SFRA - Theory and Method - Standards_120911

8/10/2019 SFRA - Theory and Method - Standards_120911

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Advanced Transformer Testing 2012

Sweep Frequency Response Analysis


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Transformer Diagnostics

Transformer Diagnostics is about acquiring accuratemeasurement data and other information in order to make

the correct decision about what to do with the actual unit

TTRSFRA

FDS

WRM


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SFRA testing basics

Off-line test

The transformer is seen as a complex

impedance circuit [Open] (magnetization impedance)

and [Short] (short-circuit impedance)

responses are measured over a wide

frequency range and the results are

presented as magnitude response

(transfer function) in dB

Changes in the impedance/transfer

function can be detected and

compared over time, between test

objects or within test objects

The method is unique in its ability todetect a variety of winding faults, core

issues and other electromechanical

faults in one test


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Rponse et analyse dun balayage en

frquenceSFRA mathematics basics

in

out

V

VdBG 10log20)(

Gain, dB Phase,

Generator test voltage Measured voltage


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Standards Summary


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SFRA Standards and Recommendations

Frequency Response Analysis on Winding Deformation of PowerTransformers, DL/T 911-2004, The Electric Power Industry Standard of

Peoples Republic of China

Mechanical-Condition Assessment of Transformer Windings Using

Frequency Response Analysis (FRA), CIGRE report 342, 2008

IEC 60076-18, Power transformersPart 18: Measurement offrequency response, 2012

IEEE PC57.149, Guide for the Application and Interpretation of

Frequency Response Analysis for Oil Immersed Transformers, 2012

Internal standards by transformer manufacturers, e.g. ABB FRA

Standard v.5


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SFRA - Theory and method


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FRA definitions

Frequency response The amplitude ratio and phase difference between voltages measured at

two terminals of the test object over a range of frequencies when one of the

terminals is excited by a voltage source. The frequency response

measurement result is a series of amplitude ratios and phase differences at

specific frequencies over a range of frequency.

As Vout/Vinvaries over a wide range, it is expressed in decibels (dB). The

relative voltage response in dB is calculated as 20 x log10(Vout/Vin)

Frequency response analysis (FRA) The technique used to detect damage by the use of frequency response

measurements.


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FRA history

1960: Low Voltage Impulse Method. First proposed by W. Lech & L. Tyminski

in Poland for detecting transformer winding deformation. 1966: Results Published; Detecting Transformer Winding Damage - The

Low Voltage Impulse Method, Lech & Tyminsk, The Electric Review, UK

1978: Transformer Diagnostic Testing by Frequency Response Analysis,

E.P. Dick & C.C. Erven, Ontario Hydro, IEEE Transactions of Power Delivery

1980 - 1990s : Proving trials by utilities and OEMs, the technology cascadesinternationally via CIGRE, and many other conferences and technical

meetings

2004: First SFRA standard, Frequency Response Analysis on Winding

Deformation of Power Transformers, DL/T 911-2004, is published by The

Electric Power Industry Standard of Peoples Republic of China

2008: CIGRE report 342, Mechanical-Condition Assessment of TransformerWindings Using Frequency Response Analysis (FRA) is published

2012: IEC60076-18 and IEEE PC57.149 are released


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Transformer mechanics basics A transformer is designed to handle certain (high!) mechanical

forces.

Design limits can be exceeded due to

Excessive mechanical impact

Transportation

Earthquakes

Over currents caused by

Through faults

Tap-changer faults

Faulty synchronization

Mechanical strength weakens as the transformer ages

Less capability to handle high stress/forces

Increased risk of mechanical problems

Increased risk for insulation problems


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To detect core displacement and windingdeformation due to e.g.

Large electromagnetic forces from fault current

Transformer transportation and relocation

If these faults are not detected they may developinto dielectric or thermal faults which normally

results in the loss of the transformer

Periodic testing is essential!

Why assess the mechanical condition?


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Detecting Faults with SFRA

Winding faults

Deformation Displacement

Shorts

Core related faults

Movements Grounding

Screens

Mechanical faults/changes

Clamping structures Connections

And more...


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SFRA measurement circuitry


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A large number of low voltage signals with varying

frequencies are applied to the transformer The input and output signals are measured in amplitude

and phase

The ratio of the two signals gives the frequency

response or transfer function of the transformer From the (complex) transfer function you can derive a

number of entities as function of frequency e.g.

Magnitude

Phase

Impedance/admittance

Correlation

SFRAHow does it work (1)


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The RLC network has different impedance at different

frequencies.

The transfer function for all frequencies is the measure

of the effective impedance of the RLC network.

A geometrical deformation, changes the RLC network,

which in turn changes the impedance/transfer function

at different frequencies.

These changes gives an indication of damage within a

transformer.

SFRAHow does it work (2)


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SFRA resultsFrequency regions

Transformer issues can be

detected in different frequencyranges

Low frequencies

Core problems

Shorted/open windings

Bad connections/increased

resistance

Short-circuit impedance

changes

Medium frequencies

Winding deformations

Winding displacement Highfrequencies

Movement of winding and tap

leads

Winding

interaction and

deformation

Winding

and tap

leads

Core + windings


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Frequency regions by IEC and IEEE

101

102

103

104

105

106

107

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

Frequency, Hz

Magnitu

de,

dB

A phase

B phase

C phase

Core

influence

Interaction

between

windings

Winding

structure

influence

Earthing

leads

influence


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SFRA measurement frequency ranges

IEC60076-18

Category Low frequency limit High frequency limit

Power transformers, Uw < 72.5 kV < 20 Hz > 2 MHz

Power transformers, Uw > 72.5 kV < 20 Hz > 1 MHz

Comparing older measurements

and/or methods/practices not

following IEC method 1 (CIGRE 342)

standard for signal shield grounding

< 20 Hz 500 kHz


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SFRA measurement frequency ranges

Examples

Standard Low frequency limit High frequency limit

Eskom standard 20 Hz 2 MHz

ABB standard 10 Hz 2 MHz

Japan (impedance) 100 Hz 1 MHz

DL/T-911 2004 1 kHz 1 MHz

Typical instrument default values are 20 Hz2 MHz


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Comparative tests

Transformer A

Transformer A Transformer B

Time based

Type based

Design based


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Comparisons

Time Based (Tests performed on the same transformer over time) The most reliable test

Deviations between curves are easy to detect

Type Based (Tests performed on transformer of same design) Requires knowledge about test object/versions

Small deviations are not necessarily indicating a problem

Design based (Tests performed on winding legs and bushings ofidentical design)

Requires knowledge about test object/versions

Small deviations are not necessarily indicating a problem


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SFRA Measurement philosophy

New measurement = Reference measurement

Back in Service

New measurement Reference measurement

Further Diagnostics Required


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Reference measurements

When transformer is new Capture reference data at commissioning of

new transformers

When transformer is in known good

condition Capture reference data at a scheduled routine

test (no issues found)

Save for future reference

Start Your Reference Measurements ASAP!

