Introduction to Doble

Transformer Testing
Doble Engineering Diagnostics
Toolbox
Why do we test?...

Doble Conference 2009 2

To try to prevent this!...

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Transformers fail because…
• Transformers are subject to immense stresses
during service – impact, nearby or internal
faults, overloads
• These stresses may cause winding deformation,
core damage, insulation deterioration
• Allowing a transformer to enter service without
detecting these types of problem is risking
catastrophic failure
• An adequate Diagnostics Toolbox allows
condition assessment in a timely manner.

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Transformer Condition Assessment...

Doble Test philosophy:

a single test does not tell the whole story

– Review multiple tests to get the “big picture”

– Acceptable test protocol
– Diagnose the transformers condition
– Find the cause and understand the symptoms
– Doble Engineering Diagnostics Toolbox

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Identifying Failure Modes…
Transformers fail for two broad reasons:
• Quality of Insulation: something is
contaminated and then flashes over
– Contamination, Corrosion, Wet
• Physical Integrity: something is deformed,
no longer operates within design
specifications and then fails
– Deformed, shorted, broken
Consider which tests you would choose...

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Doble Testing Protocol for Transformers

• Power Factor & Capacitance:

– Insulation Deterioration - Winding Distortion

• Excitation Current:
– Core or Winding Problems (open circuit, multiple
core grounds, shorted turns, shorted laminations)
– LTC Problems
– Manufacturing Defects

• High Voltage TTR:

– Shorted Turns - Open Circuits

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Doble Testing Protocol for Transformers
(cont.)
• Leakage Reactance:
– Winding Distortion
– Nameplate Impedance Confirmation
• Frequency Response Analysis (SFRA):
– Shorted Turns
– Coil and Core Movement
– Localized Winding Distortion
• Materials Laboratory Analysis:
– Incipient Fault Detection
– Insulation Deterioration
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Broad and Narrow Focused Test

– Broad
• Power Factor and Capacitance
• Excitation Current
• TTR (High Voltage and Low Voltage)
• Dissolved Gas Analysis (DGA)

– Narrow
• Sweep Frequency Response Analysis (SFRA)
• Leakage Reactance

These independent diagnostic methods have their place in ascertaining

transformer condition

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Power Factor and
Capacitance

The Traditional Doble Test

Power and Distribution Transformers

The Doble Test as applied to transformers is the most comprehensive

tests for insulation assessment
Moisture/Contamination

Doble Test Carbonization

Mechanical Failure
The test technique makes it possible to segregate the specimen into
major components for more effective analysis of test results

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Transformer General Types
Power and distribution transformers may be either single-phase
or three-phase
• Two-Winding
• Three-Winding
• Autotransformer (with or without a tertiary winding)

They may be liquid-insulated, gas-insulated, or dry-type

For test purposes, the procedure used depends on the

number of accessible, separate windings

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Doble Tests Performed on Transformers
• Power Factor and Capacitance
• Overall Insulation
• High-Voltage windings
• Low-Voltage windings
• Inter-winding
• Bushings
• Main Core Insulation
• Tap Insulation
• Hot-Collar
• Oil
• Other Doble Protocol Tests
• Dielectric loss measurements on arresters
• Single phase Excitation Currents
• Single phase High Voltage TTR tests
• Leakage Reactance (three-phase and per phase)
• Sweep Frequency Response Analysis (SFRA)

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Deterioration of Insulation. . .
Basically loss of Dielectric Strength
• Changes in original characteristics:
– Electrical
– Mechanical
– Chemical

• Detected by:
– Destructive Tests
– Non-Destructive Tests

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Doble Test Parameters. . .

• Current
• Dielectric Loss
• Capacitance (Total Charging Current)
• Power Factor
• Power Factor Change with Test Voltage

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 Transformers Overall
Tests Procedures
(Two-Winding)

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Transformer Insulation System
For simplicity, transformer
High insulation is grouped into three
Voltage representative insulation
Windings systems

CH
CH
CHL CL
CL CHL

Low Rather than measure all the

Voltage insulation systems together, the
Windings Doble Test allows the testers
to measure them individually.

