The high seasonality and variability of electricity production from renewable sources has enormous
impact on both production and distribution networks. Among the many aspects, several traditional
power plants, especially, combined cycle plants, but also coal-fired plants, now operate
intermittently with variable set points due to fluctuations of energy load which they are requested to
deliver [1]. Thus, a specific analysis of control system performance during these transient cyclic
phases, as reference changes, operations of start up and shutdown, is highly desirable.
Generally speaking, monitoring and assessment of performance of control systems of industrial
plants are important topics in process control. The deterioration in performance is, in fact, a fairly
common phenomenon and manifests with sluggish or oscillating trends of control variables.
Oscillations in control loops can cause many problems which affect normal operation of process
plants. Typically, fluctuations increase variability of product quality, accelerate wear of equipment,
move operating conditions away from optimality, and, in general, cause excessive or unnecessary
consumption of energy and raw materials [2].
This paper introduces a technique for the analysis of performance of basic control loops when
process is subject to changes of operating conditions. The method employs the well-established
approach of Internal Model Control, IMC. After establishing lower limit for the absolute value of
the integral (IAE) of control error and the total variation (TV) of control action, such limits are
assumed as reference values for a control considered “optimal”, or anyway “good”. A performance
index is thus based on IMC and is properly defined with respect to lower limit of IAE and TV. With
this approach, the validity of tuning of PID-type controller in response to any reference change can
be assessed. In particular, one can successfully evaluate closed-loop performance for setpoint
changes, as steps, ramps, or generic trends, as for the common case of preset programs of variable
load of power plants