Basic Insulation & Power Factor Theory
Power Factor Theory --------------------------------------Aradhana Ray Consulting Engineer Doble Engineering, India
Basic Insulation & Power Factor Theory
The underlying principle of the Doble M4000 Test is to measure the fundamental AC electrical characteristics of insulation.
Insulation IEEE Defines Insulation as:
•Changes in the electrical characteristics of insulation can indicate: - An increased or decreased size of the insulation system, - Presence or absence of an insulation component or the movement of the conductors. -Indicate presence of moisture, insulation deterioration, destructive agents or ionization
“Material or a combination of suitable nonconducting material that provides electrical isolation of two parts at different voltages.”
- These
changes can effect the performance of the insulation system.
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Examples of Material With Insulating Properties
Dielectric implies that the medium or material has specific measurable properties, such as: Dielectric Strength, Dielectric Absorption, Dielectric Constant, Dielectric Loss & Power Factor
Insulation
Gaseous
Liquid
Solid
High Vacuum
HydrocarbonBased Oil
Cellulose
Air
Silicone Oil
Porcelain
Sulfur Hexafluoride (SF6)
Distilled Water
Phenolics
Fundamental AC Electrical Characteristics
Insulation is basically two plates separated by one or more dielectrics. One plate is at a high potential and the other at a lower or ground potential. Current generated by polar contaminants in the dielectric insulation dielectric shows up as Watts.
•Total Charging Current •Capacitance •Dielectric-Loss •Power Factor •Power Factor Tip-up
Heat/Watts
2
Typical Insulation System IT IC
IR ~ 0
Good Insulation: Has a very low power factor C P
Total Charging Current
RP
• IR<
→
→
→
Voltage and Currents
IT = IC + IR
1.5
1
0.79
0.75 The capacitance current and the resistive current cannot be just arithmetically added together because the quantities vary in time and are not in phase.
Magnitude
0.5
Voltage
0.25 0
0
90
180
270
360
-0.5
450
540
630
IR IC IT
-1
-1.5 Angle
3
E
Basic Circuit IT IC
Capacitance
IR
CP
Parallel Circuit
The Perfect Capacitor Capacitor Current
The perfect capacitor
1.5
passes no Direct Current Alternating Current leads the voltage by 90 degrees has a Power Factor of 0% by definition
1
Magnitude
0.5
Perfect Capacitor % PF = 0%
Voltage
0
Current
0
90
180
270
360
450
540
630
-0.5
-1
-1.5 Angle
4
The Capacitor Capacitor Current Vector Diagram
IC
Plates zAlternating
capacitance current leads the voltage by 90 degrees
A
d
Dielectric
θ=90ο
Two conducting plates with area A separated by a dielectric with a thickness of d and dielectric constant ε
E
Capacitance
Aε C= 4πd
Dielectric Constant
A
C = Capacitance ε = dielectric constant d = Distance between plates
d
In 1836, Michael Faraday discovered that when the plates between a capacitor were filled with another insulating material, the capacitance would change. This factor is the dielectric constant ε By definition the dielectric constant of a Vacuum is 1.0. All other dielectric constants are referenced to this standard.
Oil ε=2.2 Vacuum
•All of these variables are Physical Parameters
Cvacuum=10 pF
Coil = ε x CVacuum = 22 pF
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Typical Insulation System Various Dielectric Constants Material Dielectric Constant Vacuum 1.0 Air 1.000549 Mica 5.4 Oil Impregnated Paper 3.7 Porcelain 7 Rubber 2.4 - 3.7 Oil 2.24 Silicone Fluid 2.75
High-Voltage Test Cable
I Dielectric Strength (kV/mm) 3 10 16 5.7 12 12 15
Current & Loss Meter Guard Test Mode-GST Ground
Test Ground
Test-Set Ground Lead
Oil ∈ = 2.1 Porcelain ∈ = 7.0
Paper ∈ = 2.0
Measuring the Dielectric Constant of a Material
IVac Vacuum
E∼
E∼
IOil
Example: Oil leaking from an Insulation System
Oil
Oil = 2.1 Porcelain = 7.0 Paper = 2.0 Air = 1.0
Given three dielectrics in series the dielectric constant ε is:
εbefore =
2.1 x 7.0 x 2.0
= 2.65
2.1 + 7.0 + 2.0
If the Oil leaks out and is replaced by air...
An alternating current of the same voltage is applied to the capacitor for both tests.
IOil IVac
= εoil = 2.24 =
COil CVac
εafter =
C
1 x 7.0 x 2.0
= 1.4
1 + 7.0 + 2.0
=>
It
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Capacitive current and Capacitance IC
IC = EωC d
A
ε (Dielectric constant) & d (Distance between plates) are constant.
