analysis of electrical insulation systems: capacitance and ...engineering.richmondcc.edu/courses/eus...
TRANSCRIPT
Analysis of Electrical Insulation
Systems: Capacitance and Power Factor
Robert Brusetti, Doble Engineering
Doble Client Conference 2012
What is the Principle of the Power Factor Test?...
• The Underlying Principle of the Power
Factor Test, is to Measure the
Fundamental AC Electrical Characteristics
of Insulation.
• Overall assessment of the condition of the
insulation
• AC high voltage low current measurement
– Non-destructive.
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Electrical Characteristic
• Changes can indicate an increased or
decreased size of the insulation system, the
presence or absence of an insulation component
or the movement of the conductors. These
changes can effect the performance of the
insulation system.
• Can also indicate moisture, insulation
deterioration, destructive agents or ionization
that can effect the dielectric strength and
serviceability of the insulation.
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Terminology
• Insulator
– The electrons are held firmly by the nucleus, and a relatively high potential difference produces only a very small movement of electrons from atom to atom.
• Conductor
– The electrons are loosely held by the nucleus and are able to move readily from atom to atom.
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Terminology
• Insulation vs. Dielectric
– Insulation
• Def: Material or a combination of suitable non-conducting
material that provides electrical isolation of two parts at
different voltages.
• Medium’s ability to prevent the flow of current, I.e. poor
conductor
– Dielectric
• Def: Medium in which it is possible to produce and maintain
an electric field with little or no supply of energy
• Specific measurable properties such as: Dielectric Strength,
Dielectric Constant, Dielectric Loss and Power Factor.
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•Dielectric Strength (Normally determined
empirically, i.e., we measure
it in the lab)
•Dielectric Constant (Intrinsic characteristic or
property of a material)
•Dielectric Loss
•Power Factor
Dielectric Properties:
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Resistor
Real Power Watts
Reactive Power
VARs
Reactive Power
VARs
Capacitor Inductor
• V dependent
• Leading • Energy storing
• I dependent
• Lagging
• Energy storing
• Dissipates energy/heat
• V and I in-phase
P Q Q
Three Basic Elements of
Substation/Insulation
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Modeling
• To model an electrical insulation system
using only on electrical component (e.g.
resistor, capacitor, inductor) which would
best represent the system
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Perfect Insulator
The Capacitor
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Properties of a Capacitor
• Capacitors allow positive and negative
charges to be stored on the electrodes
• The movement of electrons from the
negative plate to the positive is greatly
restricted by the dielectric medium.
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Conditions Required for Charges to
Form on the Plates of Capacitors
• There must be a potential difference across the plates
• The plates must be in relative proximity to each other
• The amount of charge that develops on the plates of a capacitor is proportional to the voltage across the plates. The relationship between voltage & charge is expressed by the equation: Q = E C
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Perfect Vacuum +
-
d + + + + + +
- - - -
- - - -
- - - -
Perfect Vacuum
Distance
To change
capacitance,
change voltage
or change d.
+
-
Both conditions
for charge
formation have
not been met
NO CHARGE
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, d Q C Capacitance is inversely
proportional to distance
Distance and Area
• At an infinite distance apart, there is no charge formed on the plates.
• As the plates are brought together, more & more charges are formed on the plates. Therefore charge, Q, is inversely proportional to distance between the plates, d.
• It is reasonable to assume that the larger the area, A, of the plates, the more charge accumulation, Q, we have.
• Recall that: Q = E C (charge is proportional to capacitance)
A Q C
Capacitance is directly
proportional to Area
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Dielectric Material
• What happens when different insulating materials are inserted between the plates?
• Some insulating mat’l enhances the capacitance & therefore, charge formation.
• 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 e
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Dielectric Constant
• Def: Ratio of the capacitance of a give configure with a medium as the dielectric to that of vacuum as the dielectric
• The dielectric constant of a Vacuum is 1.0. All other dielectric constants are referenced to this standard.
Vacuum
Cvacuum=10 pF
Oil e=2.2
Coil = e x Vacuum = 22 pF
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Capacitance
C = Capacitance
e = dielectric constant
d = Distance between plates
All of these variables are Physical
Parameters
A d
C= Ae
d
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Current is directly related to capacitance:
Ic = E 2pf C
where E = test voltage in 10kV equivalents
So changes in either reading indicate physical changes in the insulation system.
