All About Transformers
and Transformer Faults
Root Cause Analysis
Qazi Arsalan Hamid
Power transformer a static
piece of apparatus with two or more windings which, by
electromagnetic induction, transforms a system of alternating
voltage and current in to another system of voltage and
current usually of different values and at the same
frequency for the purpose of transmitting electrical power
[IEV 421-01-01, modified]
Auto-transformer
’a
transformer in which at least two windings have a
common part [IEV 421-01-1 11
Booster transformer a transformer
of which one winding is intended to be connected in series
with a circuit in order to alter its voltage and/or shift its
phase. The other winding is an energizing winding [IEV 42101-12, modified]
Oil-immersed type
transformer a transformer of which the
magnetic circuit and windings are immersed in oil [IEV
421-01-141 NOTE For the purpose of this part any
insulating liquid, mineral oil or other product, is regarded
as oil.
Dry-type transformer a transformer
of which the magnetic circuit and windings are not
immersed in an insulating liquid [IEV 421 -01 -161
Terminals and neutral
point
Terminal a conducting element intended
for connecting a winding to external conductors
line terminal l a terminal
intended for connection to a line conductor
of a network [IEV 421-02-011
Neutral terminal
a) For three-phase transformers and
three-phase banks of single-phase
transformers: The terminal or terminals
connected to the common point( the
neutral point) of a star- connected or
zigzag connected winding.
b) For single-phase transformers: The
terminal intended for connection to a
neutral point of a network[ IEV4 21-0202, modified]
neutral point the point of
a symmetrical system of voltages which is
normally at zero potential
corresponding
terminals terminals of
different windings of a transformer,
marked with the same letter or
corresponding symbol [IEV 421 -02-031
Windings
winding
The assembly of turns forming an electrical circuit
associated with one of the voltages assigned to
the transformer NOTE For a three-phase
transformer, the 'winding' is the combination of the
phase windings (see 3.3.3). [IEV 421-03-01,
modified]
tapped winding a
winding in which the effective number of
turns can be changed in steps
phase winding the
assembly of turns forming one phase of a
three-phase winding NOTE The term
'phase winding' should not be used for
identifying the assembly of all coils on a
specific leg. [IEV 421-03-02, modified]
high-voltage
winding* the winding having the
highest rated voltage [IEV 421-03-031
low-voltage
winding' the winding having the
lowest rated voltage [IEV 421 -03-041
NOTE For a booster transformer, the
winding having the lower rated voltage may
be that having the higher insulation level.
tapping in a transformer having a tapped
winding, a specific connection of that winding,
representing a definite effective number of turns
in the tapped winding and, consequently, a
definite turns ratio between this winding and any
other winding with fixed number of turns
NOTE One of the tapping is the principal tapping,
and other tappings are described in relation to the
principal tapping by their respective tapping
factors. See definitions of these terms below.
principal tapping the
tapping to which the rated quantities are
related [IEV 421-05-021
tapping factor
(corresponding
to a given tapping)
The ratio:
rated voltage of the winding / the voltage which
would be developed at no-load at the terminals of
the winding, at the tapping concerned, by
applying rated voltage to an untapped winding.
Plus tapping a tapping
whose tapping factor is higher than 1
[IEV 421-05-041
minus tapping a tapping
whose tapping factor is lower than 1
[IEV 421-05-051
tapping step the difference between the
tapping factors, expressed as a percentage, of two
adjacent tappings [IEV 421 -05-061
tapping range the variation range of
the tapping factor, expressed as a percentage,
compared with the value '1 00' NOTE If this factor
ranges from 100 + a to 100 - b, the tapping range is said
to be: +a %, -b % or +-a %, if a = b. [IEV 421-05-071
tapping voltage ratio (of a pair of
windings) the ratio which is equal to the rated voltage
ratio: - multiplied by the tapping factor of the tapped
winding if this is the high-voltage winding; - divided by the
tapping factor of the tapped winding if this is the lowvoltage winding. [IEV 421 -05-081 NOTE While the rated
voltage ratio is, by definition, at least equal to 1, the tapping
voltage ratio can be lower than 1 for certain tappings when
the rated voltage ratio is close to l.
