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Load model in power system can be done by three ways;

1. constant impedance model (Z)

2. constant current model (I)

3. constant power model (P)

Commonly, it is known as ZIP model.

For more details go through the following publication;

A. F. Naiem, Y. Hegazy, Member, IEEE, A. Y. Abdelaziz, and M. A. Elsharkawy "A Classification Technique for Recloser-Fuse Coordination in Distribution Systems With

Distributed Generation" IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 1, JANUARY 2012.