Download Issues to Consider when Substituting Large Power Transformers in

Relu Ilie, Israel Electric Corporation, and Isidor Kerszenbaum, Exponent, Inc.

Abstract—For availability reasons, many generating utilities
keep in storage custom designed spare transformers, readily
available and identical to their critical large power transformers.
However, in case an exact replacement is not available, the only
option the generating utility has is to search for a substitute
transformer that, as a minimum, is able to offer a temporary
solution. The purpose of this paper is to indicate the most
important aspects to be considered when checking the
interchangeability of such substitute transformer, based on
authors' experience and on various standards requirements.
transformer (UAT) and the station service/reserve
transformer (SST). A failure of any one of these transformers
may lead to unit shutdown or start-up unavailability.
II. GENERAL LAYOUT
Issues to Consider when Substituting Large
Power Transformers in Generating Stations
The UT may consist of a single three-phase unit, two halfsize three-phase units, or three single-phase units. This is the
most evident aspect to consider when a substitute transformer
is needed.
When designing a new plant, the selection among these
alternatives
is generally based on consideration of some form
Index Terms—power plants, power transformers.
of strategic reserve, as well as available space. Two half-size
transformers
may be selected
in place
Relu
Israel Electric Corporation, and Isidor
Kerszenbaum,
Exponent,
Inc. of a single full-size
I. Ilie,
INTRODUCTION
transformer in order to reduce the cost of the spare. For
HE power transformer is a reliable device, yet not failuresimilar reasons, a generator transformer may be made as a
free. It has no moving parts, consequently neither the
bank of single-phase units. Normally the cost, mass, and loss
typical faults of rotating machines. On the other hand, the
of such a solutions
are larger may
than be
formade
a single
three-phase
reasons,
generator transformer
as a bank
of singleAbstract—For
availability
reasons,
many
generating
utilities
large transformers are oil-immersed and suffer from other
transformer;
however,the
theycost,
maymass,
be preferable
if transport
size
phase
units. Normally
and loss of
such solutions
keep in storage custom designed spare transformers,
faults, mainly chemically or electrically related. A transformer
are
for aapply.
single The
three-phase
transformer; however,
they
readily available and identical to their critical large power
or larger
weightthan
limits
three single-phase
transformers
internal fault
may beinvery
to locate
and to is
repair.
be preferable
if magnetic
transport circuits
size or weight
limitsXI
apply.
The
transformers.
However,
casedifficult
an exact
replacement
not In may
provide
independent
(see Section
below),
many the
cases,
the
ownerthe
may
decide that
it ishas
faster
and
cheaper three single-phase transformers provide independent magnetic
available,
only
option
generating
utility
is to
search
representing high magnetizing impedance for zero-sequence
buy a new
transformer
than
repair an old
damaged
Section XI
below), representing
high magnetizing
for ato
substitute
transformer
that,
as to
a minimum,
is able
to offerone. circuits
voltage (see
components.
Therefore,
a delta equalizer
winding is
Even when
the repair
is worthwhile,
it mayis to
lastindicate
for many impedance for zero-sequence voltage components. Therefore, a
a temporary
solution.
The purpose
of this paper
normally
provided,
implemented
by
external
connection
delta equalizer winding is normally provided, implemented by
the most
important aspects to be considered when checking
months.
betweenconnection
phase units.
between phase units.
the interchangeability
of such
substituteused
transformer,
on are external
Almost all large
transformers
in powerbased
plants
Some
layouts may
maypose
poseother
other
complications,
Some
layouts
complications,
likelike
UATUAT
with
authors’
experience
and
on
various
standards
requirements.
custom-designed. Keeping in storage suitable spare
with three-winding
design.
three-winding
design.
Index Terms—power plants, power transformers.
transformers is a common practice, for the purpose of
avoiding long unplanned
outages and high economic losses. In
I. Introduction
III.DD
imensionsAND
and WEIGHT
eight
III.
IMENSIONS
case
an
exact
replacement
is not device,
available,
option the
HE power transformer is a reliable
yet the
not only
failure-free.
Any physical size and weight limitations should be checked,
physical
size andonweight
limitations
should
be
utilityparts,
has consequently
is to search for
a substitute
transformer
Itgenerating
has no moving
neither
the typical
faults
for Any
example
for installation
an existing
foundation.
Special
checked,
for
example
for
installation
on
an
existing
that, asmachines.
a minimum,
to offer
temporary
solution than installation space restrictions may influence the insulation
of rotating
On is
theable
other
hand, athe
large transformers
foundation. Special installation space restrictions may
are oil-immersed
andsome
sufferoperational
from other constraints.
faults, mainly
chemically
may introduce
The
goal of this clearan
or electrically
A transformer
internal
fault may
be very
paper is related.
to mention
the most
important
aspects
to be influence the insulation clearances and terminal locations on
difficult
to locatewhen
and tochecking
repair. In the
many
cases, the owner of
maysuch
EHV
EHV/HV
EHV
considered
interchangeability
decide that it is faster and cheaper to buy a new transformer than
substitute transformer. The discussion is based on the various
to repair an old damaged one. Even when the repair is worthwhile,
requirements
it may
last for manyincluded
months. in relevant American and European
UT
UT
standards.
Almost
all large transformers used in power plants are customThe
most
common
generating
station
arrangements
are
designed. Keeping in storage suitable spare transformers is a
common
practice,
for the
avoiding long unplanned
shown
in Fig.
1: purpose
a unit of generator-transformer
block
GCB
outages
and high economic
losses.
In case an exact replacement
configuration,
and a unit
generator-transformer
with generator
UAT
SST
UAT
is notbreaker.
available,
only
option
the generating
utility plant
has isare
to the
Thethe
vital
large
transformers
in a power
search
forstep-up/main/generator
a substitute transformer transformer
that, as a minimum,
is able
to
unit
(UT), the
auxiliary
offer a temporary solution than may introduce some operational
MV
MV
MV
constraints. The goal of this paper is to mention the most important
aspects to be considered when checking the interchangeability
a) Unit generator-transformer
b) Unit generator-transformer
configuration
with generator breaker
of such substitute transformer. The discussion is based on the
cesblock
and terminal
locations on the transformer.
various requirements included in relevant American and European
Fig. 1. Common generating station arrangements.
standards.
The most common generating station arrangements are shown
in Fig. 1: a unit generator-transformer block configuration, and
UT and UAT are connected to the generator through isolated
a unit generator-transformer with generator breaker. The vital
phase bus ducts. The high/extra high voltage terminals of the
large transformers in a power plant are the unit step-up/main/
UT may be connected to gas insulated switchgear. The medium
generator transformer (UT), the auxiliary transformer (UAT) and
voltage connections to plant auxiliaries are also normally done via
the station service/reserve transformer (SST). A failure of any
rigid, non-segregated phase bus bars or cables. All these aspects
one of these transformers may lead to unit shutdown or start-up
must be checked and solved when looking for a transformer
unavailability.
replacement.
T
T
II. General Layout
The UT may consist of a single three-phase unit, two halfsize three-phase units, or three single-phase units. This is the
most evident aspect to consider when a substitute transformer is
needed.
When designing a new plant, the selection among these
alternatives is generally based on consideration of some form
of strategic reserve, as well as available space. Two half-size
transformers may be selected in place of a single full-size
transformer in order to reduce the cost of the spare. For similar
IV. Rated Frequency
In the unlikely situation a transformer designed for a different
frequency is considered, the following applies. The rated frequency
radically affects the transformer design and operation. The general
formula of voltage e induced by a variable flux φ in a coil with N
turns is e = -N dφ/dt. Assuming a sinusoidal flux φ = Φm cos ωt, the
induced voltage becomes e = ω N Φm sin ωt. Its rms value will be,
in terms of core cross section A and flux density Bm [1]:
E = 2π/√2 f N Φm = 4.44 f N A Bm.
2013 ‫ אוגוסט‬,46 ‫גליון‬
(1)
62
According to (1), operating at 50 Hz a transformer designed for
60 Hz means that the same voltage can be achieved only by a
substantial increase in the flux density. The core iron will become
heavily saturated, the excitation current will rise and also the
hysteresis losses (proportional to the area of the hysteresis loop),
which could severely overheat and damage the laminations. A 50
Hz transformer needs roughly 12% more iron in its core than a 60
Hz unit, and also more winding turns.
When operating frequency is 60 Hz in contrast to a 50 Hz
design, there is too much iron in the core. The hysteresis losses
will be higher than the designed value (because iron volume and
increased frequency), thus decreasing the efficiency. More importantly, the eddy currents losses will heat up the laminations,
because they tend to increase as the square of the frequency. Operation of a 50 Hz rated transformer at 60 Hz may be sometime
possible, but it may have to be derated from its nameplate MVA
rating [2], depending on the new versus rated voltage.
V. MVA Rating
The substitute transformer rated MVA is obviously a first parameter to consider. When a new plant is designed, the UT rating
is chosen to ensure that it will not represent a bottleneck of unit
capability, under any possible operating condition. However, a
substitute UT poses a different perspective: using a smaller MVA
rating than the original one means generating unit derating, nevertheless it is normally preferable to a complete shut-down, taking into consideration the long time needed to obtain an exact
replacement.