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Core Transformer Winding Details

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Physical Representation of 3-Phase
2-Winding Transformer

High-Voltage
Windings
Low-Voltage
Windings

CH CHL CL One of
Core
Leg Three
Phases
Shown

Transformer Tank

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Dielectric Representation of 3-Phase
2-Winding Transformer

High
CH

Tank and
CHL Core

CL

Low

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Insulation subsystems components

CH - Insulation between the High Voltage

Winding Conductors and Grounded
Tank & Core

– High Voltage Bushings

– Winding Insulation
– Structural Insulating Members
– DETC Insulation
– Insulating Fluid

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Insulation subsystems components

CL - Insulation Between the Low Voltage

Winding Conductors and Grounded
Tank & Core

– Low Voltage Bushings

– Winding Insulation
– Structural Insulating Members
– LTC Insulation
– Insulating Fluid

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Insulation subsystems components

CHL - Insulation between the High and

Low Voltage Windings

– Winding Insulation Barriers

– Structural Insulating Members
– Insulating Fluid

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The Overall Test
Before you start the overall test:

• Short Circuit High Voltage Windings

• Short Circuit Low Voltage Windings
• Disconnect the neutral bushing from ground
CH
CHL
CL

This is one of the most

common sources of error
in test results!
Ensure that the Neutral
Bushing is also shorted and
disconnected from Ground

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Two Winding Transformer

H1 H2 H3 High Low
H2 CHL
X2

X1
X0
X1 X2 X3 H1 H3
X3
X0
CH CL

Tank & Core

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Two-Winding Transformer
Test Procedure (HV side)
HV Cable
High Low

Tank & Core

Test Set

Ground
Lead

Low Voltage
Guard Lead
I & W Meter

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Test No. 1: Measure CH + CHL
ICHL
HV Cable
High Low
ICH
Tank & Core
Test Set
ICH

GST-Ground Ground ICHL

Lead
ICH + ICHL
Low Voltage Lead
Guard
I & W Meter

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Test No. 2: Measure CH
ICHL
HV Cable
High Low
ICH
Tank & Core
Test Set
ICH

GST-Guard Ground ICHL

Lead
ICH
Low Voltage Lead
Guard
I & W Meter

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Test No. 3: Measure CHL
ICHL
HV Cable
High Low
ICH
Tank & Core
Test Set
ICH

UST Ground ICHL

ICH Lead
ICHL
Low Voltage Lead
Guard
I & W Meter

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Test No. 4
Test Summary and Calculated CHL

ILINE4 = ITEST1 - ITEST2

WLINE4 = WTEST1 - WTEST2
%PFLINE4 = 10 * WLINE4 / ILINE4
CAPLINE4 = CAPTEST1 - CAPTEST2

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Two-Winding Transformer
Test Procedure (Low side)
HV Cable
High Low

Tank & Core

Test Set
Low Voltage
Lead

Ground Lead

Guard
I & W Meter

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Test No. 5: Measure CL + CHL
HV Cable
High Low
ICHL
Tank & Core

Test Set
Low Voltage ICL
GST-Ground Lead

ICL + ICHL Ground Lead

Guard
I & W Meter

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Test No. 6: Measure CL
HV Cable
High Low
ICHL
Tank & Core

Test Set
Low Voltage ICL
GST-Guard Lead

ICL Ground Lead

Guard
I & W Meter

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Test No. 7: Measure CHL
HV Cable
High Low
ICHL
Tank & Core

Test Set
Low Voltage ICL
UST Lead

ICHL Ground Lead

Guard
I & W Meter

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Test No. 8
Test Summary and Calculated CHL

ILINE8 = ITEST5 - ITEST6

WLINE8 = WTEST5 - WTEST6
%PFLINE8 = 10 * WLINE8 / ILINE8
CAPLINE8 = CAPTEST5 - CAPTEST6

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 Transformers Overall
Tests Procedures
(Three-Winding)

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Dielectric Representation of 3-Phase
3-Winding Transformers

High
CH
CHL
Tank
CHT Low and
Core
CL
CLT

Tert
CT

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Comparison between Insulation
subsystems components (2W vs. 3W)

High
CH
Tank
CHL and
Core
CL
Low
High

CH
CHL
Tank
CHT Low and
Core
CL
CLT

Tert

CT

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Insulation subsystem components
CH : CL : CT
Insulation between the Winding Conductors
and the Grounded Tank and Core

– Winding Insulation
– High, Low, Tertiary Voltage Bushings
– Structural Insulating Members
– DETC Insulation
– LTC
– Insulating Fluid

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Insulation subsystem components

CHL : CHT : CLT

Insulation Between Two Windings or
Inter-winding Insulation

– Winding Insulation Barriers

– Structural Insulating Members
– Insulating Liquid

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Three-Winding Transformer
Test procedure (HV side)
HV Cable
Test Set
High
CH
CHL
CL Tank
Low
Low Voltage Lead and
Core

CLT CHT
Low Voltage Lead
Guard Tert
I & W Meter
Ground Lead CT

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Test #1: Measure CH+CHL

Test Set HV Cable

High
GST CH
Guard R CHL
Ground B Tank
Low Voltage Lead Low and
ICH + ICHL CL Core
CLT CHT
Low Voltage Lead
Guard Tert
I & W Meter
Ground Lead CT

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Test #2: Measure CH

Test Set HV Cable

High
CH
GST
Guard-RB CHL
Low Voltage Lead Tank
Low and
ICH CL Core
CLT CHT
Low Voltage Lead
Guard Tert
I & W Meter
Ground Lead CT

