bernstein danikas
TRANSCRIPT
-
7/30/2019 Bernstein Danikas
1/68
ELECTRICAL PROPERTIES
OF
CABLE INSULATION MATERIALS
AND SOME FURTHER COMMENTS
BRUCE S. BERNSTEIN
MICHAEL G. DANIKAS
-
7/30/2019 Bernstein Danikas
2/68
Paper-Insulated Lead Covered Cables
PILC-Fundamentals
-
7/30/2019 Bernstein Danikas
3/68
INSULATION MATERIALS
MEDIUM VOLTAGE
Polyethylene[PE]
Crosslinked PE [XLPE]
Tree Retardant CrosslinkedPE [TR-XLPE]
Ethylene-Propylene
Elastomers [EPR]
PILC
HIGH VOLTAGE
Crosslinked Polyethylene
PAPER/OIL
Paper/Polypropylene [PPP]
SF6 Gas
-
7/30/2019 Bernstein Danikas
4/68
PILC
Cable is comprised of Paper strips wound over
conductor with construction impregnated with
dielectric fluid (oil)
Long Service History
Reliable/used since late 1800s
Gradually being replaced by Extruded Cables
-
7/30/2019 Bernstein Danikas
5/68
PILC
Paper derived form wood
Wood
Cellulose 40%
Hemicellulose 30% Poor Electrical Properties
Lignin 30% Serves as adhesive
Cellulose must be separated from others
Separation by bleaching
sulfate/sulfite process
-
7/30/2019 Bernstein Danikas
6/68
PILC
CELLULOSE-Insulation Material
HEMICELULOSE-Non-fibrous
more polar
losses higher vs. cellulose
LIGNIN-Amorphous
binds other components in the wood
-
7/30/2019 Bernstein Danikas
7/68
Paper/Oil
Cellulose chemical structure more complex vs. PE or
XLPE
Oil impregnates the cellulose/superior dielectric
properties
Different cable constructions for Medium vs. High
Voltage
-
7/30/2019 Bernstein Danikas
8/68
Paper/Oil
LEAD SHEATH over cable construction
Protects cable core
Benefit: Superior barrier to outside environment
-
7/30/2019 Bernstein Danikas
9/68
COMPARISON OF CABLE INSULATION
MATERIALS
PE / XLPE / TR-XLPE /EPR / PILC
-
7/30/2019 Bernstein Danikas
10/68
Polyethylene
Low Permittivity: limits capacitive currents
Low Tan Delta: Low Losses
Very High dielectric strength (prior to aging) Easy to process/extrude
-
7/30/2019 Bernstein Danikas
11/68
Crosslinked Polyethylene
All of the above PLUS
Improved mechanical properties at elevated
temperature
does not melt at 105C and above
thermal expansion
Improved water tree resistance vs. PE
-
7/30/2019 Bernstein Danikas
12/68
Tree-retardant XLPE
All of the above PLUS
Superior Water tree resistance to XLPE
TR-XLPE properties brought about by Additives to XLPE
Modifying the PE structure before crosslinking
Both
-
7/30/2019 Bernstein Danikas
13/68
Paper/Oil PILC
Long history of reliability
some cables installed 60 or more years !