tapping duty the numerical values
assigned to the quantities, analogous to rated
quantities, which refer to tappings other than the
principal tapping [ IEV4 21-05-09, modified
on-load tap-changer a device
for changing the tapping connections of a winding,
suitable for operation while the transformer is
energized or on load [IEV 421 -1 1-01
Losses and no-load current
no-load loss the active power
absorbed when rated voltage (tapping voltage) at
rated frequency is applied to the terminals of one of
the windings, the other winding or windings being
open-circuited [IEV 421-06-01, modified]
no-load current the r.m.s. value of the
current flowing through a line terminal of a winding when
rated voltage (tapping voltage) is applied at rated frequency, the
other winding or windings being open- circuited
NOTE 1 For a three-phase transformer , the value is the
arithmetic mean of the values of current in the three phases.
NOTE 2 The no-load current of a winding is often expressed
as a percentage of the rated current of that winding. For a
multi-winding transformer this percentage is referred to the
winding with the highest rated power. [IEV 421-06-02,
modified]
load loss the absorbed active power at rated
frequency and reference temperature, associated with a
pair of windings when rated current (tapping current)
is flowing through the line terminals of one of the
windings , and the terminals of the other winding are
short- circuited. Further windings, if existing, are opencircuited
total losses the sum of the no-load loss
and the load loss
Temperature rise
The difference between the temperature
of the part under consideration and the
temperature of the external cooling
medium. [IEV 421-08-01, modified]
star connection
Delta Connection
zigzag connection (&connection) the
winding connection in which one end of each phase
winding of a three-phase transformer is connected to a
common point (neutral point), and each phase winding
consists of two parts in which phase-displaced voltages
are induced NOTE These two parts normally have the
same number of turns. [IEV 421-10-04, modified]
Tests
10.1 General requirements for routine, type and special tests
Transformers shall be subjected to tests as specified
below.
• Tests shall be made at any ambient temperature between
10 "C and 40 "C and with cooling water (if required) at any
temperature not exceeding 25 "C.
• Tests shall be made at the manufacturer's works, unless
otherwise agreed between the manufacturer and the
purchaser.
• Tapped windings shall be connected on their principal
tapping, unless the relevant test clause requires
otherwise or unless the manufacturer and the purchaser
agree otherwise.
• The test basis for all characteristics other than insulation
is the rated condition, unless the test clause states
otherwise.
• All measuring systems used for the tests shall have certified,
traceable accuracy and be subjected to periodic calibration,
according to the rules of 4.1 1 of IS0 9001,
Routine Test
•
•
•
•
•
•
Measurement of winding resistance
Measurement of voltage ratio and check of phase displacement
Measurement of short-circuit impedance and load loss
Measurement of no-load loss and current
Dielectric routine tests (IEC 60076-3)
Tests on on-load tap-changers, where appropriate
Type tests
a) Temperature-rise test
b) Dielectric type tests (IEC 60076-3)
Maintenance and
inspection of the
transformer bushings
Six different inspections
There are six different inspections to be performed on the
bushings of the power transformer:
1.Routine inspection
2.Regular inspection (Once every two years)
3.Inspection due to excessive partial heatings
4.Local damages inspection (fissures) on the bushings
5.Inspection for oil leaks
6.Storage
1. Routine inspection
Excessive local heating
Pay attention to the clamping section of the terminals. It is
convenient to paint this section with heat indicating paint.
Pollution
When there is much dust and salt, a clean up must be performed and
to do so, the transformer must be place out of service and use water,
ammonia or carbon tetrachloride. If they are very dirty, use
concentrated hydrochloric acid diluted 40 or more times in water.
The solution should not be in contact with any metallic part; after the
cleaning the porcelain parts, these must be neutralized with water
that contains sodium bicarbonate in a proportion of 30 grams by liter.
2. Regular inspection
(Once every two years)
Evaluation of the deterioration of the insulation
The methods to detect the deterioration of the insulation are the
measurement of the insulation resistance and tan ∆.
The measurement of the tan ∆ is also difficult, since the bushings
should be separated from the transformer in most cases. The
evaluation of the result of the measurement should not depend solely
on the absolute values obtained, but on the values obtained each
year and from the variation among them. If there are large
discrepancies in the values, special attention is necessary.
