Sweep Frequency Response Analysis

Together og ether We Power The Wor World ld

SFRA SFR A Tra raini ining ng

SFRA History Initial research started in the 60’s. In the 80’s Engineers mainly used HP network analyzers First generation of SFRA purpose built test sets developed by Doble in 1990. Now SFRA technique has gained world wide acceptance with several manufacturers producing dedicated instruments for SFRA

SFRA History

Today Doble’s SFRA has become the de facto SFRA standard in North America and wordwide with > 700 Instruments sold Doble has been cataloging results for ten years Doble has developed standardized SFRA procedures for IEEE & CIGRE Doble develops support network for SFRA analysis and interpretation

National Grid FRA experience

• started ev evaluating FR FRA in in late 1980’s • initially used an impulse technique (KEMA digital LVI) • changed to swept frequency ( Ontario Hydro ) • hundreds of tests • several ex examples of failures • standardised test procedure ( ‘Euro Doble’ ) • Meth ethod no now used by many other her utilities in Europe, Far East, Australia, North America

Support: Integration

SFRA is one tool in the tool box Capacitance

OC: Open Circuit

Exciting Currents

DC Resistance

SFRA

SC: Open Circuit

Leakage Reactance


SFRA SFR A Tra raini ining ng Why Use SFRA?

Doble Doble SFRA SFRA – summar summary y SFRA is a test set for transformers, reactors, and rotating machines SFRA can tell you if anything is damaged or broken inside your transformer without going inside. SFRA is used alongside other o ther electrical tests such as the M4000 and oil analysis

Transformer Decisions • There are many decisions relating to transformers: – Is it safe to go back in service after a fault? – Has it been damaged in transit? – Is it deteriorating as it ages? – Will it fail unexpectedly? • Decision making for transformers is not easy • Quality data and engineers support good decisions

Transformer Data • Data may relate to different transformer areas: – Dielectric – ability to carry voltage – Mechanical – ability to carry current – Thermal – ability to sustain power transfer • Field Testing provides the engineer with a group of tools to assess the condition of a transformer in each area • Engineer should choose the right tool for the job

Which Transformer Tests?

• Tests depend on the decision you need to make!

Motivation: SFRA When & Why? • Acceptance • Establish a Baseline • Assess Condition after Electrical Disturbance • Assess Condition after a Relocation • Asset Management - Ranking and Prioritizing • Reduce Catastrophic Failures • Winding technology hasn’t changed much in 100 years • Available tools have changed a great deal

Why Do Things Move?

• Mechanical Shock during Shipping • Loosening of Internal Structural Components – loss of clamping pressure • Fault Duty

Mechanical Failure

Mechanical Failure

Failure Mode •

The very large electromagnetic forces on windings during fault conditions can cause winding movement and even permanent winding and core deformation

•

This may result in insulation damage.

•

Turn to turn faults are the most likely.

•

Insulation damage usually results in a failure of the transformer which is uneconomic to repair.

Design issues

• Transformers should be designed to withstand through faults. • However, transformers are rarely short-circuit tested because of the costs involved. • So the short-circuit strengths of designs are not often checked. • Many new transformers fail short circuit tests, so the shortcircuit strength of many designs must be suspect.

Design limitations

• Desi Design gn cal calcu cula lati tion onss don’ don’tt take take int into o acco accoun untt the the effe effect ctss of twisting forces. • Desi Design gner erss can can’t ’t desi design gn agai agains nstt tap tap to tap tap fau fault lts. s. • Tap Tap wind windin ings gs are are dif diffi ficu cult lt to des desig ign n for for shor shortt-ci circ rcui uitt strength.

Specification

• Impedance

20 %

5%

• Short-circuit current

5 p.u.

20 p.u.

• Short-circuit fo forces

1

:

16

Detection • Dissolved gas analysis (DGA) will only indicate a problem when the insulation has been damaged (usually too late to repair). • Internal visual inspections often inconclusive • Winding failures can usually be diagnosed by various electrical tests ( but not always ). • But latent damage is much harder to detect. So your transformer may be critically damaged without you being aware of it !

Service • The number and severity of short-circuit events suffered are important. • Close-up lightning strikes are a common cause of winding movement failures. • Tap-changer faults can cause tap winding failures. • Faulty synchronisation can cause winding damage and failure.

Ageing

• As a transformer ages th the insulation shrinks and clamping pressure is lost. - reduces strength. • Any Any min minor or wind windin ing g def defor orma mati tion on usua usuall lly y results in mis-alig mis-alignmen nmentt of electrom electromagne agnetic tic centres - increases stresses during

subsequent faults.

Failure Mode • Hoop buc uck klin ing g of inn nne er winding • Conductor ti tipping • Conductor tele lesscopin ing g • Coil cla lam mpin ing g fail ilu ure • End insulation collapse

• Spiral ti tightening • Lead displacement

Bushing failure….

This bushing failed catastrophically

This bushing was hit by porcelain and failed Neutral bushing lifted from turret and is no longer vertical SFRA 2009

Bushing….

But did the windings move because of the pressure wave in the oil? SFRA showed that the windings were in good shape. SFRA 2009

Transformer impact….

Or maybe we hit a bridge?

Bridge Impact

Paint scrapes on delivery are suspicious? How did the scrapes get there, and what does the impact recorder say? Do we know if anything moved inside the transformer?

Transport Issues

t

Transformer Arrives by Barge

Almost!

Symptoms - black box scenario

• How can we tell if anything has moved?

SFRA as a transformer test

Sweep Frequency Response Analysis

A method to measure the frequency response of the passive elements (RLC) of a transformer. The result is a transfer function which produces a fingerprint related to the mechanical geometry of the transformer. SFRA relates to Mechanical Integrity

SFRA – What do you get?

Main winding

Tap leads Core

Tap windings

Conclusions SFRA is a means to provide data about transformer mechanical integrity Good results lead to good decisions Doble support means you can extract value from your SFRA measurements

Together We Power The World

SFRA Training Introduction: RLC, dB & MHz

SFRA Theory and Practice

• In a passive device there are three basic components: – resistors – capacitors – inductors • They each have a different response to an AC signal • Their response is closely related to their geometry: both internal and in relation to other components

FRA theory

What is a winding ?

