Subject Of Previous
Articles

Article

Glossary of
Sizing Power and Distribution Transformers,


Resources used
to calculate basic ratings of power and distribution transformers


the selection
factors for the Power and Distribution Transformers


Applicable
calculations procedures for sizing of power and distribution transformers


1Applicable procedures for calculating power and distribution transformer ratios,
2Applicable procedures for calculating power transformer efficiency,
3Applicable procedures for calculating power transformer voltage regulation,
4Special Cases In Transformers Sizing Calculations: Secondary Unit Substations


Note: I’d like
from all of you to review our course “EP3: Electrical Procurement – Transformers Course” to be more
familiar with the contents of our new articles about the Power and
Distribution Transformers sizing calculations.


Today, we
will explain other special cases for Power and Distribution Transformers sizing
calculations; Paralleled Transformers.

Special
Cases In Transformers Sizing Calculations
2
Paralleled Transformers

Here we will explain
the limiting conditions of connecting transformers in parallel and
loading/sizing considerations that must be followed. Firstly, let’s see what
is meant by Paralleled Transformers?
If two or
more transformers are connected to a same supply on the primary side and to a
same load on the secondary side, then it is called as Paralleled
transformers.
Fig1: Paralleled
Transformers
Figure1
shows two transformers connected in parallel. The transformers are connected
in parallel by connecting the similarly marked terminals together. For
example, terminal X1 of T1 to terminal X1 of T2, terminal X2 of T1 to X2 of T2,
etc.

Importance
for using Paralleled Transformers
1 Increased Load:
When load is increased and it exceeds
the capacity of existing transformer, another transformer may be connected in
parallel with the existing transformer to supply the increased load.
2 Nonavailability of large
transformer:
If a large transformer is not
available which can meet the total requirement of load, two or more small
transformers can be connected in parallel to increase the capacity.
3 Increased reliability:
If multiple transformers are running
in parallel, and a fault occurs in one transformer, then the other parallel
transformers still continue to serve the load and the faulty transformer can
be taken out for the maintenance.
4 Transportation is easier for small
transformers:
If installation site is located far
away, then transportation of smaller units is easier and may be economical.

Conditions
for Parallel Operation of Transformer
1 Mandatory conditions for Parallel
Operation:
2 Mandatory Conditions for Optimal
Parallel Operation:

The factors
that should be considered when paralleling transformers

1 Increased
Fault Levels
The
paralleling of the secondaries of transformers increases the short circuit
current available, and, therefore necessitates higher interrupting capacity
and more expensive secondary switchgear (bus, breakers, etc.).
Example#1:
Fig2
Referring to
Figure2, what is the total transformer letthrough current at the secondary
bus?
Answer:
1. IT = I_{LT}(T1)
+ I_{LT}(T2) = 2I_{LT}(T1) = 2 [1500/( 3 x .48 x 0.055) = 2
(32803) = 65.6 kA

2 Circulating
Currents
If the Mandatory
Conditions for Optimal Parallel Operation do not exist, circulating currents
will flow between and through the secondary windings of both transformers.
Definitions:
The circulating current is the
current flowing at no load in the high and low voltage windings, excluding
exciting currents.
Full load current is the current
flowing in the transformer with a load connected, absent of exciting and
circulating currents.
Disadvantages of Circulating Currents:
The magnitude
of the circulating current
The magnitude
of the circulating current that flows depends on which of the above optimal
conditions do not exist. Let’s consider each of these optimal conditions to
determine what occurs if they do not exist as follows:
a If the
turns ratios are not equal
A circulating
current flows even if there is no load on the transformers. This circulating
current flows because the voltage is different on the secondary side of the
transformer. In figure1, if
transformer T1 has a higher turns ratio than transformer T2, then T1 will
have a lower secondary voltage than T2.
b If the
transformers do not have equal impedances
Transformers
will not equally share the load. Current will divide between the two transformers,
but not necessarily equally. . In
figure1, if transformer T1 has a higher impedance than transformer T2, then
more current will flow through T2 than T1. This is the same principle as the
principle of current dividing between two paralleled impedances. If unequal
impedances exist, one transformer can easily overload.
c Different
Phase shift and polarity
Paralleling
two transformers with different phase shifts (e.g., ΔΔ to ΔY) or different polarities
(e.g., subtractive to additive) will cause large secondary circulating
currents to flow.

3 Limiting
kVA
Two
dissimilar transformers (where one of the Mandatory Conditions for Optimal
Parallel Operation doesn’t exist) may be operated in parallel, but the
following two conditions must be met:
Example#2:
Referring to
Figure1, what is the circulating current magnitude that flows between the
paralleled transformers, and what is the limiting kVA of the two transformers
that are being operated in parallel?
Answer:
I_{circ}
= (kV1  kV2)/[(Z1kV1)/(100*I1) + (Z2kV2)/(100*I2)]
Where:
I1 = kVA1/( 3
x kV1) = 12000/( 3 x 14.0) = 494.9 A
I2 = kVA2/( 3
x kV2) = 10000/( 3 x 13.9) = 415.4 A
I_{circ}
= (14.0  13.9)/[(9 x 14.0)/(100 x 494.9) + (8 x 13.9)/(100 x 415.4)] = 19.15
A
The
circulating current must be less than 10% of the smaller of I1 or I2.
I_{circ}
<(0.10 x I1)<(0.10 x 494.9) = 49.5 A
I_{circ }<(0.10
x I2)<(0.10 x 415.4) = 41.5 A
The circulating
current is less than 41.5 A and therefore it is acceptable.
kVA_{limit}
= Z_{min} x [(kVA1/Z1) + (kVA2/Z2)] x 0.9 = 8 x [(12000/9) +
(1000/8)] x 0.9 = 18600 kVA = 18.6 MVA
So, Total
power supplied to the secondary bus cannot exceed 18.6 MVA. If more than 18.6
MVA of load is connected, transformer T2 will overload resulting in possible
damage.

Additional precautions
must be considered before paralleling transformers

Summary
for Parallel Transformers Connections

In the next article, we will continue
discussing other special cases for Power and Distribution Transformers sizing
calculations which are:
 KFactor Transformers,
 Transformers with Large Motor Loads.
So, please keep following.
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