Capacitance is affected by
physical changes:
Cp = Ae/d where A = area of capacitor plates
e = dielectric constant (ratio
of material
to air’s ability to conduct
electricity)
d = distance between
capacitor plates
Porcelain: e = 7.0
Test Mode-GST
Ground
Guard
Current & Loss
Meter
Test
Ground
Test-Set Ground
Lead
High-Voltage Test Cable
Ic Oil: e= 2.1
Paper: e = 2.0
Center Conductor
Flange
Ideal Loss-less
Specimen
Cp
Why Analyze Current & Capacitance
Readings
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Relationship between Current and
Capacitance
*10KV equivalent measurement or actual 10KV test potential
**Current in mA, Voltage in KV and Capacitance in pF
**
**
*
50318
60265
10
2
HzforIC
HzforIC
KVE
fw
Ew
IC
EwCI
C
C
C
C
=
=
=
=
=
=
p
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Measuring the Dielectric Constant of a
Material
An alternating current of the same voltage is applied to
the capacitor for both tests.
E
IOil Oil
IOil COil
CVac
= = eoil = 2.24 IVac
Vacuum
IVac
E
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Id
E d
I2d
E 2d
C = Ae 4pd
I2d = EwC/2) Id = EwC
Ae
4p2d) =
C
2
Double the
distance
= Id/2
Distance Between the Plates “d”
of the Capacitor
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Test Mode-GST
Ground
Test
Ground
Test-Set Ground
Lead
High-Voltage Test Cable
Oil = 2.1
Porcelain = 7.0
Paper = 2.0
Guard
Current & Loss
Meter
I
Typical Insulation System
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Oil leaking from an Insulation System
Oil = 2.1 Porcelain = 7.0 Paper = 2.0
Given three dielectrics in series the
dielectric constant e is:
ebefore = 2.1 x 7.0 x 2.0
2.1 + 7.0 + 2.0 = 2.65
If the Oil leaks out and is replaced by air...
Air = 1.0
eafter = 1 x 7.0 x 2.0
1 + 7.0 + 2.0 = 1.4
C => It
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Current and Capacitance
Characters of the test specimen (insulation system)
Total Current IT
Capacitance C
Dielectric-Loss W
Dissipation Factor% DF or DF
Power Factor %PF or PF
} Evaluate physical
makeup of specimen,
size dependent
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Ideal Insulation System
Evaluating Insulation System
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Current Lead Voltage
Purely Capacitive Circuit
-1.5
-1
-0.5
0
0.5
1
1.5
0.1 0.8 1.5 2.2 2.9 3.6 4.3 55.7 6.4 7.1 7.8 8.5 9.2 9.9
10.611.3 12
Time
Mag
nat
ud
e
Voltage
Current
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E
q=90o
IC
Out of Phase
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Review
• The Insulation model consist of a,
capacitor or an energy-storing device.
• While energy is stored in a dielectric,
energy is also dissipated in a dielectric.
– The model is somewhat deficient.
• Improve the model with the addition of
another electrical element. Which one?
• Resistor
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Real Insulation Model
Simplified Equivalent Circuits of an Insulation Specimen
Series Circuit
RS
CS
CP
Parallel Circuit
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Parallel Circuit
You can see why the
total charging current, IT,
capacitive current, IC.
However, since IR is
typically very small in
an insulation system, it
is considered
negligible. IT IC
CP RP
E
IT
IR IC
IT = IC + IR
Equivalent Circuit of an Insulation
System
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Resistor
The Resistor
The Resistor represents the
energy-dissipating tendency of
the insulating material (i.e., the
Dielectric Loss portion of the
insulating material).
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In Phase
The Perfect Resistor
-1.5
-1
-0.5
0
0.5
1
1.5
0.1
0.8
1.5
2.2
2.9
3.6
4.3 5
5.7
6.4
7.1
7.8
8.5
9.2
9.9
10.6
11.3 12
Time
Mag
nit
ud
e
Voltage
Current
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Resistive Component
The Resistor
IR
IR = IT
IR = E/R
W=EIR
q=0o
E
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Known that: S = P + jQ
Apparent Power
VARS
Average Power (Watts)
Dielectric Loss, Watts
• Dielectric loss has dimension of watts.
• The energy expended on dielectric losses is in the form of heat. In fact, dielectric loss is: how fast electrical energy is transformed into heat when the dielectric is subjected to an electric field
• The current that is in phase with the applied voltage produces the dielectric loss of the specimen.