Inductance

Series capacitance

Shunt capacitance

High frequency model

Winding

• inductance • series capacitance ( turn to turn ) • shunt capacitance ( turn to earth )

• winding geometry determines values of L and C

Winding frequency responses

• windings have frequency dependent response for transmitted signals • the detailed form of the frequency response depends on winding geometry • a change in geometry will change the frequency response

FRA theory

If we can measure changes in the frequency response of a winding caused by winding movement, then we can detect winding movement

SFRA Theory and Practice • Impedance of an ideal resistor, capacitor and inductor 450 400

Resistance:

s350 m h300 O , e250 c n a200 d e150 p m100 I

flat response v. frequency

Inductor: Increased impedance with increased frequency; dead short at low frequency

50 0 0.1

Capacitor:

1

10

100

1000

Frequency, Hz

Reduced impedance with increased frequency; open circuit at low frequency


• dB’s: as impedance increases, Vout falls V in

V out Impedance, Z

Response in dB’s = 20 log10 (Vout /Vin)

• Each 20 dB drop means we are looking at a tenth of the previous Vout/Vin

SFRA Theory and Practice Each 20 dB drop means we are looking at a tenth of the previous Vout/Vin Response in dB’s = 20 log10(Vout/Vin) +20 dBs 0

Vout = 10 * Vin

-20

Vout = 0.1 * Vin

Vout = Vin

-40

Vout = 0.01 * Vin

-60

Vout = 0.001 * Vin

SFRA Theory and Practice Need to reference each measurement to ground V signal

50  Coax Signal Lead

V in

V out Im pedance, Z

50  Coax Reference Measurement Lead

50  Coax Test Measurement Lead

Means we get a consistent measurement


Response of a short circuit 0 Response, dB -25

Response

-50 Frequency, Hz

0 dB across the frequency range

SFRA Theory and Practice Response of ideal resistors - no inductance or capacitance present 0 Response, dB

50  Response

-25 500  Response -50 Frequency, Hz

Flat across the frequency range


Response of an ideal inductor

0 dB down at low frequency means it looks like a dead short

Inductive roll off

Larger inductances start to roll off at lower frequencies


Response of ideal capacitor ‘Knee’ point depends on size of capacitor

Low frequency response is like an open circuit

Capacitive climb back

0 dB down at high frequency like a dead short

SFRA Theory and Practice Parallel RLC Circuit:

Constant dBs down v. frequency

Dead short at low frequency

Open circuit at high frequency

Open circuit at low frequency

Dead short at high frequency

SFRA Theory and Practice Impedance of the Parallel RLC Circuit:

R affects size of resonance peak

Resonant frequency depends only on L and C values 6 s 5 m h O4 , e c 3 n a d 2 e p m1 I

0 0.1

1

10 Frequency, Hz

100

1000

SFRA Theory and Practice Response of the RLC Circuit:

Inductive roll off

Capacitive climb back

Resonance

4. Understanding SFRA - Basic Principle Response of Multiple Circuits:

System 1

Vin

Vout

System 2

0 -20 -40 -60 -80 -100

Resonance ?

Resonance ? 100

Resonance ?

1010

SFRA Theory and Practice • We get a resonance for an inductor-capacitor (LC) combination • Changing L or changing C gives a new resonance • L and C are dependent on geometry • Changing R changes the size of the resonance

Real World Measurements

HV

A real C Winding C transformer has R many inductance and C capacitances

T

H

C

CT

H

LH

C H

LH

RH

H

H

L

RL

LV Winding

CL

Inter CH

CH

L

L

LL CT

CL

RL

LL

Winding

Each LC pair C givesC a resonance T

L


• Need to reference each measurement to ground V signal

50  Coax Signal Lead

V in

V out Im pedance, Z

50  Coax Reference Measurement Lead

 Means we get a consistent measurement

50  Coax Test Measurement Lead

SFRA is really many measurements

Some individual resonances

Hardware Test Leads • Three lead system Signal + Reference Measure Ground Ground Signal

M5x00

Signal & Reference Leads Measurement lead

5. SFR SFRA A – M5 M5000 000 Se Seri ries es

M5200

M5300

M5400

5. M5000 Test Cable At Test Test Set Set - Color Color Cod Coded ed Lead Leadss Yellow - Sign Signal al (Giv (Gives es Vin Vin))

Red -- Refere Red Reference nce (Mea (Measur sures es Vin) Vin) Black Black - Test Test (Meas (Measure ure Vout Vout))

At Transformer Red => Red => Vin Black => Vout Lead Grounds Lead Grounds to Base of Bushings Safety Safety Ground Ground - Twist Twist Lock Lock to to Tran Transfo sforme rmer r

A note about Test Leads

• 60 ft /18 m • Cabl Cablee Trun Trunk k

30 ft to the the spli splitt

• Shie Shield ld Grou Ground nd

12 ft

• Appl Applic icat atio ion n

use use if <= 362 362 kV

• 100 ft /30 m • Cabl Cablee Trun Trunk k

55 ft to the the spli splitt

• Shie Shield ld Grou Ground nd

18 ft

• Appl Applic icat atio ion n

use use if > 362 362 kV

• No long longeer usi using ng the the whi white te jum jumpers pers

Hardware Test Leads IEC Definitions Source lead The lead connected to the voltage source of the measuring equipment used to supply an input voltage to the test object. Reference lead (Vin) The lead connected to the reference channel of the measuring equipment used to measure the input voltage to the test object ( Vin). Response lead (Vout) The lead connected to the response channel of the measuring equipment used to measure the output voltage of the test object ( Vout)

FRA test leads and equipment • Hig igh h fre frequ quen ency cy coo-ax axia iall ca cabl blee • Impe Impeda danc ncee mat match ched ed ( 50 50 ohm ohmss ) at eq equi uipm pmen entt (to avoid reflections in test leads) • Separate S and R leads for applying and measuring signal at input terminal Only then will layout of test leads have no effect on measurement

• Any length ( 18 m popular )

Applied and measured signals

Vo

Vi

Res Re s p o n s e = Vo /V /Vii

Freque requenc ncy y Resp spon onse se An Ana aly lysi sis s

Sweep Frequency Method

 V out   H (dB )  20 log10   V in  1  V out   H ( )  tan   V in 

Most Useful Frequency Range 20 Hz - 2 MHz MHz

Tutorial Session - Frequency Response Analysis

Achieve a Two Port Network

Hardware Range & Resolution

Signal Generation: Range & Resolution Two independent measurement channels Oversample at up to 100 MS/s 20 V p-p 10 Hz-25 MHz Log Scale at 1.02% of frequency

Conclusions RLC components have different SFRA responses Even simple circuits may have complex responses ‘Real’ transformers have many resonances


SFRA SFR A Tra raini ining ng Test Procedures

SFRA Test Procedures There is a basic set of recommended tests for any transformer. Doble software comes complete with templates which have IEEE and CIGRE compliant tests. Further tests may be performed for diagnostic purposes: per phase short circuit circuit interwinding tests Reverse short circuit etc

Setting up the transformer

• Tran Transf sfor orme merr sho shoul uld d be be dis disco conn nnec ecte ted d from system. Bu Busb sbar arss re remo move ved d if possible. Line, neutral and any tertiary line connections shall be disconnected but tank earth, internal auxiliary equipment and internal current transformer connections shall remain connected. • Tran Transf sfor orme merr shou should ld be in norm normal al service condition (assembled, oilfilled).


