16-QMP-05-3B
DESIGN GUIDE
FOR
TRANSFORMER SIZING
3 29.07.11 General Revision due to change of Organization name and
MUKK AV BPN
Issued for Implementation
Rev. Date Description Prpd. Chkd. Appd.
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
New Delhi Sheet 1 of 8
16-QMP-05-3B
REVISION HISTORY SHEET
Rev. Date Description Prpd. Chkd. Appd.
0 01-09-04 First issue IB ASA NS
General Revision And Issued due to change in organization
1 05.01.07 AP JKK RNS
name
2 24.03.09 General Revision And Issued for implementation MOK DBS RNS
3 29.07.11 General Revision due to change of Organization name and
MUKK AV BPN
Issued for Implementation
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 2 of 8
16-QMP-05-3B
CONTENTS
1.0 PURPOSE 4
2.0 PROCEDURE 4
3.0 STEPS FOR TRANSFORMER SIZING 4
4.0 CONCLUSION 8
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 3 of 8
16-QMP-05-3B
1.0 PURPOSE
The purpose of this document is to lay down the guidelines and design procedure for
performing the distribution transformer sizing calculation, rather to say verification
of kVA rating and fixing of transformer ‘%Z’ and/or starting method of big motors as
generally kVA rating of transformer is fixed during conceptual engineering.
2.0 PROCEDURE
The sizing of the transformer is selected based on the following conditions.
a) Peak load calculated from continuous, intermittent & standby loads
b) Spare capacity for any falling load growth minimum 20% indicated by client or
according to project requirement.
c) Actual load requirement with highest rated motor started on DOL
3.0 STEPS FOR TRANSFORMER SIZING
3.1 Step-1 - Calculation of Total Peak Load
Peak (maximum) load is calculated from continuous, intermittent & standby load as
mentioned below: -
Maximum load = continuous loads * X + Intermittent * Y + (Standby * Z or highest
Rating of standby load/ motor)
Where coincidence factor X, Y & Z are:
For Continuous load X = 1.0
For Intermittent Load Y = 0.3
For Standby Load Z = 0.1
Continuous, Intermittent & Standby loads are obtained from load list. The values of
required load (kW), motor rating (kW) & Load factor are obtained from load List.
On the basis of load list, load Analysis is performed taking values of efficiency &
power factor from in house data. The coincidence factors as mentioned above shall be
considered for load analysis. The values of efficiency & power factor will be revised
after receipt of vendor data.
From the load analysis, the values of peak load for various voltage levels are
obtained.
3.2 Step-2 – Calculation Of Transformer Capacity
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 4 of 8
16-QMP-05-3B
From the value of peak load obtained in step-1, transformer capacity is calculated as
mentioned below: -
Transformer Capacity = (peak Load) *1.2 [considering 20% spare capacity]
In case 20% margin does not fulfill the voltage regulation limit then the margin can
be increased up to 30%.
3.3 Step-3 – Determination of allowable % voltage drop at different section and
target transformer regulation
3.3.1 Prior to transformer regulation calculation allowable voltage drops at different section
of electrical distribution system are to be determined and tabulated in a proper tree
structure as shown below.
Say allowable voltage drop at any consumer end is 6% of equipment rated voltage
(as applicable for offshore jobs or according to class society requirement)
CAUTION: Switchboard rated voltage and electrical motor rated voltage may
be different. In such case no load ratio of transformer should be selected judiciously
so that motor end terminal voltage does not exceed the permissible voltage during
start-up of plant i.e. very less load.
Figure inside rectangle indicates an example of voltage at that node considering
cumulative drop of upstream sections and % drop as indicated above.
3.3.2 Depending on electrical distribution configuration, target distribution transformer
regulation % to be fixed say 4 % according to clause .3.3.1
3.4 Step-4- Transformer regulation at running condition
At Running conditions following parameters are to be fixed / derived from load
analysis report.
(a) Maximum running load in kW at transformer secondary bus (kWr)=say 2300kW
(b) Pf of running load in load (Pfr) = say 0.8
(c) Low voltage rating of transformer secondary (kVt) =say 0.433kV
(d) Voltage rating of LV switchboard bus (kVb) =say 0.415 kV
(e) Transformer rating calculated (kVA) = say (2300/0.8)*1.3=3737
~say 4000 kVA(selected)
In case short circuit calculation at LV level is not performed prior to this
calculation , % impedance of transformer to be determined approximately by
following with due consideration of standard values as per relevant standrad
subject to further verified through short circuit calculation latter.
