## 2 Analysis of PV PCC Harmonics  
To analyze the PCC harmonics of high-power PV plant with multiple grid-tied inverters, a micro-grid containing photovoltaic and hydroelectricity was addressed. This micro-grid is located in Batang Town, Yushu Prefecture, Qinghai Province, in western China, and the distance from large grid to the micro-grid is more than 400 km. The capacity of hydroelectricity as the main power source of the micro-grid is 12.8 MW. The load peak of this micro-grid is 12 MW. Under the influence of the flood season and the dry season, hydroelectricity cannot meet the needs of power in winter. The power supply gap is large, especially during the peak hours in the evening. Therefore, the construction of a PV/energy-storage generation system was implemented to improve the power supply of the micro-grid. However, significant harmonics appear frequently at PCC of the PV plant when PV inverters are working. The micro-grid and PV plant are depicted in Figure 1.  
Figure 1: Photovoltaic (PV) plant of an isolated micro-grid: (**a**) the topology of Yushu water-photovoltaic micro-grid; (**b**) the aerial view of the PV plant in micro-grid; (**c**) the details of PV plant; and (**d**) topology of the PV plant. As illustrated in Figure 1, the capacity of the PV plant was 2 MWp, and the rated power of each inverter was 100 kW. The topology of these inverters was a standard current-source converter (CSC) and the output filter consisted of an inductor and a capacitor. The capacitance and conductance of inverter filter were 47 \(\times\) 10\({}^{-6}\) F and 0.25 mH, respectively. Each grid-tied transformer was connected by 3-6 PV inverters in parallel, and the transformers are connected to PV PCC of the microgrid. The ratio of these grid-tied transformers was 10 kV/400 V, and the ratio of isolating transformer in PV inverters was 400 V/215 V. The proportional integral (PI) regulators in _dq_ reference frame were adopted by PV inverter controllers. The capacity of energy storage system was 15.60 MWh. As a result, the PV inverters were working with an ideal direct current (DC) supply and the harmonic analysis could focus on the PCC of PV plant. The grid-tied inverters and energy storage of PV plant are presented as Figure 2.  
PV inverters with different generation power were operated. The voltage and current waveforms of PCC with low output power of PV inverters are depicted as Figure 3.  
As illustrated in Figure 3, the PCC waveforms were perfect. The harmonic waveforms, when the output power of PV plant was about 65% of rated value,  
Figure 3: Waveforms of PV point of common coupling (PCC): (**a**) HIOKI-3196 power quality analyzer (HIOKI E.E. Corporation, 81 Koizumi Ueda, Nagano 386-1192, Japan); and (**b**) voltage and current waveforms at PCC of micro-grid. Figure 2: Equipment of PV plant: (**a**) PV inverters; and (**b**) energy storage systems. As illustrated in Figure 4, the waveforms of PCC voltage and current were recorded by data acquisition instrument YOKOGAWA DL850 ScopeCorder (Yokogawa Test & Measurement Corporation, Musashino-shi, Tokyo 180-8750, Japan) and the sample frequency was 20 kHz. The main harmonic components were steady-state, and the harmonics were significant (total harmonic voltage distortion (THDU) of 15% and total harmonic current distortion (THDI) of 40%). Total power factor was 0.95 and the power factor of fundamental signals was 0.99. However, the voltage and current waveforms in Figure 4 were not symmetrical perfectly, and this phenomenon revealed the existence of inter-harmonics at PV PCC. There are spectral leakage and fence effects when any inter-harmonics are analyzed using discrete Fourier transform (DFT) or fast Fourier transform (FFT) algorithms. Therefore, the waveform measurement data in the PV plant of the micro-grid were analyzed using windowed interpolation Fourier transform (IpFFT) method. IpFFT is an improved FFT algorithm with high frequency resolution and accurate analysis of signal phases and amplitudes [34; 35].