Cable Tests (115 kV)
MEA Standards for Cable Tests (24 kV)
MEA Standards for Cable Tests (24 kV)
MEA Standards for Cable Tests (24 kV)
MEA Standards for Cable Tests (24 kV)
“Thank you”
asawinraja@gmail.com