• Make Make conn connec ecti tion onss as as det deter ermi mine ned d by by winding configuration. Phases not under test are usually left floating. • In the the cas casee whe where re two two con conne nect ctio ions ns to one corner of a delta winding are brought out, the transformer shall be measured with the delta closed but not with the earth connected.


• If spec specia iall conn connec ectio tions ns have have been been specif specified ied and are provided on the test object to enable a frequency response measurement to be made when it is arranged for transport then the measurement must be made in both the fully assembled (fluid filled) and transport configurations (drained if required for transport) before transport and subsequently as specified by the purchaser. • Bewa Beware re of diff differ eren entt state statess of of oil oil dra drain ining ing


• It is important that SFRA measurements are always made in a consistent way and that all details of the measurement method are systematically recorded. This will help to avoid false discrepancies and ensure the compatibility of frequency responses during comparison.

Typical Measurements – two winding unit

• Connections - look at each winding separately: • Make measurements at extreme raise LTC and nominal DETC • With previous results – repeat those procedures HV - H1-H2, H2-H3, H3-H1 LV - X1-X0, X2-X0, X3-X0 Short Circuit - H1-H2, H2-H3, H3-H1 with X1-X2-X3-X1 shorted (all three phases, not X0)

FRA test connections

FRA test connections

Measurements

5. SFRA Test Procedure • Open-Circuit (OC) Tests • Short-Circuit (SC) Tests • Supplementary Test: Inter-Winding Tests  LTC at extreme rise (16R or 1).  DETC at nominal position.  If possible to do additional tests - e.g. various tap

positions, short circuit tests, tertiary tests etc.

5. Typical Test Procedure Delta-Wye – Open Circuit (OC) Tests Red H1 H2 H3 -

Black H3 H1 H2

X1 X2 X3 -

X0 X0 X0

Supplementary H1 - X1 H2 - X2 H3 - X3

HV Winding Tests

LV Winding Tests

Inter-Winding Tests

5. Typical Test Connections Delta-Wye – Short Circuit (SC) Tests Red

Black

H1 - H3 H2 - H1 H3 - H2

Short-Circuiting X1X2X3X1 X1X2X3X1 X1X2X3X1

Other Test Connections

Software Nameplate Test Template

SFRA Trace Shorted Leads What – kind of trace?

SFRA Trace – Open in Test Circuit What kind of trace?

Open Circuit inside test specimen

Open Circuit within test leads

What would be the measurements ? Single phase two winding unit? Three phase three winding D-D-Y unit? Check Latest Doble connection guide

Measurements Example from Manual

S/C Connections

Measurements Example from Manual

Measurements

Typical Results - Conclusions

• Results should be expected form • Make sure connections are appropriate for transformers • Check with previous results for consistency

Troubleshooting

• Shorted cable measurement • Shorted cable – open ground measurement • Open cable measurement • Ground check on transformer • Repeat measurements on ‘good’ winding • Does it make sense????

Troubleshooting

If differences are observed when comparing with a finger print result, it is important to first verify the measurement by repeating to ensure that the differences are not caused by bad measurement practice or by making different measurement connection.


SFRA Training Typical Results

Wye-wye : HV’s Note low frequency variations

Wye-wye: LV’s Less dB down than HV

Wye-wye HV Short Circuit LV windings shorted

Another HV Wye HV winding – three phases one DETC positions

Another HV Wye Detail – three phases Center phase Two outer phases

Another HV Wye Detail – three phases

Center phase

Two outer phases

Delta-wye HV’s Typical response at low frequencies

Delta-Wye: LV’s Less dB down than HV

4. Simple Transformer Model Which Winding has higher impedance?

Which trace is highest and lowest? • OC HV Trace • OC LV Trace • SC Trace

Lowest Middle Highest

Open Circuit (OC) HV

LV

Vi

Vi

Vo

Vo Short Circuit (SC) HV

Vi Vo

LV

4. Simple Transformer Model

HV

• Which trace is VH

LV

Vi

Vi

VH

ZH > ZL: VH: Lower response or more attenuated

0 -20 VL -40 -60 -80 -100

VL

VH 100

1010

Delta-Wye HV Short Circuit LV windings shorted together

Autotransformer HV HV LV LV and Terti Tertiary ary compared compared

Tertiary winding

LV (common) winding

HV (series) winding

Measurement Sense: H1-H0 v. H0-H1

Effect Effect is smaller when the two bushings are simila similarr – e.g. e.g. H1-H H1-H3 3 v H3-H H3-H1 1 H0-H1

H1-H0

Variation with LTC position 16 Lower through 8 Lower for one winding

Variation with LTC position Mid frequency detail

16 Lower

8 Lower

Variation with and without oil LV winding

With oil – Lower resonant frequencies

Without oil Higher resonant frequencies

Effect of Bad grounds Original and Bad Red lead ground

Bad red lead ground

Original

Effect of Bad grounds Original and Bad Black lead ground

Original

Bad red lead ground

Effect of Magnetization Low frequency variation is severe – but identifiable Magnetization affects the core response

Magnetization & Grounding variation

HV winding High frequency variation due to grounding

Magnetized center phase

Magnetization affects the core response

Sister Units showing Magnetization HV winding

Magnetization variation

Sister Units showing Magnetization HV winding – same unit, more magnetization

Magnetization variation

HV Wye winding: Open and Short HV winding – same unit, more magnetization

Short circuit wye winding

Open circuit wye winding Very similar at high frequencies

HV Delta winding: Open and Short HV winding – same unit, more magnetization

Short Circuit

Open Circuit

Similar at high frequencies

Typical Results - Conclusions

• Results vary between units – depending on size and type of unit • Results vary with magnetization & grounding • Results vary with tap changer positions • Results vary with oil level • Know what to expect – see next slide!!!

