Transformer Excitation Current Testing

Summary

In this webinar, Tom Sandry, the director of Workforce Development at Vector Power, dives into the intricacies of transformer excitation current testing. He refreshes participants on magnetism, magnetic induction, and Faraday's and Lenz's laws, setting the stage for a deeper understanding of the test's purpose. Sandry explains how excitation current tests can detect winding and core issues in transformers, even when other tests show normal results. He discusses the physics behind the test, the significance of phase patterns, and how to perform the test with necessary safety precautions. The webinar concludes with a quiz to reinforce the key learnings and a Q&A session, providing a comprehensive guide for professionals in the field.

Takeaways

• 🧲 The webinar by Tom Sandry from Vector Power focused on Transformer excitation current testing, which is crucial for diagnosing transformer health.
• 🔍 Magnetism is foundational to understanding excitation current testing, with magnetic fields, flux, and lines of force being key concepts.
• 👐 The left-hand rule helps determine the relationship between current flow and the direction of magnetic lines of force around a conductor.
• 📚 Magnetic permeability is analogous to electrical conductivity, with high permeability materials concentrating magnetic flux lines effectively.
• 🔗 The relationship between magnetism and electrical current is demonstrated through Ohm's law in the magnetic world, relating MMF, reluctance, and flux.
• 🌐 Electromagnetic induction, discovered by Michael Faraday, is the process where a voltage is induced in a conductor that cuts through magnetic field lines.
• 👣 Lenz's Law explains the polarity of induced voltage, which opposes the change causing the induction, setting up a magnetic field around the conductor.
• 🚀 The excitation current test is valuable for detecting winding and core problems in transformers, even when other tests seem normal.
• 🔍 The test can identify issues like abnormal core grounds, winding faults, low tap changer problems, and manufacturing defects.
• 📈 Phase patterns in three-phase transformers can indicate potential problems; normal patterns are high-low-high or low-high-low, with deviations suggesting issues.
• 🛠️ The procedure for performing excitation current tests involves careful setup, safety considerations, and comparison of results to previous tests or benchmarks.

Q & A

What is the main focus of the webinar presented by Tom Sandry from Vector Power?

The webinar focuses on Transformer excitation current testing, covering topics such as magnetism, magnetic induction, the physics behind the test, and how to identify faults in transformers using this method.

Why is a refresher on magnetism important for understanding excitation current testing?

A refresher on magnetism is important because the theory behind excitation current testing is deeply rooted in the understanding of magnetic fields, magnetic flux, and the principles of electromagnetism.

What is magnetic flux and how is it represented?

Magnetic flux is the group or number of magnetic field lines emitted outward from the North Pole of a magnet. It is represented by the symbol 'Φ' (phi), and its SI unit is the Weber.

Can you explain the left-hand rule for current-carrying conductors?

The left-hand rule helps determine the relationship between the current flow through a conductor and the direction of the magnetic lines of force around it. If you point your thumb in the direction of the positive potential and wrap your fingers around the conductor, your fingers indicate the direction of the magnetic field.

How does adding an iron core to a coil affect the magnetic field?

Adding an iron core inside a coil increases the flux density. The iron core becomes magnetized, enhancing the magnetic field around the conductor.

What is magnetic permeability and how is it related to relative permeability?

Magnetic permeability (mu) refers to the ability of a material to concentrate magnetic lines of flux. Relative permeability (Mu_r) is the ratio of the permeability of a material to the permeability of a vacuum (mu_0). It indicates how easily a material can be magnetized compared to a vacuum.

What is the significance of the excitation current test in transformer maintenance?

The excitation current test is significant for detecting possible winding or core problems in transformers, even when other tests like turn ratio and winding resistance tests appear normal. It can identify issues such as abnormal core grounds, winding faults, and manufacturing defects.

Why can't we measure the magnetic flux directly in a transformer?

Direct measurement of magnetic flux is not possible because it would require being inside the transformer while it is energized. Instead, magnetizing current, which is easier to measure and record, is used as an indicator.

What are the typical phase patterns observed during excitation current testing on three-phase transformers?

The typical phase patterns observed are the high-low-high pattern, the low-high-low pattern, and the pattern where all phases provide similar measurements. These patterns are tied to the core configuration and the interaction of the magnetic field with the core.

