PROBLEM - 01
A three phase three winding 50 Hz, transformer rated 110/11/6.6kV as the measured impedances between windings referred to 30 MVA as Zps = j0.16 p.u, Zpt = j0.1 p.u and Zst = j0.09 p.u. A balanced load supplied from 6.6kV secondary winding draws a current of 2kA, 0.8 power factor lagging, and a star connected capacitor bank with 50 ohms reactance per phase is supplied from 11kV tertiary winding. Find the voltage required at the 110kV primary terminals to maintain 6.6kV across the secondary winding. Draw the equivalent single phase circuit marking all parameters thereon.
ADVANTAGES OF PER UNIT SYSTEM
1. Manufacturers usually specify the impedance value of equipment in per unit of the equipment rating. If any data is not available, it is easier to assume it’s per unit value than its numerical value.
2. The circuit laws are valid in p.u. systems and the power and voltage equations are simplified since the factors of root 3 and 3 are eliminated.
3. The per-unit system gives us a clear idea of the relative magnitude of various quantities, such as voltage, current, power, and impedance.
4. The per-unit value of impedance, voltage and current of a transformer are same regardless of whether they are referred to the primary or the secondary side.
5. The p.u. impedance is the same on the two sides of the transformers. i.e., Star or Delta.
6. For computerized analysis and simulation of complex power problems, per-unit system representations are ideal.
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