## 2 Test Procedure for Islanding Detect  
The standards for anti-islanding tests for PV system inverters are IEC61727:2004 (photovoltaic systems characteristics of the utility interface) and IEC 62116 (test procedure for islanding prevention measures). These standards describe the safety test methods for PV inverters, including islanding prevention. Figure 2 shows a circuit configuration of a test device for islanding detection. The test was carried out according to various test conditions set in the standards regarding the ratio of active power to reactive power. \begin{table}
\begin{tabular}{c} Setting Value \\ **Voltage** \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Reference standards for anti-islanding. IEC 62116 is a test standard for islanding detection of one PV inverter. However, when PV and EESS are connected to a grid, the PV inverter and EESS inverter are used together. In this paper, we simulated this situation and experimented by connecting two inverters (PV and EESS) in parallel based on IEC 62116. Also, among test condition A of IEC 62116, the experiment was conducted using the (0,0) condition. The configuration of the experimental equipment consisted of one test inverter (EESS) and one DC power source in parallel, diagrammed in Figure 3. The generator and loads could be any combination of customer-owned and utility-owned. In this work, the experimental circuit was configured as shown in Figure 3 to test islanding detection while two inverters (PV inverter and EESS inverter) were operated. The DC power source, which can be operated in constant voltage (CV) mode (red dotted line), can supply power and consume the load like a battery system. Tests were carried out under the condition that the quality factor was one, not under a variety of conditions. Equation (1) shows how the quality factor is calculated:  
\[Q_{f}=\frac{\sqrt{Q_{L}\times Q_{C}}}{P_{R}} \tag{1}\]  
Figure 4 shows the specifications and configuration of the experimental equipment used for islanding detection testing. The power meter measured the active power from the test inverters and the active and reactive power consumed by the RLC loads. The scope meter measured and recorded the waveforms for voltages, currents and frequency at the test-inverter output terminals and the waveform of the voltage of utility power line. The experiment was carried out when the test inverters were in islanding condition. The switch of the main power line was turned off in this state and the waveforms were measured and recorded. Then, it was checked whether the inverter stopped normally  
Figure 3: Test configuration diagram of two inverters connected to a utility grid in parallel for islanding detection. Figure 2: Test circuit for islanding detection in a PV inverter. within 0.5 seconds according to the Korean grid code. The two test inverters used in the testing were commercial products and the specifications of the inverters are described in Table 3. Table 4 shows each test case. Experimental cases 1-2 were tests to check whether each inverter could detect an islanding operation with an output power of 20 kW. In experimental case 3, when the PV inverter and EESS inverter supplied power to the grid line (including the loads), the function of island detection was tested. In case 4, the PV inverter supplied power to the grid and the EESS inverter charged its batteries (in the energy storage-system simulator in Figure 4). Because the PV inverter and EESS inverter were operated in parallel at the grid connection point, the characteristics of islanding detection had to be tested under various conditions according to the active power of the inverters.