SFRA t Wh ?


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SFRA measurementsWhen?

Manufacturing tests

Quality check during manufacturing

Proofing the transformer after short-circuit test Before shipping

Installation/commissioning

Relocation

After a significant through-fault event

Part of routine diagnostic test

Catastrophic events

Earth quakes

Hurricanes/tornadoes

Trigger based test/transformer alarms

Buchholz

DGA High temperature

Before-after maintenance


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Transformer fault detection


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26

Prior to SFRA the mechanical integrity of the

transformer was assessed with the following standard

methods:

Winding capacitance

Excitation current

Leakage reactance measurements

Each of these methods have drawbacks

Detection of Winding Movement (1)


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Winding Capacitance

Successful only if reference data is available

Limited sensitivity for some failure modes

Excitation Current

Excitation current is an excellent means of detecting turn-to-turn

failure as a result of winding movement

If a turn-to-turn failure is absent, winding movements can remain

undetected.

Leakage Reactance

Per phase leakage reactance measurements generally shows

no or little correlation between the phases and nameplate

Discrepancies from nameplate value of 0,5 % to 3 % can be a

reason for concern

The range of defect detection is to large for an accurate

assessment

Detection of Winding Movement (2)


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Comparing diagnostic techniques (CIGRE)

Diagnostic technique Advantages DisadvantagesMagnetizing (exciting) current Requires relatively simple equipment.

Can detect core damage

Not sensitive to winding deformation.

Measurement strongly affected by core

residual magnetism

Impedance (leakage reactance) Traditional method currently specified in

short-circuits test standards.

Reference (nameplate) values are

available

Very small changes can be significant.


(best for radial deformation)

Frequency Response of Stray Losses

(FRSL)

Can be more sensitive than impedance

measurement.

Almost unique to detect short circuits

between parallel strands

Not a standard use in the industry

Winding capacitance Can be more sensitive than impedance

measurements.

Standard equipment available


(best for radial deformation).

Relevant capacitance may not be

measurable (e.g. Between

series/common/tap windings for auto

transformers)

Low Voltage Impulse (LVI) (time domain) Recognized as very sensitive Specialist equipment required.

Difficult to achieve repeatability.

Difficult to interpretFrequency Response Analysis Better repeatability than LVI with the

same sensitivity.

Easier to interpret than LVI (frequency

instead of time domain).

Increasing number of users

Standardization of techniques required.

Guide to interpretation required


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Comparing SFRA and other traditional

transformer measurements

End-to-End [Open], (Open Circuit Self Admittance] Example: 1U - 1N [open]

Excitation current as function of frequency

End-to-End [short], (Short Circuit Self Admittance) Example: 1U1N [short]

Leakage reactance/short-circuit impedance as function of frequency (compareIEEE 62 measurements at 50/60 Hz)

FRSL, Frequency Response of Stray Losses (SFRA 20600 Hz) Input Impedance

Measurement of impedance to ground for a certain configuration (Japanesestandard, common in South America, common in China before DL/T 911)

Can be performed for grounded objects with the active impedance probe

Capacitive Inter-winding [Inter-Winding] Capacitance as a function of frequency

Inductive Inter-winding [Transfer Admittance] Turn-ratio measurement (voltage ratio) as a function of frequency

Possible to perform at various impedances with the active voltage probe


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SFRA vs Excitaion current

Example; U1 - N1 [open] Excitation current as function of

frequency

Please note that excitation current is

voltage dependent!

At low voltages the inductance is low and

increasing with voltage

At high voltages the core gets saturated and

the inductance decreases

Non-linear phenomena...

SFRA


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SFRA vs short-circuit impedance/leakage reactance

Example; U1N1 [short] Short-circuit impedance/Leakage reactance as a function

of frequency (IEEE50/60 Hz @ 200 V)

Leakage reactance is not voltage dependent. However, in certain

configurations the magnetizing impedance can influence the results

at lower test voltages FRSL, Frequency Response of Stray Losses (SFRA 20

600 Hz @ ~200 V)


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Frequency Response of Stray Losses (FRSL)

End-to-End (short-circuit), [Short

Circuit Self Admittance]

Impedance changes may be caused

by;

Inductance changes e.g winding

movement

Resistance change (DC) due to badcontacts, soldering issues etc

Resistance change at higher

frequencies (Rstray) due to stray losses

caused by;

Winding deformation

Shorts between parallel strands Ref: L. BOLDUC, et. Al DETECTION OFTRANSFORMER WINDING DISPLACEMENT

BY THE FREQUENCY RESPONSE OF STRAY

LOSSES (FRSL), CIGRE session, 2000.


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FRSL160 MVA transformer with contact resistance problem

HV [short], Transformer G2-1

HV [short], Transformers G2-1 and 3


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FRA Methods

Sweep Frequency Response Impulse


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Impulse FRA vs. SweepFRA

Impulse FRA

Injects a pulse signal andmeasure response

Convert Time Domain to

Frequency Domain using Fast

Fourier Transform (FFT) algorithm

Low resolution in lower frequencies

SFRA

Injects a single frequency signal

Measures response at the same

frequency No conversion

High resoultion at all frequencies

Impulse FRA


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Comparing Impulse & SweepFRA

SFRA (Sweep frequency response analysis)provides good detail data in all frequencies

Black = Imported Impulse measurement

(Time domain converted to Frequency Domain)

Red = SFRA Measurement

Deviations Low Frequency = Method

Deviation High Frequency = Cable practice


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Zoom View of impulse vs. SFRA

Impulse instrument sample rate limts

frequency resolution to 2kHz.