4. Simple Transformer Model

HV

Vi VH

• Which trace is VCHL

LV

VCHL CHL

CHL: High-to-Low or Inter-Winding

0 -20 -40 -60 -80 -100

VCHL

VH 100

1010

One Transformer: HV, LV, SC & IW

IW = Interwinding measurement Short Circuit

LV Open Circuit

HV Open Circuit H-X interwinding

Wye : HV open circuit Note low frequency variations 2Highs and 1 Low V and W shape

Interpretation • Experience: Certain Frequency Bands Indicate Different Problem Conditions 400kHz to 2MHz: Movement of main and tap winding leads

<2kHz: Core Deformation, Open Circuits, Shorted Turns & Residual Magnetism

2kHz to 20kHz: Bulk Winding Movement Relative to Each Other, clamping structure

20 kHz to 400kHz: Deformation Within the main and tap windings

4. Simple Transformer Model High W shape = A/C Phase

Low

A Vi

V shape = B Phase

Vo

=> Exciting Currents = 2 similar Highs & 1 Low

B

High

C

2. SFRA – Logarithm Graph

100 Hz

1 MHz

2. What is SFRA? – Linear Graph

Impossible to see 100 Hz

Interpretation

Power of a Null Result

• 600 MVA GSU Transformer • SFRA results from factory and initial field • New SFRA results taken after the fire

Fire: factory to field comparison HV and LV results overlay almost exactly up to 2 MHz

Short circuit results to within 0.01 dB

Low frequency variations are due to core magnetization

Post fire conclusions • Results show no variation to 2 MHz using different: – Doble test sets – test personnel – test leads – lead positions etc

Post fire conclusions • SFRA results show no indication of a mechanical problem within the transformer – this is a NULL result • SFRA data was key in the decision to return the unit to service • Full details in 2005 Doble Conference Paper


SFRA Training Open and Short Circuit Test Variation

SFRA Typical Results This presentation gives typical results for open and short circuit tests It explains why the two tests are different and the value in each test A T-model of a transformer is used to make things clearer Only low frequencies are considered – no capacitive effects

Test Connections - SFRA Open Circuit Normal test on HVs the LVs float HV Winding Signal & Reference

LV Winding

R small

R small

Core

M5100

R high

Test

Model is relevant for LOW FREQUENCIES

Test Connections - SFRA Short Circuit Short circuit test on HVs - LVs shorted HV Winding Signal & Reference

LV Winding

R small

R small

Core

M5100

LV Short

R high

Test

Model is relevant for LOW FREQUENCIES

Short Circuit v. Open Circuit Open circuit responses dominated by core at low frequency – as with Exciting current Different magnetic paths lead to different responses – typical outer v center phase variation Short circuit responses remove effect of core at low frequencies – as with leakage Reactance All three short circuit responses should be identical

Short Circuit Test - Setup

• Short secondaries together, use proper size jumpers, close the delta, X1-X2-X3-X1 • Do not short multiple secondaries together • Test each phase – Phase A – Phase B – Phase C

Short Circuit (SC) Analysis

• Analyze the “inductive roll-off” region • This roll-off defines the primary winding inductance model • Very similar to the LRT

SC Analysis • The SC test the phases should be no further than 0.1 dB from each other • Roll-off should be close to -20dB/decade • Analysis will require you to zoom in very close

Step 1: Overlay the three HVSC Test Results

Step 2: Zoom in on the LF Region that shows the “Inductive Roll-off”

Step 3: Identify the key artifacts A) LF DC Section B) Inductive Roll-off Knee (Low-Pass Filter Cut-off Frequency) C) Inductive Roll-off Section D)End of Inductive Roll-off

Step 4: Analyze LF DC Section This is the few points that are at the very beginning – 20 Hz There should be very close offset between the three results. If there is a fan-out at the beginning this could indicate a winding resistance problem. Recommend a winding resistance test

This example is good

Step 5: Locating the Knee point. Starting at 20 Hz, look for maximum deflection point where the traces go into the inductive roll-off section (next slides explains this more) The LP Filter cut-off frequency by definition is the point where the frequency response goes from flat to -3db down

Start

“Inductive Roll-off”

Step 5: Find the start and end points of the “Inductive Roll-off” The inductive roll-off is the ramped linear section that drops at -20dB/Decade. On a logarithmic plot it appears as a linear ramp. One of the reasons logarithmic display is better than linear.

Stop

Use the ruler-method if you need to find where the ramp stays linear (straight line ramp down to the right). Find the start and stop points where is stays on a straight edge

“Inductive Roll-off”

Zoom in here

Step 6: Choose a point to analyze Select a point somewhere very close to the half way points between Start and Stop and zoom in very tight

Step 7: Calculate the offset 12.64-12.57=0.07dB = GOOD! In general, most transformers will be below 0.1 db offset, but up to 0.2 dB is not uncommon. This analysis is sensitive to the same issues as Leakage Reactance.

HV Wye winding: Open and Short HV winding – same unit, more magnetization

Short circuit wye winding

Open circuit wye winding Very similar at high frequencies

Good Short Circuit Results Good Transformer – Good Results – Lo Frequency

All three have same roll off All three have very similar resistance element at 20 Hz Resistive section in this case is not horizontal. This is common

SC Analysis

• If SC results do not match.... – at 20 Hz -> Check Winding Resistance Variation in results may occur near 20 Hz due to variation in resistance element of shorting leads. May indicate a need to check transformer with DC resistance tests

– Compare LRT results throughout roll off – Variation in inductive roll off is due to variation in winding impedance

• Center phase may be slightly different • We expect all three phases to be within 0.1 dB

Short Circuit Variation Recommended is a three phase equivalent short circuit test Can do a per phase equivalent by repeating tests and just shorting appropriate LV winding Can do LV short circuit (with HV’s shorted) Per phase and LV measurements are useful in diagnostic investigations

Short Circuit v. Open Circuit At open circuit the load on the LV side is ‘infinite’ At short circuit the load on the LV side is zero We should expect a result to lie between the open and short circuit results!

Case Studies 1 Shorted Turn 2 Hoop Buckling 3 Axial Collapse 4 Clamping Failure 5 Bushing Failure

Case 1: Shorted Turn

• Demonstrates need for good low frequency resolution • Many electrical tests should show a shorted turn • No baseline data required for diagnosis

Case 1: Normal Response Three normal open circuit responses

Results as expected and

Three phases respond differently at low frequencies acceptable

Case 1: Shorted Turn After an incident - one year later... One phase has clear inductive roll off associated with short circuit measurement

Variation is clear indicator of shorted turn on A phase

Case 1: Shorted Turn Low frequency responses clearly show inductive roll off associated with short circuit measurement Another transformer - no reference results

Results indicate shorted turn on one phase

Case 2: Hoop Buckling • Compressive Failure of Winding - also known as Hoop Buckling – Need repeatability to within 1 dB – Need low frequency repeatability to within 0.3 dB

• Expect increase in LC combination of winding bulk - seen as left shift of resonance at mid range • Consequent increase in winding impedance - seen in short circuit results

Case 2: Hoop Buckling

• Example here of two 28 MVA sister units • One known bad after internal inspection • Other unit suspect based on Dissolved Gas Analysis


Inspection of bad transformer revealed hoop buckling on TWO windings

Clear bulge in winding

Case 2: Hoop Buckling Open Circuit LV results

Two bad phases have shift to lower frequency at this frequency

The B phase is different which may be due to design and construction

By itself, this is not conclusive evidence as we have no reference results


Low frequency short circuit results show consistent increased impedance on bad phases

The variation was consistent and repeatable: attributable to variation within the transformer

Results are anomalous but symmetrical evidence is inconclusive

0.2 dB delta is significant here!


