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

Cellulose Insulation

Is a Renewable Resource

Solid Insulation

Barriers, Spacers, Clamps Winding Insulation

ref. Weidmann Insulation Model

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.

Cellulose Molecule

Single Glucose Ring

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

Degree of polymerization = measurement of ‘n’.

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.

Transformer’s Forces

Radial Forces Axial Forces

Core

Inner WindingOuter Winding

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 Configurations

For Distances “a” and “b”

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 Stress Analysis

Typical Creep Stress Areas

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

Hybrid Insulation System with NomexTM

10%-50% Increase in

MVA

• 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

Thank You for Your Attention