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SFRA Measurement Technique, part 1

- Measurement setups


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SFRA test setup


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FRAX measurement circuitry


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Test typesEnd-to-end (open)

Test signal is applied to one end of a winding and thetransmitted signal is measured at the other end

Magnetizing impedance of the transformer is the main

parameter characterizing the low-frequency response

(below first resonance) in this configuration

Commonly used because of its simplicity and the possibility

to examine each winding separately


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End-to-end (open) - Example

Low frequencies

May vary between measurements pending magnetization

Typical dubbel-dip response B-phase should be below A and C-phase (Y)


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Test typesEnd-to-end short-circuit

The test is similar to the end-to-end measurement, but witha winding on the same phase being short-circuited

The influence of the core is removed below about 10-20

kHz because the low-frequency response is characterized

by the short-circuit impedance/leakage reactance instead ofthe magnetizing inductance

Response at higher frequencies is similar to end-to-end

(open) measurements


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End-to-end (short) - Example

Low frequencies

All phases should be very similar. > 0.25 dB difference may indicate leakagereactance/winding resistance/connection/tap-changer problems

T C i i i i di (IW)


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Test typesCapacitive inter-winding (IW)

Test signal is applied to one end of a winding andthe response is measured at one end of another

winding on the same phase (not connected to the

first one)

The response using this configuration is dominatedat low frequencies by the inter-winding capacitance

T t t I d ti i t i di (TA)


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Test typesInductive inter-winding (TA)

The signal is applied to a terminal on the HV side, andthe response is measured on the corresponding

terminal on the LV side, with the other end of both

windings being grounded

The low-frequency range of this test is determined bythe winding turns ratio

I t i di t E l


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Inter-winding measurements - Example

IW (red) is capacitive at low frequencies

TA (black) reflects turn ratio at low frequencies (135 MVA, 160/16 Dd0)

Similar response at high frequencies


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SFRA Measurement Technique, part 2

- How to achieve high quality results

Test results always comparisons


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Test resultsalways comparisons

Repeatability is of utmost importance!

Core NOT grounded

Core grounded


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Example of repeatability

105 MVA, Single phase Generator Step-up (GSU)transformer

SFRA measurements with FRAX 101 before and

after a severe short-circuit in the generator

Two different test units

Tests performed by two different persons

Test performed at different dates

B f (2007 05 23) d ft f lt (2007 08 29)


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Before (2007-05-23) and after fault (2007-08-29)

LV winding

HV winding


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105 MVA, Single phase GSU

Measurements before and after were virtuallyidentical

Very good correlation between reference and after

fault

Conclusion: No indication of mechanical changes in the transformer

Transformer can safely be put back in service


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Potential compromising factors

Measurement signal connection quality

Shield grounding practice

Instrument dynamic range/internal noisefloor

Understanding core property influence inlower frequencies in open - circuit SFRAmeasurements


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Bad connection

Bad connection can affect the curve at higher frequencies


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Good connection

After proper connections were made

FRAX C Cl i ti lit


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FRAX C-Clampensuring connection quality

C-Clamp ensures goodcontact quality

Penetrates non conductive

layers

Solid connection to round orflat busbars/bushings

Provides strain relief for cable

Separate connector for single

or multible ground braids


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Proper ground connection ensures

repeatability at high frequencies

Good grounding practice;use shortest braid from cable

shield to bushing flange.

Poor grounding practice


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Shield grounding influence

C. Homagk et al, Circuit design for reproducible on-site measurements of

transfer function on large power transformers using the SFRA method, ISH2007

FRAX bl t d di


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FRAX cable set and grounding

Always the same ground-loop

inductance on a given bushing


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Instrument performance

Transformers have high impedance/largeattenuation at first resonance

Internal instrument noise is most often the main

limiting source, not substation noise

Test your instruments noise floor by running asweep with open cables (Clamps not connected to

transformer)

I t l i l l N i fl


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Internal noise levelNoise floor

Open/noise floor measurements

Red = Other brand

Green = FRAX 101

Example of internal noise problem


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Example of internal noise problem

H1H2 (open & short) measurements

Black = Other brand

Red = FRAX 101

Wh d t l t 100 dB


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Why you need at least -100 dB...

Westinghouse 40 MVA, Dyn1, 115/14 kV, HV [open]

I fl f


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Influence of core

Try to minimize the effect, however, somedifferences are still to be expected and must beaccepted (magnetic viscosity).

Preferably:

perform SFRA measurements prior to winding

resistance measurements (or demagnetize thecore prior to SFRA measurements)

use same measurement voltage in all SFRAmeasurements

f S


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Run winding resistance test after SFRA!

After

demagnetization

H1-H2 [open]

After winding resistance test

Core magnetization by Doble


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Core magnetization by Doble

Trace A shows the fingerprint response of the transformer and trace Bshows the response as a result of magnetized core (caused by WRM

measurements)

Magnetization status over time


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Magnetization status over time

Lachman et al, Frequency ResponseAnalysis of Transformers and Influence

of Magnetic Viscosity, Doble 2012

Effect of applied measurement voltage


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10V peak-to peak

H1-H0 [open]

0.1 V peak-to-peak

Influence of applied

voltage is morepronounced on LV

windings

Effect of applied measurement voltage

Measurement voltage by Tettex


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Measurement voltage by Tettex


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Measurement voltage effectin practice

2.8 V

Omicron

10 V

FRAX, Doble and others

FRAX101 h dj t bl t t lt !


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FRAX101 has adjustable output voltage!

Omicron (2.8 V)

FRAX, 2.8 V

Influence of tap changers


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Influence of tap changers

The tap windings in a transformer add in one section at a time

- affecting the low frequency (magnetization impedance)response and the mid-frequency (winding) response

Tap lead responses will be seen at higher frequencies than

the tap windings. They are less organized but are still

repeatable

Some tap-changers have a neutral position which is moredifferent than the difference between consecutive taps. Avoid

using the neutral position as reference measurement

Distribution transformer with 5 HV taps


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Distribution transformer with 5 HV taps

Low frequency

effect

Tap winding

Tap changer measurements by Doble


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Tap changer measurements by Doble

Low frequency

effect

Tap winding

Tap leads

System integrity test


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System integrity test

Field verification unit with known

frequency response is

recommended in CIGRE andother standards to verify

instrument and cables before

starting the test

Summary


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Measurement quality and repeatability

The basis of SFRA measurements is comparison andrepeatability/reproducibility is of utmost importance

To ensure high repeatability; Select a high quality, high accuracy instrument with high dynamic

range and input/output impedance matched to the coaxial cables(e.g. 50 Ohm)

Make sure to get good signal connection and connect the shieldsof coaxial cables to flange of bushing using shortest braidtechnique

Use the same applied voltage in all SFRA measurements

Be careful about WRM testing and other tests that can magnetize

the core. Perform after SFRA or demagnetize prior to SFRA Make good documentation, e.g. make photographs of connections

and note tap settings


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SFRA Analysis



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Winding faults

Deformation Displacement

Shorts

Core related faults

Movements

Grounding

Screens

Mechanical faults/changes

Clamping structures

Connections

And more...

