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arallel Operation of Transformers
Introduction:
§
For supplying a load in
excess of the rating of an existing transformer, two or more transformers may
be connected in parallel with the existing transformer. The transformers are
connected in parallel when load on one of the transformers is more than its capacity.
The reliability is increased with parallel operation than to have single larger
unit. The cost associated with maintaining the spares is less when two
transformers are connected in parallel.
§
It is usually economical
to install another transformer in parallel instead of replacing the existing
transformer by a single larger unit. The cost of a spare unit in the case of
two parallel transformers (of equal rating) is also lower than that of a single
large transformer. In addition, it is preferable to have a parallel transformer
for the reason of reliability. With this at least half the load can be supplied
with one transformer out of service.
Condition for Parallel Operation of
Transformer:
§
For parallel connection
of transformers, primary windings of the Transformers are connected to source
bus-bars and secondary windings are connected to the load bus-bars.
§
Various conditions that
must be fulfilled for the successful parallel operation of transformers:
1.
Same voltage Ratio
& Turns Ratio (both primary and secondary Voltage Rating is same).
2.
Same Percentage
Impedance and X/R ratio.
3.
Identical Position of
Tap changer.
4.
Same KVA ratings.
5.
Same Phase angle shift
(vector group are same).
6.
Same Frequency rating.
7.
Same Polarity.
8.
Same Phase sequence.
§
Some of these conditions
are convenient and some are mandatory.
§
The convenient are: Same
voltage Ratio & Turns Ratio, Same Percentage Impedance, Same KVA Rating,
Same Position of Tap changer.
§
The mandatory conditions
are: Same Phase Angle Shift, Same Polarity, Same Phase Sequence and Same
Frequency.
§
When the convenient
conditions are not met paralleled operation is possible but not optimal.
1.Same
voltage Ratio & Turns Ratio (on
each tap):
§
If the transformers connected in parallel have slightly different
voltage ratios, then due to the inequality of induced emfs in the secondary
windings, a circulating current will flow in the loop formed by the
secondarywindings under the no-load condition, which may be much greater than
the normal no-load current.
§
The current will be
quite high as the leakage impedance is low. When the secondary windings are
loaded, this circulating current will tend to produce unequal loading on the
two transformers, and it may not be possible to take the full load from this
group of two parallel transformers (one of the transformers may get
overloaded).
§
If two transformers of
different voltage ratio are connected in parallel with same primary supply
voltage, there will be a difference in secondary voltages.
§
Now when the secondary of
these transformers are connected to same bus, there will be a circulating
current between secondary’s and therefore between primaries also. As the
internal impedance of transformer is small, a small voltage difference may
cause sufficiently high circulating current causing unnecessary extra I2R
loss.
§
The ratings of both
primaries and secondary’s should be identical. In other words, the transformers
should have the same turn ratio i.e. transformation ratio.
2. Same percentage impedance and X/R ratio:
§
If two
transformers connected in parallel with similar per-unit impedances they will
mostly share the load in the ration of their KVA ratings. Here Load is mostly
equal because it is possible to have two transformers with equal per-unit
impedances but different X/R ratios. In this case the line current will be less
than the sum of the transformer currents and the combined capacity will be
reduced accordingly.
§
A difference in the
ratio of the reactance value to resistance value of the per unit impedance
results in a different phase angle of the currents carried by the two
paralleled transformers; one transformer will be working with a higher power
factor and the other with a lower power factor than that of the combined
output. Hence, the real power will not be proportionally shared by the
transformers.
§
The current shared by
two transformers running in parallel should be proportional to their MVA
ratings.
§
The current carried by
these transformers are inversely proportional to their internal impedance.
§
From the above two
statements it can be said that impedance of transformers running in parallel
are inversely proportional to their MVA ratings. In other words percentage
impedance or per unit values of impedance should be identical for all the
transformers run in parallel.
§
When connecting
single-phase transformers in three-phase banks, proper impedance matching
becomes even more critical. In addition to following the three rules for
parallel operation, it is also a good practice to try to match the X/R ratios of the three series
impedances to keep the three-phase output voltages balanced.
§
When single-phase
transformers with the same KVA ratings are connected in a Y-∆ Bank,
impedance mismatches can cause a significant load unbalance among the
transformers
§
Lets examine following different type of case among Impedance, Ratio and
KVA.
§
If single-phase
transformers are connected in a Y-Y bank with an isolated neutral, then the
magnetizing impedance should also be equal on an ohmic basis. Otherwise, the
transformer having the largest magnetizing impedance will have a highest
percentage of exciting voltage, increasing the core losses of that transformer
and possibly driving its core into saturation.
Case 1: Equal Impedance, Ratios and Same kVA:
§
The standard method of connecting transformers in parallel is to have
the same turn ratios, percent impedances, and kVA ratings.
§
Connecting
transformers in parallel with the same parameters results in equal load sharing
and no circulating currents in the transformer windings.
§
Example: Connecting two 2000 kVA, 5.75% impedance transformers in parallel, each
with the same turn ratios to a 4000 kVA load.
§
Loading on the
transformers-1 =KVA1=[( KVA1 / %Z) / ((KVA1 / %Z1)+ (KVA2 / %Z2))]X KVAl
§
kVA1 = 348 / (348 + 348) x 4000 kVA = 2000 kVA.
§
Loading on the
transformers-2 =KVA1=[( KVA2 / %Z) / ((KVA1 / %Z1)+ (KVA2 / %Z2))]X KVAl
§
kVA2 = 348 / (348 + 348) x 4000 kVA = 2000 kVA
§
Hence KVA1=KVA2=2000KVA
Case 2: Equal Impedances, Ratios and Different kVA:
§
This Parameter is not in common practice for new installations,
sometimes two transformers with different kVAs and the same percent impedances
are connected to one common bus. In this situation, the current division causes
each transformer to carry its rated load. There will be no circulating currents
because the voltages (turn ratios) are the same.
§
Example: Connecting 3000 kVA and 1000 kVA transformers in parallel, each with
5.75% impedance, each with the same turn ratios, connected to a common 4000 kVA
load.
