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

This paper considers a power system where about half the demand comprises industrial loads and some include large rotating machines. Therefore, the behaviour of these loads during system disturbances has to be considered for system stability studies. This paper describes a practical approach to the development of a Laplace transfer function (TF) model of a (paper) mill load connected to this power system. A PSCAD/EMTDC^TM simulation model of the mill (the detailed model) was built from available information on the types of load within the mill. This detailed model was verified in a piece-wise manner, e.g. against individual motor starts. Instead of using the available generic load models, system identification theory is applied to evaluate the transfer function load model from simulated small signal disturbance responses. The model is developed for both voltage and frequency changes, and considers the actual frequency range of inter-area oscillations within the power system. In this paper the focus of the work is to accurately represent the mill load behaviour for small signal disturbances. It is recognised that the large signal requirements of a load model for transient stability studies are also important but the inevitable non-linearity of such a model would tend to saturate the loads¡¦ behaviour during lesser disturbances. The performance of the developed small signal load model is tested using some 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 is currently scant. However, modern high bandwidth monitoring has recently come on-line for the mill and the TF load model will be fully verified over time as disturbance data builds up.