Matz OhlenDirector – Transformer

Test Systems

Megger

Sweden

Frequency response analysis of power transformers

Measuring and analyzing data as function of frequency, “variable frequency diagnostics”

• Impedance vs frequency – FRA/SFRA (Sweep Frequency Response Analysis)• Magnitude/phase vs frequency• Magnitude/phase vs frequency

• Typical frequency range 20 Hz – 2 MHz

• Insulation characteristics vs frequency – DFR/FDS (Dielectric Frequency Response/Frequency Domain Spectroscopy)• Capacitance and dissipation factor vs frequency

• Typical frequency range a few mHz to 1 kHz

Transformer Diagnostics

• Diagnostics is about collecting reliable information to make the correct decision

• Making the correct decisions improves reliability and saves money

TTR

SFRA

FDSWinding

Resistance

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 measured over a wide frequency range and the results are presented as magnitude response curves (“filter response”)

• Changes in the impedance can be detected and compared over time, between test objects or within test objects

• The method is unique in its ability to detect a variety of winding faults, core issues and other electrical faults in one test

SFRA measurement circuitry

SFRA analysis tools

• Visual/graphical analysis

• Starting dB values for• [Open] (excitation impedance/current)

• [Short] (short-circuit impedance)

• The expected shape of star and delta configurations

• Comparison of fingerprints from;• Comparison of fingerprints from;• The same transformer

• A sister transformer

• Symmetric phases within the same transformer

• New/missing resonance frequencies

• Correlation analysis

• DL/T 911 2004 standard

• Customer/transformer specific

Typical response from a healthy transformer

HV [short] identical

between phases

LV [open] as

expected for a ∆Y tx

HV [open] as expected for

a ∆Y tx. ”Double dip” and

one response lower

Very low deviation

between phases for

all tests – no winding

defects

Transformer with serious issues...

Large deviations

between 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

SFRA standards and recommendations

• DL/T 911-2004, Frequency Response Analysis on Winding Deformation of Power Transformers, The Electric Power Industry Standard of People’s Republic of China, 2004

• Cigre brochure 342 (2008), Mechanical Condition Assessment of Transformer Windings Using Frequency Response (FRA)Response (FRA)

• IEEE PC57.149™/D7 (2009), Draft Trial-Use Guide for the Application and Interpretation of Frequency Response Analysis for Oil Immersed Transformers (Draft)

• IEC 60076-18 Ed1.0 (2010), Power Transformers – Pert 18. Measurement of Frequency Response (Draft)

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

SFRA standards – Key points

Standard Dynamic range Accuracy Signal cable grounding

EPIS PRC DL/T 911 -100 to +20 dB ± 1 dB @ -80 dB

Wire, shortest length to

transformer core

grounding

CIGRE brochure 342-100 to +20 dB

(measurement range)± 1 dB @ -100 dB “Shortest braid principle”CIGRE brochure 342

(measurement range)± 1 dB @ -100 dB “Shortest braid principle”

IEEE PC57.149/D7 (draft)

"Sufficient dynamic

range to

accommodate all

transformer test

objects"

"Calibrated to an

acceptable standard"

Grounded at both ends,

documented and

repeatable procedure

IEC 60076-18 (Draft)-100 to +10 dB

min 6 dB S/N

± 0.3 dB @ -40 dB

± 1 dB @ -80 dB

Smoothing not allowed

“Shortest braid principle”

ABB FRA Technical StandardBetter than

-100 to +40 dB± 1 dB @ -100 dB “Shortest braid principle”

SFRA measurement Range - Why you need at least -100 dB...

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

Signal cable connection – ”Shortest braid principle”

Source:IEC 60076-18 (draft)

SFRA – Summary and conclusions

• SFRA is an established methodology for detecting electromechanical changes in power transformers

• Collecting reference curves on all mission critical transformers is an mission critical transformers is an investment!

• Ensure accuracy by selecting a high-quality instrument

• Ensure repeatability by following international standards and practices

5

6

7

FDS/DFR

Insulation Testing –Dielectric Response Methods

0

1

2

3

4

0,000001 0,00001 0,0001 0,001 0,01 0,1 1 10 100 1000 10000

FDS/DFR

HV Tan Delta

VLF

PDC

Polarization Index

"DC"

Frequency, Hz

Dielectric Frequency Response Measurements – Tan delta from mHz to kHz

V

A

Hi

Lo

A

GroundCHL

CL CH

( )( )( )ω

ωω

I

UZ = ( )

( )εεω

′′′⇒

and

PF tand,,C Z

Measure at several frequenciesUse Ohms law:

Why perform dielectric frequency response measurements...