In the case of a transformer (especially UT) substitution, it
is essential to pay attention to the differences among the various
standards about the definition of rated power. The IEC 60076-1
[3] definition implies that rated MVA is the apparent input power,
received when rated voltage is applied to the primary winding and
rated current flows through the terminals of the secondary winding; the output power is, in principle, the rated power minus the
power consumption in the transformer (active and reactive losses).
Differently, by the North America conventions (IEEE C57.12.80
[4]), the rated MVA is the output power that can be delivered at
rated secondary voltage.
If the transformer nameplate mentions a certain rated power and
if it was designed in conformity with the American standards, the
significance is that its primary (connected to the generator) would
be able to receive a higher than rated power. If for instance, the
UT impedance is 15%, it should be able to accept a primary MVA
higher up to about 15% than the rated one, including load and magnetizing losses (in the worst conditions of lagging power factors).
Exact values can be obtained by using the equivalent circuit of the
transformer. Contrarily, such intrinsic capability will not be available in transformer exhibiting the same rated MVA, but designed
to IEC requirements. The situation inverts under leading power
factors (Mvar absorbed from the system) but this is a less likely
regime, especially when a substitute transformer is involved.
A transformer may be able under some limitations to carry
loads in excess of nameplate rating. In the case of a substitute
transformer, the expected regime is a long-time emergency loading that may persist for months. Both IEEE and IEC have standards specially dedicated to such loading aspects (IEC 60076-7
[5], IEEE C57.91 [6]). The application of a load in excess of
nameplate rating involves accelerated ageing and risk of premature failure, for instance: deterioration at high temperatures of
conductor insulation, other insulation parts and oil, overheating of
metallic parts, increased gassing in the oil, high stresses in bushings, tap-changers, connections and current transformers, brittleness of gasket materials. In general, the larger the transformer the
more vulnerable.
Both above mentioned standards give similar maximum
61
2013 ‫ אוגוסט‬,46 ‫גליון‬
temperature limits that should not be exceeded in case of long–
time loading beyond the nameplate rating: current 1.3 per-unit,
winding hot-spot temperature 140 ºC (instead of 120 ºC at
normal loading), top-oil temperature 110-115 ºC (instead of 105
ºC at normal loading). The increased ageing rate due to a hotspot temperature of 140 ºC is 17.2 times higher than normal for
upgraded paper insulation and 128 times higher than normal for
non-upgraded paper insulation [5].
VI. Rated Voltages
Finding a substitute transformer with suitable primary and
secondary rated voltages is a challenging task, difficult to be selected intuitively. Additionally, a transformer design for a certain
voltage determines the size of the core and has a significant impact on the overall transformer size and cost.
The system and transformer medium/high/extra high voltage ratings are well standardized and correlated in ANSI C84.1
[7] and IEC 60038 [8]. However, the generators standards do not
specify standard series, neither preferable values for the rated stator voltage. The generator stator voltage rating is normally fixed
by agreement, and in many cases it is simply the generator manufacturer decision, according to its available design. Therefore, it
is quite difficult to match between UT/UAT primary rated voltage
and generator rated voltage.
Section VII deals in details with transformers having rated
voltages lower than expected operational voltages. Contrarily, it
may be possible to use transformers with rated voltages higher
than the operational ones, providing that the rated current will not
be exceeded. Practically in such case the substitute transformer
might be oversized and then unsuitable, because of larger core
size and longer insulation distances.
Ideally, it is desirable for a generator to be able to absorb Mvar
to its limit when the system voltage is at its highest expected level
and to produce Mvar to its limit, when the system voltage is at
its lowest expected level. This is seldom possible for a fixed tap
setting in the UT, and thus a compromise may have to be made
by selecting the appropriate tap rating to meet the most likely operating condition. It is important to note that the reactive power
consumed by the UT will absorb a significant part of the generator
Mvar output under most conditions.
IEEE C57.116 [9] recommends selecting the main parameters
of UT (rated voltages, rated MVA, impedance, and over-excitation) by portraying graphically their effect under various operation conditions. While this method is mainly dedicated to a new
plant project, it may be also used in case of a substitute transformer. A typical graph (Fig. 2) shows the change in generator voltage
with generator reactive load, for various constant transmission
system voltages. The graph allows predicting the Mvar capability
(both lead and lag) at any given system voltage, keeping the ±5%
generator voltage limits, as required by IEEE C50.12 [10], IEEE
C50.13 [11] and IEC 60034-1 [12].
The example in Fig. 2 was built on a spreadsheet using equations
based on transformer phasors diagram [9]. For a potential substitute
transformer, such graphs should be drawn for any available tap/ratio
at anticipated MW, and analyzed in order to choose the most suitable
tap voltages (according to the forecasted system voltage profile) that
will pose minimum restrictions to unit operation: range of available
Mvar, synchronization ability, etc. Of course, a substitute transformer
may not allow a full range of loading; normally some limitations (e.g.
in reactive capabilities) may be acceptable.
References [13] and [14] propose other graphs, more complete
but also more complex, which take in consideration additional
restrictions, such as: generator maximum excitation limit,
generator under-excited reactive ampere limit, UT limits (at lower
than rated tap voltage), turbine MW limit, and auxiliary bus-bars
(motors) voltage limits. Such graph shows the reactive power
33
UT
UTRatio
Ratio2222/ 399
/ 399kV
kV(Tap
(Tap5),5),Generator
GeneratorNet
NetOutput
Output576
576MW
MW
UT
UTRating
Rating650
650MVA,
MVA,UT
UTImpedance
Impedance17%,
17%,Generator
GeneratorRated
RatedVoltage
Voltage2222kVkV
Generator Voltage (per-unit)
Generator Voltage (per-unit)
1.05
1.05
1.04
1.04
1.03
1.03
1.02
1.02
1.01
1.01
1.00
1.00
0.99
0.99
420
420
kVkV
System
System
Voltages:
Voltages:
412
kVkV
412
410
kVkV
410
408
408
kVkV
406
kVkV
406
400
400
kVkV
380
380
kVkV
0.98
0.98
0.97
0.97
0.96
0.96
0.95
0.95
-100
-100-80-80-60-60-40-40-20-20 0 0 2020 4040 6060 8080 100
100120
120140
140160
160180
180200
200220
220240
240260
260280
280300
300
Generator
GeneratorReactive
ReactivePower
PowerOutput
OutputNet
NetValue
Value
Transferred
Transferredtoto(+)
(+)ororfrom
from(-)(-)UT
UT(Mvar)
(Mvar)
is not meant to be systematically utilized in normal service, but
should
be reserved
relatively
rare
casesother
of emergency
References
References
[13]
[13]for
and
and
[14]
[14] propose
propose
other
graphs,
graphs,service
more
more
under limited
periods
ofcomplex,
time. IEEE
C57.12.00
defines the
complete
complete
but
butalso
also
more
more
complex,
which
which
take
takeinin[16]
consideration
consideration
over-fluxrestrictions,
capability
insuch
asuch
different
way: secondary
voltage
and V/
additional
additional
restrictions,
as:
as:generator
generator
maximum
maximum
excitation
excitation
Hz up to 105% of rated values when load power factor is 0.80
limit,
limit,
generator
generatorunder-excited
under-excitedreactive
reactiveampere
amperelimit,
limit,UT
UTlimits
limits
or higher (lagging). Taking into account the previous consider(at
(at
lower
lower
than
than
rated
rated
tap
tap
voltage),
voltage),
turbine
turbine
MW
MW
limit,
limit,
and
and
ations, this additional requirement may result in 10-15% primary
auxiliary
auxiliary
bus-bars
bus-bars
(motors)voltage
voltagefor
limits.
limits.
Such
Such
graph
graph
shows
shows
overvoltage
(and (motors)
over-excitation)
a fully
loaded
generator
step-up
transformer
built
under IEEE.
Fortunately,
generators
are
the
the
reactive
reactive
power
powertransferred
transferred
to/from
to/from
the
thegrid
grid(at
(atUT
UThigh
high
only capable
of
operation
5% above
or
5% below
voltage
voltage
side)
side)as
asacontinuous
afunction
functionof
ofsystem
systemuntil
voltage,
voltage,
and
andforecast
forecast
the
the
their
rated
voltage,operation.
by [10]-[12]
(i.e.graph
V/Hz can
until
105%
atobtained
generator
area
area
ofof
allowable
allowable
operation.
This
This
graph
canbe
bealso
also
obtained
base) so the above overvoltage capabilities are rarely exploited.
ininFor
aaspreadsheet
spreadsheet
using
using
suitable
suitable
equations
equations
(Fig.
(Fig.
3).
3).
UAT and SST the impedance is usually smaller than for UT,
they often work not fully loaded and the primary voltage will norVII.
VERVOLTAGE
VERVOLTAGE
LLIMITS
IMITS
mally not rise byVII.
moreOO
than
a few percentage
points for a secondary
increase
of
5%.
