Power And Distribution Transformers Sizing Calculations – Part Four



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:

  • Glossary of Sizing Power and Distribution Transformers,
  • 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”.

 
Also, in ArticlePower and Distribution Transformers Sizing Calculations – Part Two” , we indicate that Our study for the Power and Distribution Transformers sizing calculations will include the explanations of the following points:

  • Resources used to calculate basic ratings of power and distribution transformers,
  • Selection Factors,
  • Calculations procedures For Sizing of Power and Distribution Transformers,
  • Special cases.

 And we explained in this article the Resources used to calculate basic ratings of power and distribution transformers while we explained the selection factors for the Power and Distribution Transformers in article “Power and Distribution Transformers Sizing Calculations – Part Three

Today we will explain the calculations procedures for sizing of Power and Distribution Transformers.

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.

 
 
 
Calculations Procedures For
Sizing Of Power And Distribution Transformers.
 

  
 

In power systems, there are many devices whose proper size is critical to the design of a power delivery system. One of the most important is the power transformer.

There are several factors involved in the process of sizing a transformer which were explained in article “Power and Distribution Transformers Sizing Calculations – Part Three, these factors can be used to determine if a transformer can handle its required operating load or not. Inadequately sized transformers may shorten the equipment's operating life or cause overloading failures.
 
Now, we will explain step by step the accurate sizing calculations of Power and Distribution Transformers which will include the following:

  1. Applicable calculations procedures for sizing of power and distribution transformers,
  2. Applicable procedures for calculating power and distribution transformer ratios,
  3. Applicable procedures for calculating power transformer efficiency,
  4. Applicable procedures for calculating power transformer voltage regulation.

 
 
1- Applicable Calculations Procedures
For Sizing Of Power And Distribution Transformers.
 

  

 
Summary Of Selection/Calculation Procedures
 

 

 
  1. Select Power or Distribution Transformer,
  2. Select Liquid-Filled or Dry-Type Transformer,
  3. Select Primary Voltage,
  4. Select Secondary Voltage,
  5. Select BIL Ratings,
  6. Calculate Load kW and kVAR,
  7. Calculate Site kVA,
  8. Calculate Adjusted Site kVA,
  9. Select Transformer kVA Ratings from Standard Sizes,
  10. Select Transformer Cooling Class kVA Ratings.
 

  

 
Step# 1
Selected data:
 
  • Power or Distribution?
  • Liquid-Filled or Dry-Type?
 

  

 
Select power or distribution transformer based on given information then select a liquid-filled or a dry-type.
 
Selection Guide:
 
  • Power transformer: used between the generator and the distribution circuits and these are usually rated at 500 kVA and above. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. See Fig.1
Fig.1
 
  • Distribution transformers: Using distribution transformers, the primary feeder voltage is reduced to actual utilization voltage for domestic/ industrial use. See Fig.2
 
Fig.2
 
 
How to select between Liquid-Filled or Dry-Type Transformers?
 
 
Liquid-Filled Transformers
Dry-Type Transformers
Applications
Suitable For Very High Voltages And Outdoor Applications
Suitable For Indoor Applications Of 600V And Below
Location To The Load
Further
Closer
Fire Hazard
Higher
Lower
Environmental Contamination On Surroundings And People
Higher
Lower
Size For The Same Rating
Smaller
Bigger
First Cost For The Same Rating
Lower
Higher
Installation Cost For The Same Rating
Higher
Lower
Over-Load Capability For The Same Rating
More
Less
Life Expectancy For The Same Rating
More
Less
No-Load Losses For The Same Rating
Less
More
Audible Sound Level For The Same Rating
Less
More
 

  

 
Step# 2
Selected data:
 
  • Primary Voltage?
  • Secondary Voltage?
 

  

 
Select the primary and secondary voltage ratings from Table-1 and the given information.
For three-phase transformers, assume 3Ø, 3-wire, delta-connected primaries and 3Ø, 4-wire, wye-connected secondaries.
 
