3-phase trasnformers (1).pptx For electrical engineering

Transformer
electrical transformer is a static electrical machine which transforms electrical power from one circuit to
another circuit, without changing the frequency. Transformer can increase or decrease the voltage with
corresponding decrease or increase in current.
Working principle of transformer
The basic principle behind working of a transformer is the phenomenon of mutual induction between two
windings linked by common magnetic flux. The figure at right shows the simplest form of a transformer.
Basically a transformer consists of two inductive coils; primary winding and secondary winding. The coils are
electrically separated but magnetically linked to each other. When, primary winding is connected to a source of
alternating voltage, alternating magnetic flux is produced around the winding. The core provides magnetic path
for the flux, to get linked with the secondary winding. Most of the flux gets linked with the secondary winding
which is called as 'useful flux' or main 'flux', and the flux which does not get linked with secondary winding is
called as 'leakage flux'. As the flux produced is alternating (the direction of it is continuously changing), EMF
gets induced in the secondary winding according to Faraday's law of electromagnetic induction. This emf is
called 'mutually induced emf', and the frequency of mutually induced emf is same as that of supplied emf. If the
secondary winding is closed circuit, then mutually induced current flows through it, and hence the electrical
energy is transferred from one circuit (primary) to another circuit (secondary).
Basic construction of transformer
Basically a transformer consists of two inductive windings and a laminated steel core. The coils are insulated
from each other as well as from the steel core. A transformer may also consist of a container for winding and
core assembly (called as tank), suitable bushings to take our the terminals, oil conservator to provide oil in the
transformer tank for cooling purposes etc. The figure at left illustrates the basic construction of a transformer.
In all types of transformers, core is constructed by assembling (stacking) laminated sheets of steel, with
minimum air-gap between them (to achieve continuous magnetic path). The steel used is having high silicon
content and sometimes heat treated, to provide high permeability and low hysteresis loss. Laminated sheets of
steel are used to reduce eddy current loss. The sheets are cut in the shape as E,I and L. To avoid high reluctance
at joints, laminations are stacked by alternating the sides of joint. That is, if joints of first sheet assembly are at
front face, the joints of following assemble are kept at back face.
Open circuit and Short circuit Test on transformer
These two transformer tests are performed to find the parameters of equivalent circuit of transformer and losses
of the transformer. Open circuit test and short circuit test on transformer are very economical and convenient.

1. Electrical Machines Subject code- EE 332 M1(Sec) - M3B1(Batch)

2. 3-Phase transformers A three-phase transformer is a fundamental component in electrical power distribution systems that deal with three-phase alternating current (AC) power. It plays a critical role in stepping up or stepping down voltage levels to meet the needs of various industrial, commercial, and residential applications. Unlike single-phase transformers, which handle single-phase AC power, three-phase transformers are designed to handle the more complex and efficient three-phase power systems.

3. Fig. 1. Bank of three single phase transformers connected in star-delta connection 3-Phase transformers

4. Fig. 2. Three phase core type transformers

5. Fig. 3. Three phase shell type transformers

6. Three-Phase Transformer Connections Depending on the type of connections of the two windings and the phase displacement between them, three phase transformers can be classified into following four phasor groups. Group 1: zero phase displacement ( Yy 0, Dd 0) Group 2: 180 degrees phase displacement ( Yy 6, Dd 6) Group 3: 30 degrees lag phase displacement ( Dy 1, Yd 1) Group 4: 30 degrees lead phase displacement ( Dy 11, Yd 11)

7. Fig. 4. Connections of Y/y with zero phase displacement Fig. 5. phasor diagrams of the circuits shown in Fig. 4.

8. Fig. 6 . Connections of HV and LV in delta with zero phase displacement Fig. 7. phasor diagrams of the circuits shown in Fig. 6.

