Handbook of Electrical Engineering

FAULT CALCULATIONS AND STABILITY STUDIES 297

Figure 11.14 One-line diagram of a two-platform power system.

The star and delta configurations use reactors that have lower reactances than the simple method
of Figure 11.11. This will give rise to better stability in both the steady state and the transient state.
In addition the AVR set-point circuits should not need to be modified.

11.9.1 Worked example

Consider the systems shown in Figures 11.11 to 11.13. Assume that all the generators are rated at
20 MVA, with sub-transient reactances of 0.15 pu and the main switchboard operates at 11 kV. The
symmetrical making current duty of the switchboard is 30,000 amps. Ignore motor contribution in this
example. Calculate the per-unit reactanceXrrequired for each reactor in the three different systems.

a) Case A: Simple system

The 1 pu current of each generator is,

I 1 =

S

√

3 V

=

20 , 000 , 000

√

3 × 11 , 000

=1049.8 amps

The short-circuit current available from each generator is,

Ifg=

E′′I 1

VX′′d

=

1. 1 × 1049. 8

1. 0 × 0. 15

=7698.2 amps

Without reactors and with five generators operating the total fault current is

Ifa= 5 × 7698. 2 =38,491 amps

which exceeds the duty of the switchgear by 8491 amps.