New control scheme in the local system stabilizing controller

A new control scheme for the local system stabilizing controller (SSC), an adaptive relay system to prevent blackout and maintain frequency of split subsystems with generator shedding and/or load shedding, is described. The most distinctive feature of the scheme is its multiple shedding steps; i.e., primary step and additional steps. This reduces the total amount of control and minimizes control error which may be caused by unmeasured factors such as load dropout by undervoltage relays. In the primary step, shortage control is implemented which is not enough to make the frequency deviation zero, but sufficient to suppress the frequency maximum predicted in real time analysis of dynamic characteristics. In the additional steps, multiple adjustment controls are implemented to revise the frequency deviation of the primary step. The true frequency value is estimated with plural values of measured frequency to deal with load dropout or recovery. The scheme was tested in a power system simulator. Test results showed the amount of shedding was cut 14.9% compared to a conventional scheme.