How should the excitation current test be performed on a three-phase transformer with a Y connection?

For a three-phase Y connection, the test should be performed by measuring each phase (H1 to Ho, H2 to Ho, H3 to Ho) in the ungrounded specimen test (UST) mode. Safety should be observed for other windings, and terminals that are normally grounded should be grounded during the test.

What does an unusual excitation current test result suggest?

Unusual results from the excitation current test may suggest potential problems with the transformer, such as residual magnetism, winding faults, or core issues. It may be necessary to demagnetize the transformer and repeat the test for accurate results.

Outlines

00:00

🎓 Introduction to Transformer Excitation Current Testing

The webinar, directed by Tom Sandry, delves into the principles of transformer excitation current testing. It covers the basics of magnetism, magnetic flux, electromagnetism, and the importance of understanding these concepts for the test. The session explains the role of magnetic fields, the concept of magnetic flux denoted by Φ (Phi) measured in Weber, and the impact of materials on magnetic permeability. It also demonstrates how an electromagnet works and touches on the concepts of magnetomotive force (MMF), reluctance, and their relationship with magnetic flux.

05:01

🧲 Magnetic Concepts and Transformer Operation

This segment continues the discussion on magnetism, introducing the concept of magnetic permeability and its relevance to materials' ability to concentrate magnetic flux. It explains the calculation of relative permeability (μ_r) and the impact of an iron core on a coil's magnetic flux. The paragraph also covers the principles of magnetomotive force (MMF), reluctance (R), and their interplay in a magnetic circuit, drawing parallels with electrical concepts like EMF and resistance.

10:02

🌀 Faraday's Law and Lenz's Law in Electromagnetic Induction

The third paragraph focuses on electromagnetic induction, a phenomenon discovered by Michael Faraday. It explains how a voltage is induced in a conductor when it cuts through magnetic lines of force. Faraday's law of induced voltage is introduced, highlighting its dependency on the number of coil turns and the rate of change of magnetic flux. Lenz's law is also discussed, illustrating how the induced voltage opposes the change causing the induction, with a demonstration showing the effect of a magnetic field on a falling magnetized object.

15:04

🔌 Excitation Current Testing for Transformers

This section discusses the importance of excitation current testing in transformers, which can reveal winding or core issues even when other tests seem normal. It explains the physics behind the test, where applying a voltage to a transformer winding generates a magnetic field that affects the transformer's core. The concept of excitation current is introduced, which is the current needed to establish magnetic flux when the secondary side is open. The paragraph also touches on the effects of loading the transformer and how it influences the excitation current.

20:07

🔍 Detecting Faults Using Excitation Current Test

The paragraph explains how the excitation current test can be used to identify various types of faults in transformers, such as turn-to-turn faults and grounded windings. It describes how a fault in the secondary winding increases the excitation current due to the opposing flux created by the fault. The effects of an autotransformer and core problems on the excitation current are also discussed, with an emphasis on the patterns that may indicate issues within a three-phase transformer.

25:08

📈 Phase Patterns and Test Procedures for Transformers

This section delves into the analysis of phase patterns in three-phase transformers, which can indicate the health of the transformer's core configuration. It outlines the expected patterns and what deviations from these might suggest in terms of potential problems. The procedures for performing excitation current tests on both single-phase and three-phase transformers are detailed, emphasizing safety and the importance of correct test setup and execution.

30:10

📊 Analyzing Test Results and Conducting the Webinar Quiz

The final paragraph focuses on the analysis of excitation current test results, providing criteria for evaluating the differences in readings and what they might indicate about the transformer's condition. It also mentions the importance of comparing results with previous tests and performing alternate tests if unusual results are obtained. The webinar concludes with a quiz to reinforce the understanding of key concepts covered during the session.

35:10

🏁 Conclusion and Q&A Session

The webinar concludes with a thank you note to the attendees for their participation throughout the year. The director expresses gratitude and emphasizes the pleasure of conducting these educational sessions. The final part of the webinar is dedicated to a Q&A session where participants can ask questions and seek clarifications on the discussed topics.