When the insulation resistance is superior to 1000 MΩ at normal
temperatures, it can be considered as good condition, but the
value of the tan also must be taken into consideration for the
evaluation.
3. Inspection due to excessive
partial heating
The excessive heating of the terminals in most cases is due to
loosening. If this condition is observed, eliminate the dust or dirt from
the parts from contact and tighten firmly.
4. Local damages inspection
(fissures) on the bushings
The cleaning of the bushings must be done according to
what was mentioned. If the damages are very serious it must
be replaced with new ones.
5. Inspection for oil leaks
Check the various pieces of the bushings to see if there is any
oil leaks. If oil is leaking through the gasket, tighten it or replace
it. If there are bushings of immersed in oil type and the oil leak is
through other part of the bushing, report it to the manufacturer.
6. Storage
Keep the bushings in a vertical position and in a dry
place. It is recommended to keep them in their original
packaging.
Protection against Over
excitation
Increase of the magnetizing
current
Over excitation of a transformer means that the magnetic flux
in the core is increased above the normal design level.
This will cause an increase of the magnetizing current and
the transformer can be damaged if this situation isn’t taken
care of.
For step-up transformers connected to
generators during start-up, over excitation
can occur since the flux is dependent of
the factor voltage/frequency. This means
that the voltage must be gradually
increased, with increasing frequency, in
order not to overexcite the transformer.
The differential protection must therefore be stabilized under
these conditions as tripping of transformers and thus load will
only mean that the overvoltage condition in the network is
becoming worse.
The current during over excitation has a lot of fifth harmonic,
see figure 1. This fact is utilized in modern transformer
protection to stabilize the transformer against unwanted
functions during these kind of conditions.
If over excitation of the transformer
due to overvoltage or under
frequency is likely to happen, a
separate over excitation protection
should be supplied. This protection
has inverse characteristics
according to the transformers
capability to restrain over excitation
“V/Hz”.
This protection must be connected to a transformer
winding with fixed number of turns.
Condition Monitoring of
Transformer
Detecting early signs of
deterioration
It is possible to provide transformers with measuring devices to
detect early signs of degradation in various components
and provide warning to the operator in order to avoid a
lengthy and expensive outage due to failure.
The technique, which can be applied to other plant as well as
transformers, is called condition monitoring, as the intent is
to provide the operator with regular information on the
condition of the transformer.
By reviewing the trends in the information
provided, the operator can make a better
judgment as to the frequency of
maintenance, and detect early signs of
deterioration that, if ignored, would lead to
an internal fault occurring.
The extent to which condition monitoring is applied to
transformers on a system will depend on many factors, amongst
which will be the policy of the asset owner, the suitability of the
design (existing transformers may require modifications involving a
period out of service – this may be costly and not justified), the
importance of the asset to system operation, and the general
record of reliability.
Therefore, it should not be expected that all transformers would be,
or need to be, so fitted.
A typical condition monitoring system for an oil immersed
transformer is capable of monitoring the condition of various
transformer components (bushings, tank, tap changer, coolers
and conservators) as shown in Table 1 below.
There can be some overlap with the measurements
available from a digital/numerical relay.
When a Transformer has
stomach pain and want
to…..
About stomach pain
During normal operation, transformer internal structures and
windings are subjected to mechanical forces due to the
magnetic forces. These forces are illustrated in Figure 1.
By designing the internal structure very strong to withstand
these forces over a long period of time, service life can be
extended. However, in a large transformer during a “through
fault” (fault current passing through a transformer), forces can
reach millions of pounds, pulling the coils up and down and
pulling them apart 60/50 times per second.
Notice in Figure 1 that the internal low-voltage coil is being
pulled downward, while the high-voltage winding is pulled up,
in the opposite direction.
At the same time, the right-hand part of the figure shows that the
high- and low-voltage coils are being forced apart.
Keep in mind that these forces are reversing 50/60 times each
second. It is obvious why internal structures of transformers must be
built incredibly strong.
Many times, if fault currents are high, these forces can rip a
transformer apart and cause electrical faults inside the
transformer itself. This normally results in arcing inside the
transformer that can result in explosive failure of the tank, throwing
flaming oil over a wide area.