The
The purpose
purpose ofof this
this section
section isis toto show
show how
how low
low the
the
Another aspect related
to an
overvoltage
isorder
whether
substitute-transformer
substitute-transformer
rated
rated
voltages
voltages
may
maycondition
be
be inin order
toto
the UT and UAT will be subjected to load rejection. Sudden loss
withstand
withstand
the
thehighest
highestexpected
expectedvoltages
voltageson
onitsitsterminals.
terminals.
of load can subject these transformers to substantial overvoltage.
The
The
standards
standards
define
define
the
the
maximum
maximum
winding
winding
voltage
voltage
based
If saturation occurs, substantial exciting current will
flow,based
which
on
on
the
the
insulation
insulation
withstand
withstand
capability
capability
(in
(in
IEC
IEC
60076-3
60076-3
[15]
[15]
itit
may overheat the core and damage the transformer. A sudden
isisunit
called
called
the
thehighest
highest
for
forequipment,
equipment,
while
ininANSI
ANSI
unloading
duringvoltage
avoltage
fault may
be
caused bywhile
the
clearing
of a
C84.1
C84.1
[7]
[7]
ititisisand,
the
themaximum
maximum
systemvoltage).
voltage).
system
fault
hence,
the system
machine
may be at the ceiling of its
excitation
system;
the
may bethe
excited
with and
voltAccording
According
toto (1),
(1),unit
the
thetransformer
ratio
ratio between
between
the
voltage
voltage
and
ages exceeding
130%
of system
normal
[18],
[19].the
With
the excitation
frequency
frequency
(V/Hz)
(V/Hz)
ofofthe
the
systemto
towhich
which
thetransformer
transformer
isis
control indetermines
service,
thethe
over-excitation
will
generally
be
reduced
connected
connected
determines
thenominal
nominalflux
flux
density
density
atatwhich
which
the
the
to safe limits
in a few(assuming
seconds;
with
thenumber
excitation
transformer
transformer
operates
operates
(assuming
the
the
number
ofofcontrol
turns
turns out
atat aofa
service, the over-excitation may be sustained and damage can ocparticular
particular
tap
tap
will
will
remain
remain
constant).
constant).
Normally,
Normally,
the
the
transformer
transformer
cur (unless dedicated V/Hz protection exists). Both IEC 60076-1
isis[3]
economically
economically
designed
designedto[16]
tooperate
operate
atasashigh
highasasoperation
possible
possibleflux
and IEEE C57.12.00
allowatcontinuous
atflux
no
density,
density,
while
while
avoiding
avoiding
saturation
saturation
of
of
the
the
core.
core.
System
System
load at a voltage or V/Hz up to 110% of the rated values. For the
frequency
frequency
normally
normally
controlled
controlled
within
withinclose
close
limits;
limits;
thus,
thus,the
the
particularisis
case
of transformers
connected
directly
to generators
system
systemvoltage
voltageisisthe
themain
mainfactor
factorresponsible
responsiblefor
forover-fluxing
over-fluxing
System
SystemVoltage
Voltage(kV)
(kV)
320
320
1200
1200
1100
1100
1000
1000
900
900
Reactive Power Transferred to (+) or from (-) System (Mvar)
Reactive Power Transferred to (+) or from (-) System (Mvar)
between
betweenUT/UAT
UT/UATprimary
primaryrated
ratedvoltage
voltageand
andgenerator
generatorrated
rated
voltage.
voltage.
transferred to/from the grid (at UT high voltage side) as a function
Section
Section
VII
VIIdeals
deals
indetails
detailsthe
with
with
transformers
transformers
having
having
rated
rated
of system
voltage,
andinforecast
area
of allowable
operation.
This
graph
can be
also
obtainedoperational
in a spreadsheet
using
suitable
voltages
voltages
lower
lower
than
than
expected
expected
operational
voltages.
voltages.
Contrarily,
Contrarily,
equations
(Fig.
3).
it
it may
may be
be
possible
possible
toto use
use transformers
transformers with
with rated
rated voltages
voltages
higher
higher than
than the
the operational
operational ones,
ones, providing
providing that
that the
the rated
rated
Overvoltage
Limits inin such
current
current will
will not
notVII.
bebe exceeded.
exceeded.
Practically
Practically
such case
case the
the
The purpose of this section is to show how low the substitutesubstitute
substitute
transformer
transformermight
mightbe
beoversized
oversizedand
andthen
thenunsuitable,
unsuitable,
transformer rated voltages may be in order to withstand the highbecause
because
ofoflarger
largercore
core
sizeterminals.
and
andlonger
longerinsulation
insulationdistances.
distances.
est
expected
voltages
onsize
its
Ideally,
Ideally,
it
it
is
is
desirable
desirable
for
for
a
a
generator
generator
to
to
be
be
able
able
totoabsorb
absorb
The standards define the maximum winding voltage
based
Mvar
Mvar
to
to
its
its
limit
limit
when
when
the
the
system
system
voltage
voltage
is
is
at
at
its
its
highest
highest
on the insulation withstand capability (in IEC 60076-3 [15] it is
expected
expected
level
leveland
and toto produce
produce
Mvar
Mvarwhile
toto itsits
limit,C84.1
when
when[7]
the
the
called
the highest
voltage
for
equipment,
in limit,
ANSI
itsystem
is the maximum
voltage).
system
voltage
voltageisissystem
atatitsitslowest
lowestexpected
expectedlevel.
level.This
Thisisisseldom
seldom
According
the tap
ratio
between
the
voltage
and
frequency
possible
possible
for
for to
aa (1),
fixed
fixed
tap setting
setting in
in the
the UT,
UT,
and
and
thus
thus aa
(V/Hz)
of themay
system
to to
which
the transformer
isthe
connected
decompromise
compromise
mayhave
have
tobe
bemade
made
by
byselecting
selecting
theappropriate
appropriate
termines
thetoto
nominal
flux
density
at which
the transformer
opertap
taprating
rating
meet
meetthe
themost
mostlikely
likely
operating
operating
condition.
condition.
ItItisis
ates
(assuming
the that
number
turns atpower
a particular
tap will
important
important
totonote
note
thatthe
theof
reactive
reactive
power
consumed
consumed
by
byremain
the
theUT
UT
constant). Normally, the transformer is economically designed to
will
will
absorb
absorb
a
a
significant
significant
part
part
of
of
the
the
generator
generator
Mvar
Mvar
output
output
operate at as high as possible flux density, while avoiding saturaunder
under
most
conditions.
conditions.
tion
ofmost
the
core.
System frequency is normally controlled within
IEEE
IEEE
C57.116
C57.116
[9]
[9] recommends
recommends
selecting
the
the
main
main
close limits; thus, the system
voltage is theselecting
main factor
responparameters
parameters
ofofUT
UT(rated
(rated
voltages,
voltages,rated
rated
MVA,
MVA,
impedance,
and
and
sible
for over-fluxing
(over-excitation).
When
the impedance,
V/Hz
ratios are
over-excitation)
over-excitation)
by
byofportraying
portraying
graphically
graphically
theireffect
effectunder
under
exceeded,
saturation
the magnetic
core of thetheir
transformers
may
occur,
and
stray fluxconditions.
may be induced
inthis
non-laminated
various
various
operation
operation
conditions.
While
While
this method
method isiscompomainly
mainly
nents
that to
are
designed
to carry
flux.
This
canused
cause
severe
dedicated
dedicated
toaanot
new
new
plant
plantproject,
project,
ititmay
maybe
bealso
also
used
inincase
caseofof
overheating
in theAtransformer
and eventual
alocalized
asubstitute
substitute
transformer.
transformer.
Atypical
typicalgraph
graph
(Fig.
(Fig.2)2)breakdown
shows
showsthe
the
of
the
core
assembly
and/or
winding
insulation.
change
changeiningenerator
generatorvoltage
voltagewith
withgenerator
generatorreactive
reactiveload,
load,for
for
As mentioned before, the IEC 60076-1 [3] definitions imply
various
various
constant
constant
transmission
transmission
system
system
voltages.
voltages.
The
The
graph
graph
that rated voltage is applied to primary winding, and the voltage
allows
allowsthe
predicting
predicting
the
Mvar
Mvarcapability
capability
(both
leadand
and
lag)
lag)atat
across
secondarythe
terminals
differs
from(both
rated lead
voltage
(defined
any
any
given
given
system
system
voltage,
voltage,
keeping
keeping
the
the
±5%
±5%
generator
generator
voltage
voltage
in no-load condition) by the voltage drop/rise in the transformer.
limits,
limits,asasrequired
required
by
byIEEE
IEEE[16]
C50.12
C50.12
[10],
[10],
IEEE
IEEE
C50.13[11]
[11]
Contrarily,
by
IEEE C57.12.00
the rated
output
isC50.13
delivered
at
and
andIEC
IEC
60034-1
60034-1
[12].
[12].according to IEEE definition, allowance
rated
secondary
voltage;
for voltage
drop hasinin
toFig.
be
made
in the
design
that
the necessary
The
Theexample
example
Fig.
