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 Course” to 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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