There are protective relaying systems to protect against this
possibility, although explosive failures do occur occasionally.
How to prevent pain
Through Fault – Short Circuit withstand considerations
The windings are subject to both radial and axial forces related to
the current and flux interactions. Radial forces in the inner
winding (normally the LV winding) are in compression while the
outer winding (normally the HV winding) forces are in tension.
Design of the windings and bracing must consider the
magnitude of these forces and provide adequate strength to
withstand them without significant mechanical deformation which
could result in a dielectric failure.
The picture below is an example of a free bucking mechanical
failure of an inner winding resulting from radial forces in compression
on the winding. Note, even though there is mechanical failure, there
wasn’t a dielectric failure of this winding.
Flux fields are dependent of
the balance of the ampere
turn distribution of the HV
and LV windings. When the
ampere turns of the HV and
LV windings are equal and
balanced, the only forces
are radial.
DETC taps (De Energized Tap Changer) in the HV windings and
LTC (Load Tap Changer) operation result in changes in the
ampere turn distribution resulting in axial forces. If the HV and LV
windings are not aligned axially or one winding is physically
shorter than the other, ampere turn balance is significantly
affected and axial forces are magnified.
Autotransformers, low impedance,
motor starting duty, transformers
with multiple voltages by
reconnecting the transformer
windings in series and parallel
configurations, three
winding transformers with dual
secondary windings for start up or
unit auxiliary service at power
plants – all can result in
increased axial and radial forces
during a short circuit and require
special consideration.
Some Viva Questions
Why Delta Star Transformers
are used for Lighting Loads?
•For lighting loads, neutral conductor is must and hence the
secondary must be star winding and this lighting load is always
unbalanced in all three phases.
•To minimize the current unbalance in the primary we use delta
winding in the primary So delta / star transformer is used for
lighting loads.
How do we select
transformers?
•Determine primary voltage and frequency.
•Determine secondary voltage required.
•Determine the capacity required in volt-amperes. This is done by
multiplying the load current (amperes) by the load voltage (volts)
for single phase.
•For example: if the load is 40 amperes, such as a motor, and the
secondary voltage is 240 volts, then 240 x 40 equals 9600 VA. A
10 KVA (10,000 volt-amperes) transformer is required.
•Always select Transformer Larger than Actual Load. This is done
for safety purposes and allows for expansion, in case more loads
is added at a later date. For 3 phase KVA, multiply rated volts x
load amps x 1.73 (square root of 3) then divide by 1000.
•Determine whether taps are required. Taps are usually specified
on larger transformers.
Why Small Distribution
Transformers not used for
Industrial Applications?
•Industrial control equipment demands a momentary overload
capacity of three to eight times’ normal capacity. This is most
prevalent in solenoid or magnetic contactor applications where
inrush currents can be three to eight times as high as normal
sealed or holding currents but still maintain normal voltage at
this momentary overloaded condition.
•Distribution transformers are designed for good regulation up to
100 percent loading, but their output voltage will drop rapidly on
momentary overloads of this type making them unsuitable for
high inrush applications.
•Industrial control transformers are designed especially for
maintaining a high degree of regulation even at eight time’s
normal load. This results in a larger and generally more
expensive transformer.
Can 50 Hz transformers be
used at higher frequencies?
•Transformers can be used at frequencies above 50 Hz up
through 400 Hz with no limitations provided nameplate
voltages are not exceeded.
• However, 60 Hz transformers will have less voltage
regulation at 400 Hz than 50 Hz.
What is meant by regulation in a
transformer?
•Voltage regulation in transformers is the difference between the no
load voltage and the full load voltage. This is usually expressed in
terms of percentage.
•For example: A transformer delivers 100 volts at no load and the
voltage drops to 95 volts at full load, the regulation would be 5%.
Distribution transformers generally have regulation from 2% to 4%,
depending on the size and the application for which they are used.
What are taps and when are
they used?
•Taps are provided on some transformers on the high voltage
winding to correct for high or low voltage conditions, and still
deliver full rated output voltages at the secondary terminals. Taps
are generally set at two and a half and five percent above and
below the rated primary voltage.
Can Transformers be reverse connected?
