Three-Phase Transformers

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When it is about the distribution of voltages, the system with whose help it is distributed is termed as poly-phase system. Under this system, three-phase transformations take place. This transformation can be successfully made when all the single-phase transformers are connected in a proper manner.

There are mainly 5 connections that are used extensively in this field. They are:

  1. Primary Δ – Secondary Δ
  2. Primary and secondary open Δ
  3. Primary X – Secondary Y
  4. Scott connection,e., Primary T secondary T
  5. Primary Δ – secondary Y, or Secondary Y – Primary Δ

As per the above 5 connections, the common ones are:

  • Δ-Δ connection
  • Δ-Y connection
  • Y-Y connection
  • T-T connection
  • Y-Δ connection
  • V-V connection

Here, T-T connection is known as Scott connection, and V-V connection is known as anopen delta.

All of these connections are used extensively in commercial field.

  1. Δ-Δ connection

In these types of connections, in both primary and secondary sides,abank of transformer is connected. The connection is in Δ. In this connection, there are three-phase lines at the upper section, and lower side depicts the load. In most cases, one can find this connection being used in systems where the voltages are not too high.

Also, you can see this system being used in those areas where a constant supply of voltage is required to be maintained. In such situations where there are chances of a transformer to fail, Δ – Δ connection is used.


  • In case there is an unbalanced load on the secondary section or side, a transformer will not face any complication.
  • Even if there is a failure in one of the transformers, you can switch it offoperation and out of the line. The main operation will continue but only at a lower power level. This operation is known as V – V or open delta.
  • If taken in considerationto anunbalanced load, system voltages generated by this connection are more stable.
  • Due tothird harmonic component of magnetizing of unbalanced load, distortion of flux is absent.As a result of this, the flow of current is possible in delta connected primary windings. And this procedure takes place without that current flowing through the line wires.


  • The snag factor in this connection is the omission of star point. In case by default, if one of the lines get earthed, maximum voltage between core and windings will have full line voltage.
  • More insulation is required if this connection is compared with Y – Y connections.
  1. Δ-Y connection

This connection has 3 phase line at the upper portion and 3 load points at the lower section. In areas where it is required to step up that voltage for smooth transformer operation, this type of connection is used. One of the instances can be the initiation of a high-tension transmission system.

As per this system, we can see,

  • The transformation ratio of every transformer is certain times the ratio of its secondary to primary voltage.
  • For the distribution of (3-phase) and (4 wire) service, the neutral of secondary voltage is established.Beinga dual serving connection, it is used for serving both single-phase lighting circuit and3-phase power equipment. This connection is prevalent in both these areas.

Due to the existence of a Δ connection, this connection does not showcase any objection regarding voltage distortion and neutral floating voltage. The reason for such situation is because of Δ connection which permits third harmonics current to flow through that connection.

If a careful observation regarding this connection is made, you can find that line currents and secondary, as well as primary line voltages,are out of phase. The degree angle of those two aspects is only by 30°.

This minor shift of 30° is the main reason why Y – Y bank of transformers or Δ – Δ bank of transformers cannot be banked for a parallel connection. Even if that adjustment of the voltage ratios is properly made, this parallel banking is beyond the bounds of possibility.

  1. Y-Y connection

As per this connection, there is an involvement of a bank of three transformers. Their connections are both on primary side and secondary side in Y section. The existence of similar ratio between theprimary side and secondary side and line voltage is only possible when transformation ratio of every transformer comes to be K. Only in the case of a balanced three phase load, this connection will provide a complete service which can be deemed as satisfactory.

In case this load comes to be unbalanced, there will be a shift in an electrical neutral from its precise center point. That shift will be to a certain point which has the capacity to make neutral voltage and line voltage variable or unbalanced. This Y – Y connection has a rival in the form of Δ – Δ connection.