• For the bad transformer we have picked out two possible indicators of variation for the two bad phases • For the suspect unit, we look at the same areas

Case 2: Hoop Buckling Open circuit results show same effect on ONE phase

Clear shift left at same frequency range

In the known bad unit two phases had Now we have just one phase shift left a shift left and both had hoop buckling

Small change in frequency and dB


Both units - displayed for comparison

This is good evidence that one phase may have hoop buckling

Case 2: Hoop Buckling Suspect unit shows same increased impedance (more dB’s down) for one phase - the same one with the shift left on open circuit results

One phase appears to have symptoms of hoop buckling

0.2 dB delta is significant here!

Case 2: Hoop Buckling • Diagnosis is that suspect unit has one bad phase through hoop buckling • Transformer is bent, but not broken • Unit was returned to service for some months, under advice that it was less likely to withstand a close in fault • Subsequent internal inspection revealed hoop buckling on the suspect phase, as expected and predicted

Case 2: Hoop Buckling Inspection showed hoop buckling on suspect phase - as predicted

Transformer returned to service before being rewound


• Need good resolution - especially below 1 kHz for short circuit results • Need good repeatability over a range (less than 0.3 dB) • Use of results from similar unit as reference was key for diagnosis

Case 3: Axial Shift of a Winding

• Example uses reference results taken 7 years previously • Need range up over 1 MHz • Need repeatability to within 1% • Need confidence in results to avoid a false negative • Failure mode is reduction in LC combination • Resonances should shift right at higher frequencies as a consequence of LC reduction


Scattered blocking under a collapsed winding

Case 3: Axial Shift of a Winding 0 Results from one GOOD phase taken in 1994 and 2001 -10 -20 B d n i -30 e d u t i l -40 p m A -50 -60

Small dB variations are acceptable: no 0.5 betweenFrequency tracesin implies MHz LC variation implied

-70 0

No variation that THIS PHASE has little change 22/11/94

8/11/01

1

Case 3: Axial Shift of a Winding 0 Results from bad phase taken in 1994 and 2001 -10 -20 B d n i -30 e d u t i l -40 p m A -50 -60

Low frequency results, up to -70 over 200 0 kHz, are acceptable

Clear and consistent shift to higher frequencies of several resonances 0.5

Variation implies we have Frequency in MHz a substantial problem 22/11/94

8/11/01

1

Case 3: Axial Shift of a Winding 0 -10

Good SFRA repeatability gives confidence in the integrity of the good phase

-20 B d n i -30 e d u t i l -40 p m A -50 0

-60 -10

-70 0

0.5

Good SFRA repeatability allows confident diagnosis of bad phase

-20 B d n i -30 e d u t i l -40 p m A -50

1

-60 -70 0

0.5

Frequency in MHz

1


• Bad phase clearly identified • SFRA results up to >1 MHz required • SFRA results taken with different test sets by different test teams • Repeatability is key

Case 4: Clamping Failure

• 750 MVA transmission transformer • Close in fault caused center winding to ‘jump’ and break the clamping • Stress shield dropped on to winding, altering capacitance of that winding


Clamping beam broken

Stress shield collapsed on to center phase


Three HV phase results shown together 0

Center phase shows shift to lower frequency for first resonances

-10

-20

-30

B d n i e d -40 u t i l p m A

-50

-60

-70

-80

Low frequency results show expectedfurther form Transformer has a problem investigation required

0

5

Frequency in kHz10

15

20


• Initial low frequency (< 2kHz) core related resonance unaffected • Resonances usually associated with bulk capacitance of windings to ground are moved to lower frequencies

Case 5: Bushing Failure

• Transmission Transformer had a bushing failure • No reference results available • Did the failure move the windings? • Sister unit available for reference

Case 5: Bushing Failure Minor variations in suspect unit at high frequencies may relate to some internal lead movement

Variation is small - but needs further investigation

Low frequency variation as expected between phases

Case 5: Bushing Failure Six traces here - suspect unit and sister unit

Variation is still a concern - need to do a phase-by-phase comparison of two sister units

Case 5: Bushing Failure There are two traces here - suspect phase of suspect unit and same phase of sister transformer

The two traces are clearly very similar can just see one trace behind the other

Sister unit has exactly the same variation - it must be design related as traces implies this sister is known good

No variation between phase is OK, despite bushing failure


• Suspect phase does show some variation at high frequency • Sister unit shows same variation known to be a good transformer • Consequently - SFRA variation is due to design and construction


• Sister Transformers may show variation between corresponding phases on each transformer • If two phases are similar between the two units, it is likely that the third phase is also similar • So we can use sister units as a reference, even if there are variations in response between the units

Interpretation • Experience: Certain Frequency Bands Indicate Different Problem Conditions

400kHz to 2MHz: Movement of main and tap winding leads

<2kHz: Core Deformation, Open Circuits, Shorted Turns & Residual Magnetism

2kHz to 20kHz: Bulk Winding Movement Relative to Each Other, clamping structure

20 kHz to 400kHz: Deformation Within the main and tap windings


SFRA Training Using the Software

Using the Software Communication protocol – Ethernet cable

Using the Software Communication protocol – CSMUAB (USB) Cable

USB Drivers

• Communication: Ethernet or USB over Ethernet • Ethernet 192.168.1.0 • USB over Ethernet 192.168.1.1 • Windows will ask to install the USB driver when the USB cable is plugged into the M5X00 for the first time. • Point to location of USB drivers

Windows Vista

• SFRA V5.1 will operate on Windows VISTA • The communication limitations still apply, regarding profiles and settings • One issue has surfaced.

VISTA Workaround

• Some people have had issues even when logged in as an administrator. • The workaround so far has been to tell the user to turn off the 'User Account Control' in the Vista User Manager. • Turning this off has fixed every issue we have had so far.