Doble Conference 2009 43

Test #3: Measure CHL
HV Cable
Test Set
High
CH
UST-B
CHL
Ground R
Tank
Low Voltage Lead
Low and
ICHL CL Core
CLT CHT

Tert
Guard Low Voltage Lead
I & W Meter Ground Lead CT

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Test No. 4
Test Summary and Calculated CHL

ILINE4 = ITEST1 – ITEST2

WLINE4 = WTEST1 – WTEST2
%PFLINE4 = 10 * WLINE4 / ILINE4
CAPLINE4 = CAPTEST1 – CAPTEST2

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Three-Winding Transformer
Test Procedure (LV side)
Rotate Test Leads

Test Set Low Voltage Lead

High
CH
HV Cable CHL
CL Tank
Low and
Core
CLT CHT
Low Voltage Lead
Guard
Tert
I & W Meter CT
Ground Lead

Doble Conference 2009 46

Test #5: Measure CL+CLT

Test Set Low Voltage Lead

High
GST CH
Guard R
Ground B CHL
CL Tank
HV Cable
Low and
ICL + ICLT Core
CLT CHT
Low Voltage Lead
Tert
Guard
I & W Meter
Ground Lead CT

Doble Conference 2009 47

Test #6: Measure CL

Test Set Low Voltage Lead

High
CH
GST
Guard BR CHL
HV Cable Tank
Low and
ICL CL Core
CLT CHT
Low Voltage Lead
Guard
Tert
I & W Meter Ground Lead CT

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Test #7: Measure CLT

Test Set Low Voltage Lead

High
CH
UST-B
CHL
Ground R
HV Cable Tank
Low and
ICLT CL Core
CLT CHT
Low Voltage Lead
Guard Tert
I & W Meter Ground Lead CT

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Test No. 8
Test Summary and Calculated CLT

ILINE8 = ITEST5 – ITEST6

WLINE8 = WTEST5 – WTEST6
%PFLINE8 = 10 * WLINE8 / ILINE8
CAPLINE8 = CAPTEST5 – CAPTEST6

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Three-Winding Transformer
Test Procedure (TV side)
Rotate Test Leads

Test Set Low Voltage Lead

High
CH
CHL
CL
Low Voltage Lead Low
CLT CHT
HV Cable
Guard
Tert
I & W Meter CT
Ground Lead

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Test #9: Measure CT+CHT
Test Set Low Voltage Lead
High
GST CH
Guard R CHL
Ground B
CL
Low Voltage Lead
Low Tank
and
ICT + ICHT
Core
CL CHT
HV Cable T
Guard
Tert
I & W Meter CT
Ground Lead

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Test #10: Measure CT
Test Set Low Voltage Lead
High
CH
GST
Guard BR CHL
CL
Low Voltage Lead
Low Tank
and
ICT Core
CLT CHT
HV Cable
Guard
Tert
I & W Meter CT
Ground Lead

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Test #11: Measure CHT
Test Set Low Voltage Lead
High
CH
UST-B
Ground R CHL
CL
Low Voltage Lead Tank
Low and
ICHT Core
CLT CHT
HV Cable
Tert
Guard
I & W Meter CT
Ground Lead

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Test No. 12
Test Summary and Calculated CHT

ILINE12 = ITEST9 – ITEST10

WLINE12 = WTEST9 – WTEST10
%PFLINE12 = 10 * WLINE12 / ILINE12
CAPLINE12 = CAPTEST9 – CAPTEST10

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Test #13: Measure CH +CL +CT

HV Cable
Test Set
High
CH
GST
Ground CHL
CL
Low Tank
and
ICH + ICL + ICT
Core
CLT CHT

Tert
Guard
I & W Meter
Ground Lead CT

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 Transformers Overall
Tests Procedures
(Autotransformers with and
without Tertiary)

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Auto-Transformer Without Tertiary
(or Tertiary Winding Not Accessible)

H1 H2 H3
High and Low
H2
X2
H1 X1

X1 X2 X3 H0X0 X3
X0 H3

CH

Tank & Core

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Auto-Transformer Without Tertiary
(or Tertiary Winding Not Accessible)

Doble Conference 2009 59

Auto-Transformer With Accessible
Tertiary Winding

H1 H2 H3
High and Low
H2 Tertiary
CHT
X2 Y2
H1 X1
Y3
H0X0 X3
Y2 X1 X2 X3
Y1 H3 Y1 Y3
H0X0
CH CT

Tank & Core

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Auto-Transformer With
Accessible Tertiary Winding

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 Transformers Overall
Tests Procedures
(Safety Considerations)

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Apparatus and Personal Safety

• Follow Your Company’s De-Energizing and Safety

Tagging Procedures
• Work Between Installed Safety Grounds
• The Transformer Must be Disconnected From the
Power System
• The Transformer Tank Must Be Properly Grounded
• Do Not Stand on Apparatus While the Test Voltage
is Applied
• Both Doble Test Set Safety Switches Must be
Installed and Operational

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Apparatus and Personal Safety
 Do Not Hold the High Voltage Cable While Test
Voltage is Applied (Reference: 1995 Paper, Page
1-2.1 David Train & Lawrence Melia).