(f) Fault current to be restricted at LV level = say 40 kA at 415V =
28.75MVA
(g) Considering no paralleling ,
% impedance to be restricted at lower side = 4 / 28.75=13.9%
which is not a acceptable value. So allowable fault current to be increased ,
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 5 of 8
16-QMP-05-3B
say 65kA=46.7 MVA and
% impedance recalculated = 4 / 46.7 =8.56 %
So % impedance to be considered for transformer regulation =
8.56 % +10 % tolerance = 9.42 %
(h) Running current Ir= I load = kWr / 3 * Pfr * kVb)
=2300 / (1.732 * 0.8 * 0.415)
=4000 A
Transformer secondary rated current = I rated
= kVAt / 3 * kVb = 4000/ 1.732 * 0.415
=5565A
Pfr = cosφ = 0.8 ; sinφ = 0.6
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 6 of 8
16-QMP-05-3B
Z = 9.42 %
(i) Voltage regulation of transformer in %
=[ Z sinφ + {(Z cosφ ) 2 / 200}] * (I load / I rated)
=[9.42 * 0.6 + {(9.42 * 0.8)2 / 200}] * (4000/5565)
=4.26 %
Which is beyond allowable limit. To meet the design consideration,
Alternative – 1
0.26% Voltage drop at downstream side to be decreased from value considered in
clause 3.3.1
Alternative - 2
10 % margin considered on Z in clause 3.4 (g) to be reconsidered as 5 %
i.e. Z = 8.56 % + 5 % tolerance = 8.988 %
And voltage regulation of transformer recalculated to 4.06 % as per formula in
clause 3.4(i)
(j) Assuming Alternative – 2 is envisaged, %Z acceptable limit = 8.56 % to 8.988 %
and accordingly further detailed short circuit calculation to be performed for
verification.
(k) Alternatively kVA rating of transformer can also be suitably increased to meet
voltage regulation subject to allowable fault level with in limit but generally
transformer kVA rating is fixed during conceptual engineering and not practical
to change rating to meet the voltage regulation.
3.5 Step-5- Transformer regulation at starting of biggest motor
(a) Running load (kWr) = 2050 kW
(b) Biggest motor (kWs) = 250 kW having 6 times starting current and pf=0.85
(c) Running pf (pfr) = 0.8
(d) Starting pf (pfr) = 0.3
(e) Running current (Ir)
=kWr / 3 * kVb * Pfr)
= 2050 / (1.732 * 0.415 * 0.8) at pf 0.8
= 3565 A at 0.8 pf
= (2852 + j 2139) A
(f) Starting current (Is)
=(kWs / 3 * kVb * Pfr }) * starting current
=250 / (1.732 * 0.415 * 0.85) * 6
=2455 A at 0.25 pf
= (736.5 + j 2342) A
(g) Total load current (I load) =Ir + Is
=3588.5 + j 4481
= 5740 A at 0.625 pf
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 7 of 8
16-QMP-05-3B
(h) cos φ = 0.625
sin φ = 0.78
(i) So transformer voltage regulation according to formula in clause 3.4 (i)
= [(8.988 * 0.78) + (8.988 * 0.625)2 / 200] * 5740 / 5565
=7.40 %
Which means additional voltage regulation = 3.40 % and running consumer end
terminal voltage shall be 6 + 3.40 % = 9.40 % during starting of biggest motor.
For practical purposes it is suggested to consider reduced voltage starting like star /
delta for
Motor greater than or equal to 110 kW or as per project design basis. In this case if
star-delta standby starter is considered i.e. 2 times starting current, the transformer
voltage regulation shall be 4.88 % according to formula in clause 3.4 (i) which will
not affect the operation of running loads i.e. 6.88 % terminal voltage at running
consumer end for duration of biggest motor starting time.
4.0 CONCLUSION
Unless both short circuit limit of switchboard and voltage regulation limit of
distribution transformer are achieved within acceptable limit, following major
parameters are to be reviewed and recalculation to be done
a) Transformer impedance range
b) Transformer kVA rating
c) LV switchboard short circuit limit
d) Distribution of permissible voltage drop at different sections
e) Starting method of biggest LV motor
f) No load ratio of transformer
g) Specification of on load tap changer, if applicable
Std. / Doc. Number Rev.
DESIGN GUIDE FOR
06-DG-007 3
Triune Energy Services Pvt. Ltd. TRANSFORMER SIZING
Word
New Delhi Sheet 8 of 8