User Account Control

SFRA Software The best way to become familiar with the software is to use it Topics on the following slides may help direct discussion and experimentation

To try… Software installation Connect to an M5200 Data management: settings, folders and files Test philosophy: Set up data files Select particular files Run test Review results

To try… Choosing & Using templates Select Apparatus & Run test Reviewing, overlaying and comparing data Analysis tab Filenames & e-mailing files

Conclusions Key points: Collect data in an organized manner Always make notes for tap positions Backup for your test data regularly


SFRA Training Basic Analysis

3. Are Baseline Results Necessary? Not always - but they are preferable Phase-to-phase comparisons Comparison with sister units Comparison with units from the same manufacturer Results have a generally predictable shape

SFRA Analysis is Logical Analyzing SFRA data is a logical and methodical process Examples are given here to show the approach taken

SFRA Analysis : getting Started Make sure all results are available: Current SFRA results Any previous SFRA results Any results from similar units Any electrical test or DGA data Understand the context: why are these tests being performed?

In general Trace shapes: HV LV Short Circuit Do they look as expected? Any variance?

Analysis with benchmark Have the same measurements been made? LTC and DETC the same? Oil, bushings etc

Sister Units Real sister? Example

Phase-by-phase When it gets down to basics…

Causes of variation Check two useful documents: Practical variability Field and Factory variation

Integration

SFRA is one tool in the tool box Capacitance

OC: Open Circuit

Exciting Currents

DC Resistance

SFRA

SC: Short Circuit

Leakage Reactance

Context Link SFRA to other tests: DC resistance Leakage Reactance Exciting Currents Capacitance

Conclusions Analysis takes experience and time

Using Cross-Correlation Coefficients to Analyze Transformer SFRA Traces

The Problem

• Analyzing SFRA Traces can be Challenging • No simple categorization available • Natural desire to apply “limits” to SFRA results • “Smart” SFRA analysis routines may have dubious outputs

The Reality

• Human element needed - pictures • Every transformer is an individual • No perfect “pass/fail” criteria for any range • SFRA analysis is still prone to outlandish claims from some quarters • Someone has to know what they are doing…

The Stop-Gap

• Cross-correlation coefficients (CCF) are commonly used in a variety of industries to monitor signal integrity • CCF’s provide a method to: – Quantify SFRA Trace similarities – Communicate results – Apply limits to analysis – Don’t rely on hidden or secret “black box” analysis routines

What is a CCF really?

• Take two sets of numbers • Work out a ‘sameness’ value • Same… different… opposite… • Math is simple, as shown here

Still keeping the math simple....

2

CCF

( Xi  X )(Yi  Y )    ( Xi  X ) *  (Yi  Y ) 2

2

In words, the CCF calculates the convoluted mean error and normalizes to the individual mean over a particular range of frequencies

Keeping the math simple....

• CCF is a number between 1.0 and 1.0. • For SFRA, it’s usually between 1.0 and 0.0 • The closer the value is to 1.0, the more similar the traces are. • The lower the value, the more the two traces diverge. • CCF can also be expressed in Percent

Example CCFs

CCF Good Match

0.95-1.0

Close Match

0.90-1.0

Poor Match

<.89

No or very poor match

<= 0.0

What you really need to know

Regions of Interest Region

Frequency Sub-Band

Components

1

< 2kHz

Main Core; Bulk Winding Inductance

2

2kHz to 20 kHz

Bulk Component Shunt Impedances

3

20 kHz to 400 kHz

Main Windings

4

400 kHz to ~1 Mhz

Main windings, Tap windings etc

Every transformer is different, these are just general guidelines

SFRA Regions

Select your Band and Calculate

Selecting Limits • Depending on the type of comparison , the CCFs limit should be adjusted. Starting Point! – Benchmark comparison:

– Phase to Phase (A/C) or sister units:

Case 1: Benchmark Comparison

• 675 MVA GSU built in 2002 • Suffered a fire on connected iso-phase bus • Bushing had been degraded during the fire – Expect some high frequency variation

• The unit was known to be magnetized – Expect some low frequency variation

Case 1: Benchmark Comparison: H2-H0

Frequency Sub Band

CCF

Region 1: 0 - 2 kHz

0.9879

Region 2: 2kHz – 20 kHz

0.9964

Region 3: 20 kHz – 400 kHz

0.9882

Region 4: 400 kHz – 1 MHz

0.9988

Case 2: Phase-to-phase : Bent Unit • 1960’s vintage GE 50 MVA transformer • Tripped out of server on protection • No reference SFRA results available • Phase to phase analysis required

Close up view of LV winding shift

Case 2: Bent Transformer: A/C Phase comparison

Frequency Sub Band

CCF

Region 1: 0 - 2 kHz

0.9831


0.9868


0.8262


0.9567

Case 3: Two Large Sister Units

• Two 370 MVA Alstom transformers • 345/14 kV • Routine health assessment of the units was conducted • SFRA was used to ascertain construction consistency between the units • Determines if two units are in fact identical

Case 3: Sister Units

Frequency Sub Band

CCF

Region 1: 0 - 2 kHz

0.9898


0.9994


0.9914


0.9923

Considerations

• Case studies did not change region limits – Not necessarily true in all cases – May change limits for different designs

• Recommended limits are starting points – Can adjust limits; situation may require fine tuning

• Still does not replace the trained SFRA user • Poor test technique can throw off results

Conclusion • CCF can help in the analysis of traces by drawing attention to various regions • CCF assign a “Figure of Merit” that can be used to quantify SFRA traces similarities • CCF can not diagnose the failure mode, only the trained human eye and experience can do that • Still does not replace the trained SFRA user

7. Conclusions

• SFRA is a useful and sensitive tool • Use in Conjunction with other Tools • SFRA is a reliable and repeatable means of making FRA measurements • Use on relocation or after an incident • Use as part of factory QA

7. Notes...

• Need for good grounds • Make sure transformer is in the same state as last time • Use a reference library

SFRA Case Studies: Dealing With Noise

Presented by Doble Engineering Company

Signal To Noise Ratio (SNR) Neighboring Transformers

The Test signal can become “swallowed” by neighboring sources of interference The Real World has Real Sources of Line interference Doble Provides 20 Vpp to prevent this problem.

Substation Equipment

Overhead Lines

Ground Grids 239

SNR: Why it’s important You may end up measuring the surrounding noise.

Results for a test set using low SNR

Doble SFRA Leads Clip-1

•Tests above 1 MHz are dominated by the test leads (IEC & IEEE) • Every test connection is a chance to make a mistake and mess up results; Doble keeps it simple

Done.

•We all test in every possible environment so don’t complicate it!

Clip-2

241

Inside Transformers

Adverse Weather

At Night

Advanced Analysis: Cross Correlation & Difference

Cross-Correlation Analysis - Assess Condition by Winding Region Difference Analysis - Compare offset variations for simple quick analysis

242

Technical – automated analysis •

Diagnosis by difference and cross correlation is most advanced available

•

Training and support mean we don’t lose sight of the ‘picture’, numbers can only tell you so much

•

Cross-correlation (CC) looks at curve shape – will be the basis of the Doble expert system

•

Some systems have a single CC value for the whole trace

243

Intermediate Frequency (IF) Bandwidth

Frequency to be measured

Doble follows Laboratory Best Practices of always ensuring IF Bandwidth is < 10 % In simple Terms: We don’t try to make the results look prettier by allowing other surrounding signals to smooth the result out.