Keywords

💡Magnetism

Magnetism is a fundamental concept in the video, defined as the force of attraction or repulsion that arises between objects due to their magnetic properties. It is related to the video's theme as it underpins the understanding of transformer excitation current testing. In the script, magnetism is described through the concept of magnetic fields and lines of force, which are essential for comprehending how transformers operate and why excitation current tests are conducted.

💡Magnetic Flux

Magnetic flux, denoted by the symbol 'Φ' in the script, refers to the measure of the magnetic field that passes through a given area. It is a key concept in the video because it is directly related to the operation of transformers and the excitation current testing. The script explains that magnetic flux is the number of magnetic field lines emitted from the North Pole of a magnet, with one Weber being equal to 10^8 power magnetic field lines.

💡Electromagnetism

Electromagnetism is the physics of electromagnetic phenomena, including the relationship between electricity and magnetism. In the video, it is mentioned that the Danish scientist Ørsted discovered in 1819 that an electric current flowing through a conductor produces a magnetic field around it. This principle is crucial for understanding how transformers work and why excitation current tests are necessary for diagnosing transformer issues.

💡Magnetic Permeability

Magnetic permeability, symbolized as 'μ' in the script, is a measure of how easily a material can concentrate magnetic lines of flux. It is akin to conductivity in electrical terms. The script explains that materials with high permeability can be easily magnetized. This concept is important in the video's context as it influences the design and operation of transformers, affecting how excitation current tests are interpreted.

💡Magnetomotive Force (MMF)

Magnetomotive force, or MMF, is the driving force that produces a magnetic field in a coil, analogous to voltage in an electrical circuit. The script describes it as being dependent on the current flowing through the turns of a coil. MMF is crucial for the excitation current testing as it helps determine the strength of the magnetic field within a transformer, which is key to diagnosing potential faults.

💡Reluctance

Reluctance, symbolized by 'R' in the script, is the opposition to the production of magnetic flux in a material, similar to resistance in electrical circuits. It is calculated as the ratio of magnetomotive force to magnetic flux. The script explains that reluctance is affected by the material's permeability, the length of the coil, and the cross-sectional area. Understanding reluctance is essential for analyzing the results of excitation current tests.

💡Magnetic Induction

Magnetic induction, discovered by Michael Faraday in 1831, is the process by which a voltage or EMF is induced in a conductor when it cuts across magnetic lines of force. The script discusses Faraday's law of induced voltage, which is central to the operation of transformers and the excitation current tests. The induced voltage is a key factor in diagnosing transformer conditions through these tests.

💡Lenz's Law

Lenz's Law, as mentioned in the script, determines the polarity of the induced voltage and states that the induced current will flow in such a direction as to oppose the change in magnetic flux that produced it. This law is crucial for understanding how transformers respond to changes in current and voltage, which is directly relevant to the excitation current testing process.

💡Transformer

A transformer, as described in the script, is a device that allows energy transfer from one electrical circuit to another. The ideal transformer would have zero losses, but in reality, there are elements that dissipate energy, such as copper losses, leakage flux, and iron losses. The script emphasizes that changes in these losses can indicate changes in the transformer's internal state, which is why excitation current tests are performed to detect potential problems.

💡Excitation Current Test

The excitation current test is a diagnostic procedure used to detect winding or core problems in transformers. The script explains that this test is conducted even when other tests like turn ratio and winding resistance tests appear normal. The test involves applying a voltage to a transformer's winding and measuring the resulting current, which can indicate issues like shorts, open circuits, or manufacturing defects. The video's theme revolves around the importance and methodology of performing this test.

Highlights

Introduction to webinar on Transformer excitation current testing by Tom Sandry, Director of Workforce Development at Vector power.

Importance of understanding magnetism for excitation current testing.

Explanation of magnetic field lines and their relation to magnetic poles.

Magnetic flux defined and its measurement in Weber.

Historical discovery of electromagnetism by Hans Christian Ørsted.

Demonstration of electromagnetism through a conductor and the left-hand rule.

Magnetic permeability and its comparison to electrical conductivity.

Magnetomotive force (MMF) and its calculation in ampere-turns.

Reluctance in magnetism and its relationship to resistance in electricity.

Magnetic circuit comparison to an electric circuit with EMF and voltage.

Faraday's law of electromagnetic induction and its formula.

