# Power And Distribution Transformers Sizing Calculations – Part Four

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:

• 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

 Select Power or Distribution Transformer, Select Liquid-Filled or Dry-Type Transformer, Select Primary Voltage, Select Secondary Voltage, Select BIL Ratings, Calculate Load kW and kVAR, Calculate Site kVA, Calculate Adjusted Site kVA, Select Transformer kVA Ratings from Standard Sizes, 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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