AM - 14:00 PM). Loads fluctuate upon activities of customer groups, for example, a peak load for industrial zones occurs in afternoon (13:00 - 17:00 PM) and a peak load for residential zones occurs in evening(18:00 - 21:00 PM). Variations from steady state conditions impact power quality such as overvoltage, over-current, harmonics, and so on. In case of transients, there are variations in inputs, loads and utility. Weather variations such as solar irradiance and temperature exhibit significant changes. Unexpected accidents happen. Local loads may sudden change due to activities of customers in each time. A utility has some faults in nearby locations which impact utility parameters such grid impedance. These conditions lead to short duration power quality problems with such spikes, voltage sag, voltage swell. In some extreme cases, abnormal conditions, such as very low solar irradiance or abnormal conditions such islanding, the grid-connected PV systems may collapse. The PV systems are black out and cut out of the utility grid. Such can affect power quality, stability and reliability of power systems.
2.3 Power converter
There are several topologies for converting a DC to DC voltage with desired values, for example, Push-Pull, Flyback, Forward, Half Bridge and Full Bridge [49]. The choice for a specific application is often based on many considerations such as size, weight of switching converter, generation interference and economic evaluation [50-51]. Inverters can be classified into two types, i.e. voltage source inverter (VSI) if an input voltage remains constant and a current source inverter (CSI) if input current remains constant [52-53]. The CSI is mostly used in large motor applications, whereas the VSI is adopted for and alone systems. The CSI is a dual of a VSI. A control technique for voltage source inverters consists of two types, a voltage control inverter, shown in Fig. 4(a) and a current control inverter, Fig. 4(b) [54].
Fig. 4. Control techniques for an inverter
3. System Identification
System identification is the process for modeling dynamical systems by measuring the input/output from system. In this section, the principle of system identification is described. The classification is introduced and particularly a Hammerstein-Wiener model is explained. Finally, a MIMO (multi input multi output model with equation and characteristic is illustrated.