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 Transformers Overall
Tests Procedure
(General Considerations)

Doble Conference 2009 65

Transformer Overall Tests

Recommended Doble Power-Factor Test Voltages for

Liquid-Filled Power and Distribution Transformers

Winding Rating L-L (kV) Test Voltage (kV)

12 and Above 10
5.04 to 8.72 5
2.4 to 4.8 2
Below 2.4 1

Doble Conference 2009 66

Transformer Overall Tests
Recommended Doble Power-Factor Test Voltages for Liquid-
Filled Type Power and Distribution Transformers Tested in
the Absence Of Insulating Fluid and Under Atmospheric or
Greater Absolute Pressure
• Determine - by direct measurement - that the Gas space and fluid
contain safe combustible gas levels for testing
• Purge with dry nitrogen to obtain <2% oxygen
• Never apply test voltage to a transformer whose windings are
under vacuum or partial vacuum
• Low test voltage is sufficient to assess the general level of dryness
of a transformer insulation system
• Maintain adequate clearance between all energized, floating and
grounded conductors when testing with some or all of the
transformer leads, core, and coils out of the insulating fluid
• Do not apply test voltages that exceed those recommended for
transformers with the insulating fluid removed

Doble Conference 2009 67

Transformer Overall Tests
Recommended Doble Power-Factor Test Voltages for Liquid-
Filled Type Power and Distribution Transformers Tested in the
Absence Of Insulating Fluid and Under Atmospheric or Greater
Absolute Pressure
Transformer Winding Rating L-L (kV) Test Voltage (kV)
Delta Windings
161 and Above 10
115 to 138 5
34 to 69 2
12 to 25 1
Below 12 0.5
Wye Windings and Single Phase w/ Neutral
12 and Above 1
Below 12 0.5

Doble Conference 2009 68

Transformer Overall Tests
Recommended Doble Power-Factor Test Voltages for
Dry-Type Power and Distribution Transformers
Transformer Winding Rating L-L (kV) Test Voltages (kV)
Delta and ungrounded Wye Windings
Above 14.4 2 and 10
12 to 14.4 2, L-to-G, 10
5.04 to 8.72 2 and 5
2.4 to 4.8 2
Below 2.4 1
Grounded Wye Windings
2.4 and Above 2
Below 2.4 1

Doble Conference 2009 69

Overall Tests Results Analysis

Data Interpretation of Modern Oil-Filled Power

Transformers (percent Power Factor
Temperature Corrected to 20°C)

• Less or equal to 0.5% -- GOOD (acceptable)

• >0.5% up to 0.7% -- DETERIORATED
• >0.7% up to 1.0% -- INVESTIGATE
• Greater than 1.0% -- BAD (unacceptable)

GENERAL GUIDELINES!

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Overall Tests Results Analysis

Considering Age and Application

Rating Type New Used
0 - 500 KVA Distribution 1.0% 2.0%
> 500 KVA Power 0.5% 1.0%

For Geometry of the Specimen

Change in Charging Current Rating
0 - 3% G (acceptable)
3 - 5% D (deteriorated)
5 - 10% I (investigate)
 10% B (unacceptable)

RECOMMENDED LIMITS!

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Other Analysis Considerations

• Previous Test Results (Rate of Change)

• Test Temperatures
• Reason for Test: Acceptance, Routine or
Suspect
• Other Test Results, e.g.
– High P.F. Readings  Tip-Up, Lab Results (DGA,
Quality)
– Change in Capacitance  LRT and/or SFRA

Doble Conference 2009 72

Example of Analysis for Oil Filled
Power Transformers CH
CH Power Factors for Power Transformers
Number of Transformers
300
240
250
200 180

150 120
100 80
60
50 20 25 20 15
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

% Power Factor at 20 Degrees C

Doble Conference 2009 73

Analysis for Dry Type Transformers

• Limits Are Determined Based on Manufacturer and

Rating
• Based on a Recent Tabulation (1999), the Following
General Guidelines Can Be Used:

Epoxy-Encapsulated
Ventilated Transformers
Transformers
CHL  2.0%
CHL  1.0%
CH  3.0%
CH  3.0%
CL  4.0%
CL  2.0%

Doble Conference 2009 74

Analysis Example

Which transformer would you be more concerned with?

