The small signal stability is the ability of a power system to maintain synchronism in case of small disturbances. In this condition, the system equations may be linearised.

Instability may result in two ways:

  1. slow increase of rotor angles due to lack of synchronising torque,
  2. rotor oscillations due to lack of damping torque.
The power system is represented by its state variables, related each other by a set of linear differential equations. From this equations the state matrix can be obtained, whose eigenvalues allow to determine the natural frequencies, or system modes, of the grid. The analysis of the eigenvalues of the state matrix determines the grid properties and verifies its small signal stability.
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