Typical power factor values @ 20° C

"New" "Old" Warning/alert limit

Power transformers, oil insulated

0.2-0.4% 0.3-0.5% > 0.5%

Bushings 0.2-0.3% 0.3-0.5% > 0.5%

IEEE 62-1995 states; “The power factors recorded for routine overall tests on

older apparatus provide information regarding the general condition of the

ground and inter-winding insulation of transformers and reactors. While the

power factors for most older transformers will also be <0.5% (20C), power

factors between 0.5% and 1.0% (20C) may be acceptable; however, power factors >1.0% (20C) should be investigated.”

Bushings 0.2-0.3% 0.3-0.5% > 0.5%

Dielectric Frequency Response- Investigating high single number PF data

Dry transformer with old

oil (high conductivity)

Wet transformer with good oil

What affects the response?

-M

ois

ture

+

- Oil Conductivity +

-M

ois

ture

+

-

- Temperature +

DFR – Moisture estimation (1-2-3)

Measured DFR

Right click

DFR response

Select Send to…MODS

DFR – Moisture estimation (1-2-3)

% Spacers

Oil

% Barriers

Capacitor model

Master curve

Measurement

DFR – Moisture estimation (1-2-3)

2. Click Auto match

1. Confirm insulation

temperature

Auto match

DFR – Moisture estimation – Result

Geometry

Moisture

Geometry

Oil conductivity

Dielectric Frequency Response- Investigating irregular shapes

CHL response

CH and CL responses

DFR analysis – irregular responses

A measured irregular shape is not a mishap – It is information!

• CH and CL has expected oil-paper response

• CHL looks “different” with higher losses at • CHL looks “different” with higher losses at mid-frequencies

• Contamination/conductive layer between windings?

• This particular transformer had a history including an LTC replacement due to seriously burned contacts...

Methods for dielectric response measurements

DC (Polarization-Depolarization Current measurements)

• Strenghts

• Shorter measurement time at very low frequencies

• Weaknesses

• More sensitive to AC

AC (Dielectric Frequency Response measurements)

• Strenghts

• Less sensitive to AC interference

• Less sensitive to DC interference

• Wide frequency range

• No discharge necessarry• More sensitive to AC interference

• More sensitive to DC interference

• Limited frequency range (PDC only)

• Data conversion necessary (combined PDC/DFR only)

• Discharge before measurement may be needed

• No discharge necessarry

• Weaknesses

• Longer measurement time for very low frequencies

Moisture assessment with dielectric response methods takes a while…

Available methods – Measurement times• PDC – Typically 0.5-3 hours

• PDC+DFR – approximately 15-25 minutes (2 mHz, with and without discharge) to 2.5-4 hours (0.1 mHz with and without discharge)

• True AC DFR/FDS – approximately 18 minutes (2 mHz) to • True AC DFR/FDS – approximately 18 minutes (2 mHz) to about 5.5 hours (0.1 mHz)

Availability – Transformer off-line in field• Typically 1 day for complete diagnostic measurements

Measurement time (minutes) for DR measurements

100

1000

1

10

1st gen FDS 3rd gen FDS PDC+FDS, no discharge PDC+FDS, with discharge

2 mHz

1 mHz

0,1 Mhz

Typical DFR results for transfomers with various moisture content

1.5% moisture

0.3% moisture

2.1% moisture

0.2% moisture

0.3% moisture

DFR results for a transfomer at various temperatures

Temp Moisture, % Oil conductivity, pS

21 2,4 10,4

27 2,3 13,8

34 2,4 22,8

49 2,3 39,349 2,3 39,3

DFR data acqusition is pending insulation temperature

10,00

100,00

Frequency, mHz

0,10

1,00

0 10 20 30 40 50 60

eV=0,9

eV=0,7

eV=0,5

Insulation Temperature

Corresponding data points

Ongoing project collecting measurement results on various transformers…

• Old distribution transformers

• New power transformers in factory

• New power transformers in the field

Typical power transformers in various • Typical power transformers in various conditions

Moisture assessment of transformers with different low frequency limits

10,0

T1, 3°C

T2, 7°C

Moisture level, %

0,1

1,0

0,1 1 10

T2, 7°C

T3, 15°C

T4, 15°C

T5, 21°C

T6, 23°C

T7, 25°C

Low frequency limit, mHz

Distribution transformerT = 23°C, f = 0.1-10mHz

Distribution transformerT = 23°C, f = 0.1mHz-10kHz

4

5

6

7

Distribution transformerT = 23°C, f0 = 0.1-10mHz

Auto geometry

0

1

2

3

4

0,1 1 10

X (auto)

