# Low-Loss Active Grid Impedance Cancellation in Grid-Connected Inverters with LCL Filter  
[PERSON]  
1 College of Automation, Hangzhou Dianzi University, Hangzhou 310027, China; [EMAIL_ADDRESS] (Y.P. ); [EMAIL_ADDRESS] (L.H. )1 College of Automation, Hangzhou Dianzi University, Hangzhou 310027, China; 172060068@hdu. parallel resonance among inverters, and series resonance with the grid. Thus, the distributed power generation system (DPGS) in Figure 1 faces severe challenges of local filter resonance, background harmonics, and multiple interactive resonance [19]. Even when a single inverter is designed to suppress the local filter resonance and comply with the stability requirements, multiple distributed inverters together can have harmonic interactions in the DPGS. Auxiliary converters based on the concept of active damping are helpful to suppress multiple resonances at the point-of-common-coupling (PCC), such as resistive-active power filter (R-APF) [20] and adaptive parallel hybrid-active power filter (P-APF) [21]. However, they do not address the dynamic interaction among sources and/or loads at the same PCC. The dynamic interaction among them impacts the stability and power quality of the DPGS. A well-established technique to analyze such interactive stability problem is by the impedance-based analytical approach: the ratio of the converter output impedance to the load input impedance must satisfy Nyquist stability criterion in order for the DPGS to be stable [22; 23]. In the DPGS, an impedance-based analytical approach is usually applied to incorporate multiple paralleled inverters into a single inverter based on the assumption that all paralleled inverters together with their control function are equal [24; 25], but it becomes difficult to use when DPG inverters are not equal because the system with different inverters requires a complicated reformulation of the system description [26]. As previously described, the existence of grid impedance is the critical factor to challenge the suppression of the local filter resonance, arouse the multiple interactive resonance, and aggravate the injected power quality of the DPGS [27; 28]. A well-known method is to utilize the PCC voltage feedforward control to ease such dynamic interaction between inverters and the grid and improve the power quality of the inverter. However, it has been proven that the unit PCC voltage feedforward control method weakens the inverter stability due to the effect of digital delay [29]. Therefore, the PCC voltage feedforward control method with reduced digital delay is studied to cancel the negative real part of the inverter output admittance according to passivity-based stability approach [30]. In references [31; 32], a full PCC voltage feedforward control technique is also proposed to counteract the effect of grid impedance and maintain wide control bandwidth of inverters through a full-order state observer, but it faces the challenge of filter parameter drifting and noise amplification due to the differentiator. Recently, unit capacitor voltage feedforward with converter-side current regulation is also presented to sustain the inverter control bandwidth and decouple the effect of grid-side impedance, wherein the digital delay is effectively compensated through the lead compensator [33]. The problem is that the resonance caused by grid-side inductor and filter capacitor still challenges the inverter stability. In reference [34], an active circuit in series at the inverter output is used to mimic the negative grid impedance and cancel the effects of grid impedance on the dynamic interaction between inverters and  
Figure 1: Typical distributed power generation system. the grid. By doing so, the stability of the grid-connected inverter is guaranteed regardless of the value of grid impedance and the number of parallel inverters. Because the additional circuit is in series at the inverter output, the cost and power loss increase with the inverter output power.