analysis of electrical insulation systems: capacitance and ...engineering.richmondcc.edu/courses/eus...

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.

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.

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.

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.

•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:

Resistor

Real Power Watts

Reactive Power

Capacitor Inductor

• V dependent

• Leading • Energy storing

• I dependent

• Lagging

• Energy storing

• Dissipates energy/heat

• V and I in-phase

Three Basic Elements of

Substation/Insulation

Modeling

• To model an electrical insulation system

using only on electrical component (e.g.

resistor, capacitor, inductor) which would

best represent the system

Perfect Insulator

The Capacitor

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.

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

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

, 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)

Capacitance is directly

proportional to Area

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

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

Capacitance

C = Capacitance

e = dielectric constant

d = Distance between plates

All of these variables are Physical

Parameters

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

Current & Loss

Ground

Test-Set Ground

High-Voltage Test Cable

Ic Oil: e= 2.1

Paper: e = 2.0

Center Conductor

Flange

Ideal Loss-less

Specimen

Why Analyze Current & Capacitance

Readings

Relationship between Current and

Capacitance

*10KV equivalent measurement or actual 10KV test potential

**Current in mA, Voltage in KV and Capacitance in pF

HzforIC

Measuring the Dielectric Constant of a

Material

An alternating current of the same voltage is applied to

the capacitor for both tests.

IOil Oil

IOil COil

= = eoil = 2.24 IVac

Vacuum

C = Ae 4pd

I2d = EwC/2) Id = EwC

4p2d) =

Double the

distance

= Id/2

Distance Between the Plates “d”

of the Capacitor

Test Mode-GST

Ground

Test-Set Ground

Oil = 2.1

Porcelain = 7.0

Paper = 2.0

Current & Loss

Typical Insulation System

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

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

Ideal Insulation System

Evaluating Insulation System

Current Lead Voltage

Purely Capacitive Circuit

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

Voltage

Current

Out of Phase

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

Real Insulation Model

Simplified Equivalent Circuits of an Insulation Specimen

Series Circuit

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

IT = IC + IR

Equivalent Circuit of an Insulation

System

Resistor

The Resistor

The Resistor represents the

energy-dissipating tendency of

the insulating material (i.e., the

Dielectric Loss portion of the

insulating material).

In Phase

The Perfect Resistor

11.3 12

Voltage

Current

Resistive Component

The Resistor

IR = IT

IR = E/R

Known that: S = P + jQ

Apparent Power

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]

Test Mode-GST

Ground

Current & Loss

Ground

Test-Set Ground

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

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

• 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

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

Basic Insulation Circuit

• Basic Power/Dissipation Factor Circuit

Voltage and Currents

0 90 180 270 360 450 540 630

Angle (360 degrees = 1 / 60th second)

Time and Angle Relationships

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

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

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 Theory

• How to Calculate Power Factor

Real Power

Apparent Power

Watts = E x IR

PF = Cosine =Watts

= Real Current

Total Current

E CP RP

P.F. = Watts

E IT P.F. =

P.F. = cosq

Capacitor

PF = 0%

Resistor

PF = 100%

Limits of % P.F.

2IC 2IT

2IR IR/IT = (IR+IR)/(IT+IT) = 2IR/2IT = IR/IT = PF

Power Factor Relationships

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

IR2 E IR1

IC1 IT1

Power Factor Is Size Independent

Doble Client Conference 2012 49

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

Overall Test Temperature Correction

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

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

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

%PF @ 25% L-G

%PF @ L-G

E 25% L-G L-G

25% L-G L-G

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:

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

Measurement Overview

• Reference

– Applied Voltage

• Measure

– Total Current Vector

• Angle

• Magnitude

• Extract

– Reactive Component

• Usually Capacitance

– Real Component

• Resistance-Watts

Capacitance

Basic Laws of Electricity

• A Difference in Potential Must Exist

Between Two Points in order for current to

• Current Always Returns to It’s Source

• Current Always Takes the Path of Least

Resistance

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.

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

Grounded-Specimen Test Mode (GST-

Guard)

Guard Test Ground

High-Voltage Cable

Low-Voltage Lead

Ungrounded-Specimen Test Mode

Test Ground

High-Voltage Cable

Low-Voltage Lead

High-Voltage Cable

Test Ground

Low-Voltage Lead Test-Set

Ground Lead

GST-Ground

Measure CA and CB

High-Voltage Cable

Test Ground

Low-Voltage Lead

GST-Guard

Measure CA Guard CB

High-Voltage Cable

Test Ground

Low-Voltage Lead

Measure CA Ground/Guard CB

High-Voltage Cable

Test Ground

Low-Voltage Leads

IA+IB +IC

GST Ground Red + Blue

Measure CA + CB + CC

High-Voltage Cable

Test Ground

Low-Voltage Leads

IB +IC

GST Gnd Red Guard Blue

Measure CB + CC

High-Voltage Cable

Test Ground

Low-Voltage Leads

GST Gnd Blue Guard Red

Measure CA + CC

High-Voltage Cable

Test Ground

Low-Voltage Leads

GST Guard Red + Blue

Measure CC

High-Voltage Cable

Test Ground

Low-Voltage Leads

UST Measure Red + Blue

Measure CA + CB

High-Voltage Cable

Test Ground

Low-Voltage Leads

UST Measure Red Gnd Blue

Measure CB

High-Voltage Cable

Test Ground

Low-Voltage Leads

UST Measure Blue Gnd Red

Measure CA

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

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

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

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.

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

End of Dielectric Theory

• Thank you for your attention

analysis of electrical insulation systems: capacitance and ...engineering.richmondcc.edu/courses/eus...

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