CALCULATION IN 3 PHASE POWER TRANSFORMER

3 Phase Electrical Power Transformer

Most power is distributed in the form of three-phase AC. Therefore, before proceeding any further you should understand what is meant by 3 phase power. Basically, the power company generators produce electricity by rotating (3) coils or windings through a magnetic field within the generator . These coils or windings are spaced 120 degrees apart. As they rotate through the magnetic field they generate power which is then sent out on three (3) lines as in three-phase power. 3 phase transformers must have (3) coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage we need and maintain the proper phasing or polarity.

 3 Phase Power Is More Efficient Than Single PhaseThree phase electricity powers large industrial loads more efficiently than single-phase electricity. When single-phase electricity is needed, It is available between any two phases of a three-phase system, or in some systems , between one of the phases and ground. By the use of three conductors a 3 phase system can provide 173% more power than the two conductors of a single-phase system. Three-phase power allows heavy duty industrial equipment to operate more smoothly and efficiently. 3 phase power can be transmitted over long distances with smaller conductor size.

3 Phase Transformer Delta and Wye Connections  

In a 3 phase transformer, there is a three-legged iron core. Each leg has a respective primary and secondary winding.  

3 Phase Transformer Winding Combination  

As can be seen, the three-phase transformer actually has 6 windings (or coils) 3 primary and 3 secondary. These 6 windings will be pre-connected at the factory in one of two configurations:  

Three primary Windings in Delta and Three Secondary Windings in Wye  

3 Phase Transformer Voltage in Delta and Wye Connections  

Different brands of 3 phase transformers handle the windings in different manners. All Federal Pacific 3 phase transformers have their primary windings pre-connected in a Delta configuration. Therefore, when connected to a three-phase source, each primary winding will have the same voltage across it.  
As you can see there is no point in a Delta at which an equal potential to all three lines and the grounded neutral can be made. This is a disadvantage of a Delta compared to a Wye secondary connection
This Delta secondary connection has only one winding (S3) with a neutral conductor. The mid-point of winding S3 is tapped which gives the XI and X3 to neutral a voltage reading of 120 volts. In a 3-phase system, winding S3 is the workhorse; it has to carry all the 120V lighting and appliance loads plus one-third of all the 3 phase loads. (The 120V loads must not exceed 5% of the nameplate KVA, and the total of the nameplate KVA must be derated by 30%). Winding S1 and S2 cannot carry any 120 volt loads as there is no neutral connection to these windings. Windings S1 and S2 can only carry one-third of the three-phase loads each, and the 240 volt single-phase loads.
*Caution: A240 volt Delta connected transformer with a 120 volt neutral tap creates a condition called “high leg” As indicated in the above diagram, the voltage between Phase B (X2) and the neutral tap will be 208 volts; therefore, no 120 volt single-phase loads can be connected between X2 and the neutral tap.  

Single Phase Transformers Connected to Form Three Phase Bank  

Normally , when 3 phase is required, a single enclosure with three primary and three secondary windings wound on a common core is all that is required. However three single-phase transformers with the same rating can be connected to form a three-phase bank. Since each single-phase transformer has a primary and a secondary winding, then 3 single-phase transformers will have the required 3 primary and 3 secondary windings and can be connected in the field either Delta-Delta or Delta-Wye to achieve the required 3 phased transformer bank.  

3 Phase Transformer: Delta-Delta  

Utilizing 3 single-phase transformers is normally not done because it is more expensive than utilizing 1 three-phase transformer. However, there is an advantage which is called the open Delta or V-Connection and it functions as follows: A defective single-phase transformer in a Delta-Delta 3 phase bank can be disconnected and removed for repair. Partial service can be restored using the remaining single-phase transformer open-Delta until a replacement transformer is obtained. With two transformers three-phase is still obtained, but at reduced power. 57.7 of original power. This makes it a very practical transformer application for temporary emergency conditions  
Open Delta 57.7%  

3 Phase Loads and Single Phase Loads  

If the load is 3 phase, then both the supply and the transformer must be in three-phase. If the load is single-phase the supply can either be single or 3 phase but the transformer need only be single-phase with the primary being connected to two lines on the three phase circuit. With single-phase loads, an attempt to use a transformer with three-phase input and only one phase connected at the output to convert the loading on the line to 3 phase is not practical.  

3 Phase Transformer Sizing with 3 Phase Loads

1) Determine electrical load
A. Voltage required by load.
B. Amperes or KVA required by load.
C. Frequency in Hz (cycles per second).
D. Verify load is designed to operate on three phase.

All the above information is standard data normally obtained from equipment nameplates or instruction manuals.

2) Determine supply voltage
A. Voltage of supply (source).
B. Frequency in Hz (cycles per second).

The frequency of the line supply and electrical load must be the same. A 3 phase transformer is selected which is designed to operate at this frequency having a primary (input) equal to the supply voltage and a secondary (output) equal to the voltage required by the load.

3) If the load nameplate expresses a rating in KVA, a transformer can be directly selected from the charts in the catalog. Choose from the group of transformers with primary and secondary voltages matching that which you have just determined.

A. Select a 3 phase transformer with a standard KVA capacity equal to or greater than that needed to operate the load.

B. Primary taps are available on most models to compensate for line voltage variations. (Refer to question #2 in the Transformer Question and Answer Section of Acme's marketplace.

C. When load ratings are given only in amperes, the following formulas below may be used to determine proper KVA size for the required transformer.

(1) To determine three phase KVA when volts and amps are known:

Three Phase KVA =Volts x Amps x 1.73 /1000

(2) To determine Amperes when KVA and volts are known: Amps = 3 Phase KVA x 1000 /Volts x 1.73

Three Phase Example
Question:

Select a transformer to fulfill the following conditions. Load is a three phase induction motor, 25 horsepower @ 240 volts, 60 Hz and a heater load of 4 kilowatts @ 240 volts single phase. The supply voltage is 480Y/277, three phase, 4 wire.

Answer: Compute the KVA required.
28.2 KVA =240 volts x 68 amps x 1.73 /1000
Heater - 4 KVA

A three phase transformer must be selected so that any one phase is not overloaded. Each phase should have the additional 4 KVA rating required by the heater even though the heater will operate on one phase only. So, the transformer should have a minimum KVA rating of 28.2 - 4 + 4 + 4 or 40.2 KVA.

A 480 delta primary - 240 delta secondary transformer may be used on a 4 wire, 480Y/277 volt supply. The fourth wire (neutral) is not Connected to the transformer. To not overload the transformer, a 45 KVA transformer should be selected.

NOTE: Any two wires of the 240 volts, 3 phase developed by the secondary of the transformer may be used to supply the heater. Any 2 wires of a 3 phase system is single phase.

Three Phase Transformers Overview

Three phase transformers are used throughout industry to change values of three phase voltage and current. Since three phase power is the most common way in which power is produced, transmitted, an used, an understanding of how three phase transformer connections are made is essential. In this section it will discuss different types of three phase transformers connections, and present examples of how values of voltage and current for these connections are computed.

3 Phase Transformer Construction:

A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation.

3-Phase Transformer Connections:

There are only 4 possible transformer combinations:

Delta to Delta - use: industrial applications

Delta to Wye - use : most common; commercial and industrial

Wye to Delta - use : high voltage transmissions

Wye to Wye - use : rare, don't use causes harmonics and balancing problems.