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Power Factor Testing - Transformers Gary Heuston Doble Engineering Company
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Transformers
Agenda • Introduction • The power factor test • Test policies • Testing the apparatus and case studies - transformers - bushings - surge arresters
Introduction • Primary interest - testing electrical insulation • Developed capabilities - expertise in testing electrical insulation - accumulation of test data - library of information
The Power Factor Test • Terminology - power factor versus dissipation factor • The “Doble” power factor test • Significance of measured parameters - dielectric loss - capacitance - power factor - power factor tip-up
Power Factor versus Dissipation Factor
Power factor (PF) = Cos
θ
Dissipation factor (DF) = Tan
= IR / IT δ
= IR / IC
Power Factor versus Dissipation Factor Comparison of the power factor and dissipation factor for a few phase angles and complementary loss angles are given below.
0.000000
90.000000
Complement Loss Angle, 0.000000
0.005000
89.713520
0.286480
0.005000
< ± 1 ppm
0.010000
89.427033
0.572967
0.010000
< ± 1 ppm
0.020000
88.854008
1.145992
0.020004
± 4 ppm
0.050000
87.134016
2.865984
0.050063
± 63 ppm
0.100000
84.260830
5.739170
0.100504
± 504 ppm
1.000000
0.0000000
90.00000
infinity
incomparable
PF, cos
Phase Angle,
DF, tan
Difference
0.000000
none
The two measurements are very nearly the same for a specimen with a power factor or dissipation factor of 0.1000 or less.
Power Factor versus Dissipation Factor Comparison of the power factor and dissipation factor for a few phase angles and complementary loss angles are given below.
0.000000
90.000000
Complement Loss Angle, 0.000000
0.005000
89.713520
0.286480
0.005000
< ± 1 ppm
0.010000
89.427033
0.572967
0.010000
< ± 1 ppm
0.020000
88.854008
1.145992
0.020004
± 4 ppm
0.050000
87.134016
2.865984
0.050063
± 63 ppm
0.100000
84.260830
5.739170
0.100504
± 504 ppm
1.000000
0.0000000
90.00000
infinity
incomparable
PF, cos
Phase Angle,
DF, tan
Difference
0.000000
none
The two measurements are very nearly the same for a specimen with a power factor or dissipation factor of 0.1000 or less.
The “Doble” Power Factor Test An insulation can be defined by its chemical, electrical, mechanical, physical and thermal characteristics. Contamination and deterioration can have various affects on any or all of these characteristics.
The “Doble” Power Factor Test is a measurement of some of the fundamental AC electrical characteristics of a insulation, to include: • Dielectric loss • Capacitance • Power factor • Power factor tip-up (power factor-voltage characteristic)
The “Doble” Power Factor Test The “Doble” M-instruments can also measure these additional transformer parameters: • Excitation current • High-voltage transformer turns ratio • Leakage reactance (requires M4000 instrument & M4110 interface) • Sweep frequency response analysis (requires M5100 instrument)
Significance of Measured Parameters
Parallel equivalent circuit and its vector diagram The capacitor represents the fundamental capacitance of an insulation and its ability to store electricity when a potential difference exists across it. The resistor represents the dissipated loss and dielectric heating that occurs in the insulation because of the presence of impurities when voltage is applied across the insulation.
Significance of Measured Capacitance Capacitance =
κ ε0 A
/d
κ
= relative dielectric constant (intrinsic property of material)
ε0
= permittivity of free space (8.85 x 10 picofarad/mm)
A
= effective area of parallel plates (mm )
d
-3
2
= separation distance (mm)
Capacitance is primarily a function of specimen’s physical geometry.
Significance of Measured Capacitance Capacitance detects movement and deformation of transformer windings.
Interwinding (CHT) capacitance of an autotransformer. Test Date 1965 1968 1974 1982
20°C % PF 0.20 0.29 0.29 0.32
Cap (pF) 2,650 2,756 3,710 5,100
Significance of Measured Capacitance Capacitance detects movement and deformation of transformer windings.
Significance of Measured Power Factor • P = EIT Cos θ and PF = Cos
θ
PF = Cos θ = P / (EIT)
• Power factor is the ratio of the dielectric loss to the applied voltamperes. • Power factor is very low for most modern insulation. • Power factor is sensitive to soluble polar, ionic and colloidal materials, to include: - moisture - products of oxidation of the mineral oil - conducting materials, e.g. carbon • Power factor is a function of the inherent characteristic of the insulation and the presence of any contamination and deterioration.