22was
was
built
built
on
onasoaspreadsheet
spreadsheet
using
using
primary
voltage
can
betransformer
applied
to thephasors
transformer
(at a secondary
equations
equations
based
based
on
on
transformer
phasors
diagram
diagram
[9].
[9].For
Foraa
load
lagging
power factor
of 0.80 such
orsuch
higher).
For
instance,
adrawn
UT
potential
potential
substitute
substitute
transformer,
transformer,
graphs
graphs
should
should
be
bedrawn
with
a
impedance
of
15%
(typical
range:
13–17%)
designed
ac-inin
for
forany
anyavailable
availabletap/ratio
tap/ratioatatanticipated
anticipatedMW,
MW,and
andanalyzed
analyzed
cording to IEEE shall withstand continuously primary voltages
order
order
to
to
choose
choose
the
the
most
most
suitable
suitable
tap
tap
voltages
voltages
(according
(according
toto
as high as 110% of the rated value when fully loaded at a power
the
the
forecasted
forecasted
system
system
voltage
voltage
profile)
profile)
that
that
will
will
pose
pose
minimum
minimum
factor of 0.80 (value calculable using the transformer equivalent
restrictions
restrictions
to unit
unit
operation:
operation:
range
range
ofof available
available
Mvar,
Mvar,
circuit).
Such to
hidden
capability
will not
be available
in the case
of
synchronization
ability, the
etc.
etc.
Of course,
course, aa substitute
substitute
asynchronization
transformer whichability,
follows
IECOf
requirements.
transformer
transformer
may
not
not60076-1
allow
allowa[3],
afull
full
rangeofofloading;
loading;
normally
Accordingmay
to IEC
a range
transformer
shall benormally
capable
ofsome
continuous
operation
up to
voltagecapabilities)
or V/Hz (the standard
some
limitations
limitations
(e.g.
(e.g.
inin105%
reactive
reactive
capabilities)
may
may be
be
meaning
is per each winding). IEC 60076-8 [17] adds that this
acceptable.
acceptable.
800
800
340
340
350
350
360
360
370
370
Generator
GeneratorOverexcited
OverexcitedLimit
Limit
Generator
Power
GeneratorUnity
Unity
PowerFactor
Factor
GeneratorUnderexcited
UnderexcitedLimit
Limit
Generator
GeneratorVoltage
Voltage105%
105%
Generator
700
700
GeneratorVoltage
Voltage100%
100%
Generator
600
600
GeneratorVoltage
Voltage95%
95%
Generator
500
500
Motors
MotorsVoltage
Voltage110%
110%
400
400
Motors
MotorsVoltage
Voltage90%
90%
300
300
UTUTLimits
Limits
200
200
100
100
00
-100
-100
-200
-200
-300
-300
-400
-400
-500
-500
-600
-600
-700
-700
-800
-800
-900
-900
-1000
-1000
-1100
-1100
-1200
-1200
Fig.
Fig.2.2. Change
Changeiningenerator
generatorvoltage
voltagewith
withgenerator
generatorreactive
reactiveload
loadfor
forvarious
various
system
systemvoltages.
voltages.
330
330
UNIT
UNITOPERATION
OPERATIONLIMITS
LIMITS
(shaded
(shadedarea)
area)
947
947
MVA
MVA UTUT
Rated
Rated
Power
Power
23.13
23.13
kVkV UTUT
Primary
Primary
Voltage
Voltage
345
345
kVkV UTUT
Secondary
Secondary
Voltage
Voltage
9.17%
9.17% UTUT
Impedance
Impedance
1.585
1.585
kAkA UTUT
Tap
Tap
Current
Current
1006
1006
MVA
MVAGen.
Gen.
Rated
Rated
Power
Power
854.9
854.9
MW
MWGen.
Gen.
Active
Active
Power
Power
530
530
Mvar
MvarGen.
Gen.
Overex.
Overex.
Limit
Limit
-346
-346
Mvar
MvarGen.
Gen.
Underex.
Underex.
Limit
Limit
2424
kVkV Gen.
Gen.
Rated
Rated
Voltage
Voltage
6060
MVA
MVA UAT
UAT
Rated
Rated
Power
Power
2424
KVKV UAT
UAT
Primary
Primary
Voltage
Voltage
7.07
7.07
KVKV UAT
UAT
Secondary
Secondary
Voltage
Voltage
7.50%
7.50% UAT
UAT
Impedance
Impedance
42.42
42.42
MW
MWUnit
Unit
Aux.
Aux.
Active
Active
Load
Load
30.71
30.71
Mvar
Mvar
Unit
Unit
Aux.
Aux.
React.
React.
Load
Load
6.66.6
kVkV Motors
Motors
Rated
Rated
Voltage
Voltage
330
330
kVkV Syst.
Syst.
Volt.
Volt.
Lowest
Lowest
Limit
Limit
362
362
KVKV Syst.
Syst.
Volt.
Volt.
Highest
Highest
Limit
Limit
Fig.3
Fig.3 Reactive
Reactivepower
powertransferred
transferredto/from
to/fromsystem
systemasasa afunction
functionofofsystem
system
voltage
voltage(operation
(operationarea
arealimit).
limit).
2013 ‫ אוגוסט‬,46 ‫גליון‬
60
4
(over-excitation). When the V/Hz ratios are exceeded,
saturation of the magnetic core of the transformers may occur,
and
stray
fluxthat
may
be may
induced
in non-laminated
components
in
such
a way
they
be subjected
to load rejection
(i.e.
that are not designed
to carrybreaker),
flux. This
canancause
severe
configurations
without generator
[3] has
additional
localized overheating
in the 1.4
transformer
and eventual
requirement:
to be able to withstand
times rated primary
voltage
for 5 seconds.
breakdown
of the core assembly and/or winding insulation.
As mentioned before, the IEC 60076-1 [3] definitions
Connection
VIII.that
Arrangement
Phase Dwinding,
isplacement
imply
rated voltage
is applied and
to primary
and the
Normally the high voltage system is grounded, leading the
voltage across the secondary terminals differs from rated
UT’s high voltage windings to be star connected, with the neutral
voltage
(defined
in no-load
condition)
byisthe
voltage
drop/rise
often
solidly
grounded.
The reason
for this
mainly
related
to
in the transformer.
Contrarily,
by IEEE
C57.12.00
[16] the
equipment
insulation level
and system
protection
requirements.
ratedsignificant
output is delivered
at rated
secondary
voltage; according
The
consequence
as regards
the transformer
high
voltage
is the
possibility
use non-uniform,
to IEEEwindings
definition,
allowance
fortovoltage
drop has tocheaper
be made
insulation
systems
neutral
terminal
insulation
is designed
in the design
so (i.e.
thatthethe
necessary
primary
voltage
can be
with
a lower
insulation
level than
for the
line
terminals).
applied
to the
transformer
(at aassigned
secondary
load
lagging
power
On the other hand, it is convenient to have the low voltage windings
factor of 0.80 or higher). For instance, a UT with a impedance
delta connected (the delta circuit provides a path for third-order
of 15% (typical
range: 13–17%)
designed
according
to IEEE
harmonics
of the magnetizing
currents,
thus reducing
the voltage
shall withstand
continuously
primary
voltages
high as
waveform
distortion;
it also stabilizes
the neutral
pointas
potential
110%
of
the
rated
value
when
fully
loaded
at
a
power
factor
in the case it is left ungrounded). The most common connection
/ofangular
used for
UTs is YNd1
[9],
0.80 (phase)
(value displacement
calculable using
thelarge
transformer
equivalent
[14],
however
is also
encountered
4). available in the
circuit).
SuchYNd11
hidden
capability
will (Fig.
not be
Forofsimilar
reasons,which
commonly
thethe
UAT
windings
case
a transformer
follows
IECprimary
requirements.
(connected
to
the
generator
side)
are
delta
connected,
while
the be
According to IEC 60076-1 [3], a transformer shall
secondary ones are star connected through a current-limiting
capable
of
continuous
operation
up
to
105%
voltage
or
V/Hz
resistor. The most common connections used for UAT are Dyn11
(the
standard
meaning
is
per
each
winding).
IEC
60076-8
[17]
[14] or Dyn1 [9] (Fig. 5). According to IEEE C57.12.00 [16], in
adds
that
this
is
not
meant
to
be
systematically
utilized
Yd or Dy transformers the low voltage shall lag the high voltage in
normal
but should
relatively matches
rare cases
by
30°; service,
the connection
Yd1beforreserved
step-upfor
transformer
this
standard, while
Dy11under
for step-down
does not;
of emergency
service
limited transformers
periods of time.
IEEE
however,
a standard
Dy1 the
UATover-flux
with phase
sequenceinexternally
C57.12.00
[16] defines
capability
a different
reversed
on both sides,
as explained
equal of
to Dy11.
The
way: secondary
voltage
and V/Hzbelow,
up tois105%
rated values
IEC standards do not have such restrictions.
when
load
power
factor
is
0.80
or
higher
(lagging).