Lenz's Law and its demonstration with a magnetized bobbin in a metallic tube.

Transformer energy transfer and the concept of an ideal transformer.

Purpose of excitation current tests in detecting winding and core problems in transformers.

Explanation of how a loaded transformer affects excitation current.

Identification of turns-to-turn faults in transformers using excitation current tests.

Detection of grounded windings in transformers through excitation current tests.

Effects of auto-transformers on excitation current tests.

Analysis of phase patterns in three-phase transformers for fault detection.

Procedure for performing excitation current tests on single-phase and three-phase transformers.

Analysis of test results and comparison with previous data for fault detection.

Quiz互动环节，增强了参与者的参与度和对主题的理解。

Webinar 结束时的问答环节，提供了解决疑惑和深入讨论的机会。

Transcripts

well hello and welcome to our webinar

today I am Tom Sandry I am the director

of Workforce Development at Vector power

and today we're going to be taking a

look at Transformer excitation current

testing

so without further too let's get

started

the topics that we will be covering

today will be a refresher on magnetism

since a great deal of the theory behind

excitation current testing resides in an

understanding of

magnetism we will also talk about

magnetic

induction lens's

law we'll discuss why do we perform

excitation current tests what type of

information

do they provide

us we'll look at understanding the

physics behind the

test we will look at finding faults

using the excitation current

test understanding phase patterns that

are created when performing the

excitation current

test and finally performing the test and

the necessary connections

now every magnet is surrounded by a

magnetic field that consists of magnetic

field lines that extend from one end of

the magnet to the other as well as

inside the magnet a magnetic field can

be thought of as consisting of lines of

force these magnetic field lines are

said to exit the North Pole of the

magnet and enter the South Pole the

forces of magnetic attraction and

repulsion move along the lines of

force next let's discuss magnetic

flux the group or number of magnetic

field lines that are emitted outward

from the North Pole of a magnet is

called magnetic

flux the symbol for magnetic flux is

fi

the international symbol of units or SI

unit of magnetic flux is called the

Weber one Weber is equal to 1 * 10 8

power magnetic field

lines now let's look at Electro

magnetism the relationship between

magnetism and electrical current was

discovered by a Danish scientist name

orad in

1819 he found that if an electric

current was caused to flow through a

conductor the conductor produced a

magnetic field around that

conductor a convenient way to determine

the relationship between the current

flow through a conductor and the

direction of the magnetic lines of force

around the conductor is the leftand rule

for current carrying conductors as seen

in this

illustration if you were to

theoretically grab the conductor with

your left hand and your thumb pointing

to the positive potential your fingers

around the conductor will indicate the

direction of the magnetic

field bending a straight conductor into

a loop has two results magnetic field

lines become denser inside inside the

loop and two all lines inside the loop

are aiding in the same direction when a

conductor is shaped into several Loops

it is considered to be a coil to

determine the polarity of a coil use the

leftand rule for coils as seen in this

illustration adding an iron core inside

a coil will increase the flux density

the polarity of the iron core will be

the same as that of the coil current

flow is from the negative side of the

voltage source through the coil and back

to the positive side of the source as

seen

here all right let's see a short

demonstration of an

electromagnet

with no potential applied no current

flows through the

coil when we apply potential current now

flows through the coil creating a

magnetic

field

once again with no potential no current

flows no magnetic field is

formed once we apply a voltage current

will

flow and magnetism is

formed so let's talk a little bit about

magnetic

permeability in the world of magnetism

magnetic permeability is very similar in

concept to conductivity in the

electrical

World permeability or mu refers to the

ability of a material to concentrate

magnetic lines of flux those materials

that can be easily magnetize are

considered to have a high

permeability relative per permeability

is the ratio of the permeability of a

material to the permeability of a vacuum

or mu

subo the symbol for relative

permeability is Mu

subr where mu subr equals mu / mu sub

o and here we can see the mathematic

representation for Mu

subo

magnetomotive

force magnetomotive force or MMF is the

strength of a magnetic field in a coil

of wire this is dependent on how much

current flows in the turns of coil the

more current the stronger the magnetic

field the more turns of wire the more

concentrated the lines of force the

current times the number of turns of the

coil is expressed in units called

ampere turns or simply a t also known as

MMF the equation shown here is the

mathematical representation for ampere

hour

turns