Significance of Measured Power Factor Power factor detects moisture in transformer. April 10, 1979 test Insulation 20°C % PF CH 0.49 CL 0.45 CHL 0.39
Cap (pF) 9,805 31,800 23,585
May 7, 1981 test Insulation 20°C % PF CH 1.0 CL 0.65 CHL 1.16
Cap (pF) 9,938 32,065 23,983
Westinghouse 138 / 21 kV, 265 MVA transformer
Significance of Measured Power Factor Power factor detects moisture in transformer. February 21, 1985 test Insulation 20°C % PF CH 2.5 CL 1.1 CHL 3.3
Cap (pF) 10,468 32,330 25,573
April 25, 1986 test, after drying Insulation 20°C % PF CH 0.74 CL 0.64 CHL 0.56
Cap (pF) 9,805 32,330 23,850
Westinghouse 138 / 21 kV, 265 MVA transformer
Significance of Measured Power Factor Power factor detects moisture in transformer. 4
3
F P %2 C ° 0 2 1
0 1979
1980
1981
1982
1983
1984
Test Date CH
Westinghouse 138 / 21 kV, 265 MVA transformer
CL
CHL
1985
1986
Test Policies It was necessary to address, in addition to test procedures, analysis of results, and investigation of questionable results, various aspects of this testing, to include: • Averaging effect • Temperature correction • Very low capacitance specimens • High levels of electrostatic interference • Power factor-voltage characteristic • Peculiarities - winding inductance - high external surface losses - negative power factor
Averaging Effect Two-winding transformer test methods Doble Method 1 – without guard circuit Test Insulation 20°C % PF 1 CH + CHL 0.48 2 CL + CHL 0.25 3 CH + CL 0.42
Cap (pF) 8,215 15,105 12,190
Doble Method 2 – with guard circuit Test Insulation 20°C % PF 1 CH 1.0 2 CL 0.25 3 CHL 0.24
Cap (pF) 2,650 9,540 5,565
Method 1 gives power factors that are a weighted average of the two insulation components. Method 2, the current procedure gives a power factor for each insulation component and increases the sensitivity of the test to localized conditions.
Averaging Effect Perform separate test on bushings.
Averaging Effect Importance of separate bushing test. Acceptance Test Insulation I (mA) CH 12.4
Watts 0.25
20°C % PF 0.20
H1 - C1 H2 - C1 H3 - C1
1.52 1.48 1.50
0.036 0.031 0.036
0.24 0.21 0.24
Later Test Insulation CH
I (mA) 12.5
Watts 0.43
20°C % PF 0.34
H1 - C1
1.6
0.22
1.4
Averaging Effect Importance of separate bushing test. Acceptance Test Insulation I (mA) CH 12.4
Watts 0.25
20°C % PF 0.20
H1 - C1 H2 - C1 H3 - C1
1.52 1.48 1.50
0.036 0.031 0.036
0.24 0.21 0.24
Later Test Insulation CH
I (mA) 12.5
Watts 0.43
20°C % PF 0.34
H1 - C1
1.6
0.22
1.4
Temperature Correction Power factor-temperature characteristics of power transformer insulation
10
F P 1 %
Doble AIEE Oil
0.1 0
10
20
30
40
Temperature, °C
50
60
Temperature Correction Power factor characteristic of oil-impregnated cellulose (by percent moisture content) 100 4.0% 2.7%
z H 10 0 6 , F P % 1
2.0% 0.90% 0.40% 0.20% 0.15% 0.10%
0.1 25
35
45
55
65
Temperature, °C
75
85
Testing the Apparatus and Case Studies • Transformer test • Transformer power factor test • Analysis of results • Case studies - case 1 - ungrounded core - case 2 - moisture - case 3 - defective pumps - case 4 - carbonized path
Transformer Test Normally includes these measurements: • Power factor - transformer insulation - bushing insulation - surge arresters (dielectric loss) - insulating fluid • Excitation current • High-voltage transformer turns ratio • Leakage reactance • Sweep frequency response analysis (SFRA)
Transformer Test It may also include these measurements: • Insulation resistance - sensitive to contamination and deterioration - sensitive to very low capacitive parallel paths with low resistance • Winding resistance - sensitive to continuity of conductors and quality of connections • Insulating fluid - physical, chemical and electrical properties - dissolved gases and metals
Transformer Test • Infra-red - sensitive to hot-spots
Transformer Power Factor Test Transformers may be either single-phase or three-phase with liquid-immersed or dry windings. The power factor test procedure is determined by the number of separate and accessible windings: • Two-winding • Three-winding • Autotransformer with a tertiary winding • Autotransformer without a tertiary winding (single-winding) • Four-winding • Five-winding
Transformer Power Factor Test Two-winding transformer dielectric circuit
CH - Insulation between high-voltage conductors and grounded core and tank (bushings, winding insulation, structural insulating members and oil). CL - Insulation between low-voltage conductors and grounded core and tank (bushings, winding insulation, structural insulating members and oil). CHL - Insulation between high- and low-voltage conductors (winding insulation, barriers and oil).
