Power and Distribution Transformers sizing calculations – part Two


In Article “Power and Distribution Transformers sizing calculations – part One”, we indicate that the contents of our articles for Power and Distribution Transformers sizing calculations will include the following points:

  • Glossary of Sizing Power and Distribution Transformers,
  • Power and distribution transformer components,
  • Power and distribution transformer classification: construction and application,
  • Three-phase power and distribution transformer connections,
  • Power and Distribution Transformers sizing calculations.


The following points were explained before ( or will be explained) in our course “EP-3:Electrical Procurement – Transformers Course” :

  • Power and distribution transformer components,
  • Power and distribution transformer classification: construction and application,
  • Three-phase power and distribution transformer connections,

So, we will not go through these points here, we will focus only on the following two points:

  1. Glossary of Sizing Power and Distribution Transformers,
  2. Power and Distribution Transformers sizing calculations.


And we already explained the Glossary of Sizing Power and Distribution Transformers in Article “Power and Distribution Transformers sizing calculations – part One” , today we will explain the second point; Power and Distribution Transformers sizing calculations.

Note:

I’d like from all of you to review our course “EP-3:Electrical Procurement – Transformers Courseto be more familiar with the contents of our new articles about the Power and Distribution Transformers sizing calculations.

  

Power and Distribution Transformers sizing calculations






Our study for the Power and Distribution Transformers sizing calculations will include the explanations of the following points:

  1. Resources used to calculate basic ratings of power and distribution transformers,
  2. Selection Factors,
  3. Special cases,
  4. Calculations procedures.



1-      Resources Used To Calculate Basic Ratings Of Power And Distribution Transformers

  • The Resources used in our study to calculate basic ratings of power and distribution transformers will be the ANSI/IEEE Standard C57 (Distribution, Power, and Regulating Transformers).
  • We will go through the important points in the ANSI/IEEE Standard C57 that will lead to the accurate sizing of power and distribution transformers as follows:

1. C57.12.00 specifies the following service conditions for liquid-immersed distribution and power transformers:

  1. Temperature should not exceed an average ambient temperature of 300C.
  2. Altitude should not exceed 3300 ft (1000 meters).
  3. Voltage should be approximately sinusoidal.
  4. Load current should be approximately sinusoidal and a harmonic content should not exceed 0.05 per unit.
  5. Frequency should be at least 95% of the rated value (e.g., 60 Hz).


2. C57.12.00 lists the following preferred continuous kVA ratings for liquid immersed power and distribution transformers (Table-1).

  


Single-Phase Transformers

Three-Phase Transformers

kVA

kVA

kVA

kVA

kVA

kVA

3


167


5000


15


1000


15,000


5


250


6667


30


1500


20,000


10


333


8333


45


2000


25,000


15


500


10,000


75


2500


30,000


25


833


12,500


112.5


3750


37,500


37.5


1250


16,667


150


5000


50,000


50


1667


20,000


225


7500


60,000


75


2500


25,000


300


10,000


75,000


100

3333

33,333

500


12,000

100,000




750




Table-1: Standard Transformer kVA Ratings (Liquid-Filled)
3. C57.12.00 lists the following basic dielectric insulation levels (BIL) for liquid-immersed transformers (Table-2).


Nominal

System

Voltage


kV

rms

Transformer

Category

BIL

Full

Wave

kV

crest

Chopped Wave

Low Frequency

Test Label


kV

rms

Minimum

Voltage


kV

Crest

Minimum

Time to

Flashover


sec

0.48 and

below


Power and

Distribution


30


36


1.0


10


2.4


Distribution


45


54


1.5


15


2.4


Power


60


69


1.5


15


4.16


Distribution


60


69


1.5


19


4.16


Power


75


88


1.6


19


13.8


Distribution


95


110


1.8


34


13.8


Power


110


130


2.0


34


34.5/19.9


Distribution


150


175


3.0


50


34.5/19.9


Power

200


230


3.0

70


69


Power

350


400


3.0

140


115


Power

450


520


3.0

185


230


Power


825


950


3.0


360

Table-2: Transformer BIL Ratings
 
 
 
4. C57.12.00 specifies that the angular placement for three-phase transformers shall be in accordance with Figure-1.





Figure-1: Phase Relation of Terminal Designations

 
5. C57.12.00 list four categories for the rating of transformers (Table -3):
 
Category
Single-Phase
kVA Ratings
Three-Phase
kVA Ratings
Ia
5 to 500
15 to 500
II
501 to 1667
501 to 5000
III
1668 to 10,000
5000 to 30,000
IV
above 10,000
above 30,000

Table-3: Transformer Categories (Liquid-Filled)


a :Category I shall include distribution transformers manufactured in accordance with ANSI C57.12.20 [B3] up through 500 kVA, single phase or three phase. In addition, autotransformers of 500 equivalent two winding kVA or less, which are manufactured as distribution transformers in accordance with ANSI C57.12.20 [B3], shall be included in Category I, even though their nameplate kVA may exceed 500.


6. C57.12.00 specifies that power transformers that are rated 200 kVA and below, and that have high voltage ratings 8660 volts and below, shall have additive polarity. All other single-phase transformers shall have subtractive polarity.


7. C57.12.00 specifies that the average winding temperature rise above ambient temperature shall not exceed 650C when measured by resistance and that the winding hottest-spot temperature rise shall not exceed 800C.