•Dry type distribution transformers can be reverse connected without a loss of
KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger
single phase, 3 KVA and larger three phases can be reverse connected without
any adverse effects or loss in KVA capacity.
•The reason for this limitation in KVA size is, the turns ratio is the same as the
voltage ratio.
•Example: A transformer with a 480 volt input, 240 volt output— can have the
output connected to a 240 volt source and thereby become the primary or input to
the transformer, then the original 480 volt primary winding will become the output
or 480 volt secondary.
•On transformers rated below 1 KVA single phase, there is a turn’s ratio
compensation on the low voltage winding. This means the low voltage winding
has a greater voltage than the nameplate voltage indicates at no load.
•For example, a small single phase transformer having a nameplate voltage of
480 volts primary and 240 volts secondary, would actually have a no load voltage
of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt
winding were connected to a 240 volt source, then the output voltage would
consequently be approximately 460 volts at no load and approximately 442 volts
at full load. As the KVA becomes smaller, the compensation is greater—resulting
in lower output voltages.
•When one attempts to use these transformers in reverse, the transformer will not
be harmed; however, the output voltage will be lower than is indicated by the
nameplate.
What is the difference
between “Insulating”,
“Isolating”, and “Shielded
Winding” transformers?
•Insulating and isolating transformers are identical. These terms
are used to describe the separation of the primary and
secondary windings. A shielded transformer includes a metallic
shield between the primary and secondary windings to attenuate
(lessen) transient noise.
Will a transformer change
Three Phases to Single
Phase?
•A transformer will not act as a phase changing device when
attempting to change three phase to single phase.
•There is no way that a transformer will take three phase in and
deliver single phase out while at the same time presenting a
balanced load to the three phase supply system.
•There are, however, circuits available to change three phase to
two phase or vice versa using standard dual wound
transformers. Please contact the factory for two phase
applications.
Can 60 Hz transformers be
operated at 50 Hz?
•Transformers rated below 1 KVA can be used on 50 Hz service.
•Transformers 1 KVA and larger, rated at 60 Hz, should not be
used on 50 Hz service, due to the higher losses and resultant
heat rise. Special designs are required for this service. However,
any 50 Hz transformer will operate on a 60 Hz service.
Can transformers be used in
parallel?
•Single phase transformers can be used in parallel only when
their impedances and voltages are equal. If unequal voltages
are used, a circulating current exists in the closed network
between the two transformers, which will cause excess heating
and result in a shorter life of the transformer. In addition,
impedance values of each transformer must be within 7.5% of
each other.
•For example: Transformer A has an impedance of 4%,
transformer B which is to be parallel to A must have
impedance between the limits of 3.7% and 4.3%. When
paralleling three phase transformers, the same precautions
must be observed as listed above, plus the angular
displacement and phasing between the two transformers must
be identical.
What is Boucholz relay and
the significance of it in to the
transformer?
•Boucholz relay is a device which is used for the protection of
transformer from its internal faults,
•it is a gas based relay. whenever any internal fault occurs in a
transformer, the boucholz relay at once gives a horn for some
time, if the transformer is isolated from the circuit then it stop
its sound itself otherwise it trips the circuit by its own tripping
mechanism.
Why we do two types of
earthing on transformer
(Body earthing & neutral
earthing)
•The two types of earthing are Familiar as Equipment earthing
and system earthing.
•In Equipment earthing: body (non conducting part) of the
equipment should be earthed to safeguard the human beings.
•The System Earthing : In this neutral of the supply source (
Transformer or Generator) should be grounded. With this, in
case of unbalanced loading neutral will not be shifted. So that
unbalanced voltages will not arise. We can protect the
equipment also. With size of the equipment ( transformer or
alternator)and selection of relying system earthing will be
further classified into directly earthed, Impedance earthing,
resistive (NGRs) earthing.
Which Point need to be
consider while Neutral
Earthing of Transformer?
•The following points need to check before going for Neutral
Grounding Resistance.
•Fault current passing through ground, step and touch potential.
•Capacity of transformer to sustain ground fault current, w.r.t
winding, core burning.
•Relay co-ordination and fault clearing time.
•Standard practice of limiting earth fault current. In case no data
or calculation is possible, go for limiting E/F current to 300A or
500A, depending on sensitivity of relay.
Thank You