9. Fig. 8. Y/y Connections with 180 degree phase displacement Fig. 9. phasor diagrams of the circuits shown in Fig. 8.

10. Fig. 10. Connections of Dd 6 ( with 180 degree phase displacement)

11. Fig. 11. 3-phase transformer connection, Dy 1 with 30 degree lag phase displacement Fig. 12. Phase diagrams of Dy 1 connection, i.e. with 30 degree lag phase displacement

12. Fig. 13. C onnections of 3-phase transformer, Dy11, i.e. 30 degree lead phase displacement Fig. 14. Phase diagrams of Dy 11 connection, i.e. with 30 degree lead phase displacement

13. Principal features of commonly used transformers connections The primary and secondary windings can be connected either in a star (Y) or delta(Δ) connection. The commonly used connections of primary and secondary windings are star - star Connection (Yy0 or Yy6) star -Delta Connection ( Dy or Yd) Delta-Delta Connection (Dd0 or Dd6)

14. Star-Star Star-star connection is generally used for small, high-voltage transformers. Because of star connection, number of required turns/phase is reduced (as phase voltage in star connection is 1/√3 times of line voltage only). Thus, the amount of insulation required is also reduced. The ratio of line voltages on the primary side and the secondary side is equal to the transformation ratio of the transformers. Line voltages on both sides are in phase with each other. This connection can be used only if the connected load is balanced

15. Star-Delta The primary winding is star star (Y) connected with grounded neutral and the secondary winding is delta connected. This connection is mainly used in step down transformer at the substation end of the transmission line. The ratio of secondary to primary line voltage is 1/√3 times the transformation ratio. There is 30° shift between the primary and secondary line voltages.

16. Delta-Delta This connection is generally used for large, low-voltage transformers. Number of required phase/turns is relatively greater than that for star-star connection. The ratio of line voltages on the primary and the secondary side is equal to the transformation ratio of the transformers. This connection can be used even for unbalanced loading. Another advantage of this type of connection is that even if one transformer is disabled, system can continue to operate in open delta connection but with reduced available capacity.

17. Three phase to two phase conversion Scott connection The Scott connection is a type of 3-phase to 2-phase transformer connection. Fig. 15. Scott-connection

18. One transformer is the main transformer, and the other is the auxiliary transformer/teaser transformer . The main transformer has a center tap, which is connected to the neutral point of the 3-phase system. The auxiliary transformer is tapped at 86.6% of its full winding, which creates a 90-degree phase shift between its primary and secondary windings . The primary windings of the two transformers are connected in delta, and the secondary windings are connected in star. The middle tap of the star winding is connected to the neutral point of the 2-phase system. The two secondary windings of the Scott connection produce two 2-phase voltages that are 90 degrees apart. The total power output of the Scott connection is equal to the power input to the 3-phase system.

19. Fig. 16. Phasor diagram of scott -connected transformers

20. Fig. 17. Phasor diagrams under balanced loading at unity power factor Behaviour under equal loading with unity power factor

21. Fig. 18. Phasor diagram under balanced loading at 0.71 power factor lagging Behaviour under equal loading at power factor of 0.71 lagging

22. Fig. 19. Phasor diagram under unbalanced loading on secondary side Behaviour under unequal loading with different power factors

23. Parallel Operation of 3-Phase Transformers The conditions for proper parallel operation of two or more three phase transformers are as follows. The polarities and phase sequence must be same Identical primary and secondary terminal voltages The phase displacement between primary and secondary voltages must be same for all the transformers The ratio of equivalent leakage reactance per phase to equivalent resistance per phase should be same for all the transformers to ensure that transformers operate at same power factor and thus share active and reactive power according to their ratings Equal per unit ratings

24. 25 The following three phase transformers cane be operated in parallel. However, transformers with +30 degrees phase displacement may be operated in parallel with those having -30 degrees phase displacement after reversing the phase sequence of both the primary and secondary winding. If a number of transformers are worked in parallel taking supply from a common source, there are some connections which cannot be employed, that is Transformer 1 Y/Y Y/ Δ Transformer 2 Δ / Δ Δ /Y Transformer 1 Δ / Δ Y/Y Transformer 2 Δ /Y Y/ Δ

25. Parallel Operation of 3-Phase Transform Pros Increased Capacity: Transformers can collectively handle higher loads, enhancing the system's overall capacity. Redundancy : If one transformer fails, others can continue to provide power, ensuring system reliability. Efficient Load Distribution : Transformers share the load based on their impedance characteristics, minimizing overloading of any single unit. Flexibility: Additional transformers can be added to meet growing demand without disrupting the system Cons Complex Synchronization: Ensuring proper synchronization and phase matching is essential to prevent circulating currents and voltage imbalances. Voltage Regulation: Variations in transformer parameters can lead to voltage imbalances if not carefully managed. Increased Maintenance: More transformers mean more maintenance tasks, which can increase operational costs. Cost: Paralleling transformers requires additional equipment and careful design, which can result in higher initial costs. 26

26. Tap changing transformers Tap changing transformers are a type of power transformer that includes a tap-changing mechanism to adjust the turns ratio (voltage ratio) between the primary and secondary windings. This feature allows for voltage regulation and compensation in power distribution and transmission systems.