SFRA analysis tools


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Visual/graphical analysis

Starting dB values The expected shape of star and delta configurations

Comparison of fingerprints from;

The same transformer

A sister transformer

Symmetric phases

New/missing resonance frequencies

Correlation analysis

DL/T 911 2004 standard

Customer/transformer specific

Typical response from a healthy transformer


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80

y y

HV [open] as expected for

aY tx

Double dip and mid

phase response lower

Very low deviation

between phases for

all testsno winding

defects

HV [short] identicalbetween phases

LV [open] as

expected for aY tx

Transformer with serious issues...


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81

Large deviationsbetween phases at mid

and high frequencies

indicates winding faults

Large deviations

between phases for

LV [open] at low

frequencies

indicates changes in

the magnetic

circuit/core defects

Transformer with winding shorted turn


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Easiest fault to recognize with SFRA

Typically produced by a through current fault

Adjacent turns lose paper and weld together resulting in a

solid loop around the core

SFRA gives clear and unambiguous diagnosis of ashorted turn

SFRA response for the shorted phase may be identified

without reference results since the variation at low

frequencies gives a clear fault signature

Shorted turn (IEEE)


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Shorted turn (IEEE)

Frequency

Range

Winding Turn-to-Turn Short Circuit

Assuming no other failure modes exist:

20 Hz10 kHz Open Circuit Tests:

The short circuit failure mode removes the effect of the coresreluctance from

the open circuit FRA results. The FRA open circuit trace assumes a similar

behavior as short circuit test. The affected winding will show the greatest

change. This failure mode will also affect the FRA responses from all other

windings, but not as much.

Short Circuit Tests:

The results will not compare well against previous data or amongst phases. The

affected winding is generally offset.5 kHz100 kHz Open Circuit and Short Circuit Tests:

This range can shift or produce new resonance peaks and valleys. The changes

will be greater on the affect phase.

50 kHz1 MHz Open Circuit and Short Circuit Tests:

This range can shift or produce new resonance peaks and valleys. The changes


> 1 MHz Open Circuit and Short Circuit Tests:This range can shift or produce new resonance peaks and valleys. The changes


Transformer with shorted turn


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84

HV [open]; B phase (red) should have lower response compared to A and

C phase but has instead higher magnitude/lower impedance

-80

-70

-60

-50

-40

-30

-20

-10

0

10 100 1000 10000 100000 1000000

Frequency (Hz)

Response(dBs)

Shorted turn by Doble


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Shorted turn by Doble

Responses of the HV and LV winding of the same transformer Significant difference in the white phase due to imbalance in the reluctance

on one of the core limbs (white phase) as a result of shorted turns

Shorted turn by IEEE


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86

y

Large impedance decrease

at low frequencies in open

circuit test

Impedance decrease at low

frequencies in HV short-

circuit test (only if short is

on HV side)

Radial winding deformationHoop buckling (IEEE)


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g p g ( )

Frequency Range Radial Winding Deformation

Assuming, no other failure modes exist:


This region (core region) is generally unaffected during radial winding deformation.


Results in an increase in impedance. The FRA trace for the affected phase

generally exhibits slight attenuation within the inductive roll-off portion.

5 kHz100 kHz Open Circuit and Short Circuit Tests:

The bulk winding range can shift or produce new resonance peaks and valleys

depending of the severity of the deformation. However, this change is minimal and

difficult identify. The changes will be greater on the affect winding, but it is stillpossible to have the effects transferred to the opposing winding. The response in

the bulk region should be used as secondary evidence to support the analysis.


Radial winding deformation is most obvious in this range. It can shift or produce

new resonance peaks and valleys depending of the severity of the deformation.

The changes will be greater on the affect winding, but it is still possible to have the

effects transferred to the opposing winding.

> 1 MHz Open Circuit and Short Circuit Tests:This range is generally unaffected in this range. However, severe deformation can

extend into this range.

Radial winding deformation by IEEE...


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Small but significant

impedance increase at

low frequencies inshort-circuit test

Resonance changes at

mid- and high

frequencies in open

circuit test

Axial winding deformationTelescoping (IEEE)


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Frequency Range Axial Winding Deformation


20 Hz10 kHz Open Circuit Tests:This region (core region) is generally unaffected during axial winding deformation.


Results in a change in impedance. The FRA trace for the affected winding causes a

difference between phases or previous results in the inductive roll-off portion.


Axial winding deformation is most obvious in this range. The bulk winding range

can shift or produce new resonance peaks and valleys depending of the severity of

the deformation. The changes will be greater on the affect winding, but it is stillpossible to have the effects transferred to the opposing winding.


Axial winding deformation can shift or produce new resonance peaks and valleys

depending of the severity of the deformation. The changes will be greater on the

affect winding, but it is still possible to have the effects transferred to the opposing

winding.

> 1 MHz Open Circuit and Short Circuit Tests:

The response to axial winding deformation is unpredictable.

Axial winding deformation by IEEE...


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Resonance changes at

mid- and high

frequencies in open

circuit test

Small but significant

iImpedance increase at

low frequencies in

short-circuit test

Core defects


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Core defects failures cause changes to the cores

magnetic circuit

Burnt core laminations

Shorted core laminations

Multiple/unintentional core grounds Lost core ground and joint dislocations.

Core defects (IEEE)


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Frequency

Range

Core Defects



These types of failures will affect the lower frequency regions generally below10 kHz. Core defects often change the primary core resonance shape. Less

weight should be placed on shifting, because identifying core defects can

sometimes be masked by the effects of core residual magnetization. If the

open circuit core appears to be loaded, (looking closer to a short circuit

response), this could indicated a core defect.


This region is generally unaffected during bulk winding movement. All phases

should be similar.


This range can shift or produce new resonance peaks and valleys.


Generally this range remains unaffected. However, if the fault is due to a core

ground issue, changes to the CL capacitance can cause resonance shifts in

the upper portion of this range.

> 1 MHz Open Circuit and Short Circuit Tests:

If the fault is due to a core ground issue, changes to the CL capacitance can

cause resonance shifts.

Core defectsExample


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Significant (and

unexpected)

differencies between

phases at low

frequencies in LV

[open] test

No differencies

between phases at high

frequenciesNowinding defetcts...

Core defects by IEEE...


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Significant changes in

the magnetic circuit at

first resonance in open

circuit test

SFRA analysisdB and Impedance


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dB-scale

Magnitude = 20*log(Meas/Ref)

Phase = Phase (Meas/Ref)

Impedance scale (Admittance Y = 1/Z)|Z| = |U/I| = 50*(RefMeas)/Mea.