____________________________________________________________
Years 5 10 15 20 25 30

CH %P.F.
#1 .3 .31 .32 .36 .39 .46

#2
 .3 .31 .32 .32 .33 .46

Doble Conference 2009 75

Diagnostic Field Testing Standards

• IEEE Std 62 - 1995

– Guide for Diagnostic Field Testing of Electric Power
Apparatus – Part 1: Oil Filled Power Transformers,
Regulators, and Reactors
• Section 3 Definitions (3.6: Power Factor - Dielectric)
• Section 6 Tests and test techniques
– 6.1.2 Ratio/Polarity/Phase
– 6.1.3 Exciting Current
– 6.1.4 Short Circuit Impedance
– 6.1.6 Capacitance, Power Factor, and Dissipation Factor
– 6.3 Insulating Fluids (Transformer-Grade mineral Oil)
– Annex A (informative): Power Factor measurements

Doble Conference 2009 76

Transformers Excitation Current
Test Procedures

Doble Conference 2009 77

Transformer Excitation Current Test

The Doble Excitation Current Test has

been effective in finding:

• Abnormal core grounds

• Winding faults: shorts, open
circuits
• LTC problems
• Manufacturing defects

Doble Conference 2009 78

Excitation Current Principles: No Load
Iexcitation = Imag
 mag

+ 
V1  V2
- 

1) When an AC Voltage Source, such as a Doble Test Set, is placed on a

transformer, a small current will flow.
2) This small current is the Magnetizing Current: the current required to

magnetize the Transformer core with the Magnetic Flux mag. This
Magnetizing Current is the Excitation Current we measure and record.
3) This Magnetic Flux will induce a voltage across the secondary
windings: V2

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Excitation Current Principles: Load

Iexcitation = Imag +  + 2
mag
2 I2
I2
+ 
V1  V2 R2
- 

1) When a Load is placed on the secondary windings a current will flow

I2=V2/R2 [ Ohm’s Law ]

2) The Current I2 will in-turn create an Opposing Magnetic Flux 2.

3) The Generator, which regulates voltage at a set level, will provide more
current to maintain the core magnetized equal to the opposing flux

Iexcitation =Imag + I2

Doble Conference 2009 80

Excitation Current Principals:
Finding a Turn-to-Turn Fault

Imag+ If mag+ fault

HV fault X1
UST H1
E1 + Ifault
- LV
H0 X0

1) If a fault develops in the secondary windings, this fault will act as a

load across the faulted windings drawing a current Ifault
2) As a result, the Excitation Current will go up due to the opposing flux
created by the fault [Φfault]

Result: A Fault will cause Excitation Current to Increase

Doble Conference 2009 81

Excitation Current Principals:
Finding Grounded Windings
mag+ fault
Iexcitation=Imag+ Ifault fault X1 Ground Fault
UST HV H1
+
Ifault
-
LV H0 Xo
Neutral Ground

1) If the secondary winding has a grounded neutral and one of the windings
develops a fault to ground, grounded windings will draw a fault current
2) As a result, the Excitation Current will go up due to the opposing flux created
by the fault [Φfault]

Result: A grounded winding on a transformer with a grounded

neutral will cause the Excitation Current to go up.

Doble Conference 2009 82

Excitation Current Principals:
Effect of Preventive Autotransformer
Iexcitation=Imag+ IAT
mag+ AT
AT X1
UST HV H1
+ IA

-
LV H0 Xo

When an autotransformer is connected across two taps it acts as a

load and the primary current goes up.

Result: When a LTC transitioning device such as a Preventive

Autotransformer is in the bridging position the
excitation current goes up.

Doble Conference 2009 83

Excitation Current Testing

Excitation Current test results present patterns:

• Three Phase Transformers:
– Between phases
(Two Similar Highs and One Low)*
– Within each Phase
(When Tap Changers are present)**
• Single Phase Transformers
– Reverse measurement for confirmation

* There are exceptions

** Pattern will depend on Tap Changer type and Manufacture

Doble Conference 2009 84

Excitation Current: “The Pattern”

In a magnetic core, the Reluctance of the steel acts much

the same to resist magnetic flux as a resistor does to current.
A magnetic core can be represented by a Reluctance Circuit

=
(a) Ferrous Core (b) Reluctance Circuit

Doble Conference 2009 85

Excitation Current: “The Pattern”

For simplicity assume that each section has a reluctance of 1 

A 1 B 1 C

1 1

1 1

In this example we have a three Phase three legged core

Doble Conference 2009 86

Excitation Current: “The Pattern”

When you energize Phase A the equivalent reluctance circuit becomes:

P1
P1
HV
HV
3.75 
-
P2 P2

A B C
-
The same holds true when you energize Phase C

Doble Conference 2009 87

Excitation Current: “The Pattern”
For Phase B the equivalent reluctance circuit becomes:
P1 P1

HV
B 2.5 
-

P2 P2

For our example, phases A and C have equivalent reluctance of 3.75 ; Phase B
has equivalent reluctance of 2.5 confirming the expected pattern of Two Highs
and One Low measurement.
This general pattern holds true for most three phase transformers.

