Post on 20-Oct-2015
Insulation Systems of Power Transformers
Conference on Electrical Power Equipment Diagnostics Bali, Indonesia Thomas Prevost
Figure 1.10 - Gaulard and Gibbs’ “secondary generator”, forefunner to the transformer. The original British patent is dated 1882. (courtesy of the Smithsonian Institution).
Figure 1.12 - Stanley and his original “converter”, now known as a shell-type
transformer. (Courtesy of General Electric Co., Men and Volts).
Early insulation materials were composed of cotton rags
dipped in varnish.
Large Power Transformer Materials (reference 250 MVA Transformer)
Material Weight (kg) % Core Steel 67165 34 Sheet Steel 45573 22 Copper Wire 24228 12 Transformer Oil 48000 24 Kraft
Pressboard 5294 2.65 Paper 1479 .74
Total 200,000
Ref. ABB
Transformer Oil
• Provide Sufficient Dielectric Strength.
• Provide Sufficient Cooling/Heat Transfer.
• Minimize the contact of oxygen with
cellulose and other materials susceptible
to oxidation.
• * Information Carrier* • DGA, Oil Quality Tests
Transformer Oil
Types: • Mineral Oil
• Naphthenic • Paraffinic • High Molecular Weight (R-Temp)
• Ester • Synthetic • Natural
• Silicon
Transformer Oil
Key properties: • Low viscosity • Low pour point • High flash point • Chemical stability • High electrical strength
Typical Properties of Dielectric Fluids (RTemp) Conventional Silicone High Molecular Natural Transformer Oil Fluid Weight Petroleum Ester Viscosity, D445, cSt. @ 100 C: 3.0 16.0 14.5 15 Density @ 20 C., kg/dm : 0.86 0.91 0.86 0.83 Appearance: yellow clear yellow clear Pour Point, D97, C.: -40 -55 -21 -10 Dielectric Breakdown, D1816 (.08”), kV: 56 60 56 52 Dissipation Factor 100 c, D924: 0.007 0.001 0.001 4.0 Dielectric Constant, D924 @ 20 C: 2.2 2.7 2.3 3.0 Neutralization Number, D974, mg KOH/g: 0.02 0.01 0.01 0.06 Fire Point, ASTM D92, C.: 165 320 308 300 Biodegradability (per BOD tests): some no yes yes
Solid Insulation
Function:
• Dielectric - Control Applied Voltage
• Mechanical - Support the Winding
• Thermal - Create Cooling Ducts
Solid Insulation
Primary insulation material used today continues to be cellulose, typically processed using the Kraft method. •Paper
•Conductor Insulation
•Pressboard •Winding forms •Conductor Spacer Material •Barriers •Clamping
Conductor Insulation Type % Elongation Use Non-Creped 2-4 Winding Cu Calendared Crepe 8-12 Winding Cu CTC Crepe 50-200 Leads
The thermal limit of transformer windings is the insulation on the conductor at the winding hot spot. The average winding rise is calculated as follows: IEEE C57.12.00 55° C Rise 65 ° C Rise Ambient 30 ° 30 ° Average Wndg Rise 55 ° 65 ° Hot Spot Differential 10 ° 15 ° Hot Spot Temperature 95 ° 110 ° * * Only attainable with thermally upgraded insulation.
Insulation Life
Thermal Upgrading Process:
Addition of chemicals to protect the cellulose
from oxidation: this is primarily achieved with nitrous compounds such as urea, melamine, dicyandiamide, and polyacrylamide.
Amine Addition - Dicyandiamide
•Chemical Additive to paper. •Consumes water as it is produced. •Neutralizes acids as they are produced.
•(ref Lundgaard) •Suppresses the self-catalyzing character of aging process by chemical reaction.
•During this process the stabilizing agent is consumed.