Plus points regarding this connection

  • In case of these winding wire, its cross connection is large. This is because its winding or phase current is found to be equal to line current. On the basis of these 2 facts, a conclusion that can be drawn regarding the winding is that its strength is high. Due to its increased strength, it has a certain capacity to bear alarge amount of stress which is imposed on it during short circuits or heavy loads.
  • As the voltage across each winding is times its line voltage, these require only a few turns per winding. This in totality makes it a much cheaper option.
  • Because of less voltage on that insulating material, the dielectric stress on it is also less. Itmakes this equated formation as times the line voltage.

Downside of this connection

  • Third harmonic components are present in the magnetizing current of a transformer. There is a connecting path between a neutral point of an alternator and a primary star point of a transformer. This magnetizing current has the ability to find the return path between the 2. But in instances when these connections are missing, an adverse resultalso evolves. The missing connection will make the flux wave distort and will force to produce a voltage that will be having a third harmonic on both secondary and primary side of the transformer. Now, on a certain condition, say if star point on secondaryside of the transformer is either grounded or earthed, you can clearly see in the secondary circuit the appearance of triple harmonic currents. These currents will flow via aneutral wire which can interfere with the phone lines in the near vicinity.
  • Suppose in the distributed network there is an unbalanced load on the secondary side. In this scenario, if the star point is not grounded then its potential can be considered of any value.This assumption can be the reason for full line voltage according in the secondary winding. In order to avoid the shifting of neutralpoint, it is necessary that a connection is made between star points of the alternator winding to the primary star point of the transformer.
  • There will be a poor regulation of phases which can occur because of one condition. That condition is entirely dependent on the fact that star points of secondary side and primary side are not grounded. This situationcan transpire only when there is an unbalanced load in the distribution network.
  • Now, if there is a special case where there is a formation of third harmonics in the alternator voltage, that third harmonics can be seen in the secondary side too (in the voltages). This sudden harmonic formation in the three-phase circuits will lead to thegeneration of triple frequency currents. The flow of such currents are additive in the neutral wire and therefore do not end in cancellation with each other. As a result, these currents will cause adisturbance in the nearby telephone lines.
  1. T-T connection

This connection is also known as Scott connection. After a proper and suitable connection between the windings of the transformers,interconnection of one type ofpolyphase system into another ofpolyphase system is attainable. In the earlier types or modelsof transformers, this type of connection (T-T connection) was used. As per the combination of Scott connection and earlier version of transformer, a 3 phase system is obtainable from a 2 phase system or a 2 phase system was obtainable from a 3 phase system.

As per the representation of this connection, suppose there are 2 single phase transformers which are denoted by T and M. the primaries of both the phases are linked to a 3 phase supply. Both the connections form a true 2 phase system when the secondary of T links with one point of the 2 phase system and the other secondary of M links with the other point.

As per the connection construction, T is termed as teaser and M is highlighted as main transformer. In the central point of the main primary, you can find on end of teaser primary. Now, as per the made connections, to 2 line wires, main primary’s 2 ends are linked. The 2 line wires comprise of 3-wire system and 3-phase. And the left third line wire is linked to a tapping X which is present on the teaser primary.

Now, if we assume that the voltages that are supplied are symmetrical, then the diagram of it will resemble a triangle of voltages. And on angle calculation, we can see it be an equilateral triangle. Let us also consider a straight vertical line from the tip of this triangle to the center of the horizontal line of it.

Suppose the name of the horizontal line is LS. As per the equation which states, the relation between 2 primaries (as per number of turns) stands as,

 NM = LS (horizontal line)

The formulation will be,

A number of turns required for the main transformer M by the number of turns that are required for the teaser T.

  1. Y-Δ connection

Where there is a principle requirement of voltage that requires being stepped down, for those cases this Y-Δ connection is used. One of the common areas where you can see this connection is theend of a transmission line. Other than this, you can also see this connection being utilized in the circuits that distributemoderately low voltage. These types of circuits are utilized for abdicating the transmission voltages from higher voltage point to lower voltage point. It can reduce the voltage level from 8000 V to 230 V or from 4000 V to 115 V.