 
Voltage
No. of Phases
No. of Wires
120/240 V
 
1 Ø
 
3
 
480 V
 
3Ø
3
 
208Y/120 V
 
3Ø
4
 
480Y/277 V
 
3Ø
4
 
2400 V*
 
3Ø
3
4160 V
 
3Ø
3
6900 V*
 
3Ø
3
13,800 V
 
3Ø
3
34,500 V*
 
3Ø
3
69,000 V
 
3Ø
3
115,000 V
 
3Ø
3
230,000 V
 
3Ø
3
 
Table-1: Standard Voltage Ratings
 

  

 
Step# 3
Selected data:
 
  • BIL (primary winding)?
  • BIL (secondary winding)?
 

  

 
Select the BIL Rating for the transformer from Table-2 for liquid-filled transformers, or from Table-3 for dry-type transformers.
 
 
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 for liquid-immersed transformers
 
 
 
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-3 BIL Ratings for Dry-Type Transformers
 

 

 
Step# 4
Calculated Data:
 
  • Calculate Load kW and kVAR,
  • Calculate Site kVA,
  • Calculate Adjusted Site kVA.
 

 

 
A- Calculate the load kW for each load or combination of loads at rated utilization voltage.
Note: Include any projected (known) future loads.
 
kWload = 3 x kV x I x p.f. or
 
kWload = kVAload x p.f.
 
B- Calculate the load kVAR for each load or combination of loads at rated utilization voltage.
 
Note: Include any projected (known) future loads.
 
kVARload = 3 x kV x I x sin (cos-1 p.f.) or
 
kVARload = kWload x tan (cos-1 p.f.)
 
C- Calculate site kVA.
 
 
kVA(site) = (kW2 total + kVAR2 total)1/2
 
D- Calculate the adjusted site kVA after derating for temperature from Table-4.
 
 
Transformer type
Actual Ambient temp.
De-rating factor
liquid-filled power transformers
 
the average ambient temperature exceeds 300C
1.5% for each 10C over 300C
1500C dry-type transformers
the average ambient temperature exceeds 300C
0.57% for each 10C over 300C
2200C dry-type transformers
the average ambient temperature exceeds 300C
0.34% for each 10C over 300C
 
Table-4: Ambient Temperature De-Rating
 
Example-1:
 
Use a design ambient temperature of 400C.
 
Then:
 
kVA (adjusted) = kVA (site)/0.85 for liquid-filled transformers,
 
kVA (adjusted) = kVA (site)/0.94 for dry-type transformers with 1500C insulation,
 
kVA (adjusted) = kVA (site)/0.96 for dry-type transformers with 2200C insulation.

 


 
 
Step# 5
Selected data:
 
  • Standard kVA rating.
 



 
Select the next standard size kVA-rated transformer from Table-5 for liquid-filled transformers, or from Table-6 for dry-type transformers.
 
Note:
 
For self-cooled transformers (OA or AA only) add 10% for growth.
 
 
 
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-5: Standard Transformer kVA Ratings (Liquid-Filled)
 
 
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-6: Standard Transformer kVA Ratings (Dry-Type)
 

 

 
Step# 6
Selected data:
 
  • Cooling class kVA ratings.
 



 
Select the cooling class kVA ratings as follows:
 
  • From Table-7 for liquid-filled transformers that are rated 750 - 12,500 kVA or
  • From Table-8 for liquid-filled transformers that are Greater Than 12.5 MVA and
  • From Table-9 for dry-type transformers.
 
 
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-7: The Cooling Class kVA Ratings For Liquid-Filled Transformers (750 - 12,500 KVA)
 
 
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-8: The Cooling Class kVA Ratings For Liquid-Filled Transformers Greater Than 12.5 MVA
 
 
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-9: The Cooling Class kVA Ratings for Dry-Type Transformers
 
 
 
Notes:
 
  • The forced-cooled kVA ratings of each transformer serving a double-ended substation shall be capable of feeding the entire load of both buses with the bus tie breaker closed.
  • Forced-air cooling shall be provided on all transformers that are rated 2500 kVA and larger.
 

 

In the next article, we will explain the following calculations procedures:

  •  Applicable procedures for calculating power and distribution transformer ratios,
  • Applicable procedures for calculating power transformer efficiency,
  • Applicable procedures for calculating power transformer voltage regulation.

  So, please keep following.

 

 

 
 

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