(ω = 2πf)
Aε 4πd
IC = Eω
Aε 4πd
Area of the plates doubles.
C= d
A
A
d
Physical
2C =
}
}
Considered Constants During Testing 10 kV, 50 Hz
C=
Capacitance Change due to Change in Area of Plate
Aε 4πd (2A)ε 4πd
Distance Between the Plates “d” of the Capacitor
Area of the Capacitor
Id
Area ↑
C↑
Ic ↑
Area ↓
C↓
Ic ↓
d
E∼
C=
Aε 4πd
Id = EωC
Double the distance
I2d
E∼
2d
Aε C = 2 4π(2d) I2d = Eω(C/2) = Id/2
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Distance Between the Plates “d” of the Capacitor
d↑
C↓
Ic ↓
d↓
C↑
Ic ↑
Example of Winding Movement
Insulation System 1995 New 1996 Fault Difference CH pF 2155 2159 0.2% CHL pF 4360 3886 -10.9% CL pF 8291 9339 12.6%
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Changes in Current/Capacitance Summary of Possible Capacitance Changes of a Capacitor
A↑ A↓ ε↑ ε↓
C↑ C↓ C↑ C↓
Ic ↑ Ic ↓ Ic ↑ Ic ↓
d↑ d↓
C↓ C↑
Ic ↓ Ic ↑
Significance Indicate a physical change Bushings - shorting of capacitive layers Transformers - movement of core/coils Arresters - broken elements Suggested Limits + 5% - Investigate + 10% - Investigate/remove from service
Basic Power Factor IT=Total Current IC=Capacitive Current E=Applied Voltage IR=Resistive Current θ =Power Factor Angle IT 0% PF I T I C
IC
Power Factor E
O IR
CP
IR
RP
100% PF E
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Power = Voltage x Current x Cos(θ)
What Is Power Factor (PF)?
P= E IT Cos(θ)
P.F. =
Watts E × IT
I P.F. = IR T
Watts = E x IR Watts = E × I × Cosine θ
IT
IT
IC
T
IR
IC E
CP
P.F. = cosθ
RP
E
Watts E×I T
E×I I = = E×I I
O IR
PF = Cosine θ = R
R
T
T
What Is Power Factor (PF)? Power Factor = W = Real Power IT *E Apparent Power
Limits of % P.F.
To express power factor in percent (% PF), multiply by 100:
% PF =
W X100 mA X10−3*10 X103
= W X10 mA
Capacitor PF = 0%
Resistor PF = 100%
10 kV equivalent values
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Changes In Power Factor Power Factor Relationships θ (°) 90 89.714 89.427 45 30 0
P. F. (%) 0 (Capacitance only) 0.5 1.0 70.7 86.6 100 (Resistance only)
Increase in Losses
Added Inductance IT I
IT IT
IC
C
IT
O
O
O
O
IR
E
IR
E
IL
Power Factor Is Size Independent
Basic Theory
Capacitors, Resistors, and Inductors
IC2
IT2
IC1
Specimen 1: 5 MVA Transformer Specimen 2: 10MVA Transformer
IT1
E ↓ IC ↓
0
IR ↓
IL ↓
90
180
IR1 270
360
IR2
E
Power Factor is an evaluation of the quality of the insulation and is size independent remains the same regardless of the size of the transformer
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Definition of Power Factor
Power factor is a measurement of the efficiency of insulation system.
Why Power Factor Instead of Dielectric Loss, Watts? • Dielectric loss is a function of volume. For a larger insulation system, there is more material to dissipate watts due to inherent losses, deterioration, and contamination.
• To analyze losses there is a need to be able to compare the size of the insulation tested, which is difficult to measure physically.
Power Factor
•
Provides an index to compare the relative losses of different sizes of insulation systems.
•
The power factor is the ratio of the real power to the apparent power or the resistor current to the total current. PReal /PApparent = IR/IT
•
A lower power factor insulation system will have lower relative losses.
•
Minimizing the power factor will provide an insulation system that is in better condition.