Taking
If the UT is YNd1 and the UAT is Dyn11, the medium voltage
into account
this the
additional
auxiliary
systemthe
has previous
zero-phase considerations,
shift compared with
high/
requirement
may
result
in
10-15%
primary
overvoltage
(and
extra high voltage system (Fig. 1a). During unit start-up or shutover-excitation)
a busses
fully fed
loaded
generator
down,
the medium for
voltage
from the
UAT and step-up
SST
secondary
windings
are briefly
(bygenerators
the fast transfer
transformer
built under
IEEE. paralleled
Fortunately,
are only
scheme),
so continuous
both must be
in phase.until
Additionally,
capable of
operation
5% abovetheorhigh
5% and
below
extra
systems
always (i.e.
in phase
so the
SST105%
must at
their high
ratedvoltage
voltage,
by are
[10]-[12]
V/Hz
until
produce zero-phase displacement and therefore, usually it is a
generator base) so the above overvoltage capabilities are
star-star as YNyn0 transformer. In same cases it will have a deltararely exploited.
For UAT
thementioned
impedance
is usually
connected
tertiary winding,
forand
the SST
reasons
above.
For
smaller
than
for
UT,
they
often
work
not
fully
loaded
and the
modern transformers, it is matter of the grid configuration and/or
primary voltage
will whether
normallythenot
by morewith
than
a few
protection
requirements
SSTrise
is provided
a delta
tertiary
or not.
percentage
points for a secondary increase of 5%.
The
connection
configurations
groups condition
are drawn is
Another
aspect
related to and
an phasor
overvoltage
in
Fig. 4 and
according
to IEC
conventions
[3].rejection.
Since
whether
the 5UT
and UAT
will60076-1
be subjected
to load
that standard mentions that terminal marking on the transformers
Sudden loss of load can subject these transformers to
follows national practice, Fig. 4 and 5 use the American practice
overvoltage.
If saturation occurs, substantial
-substantial
IEEE C57.12.70
[20].
exciting
current
will
flow,
whichbemay
overheat
the core and
If the original transformer should
replaced
by a non-identical
damage the
transformer.
A sudden connections
unit unloading
substitute,
matching
the three-phase
and during
phase- a
angle
be a by
complicated
or even
impossible
task.
fault relations
may be may
caused
the clearing
of an
a system
fault
and,
Normally
it ismachine
not possible
substitute
UT or UAT
having
the
hence, the
mayto be
at the aceiling
of its
excitation
vector
group
11 with a vector
transformer
system;
thenumber
unit transformer
maygroup
be number
excited 1with
voltages
(or
the opposite);
explained
above:thethese
two
exceeding
130% the
of reason
normalwas
[18],
[19]. With
excitation
transformers link between two rigid phasor systems. Only in the
control
in equipped
service, with
the generator
over-excitation
will1b),
generally
case
of a unit
breaker (Fig.
it may be be
reduced
to
safe
limits
in
a
few
seconds;
with
the
excitation
possible to use a YNd11 UT instead of a YNd1 one (or contrary)
control out
service,
over-excitation
may
be sustained
assuming
no of
rapid
transferthe
is performed
to other
auxiliary
busand However,
damage such
can substitution
occur (unless
dedicated
V/Hz protection
bars.
will affect
the secondary
circuits
(at
least the
differential
protection)
and
changes
will be required
exists).
Both
IEC 60076-1
[3] and
IEEE
C57.12.00
[16] allow
in
relays settings
and/or at
matching
transformers.
continuous
operation
no loadbyatintermediate
a voltage or
V/Hz up to
It110%
is recommended
to check
alsoFor
any the
potential
influence
of the rated
values.
particular
caseon of
synchronizing circuits; it is a good practice to use supervision
sync-check relay with two or three phase-sensing circuits.
59
2013 ‫ אוגוסט‬,46 ‫גליון‬
transformers connected directly to generators in such a way
that they may be subjected to load rejection (i.e.
configurations
generator
breaker),
[3] has
Theoretically, itwithout
is possible
to keep the
original vector
group an
additional
to betransformer.
able to withstand
1.4 times
rated
while usingrequirement:
a different group
For example,
an UAT
primary
voltage
seconds.
with vector
groupfor
11 5may
be used in place of an original device
with vector group 1 (or inverse), by reversing the phase sequence
on both VIII.
sides of
the transformer.
Such changeAND
is shown
ARRANGEMENT
PHASEin Fig.
CONNECTION
6a: as viewed from theDexternal
line connections, the Dy11
ISPLACEMENT
transformer became a Dy1 one. Unfortunately, the rigid isolated
Normally
voltage side
system
grounded, leading
phase
bus barsthe
on high
the generator
and is
non-segregated
bars onthe
medium
voltage
side windings
will not allow
UT’s
high
voltage
to besuch
starcross-connections
connected, withinthe
most cases.
neutral
often solidly grounded. The reason for this is mainly
The substitution may be complicated by the transformer layout
related to equipment insulation level and system protection
terminal marking and sequence. According to IEEE C57.12.70
requirements.
Thearesignificant
consequence
as H1
regards
[20], the terminals
marked as in
Fig. 7a, i.e. the
lead isthe
transformer
voltage windings
is seen
the possibility
brought out ashigh
the right-hand
terminal as
when facingtotheuse
high voltage side.
Other countries
use different
non-uniform,
cheaper
insulationmay
systems
(i.e. standards;
the neutral
for instance,
the English
practice with
is to locate
theinsulation
high voltage
terminal
insulation
is designed
a lower
level
terminals from left to right when facing that side [21]; the German
than assigned for the line terminals). On the other hand, it is
DIN 42402 rule is the terminals are arranged from right to left as
convenient
lowside
voltage
windings
delta
connected
viewed fromto
thehave
low the
voltage
[22] (Fig.
7b). For
instance,
if
(the
delta
circuitaccording
providestoa the
path
for third-order
harmonics
an UAT
designed
German
standard with
a vector of
group
Dy11 is installed
in thus
placereducing
of an original
UAT designed
the
magnetizing
currents,
the voltage
waveform
according toitanalso
American
one, without
any change
in the external
distortion;
stabilizes
the neutral
point potential
in the
connections, it will externally appear as a Dy1 transformer (Fig.
case
it is left ungrounded). The most common connection /
6 b).
angular
(phase)
displacement
used
for and
largeterminal
UTs is sequence
YNd1 [9],
Taking
in account
both vector
group
[14],
however
is also
encountered
(Fig. 4).
aspects,
an Yd1YNd11
American
transformer
is interchangeable
with a
European
Yd11,reasons,
without any
external modifications.
For similar
commonly
the UAT primary windings
(connected to the generator side) are delta connected, while
IX. Tare
aps and Tap-Changer
the secondary ones
star connected through a currentThe original transformer may be equipped with on-load taplimiting
resistor.
The
most
common
connections
usedforfor
changer or de-energized tap-changer.
Normally
a substitution
UAT
are period
Dyn11of[14]
Dyn1
[9] (Fig.
5). According
to will
IEEE
a limited
timeorwith
a fixed
turns-ratio
transformer
be possible in[16],
an emergency.
choosing the most
tap
C57.12.00
in Yd orWhen
Dy transformers
the suitable
low voltage
of thelag
substitute
transformer,
is indispensable
to check
shall
the high
voltage byit 30°;
the connection
Yd1whether
for stepit istransformer
a full-power matches
tapping (e.g.
for awhile
current
equalfor
to the
up
thissuitable
standard,
Dy11
steprated power divided by the tap voltage).
down transformers does not; however, a standard Dy1 UAT
H0 H1 H2 H3
H1
H0 H1 H2 H3
H1
SYSTEM SIDE
H0
X1 X2 X3
H3
H0
H2
X1
X3
GENERATOR
SIDE
X1 X2 X3
H3
X1
H2
X2
X2
X3
a) YNd1
b) YNd11
Fig. 4 Most common connection/phase displacement used for UTs.
H1 H2 H3
H1
H1 H2 H3
H1
GENERATOR
SIDE
X0 X1 X2 X3
H3
X1
X0
H2
X2
X0 X1 X2 X3
H3
AUXILIARIES
SIDE
X3
X3
a) Dyn11
X1
X0
X2
b) Dyn1
Fig. 5 Most common connection/phase displacement used for UATs.
