Closed Loop Control via Data Transmission Bus in Decentralized Power Plant Control Systems

Abstract

Modern control systems in large power plants have a decentralized structure with data acquisition and control function performed by microcomputers, which are connected by a data transmission bus. In order to realize closed loop control via the bus, data transmission of an analog signal is to be caused, if the analog signal has changed its value more than a well defined threshold since the last data transmission. Furthermore it is wanted to cause a data transmission only beyond a well defined time stop after the last data transmission.Considering the controlled system together with the control algorithm as linear sampled data system, the threshold with time stop is an additiv nonlinearity whose influence on the time response of the closed loop is to be analysed. To give a general solution to this problem the sampled data control system is written by its w-transform with the relation (T/2)W=(Z-1)(Z+1). As the w-transfer function gives the relation between input and output at sinusoidal steady state, now it is the idea to give a describing function for the nonlinearity to solve the stability problem for sampled data systems in the same way as for continuous systems. As results there are given the conditions for and characteristics of limit cycles depending on the threshold, time stop and sampling period for a considered w-transfer function of the sampled data system.