Solved Example for Power System Modeling

With this discussion now let us model the power system shown in figure by using pu values. 

To start with let the base selected as 100 MVA for modeling the power system. You can select any value but it is normal practice to select 100 MVA as base MVA. Select base kV as 11kV for generator. This will fix the base kV of other elements of power system to its rated kV considering transformer voltage ratios in the power system. Thus base kV for 132kV line would be 132kV and for 220kV line it would be 220kV. Now we will decide abbreviation for pu reactance (neglect resistance for fault calculations) for different elements as below

• 1)    Generator                                                      Xg
• 2)    Generator Transformer                               Xgt
• 3)    132kV Lines                                                  XL1, XL2
• 4)    220kV Lines                                                  XLa, XLb
• 5)    220/132kV ICTs                                            XT1, XT2

Using different formulae now we will calculate pu values of respective elements for base MVA as 100 and base kV as rated kV of the element. Thus

1)    Generator and Generator transformer

Xg(new) = Xg(given)*(Base MVA/Rated MVA)

            Xg(new) = 0.15*100/60 = 0.25 or 25%

            Xgt(new) = 0.15*100/100 = 0.15 or 15%

2)    132kV Transmission lines

Ohmic value of line reactance = 0.4 * 35 = 14 Ω

Pu value XL1 = XL2 = 14*100/(132)² = 0.0803 or 8.03%

3)    220/132kV ICTs

XT1 = 0.14*100/200 = 0.07  (or 7%)

XT2 = 0.127*100/100 = 0.127 (or 12.7%)

4)    220kV Transmission lines

Ohmic value of line reactance = 0.4 * 55 = 22 Ω

Pu value XLa = XLb = 22*100/(220)² = 0.0454 or 4.54 %

5)    Power Grid

Using formula

 =

X_th% = 100x100/1200 = 8.33%

Thus now system model with percent values will be

The impedance equivalent to grid is generally referred as source impedance