Transformers and Substations Handbook 2014

• Stray losses: Owing to stray magnetic fields causing eddy current in the conductors or in the surrounding metal, eg tank • Dielectric losses: In the insulating materials, particularly in the oil and solid insulation of high voltage transformers Regulation The voltage regulation is defined for any given load current as the arithmetic difference between the secondary no-load voltage E 2 and the load voltage V 2 expressed as a fraction of the no-load voltage. Regulation % = {(E 2 -V 2 )/E 2 } x 100 No-load losses On no-load the secondary circuit is open and, consequently, the primary current is I o only. The I 2 R losses owing to this are negligible. (At full load the I 2 R losses would be approximately 1% or less and since the no-load current is of the order of one twentieth of the full load current the I 2 R losses would be 1/400 x 1%= one four hundredths of a percent.) Consequently, the power input on no-load is concerned with the core and dielectric loss, the latter being negligible except in very high voltage transformers. The no-load losses measured on open circuit secondary represent the core and dielectric losses; the dielectric losses are generally negli- gible compared to the iron losses. Copper losses (I 2 R losses) As the voltage has to be reduced to a very low value if the secondary terminals are short-circuited, the current in the secondary could be full load current while the secondary voltage would be zero because of the short-circuit. The primary voltage would be small and the flux F would likewise be small. At full load the input voltage would be 0,05 to 0,1 of the rated voltage. The core loss is approximately proportional to the square of the flux and would be very small. Therefore, the core losses would be negligible. Transformer connections In three phase transformers there are five types of winding connections.

secondary to be earthed di- rectly or through a suitably sized resistor. The delta wind- ing inherently suppresses any triplen harmonics, that may occur in the magnetising current and distort the volt- age. The numerical number associated with the configura- tion indicates the phase angle relationship.

B

b

Dyn11 connection

30°

c

n

1

C

A

a

Figure 12: Delta-star vector diagram.

B b

Yd11 connection

30°

Star-delta connection Essentially used in situations where the secondary is not to be earthed and cannot be used where single phase voltage is required, such as domestic or small light indus- try connected to the secondary supply. Again, the connection can ac- commodate various vector phase angle relationships. Auto-wound transformers Auto wound transformers share a common star point and thus a com- mon earth and the systems are not isolated from each other. Auto-trans- formers comprise two windings; series and common. Auto-transform- ers are typically used as high voltage system interconnecting trans- formers and in reduced voltage starting systems for large motors. Zig-zag A a c C Figure 13: Star-delta vector diagram.

B

connection This configura- tion is typically used where a specific phase angle shift is re- quired, for exam- p l e , i n mu l - tiphase rectifier transformers

b

Zig-Zag connection Yzn

n

c

A

C

a

and where it is necessary to have a positive sequence impedance higher than the zero sequence impedance. Conclusion The subject matter on transformer design is extensive and this article briefly outlines some theory and factors to be considered in the design. Reference [1] Przybysz P, Transformer Fundamentals. Eskom publication. Figure 14: Star zig zag vector diagram.

The choice of connection depends on the function of the transformer in an integrated power system. Star-star connection This connection is used where the phase relation- ship is required to remain the same and earths are

B

b

Yy0 connection

a

c

A

C

Figure 11: Star-star vector diagram.

required in both sides. It is mainly used in small transformers and large transmission transformers. The transformers are frequently equipped with an additional set of winding connected in delta to suppress any triplen harmonics. Delta-star connection Dy11, Dy1 and Dy5 are commonly used configurations enabling the

Bibliography • Waterhouse T, Design of Transformers.

• Transformers. Bharat Heavy Electrical Limited. McGraw Hill. • Flanagan WM, Handbook of Transformer design and application. McGraw Hill.

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Transformers + Substations Handbook: 2014