**3.1 Principle of system identification**

A dynamical system can be classified in terms of known structures and parameters of the system, shown in Fig.5, and classified as a "White Box" if all structures and parameters are known, a "Grey Box" , if some structures and parameters known and a "Black Box" if none are known [55]**.** 

Steps in system identification can be described as the following process, shown in Fig. 6.

Fig. 6. System identification processes

Each step can be described as follows

#### **3.1.1 Goal of modeling**

The primary goal of modeling is to predict behaviors of inverters for PV systems or to simulate their outputs and related values. The other important goal is to acquire mathematical and physical characteristics and details of systems for the purposes of controlling, maintenance and trouble shooting of systems, and planning of managing the power system.