Phase = Phase (Z)

SFRA standard magnitude response in dB


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Magnitude (dB) and Admittance (S)


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Second resonance

looks normal on LV...

Second resonance

decreased on LV...

Magnitude (dB) and Impedance ()


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Low resolution on LV

magnitude

High resolution with LV

impedance

Admittance (S) and Impedance ()


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101

FRAX

The Features And Benefits

FRAX 101Frequency Response Analyzer


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FRAX101Frequency Response Analyzer


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BluetoothOn FRAX101

USB Port

On all models

Power Input

11-16VDC,

internal battery

(FRAX 101)

Rugged Extruded

Aluminum Case

Most feature rich and accurate

SFRA unit in the world!

Generator,

reference and

measure

connectotsAll

panel mounted

Active probe

connector onFRAX101

SFRA test setupEasy to connect

h t t b id bl


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Industrial grade class 1

Bluetooth (100m)

USB for redundancy

Optional Internal Battery

Over 8h effective run time

shortest braid cables

Search Database Feature


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Data files stored in XML format

Index function stores all relevant data in a small database

Search function can list and sort files in different locations

Import formats


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Fast testing


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Less points where it takes

time to test and where highfrequency resolution is not needed

More points where

higher frequencyresolution is useful

Traditional test

about 2 min

vs.FRAX fast test

< 40 seconds

Decision support with correlation analysis


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Unlimited analysis


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Unlimited graph control

Lots of available graphs

Ability to create custom

calculation models using any

mathematic formula and the

measured data from all

channels

Turn on and off as needed

Compare real data with

calculated model data

FRAX150


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As FRAX-101 except:

Internal PC/stand-alone

No internal battery option

No Bluetooth

FRAX99


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As FRAX 101 except:

No internal battery option

No Bluetooth

Dynamic range > 115 dB

Fixed output voltage

9 m cable set

No active probes

FRAX product summary


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Light weight

Rugged Battery operated (FRAX101)

Wireless communication (FRAX101)

Accuracy & Dynamic Range/Noise floor

Cable Practice Easy-to-use software

Export & Import of Data

Complies with all SFRA standards and recommendations

Only unit that is compatible with all other SFRAinstruments


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Application Examples

Time Based Comparison - Example


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1-phase generator transformer, 400 kV

SFRA measurements before and afterscheduled maintenance

Transformer supposed to be in good conditionand ready to be put in service

Time Based Comparison - Example


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Obvious distorsion as by DL/T911-2004 standard (missing core ground)

Time Based ComparisonAfter repair


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Normal as by DL/T911-2004 standard (core grounding fixed)

Type Based Comparisons (twin-units)


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Some parameters for identifying twin-units: Manufacturer

Factory of production

Original customer/technical specifications

No refurbishments or repair Same year of production or +/-1 year for large units

Re-order not later than 5 years after reference order

Unit is part of a series order (follow-up of ID numbers)

For multi-unit projects with new design: reference transformer should

preferably not be one of the first units produced

Type Based Comparison - Example


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Three 159 MVA, 144 KV single-phase transformersmanufactured 1960 (shell-form)

Put out of service for maintenance/repair after DGAindication of high temperatures

Identical units

SFRA testing and comparing the two transformerscame out OK indicating that there are noelectromechanical changes/problems in thetransformers

Short tests indicated high resistance in one unit(confirmed by WRM)

Type Based Comparison3x HV [open]


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Type Based Comparison3x LV [open]


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Type Based Comparison3x HV [short]


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Design Based Comparisons


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Power transformers are frequently designed in multi-limb

assembly. This kind of design can lead to symmetricelectrical circuits

Mechanical defects in transformer windings usually

generate non-symmetric displacements

Comparing FRA results of separately tested limbs can bean appropriate method for mechanical condition

assessment

Pending transformer type and size, the frequency range

for design-based comparisons is typically limited to about1 MHz

Design Based Comparison - Example


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40 MVA, 114/15 kV, manufactured 2006 Taken out of service to be used as spare

No known faults

No reference FRA measurements from factory

SFRA testing, comparing symmetrical phasescame out OK

The results can be used as fingerprints for

future diagnostic tests

Designed Based ComparisonHV [open]


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Designed Based ComparisonHV [short]


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Designed Based ComparisonLV [open]


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Design Based ComparisonAfter Suspected Fault


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Power transformer, 25MVA, 55/23kV,manufactured 1985

By mistake, the transformer was energizedwith grounded low voltage side

After this the transformer was energized againresulting in tripped CB (Transformer protectionworked!)

Decision was taken to do diagnostic test



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HV-0, LV open

A and C phase OK, large deviation on B-phase (shorted turn?)

-80

-70

-60

-50

-40

-30

-20

-10

0

10 100 1000 10000 100000 1000000

Frequency (Hz)

Re

sponse(dBs)



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HV-0 (LV shorted) A and C phase OK, deviation on B-phase

-60

-50

-40

-30

-20

-10

0

10 100 1000 10000 100000 1000000

Frequency (Hz)

R

esponse(dBs)

And how did the mid-leg look like?


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Insulation cylinder

Core limb

LV winding

Rponse et analyse dun balayage enfrquenceSFRA for testing filter circuits (Line traps)


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Rponse et analyse dun balayage enfrquenceTypical line trap circuit


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The filter circuit is an RLC network

Rponse et analyse dun balayage enfrquenceMeasurement principle


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in

out

V

VdBG 10log20)(

Attenuation, dBPhase shift,

Generator signal Measurement signal

Rponse et analyse dun balayage enfrquence225 kV line trap


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225kV, 850A, 17mH

Verification of cut-offfrequency

Rponse et analyse dun balayage enfrquenceNo capacitors connected


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0

-10

-20

-30

-40

-50

-60

-70

-80

Mag

nitude(dB)

100 1 k 10 k 100 k 1 MFrequency (Hz)

[A-a1 [open]]

Rponse et analyse dun balayage enfrquenceOne capacitor connected


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0

-5

-10

-15

-20

-25

-30

-35

-40

Magnitude

(dB)


[C-c1 [open] (2)]

Rponse et analyse dun balayage enfrquenceTwo capacitors connected


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0

-10

-20

-30

-40

-50

Ma

gnitude(dB)


[C-c1 [open] (4)]


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Standards

SFRA Standards and Recommendations


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Frequency Response Analysis on Winding Deformation of Power

Transformers, DL/T 911-2004, The Electric Power Industry Standard ofPeoples Republic of China

