Polyphase transformer pdf

When we require large scale generation of electrical power, 3-phase transformers are generally used. For higher voltages like V, V, V, 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. Similar to single-phase transformer, 3-phase transformer is also of two types; core and shell type.

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 o 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. 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 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-. 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 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 transformer output power in VA for balanced delta connection is:.

But for open delta connection:. Taking ratio of open delta to closed delta is 0. Scott Connection:. 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. 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 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 Also, the teaser transformer has primary voltage rating 0. 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.

Parallel connection:. 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 a kind of leakage transformer the leakage inductance of the secondary winding is used in combination with external capacitors so that it can create one or more resonant circuits. Tesla coil is a resonant transformer which is capable to produce very high voltages and it is capable to produce high current than the electrostatic high voltage generation machines. It is used in audio circuits and it can block the radio frequency interference or the DC component of audio signals.

It could provide impedance matching between high and low impedance circuit. It is a magnetic device so it is susceptible to external magnetic fields. It could split or combine audio signals, it can do impedance matching between a high impedance instrument output and a low impedance input of a mixing console.

It is used to measure the current and voltage in an electrical power system and for the protection and control of power system.

Isolation of measurement protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled. Instrument transformers have predictable characteristics on overloads, voltage or current is too large to be conveniently used by an instrument so it can be scaled down to a standardized low value.

Large transformers are used in power distribution or electrical substations and its core and coils are immersed in oil which is done for the cooling. The oil will circulate through the duct in the coil and around the coil and core assembly.

Oil is cooled outside of the tank in small ratings and for large ratings air-cooled radiator is used. If the transformer is used in underground or in a building. Oil pumps are used to circulate the oil. It can be used for transmitting rectangular electrical pulses it has a duty cycle which is less than 1.

The pulse shape of a pulse transformer must have low values of leakage inductance and distributed capacitance and a high open-circuit inductance to reduce the distortion. Mostly electric transformers are used to transform voltage from a higher level to lower voltage. Transformers can also be used to change the impedance of an electrical circuit to reduce the short circuit current.

The properties of the isolation transformer can be applied to any transformer where there is no direct connection between the primary and the secondary windings. It provides isolation rather than voltage transformation. It is a measurement device which is designed to provide a current in its secondary coil proportional to the current flowing in its primary. These transformers are commonly used in metering and protective relaying.

These transformers safely isolate measurement and control circuitry from high voltages. Wideband current transformers with an oscilloscope can be used to measure high-frequency waveforms or pulsed currents.

These transformers can be used for metering and protection in high voltage circuits, it is another type of instrument transformer.