## 3 Experimental Results
### Islanding Detection Testing with Parallel Operation of Two Inverters  
Figure 7 shows the experimental results for case 3 in Table 4. (A) is the waveform for the voltage of the grid utility side, (B) is the waveform for the inverter output voltage, (C) is the waveform for the inverter output current of manufacturer A, (D) is the waveform for the inverter output current of manufacturer B, and (E) is the waveform for frequency. For the islanding detection testing, the PV inverter was operated at its rated power of 20 kW, and the inverter for the EESS was operated simultaneously in a 20 kW discharge mode. Then, the R-load (40 kW), L-load (\(-\)40 kVA), and C-load (40 kVA) were connected in an islanding condition. Although the switch of the grid utility was shut off, the inverters failed to detect the islanding and continuously supplied power to the RLC loads. Therefore, the inverters were manually stopped. The frequency waveform in Figure 7 shows that the two inverter frequencies were combined under the islanding condition. The EESS inverter injected reactive power to detect islanding approximately every 350 ms and this showed that the frequency fluctuated. The islanding detection failed because the frequency change value fluctuated within 5% of the cutoff frequency. The algorithm and period for islanding detection are generally different for each inverter manufacturer. Even if a frequency adjustment or reactive power injection for islanding detection was performed at every specific period by each inverter, the islanding detection could still fail because the cutoff frequency range was not reached. It is possible to change this onsite by adjusting the islanding detection gain value of the specific inverter. However, if the gain value is large, there is an  
Figure 6: Test results for islanding detect of B (ESS inverter). Figure 7: Test result of case 3 (PV 20 kW, EESS \(-\)20 kW). influence on THD (Total Harmonic Distortion) or flicker occurs. Therefore, to connect a grid utility safely, it is necessary to develop a proper algorithm that can detect islanding while several inverters operate in parallel at the same connection point. Figure 8 shows the waveforms of the experimental results for islanding detection when the PV inverter was operated at a rated power of 20 kW and the EESS inverter was operated in charging mode at 20 kW. The RLC capacities were 0 kW of R-load, \(-\)20 kVA of L-load, and 20 kVA of C-load. The test time for anti-islanding of the inverters was recorded when islanding occurred after the circuit breaker of the grid side shut off. The time until the inverters stopped was approximately 357.5 ms and the inverter normally detected the islanding. Islanding detection was possible when the output power of the PV inverter was the same as the charging power of the EESS inverter. In this case, the PV inverter stopped due to an overcurrent error from an insulated-gate bipolar transistor (IGBT) component. Figure 9 shows the resulting waveforms when the PV inverter was operated at its rated power of 20 kW and the EESS inverter was operated at 10 kW output power in charging mode. This experiment simulated a state with a power load of 10 kW because the EESS inverter operated as if it was consuming the R-load. The RLC load capacities were 10 kW of R-load, \(-\)20 kVA of L-load, and 20 kVA of C-load. The time to stop was 3.197 s and the inverters failed to detect islanding within this time. Case 5 was different from case 4 because there was an R-load. It is estimated that the islanding detection time was later than case 4 due to the R-load. When the EESS was in charge mode, it functioned as an electronic load and consumed the power generated by the PV system; the R-load also consumed the power of the PV system. Figure 10 shows the resulting waveforms when the PV inverter was operated at a rated power of 20 kW and the EESS inverter operated in charging mode at 5 kW. From the experimental results, the inverters were in continuous operation and had to be stopped manually for safety.