3-level load-frequency control of power systems interconnected by asynchronous tie lines

A 3-level optimal controller for load-frequency control of a power system which is composed of several subsystems interconnected by asynchronous tie lines is presented. In addition to the conventional control (speed-changer position) of the governor, the d.c. power is considered as a new control variable. The proposed controller minimises the deviations in frequency resulting from sudden disturbance. By means of the multilevel systems concept, the controller of the interconnected power system is decomposed into three levels: d.c. power control (first level), local control (second level) and coordination (third level). Each of the local controllers in the control hierarchy controls a subsystem according to the interaction variables provided by the coordinator. The coordinator is used to improve the interaction variables to achieve global optimality, and then assigns optimal settings for the d.c. power which controls the tie-line flow via the grid control of the convertors. The proposed controller appears to be simple and fast. The work reported in this paper provides some insight into the multilevel control concept for systems with interaction via a control variable.