Note: Transformer ratings are based on a 24-hour average ambient of 300C.


8. C57.92 lists the following derating factors for 550C or 650C rise transformers that are being operated at ambient temperatures above or below 300C (Table-4).

Note: Table-4 is valid for temperatures that range from 00C to 500C.


Type of Cooling
% of Rating
Decrease Load for
each 0C
Higher Temperature
Increase Load for
each 0C
Lower Temperature
Self-cooled -- OA
1.5
1.0
Water-cooled -- OW
1.5
1.0
Forced-air-cooled -- OA/FA, OA/FA/FA
1.0
0.75
Forced-oil-cooled -- FOA, FOW and OA/FOA/FOA
1.0
0.75

Table-4: Transformer Loading on Basis of Temperature for Liquid-Filled Transformers


9. C57.12.10 lists the following OA/FA ratings for power transformers that are rated 750 - 12,500 kVA (Table-5).
 

Single-Phase (kVA)
Three-Phase (kVA)
Three-Phase (kVA)
Without Load Tap Changing
With Load Tap Changing
OA
FA
OA
FA
OA
FA
833
958
750
862
-
-
1 250
1 437
1 000
1 150
-
-
1 667
1 917
1 500
1 725
-
-
2 500
3 125
2 000
2 300
-
-
3 333
4 167
2 500
3 125
-
-
5 000
6 250
3 750
4 687
3 750
4 687
6 667
8 333
5 000
6 250
5 000
6 250
8 333
10 417
7 500
9 375
7 500
9 375
-
-
10 000
12 500
10 000
12 500

Table-5: OA/FA Ratings (750 - 12,500 kVA)


10. C57.12.10 lists the following OA/FA/FA ratings for power transformers that are rated 18 MVA and larger (Table-6).

OA
First-Stage
Second-Stage
18 000
24 000
30 000
21 000
28 000
35 000
24 000
32 000
40 000
27 000
36 000
45 000
40 000
53 333
66 667
45 000
60 000
75 000

Table-6: OA/FA/FA Ratings (Greater Than 12.5 MVA)


11. C57.12.01 specifies the same usual service conditions for dry-type transformers as for liquid-filled transformers.


12. C57.12.01 lists the following continuous kVA ratings for dry-type power and distribution transformers (Table-7):




Single-Phase Transformers

Three-Phase Transformers

kVA

kVA

kVA

kVA

kVA

kVA

1


167


5000


15


300


3750


3


250


6667


30


500


5000


5


333


8333


45


750

7500


10


500


10,000


75


1000


10,000


15


833


12,500


112.5


1500


12,000

25


1250


16,667


150


2000


15,000


37.5


1667


20,000


225


2500


20,000


50


2500


25,000





75


3333

33,333




100







Table-7: Standard Transformer kVA Ratings (Dry-Type)



13. C57.12.01 lists the following limits of temperature and temperature rise ratings for dry-type transformers (Table-8).




Average Winding Temperature

Rise by Resistance (0C)


Insulation System

Temperature (0C)

60


130


80


150


115


185


130


200


150


220



Table-8: Limits of Temperature and Temperature Rise Ratings for Dry-Type Transformers



14. C57.12.01 lists the same categories of dry-type transformers as liquid-filled transformers, except that Category IV does not exist for dry-type transformers.



15. C57.12.01 lists the following BIL ratings for dry-type transformers (Table-9).






Nominal

System

Voltage






kV rms

BIL








kV

Low

Frequency

Voltage

Insulation

Level




kV crest

Full

Wave

1.2 x 50 s






kV crest

Crest








kV crest

Impulse Levels

Chopped Wave

Minimum Time To

Flashover


Sec


0.48 and

below


10


4


10


10


1.0


2.4


20


10


20


20


1.0


4.16


30


12


30


30


1.0


13.8


60


19


60


60


1.5


34.5


150


50


150


150


2.25


Table-9: BIL Ratings for Dry-Type Transformers



16. C57.12.51 lists the AA/FA ratings for dry-type transformers (Table-10).





Self-Cooled

(AA) Ratings

(kVA)

Forced-Air-Cooled

(AA/FA) Ratings

(kVA)

750


1000


1000


1333


1500


2000


2000


2667


2500


3333


3750


5000


5000


6667


7500


10000


Table-10: AA/FA Ratings for Dry-Type Transformers



17. C57.96 lists the following derating factors for dry-type transformers that are being operated at temperatures above or below 300C (Table-11).


Note: Table-11 is valid for temperatures ranging from 00C to 500C.

 
Type of Unit
Maximum Rated
Hottest-Spot
Temperature 0C
Hottest-Spot
Temperature
300C Ambient
Percent of Rated kVA/0C
Increase for Average Ambient
Less than 300C or Decrease for
Average Ambient Greater Than 300C
Ventilated
150
140
(0.57)
Self-Cooled
185
175
(0.43)
220
210
(0.35)
Sealed
150
140
(0.65)
Self-Cooled
185
175
(0.49)
220
210
(0.40)
Table-11: Transformer Loading on Basis of Temperature for Dry-Type Transformers

In the next article, we will focus on the other points in Power and Distribution Transformers sizing calculations which are:

  • Selection Factors,
  • Special cases,
  • Calculations procedures.
So, please keep following.





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