The 8th Regional Conference of Cigre in Arab Countries &
GCC Power 2010 Conference
Session (B1 ) – Paper (87)
Using Dielectric Frequency Response Measurements to
Determine Dissipation Factor Temperature Dependence
in Power System Components
Presenter:
Authors:
Matz Ohlen, Megger
Matz Ohlen, Megger
Peter Werelius, Megger
Typical power factor values for oil
insulated transformers and bushings
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.”
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Typical power factor temperature
correction
3.50
3.00
2.50
2.00
Typical temp correction, Power
Transformers (IEEE C57.12.90)
Typical temp correction, Bushings
1.50
1.00
0.50
0.00
0
10
20
30
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50
60
70
Session (B1 ) – Paper ( 87 )
80
Temperature correction – in practice
“Experience has shown that the variation in power factor with temperature is
substantial and erratic so that no single correction curve will fit all
cases.” (quoted from IEEE C12.90-2006)
“Built-in temperature correction curves for different insulation materials are used to
recalculate the measured results to reference conditions (20°C, 68°F). The
method of correction is depending on the type of insulation and the
relevant standard” (quoted from instrument manufacturer)
Temperature correction is pending type of insulation
Temperature correction is pending status of insulation
Guessing game…
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
PF vs Temperature – Conclusions from
a project
Power factor values are affected by variation of temperature. In most cases (but not all), tan delta value
increases with increase in temperature. Rate of change is different for different makes of the transformers
and bushings
The temperature correction factors (for correcting measured power factor to 20° C) are different for
different makes. Hence temperature correction factors as given in IEEE/C57.12.90 can not be applied to
these components
“Application of Temperature Correction
Factors for Dissipation Factor Measurements
for Power Transformers – A Case Study”
Power Grid Corporation of India Limited
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Dielectric Frequency Response
Changes in insulating materials affect the power factor and
capacitance
Measurements over a frequency range, compared to
traditional power factor testing, provides a lot more
information on:
Insulation characteristics
Moisture in the cellulose insulation
Temperature dependence
Etc…
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Insulation testing - 100 years of
history
1870’s; First systematic investigations of dielectric properties (Clausius and Mosotti)
1885; The transformer is invented by Ottó Bláthy
1927; First instrument for DC insulation testing is patented and released
1990; ABB presents first results on dielectric response measurements on insulating materials
(NORD-IS 1990)
1993; Development of the first field instrument for Dielectric Frequency Response measurements
is started by Dr. Peter Werelius
1995; First field instrument for DFR delivered
1995-2005; The interest in using DFR/FDS for investigating insulation properties is rapidly
growing and numerous papers on the method and technology are presented at international
conferences
2004; CIGRE report 254, ”Dielectric Response Methods for Diagnostics of Power Transformers” is
published
2006; Project REDIATOOL reported at CIGRE, recommending DFR as a preferred method for
moisture assessment of power transformers
2010; CIGRE report 414 “Dielectric response diagnoses for transformer windings” is published
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Insulation testing/Dielectric response
methods
7
6
5
FDS/DFR
4
HV Tan Delta
VLF
PDC
3
Polarization Index
"DC"
2
1
0
Frequency, Hz
0.000001 0.00001
0.0001
0.001
GCC Power 2010Conference
0.01
0.1
1
10
100
Session (B1 ) – Paper ( 87 )
1000
10000
DFR Application Areas
Power transformers
Instrument transformers
Bushings
Motors and generators
Cables
Generic testing of insulation systems
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Dielectric Frequency Response
- Investigating high single number PF data
Dry transformer with old
oil (high conductivity)
Wet transformer with good oil
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
What affects the response?
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Typical DFR results for transfomers
with various moisture content
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
DFR and temperature dependence
Insulation properties changes with temperature
Described by the Arrhenius equation:
A measurement at e.g. 50 Hz, 20C corresponds to a
measurement at higher frequency at higher temperature
Various material have different activation energy
Dry paper typically around 1.0 eV
Oil-impregnated paper typically 0.9 – 1.0 eV
Mineral transformer oil typically 0.4 – 0.5 eV
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
DFR data acqusition is pending
insulation temperature
Measured insulation
properties are the
same @ 2 mHz, 25 °C
as @ 1 mHz, 18 °C
Corresponding data points
Cellulose, (0.9 eV)
Frequency, mHz
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
DFR measurements – oil impregnated
Kraft paper, moisture content < 0.5%
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Kraft paper – Tan Delta vs
temperature
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Bushings and power factor
The power factor is considered important for two main reasons:
Dielectric losses generate heat, which could result in premature ageing of the
insulation if the bushing is not properly designed, or even worse, could lead to a
thermal breakdown.
Quality check of the production process.
The aim is to have a power factor that:
Shows just a small variation with temperature. Increasing dissipation factor with
temperature indicates a moisture level in the main insulation above 1 %.
Remains stable during the bushing’s entire service life. Increasing dissipation
factor indicates moisture ingress and/or ageing of the insulation.
