When we require large scale generation of electrical power, 3-phase transformers are generally used. For higher voltages like 110V, 132V, 400V, 75kV, 3-phase transformers are used to step-up the generated voltage to that of transmission line. When we combine 3 single phase transformers with their cores united into a single core assembly, a single unit of 3-phase transformer network is formed. 3-phase transformer has many advantages over single phase transformer like they are reduced in size, require less oil, fewer transformer bushings, occupies less space, saving in cost and more efficient. Similar to single-phase transformer, 3-phase transformer is also of two types; core and shell type.
1. Core-type- In 3-phase transformer of core type, core consists of three legs with magnetic circuits completed through two yokes (at the top and the bottom). The primary and secondary of each phase is wound on each limb. This can be done by stack lamination. Thus, both low voltage and high voltage windings are insulated from each other. Its construction is not symmetrical so there is resultant imbalance of magnetising current. The low voltage winding is placed next to the core while high voltage winding is placed over the low voltage winding having proper insulation among them. All the three legs are placed at 120o to each other. If we supply 3 balanced sinusoidal voltage then balanced flux is produced. If these fluxes are combined, the total flux in the common leg is zero. Thus, this leg can be removed as it has no flux.
2. Shell-type- Shell type is not commonly used. It looks like three single shell type built on top of one another. It is constructed by stacking three single-phase shell transformer. But, it has one advantage over core type that it can be operated in open delta form. If one of the winding is damaged, operation can still be proceed at 58% of the rating. If the phase sequence is balanced, the flux will also be balanced. The combined flux has magnitude equal to each of its component. The imbalance in the magnetic path has very less or negligible effect on the performance of the transformer. Its windings can either be connected in delta or star as required. Area of the combined yoke is same as that of outer leg or bottom or top leg of the yoke.
Various connections and groups:
The primary and secondary of the transformer can be connected in many form either with the primary as delta-connected and secondary star connected or star-deltaa or delta or star-star. Type of connection used depends on the transformer use. When transformers are used to provide three or more than three phase then it is called polyphase transformer.
Star-delta or delta-star connection- Star (Wye) and delta (mesh) is connected to 3-phase transformer connections. As long as there is no-neutral wire, either of these connection operates substantially as in delta-delta connection but since there is a mesh connection on only one side the triplen mesh impedance is now larger and the compensation of the magnetizing current will necessitate a greater divergence of the flux wave from a true sinusoid in star delta connection (Y-D) connection. The primary carry the fundamental component of the exciting current and the secondary carry the third harmonics component whereas in delta-star (D-Y) connection the secondary voltage do not contain any harmonics components. If either of the winding is star-connected, this has advantage of reducing the voltage and thus reducing the number of turns which in turn increase the size of the conductors. This makes the coil windings easier and cheaper to insulate the delta transformers.
Delta-delta connection- Delta connection provides a closed path to triplen emfs. This circulates current in the closed path and are absorbed by the triplen leakage impedance drop so there is no resultant triplen voltage at the line terminals. The impedance to harmonics currents is mostly small and very smalls emfs are sufficient to circulate considerable harmonics currents. But, this has one advantage if one transformer of a group of three should become faulty or disabled, the remaining ones will continue to deliver three-phase power with a capacity equal to approximately two thirds of the original output from the transformer unit.
Star-star connection- The line currents when there is neutral will contain the third harmonics component of the magnitizing component current. Under balanced conditions, the third harmonics component of the three phases will be in-phase with each other and of the same magnitude. The current in the neutral wire of the supply system, therefore will not be zero but will be equal to three times the third harmonics component of one phase. While if there is no neutral, the triplen emfs are all directed simultaneously away from or towards the star point and therefore, cancel between any pair of lines. Therefore, no triplen currents can flow, and (apart from the fifth, seventh, harmonics components) the input magnetizing currents are sinusoidal. The phenomenon of oscillating neutral is highly undesirable and because of their stystar connection without neutral is not used in practice.
Open-delta connection is also known as V-connection. It is usually temporary or emergency connection of three-phase electrical circuit in which one of the three transformer is removed and its load carried by two transformers.
An open delta transformer is a type of three-phase transformer that has two primary and secondary windings, with one side of the delta is "open". This type of connection is very rare and are typically only used for small loads. Open delta transformers are three phase devices, with only two windings on each of the primary and secondary sides. It is cheaper than a conventional three winding transformer, the open delta will only deliver 57.7% of the power of a conventional transformer. If one of the three transformers is failed then the three phase circuit can remain active although the two remaining transformers are limited to about 57% of the total load. Thus, circuit works even after the failure but at a lower overall load factor.
In this connection, the voltage of any one phase at any instant is equal to the sum of the instantaneous voltage in the other two phase. So that if one transformer of the bank is disconnected, the difference of potentials between the line terminal remains unchanged and it is maintained B and C in series.