Doble Conference 2009 88

Excitation Current: Test Procedure

• Remove the short-circuit jumpers, these are single

phase tests
• Set up the Test connections on the high side,
observe SAFETY for the other winding(s)
• Proceed according to transformer winding
configuration (Delta or Wye)
• Ground any terminals that are normally grounded
during the test (i.e. neutral on a Wye transformer)
on the winding(s) that are not being energized
• All tests are performed in the UST Mode

Doble Conference 2009 89

Excitation Current: Test Procedure

When Tap Changers are present, test as follows:

Benchmark or Base Measurement: On a Routine Basis:

• LTC only: all positions • NLTC only: as found or position

regularly tapped
• NLTC only: all positions • LTC only: one full range plus neutral
and one position in opposite range
• LTC and NLTC combination: • NLTC and LTC combination:
all LTC positions with NLTC in NLTC in regularly tapped position or as
center position; then all positions found testing the LTC one full range
in NLTC with LTC in neutral plus neutral and one position in opposite
range

Doble Conference 2009 90

Excitation Current: Test Voltage
• Perform tests at the highest Voltage possible
• Test each Phase at the same Voltage
• Perform subsequent tests at the same Voltage for
comparison
• If a Preventive Autotransformer is included in the
transformer, it might not be possible to excite that
position of the LTC. In this event, testing might be
possible with the Preventive Autotransformer bypassed
or at a lower voltage
• If the test set trips, choose a lower Voltage and repeat
all three phases
• Never allow test voltage to exceed rated L-L for Delta
and L-G for Wye

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Excitation Current Test Procedure:
Managing field equipment resources
• Be aware of test equipment limitations
• Have consistent procedures
• Document special conditions, be specific
• If the Test set trips, choose a lower Voltage and repeat
all three phases
Example
M4K is rated for 300 mA at 10 kV
10 kV 300 mA
9 kV 333 mA
8 kV 375 mA
7 kV 429 mA
6 kV 500 mA
5 kV 600 mA

Doble Conference 2009 92

Excitation Current Test Procedure:
Test Voltage for Three Phase Units
Remember this:

=
(a) Ferrous Core (b) Reluctance Circuit

Because test must comply with:

• Test at the highest Voltage possible
• Test each Phase at the same Voltage
Phase demanding the highest current should be tested first

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Excitation Current Test Procedure:
How to determine Phase B

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Excitation Current Test Procedure:
Delta-Connected Winding

HV
H1
UST Test Measure
I1-2 I1-3 1 H1-H2
2 H2-H3
3 H3-H1
H3
H2
LV

When Low Voltage side is Wye configuration, remember to

ground the Neutral

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Excitation Current Test Procedure:
Wye-Connected Winding
HV
H1
UST I1-0 Test Measure
1 H1-H0
2 H2-H0
LV H0
3 H3-H0

H2 H3

When Low Voltage side is Wye configuration, remember to

ground the Neutral

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Excitation Current Test Procedure:
Single Phase

HV
H1 Two test required
UST Test Measure
I1-0
1 H1-H0
LV H0 2 H0-H1

Remember that the Excitation Current test is the only test where
the short circuit jumpers are taken off the bushings

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Excitation Current: Analysis of Test
Results
• Check the pattern: two similar high readings and one
low reading is normal (though there are exceptions)
• When Tap Changers are present, consider the pattern
within the phase
• For three-phase units, compare both high readings
with the following criterion:
– Readings <= 50 mA, difference should not exceed 10%
– Readings > 50 mA, differences should not exceed 5%
• For single phase units both readings should be
compared using the same above criterion
• Compare Normal test to alternate tests; results should
be similar for a winding in good condition
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Excitation Current: Analysis of Test
Results
• Compare Normal test to alternate tests; results should
be similar for a winding in good condition
• Compare readings to previous results. Ensure that the
same voltage was used for both tests for consistent
numerical comparison
• If the core is magnetized an irregular pattern (high,
medium, low readings) will be present and you will be
unable to compare results effectively. A true problem
could be masked, therefore core should be
demagnetized and test should be repeated
• If unusual results are obtained, consider performing an
alternate test to further investigate
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 Transformer Turn Ratio
Measurement Doble Test
Procedures

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Transformer Ratio Doble Test
• Turn Ratio of a Transformer is the ratio of the number
of turns in the HV winding to that in the LV winding
• Testing this Turn Ratio is part of recommended test
• Two test methods available:
– Voltmeter method (considered low Voltage test)
– Capacitance & Power Factor method (considered high Voltage
test 10–12 KV)

