Discontinuous Excitation Control

A properly applied power system stabilizer provides damping to both local and interarea modes  of occilations. Under large-signal or transient conditions, the stablizer generally contributes positively to first-swing stability. In presence of both locan and interarea swing modes, however, the normal stablizer response can allow the excitation to be redued after the peak of the first locan-mode swing and before highest composite peak of the swing is reached. Additional improvements in transient stability can be realozed by keeping the excitation at ceiling, within terminal voltage constraints, until the highest point of the swing is reached.
                                                           This control improves transient stability by controlling the gernerator excitation so that the terminal voltage is maintained near the maximum permissible value of about 1.12 to 1.15 pu over the entire positive swing of the rotor angle. The scheme uses a signal proportional to the change in angle of the generator rotor, in addition to the terminal voltager and rotor speed signal. However, the angle signal is used only during the transient perood of about 2 second following a severe disturbance, since it results in oscillatory instability if used continuously. The angle signal prevents premature reversal of field voltage and hence maintains the terminal voltage at a high level during the positive limiter. The discontinuous excitation control is most effective in improving the transient stability