# Power and Distribution Transformers sizing calculations – part Two

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 “

• 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

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)

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.