Doble Training

2-1

Knowledge Is PowerSM Apparatus Maintenance and Power Management

for Energy Delivery

Doble Testing

Basic Theory

2

Why Do We Test?

Doble Training

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Purpose Of Tests: To Detect…

• Overall Power Factor: Detect Moisture and

contamination in oil and deeper in cellulose

• Bushing C1: Contamination in the main body

• Bushing C2: Contamination in the oil and tap

area

• Exciting Current: Core defects, bad connections,

shorted turns; especially useful for LTC’s

• Turns Ratio: Shorted turns

• Leakage Reactance: Winding movement

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How To Pinpoint A Problem

Doble Training

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Core Form Transformer Winding Details

mA

Some currents we want to

measure…

…and some we do not.

Unmeasured current returns to

Guard without being measured.

Some currents we want to

measure…

…and some we do not.

Unmeasured current returns to

Guard without being measured.

Core Form Transformer Winding Details

mA

Some currents we want to

measure…

…and some we do not.

Unmeasured current returns to

Guard without being measured.

Core Form Transformer Winding Details

mA

Some currents we want to

measure…

…and some we do not.

Unmeasured current returns to

Guard without being measured.

Core Form Transformer Winding Details

mA

6

What We’re Measuring

mA

CHL

CH

CL

HV

LV

IT IC

IR E

O

IC IT

From the

components of

our test current,

we measure

current (IT),

capacitance (IC),

and watts (IR).

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IT IC

IR E

IC IT

IR

Relationship Between mA, pF, Watts, And

Percent Power Factor

See the relationship

clearly (Fig 1) but more

realistically (Fig 2)

Fig 1 Fig 2

We apply a voltage

across our insulation,

and measure the

current that flows (IT)

in mA. We measure the

capacitive component

(IC in pF) and the

resistive component (IR

in Watts) Cos =

PF = IR/IT

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Relationship Between Current and

Capacitance

This relationship applies when the resistive component is very

small.

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Why We Measure Capacitance

C = Capacitance

A = Area (size of capacitor)

= dielectric constant

d = Distance between plates

•All of these variables are Physical

Parameters

A d

C= A

4 d

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The Transformer As A Capacitor

Lower Plate: (the

grounded

transformer tank)

Dielectric: (oil,

porcelain, paper

products)

Upper Plate: (High Voltage Winding)

A Representation of Transformer CH Insulation: A Capacitor

A d

CHL

CH

CL

AT

BT

Doble Training

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Original Winding Capacitances Based on

Normal Distances

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Distances Changed Due To Winding

Distortions, Changing Capacitances

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Resistive component

of current in Watts

The measured Watts represent the energy-

dissipating tendency of the insulating material

(i.e., the Dielectric Loss portion of the insulating

material). This component of the measured

current is created by foreign materials not part of

the insulation system. Examples:

Why We Measure Watts (Loss)

•Moisture

•Arcing Byproducts

•Metallic Particles from forced oil

motors

•Airborne Particles

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What The Watts Measurement Means

IR

Surface leakage on a bushing with contaminated porcelain can

be highly resistive because of moisture and particles of

contamination. Since the inside of the bushing is clean and

free of moisture, the C1 current measured on the inside should

be highly capacitive.

IC

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Note that the current is

negative only when

referenced to VT; when

referenced to V2, it is

positive. So there really is

no such thing as negative

Watts.

A Word About Negative Watts...

The test set sees VT as the voltage, but

only I2 as the current. I2 referenced to

V2 would give us a positive watts

reading, but I2 referenced to VT does

not. The current IG and Resistance RG

determine the degree to which the

watts go negative.

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Watts As It Relates To VT Or V2

Without RG

With RG

The angular difference

between VT and V2 is caused

by the element

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How To Calculate Power Factor

To express power factor in percent (% PF), multiply by 100:

10 kV equivalent values Power

Factor Is:

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Know These Relationships

• % Power Factor = (Watts X 10)/mA

• Capacitance (pF) = Current (mA) X 265

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You Can Check Your Data

1. Watts X 10 =1.91

2. Divided by 9.077mA=.21%

3. %PF Measured

4. Capacitance=

9.077 mA X 265=2405 pF

EX: Line 2

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Tip-Up = Power Factor at Line-to-ground voltage -

Power Factor at 25% Line-to-ground voltage

25% L-G L-G

E

%PF

%PF @ 25% L-G

%PF @ L-G

Tip-up occurs mostly in dry-type insulation specimens such as Dry Type

Transformer, rotating machinery, and cables, but can occur in oil filled

transformers when the insulation is contaminated.

