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


Special Cases
In Transformers Sizing Calculations: Paralleled Transformers


Special Cases
In Transformers Sizing Calculations: KFactor Transformers


Special Cases
In Transformers Sizing Calculations, KFactor/Factor K Calculators


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; Transformers with Large Motor Loads .

Special Cases In Transformers Sizing Calculations
4
Transformers with Large Motor Loads

introduction
Many engineers when sizing a transformer for some loads they care only about selecting a transformer larger than the maximum demands they serve, or sometimes larger than the connected loads, so they do one of the following:
Both above methods are nonprofessional, they can be used by nonspecialist for the following reasons:
Note: for best selecting and sizing of a transformer, please
review all the articles listed in above.

Importance
of Short Circuit and Motor
Starting Calculations in Transformer Sizing
Many engineers didn’t know that Short
Circuit Calculations together with Motor Starting Calculations are also used
in Sizing Transformers and this what we will explain herein.
1 Short
Circuit Calculations Contribution to Transformer sizing
Motor
Contribution to Short Circuit Capacity
When sizing the transformer for motor loads, the fault current contribution
from the
motors will not be a consideration for sizing. However, the motor
contribution must be considered when sizing all branch circuit fuses and circuit
breakers. The interrupting capacity ratings of those devices must equal or
exceed the total short circuit capacity available at the point of
application.
Motor
Starting Calculations Contribution to Transformer sizing

Reviewing Some Motor basics
1 Inrush Current
Inrush current is higher than the normal operating current or what is called “steady state” current. An example of an electric motor inrush current is shown in Figure1. It shows the peak current for the first half cycle as being close to 30 amps and then decaying over subsequent half cycles as the motor spools up.
Fig.1: Electric Motor Inrush
Current
Fig.2: Transformer Inrush
Current
2 Motor starting KVA and
LockedRotor
Current
Lockedrotor current is the
steadystate current of a motor with the rotor locked and with rated voltage
applied at rated frequency. NEMA has designated a set of code letters to
define lockedrotor KVA/HP. This code letter appears on the nameplate of all
AC squirrelcage induction motors. KVA per horsepower is calculated as
follows:
For threephase motors:
KVA/HP = √3 x current (in amperes) x volts / (1000 x Hp)
For singlephase motors:
KVA/Hp = current (in amperes) x volts / (1000 x Hp)
Table1: NEMA code letter designations (starting KVA’s)
The lockedrotor kilovoltampereperhorsepower range includes the lower
figure up to, but not including, the higher figure. For example, 3.14 is
letter “A” and 3.15 is letter “B”.
By manipulating the preceding equation for KVA/Hp for threephase motors the
following equation can be derived for calculating lockedrotor current:
LRA = (1000 x Hp x LockedRotor KVA/Hp) / (√3 x Volts)
This equation can then be used to
determine approximate starting current of any particular motor.
Starting KVA = (1000 x Hp x LockedRotor KVA/Hp)
Starting KVA is also
known as “Locked Rotor kVA” or sometimes “Locked Rotor Amperes”.
for example, the approximate starting current for a 7 1/2 Hp, 230 volt motor
with a lockedrotor KVA code letter G would be:
LRA = (1000 x 7.5 x 6.0) / (√3 x230) = 113 Amps
Starting KVA = (1000 x 7.5 x 6.0)
= 45 KVA
For more information about Motor starting KVA and LockedRotor Current, please review our article “Motor Selection Procedures – Part One”.
3 Voltage and Frequency Variation
So, Effect of variations in voltage:
An increase or decrease in voltage may result in increased
heating at rated horsepower load. Under extended operation this may
accelerate insulation deterioration and shorten motor insulation life. The affected motors may trip off as provided
for by its protection – or if not, the motor burns.

General Rule For Sizing Transformers With Large Motor Loads
We must determine the voltage dip caused by the motor inrush on startup.
Note:
The NEMA specification
for a standard motor requires the motor to
be capable of operating at plus or minus 10% of nameplate voltage. Therefore,
the
voltage dip must not exceed 10% of nameplate voltage of the motor.

Voltage Dip Calculation
As explained before, during startup the motor absorbs a
high current which causing a big voltage drop at the secondary of the
transformer and a significant voltage drop supply network. This has an
adverse influence on the operation of other loads and can result in the
stalling and contactor drop leading to blackout of loads.
The voltage dip at the transformer terminals is proportional to the motor load required in startup. As discussed earlier, the voltage drop can be expressed as a percentage of the inrush motor load (Motor Starting kVA) compared to the maximum capability of the transformer (Motor Starting kVA + Short Circuit kVA). % Voltage Drop = (Motor Starting kVA) x 100 /(Motor Starting kVA + Short Circuit kVA)
Note: A voltage sag (U.S. English) equal to
voltage dip (British English).

Case#1: A Single Three
Phase Motor On Single Transformer
The
following calculations will determine the extra kVA capacity required for a
three
phase transformer that is used to feed a single three phase motor that is
started with full voltage applied to its terminals, or,
"acrosstheline."
The following precautions
should be followed:
Example:
A 7,500 kVA, 8.0% IZ, 694.16 kV transformer loaded with a single 2,000 hp,
4.16 kV, Code Letter J motor. Check if this transformer size is adequate or not?
Answer:
a) If the motor controller is acrosstheline (fullvoltage controller):
1 from table1 in above, Starting kVA of 2,000 hp Motor code letter J= 2,000
kVA x 7.6 = 15,200 kVA
2 ThreePhase Short Circuit Capacity of the 7,500 kVA transformer = transformer
KVA X 100 / transformer impedance= 7,500 KVA x 100 /8 = 93,750 kVA
Note: In this case, the 7.5 MVA transformer has a maximum of 93.75 MVA short
circuit capability.
3 The voltage drop on motor inrush will be:
% Voltage dip = (Motor
Starting kVA) x 100 /(Motor Starting kVA + Short Circuit kVA)
%VD at transformer Terminals = 15,200 x 100/(15,200 + 93,750) = 13.95% from
the secondary voltage 4.16 KV
So, VD at transformer Terminals = 13.95% x 4.16 KV = 580 V
transformer Terminal Volts During Motor StartUp = 4160 – 580 = 3,580 V
Result:
The transformer output voltage will drop more than 10%, so this 7.5 MVA
transformer is small for a 2,000 hp motor!
The next higher standard size transformer at 10,000 kVA 8.0% impedance would
have a short circuit output capability of 125,000 kVA which will give %VD at
transformer Terminals 10.84% . so, 12.5 MVA transformer could be the choice
which will have a short circuit output capability of 156,250 kVA which will
give %VD at transformer Terminals 8.865%.

Case#2: Multiple
Motors On A Single Transformer
The minimum transformer kVA is given by transformer manufacturers so that a transformer
may be sized properly for multiple motors.
If there are five motors on one transformer, add the
minimum kVA ratings and then add transformer capacity as necessary to accommodate
the inrush current of the largest motor.
KVA transformer min = kVA
ratings for all motors except largest + transformer KVA as necessary to accommodate
the inrush current of the largest motor (as in above Example)
The following precautions
should be followed:

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