Y (auto)

Moisture

Oil, pS

Stop freq, mHz

Power transformerT = 25°C, f = 0.1mHz-1 kHz

Power transformerT = 25°C, f = 0.1mHz-1kHz

1,2

1,4

1,6

1,8

2

Power transformer, T = 25°C, f0 = 0.1-10mHz

Auto geometry

0

0,2

0,4

0,6

0,8

1

0,1 1 10

X (auto)

Y (auto)

Moisture, %

Oil cond, pS

Min freq, mHz

DR measurement frequency range– Conclusions so far...

• Auto geometry estimation mode in MODS works good

• Limited value of measuring below 1-2 mHz at ”normal” temperatures (only a few results collected so far from measurements at < 15°C)collected so far from measurements at < 15°C)

• If geometry is (approximately) known, it may be possible to reduce measurement time

• Measurements at higher temperature can shorten the measurement time

Summary and conclusions

� Dielectric response measurement is an excellent tool for insulation diagnostics

� Moisture assessment using DFR measurements and transformer insulation modeling is a generally accepted standard diagnostic method

Transformer outage time is expensive and it is � Transformer outage time is expensive and it is necessary to minimize measurement time. DFR measurements down to 1-2 mHz seem to be sufficient for accurate moisture assessment at normal temperatures

� DFR is capable of identifying non-moisture issues like contamination/sludge and/or conductive layers

Questions and/or comments?

Sweep Frequency Response Analysis

Application Examples

Additional material

Application Examples

Time Based Comparison - Example

• 1-phase generator transformer, 400 kV

• SFRA measurements before and after scheduled maintenance

Transformer supposed to be in good • Transformer supposed to be in good condition and ready to be put in service…

Time Based Comparison - Example

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

Time Based Comparison – After repair

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

Type Based Comparisons (twin-units)

Some parameters for identifying twin-units:� Manufacturer

� Factory of production

� Original customer/technical specifications

� No refurbishments or repair� 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

• Three 159 MVA, 144 KV single-phase transformers manufactured 1960

• Put out of service for maintenance/repair after DGA indication of high temperatures

• “Identical” units• “Identical” units

• SFRA testing and comparing the two transformers came out OK indicating that there are no electromechanical changes/problems in the transformers

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

Type Based Comparison – 3x HV [open]

Type Based Comparison – 3x HV [short]

3x HV [short] - details

Higher resistance on A-phase

Type Based Comparison – 3x LV [open]

Design Based Comparisons

• Power transformers are frequently designed in multi-limb assembly. This kind of design can lead to symmetric electrical circuits

• Mechanical defects in transformer windings usually generate non-symmetric displacements

• Comparing FRA results of separately tested limbs can be an appropriate method for mechanical condition assessment

• Pending transformer type and size, the frequency range for design-based comparisons is typically limited to about 1 MHz

Design Based Comparison - Example

• 40 MVA, 114/15 kV, manufactured 2006

• Taken out of service to be used as spare

• No known faults

• No reference FRA measurements from • No reference FRA measurements from factory

• SFRA testing, comparing symmetrical phases came out OK

• The results can be used as fingerprints for future diagnostic tests

Designed Based Comparison – HV [open]

Designed Based Comparison – HV [short]

Designed Based Comparison – LV [open]

Design Based Comparison – After Suspected Fault

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

• By mistake, the transformer was energized with grounded low voltage sideside

• After this the transformer was energized again resulting in tripped CB (Transformer protection worked!)

• Decision was taken to do diagnostic test

Design Based Comparison– After Suspected Fault

-40

-30

-20

-10

0

10 100 1000 10000 100000 1000000

Re

sp

on

se

(d

Bs)

� HV-0, LV open� A and C phase OK, large deviation on B-phase

(shorted turn?)

-80

-70

-60

-50

Frequency (Hz)

Re

sp

on

se

(d

Bs)

Design Based Comparison– After Suspected Fault

-30

-20

-10

0

10 100 1000 10000 100000 1000000

Resp

on

se (

dB

s)

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

deformation?)

-60

-50

-40

Frequency (Hz)

Resp

on

se (

dB

s)

And how did the mid-leg look like…?

Insulation cylinder

Core limb

LV winding

End