paralleling power transfomer
Other
necessary condition for parallel operation
- All parallel units must be supplied from the same network.
- Secondary cabling from the transformers to the point of paralling has approximately equal length and characteristics.
- Voltage difference between corresponding phase must not exceed 0.4%
- 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.
- 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.
- There should be Directional relays on the secondary side of the transformers.
- 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
Transformer Parallel Connection 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
|
Connections
- 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.
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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 and synchronous scope. If Secondary of Transformer is not
LT Then we must use check synchronizing relay and Commission the system
properly. After connecting relay. Relay must be charges with only 1 supply and
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 and 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 and outgoing 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 any 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.
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