§
Loading on
Transformer-1=kVA1 = 522 / (522 + 174) x 4000 = 3000 kVA
§
Loading on
Transformer-1=kVA2 = 174 / (522 + 174) x 4000 = 1000 kVA
§
From above calculation
it is seen that different kVA ratings on transformers connected to one common
load, that current division causes each transformer to only be loaded to its
kVA rating. The key here is that the percent impedance are
the same.
Case 3: Unequal Impedance but Same Ratios
& kVA:
§
Mostly used this Parameter to enhance plant power capacity by connecting
existing transformers in parallel that have the same kVA rating, but with
different percent impedances.
§
This is common when
budget constraints limit the purchase of a new transformer with the same
parameters.
§
We need to understand is
that the current divides in inverse proportions to the impedances, and larger
current flows through the smaller impedance. Thus, the lower percent impedance
transformer can be overloaded when subjected to heavy loading while the other
higher percent impedance transformer will be lightly loaded.
§
Example: Two 2000 kVA transformers in parallel, one with 5.75% impedance and the
other with 4% impedance, each with the same turn ratios, connected to a common
3500 kVA load.
§
Loading on Transformer-1=kVA1 = 348 / (348 + 500) x 3500 = 1436 kVA
§
Loading on
Transformer-2=kVA2 = 500 / (348 + 500)
x 3500 = 2064 kVA
§
It can be seen that
because transformer percent impedances do not match, they cannot be loaded to
their combined kVA rating. Load division between the transformers is not equal.
At below combined rated kVA loading, the 4% impedance transformer is overloaded
by 3.2%, while the 5.75% impedance transformer is loaded by 72%.
Case 4: Unequal Impedance & KVA Same Ratios:
§
This particular of transformers used rarely in industrial and commercial
facilities connected to one common bus with different kVA and unequal percent
impedances. However, there may be that one situation where two single-ended
substations may be tied together via bussing or cables to provide better
voltage support when starting large Load.
§
If the percent
impedance and kVA ratings are different, care should be taken when loading
these transformers.
§
Example: Two transformers in parallel with one 3000 kVA (kVA1) with 5.75%
impedance, and the other a 1000 kVA (kVA2) with 4% impedance, each with the
same turn ratios, connected to a common 3500 kVA load.
§
Loading on
Transformer-1=kVA1 = 522 / (522 + 250)
x 3500 = 2366 kVA
§
Loading on
Transformer-2=kVA2 = 250 / (522 + 250)
x 3500 = 1134 kVA
§
Because the percent
impedance is less in the 1000 kVA transformer, it is overloaded with a less
than combined rated load.
Case 5: Equal Impedance
& KVA Unequal Ratios:
§
Small differences in voltage cause a large amount of current to
circulate. It is important to point out that paralleled transformers should
always be on the same tap connection.
§
Circulating current is
completely independent of the load and load division. If transformers are fully
loaded there will be a considerable amount of overheating due to circulating
currents.
§
The Point which should
be Remember that circulating currents do not flow on the line, they cannot be
measured if monitoring equipment is upstream or downstream of the common
connection points.
§
Example: Two 2000 kVA transformers connected in parallel, each with 5.75%
impedance, same X/R ratio (8), transformer 1 with tap adjusted 2.5% from
nominal and transformer 2 tapped at nominal. What is the percent circulating current
(%IC)
§
%Z1 = 5.75, So %R’ = %Z1 / √[(X/R)2 + 1)] = 5.75 /
√((8)2 + 1)=0.713
§
%R1 = %R2 = 0.713
§
%X1 = %R x (X/R)=%X1= %X2= 0.713 x 8 = 5.7
§
Let %e = difference in
voltage ratio expressed in percentage of normal and k = kVA1/ kVA2
§
Circulating current %IC
= %eX100 / √ (%R1+k%R2)2 + (%Z1+k%Z2)2.
§
%IC = 2.5X100 / √ (0.713 + (2000/2000)X0.713)2 + (5.7 +
(2000/2000)X5.7)2
§
%IC = 250 / 11.7 = 21.7
§
The circulating current
is 21.7% of the full load current.
Case 6: Unequal Impedance, KVA & Different Ratios:
§
This type of parameter would be unlikely in practice.
§
If both the ratios and
the impedance are different, the circulating current (because of the unequal
ratio) should be combined with each transformer’s share of the load current to
obtain the actual total current in each unit.
§
For unity power factor,
10% circulating current (due to unequal turn ratios) results in only half
percent to the total current. At lower power factors, the circulating current
will change dramatically.
§
Example: Two transformers connected in parallel, 2000 kVA1 with 5.75% impedance,
X/R ratio of 8, 1000 kVA2 with 4% impedance, X/R ratio of 5, 2000
kVA1 with tap adjusted 2.5% from nominal and 1000 kVA2 tapped at nominal.
§
%Z1 = 5.75, So %R’ = %Z1 / √[(X/R)2 + 1)] = 5.75 /
√((8)2 + 1)=0.713
§
%X1= %R x (X/R)=0.713 x 8 = 5.7
§
%Z2= 4, So %R2 = %Z2 /√ [(X/R)2 + 1)]= 4 / √((5)2 + 1)
=0.784
§
%X2 = %R x (X/R)=0.784 x 5 = 3.92
§
Let %e = difference in
voltage ratio expressed in percentage of normal and k = kVA1/ kVA2
§
Circulating current %IC
= %eX100 / √ (%R1+k%R2)2 + (%Z1+k%Z2)2.
§
%IC = 2.5X100 / √ (0.713 + (2000/2000)X0.713)2 + (5.7 +
(2000/2000)X5.7)2
§
%IC = 250 / 13.73 = 18.21.
§
The circulating current
is 18.21% of the full load current.
3. Same polarity:
§
Polarity of transformer
means the instantaneous direction of induced emf in secondary. If the
instantaneous directions of induced secondary emf in two transformers are
opposite to each other when same input power is fed to the both of the
transformers, the transformers are said to be in opposite polarity.