Points regarding Y – Δ connection

There is a considerablephase shift in case of Y-Δ connection. The shift is between the secondary voltages and primary voltages. This phase shift takes place at a certain angular degree (30°). As per the meaning of this shift a Δ-Δ or a Y-Y transformer bank cannot be paralleled with a Y-Δ transformer bank. This is because of the difference between phasorvoltages that takes place between the 2 systems. Its angular difference (primary to secondary) will be around sin 30°which on calculation stands as 0.5 times. This difference causes circulation of excessive current that runs between transformer banks.

  1. V – V connection

This connection is also known as open Δ connection. Suppose in a Δ-Δ bank, one of the transformers is removed and in its place, a connection is made from a three phase source. This source connectionis with the primary section. That primary section comprises of three phase voltage in three equal parts. When there is no load in the secondary terminals, measurement of this three phase voltage can be taken. This procedure of utilizing 2 transformers and then converting the three phase power is termed as V-V connection or open delta connection.

There are various beneficial points of V-V connection. 3 of the most significant ones are:

  1. Because of non-symmetry of voltage regulation which occurs and shows the effect under load, machinery comprising V – V circuits or open delta exhibits a little voltage unbalance. But such a small degree of unbalance is not usually noticed in various types of commercial loads. In fact, motor loads cannot even notice such minute change.
  2. In the previous benefit, V – V circuits are taken into consideration. A system can revert to this circuit when the primary or secondary parts of one of the transformers,which have a complete Δ – Δ transformer circuit fails. Only then this reversion can take place. Regressing to this circuit makes the operation as an automatic standby.
  • If a V – V circuits’ power handling capacity is taken into consideration, we can see its capacity certain times to the capacity of a complete Δ – Δ connection. This capacity ratio calculation is calculated with the help of same transformers. As this special feature works in both the ways, a circuit is alsoinstalled as V – V connection. The installation of this circuit is only done after understanding its power handling capacity. Another of the factor is also taken into account. The installation of V – V connection is also made by analyzing its expected capacity (power handling) in multiple levels which can be done by adding more than one transformer.

There are certain cases where V – V connection is used. Some of the cases are:

  • In a Δ – Δ bank when one of the transformers fails, it is made sure that theservice provision remains constant.Forcontinuity of work, that faulty transformer is sent for repair, and in its place, a good one is positioned.
  • When a transformer installation does not provide a warranty for a three transformer bank and on comparison too, it can be seen that the three phase load is small.
  • When itis assumed that this load can increase in the future, open Δ connection is chosen as per the assumed specific time

Uses of V – V connection

  • V – V transformer banks are mainly used in A.C. motor
  • In case of two autotransformers, the commonly used circuit in this is V – V circuit. The advantageous part of this circuit is its superior voltage efficiency and regulation and its power handling capacity regarding auto-transformers.

Downside V – V connection

  • If comparison regarding load and V – bank is taken then the average factor of his bank operation is less than its load. Of power factor is checked on the basis of balanced load factor, the percentagestands to be at 86.6%.
  • In the V – V bank, its two transformers work at different power factor. This point has to be kept in consideration except for the case of balanced unity power factor load.
  • When this load increases by a certain point and continues to get raised, the chance of secondary terminal voltages to get unbalanced also increases. This increase in level is to a certain extent.The situation when this operation takes place is the time when the load is in a perfectly unbalanced state.

Three-phase Transformer Construction


On a single common core winding of 3 single phase transformers can take place. There are mainly 3 advantages of such transformer construction. Those are:

  • It has better regulation and efficiency but just slightly more than 3 single phase transformer
  • From the point of view of budget, a single 3 phase transformer is an affordable choice compared to 3 single phase transformers
  • In case of floor space, a 3 phase transformer will occupy less area than the other option

If this choice is considered from the perspective of similar capacity or related to standby, a single 3 phase transformer is a better and economical option. It is better to have an additional 3 single phase transformer with the existing one than having two 3 phase transformers. But this option is only beneficial in budget regarding small corporations. If the requirement is in large central stations, then the beneficial choice would be 3 phase transformers.