More tolerant of some common diagnostic tests to
ascertain degree of aging
DC testing
-
7/30/2019 Bernstein Danikas
14/68
Advantages and disadvantages of paper/oil
insulation
The paper is the component most affected by temperature
The power factor increase is moderate and does not seem to beproportional to time or temperature
Cables examined after more than 20 years in service showedgood mechanical characteristics
BUT
Dielectric loss heating becomes significant compared withconductor losses for transmission voltages of 400 kV and higher
The increase of insulation thickness becomes excessive anduneconomical
-
7/30/2019 Bernstein Danikas
15/68
EPR
Compromise of extruded cable properties
Permittivity, Tan Delta > XLPEs
Dielectric strength slightly lower
High temperature properties :
Equal to or > XLPEs
Some types of EPR resistant to initiation and growth of water
and electrical trees even without additives
High thermal conductivity was also reported, higher thanXLPE at both 900 C and 1300 C (Brown 1983)
-
7/30/2019 Bernstein Danikas
16/68
Advantages of Extruded Cables
Reduced Weight vs. Paper/Oil
Accessories more easily applied
Easier to repair faults No hydraulic pressure/pumping requirements
Reduced risk of flammability/propagation
Economics
Initial and lifetime costs
-
7/30/2019 Bernstein Danikas
17/68
Extruded Cables at High Temperatures
PE/XLPE/TR-XLPE
At elevated temperatures, crystalline regions start to melt
Thermal expansion
Physical/mechanical strength reduced
At 105C crystallinity gone:
PE flows
XLPE and TR-XLPE crosslinking allows for
maintenance of FORM stability At high temperatures, crosslinks substitute for the
crystallinity at low temperature
-
7/30/2019 Bernstein Danikas
18/68
Extruded Cables at High Temperature
PE/XLPE/TR-XLPE
Although Crosslinks serve as Crystallinity substitute, they do
NOT provide same degree of
toughness
moisture resistance
Impact resistance
Crosslinking assists in maintaining form stability, but not
mechanical properties
Physical/electrical properties change as temperature
increases
-
7/30/2019 Bernstein Danikas
19/68
Extruded Cables at High Temperature
EPR
Little to no crystallinity initially
Form stability maintained due to presence of inorganic
mineral filler (clay)
Physical and electrical properties change to some extent as
temperature is increased
Present day issue: operating reliability at higher
temperatures vs. semicrystalline polymer insulation
-
7/30/2019 Bernstein Danikas
20/68
Some problems with polymeric materials in
cables
Oil composition may have an effect on polypropylene
since unlike cellulosic materials it may be soluble
in organic solvents (especially those with aromatic
molecules)
Swelling of HDPE at 900 C because of oil
composition
Non-orientated LMWPE undergo dimensional
changes following prolonged immersion in hothydrocarbon oils (Ernst and Muller 1981)
-
7/30/2019 Bernstein Danikas
21/68
Paper/Oil at High Temperature
Cellulose: No significant thermal expansion
compare with extruded cables
Oil: Some thermal expansion
Degradation mechanisms differ at elevated
temperature
-
7/30/2019 Bernstein Danikas
22/68
Thermal Degradation
Paper/Oil
Cellulose degradation
consistent from batch to
batch
Starts to degrade
immediately under thermal
stress
Moisture evolves
Follows Arrhenius model
Oil may form wax over
time(Polymerization)
Extruded
Degradation is polymer
structure related
Degradation related toantioxidant efficiency
Does not start until
stabilization system
affected
No water evolution
No proven model exists
-
7/30/2019 Bernstein Danikas
23/68
Summary: Paper/Oil
Natural Polymer
Carbon/Hydrogen/Oxygen
More polar
Not Crosslinked
Linear Fibrils/no thermal
Expansion
Oil expands thermally
Thermal degradation of
cellulose at weak link (C-O)
DC : No harmful Effect on
Aged cable-does removeweak link
-
7/30/2019 Bernstein Danikas
24/68
Summary: Extruded Materials
Synthetic Polymer
Carbon/Hydrogen
Less Polar
Branched chains Non-fibril
Partly crystalline: much less
for EPR
Mineral fillers (EPR)
Thermal expansion on
heating
Crosslinked
Degrades at weakenedregions/crosslinks hold
together form stability
DC: Latent problem -effect
depends on age (XLPE)
-
7/30/2019 Bernstein Danikas
25/68
Electrical Properties
___________________
Determined By Physical and Chemical
Structure
-
7/30/2019 Bernstein Danikas
26/68
Electrical Properties of Polyolefins