§
The transformers should
be properly connected with regard to their polarity. If they are connected with
incorrect polarities then the two emfs, induced in the secondary windings which
are in parallel, will act together in the local secondary circuit and produce a
short circuit.
§
Polarity of all
transformers run in parallel should be same otherwise huge circulating current
flows in the transformer but no load will be fed from these transformers.
§
If the instantaneous
directions of induced secondary emf in two transformers are same when same
input power is fed to the both of the transformers, the transformers are said
to be in same polarity.
4. Same phase sequence:
§
The phase sequence of line voltages of both the transformers must be
identical for parallel operation of three-phase transformers. If the phase
sequence is an incorrect, in every cycle each pair of phases will get
short-circuited.
§
This condition must be
strictly followed for parallel operation of transformers.
5. Same
phase angle shift:(zero relative phase displacement
between the secondary line voltages):
§
The transformer windings can be connected in a variety of ways which
produce different magnitudes and phase displacements of the secondary voltage. All
the transformer connections can be classified into distinct vector groups.
§
Group 1: Zero phase
displacement (Yy0, Dd0, Dz0)
Group 2:180° phase displacement (Yy6, Dd6, Dz6)
Group 3: -30° phase displacement (Yd1, Dy1, Yz1)
Group 4: +30° phase displacement (Yd11, Dy11, Yz11)
§
In order to have zero
relative phase displacement of secondary side line voltages, the transformers
belonging to the same group can be paralleled. For example, two transformers
with Yd1 and Dy1 connections can be paralleled.
§
The transformers of
groups 1 and 2 can only be paralleled with transformers of their own group. However,
the transformers of groups 3 and 4 can be paralleled by reversing the phase
sequence of one of them. For example, a transformer with Yd1 1 connection
(group 4) can be paralleled with that having Dy1 connection (group 3) by
reversing the phase sequence of both primary and secondary terminals of the Dy1
transformer.
§
We can only parallel Dy1
and Dy11 by crossing two incoming phases and the same two outgoing phases on
one of the transformers, so if we have a DY11 transformer we can cross B&C
phases on the primary and secondary to change the +30 degree phase shift into a
-30 degree shift which will parallel with the Dy1, assuming all the other
points above are satisfied.
6. Same
KVA ratings:
§
If two or more
transformer is connected in parallel, then load sharing % between them is
according to their rating. If all are of same rating, they will share equal
loads
§
Transformers of unequal
kVA ratings will share a load practically (but not exactly) in proportion to
their ratings, providing that the voltage ratios are identical and the
percentage impedances (at their own kVA rating) are identical, or very nearly
so in these cases a total of than 90% of the sum of the two ratings is normally
available.
§
It is recommended that
transformers, the kVA ratings of which differ by more than 2:1, should not be
operated permanently in parallel.
§
Transformers having
different kva ratings may operate in parallel, with load division such that
each transformer carries its proportionate share of the total load To achieve
accurate load division, it is necessary that the transformers be wound with the
same turns ratio, and that the percent impedance of all transformers be equal,
when each percentage is expressed on the kva base of its respective transformer.
It is also necessary that the ratio of resistance to reactance in all
transformers be equal. For satisfactory operation the circulating current
for any combinations of ratios and impedances probably should not exceed ten
percent of the full-load rated current of the smaller unit.
7. Identical tap changer and its operation:
§
The only important point
to be remembered is the tap changing switches must be at same position for all
the three transformers and should check and confirm that the secondary voltages
are same. When the voltage tap need change all three tap changing switches
should be operated identical for all transformers. The OL settings of the SF6
also should be identical. If the substation is operating on full load
condition, tripping of one transformer can cause cascade tripping of all three
transformers.
§
In transformers Output
Voltage can be controlled either by Off Circuit Tap Changer (Manual tap
changing) or By On – Load Tap Changer-OLTC (Automatic Changing).
§
In the transformer with
OLTC, it is a closed loop system, with following components:
§
(1) AVR (Automatic Voltage
Regulator- an electronic programmable device). With this AVR we can set the
Output Voltage of the transformers. The Output Voltage of the transformer is
fed into the AVR through the LT Panel. The AVR Compares the SET voltage &
the Output Voltage and gives the error signals, if any, to the OLTC through the
RTCC Panel for tap changing. This AVR is mounted in the RTCC.
§
(2) RTCC (Remote Tap
Changing Cubicle): This is a panel consisting of the AVR, Display for Tap
Position, Voltage, and LEDs for Raise & Lower of Taps relays, Selector
Switches for Auto Manual Selection… In AUTO MODE the voltage is controlled by
the AVR. In manual Mode the operator can Increase / decrease the voltage by
changing the Taps manually through the Push Button in the RTCC.
§
(3) OLTC is mounted on
the transformer. It consists of a motor, controlled by the RTCC, which changes
the Taps in the transformers.
§
Both the Transformers
should have same voltage ratio at all the taps & when you run transformers
in parallel, it should operate as same tap position. If we have OLTC with RTCC
panel, one RTCC should work as master & other should work as follower to
maintain same tap positions of Transformer.
§
However, a circulating
current can be flown between the two tanks if the impedances of the two transformers
are different or if the taps of the on-load tap changer (OLTC) are mismatched
temporarily due to the mechanical delay. The circulating current may cause the
malfunction of protection relays.
Other necessary condition for parallel operation
1.
All parallel units
must be supplied from the same network.
2.
Secondary cabling from
the transformers to the point of paralling has approximately equal length and
characteristics.
3.
Voltage difference
between corresponding phase must not exceed 0.4%
4.
When the transformers
are operated in parallel, the fault current would be very high on the secondary
side. Supposing percentage impedance of one transformer is say 6.25 %, the
short circuit MVA would be 25.6 MVA and short circuit current would be 35 kA.
5.
If the transformers are
of same rating and same percentage impedance, then the downstream short circuit
current would be 3 times (since 3 transformers are in Parallel) approximately
105 kA. This means all the devices like ACBs, MCCBs, switch boards should
withstand the short-circuit current of 105 kA. This is the maximum current.
This current will get reduced depending on the location of the switch boards,
cables and cable length etc. However this aspect has to be taken into consideration.