Doble

Some Others

244

Nameplate Information

• Auto Transformer with Tertiary • 20 MVA • 138/69/13.2 kV • Year of manufacture: 2011 • Test configuration: – No Oil – Spark plug (transit) Bushings

The story unfolds

SFRA tests were performed on a transformer after delivery to the site and then compared to factory results Something didn’t quite agree…..

H1-X1 Comparison to Factory

X2-H0X0 Comparison to Factory

Hey why are these different? Site Test

Factory Results

HVSC Test Results

Summary of initial SFRA analysis • Both HVOC and LVOC results were satisfactory when compared to previous • Upon evaluation it was decided the HVSC results did not compare to factory tests • An investigation followed and ultimately the transformer was tested again

The Investigation showed …. • That the shorting leads for the HVSC tests were allowed to sag and come in contact with the transformer tank. • The leads were tightened up with no sagging and the short circuit tests were performed again.

Retest Satisfactory - 9 tests shown Shorting leads resting on tank

Factory results and Retest after fixi ng the leads


• Auto Transformer with Tertiary • 100 MVA • 220/132 kV • New unit • Test configuration: – Fully dressed

The story unfolds

SFRA tests were performed on a transformer after delivery to the site and then compared to factory results Field results were different when compared to previous (factory) tests Again, something didn’t quite agree…..

Series Winding Factory vs. Field

Field

Factory

Common Winding Factory vs. Field

Field

Factory

Field retest after Demagnetization But what is this here?

This is better

Let’s get a closer look

Series Winding

Field Retest

Common winding field retest

This isn’ t good

This is better

Conclusion Effects of magnetic viscosity were evident in both the series and common windings between tests. Cause of difference in field retest – Tertiary was ungrounded for the test

SFRA Case Studies: Field LV Anomaly



• 125 MVA • 220 kV Wye Delta • Three winding transformer • In service unit, DGA starting at the end of previous year showed rising trend • SFRA was performed

Initial SFRA analysis

• SFRA results indicated problems in the LV • Even to the untrained eye one should be able to pick out the differences

LVOC SFRA Test

Houston we have a problem!

Time to go inside

An internal investigation was conducted and visible evidence of the problem was found, Pictures tell the story

Internal inspection

Internal Inspection

Internal Inspection

SFRA LVOC after repair

SFRA Example  Transformer Type: 3 Power Transformer  Manufacturer: Waukesha  Capacity: 30/40/50 MVA  Voltage: 138/27 kV  Tap Changer:

10% load tap changer; HV reconfiguration

tap changer, de-energized  Entered service July 2002  Removed from service two weeks later

Transformer

SFRA Example  Indications  Tank temperature recorded at 480 F (230 C)  Slight tank wall bulge  Tank wall discoloration  Strange noises while in service  DGA

Acetylene from 0 to >300 Hydrogen from 10s to >500 FFAs from low to high

SFRA Example

Transformer Turns Ratio

 Within 0.3% of nameplate and previous

results

SFRA Example DETC Pos C B A D E C C C C C C C C C C C C C C C C C

LTC Pos 1R 1R 1R 1R 1R N 1L 2L 3L 4L 5L 6L 7L 8L 9L 10L 11L 12L 13L 14L 15L 16L

H1-H2 X0-X2 7/22/02 8/5/02 8.809 8.811 9.033 9.035 9.258 9.260 8.575 8.574 8.336 8.340 8.861 N/P 8.919 8.911 8.976 8.964 9.033 9.018 9.090 9.075 9.149 9.133 9.208 9.190 9.268 9.250 9.329 9.310 9.391 9.370 9.460 9.433 9.516 9.495 9.580 9.559 9.645 9.623 9.711 9.688 9.785 9.745 9.846 9.821

H2-H3 X0-X3 % 7/22/02 8/5/02 8.808 -0.02% 8.809 9.032 -0.02% 9.033 9.257 -0.02% 9.258 8.572 0.01% 8.573 8.338 -0.05% 8.336 8.861 N/P N/A 8.918 0.09% 8.919 8.975 0.13% 8.976 9.031 0.17% 9.033 9.090 0.17% 9.090 9.149 0.17% 9.149 9.208 0.20% 9.211 9.268 9.268 0.19% 9.310 0.20% 9.329 9.391 0.22% 9.391 9.453 0.29% 9.453 9.518 0.22% 9.523 9.582 0.22% 9.588 9.647 0.23% 9.645 9.713 0.24% 9.711 9.780 0.41% 9.782 9.849 0.25% 9.846

H3-H1 X0-X1 % 7/22/02 8/5/02 8.808 0.01% 8.809 9.032 0.01% 9.033 9.257 0.01% 9.258 8.572 0.01% 8.573 8.338 -0.03% 8.336 8.864 N/P N/A 8.918 0.01% 8.919 8.975 0.01% 8.976 9.031 0.02% 9.033 9.090 0.00% 9.090 9.148 0.00% 9.149 9.208 0.03% 9.208 9.268 9.268 0.00% 9.310 0.20% 9.329 9.391 0.00% 9.391 9.453 0.00% 9.460 9.518 0.05% 9.516 9.582 0.06% 9.580 9.647 -0.02% 9.645 9.713 -0.02% 9.711 9.781 0.02% 9.785 9.849 -0.03% 9.849

% 0.01% 0.01% 0.01% 0.01% -0.03% N/A 0.01% 0.01% 0.02% 0.00% 0.01% 0.00% 0.00% 0.20% 0.00% 0.07% -0.02% -0.02% -0.02% -0.02% 0.04% 0.00%

Power Factor/Capacitance  CHL rose by a factor of ~3; slight

capacitance rise Current (mA)

Watts-Loss (W)

7/22/02

8/13/02

%

7/22/02

8/13/02

%

CH

9.946

9.974

0.28

0.218

0.254

16.51

CHL

23.540

23.650

0.47

0.436

1.239

184.17

CL

59.410

59.660

0.42

1.371

1.420

3.57

% PF (Corrected to 20C)

Capacitance (pF)

7/22/02

8/13/02

%

7/22/02

8/13/02

%

CH

0.18

0.21

16.67

2638

2645

0.27

CHL

0.15

0.42

180.00

6245

6275

0.48

CL

0.18

0.19

5.56

15761

15825

0.41

Excitation Currents

Excitation Currents 10 9 8

H1-H2/mA

A 7 m

H2-H3/mA

6

H3-H1/mA

5 4

N

L 2

L 4

L 6

L 8

L 1 0

LTC Position

L 2 1

L 1 4

L 1 6

ea age Reactance Nameplate 9.39% Measured 3 phase equivalent 13.19%

Per Phase Impedance

H3-H1/ 596.60 32.36%

H1-H2/ H2-H3/ 174.00 173.30 9.15% 9.11%

SFRA Example

Discoloration and bulge

Fault pressure Relief valve (did not operate)

Low Voltage

High Voltage

High Voltage Short Circuit

Black specs...