The Electrical Properties of Polyolefins may be separated into
two categories:
Those observed at low electric field strengths
Those at very high field strengths LOW FIELD
Dielectric constant/dissipation factor
Conductivity
Determines how good a dielectric is the insulation
HIGH FIELD
-Partial discharges (corona)
Controls functioning and reliability
-
7/30/2019 Bernstein Danikas
27/68
How does Polymer Insulation Respond to
Voltage Stress
Polar Regions tend to migrate toward electrodes
Motion Limited
Insulation becomes slightly mechanically stressed Charge is stored
Properties change
Next few slides seek to picture events in idealized
terms
-
7/30/2019 Bernstein Danikas
28/68
Polymer Polymer+
No field DC field applied,
polymer becomes
polarized
POLARIZATION OF A POLYMER THAT CONTAINS
MOBILE CHARGE CARRIERS
-
7/30/2019 Bernstein Danikas
29/68
Electrode
ORIENTATION OF POLYMER
UPON APPLICATION OF VOLTAGE STRESS
Alignment of Charge Carriers
Electrode Electrode ElectrodePolymerPolymer
-
7/30/2019 Bernstein Danikas
30/68
IDEALIZED DESCRIPTIONorientation of polar functionality of polymer chains under
voltage stress
No Voltage Voltage Stress Applied
-
7/30/2019 Bernstein Danikas
31/68
POLARIZATION OF A POLYMER THAT CONTAINS
SIDE GROUPS WITH PERMANENT DIPOLES
No field field applied,
polymer becomes
polarized
-
7/30/2019 Bernstein Danikas
32/68
SCHEMATIC OF SOME NORMAL MODES
OF MOTION OF A POLYMER CHAIN
First Mode Second Mode Third Mode
-
7/30/2019 Bernstein Danikas
33/68
Application of Low Voltage Stress
Dielectric Constant:Ability to hold charge
Lower Polarity -> Lower K
Dissipation Factor: Losses that occur as a result of
energy dissipated as heat, rather than electrical
energy
> Polarity leads to > Losses
-
7/30/2019 Bernstein Danikas
34/68
DC vs. AC
Under DC- Polarization persists
Under AC-Constant motion of the
polymer segments due to changing
polarity
-
7/30/2019 Bernstein Danikas
35/68
Dielectric Constant
TECHNICAL DEFINITION
In a given medium (e.g. for our purposes, in a
specific polymer insulation)it is the Ratio of
(a) the quantity of energy that can be stored,
to
(b) the quantity that can be stored in a vacuum
-
7/30/2019 Bernstein Danikas
36/68
Dielectric Constant
Relatively small if no permanent dipoles are present
Approximately proportional to density
Influenced by presence of permanent dipoles:
Dipoles orient in the electric field
inversely proportional to temperature
Orientation requires a finite time to take place
is dependent upon frequency
Relaxation time for orientation of a dipole is also temperaturedependent
-
7/30/2019 Bernstein Danikas
37/68
Dielectric Constant
The dielectric constant of the electrical-insulating
materials ranges from:
a low of about 2 or less for materials with lowest electrical-
loss characteristics, up to 10 or so for materials with highest electrical losses
-
7/30/2019 Bernstein Danikas
38/68
Dielectric Constant of Common Polymers
Polyethylene 2.28
Polypropylene 2.25
Butyl Rubber 2.45
Poly MMA 2.7-3.2 Nylon 66 3.34
PPLP (Oil-Impregnated)
2.7
Sources: M.L.Miller Structure of Polymers,Dupont and Tervakoski Literature
Cellulose Acetate 3.2-7.0
PVC 2.79
Mylar (Polyester) 3.3
Kapton (Polyimide) 3.6 Nomex (Polyamide) 2.8
Cyanoethylcellulose 13.3
-
7/30/2019 Bernstein Danikas
39/68
DIELECTRIC LOSSES
From a materials perspective, losses result from
polymer chain motion
Leads to heat evolution
Chain motion influence on electrical properties are
depicted on next few slides
-
7/30/2019 Bernstein Danikas
40/68
AND AS A FUNCTION OF FREQUENCY
log
log
max
-
7/30/2019 Bernstein Danikas
41/68
Dispersion
Dipoles RIGIDLY attached - oriented by MAIN chain motion
Dipoles FLEXIBLY attached - orientation of pendant dipoles
and/or
orientation by chain segmental motion(shows TWO dispersion regions)
at different frequencies
PE that has been OXIDIZED
PE that is a copolymer with polar monomer, e.g.,
SOME TR-XLPE
Note: 60Hz is not necessarily where these phenomena show maxima
-
7/30/2019 Bernstein Danikas
42/68
Electrical Properties of Polyolefins
Poly Olefin Structure and Dielectric Behavior
The electrical behavior of insulating materials is
influenced by temperature, time, moisture and other
contaminants, geometric relationships, mechanical
stress and electrodes, and frequency and magnitudeof applied voltage. These factors interact in a
complex fashion.