6.
There should be
Directional relays on the secondary side of the transformers.
7.
The percent impedance of
one transformer must be between 92.5% and 107.5% of the other. Otherwise,
circulating currents between the two transformers would be excessive.
Summary of Parallel Operation of
Transformer:
|
TransformerParallelConnection
Types |
Equal Loading |
Unequal Loading |
Overloading Current |
Circulating Current |
Recomm. connection |
|
Equal Impedance &
Ratio ,Same KVA |
Yes |
No |
No |
No |
Yes |
|
Equal
Impedance & Ratio But different KVA |
No |
Yes |
No |
No |
Yes |
|
Unequal
Impedance But Same Ratio& KVA |
No |
Yes |
Yes |
No |
No |
|
Unequal
Impedance & KVA But Same Ratio |
No |
Yes |
Yes |
No |
No |
|
Unequal
Impedance & Ratio But Same KVA |
Yes |
No |
Yes |
Yes |
No |
|
Unequal
Impedance & Ratio & different KVA |
No |
No |
Yes |
Yes |
No |
The combinations that will operate
in parallel:
§
Following Vector group
of Transformer will operate in parallel.
|
Operative Parallel Operation |
||
|
Sr.No |
Transformer-1 |
Transformer-2 |
|
1 |
∆∆ |
∆∆ or Yy |
|
2 |
Yy |
Yy or ∆∆ |
|
3 |
∆y |
∆y or Y∆ |
|
4 |
Y∆ |
Y∆ or ∆y |
§
Single-phase
transformers can be connected to form 3-phase transformer banks for 3-phase
Power systems.
§
Four common methods of
connecting three transformers for 3-phase circuits are Δ-Δ, Y-Y, Y-Δ, and Δ-Y
connections.
§
An advantage of Δ-Δ connection is that if one of the transformers fails or is removed from
the circuit, the remaining two can operate in the
open-Δ or V connection. This way, the bank still delivers 3-phase currents and
voltages in their correct phase relationship. However, the capacity of the bank
is reduced to 57.7 % (1 3) of its original value.
§
In the Y-Y connection,
only 57.7% of the line voltage is applied to each winding but full line current
flows in each winding. The Y-Y connection is rarely used.
§
The Δ-Y connection is used for stepping up voltages since the voltage is
increased by the transformer ratio multiplied by 3.
The combinations that will
not operate in parallel:
§
Following Vector group
of Transformer will not operate in parallel.
|
Inoperative Parallel Operation |
||
|
Sr.No |
Transformer-1 |
Transformer-2 |
|
1 |
∆∆ |
∆y |
|
2 |
∆y |
∆∆ |
|
3 |
Y∆ |
Yy |
|
4 |
Yy |
Y∆ |
To check Synchronization of
Transformers:
§
Synchronization of
Transformer can be checked by either of following steps:
§
Checked by synchronizing
relay & synchro scope.
§
If Secondary of
Transformer is not LT Then we must use check synchronizing relay &
Commission the system properly. After connecting relay. Relay must be charges
with only 1 supply & check that relay is functioning properly.
§
Synchronizing should be
checked of both the supply voltages. This can be checked directly with
millimeter between L1 phases of Transformer 1 and L1 phase of Transformer 2. Then
L2 Phase of Transformer 1 and L2 Phase of Transformer 2. Then L3 Phase of
Transformer 1 and L3 Phase of Transformer 2. In all the cases MultiMate should
show 0 voltages theoretically. These checks must be done at synchronizing
breakers only. We have to also check that breaker out going terminals are
connected in such a way that L1 Terminals of both the Breakers comes to same
Main Bus bar of panel. Same for L2 & L3.
§
Best way to check
synchronization on LT is charge complete panel with 1 source up to outgoing
terminals of another incoming breaker terminal. Then just measure Voltage
difference on Incoming & out going terminals of Incoming Breaker. It should be near to 0.
§
To check circulating
current Synchronize both the transformer without outgoing load. Then check current. It
will give you circulating current.
Advantages of Transformer Parallel
Operation:
1) Maximize electrical system efficiency:
§
Generally electrical
power transformer gives the maximum efficiency at full load. If we run numbers
of transformers in parallel, we can switch on only those transformers which
will give the total demand by running nearer to its full load rating for that
time.
§
When load increases we
can switch no one by one other transformer connected in parallel to fulfil the
total demand. In this way we can run the system with maximum efficiency.
2) Maximize electrical system availability:
§
If numbers of
transformers run in parallel we can take shutdown any one of them for
maintenance purpose. Other parallel transformers in system will serve the load without
total interruption of power.
3) Maximize power system reliability:
§
If nay one of the
transformers run in parallel, is tripped due to fault other parallel
transformers is
the system will share the load hence power supply may not be interrupted if the
shared loads do not make other transformers over loaded.
4) Maximize electrical system flexibility:
§
There is a chance of
increasing or decreasing future demand of power system. If it is predicted that
power demand will be increased in future, there must be a provision of
connecting transformers in system in parallel to fulfil the extra demand
because it is not economical from business point of view to install a bigger
rated single transformer by forecasting the increased future demand as it is
unnecessary investment of money.
§
Again if future demand
is decreased, transformers running in parallel can be removed from system to
balance the capital investment and its return.
Disadvantages of Transformer
Parallel Operation:
§
Increasing short-circuit
currents that increase necessary breaker capacity.
§
The risk of circulating
currents running from one transformer to another Transformer. Circulating currents that
diminish load capability and increased losses.
§
The bus ratings could be
too high.
§
Paralleling transformers
reduces the transformer impedance significantly, i.e. the parallel transformers
may have very low impedance, which creates the high short circuit currents.
Therefore, some current limiters are needed, e.g.
reactors, fuses, high impedance buses, etc
§
The control and
protection of three units in parallel is more complex.
§
It is not a common
practice in this industry, since Main-tie-Main is very common in this industry.
Conclusions:
§
Loading considerations for paralleling transformers are simple unless
kVA, percent impedances, or ratios are different. When paralleled transformer
turn ratios and percent impedances are the same, equal load division will exist
on each transformer. When paralleled transformer kVA ratings are the same, but
the percent impedances are different, then unequal load division will occur.
