# State Feedback Control Based Seamless Switch Control for Microgrid Inverter
## 4 Experiment Verification  
In this paper, an experimental MG system whose structure is shown in Figure 1 is established to validate the performance of the proposed control method. The experimental layout is displayed in Figure 10. The parameters of the battery inverter which implements the proposed control are shown in Table 1.  
The experiment of seamless switch from GC to SA mode with different load conditions is conducted to verify the proposed control when unintentional islanding is occurred. For this seamless transition, the islanding detection is unavoidable. The IEEE 1547 standard [22] defined the islanding detection time, which is important for the dynamic performance during the transition process. During this unintentional islanding detecting, voltage across the critical load may experience a severe transient state since the voltage is determined by the amount of the injected power and unknown load condition. Fortunately, the more transient distortion the voltage experiences, the faster the unintentional islanding is detected. Therefore, improving the sensitivity of the islanding detection may alleviate the voltage distortion during the seamless switch. In this paper, to simplify the experiment procedure, the islanding detection takes 3 ms. Figure 11a,b show the switch voltage waveform of the proposed control and the conventional PI control under idling state. The current of battery inverter is controlled to zero at GC mode and there is no feeding load in SA mode. The islanding detection time \(t_{id}\) is 3 ms. While in Figure 11c, the islanding detection takes 8 ms for the proposed method. Three criteria are considered to evaluate the performance: the maximum voltage drops,  
Figure 10: Laboratory setup for real-time experiments. voltage recovering time and the stability. For reading convenience, the variables in these figures and its values and meanings in testing is summarized in Table 2. The three criteria for the results in Figure 11a-c are summarized in Table 3. As is shown in the Figure 11 and the Table 3, the proposed control frame owns the advantages of the good dynamic response speed within switch procedure and continuous smooth voltage waveform without mutation before and after switch. It proves the favorable performance of the control frame. Furthermore, the inverter should cease to energize the local load by disconnecting the DG unit from the grid within two seconds, which is the required clearing time when the islanding is detected [14]. Therefore, the conventional mode transition from the GC mode to the SA mode will be acceptable during 10-60 cycles of fundamental frequency [3]. However, to guarantee the uninterrupted power supply for critical loads and enhance the power supply reliability of the photovoltaic source, the seamless switch is indispensable in the near future. Hence, some switch experiments are conducted under different load conditions as shown in Figure 12. The meanings of the variables in those figures are identical with those in Figure 1, and shown in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **Significance** & **Values** \\ \hline \(u_{g}\) & grid voltage & 220 V (RMS) \\ \(u\) & voltage of the battery inverter & 220 V (RMS) \\ \(i_{g}\) & current of the grid & varying \\ \(i_{hu}\) & current of the battery inverter & varying \\ \(i_{PV}\) & current of the PV inverter & 10 A (6.5 kW) \\ \(i_{nl}\) & current of the nonlinear load & 7.8 A (5.2 kW) \\ \(i_{rl}\) & current of the resistive load & 7.5 A (5 kW) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Illustration of the variables in Figure 11.  
Figure 11: Voltage waveform during seamless switch of the (**a**) proposed control with islanding detection time 3 ms, (**b**) conventional PI method with islanding detection time 3 ms, and (**c**) proposed control with islanding detection time 8 ms.