Copper exposed on lead

Factory Inspection

Case Study

Autotransformer transportation damage found using SFRA

Transformer Before Shipment With oil and actual bushings –> H – X Open Circuit test

Transformer After Shipment No oil and w/spark plug bushings –> H – X Open Circuit test

Transformer After Shipment H2 – X2 Open Circuit test before and after comparison

Transformer After Shipment No oil and w/spark plug bushings –> Tertiary Open Circuit

Case Study # 1

As a result of the Swept Frequency Response Tests a decision was made to perform an internal inspection. The following was observed:

Internal Inspection

• Center winding clamping hardware loose resulting in loss of pressure on the stack • Y1 lead laying on top of core • Loose boards were found inside transformer

Internal Inspection

Internal Inspection

Internal Inspection

Loose Clamping

Internal Inspection

Internal Inspection

SFRA Case Studies: Possible Core Variation


Low Voltage Open Circuit Traces

Unit A Unit B Unit C

High Voltage Open Circuit Traces

Unit A Unit B Unit C

Before and after de-magnetizing

The suspect unit was de-magnetized at the plant by energizing the LV winding at 110% which is 15.2KV, 20 amps for 12 hours. There was very little change.

Suspect Unit, Low Voltage

Suspect Unit, High Voltage

Conclusion

Upon further investigation it was discovered that the core steels for all units was of the same grade (M4), however two of the transformers used the steel from one manufacturer and the third unit used the steel from a different manufacturer. This could have resulted in different core characteristics (e.g., permeability).

SFRA Case Studies: Magnetic Viscosity


Effect of Magnetization Low frequency variation may be severe

Magnetization affects the core response

Magnetic Viscosity

Drift can occur from demagnetized state to lowest energy state depending on internal geometry (see Spring 2010 paper)

SFRA Case Studies: Phase-to-Phase Find


Case Study

• 120 MVA, 245kV/144kV autotransformer • There was a fault and it was unclear if there had been damage to the unit • SFRA Testing was done to assess the condition of the unit • No historical data available

Nameplate data

SFRA HVOC results

Traces do not correlate well in this area

Suggests some kind of winding issue

SFRA HVSC results

Looking for less than 0.2 db phase separation; Zoom in to check

Some variation in magnitude & resonance

HVSC inductive roll-off region

0.13 db acceptable

Course of action

Based on the SFRA results a recommendation was made for internal inspection It was discovered that the tertiary winding connections had never been bolted together, and theorized that the energy from the fault had pushed the connection apart – There had been arcing across that opening

The missing Link

Internal Connection - Delta Link

Delta Link not made up

Conclusion

Although there wasn’t any previous data, a phase to phase comparison was enough to support the indication of a problem * This might have been caught at the factory if SFRA testing had been performed at that time

SFRA Case Studies: Field SFRA & LR Puzzle


Puzzle outline

• Transformer details – nameplate and history • The failure – LV bushing in pieces • Field test results – good, bad and inconsistent • Retest • Decision?

Transformer nameplate • 230/72 kV Federal Pioneer • 50/66.7/83.3 MVA Autotransformer • Built 1994 • Transformer suffered animal intrusion and tripped out after tertiary fault in late 1990’s • No testing performed • Unit successfully returned to service • Unit had also seen “a number” of short circuit faults

2006 Incident •

Unit tripped out in July with operation of:

– R and B phase elements – Gas accumulation/surge – Gas pressure relief •

Site inspection shows X3 gaskets compromised, oil leaking from flange; conservator isolated to limit spill

•

Inspection shows lower part of X3 bushing failed catastrophically

•

Porcelain shards throughout upper portion of core/coils

•

Arcing evident from lower part of X3 bushing to top of steel frame

Transformer nameplate

Investigation • ABB contacted regarding failure of 69kV 1200 A Type O+C • ABB suspect a known defect • Service advisories from August 1998 and April 1999 • ABB offer 3 replacement bushings

Investigation •

Transformer carefully cleaned and flushed with hot oil

•

X1 & X2 bushings removed, inspected, tested and re-installed

•

Routine electrical tests performed:

– Bushing C1 & C2 & power factor measurements – Winding power factor & capacitance – HV single phase excitation – DC winding resistance – Turns ratio – Leakage reactance – SFRA

Transformer

Investigation – Doble testing

• All as expected and all ‘Good’

Investigation – Doble tests on bushings

• All as expected and all ‘Good’

Investigation – Bushing C2 tests

• Some results are ‘Investigate’ on HV C2’

Investigation – IEx & Surge Arresters

• Surge arresters all as per nameplate

Investigation – LR results

Investigation – LR results

Investigation – SFRA results




X3-X0

X1-X0 & X2-X0

Investigation – further results • Leakage reactance repeated with heavier shorting cables; results do not change much. • SFRA performed – all phases are consistent • Slight variation between phases not considered unusual; may relate to animal incident • SFRA and LR are inconsistent!!!

Time passes…

• Much thought

Time passes… more people needed

You can never have too many supervisors…

Repeat SFRA tests & LR tests

• January following year… • Repeat tests to review site procedures and confirm all test results • ALL Results confirmed • LR and SFRA still inconsistent

Variations on a theme – clutching at straws • Doble’s Lachman IEEE paper suggests perform LR tests from LV side • Wouldn’t necessarily explain the anomaly (X2 and X3 being different in LR) • LR Results confirmed from LV side as being unbalanced • LR Results reconfirmed unbalanced with three phase supply and clip on CT’s (energize H measure T)

Summary • Situation is difficult • LR indicates possible winding movement or anomaly • SFRA does not indicate significant winding movement • Other tests do not indicate a problem • What next? Internal inspection again? Energize and hope?

Action… • It was an LV bushing failure… • What could be anomalous about the tertiary? • Clutching at straws… • Everything else looks OK… • Let’s check the nameplate again…

Transformer nameplate – desperation…

Denoument…

Conclusion… • CT shorting block not shorted on tertiary • Affected high current LR • Marginal affect on low current SFRA • Everything is worth checking… • Transformer successfully returned to service

Sweep Frequency Response Analysis

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