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:
As per the above 5 connections, the common ones are:
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.
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.
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,
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.
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
Downside of this 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.
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.
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:
There are certain cases where V – V connection is used. Some of the cases are:
Uses of V – V connection
Downside V – V connection
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:
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:
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:
When we are operating with 3 phase transformer, this 3 point should be definitely kept in consideration.