Similar to advantages, 3 phase transformers also have 3 disadvantages. Those are:

  • Transportation of these transformers is a difficult issue. As these transformers are extremely heavy than its counterpart (3 single phase transformers), it is tough to move one from one place to another.
  • The construction of these 3 phase transformers is very complex. As a result, if these transformers face any technical problem or by any chance failure occurs, a complete repair is nearly impossible. In fact, the cost associated with these types of machinery repair is also too high.
  • In order to maintain the constant flow of the working procedure in large central stations, it is necessary for them to keep a working substitute. But the cost involving a single phase transformer is way less than one 3 phase transformer.

Types of 3 phase transformers

If considered in a general sense, there are mainly 2 types of 3 phase transformers which are identified and used by large stations. Its types are common to single phase transformers. But its construction is mainly dependent upon its arrangement regarding cores and windings. the types of these transformers  are categorized into shell type and core type.

3 phase shell type transformers

In case of these types of transformers, 3 shell type transformers are placed one above the other. In this condition, only the primary coil is visible. For a more clarified explanation, let us assume that the lowest transformed is named as N, the middle one as M and the topmost one as L. In this type of transformer, all the coils are wound in a similar direction.

In the core area, which is between the adjacent phases, its flux can be seen to be equal to the dual differential phase fluxes. The angular difference between the two fluxes is calculated and is found to be 120°. Now, again after further calculation, the formulation of mutual flux stands to be  x . On further equating this equation to a numeric result, the answer comes as 0.866 of the flux. The calculation states it to be in the center leg.

If throughout the transformer working procedure, its flux density is maintained at a similar level, you can find the requirement of less iron in the common core. This requirement is equal to the combined fluxes of 2 phases. In order to get the aggregate of the fluxes, there is a reversal in case of M. As per these pointers, the aggregate of these fluxes are equal to the flux present in the common core.

As per this calculative scenario of 3 phase shell type transformers, summation of two fluxes has the equal value of either one of the fluxes. In this case, its equation is 0.5 . This equated representation highlights the fact that there is more iron saving in the common core. And for all the above-stated reasons it can generally be seen that shell type 3 phased transformers are wound with a central coil. This specific winding opposes the windings of the 2 outside ones present on both the sides.

3 phase core type transformer

In order to create a common path, 3 core type transformers are placed side by side of each other. This is done for the creation of a return magnetic circuit path. As per the requirement, the windows had to be completely filled by secondary coils and primary coils. These windings have to be present in each of the legs. But because of the structure of these transformers, only the primary coils are visible on the outer portion of the legs. This depiction simplifies the construction. It also proves the actual theory with the transformer construction. Its comparison and balance are dependent on the set up between flux and primary coils.

Again in an instance, if these 3 transformers have identical properties, there will be production of three fluxes.

Parallel operation of 3-phase transformers

In case of single phase transformers, its parallel operation is similar to the conditions which are required for paralleling three phase transformers. Amongst those conditions, three are of importance with these additions. They are:

  1. The sequence of the phase should be same
  2. In casefor transformers that are required to be paralleled, it should be seen that there is equality in phase displacement in both the secondary and primary voltages.
  3. When terminal voltage of secondary and primary side is checked, its reference should be with voltage ratio.

When we are operating with 3 phase transformer, this 3 point should be definitely kept in consideration.

  • When secondary and primary winding’s impedance results are given independently, then the transference of primary impedance should be done after its multiplication of with (transformation ratio)2and then send to secondary impedance.
  • It is for a single phase only that these computations are made. The utilization of valuation of equivalent impedance is dependent on equivalent impedance per phase. This valuation is then again referred to the secondary phase.
  • If you take the case of Δ – Y transformers or Y – Δ transformers, with the help of terminal voltages, it is easy to give voltage ration. But if taken alternately, instead of ration, turn ration comes to be a little different.


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