• Consider both as each identifies different failure modes

• If Tap Changers are present, test must include tap
positions and full winding

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Transformer Ratio Doble Test

• This is considered a single phase test

• Special procedures for cases such as inaccessible
neutrals in 3-phase Wye connections units
• Turn Ratio tolerance should be within 0.5% of the
nameplate specification
• For 3-phase Wye connection this tolerance applies to
the phase-to-neutral Voltage (if not in nameplate
calculate by dividing the phase-to-phase voltage by √3)
• Comparison method also applicable

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Doble Transformer Turn Ratio
The Doble Method
(considered the HV method)
HV Lead

UST
Doble TTR
CTRUE
Capacitor

IC
CTRUE=
10 kV x 
LV Lead

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Doble Transformer Turn Ratio:
Single Phase
Doble TTR
Capacitor
HV Lead

UST CTRUE

LV Lead

IC/N
C APPARENT = N= C TRUE / C APPARENT
10 kV x 

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Doble Transformer Turn Ratio:
3-Phase Delta-Wye
H2 X2

X1
X0
HV Lead
H3
H1 X3

UST

LV Lead

Test No. 1: H1 – H3 to X1 – X0 (Schematic shown)

Test No. 2: H2 – H1 to X2 – X0
Test No. 3: H3 – H2 to X3 – X0

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Doble Transformer Turn Ratio:
3-Phase Wye-Delta

H2 X2

X1
H0
HV Lead
H1 H3 X3
UST

LV Lead

Test No. 1: H1 – H0 to X1 – X2 (Schematic shown)

Test No. 2: H2 – H0 to X2 – X3
Test No. 3: H3 – H0 to X3 – X1

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Transformer Leakage Reactance
Doble Test

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Benefits of Leakage Reactance Test

• Assess physical condition of the

transformer (Geometry of Specimen)

• Investigate Winding Deformations

– Due to through Faults
– Due to rough transportation handling

• Confirm Nameplate Impedance

• Establish Impedance on a per-phase basis

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Leakage Reactance Principals
The combined action of both currents results in some of the flux
being present in the unit permeability space

L

The unit permeability space includes the space between the

windings, within the windings & between the windings and the tank

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Leakage Reactance Principals:
Leakage Flux
• The “primary” winding is linked by the
magnetizing flux and almost all the leakage flux
• The “secondary” winding is linked by the
magnetizing flux but very little of the leakage
flux
• Therefore the “primary” has a greater Voltage
induced in each of its turns under load than the
“secondary” winding
• We can account for this Voltage drop by
inducing a Leakage Reactance

Doble Conference 2009 110

Leakage Reactance Equivalent Circuit

R DC-1 R L-1 L 1 L 2 R L-2 R DC-2

Lm Rm
E1 CUST E2

Short-Circuit Impedance

R DC-1 R L-1 L 1 L 2 R L-2 R DC-2

E1 Leakage Reactance E2

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Leakage Channel
The leakage flux path includes the regions occupied by the windings.
The leakage reactance may be sensitive to deformations in the
windings.
Top yoke

Leakage channel
Outer winding
Core leg
Inner winding

Bottom yoke

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Leakage Reactance Equivalent Circuit

• Leakage Reactance for most transformers is constant

• Leakage Reactance can be measured without the
presence of the “full load” leakage flux that requires full
load current
• As in Short Circuit Impedance, Leakage Reactance can
be expressed in Ohms but:
– Can vary over several orders of magnitude
– Values may vary when measured from different sides of the
Transformer

• Preferred expression is in terms of Percent or per unit

(as Impedance in nameplate)

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Leakage Reactance and other tests
• Capacitance:
– Sensitive to temperature and contamination
– Normally involves all three phases

• Excitation Current:
– More sensitive to Core problems than winding
deformations

• Leakage Reactance:
– Not sensitive to temperature and contamination
– Can be performed on a Per-Phase basis
– Better sensitivity to winding deformations
– Can compare test results to Nameplate Impedance

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Leakage Reactance Test Procedures

Two parts test process:

• Three-Phase Equivalent Test

– LV side short-circuited
– Nameplate confirmation measurement

• Per-Phase Tests
– Corresponding LV side connection short-circuited
(minding your vectors)
– To establish Per-Phase Benchmarks measurements
(less physical area, better focus)

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Leakage Reactance Test Procedures

Divided in two steps:

• Initial Test
– Three-Phase Equivalent measurements (for
nameplate confirmation)
– Per-Phase measurements (for individual benchmark
of each phase)

• Subsequent Tests
– Only Per-Phase tests for comparison to established
Benchmark

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Test Connections:
(Doble M4000 – M4110)

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Measured During Test

• Current (injected and measured)

• Voltage (measured through sensing leads)
• Losses (Watts)
• Power-Factor (Percentage)
• Inductance (Henrys)
• Resistance (Ohms)
• Impedance (Ohms)
• Reactance (Ohms)
All should be consistent in all Phases for all Tests

