Guidelines for Tuning Cascade Controllers in Power Plants

摘要

The use of cascade control in certain power plant applications, such as drum level to feedwater flow control, is widely accepted, while its use in other applications, such as desuperheater temperature to spraywater flow control, is seldom used. Power plant consultants and academics have indicated that cascade control systems are difficult to tune and keep stable. Furthermore, opinions differ widely about the minimum safe ratio of inner loop speed to outer loop speed and how this ratio affects the stability and disturbance rejection of cascade control systems. This paper explores the effect of tuning and process dynamics on the stability of cascaded control loops using examples from power plant boilers. It explains how process characteristics and tuning methods are interrelated and when stability issues can be expected. The paper then presents guidance on the use of certain tuning rules and the minimum ratio between the speed of response of the outer and inner loops.