H2
sides, as
ds do not
medium
ared with
unit startfrom the
lleled (by
n phase.
stems are
ero-phase
as YNyn0
connected
or modern
on and/or
ed with a
oups are
nventions
ng on the
5 use the
by a nontions and
even an
tute a UT
a vector
ason was
ween two
pped with
a YNd11
no rapid
However,
t least the
in relays
mers. It is
uence on
pervision
cuits.
al vector
example,
ace of an
reversing
mer. Such
ernal line
Dy1 one.
s on the
m voltage
ses.
ansformer
to IEEE
7a, i.e. the
een when
different
ocate the
that side
5
[21]; the German DIN 42402 rule is the terminals are arranged
from right to left as viewed from the low voltage side [22]
C57.12.00if [16],
whenever
transformerto is
(Fig.By7b).IEEE
For instance,
an UAT
designeda according
the
provided standard
with de-energized
taps, group
they shall
be isfullinstalled
capacityin
German
with a vector
Dy11
taps. Transformers with on-load tap-changer shall be capable of
place
of anrated
original
according
American
delivering
MVAUAT
at thedesigned
rated voltage
and ontoallantaps
above
one,
without
any
change
in
the
external
connections,
will
rated voltage. However, for taps below the rated voltage, they itshall
externally
a Dy1 transformer
(Fig.related
6 b). to the rated
be capableappear
of just as
delivering
the rated current
voltage
(i.e.
taps both
may be
of reduced
unless specified
Taking
inthese
account
vector
group MVA,
and terminal
sequence
otherwise).
By IEC
60076-1transformer
[3] all taps shall
be full-power taps,
aspects,
an Yd1
American
is interchangeable
with
except when specified otherwise.
a European
Yd11,
without
any
external
modifications.
Almost all transformers used in power plants are at least
equipped with de-energized tap-changers. Sometimes, especially
IX. TAPS
AND
TAP-CHANGER
in case of units equipped
with
generator
breakers, it is difficult to
ensure the unit auxiliaries suitable voltage under any operation
The original transformer may be equipped with on-load
regime. To overcome this problem, UAT with on-load tap-changer
tap-changer
or In de-energized
tap-changer.
Normally
may be required.
some cases, UT or
SST are equipped
with on- a
substitution
a limited period
timeforwith
fixed turnsload in placefor
of de-energized
taps toof
allow
largeavariations
in
transmission
systemwill
voltage.
ratio
transformer
be possible in an emergency. When
choosing the most suitable tap of the substitute transformer, it
Impedance
X. whether
is indispensable to check
it is a full-power tapping
When designing a new plant, the selection of transformer
(e.g.
suitable
for
a
current
equal
to
the
rated power
short-circuit impedance (in fact the reactance,
the divided
resistanceby
the
tap negligible
voltage). for large transformers) is subject to conflicting
being
demands:
low C57.12.00
enough to limit
voltage drop
and to meetis
By IEEE
[16], thewhenever
a transformer
stability requirements,
but alsotaps,
suitable
setfull
the capacity
system
provided
with de-energized
theyhigh
shalltobe
short
circuit
levels
according
to
economic
limitations
of the
taps. Transformers with on-load tap-changer shall be capable
switchgear and other connected plant. If a generator breaker is
ofused,
delivering
rated
at with
the rated
voltage and
on should
all taps
regulation
of MVA
the UAT
the generator
offline
above
voltage.
for be
taps
the for
rated
also be rated
considered.
TheseHowever,
aspects should
alsobelow
considered
a
replacement
voltage,
theytransformer.
shall be capable of just delivering the rated
Sincerelated
reactance
is arated
resultvoltage
of leakage
reactance
current
to the
(i.e. flux,
theselow
taps
may beisof
obtained
by
minimizing
leakage
flux
and
doing
this
requires
a
reduced MVA, unless specified otherwise). By IEC 60076-1
large core and an expensive transformer. Conversely, if high
[3]
all taps
be full-power
when specified
reactance
canshall
be tolerated,
a smallertaps,
core except
can be provided
and so
otherwise.
a less expensive transformer.
Almost
usedthe
in rated
power
plants
at least
It shouldallbetransformers
noted that since
MVA
(S) are
appears
in
the numerator
of the
expression fortap-changers.
percentage impedance
(z%)
equipped
with
de-energized
Sometimes,
calculated in
from
(Zohmwith
), thegenerator
value of percentage
especially
caseitsofohmic
units value
equipped
breakers, it
impedance tends to increase as the transformer rating increases:
is difficult to ensure the unit auxiliaries suitable voltage under
any operation regime. To overcome this problem, UAT with
A
H1
a
X1
B
H2
X2
C
A
H3
b
c
C
X3
B
C
1W 1V 1U
A
B
a
a
b
c
A
C
2W 2V 2U
c
B
a
c
b
a) Dy11→Dy1
By reversing the phase sequence
on both sides of the transformer
b
b) Dy11→Dy1
By using a substitute transformer
with different terminals layout
Fig. 6. Modifications of transformer phase displacement.
H0 H1 H2 H3
1W 1V 1U 1N
X0 X1 X2 X3
2W 2V 2U 2N
a) American practice (IEEE C57.12.70)
b) German practice (DIN 42402)
Fig. 7. Transformer layout terminal marking.
z% = Zohm x S / U2.
(2)
That means that large MVA at low impedances may require
significant bulky transformers, and permissible transport limits of
dimensions and weight may be reached. It is at this stage that the
use of single-phase units may need to be considered.
XI. Unbalance when Using Single Phase UT
In addition to all the issues discussed in this paper that must be
considered when selecting a substitute transformer, single-phase
transformer replacement from a three-phase step-up unit introduces
an additional concern: generator neutral current unbalance.
Neutral current of the main generator is monitored in many
power plants, in particular those with large generating units.
Neutral current can be the result of a ground fault on the generator
stator windings, on the circuit (isolated phase bus or cables)
between the generator and UAT(s) and UT, or on the deltaconnected windings of the UT or UATs. Upon exceeding certain
amplitude, the generator neutral current alarms and might also trip
the unit, by an overcurrent relay in the neutral circuit, or, by an
overvoltage relay connected across the neutral resistor.
A slightly different turns ratio and/or different impedance
(resulting in different regulation) between the substituted
transformer and the other two will result in voltage unbalance
and thus, negative sequence voltages and currents. This condition
will not lead to an increase in generator neutral current, although
it introduces other problems discussed elsewhere in this paper.
However, different winding capacitances to ground between the
substituted and the other two transformers will result in an increase
in neutral generator currents, with possible adverse impact on the
protective scheme of the generator neutral.
Fig. 8 shows the typical arrangement of a large generator
grounded at the neutral via a single phase grounding transformer
with a resistive load connected to the secondary winding, sized
to reduce any fault current to about 15 to 25 Amps. The sizing
of the neutral resistor is a simple calculation that can be found in
any good book on generator protection or in the standard IEEE
C62.92.2. It is mainly dependent on the value of the capacitance to
ground of the generator as well as all other equipment connected
to its stator leads. Fig. 8 shows the capacitances to ground of all
windings and the isolated phase bus for each of the three phases.
In the figure, the capacity to ground of the UT and UAT(s) are
lumped in a single delta-connected component.
Under normal conditions, all the capacitances to ground are
balanced among the phases and the neutral current is very close to
zero. However, if a substitute one-phase transformer is installed
with different capacitance, the circuit becomes unbalanced, and
the neutral current will grow. Finding the value of the unbalanced
current requires solving the unbalanced circuit by any of several
available methods. One such method requires the delta connection
of capacitors to be replaced by its wye (star) equivalent. All series
resistances and reactance are neglected. Then, Fig. 8 can be
simplified to the circuit shown in Fig. 9. In the circuit, all the
variables are vector quantities, with exception of Rr which is the
grounding resistance referred to the primary side. Voltages are
assumed balanced and generator / isolated phase bus capacitances
to ground equal in each phase. Given that the capacitance
reactance to ground is much higher than the series impedance of
the windings/buses, these last are disregarded. Following, and
using superposition, the total neutral current IN can be calculated
by solving for each phase a circuit as shown in Fig. 10 (example
for phase A) and then summing together.
Following the calculation of the neutral unbalanced current,
proper setting of the neutral protection can be done.
XII. Overcurrent Considerations
2013 ‫ אוגוסט‬,46 ‫גליון‬
58
6
es, UT or
ized taps
oltage.
ansformer
resistance
ubject to
drop and
to set the
mitations
generator
generator
hould be
actance is
equires a
y, if high
vided and
ppears in
ance (z%)
ercentage
er rating
(2)
ay require
transport
is at this
ed to be
elsewhere in this paper. However, different winding
The mechanical
force and
the thermal
short-circuit and
requirements
capacitances
to ground
between
the substituted
the other
described in transformer standards are normally satisfactory for
two
transformers
will
result
in
an
increase
in
neutral
generator
UTs.
currents,
withthepossible
adverse
impactto on
the protective
However,
UAT must
be designed
mechanically
and
scheme
of
the
generator
neutral.
thermally withstand the environment in which it operates. The
standard
requirements
for network
applications
maygenerator
be not
Fig. 8 shows
the typical
arrangement
of a large
be adequateat forthecertain
typesvia
of three-phase
through-faults
on
grounded
neutral
a single phase
grounding
the secondary of the UAT, because of the following: slower dc
transformer with a resistive load connected to the secondary
component decrement, longer short-circuit duration, possibility of
winding,
sized voltage
to reduce
any fault
currenttrip
to (load
aboutrejection)
15 to 25
higher primary
subsequent
to breaker
Amps.