Transformer Power Factor Test Doble test procedure for two-winding transformer Test 1 2 3
Energize LV Lead Test Mode Measure High Low GST - Ground CH + CHL High Low GST - Guard CH High Low UST CHL Calculated Results for CHL (Test 1 minus Test 2)
4 5 6
Low High GST - Ground CL + CLH Low High GST - Guard CL Low High UST CLH Calculated Results for CLH (Test 4 minus Test 5)
Transformer Power Factor Test DTA two-winding transformer electronic form: Overall test screen
Transformer Power Factor Test Three-winding transformer dielectric circuit
CH, CL, CT - Insulation between conductors and grounded core and tank (bushings, winding insulation, structural insulating members and oil). CHL, CLT, CHT - Insulation between conductors (winding insulation, barriers and oil).
Transformer Power Factor Test Doble test procedure for three-winding transformer Test
Energize
LV Lead Test Circuit Red Blue L T GST Guard Blue Ground Red L T GST Guard Red and Blue L T UST Red Ground Blue Calculated Results for C HL (Test 1 minus Test 2)
1 2 3
H H H
4 5 6
L L L
T H GST Guard Blue Ground Red T H GST Guard Red and Blue T H UST Red Ground Blue Calculated Results for C HL (Test 4 minus Test 5)
C L + CLT CL CLT
7 8 9
T T T
H L GST Guard Blue Ground Red H L GST Guard Red and Blue H L UST Red Ground Blue Calculated Results for C HL (Test 7 minus Test 8)
C T + CTH CT CTH
10
H, L, T
GST Ground Red and Blue
Measure C H + CHL CH CHL
CH + CL + CT
Transformer Power Factor Test DTA three-winding transformer electronic form: Overall test screen
Analysis of Results The measured power factor and capacitance should be compared: • with factory data (acceptance test) • with earlier tests (especially the first acceptable test) • with similar transformers • with Doble’s database and typical results
Analysis of Results CH power factors for power transformers Number of transformers - 660 total 240 180 120 80
60
20
0
0.1
0.2
0.3
0.4
0.5
0.6
20°C % PF
25
20
15
0.7
0.8
0.9
1
Analysis of Results General guidelines for modern oil-filled power transformers 20°C Power Factor 0.5 % and less Higher than 0.5 % to 0.7 % Higher than 0.7 % to 1.0 % Higher than 0.5 % to 1.0 % and increasing
Good Deteriorated Investigate Investigate
Capacitance (compared to acceptance test) Less than 10 % 10 % and higher
Good Investigate
Case 1 - Ungrounded Core Core connected to ground Test Insulation 1 CH + CHL 2 CH 3 CHL 4 CL + CLH 5 CL 6 CLH
20°C % PF 1.5 0.3 0.5 0.3
Cap (pF) 9,977 3,439 6,538 15,641 9,095 6,534
Core not connected to ground Test Insulation 20°C % PF 1 CH + CHL 2 CH 1.6 3 CHL 0.3 4 CL + CLH 5 CL 0.7 6 CLH 0.3
Cap (pF) 9,975 3,335 6,640 12,040 5,394 6,636
ABB, 13.8 / 0.480 kV, 5 MVA, oil-filled, two-winding transformer
Case 2 - Moisture Test 1 2 3 4 5 6
Insulation CH + CHL CH CHL CL + CLH CL CLH
kV 10 10 10 2 2 2
I (mA) 28.5 7.05 21.5 39.5 17.65 21.5
Watts 1.45 0.38 0.95 5.5 2.85 3.55
20°C %PF 0.39 0.32 1.18 1.2
High CL power factor and disagreement between C HL and CLH power factors.
FPE, 13.8 / 4.3 kV, 7 MVA, oil-filled, two-winding transformer
Case 3 - Defective Pumps Test Date 1972 1975
CH 0.50 1.60
20°C % PF CL 0.59 0.57
CHL 0.47 1.44
A similar unit was tested and did not display a similar increase.