§
The same is true for
unequal percent impedances and unequal kVA. Circulating currents only exist if
the turn ratios do not match on each transformer. The magnitude of the
circulating currents will also depend on the X/R ratios of the transformers. Delta-delta
to delta-wye transformer paralleling should not be attempted.
References
§
Say, M.G. The
performance and design of alternating current machines.
§
Application Guide,
Loading of Transformer,
§
Toro, V.D. Principles of
electrical engineering.
§
Stevenson, W.D. Elements
of power system analysis.
§
MIT Press, Magnetic
circuits and transformers, John Wiley and Sons.
Parallel
Transformers: Balancing Act
From Electrical World Magazine, "Engineer’s
Notebook" by Walter Sass
March/April
2001 — While there are few conceptually simpler ways to get more
useful capacity from transformers than to strategically parallel them, in
reality, transformer paralleling is anything but simple. For example, two
40–MVA transformers could not individually service two buses with 10– and
50–MVA loads. They could if they were tied together, but virtually any
difference between them (from winding variations to MVA ratings) could lead to
equipment overloads, wasted energy, and operational instability.
Parallel configurations where the primaries are connected to different buses,
pose even graver difficulties. And combining voltage–regulating controls that
were designed to work on independent transformers may not work at all.
Problem Explained
Power supply sharing is one of the oldest and most persistant problems in
electrical engineering. Whether the power sources are transistors or
hydro–electric generators, problems arise when their outputs get connected
together.
For example, it might be tempting to combine two independent circuits, each
consisting of one battery powering one lightbulb, to best distribute the energy
from both batteries. The combined circuit of parallel batteries and bulbs might
work, but there may be unintended consequences. The stronger battery may charge
(dump energy into) the weaker one. How much source–to–source energy transfer
takes place is a function of the source impedances and voltage characteristics.
In specific cases, such as identical batteries with identical states of
discharge, there might be no source–to–source currents. Otherwise, the
undesired energy transfer would occur unchecked, unless some form of regulating
circuitry – like blocking diodes for example – stopped it.
In the case of output–interconnected distribution transformers, undesired
energy transfer can similarly occur. Unlike the battery example, there is no
possibility of adding diodes or other regulating circuits. In fact, the only
regulating means available are usually the load tap changers (LTCs) that
regulate the line voltages going to the loads.
LTCs can’t change transformer impedances or block currents, and their primary
line regulation functions cannot be ignored, so their effectiveness in
facilitating transformer paralleling is inherently limited. In fact, transformers
with substantial impedance differences (10% or greater expressed on the same
base) may not be paralleled safely.
But let’s assume it’s possible to use sophisticated controls to operate
paralleled LTCs to somehow optimally minimize energy transfer (circulating
current) and maintain line regulation. In the example above, this would be
equivalent to paralleling two batteries by adjusting small series
potentiometers to match voltage outputs of each. The pots could be adjusted
jointly to regulate the voltage going to the lightbulb loads, and adjusted
differentially to reduce circulating current between the batteries.
Unfortunately, AC power circuits are more complicated. Using LTCs to regulate
the output voltages of parallel transformers primarily acts to share reactive
power (VAr) between them, not real power. The optimal paralleled LTC control
scheme may be required to restrict VAr burdens on each transformer, further
compromising its ability to distribute the real power loads as intended.
Despite these problems and limitations, transformer paralleling is becoming an
increasingly common utility practice. Even if inevitable circulating currents
waste energy and heat up the transformers, or if shifted VAr burdens run the
transformers nearer their VA limits, the possibility of picking up more load with existing equipment is very compelling.
The key to operating transformers in parallel safely will be more LTC control
strategies that truly optimize line regulation, minimize circulating currents,
and limit VAr burden. Such control strategies will require distributed
observation of bus interconnections at the substation and coordinated operation
of LTCs that bypass or eliminate their local controllers. Further, transformer
manufacturers can expect their gear to run hotter and experience more
overstress as paralleling becomes more common.
50 MVA 138KV
AUTOTRANSFORMER SPECIFICATION SAMPLE
Type: Outdoor use, Oil-immersed, OA/FA/FA, 3 Windings with rubber
diaphragm
conservator vented via silica
gel dehydrating breather, On-load-tap changer,
manufactured according to ANSI
C57.12.00 Std., All Copper Windings,
For use as a Step-down transformer in
an electric utility transmission substation.
Complete with standard accesories.
Rating:
HV -
30/40/50 MVA
LV -
30/40/50 MVA
TV -
12/16/20 MVA
Cooling Method: OA/FA1/FA2
Rated Voltage: HV - 138KV
LV - 69KV
TV - 13.2KV
Tap Voltage: HV Side OLTC: 138 KV + 8, - 12 x 1.0%, 21 Taps
OLTC: ABB type UZFRT 550/300, 138,000 Volts,
3 Phase, 60 Hz, 21 positions,
With Motor Drive Mechanism type BUF
3,
Motor : 460 Volts, 3 Phase, 60 Hz
Contactors: 230 Volts AC,
Position Transmitter: 230 Volts AC
Heating Element :
230 Volts AC
With Manual/Automatic Change
over-switch, Raise & Lower pushbuttons.