The kVA rating of each transformer = VI/1000
The transformer output power (in VA) for balanced delta connection is:
VA = 3 VLIPh
But for open delta connection:
VA = (3)1/2 VLIPh
Taking ratio of open delta to closed delta is 0.577 (or 57.7%).
Scott connection is popularly known as T-connection is the method of connecting two single phase transformer to perform the 2 phase conversion to 3 phase conversion or vice-versa. Among two transformer, one is called main transformer and other is called auxiliary or teaser transformer. Both the transformers are connected electrically but not magnetically. In main transformer, there is 50% while in teaser transformer, there is 86.6%.
One end of the primary winding of the teaser transformer is connected to the centre tapping provided on the primary winding of the main transformer. The two ends of the primary winding of the main transformer and 86.6% tapping point on the teaser transformer is connected to a balanced three phase supply. The voltage per turn is same both in primary of the main and teaser transformer. With the equal number of turms on secondaries of both the transformers, the secondary voltage will be equal in magnitude which results in symmetrical two phase system.
Note:- The neutral point N divides the primary of the teaser transformer in the ration 2:1. Also, the teaser transformer has primary voltage rating 0.866 of voltage rating of the main transformer.
It is used in an elecric furnace installation.
It is also used to supply single phase loads like electric train.
It is used to link a three phase system with the two phase system
For parallel operation in transformer, primary winding is connected to common supply while secondary winding is connected to common load. Parallel operation increases the efficiency of the system, make it more flexible and reliable. It is necessary for having the parallel connection in transformer due to follwing reasons:
For large loads, it become impractical to use single transformer while parallel connection makes it easier.
If any of the transformer is found faulty or get damaged still the supply will not be disturbed.
If there is some in the load in future, it becomes easy in parallel connection for expansion of substation.
It reduces the spare capacity of the substation.
If any of the transformer is taken out or removed still there will be no interruption in the supply.
For the suitable and satisfactory parallel connection, necessary conditions are:
The transformer should be connected properly as far as their polarities are connected so that the net voltage around the local loop is zero. A wrong polarity connection result in a short circuit.
The transformers must have the same voltage ratio to avoid no-load circulating current.
Three transformers must have zero relative phase displacement on the secondary sides and also be connected in a proper phase sequence on the transformer of same phase group can be paralleled. Transformer, however be paralleled by reversing phase sequence of one of them.
A type of transformer in which a part of the winding is common to both the primary and secondary circuits is called an auto-transformer. In auto-transformer, one single winding is used and common for both primary and secondary winding.
Let N1 and N2 be the number of turns of primary and secondary winding. When voltage is applied to winding, an exciting current starts flowing through the full winding.
Voltage/turns for primary winding V1/N1 and that across secondary is V2/N2
Let I2 current flows through the load and I1 through the source. Neglecting losses
Input power = output power
V1I1 cost1 = V2I2 cost2
which in turns gives
V2/ V1 = I1/I2 = N2/N1 =K
The weight of conductor (copper or aluminium) for any winding depends upon the cross-sectional area and length of the conductor.
Owing to the reduction in conductor and core material the ohmic losses in conductor and the core loss are lowered.
An auto-transformer has higher efficiency than a two winding transformer of the same output.
Reduction in the conductor material means lower value of ohmic resistance. A part of the winding being common, leakage flux and leakage reactance is less. In other words, an autotransformer has lower value of leakage impedance and has superior voltage regulation than a two winding transformer of the same output.
lf the ratio of transformation K differs far from unity, the economic advantages of autotransformcr over two winding transformer decrease.
There is a direct electrical connection between the low-tension and high tension sides.
The short circuit current in an autotransformer is higher than that in a corresponding two-winding transformer.
Magnetising harmonics and their effects:
We know that even though the secondary is open, the transformer draws current from the supply, when primary is excited by rated voltage. The current is not load current and is basically required to produce core flux. But due to non-linearities of core material, the no load current is non-sinusoidal in nature. No load current has fundamental and odd harmonics. The strongest is third harmonics which is about 40% of the fundamental. This current leads flux by some small angle. This phase shift is caused due to hysteresis. The inrush current in 3-phase transformer occur more often but are less in intensity than in 1 phase transformer. This magnetising current depends on the transformer connections.
If flux wave is sinusoidal, the magnetisation current contains odd harmonics and their percentage with respect to fundamental will increase as the peak flux density increase. On the other hand, if magnetising current is sinusoidal the induced emf become non-sinusoidal and flat-topped.
Effects of transformer harmonics-
Effects are as follows:-
Additional electrical (12R) loss owing to circulating currents.
Increased core loss.
Magnetic interference with communication circuits and protective gear.
Harmonics voltages depending on their magnitude and frequency may cause (i) increased dielectric stresses (ii) electrostatic interference with communication circuit. (iii) resonance between the inductance of the transformer windings and the capacitance of feeder to which these are connected.