Power and Distribution Transformers Sizing Calculations – Part Three

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 Article “Power 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,

• Special cases.

 And we explained in this article the Resources used to calculate basic ratings of power and distribution transformers, today we will explain Selection Factors for 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.

 

Selection Factors  for Power and Distribution Transformers sizing calculations

 

 

Before going through the equations and calculations for sizing power and distribution transformers, we must know the factors which must be considered before making these calculations. Noting that without fulfilling these factors, your calculations will not be so accurate. These factors we called them the selection factors which are:

1. Voltage class,
2. Load type,
3. Ambient Temperature,
4. Standard Sizes,
5. Cooling Classes,
6. Temperature Rise,
7. Altitude.

 

First: Voltage class:

 There are three classes of voltage: low, medium, and high voltage.

 1- Low Voltage, Three-Wire:

  ANSI C84.1 lists four voltages in this class

 

Low Voltage, Three-Wire
Volt	Application
120/240	Single-phase, three-wire, nominal system is primarily used for residential areas and light industrial loads.
240	Three-phase, three-wire, nominal system is primarily used for residential areas and light industrial loads.
480	Three-phase, Three-wire, nominal system is primarily used for supplying motor loads.
600	Three-wire, nominal system is primarily used for supplying motor loads.

 

2- Low Voltage, Four-Wire, 3 phase:

  ANSI Standard C84.1 lists three voltages in this class:

 

Low Voltage, Four-Wire, 3 phase
Volt	Application
208Y/120	It is typically used in commercial or very light industrial applications.
240/120	It is typically used in commercial or very light industrial applications.
480Y/277	It is used in most industrial applications and very large commercial applications (e.g., large office complexes, commissaries, etc.).

 

3- Medium Voltage, Three-Wire, 3 phase:

 ANSI Standard C84.1 lists 9 nominal voltages in this class:

 

Medium Voltage, Three-Wire, 3 phase
Kilo-Volt	Application
2.4	Voltages in this class are used to distribute large blocks of power and as a utilization voltage for large motors (isolated neutral).
4.16
4.8
6.9
13.8
23
34.5
46
69

  

4-High Voltage, Three phase, Three-Wire:

 ANSI Standard C84.1 lists four voltages in the class:

 

Low Voltage, Four-Wire, 3 phase
Kilo-Volt	Application
115	Nominal system voltages within this class to transmit large blocks of power over long distances.
138
161
230

 

 

Second: Load type

 

1- Normal Load:

 It means that the minimum self-cooled kVA rating of each OA/FA transformer shall be equal to the maximum normal operating load plus projected future load.

 

2- Emergency Load:

Transformers may be operated under emergency conditions at ratings above normal load ratings. However, there will be some sacrifice of life expectancy. ANSI/IEEE Standard C57.91 and C57.92 provide methods for determining the life expectancy of power transformers when they are operated at loads above their listed ratings.

 3- Future Growth Load:

It means that for self-cooled transformers (OA or AA) only, a 10% load growth factor should be added to the calculated load (normal maximum operating load plus projected future load).

 

4- Projected Future Load:

 It is a known load that will be added in the future. Projected future load should not be confused with the 10% load growth factor that was discussed in the immediate previous paragraph.

 

Third: Ambient Temperature

 

• The temperature rise ratings of transformers are all based on an ambient temperature of 30⁰C averaged over a 24-hour period, and the temperature not to exceed 40⁰C at any time.

 

• If the transformer is operated at rated load and at temperatures greater than an average ambient temperature of 30⁰C, some decrease in life expectancy will occur.  

 

• To avoid this decrease in life, ANSI/IEEE Standard C57 requires that the transformer be de-rated as follows:

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

 

Fourth: Standard Sizes

After an initial calculation to determine the kVA load requirements, the next standard (ANSI C57) size transformer is selected for a particular application. The complete lists of standard size liquid-filled and dry-type single-phase and three-phase transformers are as follows:

 

 

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)

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)

 

Fifth: Cooling Classes

 

Table- lists the cooling classes for liquid-filled transformers and Figure 55 lists the cooling classes for dry-type transformers.

Class Code	Method Of Cooling
OA	Liquid-immersed, self-cooled
OA/FA	Liquid-immersed, self-cooled/forced-air-cooled
OA/FA/FA	Liquid-immersed, self-cooled/forced-air-cooled/forced-aircooled
OA/FA/FOA	Liquid-immersed, self-cooled/forced-air-forced-liquid-cooled

Table- : Cooling Classes for Liquid-Filled Transformers

 

Class Code	Method Of Cooling
AA	Ventilated, self-cooled
AFA	Ventilated, forced-air-cooled
AA/FA	Ventilated, self-cooled/forced-air-cooled
ANV	Non-ventilated, self-cooled
GA	Sealed, self-cooled

 

Table-: Cooling Classes for Dry-Type Transformers

 

Sixth: Temperature Rise

 

The rated kVA of a transformer is the kVA output that can be delivered for a specified time, at rated secondary voltage and rated frequency, without exceeding the specified temperature rise under prescribed conditions.

 

1- For liquid-filled power transformers:

 

• In liquid-filled power transformers, the specified time is continuous operation. The rated secondary voltage and frequency are as stated on the transformer nameplate.

• The specified temperature rise is the average rise of temperature in the windings, which is either 55⁰C or 65⁰C and is stated on the nameplate.

• The corresponding hottest spot winding temperatures are 65⁰C and 80⁰C and are not stated on the nameplate.

• Sixty-five degrees average rise is the “preferred” rating specified for modern transformer designs.

• Some transformers have a dual temperature rise rating of 55⁰/65⁰C, with a corresponding dual kVA rating specified on the nameplate.

• Prescribed conditions are an ambient temperature not to exceed 30⁰C averaged over a 24 hour period and not to exceed 40⁰C at any time.

• The kVA ratings of outdoor transformers account for the warming effects of full sunlight during daytime hours.

 

 

2- For dry type transformers:

 

There are 5 classes of insulation:

 

• Class 130⁰C with a 60⁰C average rise,

• Class 150⁰C with an 80⁰C average rise,

• Class 185⁰C with a 115⁰C rise,

• Class 200⁰C with a 130⁰C average rise,

• Class 220⁰C with a 150⁰C average rise.

 

The kVA ratings for dry type transformers have the same ambient temperature basis as for liquid-filled transformers.

   

 

Seventh: Altitude

   

To allow for reduced cooling at higher elevations de-rate the transformer nameplate kVA by 0.3% for each 330 feet over 3300 feet above sea level.

    

 

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

• Calculations procedures,

• Special cases.

 

 So, please keep following.

 

  

 

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