BIL:
Winding :
HV – 650 KV
LV
- 350 KV
Neutral
– 150 KV
TV
- 110 KV
Bushing : HV
- 650 KV
LV - 350 KV
Neutral - 150 KV
TV
- 150 KV
Frequency : 60 Hz
Connection:
HV - Star with Neutral
(Auto-Star) brought out to a bushing
LV - Star with Neutral
(Auto-Star) brought out to a bushing
TV - Delta
Vector Group : Yyna0d1
Guaranted Losses at rated voltage, frequency, unity pf & @ 85 deg C (50
MVA):
No -Load Loss:
18.6 KW
Load Loss @ 50 MVA: 157.6 KW
Efficiency :
99.65% @ 50 MVA( Without
Auxiliary Loss)
Temperature Rise Limits:
Oil
- 65 deg C
Winding – 65 deg C
% Impedances @ 85 deg
HV – LV@ 50 MVA
HV - TV@ 20 MVA LV - TV@ 20 MVA
8L - 149,040 V -
10.28 -
10.45
N - 138,000 V -
10.50 -
10.31 -
5.6
12R - 121,440 V - 11.09
- 10.52
Audible Sound Level @ 50 MVA with all fans running: 72dB
Service Condition:
Maximum ambient air
temperature:
40 deg C
Average ambient air
temperature for any 24h period:
30 deg C
Maximum altitude above
sea level:
1000 meters
Maximum ambient relative
humidity:
88%
Mean annual rainfall:
2400 mm
Maximum wind velocity:
220 km/hr
Maximum seismic factor:
0.45g
CONSTRUCTIONS
a) Core:
The core of the transformer will be constructed of the highest quality,
non-aging high permeability, cold-rolled gain-oriented silicon steel
sheet especially suitable for the purpose. Every care will be taken during
slitting and cutting process to avoid burrs. Both sides of each sheet will be
special glass film insulated on to minimize eddy current losses. The cores will
be carefully assembled and rigidly clamped to ensure adequate mechanical
strength to support the windings and also reduced vibration to minimum under
operating conditions.
b) Windings:
The winding of the transformer shall be made of
high tensile strength electrolytic copper of a high conductivity
(Class A, in accordance with ANSI)
and insulation material of high quality shall be used. The windings shall
be free from burrs, scales and splinters.
The insulation material of windings and connections shall not shrink, soften or
collapse during service. Thermally upgraded paper shall be used for conductor
insulation. The design, construction and treatment of windings shall give
proper consideration to all service factors, such as high dielectric and
mechanical strenght of insulation, coil characteristics, uniform electrostatic
flux distribution, prevention of corona formation, and minimum restriction to
oil flow.
Moreover, under any load condition, none of the material used shall disintegrate, carbonizer or become brittle under the action
of hot oil.
The coils must be capable of withstanding movement and distortion caused by
abnormal operating conditions. Adequate barriers shall be provided between
windings and core as well as between high voltage and low voltage windings. All
leads or bars from the windings to the terminal boxes and bushings shall be
rigidly supported. Stresses on coils and connections must be avoided.
Due to very unfavorable short-circuit conditions and numerous short-circuits in
the network, special measures have to be taken to increase the capability of
the winding to withstand short-circuit currents. Winding and arrangement of
coils shall be designed so as to unify the initial potential distribution
caused by impulsive traveling waves, as much as possible, to avoid potential
oscillation and in order to withstand abnormal high voltage due to switching.
To increase the capability of the transformer windings to witstand
electromagnetic forces under short circuit conditions, modern technology in
design and construction shall be applied. (e.g. low
current density, provision of pressure limiting devices and spring elements, use
of perfectly dried pre-compressed pressboard, maintaining a balance of
ampere-turns between windings, ets.)
Measures against coil displacement as generated by the radial and longitudinal
forces shall be considered. Computation of strength against these forces
including the description of the method being applied shall be submitted in
detail.
The tank, conservation, coolers and bushings shall be adequately braced to
withstand ocean shipment, and earthquake with seismic coeffecient of 0.45 g
(horizontal)
c) Short Circuit Withstand Capability
The transformer shall withstand the combined effects of thermal, mechanical and
electromagnetic stresses arising under short-circuit conditions based on the
maximum durations of fault:
Primary Winding: 2 seconds
Secondary Winding: 2 seconds
Tertiary Winding
2 seconds
The maximum sustrained short-circuit current in each windings shall be stated
by the manufacturers. The maximum temperatures of the windings shall not exceed
250 deg C within the seconds duration of fault. All
transformer accessories, parts, components (CT's, bushings, tap-changer, etc.)
shall be capable of withstanding the cumulative effects of repeated mechanical
and thermal over-stressing as produced by short-circuits and loads above the
nameplate rating.
For design purposes, the following network data shall be take
into consideration. The available system fault currents as as follows (in rms):
138 KV: Ik" 60 KA
69 KV : Ik" = 50 kA 13.2 KV : Ik" = 40 kA
The transformer shall be capable of withstanding the resulting successive
short-circuits, without cooling to normal operating temperature between
successive occurence of the short-circuit, provided the accumulated duration of
short-circuit does not exceed the maximum duration permitted for single
short-circuit defined above.
The upper limits of the symmetrical overcurrent due to such
short-circuits as a multiple of rated current shall also be specified by
the manufacturer.
d) Overload Capability
The short-time overload rating and operation of the transformer shall be in
accordance with ANSI C57.92 or IEC 354. All other auxiliary equipment
(bushings, CT's, etc) affected shall be rated to match the transformer overload
rating.
e) Transformer Tanks:
The
tank should have sufficient strength to withstand full vacuum and internal
pressure of 1.0 kg/cm2, with cooling equipment & conservator connected. The
tank cover will be clamped with bolts and nuts, and will be provided with
handhole or manholes of suitable size. All seams and jointwill be oil tight.
Guides within the tank will be furnished to facilitate tanking and untanking, and
to prevent movement of the core and coil assembly, in transit. The casing will
be provided with suitable lugs for lifting the completely assembled transformer
filled with oil. All gaskets will be synthetic rubber bonded cork.
f) Radiators:
The
transformer will be provided with a number of sufficient radiators for
self-cooled (OA) operation. The radiator will be installed on the tank via
radiator valves, so that each radiator can be detached from the tank
independently of the oil in the main tank. The radiator valves will have the
open and close positions clearly marked. Radiators will be equipped with
provisions for draining. Radiators shall be made of galvanized steel.
g) Forced-air-cooling system:
For
forced-air-cooled (FA) operation, the transformer will be provided with
automatically controlled three phase motor-fans actuated from winding
temperature. Fan motor, weather proofed, three phase, Hz, and will be thermal
protected. The cooling-fans will be mounted on the radiators and the control
box will be mounted on the wall of the tank. Motor Voltage: 460 VAC, 3 phase, 60 Hz.
h) On-load tap-changer:
The following tap-changer will be equipped on H.V. side for the regulation of
voltage under loading conditions.