Aging Curves
Thermally upgraded paper Regular Kraft paper
Source: Westinghouse/ABB Brochure on Insuldur®
Aging Curves
(Paper severely aged below this line)
Degree of Polymerization
Measurement of intrinsic viscosity after dissolving the cellulose in a specific solvent.
Gives an average measurement of the number of glucose units per molecular chain.
•DP of Insulation Components prior to processing ~1200
•DP of Insulation Components following processing ~1000
•DP level considered as “over-processed” ~800
•DP level considered end of life ~200
Effects of aging: - darkening of color - loss of electrical and mechanical strength; trans. failure - shortening of cellulose chains – DP lowered - paper becomes wetter, and acidic - by-products contaminate the oil Source ABB Power Technologies, Inc.
Progressive aging with time
Paper Insulation Aging in Mineral Oil
DP DP DP DP DP 1000 733 549 405 309
DP
181
Brittle & dark End of mech str.
IEEE Transformer Committee Panel Session – October 25, 2005
Transformerboard Mechanical Role
• Support Windings During Short Circuits
• Maintain Dielectric Clearances
• Support High Voltage Leads
• Support Auxiliary Equipment
- LTC, DETC, Bus Bar etc.
F
Frigid clamping distance
transformerwindingcoil
pressboardpresspapercopper
F Clamping Pressure = f(moisture,temperature,age)
Cellulose Insulation - Dielectric
• Excellent Dielectric Insulator
- H O < 0.5% by weight
• Low Dissipation Factor
• High Volume Resistivity
• Low Dielectric Constant
2
Insulation Strength Turn to Turn (impulse and 60 cy)
1
10
100
1000
0 1 2 3 4
Turn to Turn Insulation Thickness (b)
60 H
z.R
MS
& Im
puls
e D
esig
n Vo
ltage
(KV)
t = 0.2µsec
t = 1.0µsec
FW 1.2 x50 µsec
60 cy
t measured at 75% of crest
Insulation Strength Coil-Coil (Impulse and 60 cy)
10
100
1000
0 5 10 15 20Coil-Coil Spacing "a" (mm)
Bre
akdo
wn
Volta
ge (k
V)
t = 0.2µsec
t = 1.0µsec
t = 3.0µsec
FW 1.2 x50 µsec
60 Cycle
t measured at 75% of crest
Electrical Strength of Transformer Oil
1
10
100
0.1 1 10 100 1000Oil Duct Width (mm)
Parti
al D
isch
arge
- In
cept
ion
Fiel
d St
reng
th (k
V/m
m)
Degassed Oil, Non-Insulated Electrode
Degassed Oil, Insulated Electrode
Creep Strength of Transformerboard
1
10
100
0.1 1 10 100 1000
Creep Path Length (mm)
Grad
ient (k
V/mm)
- RMS
-60 H
z.
Creep Strength
High Moisture Content in Insulation Can Cause:
• Accelerated Aging of the Cellulose
• Significant Reduction in Dielectric Strength
• Bubble Formation and Dielectric Failure
• Partial discharges in the Insulation
Dry = Cellulose < 0.5% by weight
& Oil < 10 ppm H2O
Other Insulation Materials
Aramid papers and pressboards Advantage:
• High temperature rating (220 °C) • Good dielectric strength • Excellent mechanical strength • Low moisture content
Disadvantage: •Higher relative cost
• IEEE Std. 1276-1997 “Guide for the Application of High-Temperature Insulation Material in Liquid Immersed Transformers” recommends:
– A max. copper rise of 950 C – A max. hot spot temp. of 1700 C – A max. top oil temp. of 1050 C
What parts are Nomex ?
• Conductor Wrap • Key spacers and sticks • Tap and Lead shields • FOA barriers (zig-zags) • Crepe tape on leads, etc.
What other High Temperature Issues ?
• Bushing capacity • Tap changer capacity • Oil Expansion • Cable capacity • Stray flux heating • High temperature adhesives • Gaskets, Gauges and Relays