Saturated hydrocarbons are non-polar
Dielectric constants are low Dielectric constants essentially frequency-independent (if
pure)
Dielectric constants change little with temperature
The change that occurs is related to density changes
-
7/30/2019 Bernstein Danikas
43/68
ELECTRIC BREAKDOWN
INABILITY OF INSULATION TO OPERATE or HOLD
CHARGE UNDER STRESS
Insulation inherent properties
Thermal
Stream of electrons released
Discharge-
Preceded by Partial Discharge
-
7/30/2019 Bernstein Danikas
44/68
Electric BreakdownTypes of Breakdown
Failure of a material due to the application of avoltage stress called the dielectric strength
expressed as kV/mm or V/mil Electric breakdown occurs when the applied voltage
can no longer be maintained across the material in astable fashion without excessive flow of current andphysical disruption
Theoretical understanding not clear even now What is clear is that there are several mechanisms of
failure
-
7/30/2019 Bernstein Danikas
45/68
Electric BreakdownTypes of Breakdown
Intrinsic Breakdown
Defined by the characteristics of the material itself in pure and
defect-free form under test conditions which produce breakdown
at the highest possible voltage. Never achieved experimentally.Thermal Breakdown
Occurs when the rate of heating exceeds the rate of cooling by
thermal transfer and thermal runaway occurs under voltage
stress
-
7/30/2019 Bernstein Danikas
46/68
Electric BreakdownTypes of Breakdown
Discharge-Induced Breakdown
Occurs when electrical discharges occur on the
surface or in voids of electrical insulation. Ionization
causes slow degradation. Corona, or partialdischarge, is characterized by small, local electrical
discharges
Treeing-Electrical
Results from partial discharge
-
7/30/2019 Bernstein Danikas
47/68
Two main types of trees
Electrical trees consisting of hollow channels whichare branched and have the general appearance ofbotanical trees
Water trees having fuzzy appearance andconsisting of water-filled microvoids (McMahon,1981)
Besides thosethere are electrochemical trees(similar to water trees) and chemical trees (short andvery dense)
-
7/30/2019 Bernstein Danikas
48/68
AC Breakdown Strength of 15 kV XLPE Cable
Vs. Position on Cable Run
For One 5000 Foot Reel of 50,000 Foot Run
200
400
600
8001000
1200
1400
0 80 160 240 320 400 480
Position on Cable Run (sample number)
ACBreakdown(volts/mil)
-
7/30/2019 Bernstein Danikas
49/68
WEIBULL
Commonly used to characterize time to failure
information
Defines
Characteristic Time to Failure
Scale Factor/ 63.2% probability of failure
Shape parameter/slope of failure times
Called two parameter Weibull distribution
Controversial: Most physical models do predict this type ofdistribution for failure as a function of time (but not necessarily of
voltage stress) Dissado and Fothergill, Page 323
Probability - Weibull
-
7/30/2019 Bernstein Danikas
50/68
10.00 100.00
1.00
5.00
10.00
50.00
90.00
99.000.9
1.0
1.2
1.4
1.6
2.0
3.0
4.0
6.0
Probability - Weibull
Time, (t)
Cumulative%o
fSamplesFailed
WeibullData 1
W2 RRX - SRM MEDF=8 / S=4CB[FM]@90.00%
2-Sided-B [T1]
-
7/30/2019 Bernstein Danikas
51/68
Short-Time Voltage Breakdown
of Polyethylene
0
10
20
30
40
50
60
70
0 50 100 150 200
Thickness
PeakVoltage(k
V)
AC and DC at -196C
DC at room temperature
AC (60 Hz) at room temperature
-
7/30/2019 Bernstein Danikas
52/68
Dielectric Strength
Important Points to Remember !
AC breakdown strength value NOT absolute
Related to rate of rise of the applied voltage stress
5 minute step rise
10 minute step rise
30 minute step rise
Ramp
Real world/stress is constant Voltage endurance
-
7/30/2019 Bernstein Danikas
53/68
Presence of micro-voids
Temperature
Pressure
Nature and morphology of material under
investigation
Parameters of electrical circuit
Damage path (surface of volume) Intensity, duration, waveform of applied voltage
-
7/30/2019 Bernstein Danikas
54/68
Electric BreakdownTypes of Breakdown
Treeing- Water
Water tree growth induced by water in presence of
voltage stress. Water trees generate at much lower
stresses than electrical trees. Not a direct cause ofbreakdown
-
7/30/2019 Bernstein Danikas
55/68
Water Treeing
First reported in 1968
Bahder, et al. 1972: First distinguished between
water and electrical trees
Water trees/electrochemical trees
Water and Voltage stress required
Lead to reduced dielectric strength
Cleanliness required
All known since mid-late 1970s
Progress (?)
Ions influence
Jackets
-
7/30/2019 Bernstein Danikas
56/68
Original publications by Vahlstrom et al.