reluctance as we may have learned in the

fundamentals of electricity resistance

is the opposition to current flow in

magnetism the opposition to the

production of flux in a material is

called

reluctance which corresponds to

resistance the symbol for reluctance is

capital letter r and it has the units of

ampere turns per

Weber reluctance is related to

magnetomotive force MMF and flux F by

the relationship shown in the equation

equ seen

here reluctance is inversely

proportional to

permeability iron cores have high

permeability and therefore low

reluctance air has low permeability and

therefore High

reluctance as we see permeability

reluctance and flux in the magnetic

world are very similar similar in

concept to conductivity resistance and

current in the electrical

World a magnetic circuit can be compared

with an electric current in which EMF or

voltage produces a current flow the

Ampere turns or magnetomotive force will

produce a magnetic flux as we see in

this

illustration

the MMF can be compared with EMF and

flux can be compared to current the

equation shown is the mathematical

representation of magnetomotive force

derived using ohms law or current equals

voltage / by

resistance in the magnetic world the

magnetic flux will

equal the magnet in a motive force or

MMF divided by the

reluctance reluctance will be affected

by the length of the coil permeability

of the magnetic material and the

cross-sectional area of the

coil next let's take a refresher on

magnetic

induction a electromagnetic induction

was discovered by Michael Faraday in

1831 Faraday found that if a conductor

cuts across lines of magnetic force or

if magnetic lines of force cut across a

conductor a voltage or EMF is induced

into the

conductor consider a magnet with its

lines of force from the North Pole to

the South Pole inserted in and out of a

coil or conductor that is connected to a

galvanometer which can detect the

presence of voltage or an EMF when the

magnet is not moving zero EMF is

indicated by the

galvanometer when the magnetic is either

inserted or retracted a voltage is

induced into the

conductor now let's take a close look at

Faraday's law of induced

voltage the magnitude of the induced

voltage depends on two factors one the

number of turns of a coil and two how

fast the conductor cuts across the

magnetic lines of force or flux the

equation shown is the mathematical

representation for Faraday's law of

induced

voltage where V sub IND D is the induced

voltage in

volts n is the number of turns in a

coil Delta F / delta T is the rate at

which the flux cuts across the

conductor and it is expressed in Webbers

per

second next let's look at lens's Law

lens's law determines the polarity of

the induced voltage induced voltage has

a polarity that will oppose the change

causing the induction when current flows

due to induced voltage a magnetic field

is set up around that conductor so that

the conductor's magnetic field reacts

with the external magnetic field this

produces the induced voltage to oppose

the change in the external magnetic

field field the negative sign in the

equation shown is an indication that the

EMF is in such a direction as to produce

a current whose flux if added to the

original flux would reduce the magnitude

of the

EMF lens's loss States an

electromagnetic field interacting with a

conductor will generate electrical

current that induces a counter magnetic

field that opposes the magnetic field

generating the

current next let's watch a demonstration

of lens's

law we have two bobbits of equal size

and weight one is not

magnetized the other is

magnetized

taking the non-magnetized

bobbit we will drop it down the center

of a metallic

tube notice how it drops based on a

speed created by its weight and

gravity now taking the magnetized we

will insert it and drop it down the

metallic tube or conductive

tube here we can see the magnetic field

opposing the

induction and that is lens's

law all right moving on from magnitud

ISM let's talk a little bit about the

Transformer or the ideal

Transformer Transformers allow energy

transfer from one electrical circuit to

another for a perfect Transformer there

would be zero losses or simply energy in

would equal the energy

out now in reality a Transformer has

lots of elements that dissipate energy

while the unit is energized these losses

include copper losses leakage flux

reluctance and iron

losses losses are all tied to the design

and construction of the

Transformer a change in the loss is

measured therefore shows us a change in

the internal state of the

Transformer

so let's talk a little bit now about why

we perform excitation current

tests well Transformer excitation

current tests are helpful in determining

possible winding or core problems in

Transformers even when turns ratio and

winding resistance tests appear to be

normal the test allows us to

detect abnormal core

grounds winding faults such as shorts

and open

circuits load tap changer

problems and Manufacturing

defects excitation tests are commonly

conducted routinely along with power

factor testing of the

Transformer under understanding the

physics when we apply a voltage to one

winding of a transformer we cause a

current to flow through that winding the

current generates a magnetic field

