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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