3-phase transformer

Introduction of 3 phase transformer:

As we all know the transformer is a piece of static apparatus that converts an electrical power with the desired change in current and voltage from one alternating-current circuit to another without any change in frequency. It works on the principle of mutual induction. So, does the 3-phase transformers.

Basically, the power generation is usually in 3-phase and it comprises high current and voltages such as kV or MV with the high current rating. For stepping up and down, it is really necessary to have 3-phase transformers because they prove to be economical in the transmission and utilization of electrical energy. In previous times, it was a usual practice to use three separate transformers but now, in the modern age, we use only one 3-phase transformer rather than three single-phase transformers separately.

Advantages over Single Phase Transformer:

Some of the prestigious advantages are as follow:

  • They occupy less space rather than three separate single-phase transformers.
  • They are cost-effective.
  • Only one unit is required for the operator to handle.
  • They weigh less than three separate single-phase transformers.
  • Due to being a single unit, its transportation is easy.
  • The core size gets smaller and less material is required for the core because of being a single unit.

Operation Principle:

The 3-phase transformer consists of three cores arranged and joined together at 120­0 and there forms a common leg among them. Due to this, three separate primary windings are wound on each core and are connected to three-phase AC power of connection RYB carrying currents I­R, IY & IB producing the fluxes ФR, ФY & ФB respectively and the common leg carries the sum of these currents and fluxes which is zero at any instant due to which there is no current or flux on the common leg. If the common leg gets removed then there is no big difference in other transformer conditions because any two cores will provide the return path for the current and the flux in the third core. This is the general principle of operation of 3-phase transformers.

Types of 3 Phase transformer:

Unlike single-phase transformers, 3-phase transformers can be of types:

  1. Core Type: The core comprises three legs having a magnetic circuit completed through two yokes through top and bottom. One of each limb has primary and secondary windings wound with the circular cylindrical coils concentrically. Each limb or leg comprising of primary and secondary winding makes one phase of AC power. Flux flows up each leg in turn, and down the other two legs in general so that the magnetic circuits of different phases become in series and get independent. This type is comprised only of two windows in which each one contains two primary and secondary windings, respectively.
  1. Shell Type: In this type, three phases are much important rather than the core types. There is a reason behind this because, in shell type, each phase has its independent individual magnetic circuit. Its construction is similar to that of single-phase shell type transformer built on the top of each other and its usage is in a rare manner. This type comprises phase magnetic circuits that are parallel to each other and hence independent of saturation effects in the common paths are neglected.

Connection Types:

The primary and secondary windings of 3-phase transformers can be connected in different ways such as delta or star one. With the appropriate connection, power can be stepped up or down. Some basic types are discussed below:

  1. Star-Star Connection: In this type of connection, primary and secondary windings connections are made in a star fashion. This type of connection has proven to be economical for small high voltage transformers as phase voltage is 0.578 times the line voltage and hence due to it the number of turns per phase and the insulation quantity required is minimum. The transformation ratio of each phase is the same as because the line voltage on the primary and secondary sides is the same. The main thing for being noted is that there is a phase shift of 300 between phase and line voltages and the primary voltages and line voltages on both sides are in phase with each other.
  1. Delta-Delta Connection: In this type of connection, both the three-phase primary and secondary windings are connected in delta fashion. This connection type has proven to be economical for large low voltage transformers as the number of turns increases per phase and there is no phase difference between primary and secondary voltages.
  1. Star-Delta Connection: In this type of connection, the primary connection is of star type while the secondary connection is made in delta fashion. This connection type is usually used at the substation end of the transmission line which basically, steps down the voltage. There is a phase difference of 300 between primary and secondary line voltages and the neutral on the primary side is grounded. The delta connection on the secondary side allows third harmonic current to flow which provides the sinusoidal flux.
  1. Delta-Star Connection: In this type of connection, the primary connection made is of delta type while the second one is connected in a star fashion. The main use of having this type of connection is to step up the voltages at the beginning of the high-tension transmission line. The main thing of notice is that there is a phase difference of 300 between primary and secondary line voltages and the secondary ones are the leading one which uses three phase four wire systems due to which both single and 3-phase loads can be supplied with this type of connection.

 Voltage and Current Relationships:   

The following assumptions have been made for deriving the voltage and current relationships.

  • The line voltages VL are 2 volts and the primary line currents are IL.
  • The transformation ratio is K which is

                                                            K = V2/V1 = N2/N1         where V1 & V2 are phase voltages.

  • Balanced loads have assumed.
  • Loads are purely resistive having unity power factor.
  • The transformers have been assumed of ideal behavior having no losses.

Type of Connection
 Primary Side    Secondary Side    
Parameters Line Voltage Phase Voltage Phase Current Phase Voltage Phase Current Line Voltage Line Current
Star-Star VL 0.578* VL IL 0.578* K*VL IL/K K*VL IL/K
Delta-Delta VL VL 0.578*IL K*VL 0.578*IL/K K*VL 0.578*IL
Star-Delta VL 0.578* VL IL 0.578* K*VL IL/K 0.578* K*VL 0.578*IL
Delta-Star VL  VL 0.578*IL K*VL 0.578*IL/K 1.732*K*VL 0.578*IL/K

Related Topics;

  1. All About Transformer
  2. Equivalent circuit of the transformer
  3. Efficiency and losses of a transformer
  4. Characteristics of an ideal transformer
  5. Transformer on load and no load