Dynamic load modelling of a paper mill for small signal stability studies

This study considers a power system with half the demand comprises of industrial loads with large rotating machines. The dynamic behaviour of these loads during disturbance is crucial to system stability. This study describes a practical approach to develop Laplace transfer function (TF) model of a mill load connected to this power system. A PSCAD/EMTDC™simulation model of the mill (‘the detailed model’) is built from information on the types of load within the mill and verified in a piece-wise manner, i.e., against individual motor starts. Instead of using generic load models, system identification theory is applied to evaluate the TF load model from simulated disturbance responses for both voltage and frequency changes. The main objective is to accurately represent the load behaviour particularly during small signal disturbances. It is recognised that large signal requirements of a load model for transient stability studies are important but the inevitable non-linearity of such a model would tend to saturate the loads’ behaviour during smaller disturbances. The developed small signal load model is then tested using practical disturbance profiles from this power system. This modelling approach allows development work to commence before real-life disturbance measurements are available since such data are currently scant.