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Transformer Testing

Matz Ohlen

M.Sc.EE

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Life of a transformer – with and without diagnostics and maintenance actions

(Mechanical/dielectrical strength)

(due to increased loading etc)

Diagnostic testing

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(Transformer) Modeling

� “Make it as simple as possible; but not simpler than

that”, Albert Einstein

� Generally in science

• Observe and measure

• Create a model

• Verify model by experiments

• Extend model (only) if necessary

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Transformer – “The electric gear box”

� Fixed ratio, e.g. 100 kV / 10 kV

� LTC (Load Tap Changer)

• 100±9x1.67% / 10 kV

� DETC (De-Energized Tap Changer)

• Tuning voltage level

• Change configuration, e.g. from Dzn10 to Yzn11

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Basic transformer model (impedance)

� Primary side:

• Winding resistance,

• Leakage Inductance (X1+(N1/N2)2*X2)

• Magnetization current (excitation current)

� Secondary side

• Winding resistance

• Inductance

� Primary to secondary

• Turn ratio (E1/E2)

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Measuring the parameters

� Excitation/magnetization

current (field) and no-load loss

(factory) measurements

� Winding resistance

measurements (factory and

field)

� Load-loss measurements

(factory)

� Short-circuit impedance

(voltage) measurements

(factory and field)

� Turn ratio (factory and field)

R+jX = R1+jX1 + (N1/N2)2 x (R2+jX2)

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Summary - Impedance modeling

� The ideal transformer combined with the

simple equivalent circuit describes the

physics of the transformer well.

� The important measurement parameters

are:

� Magnetization/Excitation current

(voltage dependent)

� Winding resistance (temperature

dependent)

� Short-circuit impedance/Leakage

reactance

� Turns ratio

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Insulation modeling and measurement

V

A

Hi

Lo

GroundCHL

CL CH

It is always a capacitor!

Measure with:

� DC

� AC (single freq)

� AC (multi freq)

� High or low

voltage

Use Ohms law to calculate:

� Insulation resistance

� Tan delta/Dissipation factor

� Power factor

� Capacitance

� Permittivity

� Etc

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Oil modeling and measurement (electrical)

V

A

Hi

Lo

GroundCHL

CL CH

Dielectric test:

� Measure voltage and current (test cell)

� Calculate conductivity (mS/m) or

dissipation factor at power frequency

Dielectric strenght:

� Measure the breakdown voltage (test cell)

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Transformer designs:Shell form and core form transformers

Source:

ABB Transformer Service

Handbook

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Shell form and core form transformers

� Advantages of Shell Form Transformers (from www.meppi.com )

• High short-circuit withstand capability

• High mechanical strength

• High dielectric strength

• Excellent control of leakage magnetic flux

• Efficient cooling capability

• Flexible design

• Compact size

• Highly Reliable Design

� Disadvantages of Shell Form Transformers• Complicated manufacturing process, i.e. more expensive

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Types of transformers

� Power Transformers

� Distribution Transformers

� Phase-Shifting Transformers

� Rectifier Transformers

� (Dry-Type Transformers)

� (Instrument Transformers)

� (Step-Voltage Regulators)

� (Constant-Voltage Transformers)

� (Reactors)

Electric Power Transformer Engineering, CRC Press, 2004, ISBN 0-8493-1704-5

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Type of transformers

� Power Transformers

• One-phase

• Three-phase

� Distribution Transformers

• Three-phase

• “Two”-phase (mainly in the U.S)

� Phase-Shifting Transformers

• Three-phase

� Rectifier Transformers

• Six-phase, twelve-phase

Electric Power Transformer Engineering, CRC Press, 2004, ISBN 0-8493-1704-5

Why three-phase?

The sum of the power in the three phases is constant, i.e. the torque is constant.

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Configurations

� One phase, two windings or three windings

� Three phase, two windings or three windings. e.g. YNyn0yn6

� Auto transformers with or without tertiary, e.g. YNa0

� Rectifier transformers: Six-phase, twelve phase

� Phase-shifting (uses LTC on a different core leg)

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Transformer stress factors� Thermal Stress

• Develops across all assembly components. It ages the dielectric material (even at rated

temperatures). Stress to insulation worsened by high temperature and repeated changes in

temperature. Temperature is one of the most important factors affecting transformer life!

� Chemical Stress

• Cellulose decomposes and create water in the solid insulation

• Oil additives may have a deteriorating effect on insulation components.

� Electrical Stress

• Develops within the insulation material separating conductors at different potentials. It ages

the dielectric materiel (even at rated voltages)

� Mechanical Stress

• Deforms/displaces assembly components. High shock events during shipping displaces

components. High current events (short-circuit faults) creates high impact forces and may

cause winding deformation.

� Environmental Stress

• Water may enter the transformer through leaking gaskets

• Salt and other corrosive substances in the environment may deteriorate tanks and

peripheral equipment.

High temperature and water/moisture will finally kill every transformer!

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Transformer fault statistics - Fault reasons

Source:Cigré, IEEE, Hartford S&BUniv of Queensland, ZTZServiceCanadian El Assosiation, Doble mfl

What was the reason for the transformer fault?