• E IR(in-phase current) = Power [watts]
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IT IC
IR E
O
Test Mode-GST
Ground
Guard
Current & Loss
Meter
Test
Ground
Test-Set Ground
Lead
High-Voltage Test Cable
It
Ir
R Ic
Real World
Specimen With
Slight Loss
Component R
Measured current, with capacitive
and resistive components (with
greatly exaggerated resistive
component for visual clarity)
The resistive component is very small in most
insulation systems. An increase in watts-loss indicates
contamination in the insulation system
Why We Analyze Watts-Loss
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Dielectric loss is the power created by any combination
of the following sources:
• Inherent resistance of the dielectric material.
• Deteriorated dielectric material.
• Polar molecules (including water).
• Ionization of gas. Corona.
Contamination
Normal
Deterioration
Deterioration
Dielectric Loss, Watts
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Dielectric Loss, Watts
• In a no-loss insulation system, the
current is purely due to the capacitance.
• As the insulation deteriorates, the watts will increase.
• In general a low loss or a lower loss insulation system is a better insulation system.
• A large insulation system will have higher losses than a smaller insulation system in the same condition
IR = 0
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Real Component
Characters of the test specimen (insulation system)
Total Current IT
Capacitance C
Dielectric-Loss W
Dissipation Factor %DF or DF
Power Factor %PF or PF
} Evaluate quality of the
dielectric material, size
dependent
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Basic Insulation Circuit
• Basic Power/Dissipation Factor Circuit
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Voltage and Currents
-1.5
-1
-0.5
0
0.5
1
1.5
0 90 180 270 360 450 540 630
Angle (360 degrees = 1 / 60th second)
Mag
nit
ud
e
IC IT
IR E
Time and Angle Relationships
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Power Factor
• What is a Power Factor/Dissipation
Factor/Tangent Delta Test?
– IEEE Definition of Power Factor is the cosine of the
phase angle between a sinusoidal voltage applied
across a dielectric (or combination of dielectrics) and
the resulting current through the dielectric system.
– Overall assessment of the condition of the insulation
– The underlying principle of this test is to measure the
fundamental AC electrical characteristics of
insulation.
– AC high voltage low current measurement
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Definition
• The Term Power/Dissipation Factor Describes
– The phase angle relationship between the applied
voltage across and the total current through a
specimen.
– The ratio of the real power to the apparent power.
– The relationship between the total and resistive
current
– The efficiency of a power system in terms of real &
reactive power flows
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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.
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Power Factor Theory
• How to Calculate Power Factor
=
=
E I
E I
I
IR
T
R
T
Real Power
Apparent Power
Watts = E x IR
PF = Cosine =Watts
qE I
T
= Real Current
Total Current
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IT IC
IR E
O
E CP RP
IT
IC IR
P.F. = Watts
E IT P.F. =
IR
IT
P.F. = cosq
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Capacitor
PF = 0%
Resistor
PF = 100%
Limits of % P.F.
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IC IT
IR E
IC IT
IR E
2IC 2IT
2IR IR/IT = (IR+IR)/(IT+IT) = 2IR/2IT = IR/IT = PF
Power Factor Relationships
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IC2 Specimen 1: 5 MVA Transformer
Specimen 2: 10MVA Transformer
remains the same regardless of the size of the transformer
Power Factor is an evaluation of the quality of the insulation
and is size independent
IT2
IR2 E IR1
IC1 IT1
Power Factor Is Size Independent
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Power Factor Is . . .
• Independent of voltage
• Independent of specimen size
• Temperature sensitive
– This is why we apply a correction factor to the
measured power factor recorded for some
types of apparatus.
• Performed at apparatus frequency
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Overall Test Temperature Correction
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Dissipation/Power Factor
• Characters of the test specimen (insulation
system)
– Capacitance C
– Dielectric-Loss W
– Dissipation Factor %DF or DF
– Power Factor %PF or PF
} Overall evaluate
of the insulation
(physical and
quality) size
independent
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Voltage sensitive characteristics
• When we closely examine insulation, very
small gaps or “voids” could exist. These
voids develop an electrostatic potential on
their surfaces. These small gaps become
ionized: Partial Discharge/Corona. Voids
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Power Factor vs. Test Potential
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
Tip-up occurs in dry-type insulation specimens such as Dry Type Transformer, rotating machinery, and cables.
25% L-G L-G
E
%PF
%PF @ 25% L-G
%PF @ L-G
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E 25% L-G L-G
E
%PF
25% L-G L-G
%PF
Ideal
Generators
Typical Dry-type
Insulation
If permissible, perform an additional test at 110% or
125% of the Line to Ground rating. This can provide an
indication of future test expectations.
