Predictive Control of Power Converters and Electrical Drives

10.4 Hard Constraints

One of the advantages of predictive control is the possibility of achieving direct control of the output variables without the need for inner control loops. However, it can be found in several cases that as the internal variables are not controlled, they can reach values that are outside their allowed range. In a traditional cascaded control scheme, this kind of limitation of the internal variables is considered by including saturation levels for the references of these variables. In a predictive control scheme, these limitations can be included as an additional term in the cost function.

As an example, the predictive torque control explained in Chapter 8 will be considered. In this control scheme, the electrical torque T_e and the stator flux magnitude are directly controlled using the following cost function [7]:

10.21

Here, the stator currents are not directly controlled and at steady state they are sinusoidal and their magnitude is within the allowed limits. However, during some transients these currents can be very high, damaging the inverter or the machine. The startup of the induction machine using this control scheme is shown in Figure 10.7. It can be seen in this figure that during the initial transient the stator currents can be considerably higher ...