Westinghouse, 3-Phase, 345 / 19 kV, 343 MVA Shell-Form Transformer
Case 3 - Defective Pumps Results of internal inspection: • A gray film was deposited over the lower section of the high-voltage bushings, DETC contacts and neutral bus. • A combination of this film and small, rough-edged shavings was deposited on the paper insulation and insulating boards, with the heaviest deposits in C-phase area. • This film and shavings was partially magnetic. Oil pumps (total of 9) were believed to be the source of contamination. • Oil pump No. 8 (located in the area of the C-phase high-voltage bushing) was severely deteriorated. The outer bearing sleeve, the pump housing, and 1/2 the width of the impeller blade were destroyed. • One other pump was found to have appreciable bearing wear and the remaining 7 pumps had varying, lesser degrees of bearing-wear. • After clean-up and repair, the unit was returned to service. It failed 4 months later. It is believed the failure resulted from the magnetic contamination introduced earlier. Westinghouse, 3-Phase, 345 / 19 kV, 343 MVA Shell-Form Transformer
Case 4 - Carbonized Path Test Date May 12 1969 Apr 11 1977 Nov 15 1979 Aug 30 1984
CH 0.26 0.12 0.42 0.23
20°C % PF at 10 kV CL 0.34 0.28 0.69 0.28
Test kV 2 4 6 8 10
CHL Insulation 20°C % PF 2.64 2.96 3.32 3.60
Nov 20, 1984
CHL 0.26 0.23 0.46 1.64
% PF Tip-Up
Allis-Chalmers 115 / 13.2 kV, 12/16/20 MVA, oil-filled, two-winding transformer (1969 - vintage)
0.96
Case 4 - Carbonized Path Test Date May 12 1969 Apr 11 1977 Nov 15 1979 Aug 30 1984
CH 0.26 0.12 0.42 0.23
20°C % PF at 10 kV CL 0.34 0.28 0.69 0.28
Test kV 2 4 6 8 10
CHL Insulation 20°C % PF 2.64 2.96 3.32 3.60
Nov 20, 1984
CHL 0.26 0.23 0.46 1.64
% PF Tip-Up
Allis-Chalmers 115 / 13.2 kV, 12/16/20 MVA, oil-filled, two-winding transformer (1969 - vintage)
0.96
Case 4 - Carbonized Path Transformer history: • The May 12, 1969 test is the acceptance test. • The X1 bushing (Lapp Type POC-A) failed violently. A replacement bushing was installed. The transformer was tested on April 11 and returned to service on April 16, 1977. • On November 14, 1979 the X3 bushing failed violently. A replacement bushing was installed. The transformer was tested and returned to service on November 15, 1979. The CL power factor increased. • The transformer was returned to service due to high load conditions in the area. Frequent sampling of the nitrogen blanket and oil for combustible gases was performed. No detectable problems were revealed by these tests. • The transformer was tested on August 30, 1984 during a tap-changer inspection. • On November 20, 1984 the transformer was removed from service for testing. Power factor tip-up tests were performed and the C HL power factor had increased to 3.6 % at 10 kV. Allis-Chalmers 115 / 13.2 kV, 12/16/20 MVA, oil-filled, two-winding transformer (1969 - vintage)
Case 4 - Carbonized Path Results of inspection during disassembly: • After removal from tank, the high-voltage coil assembly looked good except for misalignment of the coil spacers in the tap winding. • After the high-voltage coils were removed, large deposits of carbon were found adhering to the vertical spacers and low-voltage coils on all phases. • This transformer was rewound and returned to service in 1985.