The
sizing
of
the
neutral
resistor
is
a
simple
calculation
in block schemes [9].
eventuality
the fast
transfer
of load from
UAT toor
that Another
can be found
in anyisgood
book
on generator
protection
vice-versa),
may lead,
under certain
conditions,
inSST
the(or
standard
IEEEwhich
C62.92.2.
It is mainly
dependent
on the
to high-circulating currents flowing through the two transformers
value
of the capacitance to ground of the generator as well as
exceeding their mechanical design capability [9].
all other
equipment
connectedofto the
its stator
leads. Fig. requires
8 shows
In general,
examination
aforementioned
the
capacitances
to
ground
of
all
windings
and
the
isolated
consulting with the manufacturer.
phase bus for each of the three phases. In the figure, the
Secondary
XIII.
ircuits are lumped in a
capacity to ground
of the
UT andCUAT(s)
When using a substitute transformer, the protection circuits
single
delta-connected component.
may be required to be modified because the transformer’s rated
Underchanged,
normal conditions,
all thecurrent
capacitances
to ground
are
power
and/or bushing
transformers
have
balanced
among
the
phases
and
the
neutral
current
is
very
different turns ratio, and/or differing secondary current or burden
capabilities,
close
to zero.etc.
However, if a substitute one-phase transformer
Substitute-transformer’s
currentthe
transformers,
with
is installed
with different bushing
capacitance,
circuit becomes
different rated
secondary
current,
ratio or
those of
unbalanced,
and
the neutral
current
willburden
grow.than
Finding
the
the original one, may lead to saturation and wrong operation of
value
of
the
unbalanced
current
requires
solving
the
the differential protection. If the turns ratio or vector group of
unbalanced
circuit
by any isofdifferent
several from
available
methods.
the alternative
transformer
the original
unit,One
it
should
be taken
into account
regarding
currentoftransformers
such
method
requires
the delta
connection
capacitors ratio
to be
and connections.
Some
differential
relays
replaced
by its wye
(star)
equivalent.
All(mainly
series numerical)
resistancescan
and
internally are
accommodate
phase
of be
the simplified
transformer,
reactance
neglected. the
Then,
Fig.shift
8 can
to or
the
differences in these ratios. Other relays (mainly electromechanical)
circuit
shown
in
Fig.
9.
In
the
circuit,
all
the
variables
are
do not have this versatility, and pose difficulties or need external
vector
quantities,
with exception of Rr which is the grounding
auxiliary
current transformers.
In casereferred
of transformer
replacement
is important
verify
resistance
to the primary
side.it Voltages
aretoassumed
adequacy and
of the
transformer
over-excitation
protection.
Often,to
balanced
generator
/ isolated
phase bus
capacitances
GENERATOR
UT
aper that
nsformer,
hase stepor neutral
d in many
ing units.
lt on the
ase bus or
or on the
s. Upon
l current
t relay in
connected
mpedance
ubstituted
unbalance
nts. This
r neutral
discussed
ISOLATED PH. BUS
C GEN
C LV2
C LV3
N1/N2
R
Fig. 8 Equivalent Circuit of the generator, isolated phase bus, and the deltaconnected windings of the UT and UATs.
IA
CA
V APH
IN
Rr
V BPH
IB
CB
V CPH
IC
CC
Fig. 9 Simplified circuit for calculating the unbalanced neutral current.
57
2013 ‫ אוגוסט‬,46 ‫גליון‬
voltage is lower than the rated generator voltage and common
protection may not be applicable; in such a case, it is desirable to
XIII. SECONDARY CIRCUITS
provide supplementary protection for the transformer.
When using a substitute transformer, the protection circuits
XIV. Atodditional
Considerations
may be required
be modified
because the transformer’s
Additional
aspects
that should
be checked
deciding
for a
rated
power
changed,
and/or
bushingwhen
current
transformers
transformer substitution are:
have
different
turns
and/or of
differing
secondary
current
- Details
of type
andratio,
arrangement
terminals,
for example
or
burden
capabilities,
etc.
connections to overhead line, isolated phase bus, or cable box or
gas insulated
bus bar.
Substitute-transformer’s
bushing current transformers, with
- Isolated
phase
bus ducts
with
accompanying
strong
different
rated
secondary
current,
ratio
or burden than
those of
magnetic
fields may
currents
transformer
the original
one,cause
may high
leadcirculating
to saturation
andinwrong
operation
tanks and covers which may result in excessive temperatures when
of the differential
If in
thetheturns
ratio or vector group
corrective
measures areprotection.
not included
design.
of
the
alternative
transformer
is
different
from theoveroriginal
- Unusual voltage conditions including transient
unit, itresonance,
should be
taken surges,
into account
current
voltages,
switching
etc. whichregarding
may require
special
consideration
insulation
design. Some differential relays
transformers
ratioinand
connections.
- Derating
due to high
harmonic
load accommodate
current.
(mainly
numerical)
can
internally
the phase
- Unusual environmental conditions (altitude, special cooling
shift
of
the
transformer,
or
differences
in
these
ratios.
Other
air temperature, explosive atmosphere, etc.).
relays
(mainly
electromechanical)
do
not
have
this
versatility,
- Details of auxiliary supply voltage (for fans and pumps,
and posealarms,
difficulties
tap-changer,
etc.). or need external auxiliary current
- Sound-level restrictions.
transformers.
- In
Losses
(usually notreplacement
relevant in case
an emergency
case level
of transformer
it isofimportant
to verify
situation transformer substitution).
adequacy of the transformer over-excitation protection. Often,
IN
Rr
V APH
CB
CC
IA
CA
UT & UAT GEN. SIDE
C LV1
C IPB
requirements described in transformer standards are normally
satisfactory for UTs.
However, the UAT must be designed to mechanically and
thermally withstand the environment in which it operates. The
standard requirements for network applications may be not be
adequate for certain types of three-phase through-faults on the
secondary of the UAT, because of the following: slower dc
component decrement, longer short-circuit duration,
possibility of higher primary voltage subsequent to breaker
trip (load rejection) in block schemes [9].
Another
is thefunctions
fast transfer
of load from
protection
and eventuality
output limiting
are provided
in theUAT
generator
equipment, but
it is amay
good lead,
practice
to apply
to SSTexcitation
(or vice-versa),
which
under
certain
additionally
V/Hz protection.
The curves
thatthrough
define the
conditions,separate
to high-circulating
currents
flowing
generator and transformer V/Hz limits should be coordinated
transformers
exceeding
their
mechanical
design
capability
to two
properly
protect both.
When the
transformer
rated
voltage
is
[9].
equal to the generator rated voltage, the same V/Hz relay that is
In general,
examination
the aaforementioned
requires
protecting
the generator
may be set of
in such
way that also protects
theconsulting
transformer.
In the
some
cases, however, the rated transformer
with
manufacturer.
Fig. 10 One of the three circuits to be solved for calculating the neutral
unbalance current.
XV. Conclusions
The large transformers used in power plants have particular
characteristics and specific custom designs. Keeping in stock
spare transformers identical to the critical ones in operation is
an expensive strategy; however it ensures the lowest risk. The
advantageousness of this policy increases in the case of multiple
standardized generating units (one example of interchangeable
generator transformer is detailed in [23]).
Checking the suitability of a different substitute transformer is
a complex task. As a first feasibility check, confirm the transformer
rated frequency fits your grid, preliminarily look for an available
transformer ratio close to the ratio of secondary system voltage to
generator rated voltage (about ±5%), and roughly appreciate the
transformer largeness, heaviness and available power.
Following is a partial list of the main parameters that must be
checked:
- UT secondary tap voltage should not be lower than 95% of
the grid’s maximum expected voltage (this value typically equals
the rated system voltage).
- UT primary (low) voltage should not be lower than 95%
of generator rated voltage for transformers designed according to
IEEE C57.12.00, and not lower than generator rated voltage for
Ad
for a t
- D
exam
cable
- I
magn
transf
tempe
design
- U
voltag
specia
- D
- U
coolin
- D
tap-ch
- S
- L
emerg
Th
chara
spare
an ex
advan
multip
interc
Ch
transf
confir
prelim
the r
voltag
largen
Fo
be ch
- U
of the
equal
transformers designed according to IEC 60076-1.
- UAT primary voltage should normally match the rated
generator voltage; a lower voltage (up to 95% of generator rating)
UT primary
shouldofnot
lower
95%
may be -possible
only (low)
after voltage
investigation
theberisk
forthan
overof
generator
rated
voltage
for
transformers
designed
according
excitation.
- UT C57.12.00,
primary (low)
voltage
should
be lower
than
95%
IEEE
andphase
not lower
than not
generator
rated
voltage
- toCheck
connection and
displacement
suitability.
of
generator
rated
voltage
for
transformers
designed
according
for
transformers
designed
according
to
IEC
60076-1.
- Analyze MVA rating suitability. If taps below the rated
to IEEE
C57.12.00,
andtheir
not lower
than
generator
ratedthe
voltage
voltage
shall
be
used,
check
MVA
- UAT
primary
voltage
shouldcapability.
normally
match
rated
for
transformers
designed
according
to
IEC
60076-1.