around the core of the Transformer the

magnetic field generates changes in the

magnetic state of the Transformer's core

magnetic energy begins to circulate in

the core this is the magnetic flux when

the secondary side of the Transformer is

open only as much current as is needed

to get flux moving enters the

windings this is called the excitation

current so why don't we measure the flux

directly well simply we can't you would

have to be inside the Transformer while

it was

energized magnetizing current on the

other hand is easy to measure and record

now we need to keep in mind that the

magnetizing current will be voltage

sensitive therefore comparisons will

require that the same test voltage be

applied every

time if a load is connected the

secondary will pick up power and

transfer it through the load the load

shows up a reluctance the magnetic

equivalent of

resistance all right let's break it down

one when a load is placed on the

secondary winding a current will

flow the secondary current I sub 2 will

equal the secondary voltage V sub 2 / by

the resistance of the secondary or R sub

2 second the current on the secondary

will in turn create an opposing magnetic

flux and three the generator which

regulat Ates voltage at a set level will

provide more current to maintain the

core magnetized equal to the opposing

flux or the excitation current will

equal the magnitude of the current plus

the secondary current I sub

2 therefore a loaded Transformer would

require more current to be injected by

measuring the current we could tell if

the Transformer was intentionally

loaded so let's talk about the

excitation current test being used to

find a turns to turn

fault if a fault were to develop in the

secondary winding of the Transformer

this fault would act as a load across

the faulted windings drawing a

current I sub faal as a result the

excitation current would go up due to

the opposing flux created by the

fault the result of a turnto turn fault

in the secondary winding would be a

fault current that causes excitation

current to

increase how about finding grounded

windings the excitation current test is

also ideal for finding these grounded

windings if there is a fault in the

windings you'll get circulating current

between those turns and the magnetizing

current plus the fault

current if the secondary winding has a

grounded neutral and one of the windings

develops a fault to ground the grounded

winding will draw a fault current as a

result the excitation current will will

go up due to the opposing flux created

by the

fault a grounded winding on a

Transformer with a grounded neutral will

cause the excitation current to

increase how about the effects of

preventative autot

Transformer most perhaps all tap

changers have an autot transformer for

transitioning there are certain steps in

the low changer where this Auto

Transformers is inserted into the

windings with these low tap Changers you

will have the magnetizing current plus

the current in the autot Transformer

resulting in a higher excitation

current when an autot Transformer is

connected across two Taps it acts as a

load and the primary current goes up up

as Taps are steep through this can

create a recognizable step

pattern when a low tap changer

transitioning device such as a

preventative Auto Transformer is in the

bridging position the excitation current

goes

up now the excitation current test is

also ideal for finding core problems

they will sometimes show up on the

difference in current very few of the

tests we do on a Transformer can find a

cure problem now the swep frequency

response analysis test can do this but

you need a previous test result or a

sfra signature in order to compare

to the excitation current test results

present

patterns for three-phase Transformers

there are two patterns the first is

between phases two similar High readings

and one low reading with some

exceptions the second pattern is within

each phase when tap changes are

present this second pattern will depend

upon the tap changer type and

manufacturer

within each phase you will have similar

readings for each tap when the autot

Transformer is in your current may go up

so it may be one raised to one lower

consistently increasing current you may

see current go up and down within the

phase depending on which tap you're on

but you should see a definite

pattern for single face Transformers you

you'll need a reverse measurement for

confirmation you'll perform a forward

and reverse test and those two readings

should render very similar

results so let's talk a little bit about

phase

patterns the currents Watts measured on

a three-phase unit will show a pattern

across the

phases there are three expected patterns

the high reading low reading High

reading

pattern the low reading High reading low

reading

pattern and where All Phases will

provide similar

measurements the phase pattern is tied

to both the measurement and the core

configuration patterns arise from the

core

configuration recall that an increased

load leads to an increase reluctance and

thus an increased current

injected interacting with the core at

different points will result in

different reluctances and thus different

currents reluctance sums similar to

resistance so in a magnetic core the

reluctance of the steel acts much the

