in MATLAB there is a block named three-phase dynamic load in simscape - simpowersystem library . you can use it for the simulation purpose....Hope this may solve your problem.. the modelling of dynamic load is explained in detail in the document available in the help menu.
As a complement to previous answers, you have to specify what kind of dynamic load do you want to model. The dynamic model will be of course specific to the type of load. Some types of load are: large/small motors, discharge lightning loads, thermostat controlled loads, transformers with saturation, capacitors, etc, etc. You can use Simulink or Psim libraries to model these.
If you are looking for some detailed models equations of some of these loads, I would recommend Prabha Kundur's book on stability "Power system stability and control".
Dear Muhammad, the grid load contains active and reactive power. Your load should be likewise contains active and reactive power. We use a C-model in Simplorer as load with measured data p(t), q(t), u(t) and f(t). Thereby all possible loads are available.
Thank you everyone for the help and your input, I would also like to as my work,I am looking for only DC load and vlotage level 12 V- 24V, and peak power 65 Wp.