Type Type
UZFRT 550/300
3 phase,60 Hz, 21 positions
Number
of tap positions 21 taps positions
(138KV + 8 X 1380
V, - 12 X 1380 V)
Manufacturer ABB
Motor Drive Mechanism:
Type:
ABB type BUF 3
Motor
Voltage: 460 Volts,
3 Phase, 60 Hz
Contactors Voltage:
230 VAC
Position Transmitter:
230 VAC
Heating Element:
230 VAC
Motor-Drive Mechanism Accessories:
1. Standard Accessories
2. Phase Failure Relay
3.Circuit Breakers for Control
& Auxiliary circuits
4. Accessories
for paralleling with 2 transformers using MASTER-FOLLOWER method.
OLTC Accessories:
1. Oil
Conservator
2. Oil Level
Indicator with contacts for Alarm
3. Dehydrating
Breather
4. Pressure
Relief Valve/Device with contacts for tripping
5. Pressure Relay
with contacts for tripping
6. Oil Flow
Controlled Relay with contacts for alarm
6. Thermoswitch
Housing
7. Valve for oil
filtration mounted on the top
8. Valve for
oil filling, draining & filtration
9. Earthing
terminal
10. Prepared for on - line oil
filter unit
i) Oil preservation system:
Conservator system with sealed diaphragm will be used. Conservator with
low-profile design having a moisture-proof barrier made with an oil-resisting
diaphragm will be applied and placed at the level slightly higher than the
transformer tank.
j) Bushings:
Primary:
ABB type GOB 650-1250-0.3 Brown, Cat # 123
193-K
1250 Amps, Nominal Voltage: 170 KV rms,
Phase to Earth Voltage:145 KV rms, BIL: 650KV,
Creepage Distance: 4080 mm
Porcelain Color: Brown
Short end shield
Secondary:
ABB type GOB 380-800-0.3 Brown, Cat # 123-185-K
800 Amps, Nominal Voltage: 100 KV rms,
Phase to Earth Voltage:72.5 KV rms, BIL: 380 KV
Creepage Distance:2210 mm
Porcelain Color: Brown
Short end shield
Tertiary: CEDASPE s.p.a. Italy type Dt
30 Nf 1000
1000 Amps, Nominal Voltage: 36 KV,
Maximum Voltage to Ground: 30 KV, BIL:170 KV,
Creepage Distance: 640 mm
Porcelain Color: Brown
Threaded Extended Rod
Neutral: CEDASPE s.p.a. Italy type Dt
52 Nf 1000
1000 Amps, Nominal Voltage: 52 KV
Maximum Voltage to Gound: 52 KV, BIL 250 KV,
Creepage Distance: 1080 mm
Porcelain color: Brown
Threaded Standard Rod
Complete with the following accessories:
31. One (1) Buchholz Relay with 2 contacts for alarm
& tripping
32. Two (2) Dial type Oil Level Indicators for Main Tank &
OLTC with contacts for alarm.
33. One (1) Oil Temperature Indicator & Relay type AKM OTI series 34
for alarm.
34. Three ( 3) Winding Temperature Indicators & Relays for HV, LV
& TV windings with
3 contacts each for alarm, tripping & fan
control, AKM type WTI series 35.
35. Qualitrol type self resetting mechanical Pressure Relief Device with
contacts for tripping
36. Conservator for main Tank - Sealed Diaphragm
constant pressure type.
38. Breather type conservator for OLTC.
39. Annunciators (Marshalling Kiosk)
40. Bushing Current Transformers
HV: 300/200/100:5A;
0.6 - B 0.5
LV & Neutral:
600/500/400/300/200/100:5A; C-400
TV :
1200/1000/900/800/600/500/400/300/200/100:5A; C-400
41. Galvanized Steel Radiators
42. Bushing Terminals
HV- Universal 4 hole
NEMA Flat Terminals
LV- Universal 4 hole NEMA Flat Terminals
TV – Universal Multi -hole NEMA Flat
Terminals
43. Sets of Surge Arresters mounted nearest to the HV, LV & TV
transformer bushings,
with Surge Counters & 4/0 AWG THW Copper
conductors connected
to grounding terminals.
HV : 120 KV Voltage Rating, 98 KV MCOV, Station Class, Polymer
housing,
Metal Oxide, Line
Discharge Class 4 per IEC, 12 KJ/KV Energy capability
65 KA Pressure
Relief Capability, Grey Silicone Insulator
ABB type PEXLIM-P
Complete with top
clamps to hold a 336.4 MCM Aluminum Conductor
and 4/0 AWG THW Copper green wire ground conductor connected to ground
terminal.
LV : 60 KV voltage Rating, 48 KV MCOV,
Station Class, Polymer housing, Metal
Oxide, Line
Discharge Class4 per IEC, 12 KJ/KV Energy Capability
65 KA Pressure
relief Capability, Grey Silicone Insulator,
ABB type PEXLIM-P
Complete with top
clamps to hold a 795 MCM Aluminum Conductors
and a 4/0 AWG THW Copper green wire ground conductor
connected
to ground
terminal.
TV : 18 KV Voltage Rating, 15 KV MCOV
Station Class, Polymer housing,
Metal Oxide, Line
Discharge Class 3 per IEC, 9.0 KJ/KV energy capability
65 KA pressure
relief capability, Grey Silicone Insulator,
ABB type POLIM-S
15N
Complete
with top clamps to hold a 795 MCM Aluminum Conductors
and a 4/0 AWG THW
Copper green wire ground conductor connected to
Ground terminal.
44. Neutral Conductor: 4/0 AWG THW Copper wire colored green
connected to
ground pad.
45. Insulating Oil – Shell Diala B or equivalent
46. With provision for Built-in OLTC Insulating Oil Filter Machine, such
as mounting
brackets, connecting flange, connecting valves,
etc.
47. Cooling Fans must be 3 Phase, 460 VAC, 60 Hz, Winding
Temperature Controlled
for Automatic Operation, with automatic/manual
change over switch.
