Electrical Transformers

  Electrical Transformers

· Definition:

Is an electric static machine used in power system to transfer the electrical power from voltage and current level to another level.

Transformer is the main unit in the transformer stations; as it step up or down the voltage.

The network have many transformers which have different powers depends on the demand load.

-The values used for unified electrical net work for transformers are: 220kv/66kv 125MVA -75MVA.

66KV/11KV 25MVA-20MVA-12.5MVA.

· Principle Of Operation:

When the ac voltage is applied to the primary winding, a current will pass trough it producing AC magnetic flux in the iron core. This flux cuts the secondary winding which produce e.m.f across it. When the transformer is loaded , the current will pass trough the secondary winding.

Input power= output power+ power losses.

Input power= (√3) * i1* v1 * cosØ.

Output power=(√3) * i2* v2 * cosØ.

● Modeling of Transformer:

Generally, the model of power transformer is as shown in fig:

Where:

R_1, X ₁ : Resistance & reactance of primary side respectively.

R₂ , X₂: Resistance & reactance of secondary side respectively.

R_o: Iron-loss resistance, X_m: Magnetizing Reactance.

E₁: Induced emf in primary side.

E₂: Induced emf in secondary side.

Transformer is represented only by equivalent reactance = Z_eq.

  Where:

 Noting that:

1 - The ratio X/R is always >15 .

2- r_trans. can be negleted

3 - In most time X_trans. is added to X_Line (Series reactances).

4 - All represented impedance (or reactance or resistance) are

in per unit. Where:

 Z_p.u.= Z_actual / Z_base , Z_base = V²_base / S<sub>base



for more details read : 
http://en.wikipedia.org/wiki/Electrical_transformer</sub>