Power Factor Vs. Test Voltage

As test voltage is increased, the power factor will increase depending

on the void density.

Doble Training

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Power Factor Tip-UP

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Tipup Tests: Which Contaminants?

These will cause tipup:

– Metallic particles from pump bearings in the insulation

– Copper Sulfide

– Carbon in the insulation

These will not cause tipup:

– Water

– Normal ageing byproducts

– Corrosive Sulfur in the oil

– Carbon or metallic particles in the oil

Doble Training

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Transformers And The Tipup Test

EXAMPLES OF WHEN TO USE A TIPUP TEST

1. There is a bushing failure and you do a clean up. Check the

main insulation power factor including tip-up to see how

much conductive material might be trapped in the

insulation.

2. The oil shows a high particle metal content and we know

the bearing went on one of the pumps. Check for tip up to

see if the metal particles have gotten into the main

insulation

3. When a high power factor is measured, perform a tipup

test to see if the source of high power factor is water and/

or normal aging (no tip-up), or some other contaminant.

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Power Factor

Summary

Current (mA)

Capacitance (pF)

A-C dielectric Loss (Watts)

Percent Power Factor

Power Factor Tip-Up

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Basic Insulation & Power Factor Theory

Test Current I

Possible Red

Return Blue

Leads: Ground

How does test current return to its source?

Guard

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Basic Insulation & Power Factor Theory

Test Current I

Current returned through measuring circuit (IB) does.

Whick test currents get measured?

Current returned directly to guard (IA) does not get measured.

Guard

IA

IB

THUS WE CAN CONTROL WHICH CURRENTS TO MEASURE.

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Test Circuits

• GST means Grounded Specimen Test. At least

part of the test current is measured through the

ground lead; the rest is measured through the

LV leads, if used and not guarded.

• UST means Ungrounded Specimen Test. The

ground lead is not used for measurement. Only

the selected LV leads are measured. No

currents to ground can be measured.

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Circuits Used To Measure CH, CHL, and CL Insulation

Guard Guard

GST Test Circuit:

Currents to Ground

are measured

UST Test Circuit:

Currents to Ground

are not measured;

Ground is Guard!

Test Set Measured

Ground Lead: Not Measured

(Set automatically)

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Location Of Ground Relative To Guard:

What Is Measured

Test Current I Test Current I

Guard

Guard

GST-Ground

GST-Guard

Guarded and Ungrounded

cables not measured

Guarded and Grounded

cables not measured

UST

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Grounded Specimen Test

GST-Ground

Energize a bushing

mA

Current to ground is

measured using ground

lead

60 Hz Electrostatic

Interference IE

(returns to ground) 57/63 Hz

Doble Test

Current

(returns to

Guard)

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GST

UST

GROUNDED

SPECIMENT

TEST

UNGROUNDED

SPECIMENT

TEST

GND - RB

GAR - RB

GAR - R

GAR - B

UST - RB

UST - R

UST - B

Measure Red, Blue, & Ground

Measure Only Ground

Measure Only Blue & Ground

Measure Only Red & Ground

Measure Only Red and Blue

Measure Only Red

Measure Only Blue

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Controlling Measured Current On Ch Measurement

mA

HV LV

Without being

guarded, the LV

winding influences the

HV winding

measurement to

ground; get a mixed

measurement.

CH

CHL

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mA

HV LV

With the LV lead

connecting the LV

winding to guard, the

influence of the LV

winding is eliminated.

Get a pure

measurement.

Controlling Measured Current On Ch

Measurement

CH

CHL

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Controlling Measured Current

mA

A

B

Section A will influence the section

B measurement if not guarded. Get

a mixed measurement.

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Controlling Measured Current

mA

A

B

With Section A

guarded, the influence

of A is redirected via

the LV lead to Guard,

and doesn’t get

measured. Get a pure

measurement.

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The End