Mechanical-Condition Assessment of Transformer Windings Using

Frequency Response Analysis (FRA), CIGRE report 342, 2008

IEEE PC57.149/D4, Draft Guide for the Application and Interpretation

of Frequency Response Analysis for Oil Immersed Transformers, 2011

IEC 60076-18, Power transformersPart 18: Measurement of

frequency response, 2011 (for voting)

Internal standards by transformer manufacturers, e.g. ABB FRA

Standard v.5

SFRA StandardsShort summary

Standard Dynamic range Accuracy Signal cable grounding Self-test


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y g y g g g

EPIS PRC DL/T 911 -100 to +20 dB 1 dB @ -80 dBWire, shortest length to

transformer core groundingnot stated

CIGRE brochure 342

-100 to +20 dB

(measurement

range)

1 dB @ -100 dB Shortest braid principle

Test circuit with a known

response

Shorted leads test

IEEE PC57.149/D9 (draft)

"Sufficient dynamic

range to

accommodate most

transformer testobjects"

"Calibrated to an

acceptable

standard"

Grounded at both ends.

"Precise, repeatable and

documented" procedure

Standard test object with

a known response

IEC 60076-18

-90 to +10 dB

min 6 dB S/N

(-96 to +10 dB)

0.3 dB @ -40 dB

1 dB @ -80 dB

Three methods described:

1. Same as CIGRE (2 MHz)

2. "Old" method (500 kHz)

3. "Inversed CIGRE" (2 MHz)

Standard test object with

a known response

Shorted/open leads test

ABB FRA Technical Standard

Better than-100 to +40 dB

(measurement

range)

1 dB @ -100 dB Shortest braid principle

Condition control of FRAdevice, including coaxial

cables, is strongly

recommended

Instrumentation


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Frequency rangeAll major brands are OK

Dynamic range First transformer circuit resonance gives typically a -90 dB

response. Smaller transformers may have a first response at -100dB or lower

Note that CIGRE recommends measurement range down to -100

dB. This implies a dynamic range/noise floor at about -120 dB. Accuracy

1 dB at -100 dB fulfills all standards.

All FRAX instruments fulfills all standards for dynamicrange and accuracy!

Why you need at least -100 dB...


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Measurement voltage and internal noise


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-140.00

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

Dynamic range

Measuring voltage p-p

Measurement voltage and internal noise/dynamic range for common SFRA test sets

FRAX-101

FRAX

-99

DobleM

51000

DobleM

5200

DobleM

53000

FRAna

lyzer

Tettex5310

FRAX

-150

DobleM

54000

HP4195A

HP4395A

Measurement range comparison


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Internal noise (open) measurements

GreenFRAX-101BlueOther SFRA

-100 dB measurement

(CIGRE standard)

BlackFRAX-101

RedOther SFRA

Cable grounding practice


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The shortest wire/braid-practice is now generally accepted

All European equipment manufacturers have adapted tothis practice

Recommended grounding practice (CIGRE) Bad grounding practice (CIGRE)

Instrumentation verification


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Verification of instrument including cables Measurement with open cables (at clamp) should give a responseclose to the noise floor of the instrument (at lower frequencies,pending cable length)

Measurement with shorted cables (at clamp) should give close to0 dB response (pending cable length)

External test device with known response (FTB-101 included inFRAX standard kit)

Calibration at recommended interval FRAX; Minimum every 3 years, calibration set and SW available

Field Verification Unit


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Field verification unit with known

frequency response is

recommended in CIGRE and

other standards to verifyinstrument and cables before

starting the test


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FRAX - Benchmarking

Measurement voltage and internal noise


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-140.00

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

Dynamic range

Measuring voltage p-p

Measurement voltage and internal noise/dynamic range for common SFRA test sets

FRAX-101

FRAX

-99

DobleM

51000

DobleM

5200

DobleM

53000

FRAna

lyzer

Tettex5310

FRAX

-150

DobleM

54000

HP41

95A

HP43

95A

FRAX 101 has the highest dynamic range, -130 dB!


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Internal noise (dynamic range)


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Internal noise (open) measurementsGreenFRAX-101

RedOther SFRA 1

BlueOther SFRA 2

Measurement range


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Internal noise (open) measurements

GreenFRAX-101BlueOther SFRA 1

-100 dB measurement

(CIGRE standard)

BlackFRAX-101

RedOther SFRA 1

Field verification test (FTB101)


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Blue= Other brand

Black = FRAX101

Dynamic RangeComparison (1)


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Neutral to capacitive tap

RedFRAX-101BlackOther SFRA 1

End-to-end openGreenFRAX-101

BlueOther SFRA 1


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Dynamic Range

Measurements at first resonance


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BlueFRAX

PurpleOther SFRA 3

RedOther SFRA 1

Jiri Velek, CEPS SFRA Market Research, October 2006


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FRAX - Compatibility

FRAX vs Doble (1)


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158

5 MVA, Dyn, H2-H3 measurement

BlueDoble

OrangeFrax

FRAX vs Doble (2)


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159

YNd, H1-H0 measurement

BlueDoble

OrangeFrax

FRAX vs Tettex and Doble

H1-H0 (short) measurement


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160

( )

BlueFRAX

PurpleTettex

RedDoble

(Doble high frequency

deviation due to different

grounding practice)

Jiri Velek, CEPS SFRA Market Research, October 2006

Frax-101, 2.8 vs 10 V meas voltage


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161

2.8 V

10 V

Frax (2.8V) vs FRAnalyzer


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162

Omicron (2.8 V)

PAX, 2.8 V

Summary - conclusions


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SFRA is an established methodology for detecting

electromechanical changes in power transformers Collecting reference curves on all mission critical

transformers is an investment!

Ensure repeatability by selecting good instruments and

using standardized measurement practices Select FRAX from Megger, the ultimate Frequency

Response Analyzer!


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Additional IEC slides

IEC connection picture

B


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C

B

D

VoutVin

50

50

A

B reference lead

C response lead

D earth connection

IECFRA condition assessment


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Some examples of conditions that FRA can be usedto assess are:

Damage following a through fault or other high

current event (including short-circuit testing),

Damage following a tap-changer fault, Damage during transportation, and

Damage following a seismic event.

Damage caused by short-circuit tests

IECTest object conditionsFactory and site

The test object shall be fully assembled as for service

complete with all bushings


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complete with all bushings.

Liquid or gas filled transformers and reactors shall be filled

with liquid or gas of the same type as in service conditions

Busbars or other system or test connections shall be removed

and there shall be no connections to the test object other than

those being used for the specific measurement

If internal current transformers are installed but not connectedto a protection or measurement system, the secondary

terminals shall be shorted and earthed.

