01 Power Factor ACInsulationTheory

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AVO INTERNATIONAL

08/17/01

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Power Factor Testing Training

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Contents

AC Insulation Theory Power Factor Testing

Transformer winding insulation testing

Bushing insulation testing

Other applications: Iex, TTR, Hot Collar

Unknown facts: PF Vs V, -PF, ITC, %VDF

Delta 4000, Delta Control and PowerDB Lite

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AC Insulation Theory

Power Factor Testing



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Basic Terms

Insulation : A material having the property of insulator

used to separate parts of same or different potential.

Capacitor: A capacitor is a pair of conductors separated

by a dielectric material.

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What is it?

• A form of AC testing that applies voltage and

measures the leakage/loss current of electrical

insulation.

Vs

Ic

Insulation

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Power factor Testing

What is it ?

• Power factor testing is a type of insulation testing

used to evaluate the integrity of electrical

insulation.

CHGCLG

CHL



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Facts about Insulation

In an “ideal insulation system” connected to an AC

voltage source, the current is 100% capacitive and

leads the voltage by exactly 90 degrees.

= = Capacitor =

Vs

Ic

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Power factor is defined as Cosine of phase angle between

voltage and current.

For an ideal insulation phase angle is 90 degree.

Cos (90) = 0

Power Factor is ZERO for an ideal insulation !!

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No Insulation is ideal. A small amount of current flowthrough all insulation called as leakage current.

Leakage current comprise of two components.• Resistive current

• Capacitive current

In real life insulating conditions, in addition to thecapacitance current there appears a resistive (or loss)current in-phase with the voltage.



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In practice no insulation is perfect, but has a certain

amount of loss and the total current leads the voltage

by a phase angle

Vs

IR

Ic

Vs

Ic

VsIR

IT

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Why perform Power Factor/

Dissipation Factor Testing ?

Normal insulation resistance testing is l imited to 5kV DC; power

factor allows testing to 12kV AC

Most test specimens normally operate with AC; AC testing more

closely resembles the normal operating situation of the test

specimen.

AC capacitance is an important characteristic

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Is used for:

Evaluating the nature and quality of electrical

insulation materials and systems.

Revealing contamination, fractures, and punctures.

Detection of defects which accompany the aging of

insulation.



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What is Power Factor?

Power factor is the ratio of the resistive or in-

phase component of the current to the total

current.

Ic

VsIR

ITPower Factor= IR /IT

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What is Dissipation Factor ?

Dissipation factor is the ratio of the resistive or

in-phase component of the current to the

capacitive or 90 degree component of current

Ic

VsIR

IT Dissipation Factor= IR/IC

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POWER FACTOR

WATTS ABSORBED IN INSULATION

APPLIED VOLTAGE x TOTAL CURRENT

DISSIPATION FACTOR

IN-PHASE CURRENT (RESISTIVE)

90 DEGREE COMPONENT (CAPACITIVE)



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T

T

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Power Factor vs Dissipation Factor

Both values are similar

up to 20% power factor

and tan delta; beyond

20% they diverge.



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Where Is Power Factor Used?

Manufacturing processes:

quality assurance andquality control programs

Industrials and utilities:

scheduled maintenance

and repair programs

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Power Factor /Dissipation Factor Testing

Detect defects in electrical

insulation to minimize:

Equipment failure

Unscheduled downtime/lost revenue

Inefficiency

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Power Factor/Dissipation Factor Testers Can Be Used To

Evaluate

Transformers

Circuit breakers

Rotating machinery

Insulating liquids

HV bushings

Surge arresters

HV coils



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Test Modes

Ungrounded Specimen Test (UST)

Grounded Specimen Test (GST)

Grounded Specimen Test with Guard Connection(GST-G)

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Modeling of a Transformer

Test Modes

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=

H L

G

CHG CLG

CHL

Most physical test

objects can be

accurately represented

as

a two or three-

terminal network.

Test Modes



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Delta 4000

Four Leads

• High Voltage Lead (Black Lead)• Red Lead

• Blue Lead

• Ground Lead

Test Modes

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Ungrounded Specimen Test

This test is made when both specimen terminals

can be insulated from ground.

This test is often used to perform separate

measurements on sections of a larger insulation

system.

Test Modes

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•Measure Red

Test Modes



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Grounded Specimen Test

This is the most frequently used test connection

and involves all insulation between the high

voltage conductor and ground

Test Modes

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• Measure Red

and Ground

Test Modes

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Grounded Specimen Test with Guard

This test is used to separate the total values of

the GST test into separate parts for better

analysis.

Test Modes



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Guard

•Measure Ground

and Guard Red

Test Modes

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GST with Guard

Guard

GST UST

Test Modes

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× ×

× ×

×

Ground Red Blue

Ground Red Blue

× ×

×

×

Ground Red Blue

MeasurementTest Modes



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PF Test Results

Voltage [V]

Current [A]

PF= cos [%]

Leakage current [mA]

Watts loss [W]

Capacitance [C],

Inductance [H]

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Insulation Power Factor

Typical values for a 2-winding transformer.

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PF Test Results



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Transformer and Bushing Insulation Testing



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Two winding transformers

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Typical power factor limits for oil insulated

transformers and bushings (IEEE)

IEEE 62-1995 states; “The power factors recorded for routine overall tests on

older apparatus provide information regarding the general condition of the

ground and inter-winding insulation of transformers and reactors. While the

power factors for most older transformers will also be <0.5% (20C), power

factors between 0.5% and 1.0% (20C) may be acceptable; however, power

factors >1.0% (20C) should be investigated .”

Typical power factor values @ 20° C

"New" "Old" Warning/alert limit

Power transformers,

oil insulated0.2-0.4% 0.3-0.5% > 0.5%

Bushings 0.2-0.3% 0.3-0.5% > 0.5%

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Test results for oil-filled power transformers.

PF Test Results

For oil-filled distribution transformers the numbers can be

doubled.



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Key Fault Detection

Increases in Power Factor vs. historical values can i ndicate:

• Contamination

• Chemical deterioration

• Damage due to overheating

• Moisture

• If voltage dependent (tip-up test), can indicate PD issues

Changes in Capacitance can i ndicate:

• Mechanical deformation in core/coil structure

• Moisture

Changes in Excitation Current can indicate:

• Shorted winding turns

• Mechanical deformation in core/coil structure

• Core grounding issue

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Bushings: C1 & C2

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Test Results

C1 power and dissipation factor test data are as follows:

• Up to 2 times nameplate PF - bushing acceptable

• Between twice nameplate PF and up to 3 times - monitor

bushing closely

• Above 3 times nameplate tan delta - replace bushing

General guidelines for evaluating the C1 capacitance

data are as follows:

• Nameplate capacitance +/-5%- bushing acceptable

• Nameplate capacitance +/-5% to +/-10% - monitor bushing

closely

• Nameplate capacitance +/-10% or greater – replace bushing



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Test Results

Changes in C2 power/dissipation factor, which is not

usually included on the nameplate, are most commonly

indicative of oil contamination.

Changes in C2 capacitance are typically indicative of

physical change, such as tap electrode problems or tap

connection problems. Nameplate values for C2 are not

typically found on nameplates of bushings rated below

115 kV.

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Thank You !!

QUESTIONS ??

01 Power Factor ACInsulationTheory

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