- generator
Transformer
rateda lower
voltages
higher
the of
expected
voltage;
voltage
(upthan
to 95%
generator
- UAT
voltage
normally
match
rated
operating
ones
mean
mandatory
reducing
the MVA
avoid
rating)
mayprimary
be possible
onlyshould
after
investigation
of to
thethe
risk
for
exceeding
the rated
current.
generator
voltage;
a lower voltage (up to 95% of generator
over-excitation.
- rating)
Checkmay
dimensions
and weight
suitability.
be possible
onlyphase
after
investigationsuitability.
of the risk for
- Check connection
and
displacement
- over-excitation.
Analyze the unbalance that may be introduced when
MVA transformer
rating suitability.
If UT
tapsmade
belowofthe
rated
replacing-- Analyze
a single-phase
from
an
three
Check
connection
and phase
displacement
suitability.
voltage
shall
be
used,
check
their
MVA
capability.
sing-phase transformers.
-- Analyze
MVArated
ratingvoltages
suitability.
If taps
below
rated
Transformer
higher
thaninfluences.
the the
expected
- Estimate
transformer
short circuit impedance
voltage
shall
be
used,
check
their
MVA
capability.
operating
ones
mean
mandatory
reducing
the
MVA
to
avoid
- Confirm through fault and fast load transfer capability in
- Transformer
voltages higher than the expected
case of
UAT/SST
exceeding
thereplacement.
ratedrated
current.
operating
ones
mean
mandatory
reducing
the MVA
to avoid
- Verify
thedimensions
implications
secondary
circuits
(mainly
- Check
andon
weight
suitability.
exceeding
the
rated
current.
protection
and
synchronizing).
- Analyze the unbalance that may be introduced when
- replacing
Thoroughly
analyze and
the
synchronizing
andUTloading
- Checka dimensions
weight
suitability.
single-phase
transformer
from an
made of
capabilities
for any
available
operation
- Analyze
thetransformers.
unbalancetap,
thatunder
may various
be introduced
when
three
sing-phase
conditions,
using
graphs
as in Fig.2
or Fig.3. from an UT made of
replacing
a
single-phase
transformer
- Estimate
transformer
short
circuit impedance
- In
the
eventuality
of two units
connected
to the sameinfluences.
bus, one
three
sing-phase
transformers.
Confirm
through
fault
and
fast
load
of them having a substitute UT (with differenttransfer
MVA orcapability
ratio or in
- of
Estimate
transformer
short circuit impedance influences.
case
UAT/SST
replacement.
impedance),
the possible
effect on generators different behavior
-- Confirm
through
fault andonfastsecondary
load transfer
capability
in
Verify
the
implications
circuits
(mainly
should also
be considered.
case
of UAT/SST
replacement.
protection
and synchronizing).
-- Verify
the implications
secondary circuits
References
XVI.
Thoroughly
analyze
theonsynchronizing
and (mainly
loading
protection
and
synchronizing).
[1] M.
G.
Say,
Alternating
Current
Machines,
4th
edition,
Wiley,
capabilities for any available tap, under various operation
1976,
pp.14,using
76. graphs
- Thoroughly
analyze
synchronizing
and loading
conditions,
as intheFig.2
or Fig.3.
[2] B.
Lawrie, for
“How
frequency
affect
transformer
capabilities
any does
available
tap,connected
under
various
operation
- In the eventuality
of two units
to the same
bus,
operation”,
EC&M
Magazine,
Dec. 1992.
conditions,
using
graphs
as
in
Fig.2
or
Fig.3.
one
of
them
having
a
substitute
UT
(with
different
MVA
or
[3] Power
Transformers
– Part
1: General,
IEC Standard
- In
the
eventuality
ofpossible
two
units
connected
to the 60076same
bus,
ratio
or
impedance),
the
effect
on
generators
different
1, Ed. 3.0, April 2011.
one
of them
having
a substitute
UT (with different MVA or
behavior
should
also
be
considered.
[4] IEEE
Standard
Terminology
for Power and Distribution
ratio
or
impedance),
the
possible
effect
on generators
different
Transformers, IEEE Standard C57.12.80-2010,
Dec. 2010.
behaviorTransformers
should also be
[5] Power
– considered.
Part
7: Loading Guide for OilXVI.
REFERENCES
Immersed
Transformers,
Standard
60076-7,
First
[1] M. G. Power
Say, Alternating
Current IEC
Machines,
4th edition,
Wiley,
1976,
XVI. REFERENCES
Ed., pp.14,
Dec. 2005.
76.
[2]
Lawrie,
does Mineral-Oil-Immersed
frequency
transformer
operation",
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XVII. BIOGRAPHIES
XVII. Biographies
XVII. Breceived
IOGRAPHIES
Relu
ReluIlie
Ilie receivedthetheMS
MSdegree
degreeininelectrical
electrical
engineering from the Technical University of Iaşi,
engineering
from
theofTechnical
University
Romania.
He
is
in
charge
Power
Plant
electrical
Relu Ilie received the MS degree in electrical
of Iaşi, and
Romania.
He for
is inthecharge
Power
systems
equipment
Israel of
Electric
engineering
from
the Technical
University
of Iaşi,
Plant electrical
systems
andofPlant
equipment
Corporation.
has 30
years
experience
infor
Romania.
He Relu
is in Ilie
charge
of Power
electrical
generation
andElectric
transmission
the Israel
Corporation.
Relu
Ilie
systems
and
equipment
forfield,
themostly
Israel dedicated
Electric
to
machines
issues:
design
and
failure
haselectrical
30 years
of has
experience
generation
Corporation.
Relu
Ilie
30 years
ofin
experience
in
analysis,
maintenance
and inspections,
monitoring
generation
and transmission
field,
mostly
dedicated to
and transmission
field,
mostly
dedicated
and
testing, technical
specifications,
lifeand
extension,
toelectrical
electrical
machines
issues:
design
machines
issues:
design andfailure
failure
condition
assessment,
etc.
(email:
[email protected]).
analysis, maintenance and inspections, monitoring
analysis, maintenance and inspections,
and testing, technical specifications, life extension,
monitoring and testing,
specifications,
lifetheextension,
Isidortechnical
“Izzy”
Kerszenbaum
received
BSc EE
condition
assessment,
etc. (email:
[email protected]).
from
Technion
– Israel Institute of Technology,
condition assessment,
etc.the(email:
[email protected]).
and
the “Izzy”
MS andKerszenbaum
PhD (EE) fromreceived
the University
of the
Isidor
the BSc
EE
Witwatersrand.
His –employment
experience
includes
from
the Technion
Israel Institute
of received
Technology,
Isidor
“Izzy”
Kerszenbaum
the
working
as
large
electric
machines
design
engineer
and
theEE
MSfrom
and PhD
from the–University
of the
BSc
the(EE)
Technion
Israel
Institute
for
GEC
in
South
Africa,
and
manager
of
R&D
for
a
Witwatersrand. His employment experience includes
of Technology,
and machines
the MS
and PhD(ITC)
(EE)
power
transformer
manufacturer
workingdistribution
as large electric
design engineer
the
of
themanager
infrom
California.
He also
worked
asWitwatersrand.
protection
engineer
for
GEC
in University
South
Africa,
and
of R&D
for His
a
and
large
apparatus
consultant with
three different
employment
experience
includes
working
power
distribution
transformer
manufacturer
(ITC)
utilities.
He electric
isHe
a past
chairmanasof
the PES engineer
Electric
in
also worked
protection
asCalifornia.
large
machines
design
engineer
Machines Committee. Currently
Dr.apparatus
Kerszenbaum
is chairman
of the
IEEE
and
large
consultant
with
three
different
for
GEC
in
South
Africa,
and
manager
of
Working Group 10 on Online
Monitoring
of
Large
Synchronous
Generators..
utilities.
He
is
a
past
chairman
of
the
PES
Electric
R&D
for a power
distribution
transformer
He
is
a
Fellow
of
the
IEEE.
Dr.
Kerszenbaum
spent
2002
in
the
US
House
of
Machines Committee. Currently Dr. Kerszenbaum is chairman of the IEEE
manufacturer10(ITC)
in California.
HeTechnology
alsoSynchronous
worked
asGenerators..
protection
Representatives
aon
IEEE-AAAS
Science and
Fellow.
Working Group as
Online Monitoring
of Large
engineer
and
large
consultant
with
three
different
He
is a Fellow
of the
IEEE.apparatus
Dr. Kerszenbaum
spent 2002
in the
US House
of
utilities. He as
is a aIEEE-AAAS
past chairman
PES Electric
Representatives
Science of
andthe
Technology
Fellow. Machines
Committee. Currently Dr. Kerszenbaum is chairman of the IEEE
Working Group 10 on Online Monitoring of Large Synchronous
Generators.. He is a Fellow of the IEEE. Dr. Kerszenbaum spent
2002 in the US House of Representatives as a IEEE-AAAS
Science and Technology Fellow.
2013 ‫ אוגוסט‬,46 ‫גליון‬
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