Power Factor Relationships
IC
IT
IC
IT
IR I E R
2IC
2IT
2IRE
IR/IT = (IR+IR)/(IT+IT) = 2IR/2IT = IR/IT = PF
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Voltage and Currents 1.5
Time and Angle Relationships
IT
IC
Voltage and Currents
1
1.5
0.5
Voltage IR
Magnitude
1
Magnitude
0.5
0
0
90
180
270
360
0
90
180
270
360
450
540
630
IC IT
-1
IR
-1
E
-1.5 Angle
-1.5
%PF = 0
Angle (360 degrees = 1 / 60th second)
Voltage and Currents
Voltage and Currents
1.5
1.5
1
1
Voltage IR
0
90
180
270
360
450
540
630
-0.5
-1
IC IT
0.5 Magnitude
0.5 Magnitude
540
630
-0.5
0
450
-0.5
0
Voltage IR
0
0
90
180
270
360
450
540
630
-0.5
IC IT
-1
-1.5
-1.5 Angle
Angle
%PF = 0.5
%PF = 1.0
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Voltage and Currents
Voltage and Currents
1.5
Magnitude
0.5
Voltage IR
0
0
90
180
270
360
450
540
630
-0.5
IC IT
Power Factor Angle Magnitude
1
-1
Angle
%PF = 50
-60
-30
0
30
Voltage IR IC IT
Basic Principals of Testing: Always test the smallest piece possible
Insulation
Test contamination
• Power factor testing measure the average condition of an insulation system • Contamination would affect the total insulation system, but not to a large degree
Products of Oxidation or mineral oil Carbon (with moisture)
-0.2 -90 -0.4 -0.6 -0.8
Angle
The Dielectric Loss and Power Factor are sensitive to soluble polar, ionic or colloidal materials: Moisture (free, in cellulose, with particles in oil)
0.6 0.4 0.2 0
-1 -1.2
-1.5
Dielectric Loss and Power Factor:
1 0.8
Always break an insulation system into the Smallest possible part in order to detect insulation faults.
Metal Soaps At Higher voltage: Ionization in solid insulation
Test
The contamination becomes a “bigger piece” of the insulation and is easier to see
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Is the Doble Test Effective for Detecting Defective Insulation? PF = .5
PF = .5
PF = .5
PF = .5
2 pF
2 pF
2 pF
Is the Doble Test Effective for Detecting Defective Insulation? (continued)
1) 2 pF
PF = ∑
PF = .5
PF = .5
2 pF
2 pF
∑
c = PF =2.5
2 pF
.5 x 3 + 2 .5 = 1.0 4
2 pF
2 pF
1 pF 2
PF =
PF = .5
.5+.5+.5 =.5 3
1 pF 2
Analysis of Percent Power Factor
PF = .5
2 pF
1 1 1 1 = + + = 15 . c 2 2 2
2 pF
c=
PF = .5
PF = .5
.5 + .5 + .5 + .5 = .5 4
1 1 1 1 1 = + + + = 2 c 2 2 2 2
PF =
2)
c =.667
Power Factor vs. Dissipation Factor IC
IT
Power Factor = COS Θ =
Compare to limits published by Doble Compare to previous results Compare among similar or sister units Examine the trend Do not use PF if current is less than 300µA
δ
IR
R
T
Dissipation Factor = TAN δ =
Θ
Θ° 90 89.71 84.26 0
I I
I I
R
C
E
% PF (% COS Θ) 0 .500 10.00 100.00
δ° 0 .29 5.74 90
% DF (% TAN ∆) 0 .500 10.05 INFINITY
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Comparison of Percent Power Factor With Percent Dissipation Factor for Various Phase Angles of Θ and δ
Power Factor Vs. Dissipation Factor IC
IT Power Factor = cos θ =
δ
R
T
Dissipation Factor = tan δ =
θ IR
I I
I I
R
C
E
θ°
% PF (%COS θ )
δ°
90
0
0
0
89.71
.500
.29
.500
84.26
10.00
5.74
10.05
0
100.00
90
INFINITY
% DF (% TAN ∆)
%
θ 90 89.71 87.13 84.26 81.37 53.13 45.00 0
% PF (% cos) 0 .50 5.00 10.00 15.00 60.00 70.71 100
%
δ 0 .29 2.87 5.74 8.63 36.87 45.00 90
% DF (% tan) 0 .50 5.00 10.05 15.18 75.00 100.00 infinity
Voids and the Power Factor Tip-Up Test When we closely examine insulation, very small gaps or “voids” exist. These voids develop an electrostatic potential on their surfaces. These small gaps become ionized: Partial Discharge/Corona.
Power Factor Tip-Up
Voids
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Partial Discharge (Corona) •The phenomenon of an electrical discharge that does not completely bridge the insulation between electrodes or conductors. Corona -- accompanied by a faint glow Partial Discharge -- may not be luminous (preferred term)
Power Factor Vs. Test Voltage As test voltage is increased, the power factor will increase depending on the void density. Tip-Up = Power Factor at Line-to-ground voltage Power Factor at 25% Line-to-ground voltage %PF
•Partial discharge occurs in: %PF @ L-G
A void within an insulation system where the voltage gradient is sufficiently high -- has a damaging effect on surrounding materials.
%PF @ 25% L-G
E 25% L-G
L-G
Tip-up occurs in dry-type insulation specimens such as Dry Type Transformer, generators, etc.….
Thank You. Any questions?
[email protected]
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