Allis-Chalmers 115 / 13.2 kV, 12/16/20 MVA, oil-filled, two-winding transformer (1969 - vintage)
The Bushings and Surge Arresters
Testing the Apparatus and Case Studies • Bushing test • Bushing power factor test • Analysis of results • Case studies - case 1 - simulated failure modes - case 2 - resistive path to ground - case 3 - ungrounded conductive layer
Bushing Test Normally includes these measurements: • Power factor - C1 insulation - C2 insulation - Hot-Collar
Bushing Test It may also include these measurements: • Insulation resistance - sensitive to contamination and deterioration • Infra-red - sensitive to hot-spots • Insulating fluid - physical, chemical and electrical properties - dissolved gases
Bushing Power Factor Test The power factor test procedure is determined by whether it is equipped with a tap: • no tap - Hot-Collar • equipped with tap - C1 insulation - C2 insulation - Hot-Collar
Bushing Power Factor Test Center conductor
C1 insulation Tap electrode Tap receptacle Flange Ground Sleeve
C2 insulation
Analysis of Results The measured power factor and capacitance should be compared: • with nameplate data (acceptance test) • with earlier tests (especially the first acceptable test) • with similar bushings • with Doble’s database and typical results
Analysis of Results General guidelines for modern oil-impregnated paper-insulated capacitive-graded bushings (with original power factor of 0.5 % or less) 20°C Power Factor (acceptance test compared to nameplate) < (NP - 0.10) (NP - 0.10) and (NP + 0.20) > (NP + 0.20) and (NP + 0.50) > (NP + 0.50) and 3 % PF > 3 % PF
Investigate Good Deteriorated Investigate Bad
Capacitance (acceptance test compared to nameplate) (NP x 0.90) > (NP x 0.90) and (NP x 0.95) > (NP x 0.95) and < (NP x 1.05) (NP x 1.05) and < (NP x 1.10) (NP x 1.10)
Investigate Deteriorated Good Deteriorated Investigate
Analysis of Results General guidelines for modern oil-impregnated paper-insulated capacitive-graded bushings (with original power factor of 0.5 % or less) 20 C Power Factor (acceptance test compared to nameplate) < (NP - 0.10) Investigate Good (NP - 0.10) and (NP + 0.20) Deteriorated > (NP + 0.20) and (NP + 0.50) Investigate > (NP + 0.50) and 3 % PF > 3 % PF Bad Capacitance (acceptance test compared to nameplate) Investigate (NP x 0.90) Deteriorated > (NP x 0.90) and (NP x 0.95) > (NP x 0.95) and < (NP x 1.05) Good Deteriorated (NP x 1.05) and < (NP x 1.10) Investigate (NP x 1.10)
Analysis of Results Example, a bushing with a nameplate power factor of 0.24 % and a nameplate capacitance of 412 pF. 20 C Power Factor (acceptance test compared to nameplate) < 0.14 % Investigate Good 0.14 % and 0.44 % Deteriorated > 0.44 % and 0.74 % Investigate > 0.74 % and 3 % >3% Bad Capacitance (acceptance test compared to nameplate) Investigate 371 pF Deteriorated > 371 pF and 391 pF > 391 pF and < 433 pF Good Deteriorated 433 pF and < 453 pF Investigate 453 pF
Case 1 - Simulated Failure Modes Comparison of typical and questionable 10-kV test results for the C1 insulation of similar bushings Description Typical good bushing
20°C % PF 0.36
Cap (pF) 292
Same bushing deteriorated
0.94
297
Same bushing with shorted layers of insulation
0.40
345
Case 2 - Resistive Path to Ground
Test kV 10 ABB Type O+C 230 kV bushing
I (mA) 1.313
Watts - 0.007
20°C % PF - 0.053
Case 3 - Ungrounded Conductive Layer Test
Insulation
1
C1
2
C2
Nameplate % PF Cap (pF) 0.40 347 4,329
Measured 20°C % PF Cap (pF) 0.35 337.70 0.59
939.60
This ungrounded outer conductive layer creates an operating hazard. This increases the tap voltage, with the cover removed, from 9,609 volts to 35,108 volts. This surely renders this bushing unsuitable for use with a bushing potential device. It also places greater importance on the quality of the connections of the cover to the receptacle and to the flange. Ohio Brass Type GK-45 230 kV bushing
Surge Arresters
Surge Arresters
Operating characteristics of two type of surge arresters • Above design rating - it dissipates the impulse
• Above design rating - it dissipates the impulse
• Drops voltage to zero
• Clamps the voltage
• Generates its own impulse
• Doesn’t drops voltage to zero • Doesn’t generate its own impulse
Testing the Apparatus and Case Studies • Surge arrester test • Surge arrester dielectric loss test • Analysis of results • Case studies - case 1 - simulated failure modes
Surge Arrester Test Normally includes dielectric loss measurement.
It may also include these measurements: • Insulation resistance - sensitive to contamination and deterioration • Infra-red - sensitive to hot-spots
Surge Arrester Dielectric Loss Test The dielectric loss test procedure is determined by the number of units comprising the stack: • single unit • multiple units
Analysis of Results The measured dielectric loss should be compared: • with factory data (acceptance test) • with earlier tests (especially the first acceptable test) • with similar surge arresters • with Doble’s database and typical results
Case 1 - Simulated Failure Modes Comparison of typical and questionable 10-kV test results for similar arresters Description Typical good arrester
I ( A) 230
Watts 0.050
Same arrester contaminated with arcing byproducts
370
17.5
Same arrester contaminated with moisture
260
0.200
Same arrester, new, contaminated with factory dirt and metallic particles
250
0.240
Same arrester, with broken internal parts and open circuited
120
0.006
Transformers
Transformers
