Transformer Excitation Current Testing
Summary
TLDRIn 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
🎓 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.
🧲 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.
🌀 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.
🔌 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.
🔍 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.
📈 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.
📊 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.
🏁 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.
Mindmap
Keywords
💡Magnetism
💡Magnetic Flux
💡Electromagnetism
💡Magnetic Permeability
💡Magnetomotive Force (MMF)
💡Reluctance
💡Magnetic Induction
💡Lenz's Law
💡Transformer
💡Excitation Current 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
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