Handbook of Electrical Engineering

FAULT CALCULATIONS AND STABILITY STUDIES 307

Stability studies will help to minimise these possibilities. When planning a stability study the main
aspects that are usually included are:-

• Application of major faults on the electrical network.


• Sudden loss of a generator, e.g. due to an unexpected failure.


• Starting large induction motors direct-on-line.


• Reduced voltage methods for the starting of motors.


• Tripping large motors.


• Switching in or out interconnecting cables or overhead lines.

The performance is assessed in terms of the following:-

• Voltage recovery throughout the system.


• Frequency recovery throughout the system.


• Synchronous operation is maintained.


• Motors recover to their normal operation.


• No prolonged overloads occur.


• Generators share load changes properly.


• Hunting oscillations do not develop.


• Transient oscillations die away within a few seconds after a sudden disturbance is applied.

11.11.2.5 Depth of study – preliminary stage

A stability study should be seriously considered necessary at an early stage of a project so that
the basic configuration of the power system network may be established with confidence. This is
especially applicable to remote or self-contained power plants which have a large number of motors,
e.g. an offshore platform.

At the early stage it is acceptable to use typical data for particular plant items and a number
of simplifications are justified:

• Use typical data for generators, motors, gas turbines, pumps and compressors. This can be obtained
  as ‘budget’ data when screening vendors and manufacturers for suitable machinery.


• Neglect high voltage cable impedances unless the route distances are long.


• Use simplified block models for the turbine and generator control systems.


• Represent all the low voltage motors on a typical motor control centre by one, two or perhaps
  three equivalent motors to cover the kilowatt range. A typical selection would be 20 kW and
  100 kW. The equivalent motor would have the electrical parameters, inertia constant and pump
  characteristic of the typical machine, but would have the rating of the total of all the motors in
  the group.


• All low voltage motors would be assumed to be driving centrifugal machinery.


• Separate out any special case low voltage motors, e.g. extra large motors, large motors driving
  reciprocating machinery.