27. Tap changing transformers(key features) Voltage Regulation: Tap changing transformers are employed to regulate the output voltage levels of a power transformer. By adjusting the number of turns on either the primary or secondary winding, the voltage ratio can be altered to compensate for variations in the supply voltage, load conditions, and line losses. Load Variations: Tap changing transformers are particularly useful in scenarios where the load on the power system varies widely. They can help maintain a more consistent voltage level to meet the requirements of the connected equipment. Tapping Mechanism: The tap-changing mechanism is a feature that allows for the adjustment of the tap position on the transformer winding. This can be achieved manually, through remote control, or automatically through a control system. Common mechanisms include on-load tap changers (OLTC) and off-circuit tap changers (OCTC). On-Load Tap Changers (OLTC): These tap changers allow adjustments to be made while the transformer is in operation. This is essential for maintaining a continuous power supply during voltage adjustments. OLTCs use diverter switches and selector switches to change the tap position. Off-Circuit Tap Changers (OCTC): These tap changers require the transformer to be disconnected from the load before changing the tap position. This method is less common and is typically used in situations where the load can be temporarily disconnected.

28. 6 . Automatic Voltage Regulation (AVR): Tap changing transformers equipped with automatic control systems can regulate voltage levels without manual intervention. Sensors and feedback mechanisms monitor the system's voltage and load conditions, making necessary tap adjustments to maintain stable output voltage. 7. Voltage Boost and Buck: Tap changing transformers can raise or lower the output voltage as needed. "Boosting" refers to increasing the output voltage above the nominal value, while "bucking" involves reducing the output voltage . 8 . Long-Distance Power Transmission: In long-distance power transmission, tap changing transformers can help compensate for voltage drop along the transmission lines, ensuring that the receiving end receives the required voltage . 9. Grid Stability: Tap changing transformers contribute to maintaining grid stability by managing voltage fluctuations and preventing over- or under-voltage conditions. 10. Maintenance : Regular maintenance of tap changing mechanisms is important to ensure their proper functioning and reliability. Cleaning, lubrication, and periodic testing are essential. 11. Transformer Cooling: The tap changing mechanism can affect the cooling system design of the transformer due to the space it occupies and potential heat generation during tap changes. 12. Cost and Complexity: While tap changing transformers offer voltage regulation benefits, they can be more complex and expensive compared to standard transformers without tap-changing capabilities .

29. Tertiary winding In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformer or 3 winding transformer. Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements: It reduces the unbalancing in the primary due to unbalancing in three phase load. It redistributes the flow of fault current. Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

30. Applications of 3-phase transformers Power transmission and distribution: 3-phase transformers are used to transmit and distribute electrical power over long distances. They are also used to step up or step down the voltage of electrical power to meet the needs of different loads. Industrial applications : 3-phase transformers are used in a wide variety of industrial applications, such as powering motors, lighting, and other equipment. They are also used in power plants to convert the alternating current (AC) produced by generators into a higher voltage AC that can be transmitted over long distances. Commercial and residential applications : 3-phase transformers are used in some commercial and residential buildings to provide power for large loads, such as elevators, air conditioners, and electric stoves. They are also used in some telecommunications equipment.

31. Railway locomotives:  3-phase transformers are used to power the traction motors on railway locomotives. Electric vehicles:  3-phase transformers are used to convert the DC power from the battery pack to the AC power required by the electric motors. Wind turbines:  3-phase transformers are used to step up the voltage of the AC power generated by wind turbines to a level that can be transmitted over long distances. Solar power plants:  3-phase transformers are used to step up the voltage of the DC power generated by solar panels to a level that can be transmitted over long distances.

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