# Coordinated Mitigation Control for Wideband Harmonic of the Photovoltaic Grid-Connected Inverter
## 1 Introduction  
With the exhaustion of traditional sources and the rapid development of power electronics technology, many distributed renewable energy sources are widely connected to the distributed system [1]. Besides improving the energy supply structure, the high proportion of power electronics also brings serious harmonic pollution to the distribution system, seriously harming its safety and stability [2]. Renewable energy connects to the grid through a grid-connected inverter, including photovoltaics and wind turbines. On the one hand, the switching frequency of power electronics varies from several hundred Hz to several tens of kHz, leading to significant high-order harmonics carried in the supply current generated by such power sources [3, 4]. On the other hand, power electronic devices show the decentralization trend in the distribution network, and low-order harmonic components and the risk of parallel resonance exist simultaneously [5, 6]. The continued growth of harmonic content causes problems such as resonance, distributed power off-grid, energy supply shortage, and increased equipment losses to the distribution system [7, 8, 9]. These typical power quality problems have attracted widespread attention from scholars all over the world [10, 11, 12]. The power systems' short circuit ratio (SCR) is reduced to a certain extent due to the increasing number of distributed PVs. The system stability problems caused by harmonics in weak conditions can no longer be ignored [13, 14, 15]. LCL filters with small sizes and high-frequency attenuation capability are widely used to effectively mitigate the poor power quality of grid-connected inverters. As a high-order filter, the LCL filter has inherentresonance points [16]. The following two categories comprise the research on resonance suppression of grid-connected inverters. The first type is the passive suppression method. Adding impedance at the resonance frequency can prevent the resonance of the same frequency from occurring again. This method is easy to implement and can suppress resonance peaks well by incorporating passive components in the filter without any modification of the control strategy, so it is widely used in engineering practice [17]. However, passive damping changes the system's overall amplitude and frequency characteristics and generates additional resonance points. The second type is the active prevention method, which establishes an accurate inverter model to locate the resonance point and reduces the resonance amplitude by increasing the damping of the corresponding frequency. In the literature [18; 19], we can see the development of the LCL-type grid-connected inverters' Norton model. These researchers analyzed the influence of the system and control structures on the resonance characteristics. The literature [20] adds specific active damping based on the accurate calculation of resonance points to effectively reduce the peak resonance of grid-connected inverters. Compared with the passive approach, the active approach is more economical and has been proven to be robust in engineering. A similar method to suppress the high-order harmonics without adding any hardware changes is also used in this paper. High-frequency resonance mitigation enhances the control system's stability [21]. The system's total frequency domain characteristics can be improved with the suppression of low-frequency harmonics caused by the dead zone. The suppression methods for the 6 \(k\pm 1\) order harmonics can be divided into three categories. The first type reconstructs the modulating voltage reference to change the IGBT conduction time, thus reducing the harmonic current caused by the dead zone delay, which is also one of the most widely used harmonic suppression methods. In [22], the ripple caused by the 6 \(k\pm 1\) order phase current harmonics to the system are analyzed. In [23], the output voltage error and the dynamics of the control system are investigated separately.