Source: ABB
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Tan delta (%, 50 Hz) for bushings with
different moisture contents
10.00
4 % moisture
1.00
1% moisture
0,4 - 0,5% moisture
0.1 - 0.3% moisture
0.10
20
30
40
GCC Power 2010Conference
50
60
70
Session (B1 ) – Paper ( 87 )
80
90
Source: ABB
Temperature correction data for various
bushings
1.8
5_O_C
7_GO_25_765
1.6
9_CTF_20_60
1.4
11_CT_KF_85_330
13_B
1.2
15_F
17_L_LC_LI_LM
1
19_OF_OFI_OFM
0.8
21_S_SI_SIM
23_T_U
0.6
25_COT_COS_SOT
0.4
27_ERC
29_PRC
0.2
31_POC
33_P_PA_PB
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
DFR on 115 kV GE Type U
bushings in various conditions
Bad condition
Good
condition
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Temperature converted DFR tan
delta data for bushings (50 Hz)
100
10
GE Type U (bad)
4 % moisture
1% moisture
0,4 - 0,5% moisture
0.1 - 0.3% moisture
1
GE Type U (OK)
0.1
20
30
40
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60
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Session (B1 ) – Paper ( 87 )
80
90
Temperature correction of bushings
using converted DFR data
12.00
10.00
8.00
GE Type U, bad
6.00
GE Type U, OK?
GE Type U, OK
GE Type U, manufacturers data
4.00
2.00
0.00
0
10
20
30
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50
60
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Session (B1 ) – Paper ( 87 )
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Temperature dependence in
transformers
Temperature dependence in transformers is more complex
compared to “single-material” components e.g. bushings and
paper-insulated cables
Activation energy Wa in Arrhenius’ law, κ = κ0·exp(-Wa/kT)
Oil impregnated paper typically 0.9-1 eV
Mineral transformer oil typically around 0.4-0.5 eV
Both materials need to be considered
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Temperature correction –
Transformer
Distribution transformer, 500 kVA, 10.5/0.4 kV
Heated to different temperatures in the range 20°C-50°C
Winding temperature measured as winding resistance change
At each temperature, a DFR measurement was made and;
Moisture content of the cellulose insulation was estimated by use of MODS
Temperature correction curve was calculated, also by use of MODS
50 Hz tan-delta value at 20°C was calculated using the temperature correction curve and
compared with table from IEEE C57.12.90-2006.
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
DFR results for the transfomer at
various temperatures
Temp
GCC Power 2010Conference
Moisture, % Oil conductivity, pS
21
2,4
10,4
27
2,3
13,8
34
2,4
22,8
49
2,3
39,3
Session (B1 ) – Paper ( 87 )
Temperature correction
7.00
6.00
5.00
Tan delta (DFR 21)
4.00
Tan delta (DFR 27)
Tan delta (DFR 34)
3.00
Tan delta (DFR 49)
Measured
2.00
IEEE 57
1.00
0.00
0
20
40
60
80
Temp.
(°C)
Cap.
(pF)
PF (%),
measured
Individual
Corr. factor
PF (%)
@20°C
C57.12.90
Corr. factor
PF (%)
@20°C
21
1978
0.329
1.04
0.31
1.02
0.32
26
1976
0.367
1.20
0.31
1.14
0.32
34
1978
0.516
1.53
0.34
1.37
0.38
49
1974
0.832
2.70
0.31
1.91
0.44
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Temperature correction of transformers
using converted DFR data
12.00
10.00
Pauwels 20 MVA, 2000, new
8.00
Pauwels 80 MVA, 2005, new
6.00
Westinghouse 40 MVA, 1985,
spare
Yorkshire 10 MVA, 1977,
scrapped
4.00
Typical correction table for
transformers
2.00
0.00
0
10
20
30
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Session (B1 ) – Paper ( 87 )
80
90
Temperature correction – OIP
bushing
ABB GOB OIP bushing
Insulation
Temperature, °C
Measured Power Factor
@ 200V
@ 1-10 kV
-8
0,86
1,04 – 1,14
22
0,46
0,46
42
0,34
0,32
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )
Comments
Voltage
dependent at low
temperatures
Temperature correction
2.00
1.80
1.60
1.40
1.20
ITC43 correction
1.00
ITC22 correction
ABB correction
0.80
Measured temp dependence
0.60
0.40
0.20
0.00
-20
-10
0
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Session (B1 ) – Paper ( 87 )
50
Summary and conclusions
DFR/FDS measurement/analysis can:
Investigate increased dissipation factor in power system
components
Estimate the moisture content of oil-immersed cellulose
insulation in power transformers, CTs, bushings etc
Capability of performing individual temperature correction of
measured 50/60 Hz dissipation/power factor at various
temperatures to values at reference temperature, 20°C.
Capability of comparing test results from a new measurement at a
certain temperature to another measurement at a different
temperature
GCC Power 2010Conference
Session (B1 ) – Paper ( 87 )