(1972)
They studied 15 kV and 22 kV PE cables in service of up to
eight years
Trees originated from contaminants and voids such defects
should be reduced as much as possible
Trees originated from insulating surface open to atmosphere
grow faster than trees wholly enclosed in the insulation
Tendency for tree initiation from a contaminant was probably
more affected by the contaminant material than by the size,
location or shape of the contaminant XLPE more resistant to electrochemical treeing than HMWPE
(probably because of the cross-linking byproduct acetophenone)
-
7/30/2019 Bernstein Danikas
57/68
Further comments
Influence of water and impurities are detrimental to
electrical characteristics in extruded cable insulation
Cables exposed to wet conditions are susceptible to
electrochemical treeing impurities make thesituation even worse
PE cables were more prone to trees than XLPE
-
7/30/2019 Bernstein Danikas
58/68
-
7/30/2019 Bernstein Danikas
59/68
A Model of Water Trees in PE
cations
anionsPE outside
water tree
crystalline
amorphous
ionic end
groups
limiting
diameter
Void with
trapped saltlimiting
diameter
limiting
diameter
-
7/30/2019 Bernstein Danikas
60/68
General Procedure For Performing Electric
Breakdown Measurements
A number of specimens are tested in order that the
measurements may be a true reflection of the
statistical distribution of inhomogeneities in the
material under study The following variables must be strictly controlled and
identical in a series of tests:
Specimen thickness
Electrode shape and size Temperature
Frequency of applied field
Rate of increase of the field
-
7/30/2019 Bernstein Danikas
61/68
General Procedure For Performing Electric
Breakdown Measurements
In addition, care must be exercised in controlling the
moisture content and other pretreatment variables of
the specimen
In general, such breakdown tests are associated withsome particular conditions in which the insulation
material will be used, and these conditions therefore
become the dominant factors in designing the test
procedures
TYPICAL GEOMETRIC ARRANGEMENTS ILLUSTRATING
-
7/30/2019 Bernstein Danikas
62/68
TYPICAL GEOMETRIC ARRANGEMENTS ILLUSTRATING
THE USE OF PLASTICS AS AN INSULATOR
Uniform voltage gradient under electrode,nonuniform at edge of electrode and along
creepage path on the surface
An inset Rogowski (curved)electrode provides uniform
voltage gradient
Voltage gradient is not uniformacross the diameter; is highest
at the inner conductor
HV
Ground
HV
Ground
HV
Ground
X
t t
t
tX
R1
R2
Air, liquid, etc.Plastic
Metal electrode
Uniform voltage gradientalong surface of plastic
-
7/30/2019 Bernstein Danikas
63/68
Factors affecting the breakdown strength in
solids
Area effect
Effect of crystallinity (some materials showed a
decrease of breakdown strength with increasing
crystallinity, e.g. partially aromatic polymers)
Effect of impregnation XLPE showed improved
breakdown strength when impregnated with silicone
oil Impregnation of micro-porosities)
Temperature effect possibly the effect of thermalrunaway in localized electron system caused by the
interaction with conduction electrons)
-
7/30/2019 Bernstein Danikas
64/68
Factors affecting the breakdown strength in
solids
Interfaces very important a solid not well
impregnated is likely to breakdown at the interface
(However, carefully prepared paper-oil samples
indicated that breakdown does not necessarily takeplace at the interface, Kelly and Hebner, 1981)
Surface irregularities important particularly in thick
specimens
-
7/30/2019 Bernstein Danikas
65/68
Factors affecting liquid/solid insulation
Breakdown mechanisms
Failure mechanisms
ionic leakage current / chemical reactions of ions with
the rest of insulation
local thermal instability and possible increase of
acidity (Church and Garton - 1953, Gazzana-
Priaroggia et al. 1961, 1976)
ionizable impurities which may enhance space
charge losses (Bartnikas, Umemura et al.)
combination of moisture, temperature and high
operating electric stress (Wilkens)
-
7/30/2019 Bernstein Danikas
66/68
electochemical trees occur if adequate supply of polar liquid
(water) is exposed to a high electrical stress
swelling of polyolefins in oil indicated that swelling in partially
crystalline polymers is caused by molecules in the amorphous
regions incapable of going into true solution but enjoying a
quasi-soluble state (part of the molecule is locked into the
crystalline region and part diissolves in the immersion liquid)
contaminants are more injurious if they are near internal or
external screens than inside the insulation (Jocteur et al.
1977, Lyle and Kirkland - 1981)
-
7/30/2019 Bernstein Danikas
67/68
ELECTRICAL PROPERTIES
Survey of topics reviewed
Dielectric constant
Dissipation factor
Dielectric strength Water Treeing
Electrical Properties at low voltage stress are
dependent upon Chemical structure of Insulation
______________________________________ Reliability (Life) dependent upon other factors
Aging, local environment / beyond scope of this talk
-
7/30/2019 Bernstein Danikas
68/68
THANK YOU