Page 309 - Handbook of Electrical Engineering

FAULT CALCULATIONS AND STABILITY STUDIES 295

When power systems grow in size and complexity it often happens that the fault levels in
some parts of the existing system become too high for the equipment. Reactors can be inserted to
maintain the fault levels below the equipment limits. The most common application is in the feeders
to switchgear.
In the oil industry it is often found necessary to increase the number of generators on an
existing system. Sometimes this causes fault level problems at the generator switchboard. Rather
than replace the switchboard it may be possible to insert one or more reactors. Several solutions are
possible:-

• Insert a reactor in series with the new generator.
• Insert a reactor in series with each existing and the new generator, see Figure 11.11.
• Insert a reactor between sections of the main busbars, see Figures 11.12, 11.13 and 11.4.

The preferred solution depends on how much fault level reduction is necessary. If the change
in fault level is greater than about 20% the value of the reactance may become too large and cause
voltage regulation problems under normal operating conditions. A high reactance inserted into the
circuit between generators may cause hunting and stability problems.

Figures 11.11 to 11.14 show different methods of installing fault limiting reactors into a power
system. Figure 11.11 shows the simplest method in which one reactor is connected in series with
each main generator. This is also the least expensive because no additional switchgear is required.
However, it may not be the best technical solution because the value of reactance for each reactor
tends to be higher than other options, and this could lead to stability problems. Also the terminal
voltage of each generator under normal conditions will need to be kept slightly higher than before
due to the reactive volt-drop in the reactor. This may require some modifications to the AVR set-point
circuits.
The reactor systems shown in Figures 11.12 and 11.13 are very similar, one being a star-
connected system and the other a delta-connected system. The star system has the advantage of

Figure 11.11 One-line diagram of a simple reactor system for reducing the fault level at the switchgear in
the system.