Sabotage

Poor

worksmanship

Overload

Moisture

Contacts Joints

etc

Maintenance

reasons

Lightning

Insulation

reasons

Electric

reasons in the

net

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Different methods for fault detection

Insulation

testing

Reenergizing

Winding

resistance

External

inspecton

Exitation

current

Internal

inspection

TTR

Protectionrelay

s

How was the fault found?

Source:Cigré, IEEE, Hartford S&BUniv of Queensland, ZTZServiceCanadian El Assosiation, Doble mfl

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Testing and Standards

� CIGRE

• CIGRE Brochure 342 (SFRA-FRAX)

• CIGRE Brochure 414 (DFR-IDAX)

• CIGRE Brochure 445 (Guide for Transformer Maintenance)

� IEC, TC 14

• IEC 60076-1, Power Transformers

• IEC 60076-2, Temperature rise

• IEC 60076-3, Insulation levels, dielectric tests and external clearances in air

• IEC 60076-18, Measurement of frequency response (SFRA)

� ANSI, IEEE Transformer Committee

• IEEE C57.152 (formerly IEEE 62-1995)

• IEEE C57.12.00-2006 (under revision)

• IEEE C57.12.90-2006 (under revision)

• IEEE PC57.149 (SFRA)

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Transformer diagnostics by test set

STANDARD TEST TYPES TEST SETS

Component TestDelta IDAX MIT FRAX MLR TTR MTO CBA/LTC MoM OTS KF

Windings

Resistance X X

Ratio/polarity X

Excitation current X X X

Short-circuit impedance X X

Frequency response analysis X

Insulation resistance X

Capacitance X X

Power factor/tan delta X X

Dielectric frequency response X

Bushings

Capacitance X X

Power factor/tan delta X X

Dielectric frequency response X

Insulating oil

Water content X

Dielectric strength X

Power factor/tan delta X X

Cellulose insulation Moisture content X

Tap changers

Load

Resistance X X

Ratio X

Continuity (make before break) X X

Dynamic resistance (DRM) X

De-energizedResistance X X

Ratio X

Core/Tank

Insulation resistance X X

Frequency response analysis X

Ground test X

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WRM (winding resistance measurements)

� Winding resistance measurements

• 1, 2 or 6-ch measurement

• Single winding or SWM current injection (HV and LV simultaneously)

� Tap-changer testing

• Winding resistance/tap

• Current change detection (continuity)

� Demagnetization of transformer core

• Important to perform before excitation current and SFRA measurements

• Recommended in IEEE as standard procedure before taking the transformer in service (in-rush current risk)

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TTR measurements

� Transformer turn-ratio measurement

� Polarity and phase deviation test

� Excitation current measurement

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� Low resistance measurements of;

• HV/LV leads and connections

• Core ground connection

� Some units can perform winding

resistance measurements on small

transformers

MicroOhmmeters

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Short-circuit impedance/leakage reactance

� Short-circuit impedance/leakage

reactance measurement @ 50/60 Hz

� Test method and practice described in

e.g. IEEE C57.152

� For three-phase transformers, the per-

phase test results should be within 3%

of the average value of all three

phases.

� Some units also provides capacitor

bank testing

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SFRA

� Sweep frequency response analysis (SFRA)

from Hz to MHz

� Excitation impedance measurement [open]

� Short-circuit impedance testing [short]

� Fingerprint

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DC insulation testing

� DC insulation testing

• Insulation resistance (IR)

• Polarization index (PI)

• Dielectric absorption (DAR)

• Step voltage testing

Note: Polarization index, PI, was introduced to detect moisture in the

winding insulation of rotating machines. For the complex oil-cellulose

insulation system of transformers the PI results can be misinterpreted

and the polarization method should not be used to assess insulation

condition in new power transformers.

Polarization index for insulation liquid is always close to 1. Therefore,

the polarization index for transformers with low conductivity liquids

(e.g. new mineral oil) may be low in spite of good insulation condition.

DFR/FDS/PDC measurements are preferred for quantitative

assessments of the moisture in the insulation.

(CIGRE 445 “Guide for Transformer Maintenance” and IEEE

PC57.152 “IEEE Guide for Diagnostic Field Testing of Fluid-Filled

Power Transformers, Regulators, and Reactors”)

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Tan delta/power factor measurements

� 12 kV insulation testing

• Tan delta/power factor and capacitance

• Power transformers

• Bushings

• CT/VT’s

• Generators and motors

• Circuit Breakers

• Oil (with test cell)

• Etc...

� 50/60 Hz (power frequency)

standard test

� Some units offer testing over a

frequency range

� Excitation current measurement

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Dielectric response measurements

� Measurements • Typically 200 to 2000 V (peak) test voltage

• DC and/or AC methods

• DFR/FDS (AC)

• PDC (DC)

• RVM (DC)

• Standardized and described in CIGRE

reports

� Automatic analysis of:• Moisture in cellulose insulation

• Some units offer modeling-characterization

of tan delta temperature dependence

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Oil testing – Dielectric strength

� Two methods - D877 and D1816

� A measure of a liquids ability to

withstand electric stress without

failure

� Detects presence of contaminants:

water, dirt, fibers, conducting

particles

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Oil testing – Water in oil (Karl-Fischer)

� Measures the water content in the oil

� Standard test performed by most

transformer owners

� Results are traditionally also used to

estimate moisture in the cellulose

insulation by using equilibrium charts

� The method tends to overestimate

moisture in cellulose…

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The doctor is in...