same to resist magnetic flux as a

resistor does to current a magnetic core

can be represented by a reluctance

circuit for Simplicity assume that each

section has a reluctance of 1

ohm

so to simplify this

circuit we would start with taking the

three 1 ohm resistors in series as we

see

circled this would provide us the

equivalent Circuit of the 1 ohm resistor

now in parallel with 3 ohms providing us

75

ohms simplifying the circuit even

farther we see now we have 1 ohm in

series with 75 ohms in series with 1 ohm

resulting in

3.75 ohms of

reluctance now the same will hold true

when you energize the cphase

so both outer phases in this scenario

would result in

3.75 ohms of

reluctance

now when looking at the center

phase we can start with the three series

there three 1 ohm resistors in

series on the A and on the B side so

this would simplify down to 3 ohms in

parallel with 1 ohm in parallel with 3

ohms which would simplify down to

1.5

ohms this would provide a reluctance of

2.50

ohms so here we see the outer legs a

high

reluctance the center leg a lower

reluctance now phase pattern

characteristics the high low high

pattern outer phases have higher

currents than the center phase in this

pattern typically this is seen in

three-leg core type

Transformers the low high low

pattern the outer phases have lower

currents than the center

phase this is typically seen in a

three-leg y core type Transformers with

inaccessible

neutrals or three-leg Delta core type

without the third phase

grounded All Phases being

similar we see this in three singlephase

Transformers connected as a three-phase

bank

Transformer anything else than these

patterns points to a potential problem

or a capacity positive

winding performing the

test first remove the short circuit

jumpers that were used during the power

factor test where all the high side

windings were jumpered together and all

the low side windings were jumpered

together excitation current tests are

singlephase

tests

set up the test connections on the high

side observe safety for the other

windings proceed according to

Transformer winding configuration a

delta or

y ground any terminals that are normally

grounded during the test on the windings

that are not being energized for example

neutral on a y

Transformer ground any terminals that

would normally be be grounded normally

they would be on the low side for

example if the Y is on the low side you

would ground that

connection perform all the tests on the

power factor test set using the us or

ungrounded specimen test mode of

operation now when tap changes are

present test as

follows for benchmark or base

measurements loow tap changer only test

all

positions no load tap changer only test

all

positions load tap changer and no load

tap changer test all load tap changer

positions with the no load tap changer

in Center position then all positions in

the no loow tap changer with with the

low tap changer in

neutral for testing on a routine

basis non-load tap changer only test as

found or on the position regularly

tapped for loow tap changer

only test one full range all the raises

and lowers plus the neutral position and

one position in the opposite

range for low tap changer and non-load

tap changer again test as found or on

the position regularly tapped testing

the low tap changer one full range plus

neutral and one position in the opposite

range perform tests at the highest

voltage p

possible test the phase demanding the

highest current first we normally start

with H3 to

H1 test each phase at the same

voltage perform subsequent tests at the

same voltage for

comparison if you have previous test

data we would like the test voltage to

be the same as the pre previous year or

test

date if a preventative Auto Transformers

is included in the Transformer it might

not be possible to excite that position

of the low tap changer testing might be

possible with the preventative autot

Transformer Bypass or at a lower

voltage if the test set trips choose a

lower voltage and repeat all three

phases

never allow test voltage to exceed rate

Line to Line of delta or line to ground

for

y all right let's take a look at some

procedures let's first look at a single

phase here we will measure H1 to Ho

divided by

H2 the mode of operation on the power

factor test set is we set to the

us we will energize

H1 our measurement lead will be placed

on

ho and we will ground XO and leave X2

floating

open next we will measure ho / H2 to

H1

once again the mode will be

us we will energize the ho we will

measure on the

H1 and we will ground XO and we will

float

X2 so for our single phase here is our

reverse measurement we should render

comparable results to our previous

measurement all right our procedure for

a three-phase is

y we will measure first H1 to

Ho in US mode we will energize

H1 measurement lead on ho grounding XO

and all other terminals

floating next we will measure H2 to

H we'll energize on H2 measure on ho

grounding EXO and floating all other

Terminals and finally we will measure H3

to Ho we'll energize on

H3 measurement lead on ho grounding the

XO and all other terminals

floating all right let's look at the

procedure for three-phase Delta the a

phase so here we will measure H3 to

H1 we will energize on H3 our

