Question

A generator is connected through a 20 MVA, 13.8/138 kV step down transformer, to a transmission line. At the receiving end of the line a load is supplied through a step down transformer of 10 MVA. 138/69 kV rating. A 0.72 p.u. load, evaluated on load side transformer ratings as base values, is supplied from the above system. For system base values of 10 MVA and 69 kV in load circuit, the value of the load (in per unit) in generator circuit will be.

• A. 36
• B. 1.44
• C. 0.72
• D. 0.18

Answer 1

The correct option is A 36

Base values on load circuit,

$(MVA{)}_B=10MVA$ and

$(kV{)}_B=69kV$

Base impedance$=\frac{{69}^2}{10}=476.1$

Values of load in ohm$=Z_L$

= Value of load in p.u.$\times$ Base impedance

$\Rightarrow Z_L=0.72\times 476.1=342.8\Omega$

Base values in generator circuit,

$(MVA{)}_B=20MVA$

$(kV{)}_B=13.8kV$

Base impedance

$=Z_B=\frac{(kV{)}_B^2}{(MVA{)}_B}=\frac{{13.8}^2}{20}$

$\Rightarrow Z_B=9.522\Omega$

$\frac{Z_L}{Z_B}=\frac{324.8}{9.522}=36p.u.$