The core and frame to tank connections shall be complete

and the tank shall be connected to earth.

Measurements should be performed at ambient temperature

IECTest object conditionsSite

The test object shall be disconnected from the associated

electrical system at all winding terminals and made safe for


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electrical system at all winding terminals and made safe for

testing.

Line, neutral and any tertiary line connections shall be

disconnected but tank earth, auxiliary equipment and current

transformer service connections shall remain connected.

In the case where two connections to one corner of a delta

winding are brought out, the transformer shall be measured

with the delta closed (see also 4.4.4).

In instances where it is impossible to connect directly to the

terminal, then the connection details shall be recorded with

the measurement data since the additional bus bars

connected to the terminals may impact on the measurement

results.

IECInstrument performance check

1. Connect the source, reference and response channels of the

instrument together using suitable low loss leads, check that the


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g g ,

measured amplitude ratio is 0 dB 0,3 dB across the whole

frequency range. Connect the source and reference channelstogether and leave the response terminal open circuit, check that

the measured amplitude ratio is less than -90 dB across the whole

frequency range.

2. The performance of the instrument may be checked by measuring

the response of a known test object (test box) and checking that

the measured amplitude ratio matches the expected response ofthe test object. The test object shall have a frequency response

that covers the attenuation range -10 dB to -80 dB.

3. The correct operation of the instrument may be checked using a

performance check procedure provided by the instrument

manufacturer. This performance check procedure shall verify that

the instrument is operating at least over an attenuation range of -10

dB to -80 dB over the whole frequency range.

IECMeasurement connection check

Measurement connection and earthing The continuity of the main and earth connections shall be checked at the


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y

instrument end of the coaxial cable before the measurement is made. Poor

connections can cause significant measurement errors, attention must be paid to

the continuity of the main and earth connections. In particular, connections to boltsor flanges shall be verified to ensure that there is a good connection to the winding

or the test object tank.

Zero-check measurement If specified, a zero-check measurement shall be carried out as an additional

measurement. Before measurements commence, all the measuring leads shall be

connected to one of the highest voltage terminals and earthed using the normal

method. A measurement is then made which will indicate the frequency response

of the measurement circuit alone. The zero check measurement shall also be

repeated on other voltage terminals if specified.

The zero-check measurement can provide useful information as to the highest

frequency that can be relied upon for interpretation of the measurement.

Repeatability check

On completion of the standard measurements the measurement leads and earthconnections shall be disconnected and then the first measurement shall be

repeated and recorded.

IECMeasurement configurationwith OLTC

For transformers and reactors with an on-load tap-changer

(OLTC), the standard measurement on the tapped winding


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(OLTC), the standard measurement on the tapped winding

shall be

on the tap-position with the highest number of effective turns in circuit,

and

on the tap-position with the tap winding out of circuit.

Other windings with a fixed number of turns shall be

measured on the tap-position for the highest number of

effective turns in the tap winding. Additional measurements may be specified at other tap-

positions.

For neutral or change-over positions, the direction of

movement of the tap-changer shall be in the lowering voltage

direction unless otherwise specified. The direction ofmovement (raise or lower) shall be recorded.

IECMeasurement configurationAuto with OLTC

For auto-transformers with a line-end tap-changer, the

standard measurements shall be:


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standard measurements shall be:

on the series winding with the minimum number of actual turns of the

tap-winding in circuit (the tapping for the highest LV voltage for a linearpotentiometer type tapping arrangement or the change-over position for

a reversing type tapping arrangement, or the tapping for the lowest LV

voltage in a linear separate winding tapping arrangement),

on the common winding with the maximum number of effective turns of

the tap-winding in circuit (the tapping for the highest LV voltage), and

on the common winding with the minimum number of actual turns of thetap-winding in circuit (the tapping for the lowest LV voltage for a linear

potentiometer or separate winding type tapping arrangement or the

change-over position for a reversing type tapping arrangement).

IECMeasurement configurationDECT and OLTC

For transformers with both an OLTC and a de-energised tap-

changer (DETC), the DETC shall be in the service position if


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changer (DETC), the DETC shall be in the service position if

specified or otherwise the nominal position for the

measurements at the OLTC positions described in thisClause.

For transformers fitted with a DETC, baseline measurements

shall also be made on each position of the DETC with the

OLTC (if fitted) on the position for maximum effective turns.

It is not recommended that the position of a DETC on atransformer that has been in service is changed in order to

make a frequency response measurement, the measurement

should be made on the as found DETC tap position. It is

therefore necessary to make sufficient baseline

measurements to ensure that baseline data is available forany likely service (as found) position of the DETC.

IECFrequency range and measurement points

The lowest frequency measurement shall be at or below

20 Hz.


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The minimum highest frequency measurement for test objects

with highest voltage > 72,5 kV shall be 1 MHz.

The minimum highest frequency measurement for test objects

with highest voltage of 72,5 kV shall be 2 MHz.

Below 100 Hz, measurements shall be made at intervals not

exceeding 10 Hz

Above 100 Hz, a minimum of 200 measurements

approximately evenly spaced on either a linear or logarithmic

scale shall be made in each decade of frequency.

IECMeasurement equipment specification (1)

Dynamic range

The minimum dynamic range of the measuring instrument shall be


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y g g

+10 dB to -90 dB of the maximum output signal level of the voltage

source at a minimum signal to noise ratio of 6 dB over the wholefrequency range.

Amplitude measurement accuracy

The accuracy of the measurement of the ratio between Vin and Vout

shall be better than 0,3 dB for all ratios between +10 dB and -40 dB

and 1 dB for all ratios between -40 dB and -80 dB over the whole

frequency range.

Phase measurement accuracy

The accuracy of the measurement of the phase difference between Vin

and Vout shall be better than 1 at signal ratios between +10 dB and -

40 dB, over the whole frequency range.

Frequency range

The minimum frequency range shall be 20 Hz to 2 MHz.

IECMeasurement equipment specification (2)

Frequency accuracy

The accuracy of the frequency (as reported in the measurement record) shall be


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y q y ( p )

better than 0,1 % over the whole frequency range.

Measurement resolution bandwidth For measurements below 100 Hz, the maximum measurement resolution

bandwidth (between -3 dB points) shall be 10 Hz; above 100 Hz, it shall be less

than 10 % of the measurement frequency or half the interval between adjacent

measuring frequencies whichever is less.

Operating temperature range

The instrument shall operate within the accuracy and other requirements over a

temperature range of 0 to +45 C.