Power Factor Tip-Up:
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Test Sequence
• Apply a test voltage to an insulation specimen
– Potential across an impedance will result in current
flowing through the specimen
• Measure the current vector
– Vector has a magnitude and direction (angle)
– Reference is the applied voltage
• Calculate the impedance
– Ohms Law
– Exact the real and reactive components
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V
Measurement Overview
• Reference
– Applied Voltage
• Measure
– Total Current Vector
• Angle
• Magnitude
• Extract
– Reactive Component
• Usually Capacitance
– Real Component
• Resistance-Watts
IT
θ
Watts
Capacitance
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Basic Laws of Electricity
• A Difference in Potential Must Exist
Between Two Points in order for current to
flow
• Current Always Returns to It’s Source
• Current Always Takes the Path of Least
Resistance
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Test components of the test set
• Components of Simplified Test Circuits – Power Source
– Current & Loss Meter
– High-Voltage Test Cable
– Low-Voltage Test Cable
– Insulation Specimen
– Test Ground
• Test Set operation is based on the Relative Positions of the Power Source, Current & Loss Meter, and the Insulation Specimen with respect to the various test leads.
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Grounded-Specimen Test Mode (GST-
Ground)
Guard Test Ground
High-Voltage Cable
Low-Voltage Lead
Test-Set Ground Lead
Test-Set Step-Up Transformer
Current & Loss Meter
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Grounded-Specimen Test Mode (GST-
Guard)
Current & Loss Meter
Guard Test Ground
High-Voltage Cable
Low-Voltage Lead
Test-Set Ground Lead
Test-Set Step-Up Transformer
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Ungrounded-Specimen Test Mode
(UST)
Current & Loss Meter
Guard
Test Ground
High-Voltage Cable
Low-Voltage Lead
Test-Set Ground Lead
Test-Set Step-Up Transformer
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Lead Test-Set
Ground Lead
Current & Loss Meter
CA CB
IA IB
IA+IB
GST-Ground
Measure CA and CB
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Lead
Test-Set Ground Lead
Current & Loss Meter
GST-Guard
CA CB
IA IB
IB
Measure CA Guard CB
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Lead
Test-Set Ground Lead
Current & Loss Meter
UST
CA CB
IA IB
IA
Measure CA Ground/Guard CB
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IA+IB +IC
GST Ground Red + Blue
Measure CA + CB + CC
CA
IA
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IB +IC
GST Gnd Red Guard Blue
Measure CB + CC
CA
IA
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IA+IC
GST Gnd Blue Guard Red
Measure CA + CC
CA
IA
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IC
GST Guard Red + Blue
Measure CC
CA
IA
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IA+IB
UST Measure Red + Blue
Measure CA + CB
CA
IA
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IB
UST Measure Red Gnd Blue
Measure CB
CA
IA
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Guard
High-Voltage Cable
Test-Set Step-Up Transformer
Test Ground
Low-Voltage Leads
Test-Set Ground Lead
Current & Loss Meter
CB
CC
IB
IC
IA
UST Measure Blue Gnd Red
Measure CA
CA
IA
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Issues to Consider
• Power Requirements • Maximum amount of current at a give voltage
(12kV)
• Reliability of Measurements • Eliminate external influences
• Safety • Feature the insure the safety of the operator
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Power Requirement
• Basic Ohms Law: V=IX – The specimen under test is the load, typical insulation
system are represented as capacitor
– In order to maximize the stress on the insulation system the voltage potential across the specimen need to be as high as possible
– The only variable is the current.
• The critical factor – How much current can the instrument supply at the
maximum voltage?
– The larger the load (test specimen) the more current it will require in order to maintain a higher voltage stress
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External Influence
• Electrostatic Interference
– Define the phenomenon • De-energized conductor places with this electric
field will assume potential relative to its position within this electric field
– Obstacle to testing • Safety issue
• Auxiliary current flow through the test circuit
• Not a static effect
• Shield while necessary, will not eliminate the problem
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Issue to consider when testing
• Safety!
– Isolate and ground apparatus under test
• Work between visible grounds
– Ground Instrument
– Inspect test leads
– Connect test lead to the Instrument first
– Never come in contact with the test leads
while testing
– Once testing is complete remove leads from
specimen first.
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Summary
• Extract the Fundamental AC Characters of
the Insulation system
– Capacitance C
• Reactive Component
– Dielectric-Loss W
• Real - Resistance
– Dissipation Factor
• Power Factor %PF or PF
– Cos Θ
• Tangent Delta %DF or DF
– Tan δ
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End of Dielectric Theory
• Thank you for your attention