measurement lead will be on

H1 and we will ground

H2 and also

XO all the other terminals we will leave

floating

now the B phase we'll measure H1 to

H2 energizing on H1 low voltage lead on

H2 we will ground H3 and XO and all

other terminals

floating and finally the cphase on the

Delta we'll measure H2 to

H3 energizing on H2 measurement lead on

H3 grounding H1 and XO and all other

terminals

floating now let's take a look at

analysis of the test

results check the pattern two similar

High readings and one low reading is

normal though there are some

exceptions when T changes are present

consider the pattern within the

phase for three-phase units compare High

readings with the following

criteria readings that are less than or

equal to 50

milliamps difference should not exceed

10%

readings greater than 50 milliamps the

difference should not exceed

5% for singlephase units both readings

should be compared using the same

criteria as stated

previously readings less than or equal

to 50 milliamps difference should not

exceed

10% readings greater than 50 milliamps

difference should not exceed

5% compare normal test to alternate test

results should be similar for a winding

in good

condition if the core is magnetized an

irregular pattern for instance a high

medium low reading will be present and

you will be unable to compare results

effectively a true problem could be

masked there for the core should be

demagnetized and the test should be

repeated compare readings to previous

results ensure that the same voltage was

used for both tests for consistent

numerical

comparison if unusual results are

obtained consider performing an

alternate test to further

investigate so here we see an example of

an alternate test method being used on a

Delta

configuration so in review the

excitation current test has been

effective for detecting and confirming

winding and core faults even though in

some instances normal turn ratio and

winding resistance test results had been

obtained a Transformer has lots of

elements that dissipate energy while the

unit is energized these losses include

copper

losses leakage

flux

reluctance iron

losses losses are all tied to the design

and construction of a transformer a

change in the losses measured therefore

shows us a change in the internal state

of the

Transformer Transformer excitation

current tests are helpful in determining

possible winding or core problems in the

Transformer even when ratio and winding

resistance tests appear to be normal the

test allows us to detect abnormal core

grounds winding faults such as shorts

and open circuits

low tap changer problems and

manufacturer

defects excitation tests are commonly

conducted routinely along with power

factor

testing so as part of our review let's

run a little

quiz use

the questions function in your uh

webinar controls and type in either A B

C or

D the question is the opposition to the

production of flux in a material is

called enter a for

reluctance B for power factor C for

resistance or D

insulation enter your answer into the

questions function in the webinar

controls and the answer is a

reluctance question two the excitation

current test is also good for

finding type in a for low

frequency B for

contamination C for core problems and D

for gas filled

voids the excitation current test is

also good for

finding and the answer is C Core

problems it is one of the few tests that

will allow us to find core problems in a

Transformer question number three

which of the following measured patterns

indicate a possible

problem enter a for the high low high

pattern B low high low pattern C for All

Phases being similar or D the low medium

high

pattern which of the following measured

patterns indicate a possible

problem

and the answer should be the low medium

high

pattern this may indicate residual

magnetism and the Transformer should be

demagnetized and the test

reperform question number four which of

the following will the excitation

current test not help identify so which

of the following will the excitation

current test not help in identifying a

abnormal core grounds B oil

contamination C winding faults such as

shorts or open circuits or d low tap

changer

problems which of the following will the

excitation current test not help us

identify enter your answer into the

questions function in the webinar

controls and the answer is oil

contamination

B question number five excitation

current test should be performed using

the grounded specimen test or GST test

mode on the power factor test set enter

a for true or B for

false the excitation current test should

be performed using the GST test mode on

the power factor test

set and that is false we always run the

excitation current test using the usst

ungrounded specimen test mode on the

power factor test

set

and that concludes our webinar today on

excitation current

testing it's been an absolute pleasure

uh this is the last of the webinar for

our calendar 2023 year I truly truly

thank you all for your attendance

participation in these webinars we

really do truly enjoy uh doing these

webinars for you and we thank you for

taking the time out of your busy days to

participate with

us but without further to do how about

we get to the questions and answers

section of the webinar