Smoothing of recorded data

The output data recorded to fulfil the requirements of this standard shall not be

smoothed by any method that uses adjacent frequency measurements, but

averaging or other techniques to reduce noise using multiple measurements at aparticular frequency or using measurements within the measurement resolution

bandwidth for the particular measurement frequency are acceptable.

IECMeasurement records Test object identifier

Date

Time


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Time

Test object manufacturer

Test object serial number

Measuring equipment

The peak voltage used for the measurement.

Reference terminal

Response terminal

Terminals connected together

Earthed terminals

OLTC tap positions, current and previous

DETC position

Test object temperature

Fluid filled, yes or no.

Comments, free text to be used to state the condition of the test object

Measurement result (the frequency in Hz, the amplitude in dB and the phase in degrees) for

each measurement frequency

IECTest records (1) Test object data

Manufacturer

Year of manufacture


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Year of manufacture

Manufacturers serial number

Highest continuous rated power of each winding Rated voltage for each windings

Short circuit impedance between each pair of windings

Rated frequency

Vector group, winding configuration / arrangement

Number of phases (single or three-phase)

Transformer or reactor type (e.g. GSU, phase shifter, transmission, distribution, furnace,industrial, railway, shunt, series, etc.)

Transformer configuration (e.g. auto, double wound, buried tertiary, etc.)

Transformer or reactor construction (e.g core form, shell form), number of legs (3 or 5-leg),

winding type, etc.

Load tap-changer (OLTC): number of taps, range and configuration (linear, reversing,

coarse-fine, line-end, neutral-end, etc.)

De-Energized Tap Changer (DETC): number of positions, range, configuration, etc.

IECTest records (2) Organisation owning the test object

Test object identification (as given by the owner if any)

Any other information that may influence the result of the measurement


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Any other information that may influence the result of the measurement

Location data Location (e.g. site name, test field, harbour, etc.)

Bay identification reference if applicable

Notable surrounding conditions (e.g. live overhead line or energized busbars nearby)

Any other special features

Measuring equipment data

Working principle of device (sweep or impulse)

Equipment name and model number

Manufacturer

Equipment serial number

Calibration date

Any other special features of the equipment

Test organization data Company

Operator

IECTest records (3) Measurement set-up data

Remanence of the core: was the measurement carried out immediately following

a resistance or switching impulse test or was it deliberately demagnetised?


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a resistance or switching impulse test, or was it deliberately demagnetised?

Whether the tank was earthed

Measurement type (e.g. open circuit, short circuit, etc.)

Length of braids used to ground the cable shields

Length of coaxial cables

Reason for measurement (e.g. routine, retest, troubleshooting,

commissioning new transformer, commissioning used transformer,

protection tripping, recommissioning, acceptance testing, warrantytesting, bushing replacement, OLTC maintenance, fault operation,

etc.)

Additional information

Photographs of the test object as measured showing the position of

the bushings and connections

IECMeasurement lead connction. Method 1

The central conductor of the coaxial


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A connection clamp

B unshielded length to be made as short as possible

C measurement cable shield

D central conductor

E shortest braid

F bushing

G earth connection

H earth clamp

I tank

J smallest loop

measurement leads shall be connected

directly to the test object terminal usingthe shortest possible length of

unshielded conductor.

The shortest possible connection

between the screen of the measuring

lead and the flange at the base of the

bushing shall be made using braid. A

specific clamp arrangement or similar is

required to make the earth connection as

short as possible

In general this method may be expected

to give repeatable measurements up to

2 MHz


Method 2 is identical to method 1


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e od s de ca o e od

except that the earth connection

from the measurement leads to theflange at the base of the terminal

bushing may be made using a fixed

length wire or braid, so that the

connection is not the shortest

possible.

The position of the excess earth

conductor length in relation to the

bushing may affect amplitude (dB)

measurements above 500 kHz and

resonant frequencies above 1 MHz


In a method 3 connection, the screen of

the coaxial measurement lead is


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A connection clamp

B shortest braid or wire

C measurement cable shield

D central conductor

E earth clamp

F tank

G smallest loop

the coaxial measurement lead is

connected directly to the test object tank

at the base of the bushing and an

unshielded conductor is used to connect

the central conductor to the terminal at

the top of the bushing.

If a method 3 connection is used for the

response lead connection only then the

results are comparable with method 1.This connection may be the most

practical option if an external shunt

(measuring impedance) is used

If a common conductor is used for the

signal and reference connections then

the conductor is included in themeasurement which will therefore differ

from a method 1 measurement

IECFrequency response comparison

In order to interpret a measured frequency response, a comparison

is made between


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The measured response and a previous baseline measurement (time based

comparison) With the response measured on a twin transformer, a transformer made to the

same drawings from the same manufacturer (type based comparisons). Careful

attention should be given when using responses from sister transformers

(transformers with the same specification but with possible differences in winding

configuration even from the same manufacturer) for comparison. Improvements

and changes to the transformer design may have been introduced by amanufacturer over a period of time to outwardly similar units and this may cause

different frequency responses

For three-phase transformers, comparisons can also be made between the

responses of the individual phases (design based comparisons). When

comparing phases of the same transformer quite significant differences are

considered normal and could be due to different internal lead lengths, different

winding inter-connections and different proximities of the phases to the tank andthe other phases

IECComparisons of frequency responsesThe comparison of frequency response measurements is used to

identify the possibility of problems in the transformer. Problems are

indicated by the following criteria:


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indicated by the following criteria:

Changes in the overall shape of the frequency response;

Changes in the number of resonances (maxima) and anti-

resonances (minima);

Shifts in the position of the resonant frequencies.

The confidence in the identification of a problem in the transformer

based on the above criteria will depend on the magnitude of the

change when compared with the level of change to be expected for

the type of comparison being made (baseline, twin, sister or phase).

IECTypical frequency response


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Influence regions:

A core

B interaction between windings

C winding structureD measurement setup and lead (including earthing connection)

IECInfluence of tertiary delta connections

0

10


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101

102

103

104

105

106

-90

-80

-70

-60

-50

-40

-30

-20-10

0

Frequency, Hz

Amplitude,

dB

delta open

delta closed

IECInfluence of star neutral connections

0


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101

102

103

104

105

106

-45

-40

-35

-30

-25

-20

-15

-10

-5

Frequency, Hz

Am

plitude,

dB

neutrals open

neutrals joined

IECInfluence of measurment direction (example)

10

0


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Adva

SFRA - Theory and Method - Standards_120911

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Transcript of SFRA - Theory and Method - Standards_120911