With Circuit breakers for motor overload & short
circuit protection.
48. Grounding Pads for HV Arresters,
49. Steel Ladder with caution marking
50. All External Power & Control cables must be flexible, multicore, PVC insulated
& enclosed in conduit pipes & flexible
hoses.
51. All power & control circuits must be protected by circuit
breakers.
52. Welded Tank Cover.
53. All wiring connections & terminations must be ANSI standard using crimp
type
terminal lugs with insulator caps.
54. All wirings must be color-coded.
55. With replacement gaskets
56. Anchor Bolts
57. One (1) Spare bushing each for HV, LV & TV
58. One (1) Spare OLTC Tap Position Indicator for remote use
59. With provisions for parallel operation of existing power
transformer using
Master - Follower method of OLTC Control.
60. TESTS:
The following tests shall be carried out at the factory
with the presence
of user representative and records of testing will be
submitted.
a. Winding Resistance
Tests
b. Turns Ratio Tests
c. Polarity & Phase Relation Tests on
rated voltage
d. Measurement of no-load losses and excitation
current @ 90%, 100%
& 110% of rated secondary voltage
@ rated frequency
rated voltage connection.
e. Measurement of impedance voltage and load loss
tests at rated current
and rated frequency.
f. Low frequency tests (Applied Voltage &
Induced Voltage) including
partial discharge measurement in terms
of RIV.
g. Leak Test.
h. Routine test certificate for the bushings,
current transformers and surge
arresters shall be submitted.
i. Temperature Rise Test at OA, FA1 & FA2
ratings on the tapping
with maximum losses.
j. Lightning Impulse Test
on HV,
(Full wave, Chopped wave & reduced full wave)
k. Audible Sound level Test at no-load and rated frequency
and
with all fans operating.
l.
Measurement of zero-sequence impedance.
m. Insulation Power factor
n. Insulation Resistance Tests
at ambient temperature.
o. Vacuum test on transformer tank,
conservator & radiators; and pressure
test on tank and
oil-filled compartments.
p. Determination of Capacitances
(windings to ground & between windings)
q. Tests on auxiliary equipment &
accessories ( functional tests
including cooling control)
r. Voltage regulation
s. Measurement of the power taken by the fan
and oil pumps motors.
t. Functional tests on Tap Changer
u. Test on Current Transformers ( Check on polarity, ratio & wiring)
v. Mechanical inspection, ( check of layout,
dimensions, nameplate
data, clearances, etc)
w. Oil tests
x. Efficiency at principal tap and full load
for unity & 0.8 power factor.
61. SPECIAL TEST
Certificate of Short Circuit Test on power
transformers of similar rating
shall be submitted.
62. Other Accessories, Tools
a. Pressure gauge with nitrogen tube and automatic filling device
which
fill the transformert through the tube in
case of any leakage shall
be supplied.
b. Three-dimensional impact recorder with time period recording
chart of
at least 3 months for use during transport of the
transformers.
c. Silica -gel breathers for main and OLTC conservators.
63. Painting
Special attention should be given to the protection of all
iron-work.
The methods propised and the means adopted should be fully
described
in the offer.
All surfaces shall be thoroughly cleaned of rust, scale,
grease and dirt and
other foreign matters and all imperfections shall be removed by
means
of approved methods.
The following treatments shall be applied:
a. External surfaces
All steel surfaces shall be
sand-blasted in accordance with SIS 055900,
and shall then be painted in the following
sequence:
1. two (2) primer coats:
2 x 35 um
(micrometer)
Binder : epoxy resin hardened
with polyamide
Pigment:
titanium dioxide, zinc oxide, zinc phosphate, tinting additives
2. one (1) intermediate coat:
35 um
Binder:
epoxy resin hardened with polyamide
Pigment: titanium
dioxide, micaceous iron oxide, tinting additives
3. one (1) top coat (polyurethane base): 35 um
Binder:
epoxy resin hardened with isocyanat
Pigment: titanium
dioxide, micaceous iron oxide, tinting additives
Coating
thickness: Total
140um
The color code
shall be Munsell Gray No. N7.0
b. Internal surfaces
Inside the transformers
vessel, sand-blasting shall be performed in
accordance with SIS
055900. After that solvent-free,
oil-resistant coating
shall be applied.
The minimum dry film
thickness shall be 40 um.
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http://www.powertransformerdesign.net/2012/05/50-mva-138kv-autotransformer.html
I wish to present here the benefits of utilization and operation of
MV/LV parallel transformers in electrical distribution. Having such
experience, i will be interested to get feedbacks from other engineers.
2
MV/LV parallel transformers distribution network
For all my
experience, I’ve never met a working electrical installation, where operation
of parallel transformers was used. Once, we used this solution in the design of
welding shop, mainly to reduce the voltage drop in the network and maintain a stable level of short-circuit current. But
the project was stopped by the Employer, and the idea was gone. Below is the
list of benefits of such solution, as it is seen. It would be great if
interested engineers commented and shared experiences on this issue.
Generally, application of parallel
transformers allows achieving the following benefits:
1) Reducing the total capacity of electrical transformers (as compared to separate their work). The decrease of total installed capacity
is reached:
·
by lowering the
overall demand load to the diversity of loads connected to different transformers
·
by using a higher load
rate of parallel transformers
·
less required backup
in case of electrical transformer failure
2) Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded
transformers
3) Improving the power quality due to the stable level of short circuit current throughout the network
4) Increasing the reliability of
operation of protective devices in the case of phase-to-earth short circuits in
the network.
5) Possibility of placing electrical
transformers in operation phase-by-phase
Parallel transformers are allowed,
provided that none of the windings will be loaded by current exceeding
allowable current for that winding.
Of course, there are limitations for
using electrical transformers connected in parallel. For instance,
·
windings connections
of transformers have the same vector groups
·
the ratio of
transformers capacities of less than 1.3
·
rates of
transformation differ by no more than +/-0.5%
·
short circuit voltages
do not vary more than +/-10%
·
transformers have same phase polarity
Artem
Kropachev
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