Doble Conference 2009 118

Test Procedures: Three-Phase
Equivalent Connections

D-Y H1 H2 H3 X1 X2 X3 X0

Instrument

Test 1: H1 – H2 (schematic shown)

Test 2: H2 – H3
Test 3: H3 – H1
No Change on Low Voltage Connections

Doble Conference 2009 119

Test Procedures: Three-Phase
Equivalent Connections

Y-D
X1 X2 X3
H1 H2 H3 H0

Instrument

Test 1: H1 – H2 (schematic shown)

Test 2: H2 – H3
Test 3: H3 – H1
No Change on Low Voltage Connections

Doble Conference 2009 120

Test Procedures: Per-Phase Test
Connections

Y-D H1 H2 H3 H0
X1 X2 X3

Instrument

Test 1: H1 – H0 Short: X1 – X3 (schematic shown)

Test 2: H2 – H0 Short: X2 – X1
Test 3: H3 – H0 Short: X3 – X2

Doble Conference 2009 121

Test Procedures: Per-Phase Test
Connections

D-Y
H1 H2 H3 X1 X X X0
2 3

Instrument

Test 1: H1 – H3 Short: X1 – X0 (schematic shown)

Test 2: H2 – H1 Short: X2 – X0
Test 3: H3 – H2 Short: X3 – X0

Doble Conference 2009 122

Considerations for Leakage Reactance
Test Procedure
• Short circuiting cables (cable jumpers)
– Resistance = < winding leads
– Consider current carrying capacity
– Minimize inductance (minimize the length)

• Tap Changers positions (DETC and/or LTC)

• Test Voltage
– Manufacturers use 2 – 15 % Nominal Voltage
– Field testing at 120 or 240 volts
• Type of Test 1 versus 3

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Analysis of Test Results Initial Test

• Purpose of the Initial Test

– 3 Phase Equivalent: Compare With Nameplate
(Factory Test)
– Per-Phase: Establish a Benchmark

• When deviations are found may be related to

– Different instrumentation and test procedures
– Type of test (1 versus 3)
– Winding deformations

Doble Conference 2009 125

Analysis of Test Results Initial Test

• First or Initial 3-Phase Equivalent Test result

should be within ± 3% of Nameplate
• First or Initial Per-Phase Test results should
be within ± 3% of their average
• Winding deformation is unlikely if all three
Per-Phase Test results compare favorably
to each other
• Subsequent Per-Phase Test results should
be within ± 2% of Benchmark

Doble Conference 2009 126

Diagnostic Field Testing Standards

• IEEE Std 62 - 1995

– Guide for Diagnostic Field Testing of Electric Power
Apparatus – Part 1: Oil Filled Power Transformers,
Regulators, and Reactors
• Section 3 Definitions (3.6: Power Factor - Dielectric)
• Section 6 Tests and test techniques
– 6.1.2 Ratio/Polarity/Phase
– 6.1.3 Exciting Current
– 6.1.4 Short Circuit Impedance
– 6.1.6 Capacitance, Power Factor, and Dissipation Factor
– 6.3 Insulating Fluids (Transformer-Grade mineral Oil)
– Annex A (informative): Power Factor measurements

Doble Conference 2009 127

 DTA Transformers Tests
Procedures
(Useful tips for DTA users)

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Transformer Programs

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Transformer Tests

• Overall
• Bushings (C1, C2, Hot Collar)
• Excitation Current
• Doble Turns Ratio
• Turn Ratio
• Leakage Reactance
• Insulating Fluid
• Surge Arresters

From Doble Test Assistant

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Transformer Identification Panel

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Transformer Identification Panel

Files are Located in Directory C:\doble\dta

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Transformer Identification Panel

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Transformer Identification Panel

If using the Temperature Sensor DO NOT

Enter the Air Temperature. “Prb” notation
will be shown. When the first test is run the
probe will measure the ambient temperature and
humidity.
Doble Conference 2009 134
Transformer Identification Panel

Items required to be entered to get a proper

temperature correction factor.

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Two-Winding Transformer
Overall Test Sheet

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Overall Test Temperature Correction

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Overall Test Temperature Correction

 161
 161 kV kV

1.57 0.95

0.41 0.70

Doble Conference 2009 138

Insulation Ratings: Doble Test Assistant
Good condition, acceptable for service until
G the next scheduled test.
Deteriorated. Not as good as before.
D Re-test in a year or less to monitor
insulation condition.
Investigate. Test results show problems
I with the specimen or test procedures.
Perform additional tests or maintenance.
Bad. Not suitable for service.
B Remove from service until condition
is clarified or replace specimen.
Questionable. Nameplate data is incomplete
Q and DTA can not determine rating.

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Let’s avoid this!...

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