# Coordinated Mitigation Control for Wideband Harmonic of the Photovoltaic Grid-Connected Inverter
## 5 Simulation Case Study
### Verification of Resonance Suppression Method of LCL Filter  
In Figure 10, active damping constructed by the capacitive current feedforward method effectively reduces the resonant point gain of the LCL filter. Setting \(K_{\mathrm{c}}\) = 15 and using the second group parameters in Table 1, the time domain waveform of the three-phase grid-connected current of PV is shown in Figure 19a. Under the condition that temperature and sun irradiance are set in Table 2, the maximum grid-connected current amplitude of the PV is about 45 A.  
The experimental results show that the resonant frequency component is greatly reduced after the capacitance current feedforward. The active shunt resistance makes the character of the LCL filter present as the ideal low-pass filter. However, the waveform still has a certain degree of distortion, especially near the zone where the grid-connected current passes zero or reaches the peak. Figure 19b shows the FFT result of the PV's current. Obviously, the harmonic components at high frequencies are suppressed by the LCL filter, but the fifth and seventh harmonic currents caused by dead zone still exist, and harmonic amplitude decreases gradually with the increase of frequency. The fifth harmonic percentage is 2.436%, and the seventh harmonic percentage is 2.19%. Any other harmonic percentage is less than 1% of the fundamental amplitude, containing the 11 th and 13 th components also caused by the dead zones. The large amplitude of the current causes a  
Figure 19: The active damping experiment under the condition of 40 A.  
small proportion between the error voltage and the modulated voltage. Thus, the harmonic content still meets IEEE grid-connected standards.  
Due to the positive correlation between the modulated voltage and the inverter output current, the modulation error voltage can be considered as approximately invariant. The time domain waveform of the PV's three-phase current is shown in Figure 20a,b shows the FFT result. The percentage of the fifth and seventh harmonic currents are 4.243% and 3.052%, respectively. The THD of the PV's grid-connected current is over 5%. What is worse is that the lower current corresponds to even higher harmonics. This breaks the photovoltaic grid connection standard, which severely restricts the PVs' connection to the distributed power system.  
From Figures 19 and 20, the validity of active damping equivalent to a capacitive current feeder is verified for resonance suppression. The PV grid-connected inverters used in engineering mostly have LCL filters, so this method should be part of the general control structure of PV grid-connected inverters. In addition to resonance limiting the grid connection of new energy sources, the output current harmonic content also affects the supply power quality. Therefore, it is still necessary to verify the adaptive compensation strategy, comparing the current harmonic content before and after compensation. This part of the work is analyzed in the next section.  
Figure 20: The active damping experiment under the condition of 20 A.