# Low-Loss Active Grid Impedance Cancellation in Grid-Connected Inverters with LCL Filter
## 5 Experimental Validation
### Comparative Results with and without the Proposed LV-VSC  
Figures 20-22 present the steady-state waveforms of the inverter outputs with the active compensator from disable state (CASE I) to enable state (CASE II) under different grid impedance.  
Figure 19: Key waveforms of the inverter system.  
Figure 17: Top view of single-phase inverter prototype with the LV-VSC.  
Figure 18: Experimental inverter setup with variable grid inductance.  
They indicate that the grid impedance compensator exhibits strong harmonic rejection capability of the inverter against the grid voltage distortion even with wide variations of equivalent grid inductance.  
With the active compensator disabled, the inverter stability is vulnerable to the variation of the equivalent grid inductance, \(L_{g}\), as shown in Figures 20-22. Due to poor stability margin with large grid impedance, the measured total harmonic distortion (THD) of the grid current increases from 1.9% to 4.2%. Although the THD of the grid current at \(L_{g}=0.1\) pu still meets the standard requirement, i.e., IEEE Std 1547.2-2008, the maximum odd harmonic current distortion at 1.25 kHz is 2.2% highlighted in Figure 23 and exceeds the odd harmonic current distortion limit of 0.6% [37]. That is because the inverter becomes poor stable with the increase of grid impedance. Moreover, the voltage at the PCC, \(u_{\text{pcc}}\), is distorted due to poor quality grid current and might affect the devices at the PCC.  
Figure 21: Comparative waveforms of the inverter outputs with the active compensator from disable to enable when \(L_{g}=0.04\) pu.  
Figure 22: Comparative waveforms of the inverter outputs with the active compensator from disable to enable when \(L_{g}=0.1\) pu.  
Figure 20: Comparative waveforms of the inverter outputs with the active compensator from disable to enable when \(L_{g}=0\).  
With the active compensator enabled, the inverter maintains stable even under wide variation of equivalent grid inductance because the active damping function in the LV-VSC is activated. The measured grid current THD is sustained at 1.5%, 1.7%, and 1.5%, respectively. Moreover, no oscillation is seen at the PCC voltage. The above experimental results match with the theoretical analysis in Section 3.3.  
To further validate the effectiveness of the proposed control mechanism against the grid distortion, a nonlinear rectifier load is in shunt at the PCC to distort the grid voltage. The corresponding experimental results with highly distorted PCC voltage are presented in Figure 24. As indicated in Figure 24a, without the proposed method in CASE I, the grid current quality is poor (THDi = 10.00%) due to highly distorted PCC voltage and poor stability under relatively weak grid condition, which matches with the theoretical analysis in Figure 11a. By integrating the proposed method into the inverter, the inverter stability with large stable margin will be guaranteed as analyzed in Figure 11b, and strong harmonic suppression against the grid distortion can be ensured as shown in Figure 12. As expected, with the proposed method in CASE II, both the inverter stability and grid current quality are guaranteed (THDi = 4.22%) even under the weak grid condition with highly distorted PCC voltage as presented in Figure 24b.  
Figure 23: Frequency spectrum of the grid current without the compensator at \(L_{g}=0.1\) pu.  
Figure 24: \(Cont\).