Abstract
Due to the advantages of superior harmonics attenuation ability and reduced size, the LCL filter has been widely adopted to interface between the inverter and the grid for improving the quality of injected grid currents. However, the high-order characteristics and various constraints of the LCL filter complicate the filter design. Moreover, the stability of the internal current control loop of the individual inverter is susceptible to the inherent LCL -filter resonance peak. Meanwhile, the overall system stability would be aggravated by the external interactions between the inverter and the weak grid as well as among the paralleled inverters. Both the LCL -filter resonance peak and two types of interaction would cause severely distorted grid currents. Motivated by the existing problems, a comprehensive review on the modeling and stability analysis of the LCL -type grid-connected inverters is conducted in this paper. Concretely, the generalized parameter constraints of the LCL filter are outlined to facilitate the passive components selection, and the magnetic integration techniques of filter inductors are also introduced to reduce the weight and size of filter for increasing the power density of the system. Then, the various damping methods for enhancing the individual internal stability and the relevant application issues are also discussed. Furthermore, the impedance-based method for evaluating the system-level interactive stability is subsequently reviewed, with the emphasis on different modeling methods of inverter output impedance and online impedance measurement techniques. Finally, the future research trends on the modeling and stability analysis of LCL -type grid-connected inverters are also presented.
Highlights
The distributed power generation systems (DPGS), as shown in Fig. 1, have been widely utilized for renewable energy integration, such as solar, wind, and fuel cell, which greatly alleviate the energy crisis and environmental problems [1]–[5]
Thereafter, this paper presents a comprehensive overview on the state-of-the-art techniques of LCL-type grid-connected inverters, including the LCL-filter design, the internal and external stability of inverters
The comparison of the typical passive damping (PD) methods and the selection conditions of the damping resistances are presented in Table 3, where ωr is the resonance angular frequency of the LCL-filter, kp is the proportional coefficient of current controller, and KPWM is the gain of the inverter bridge
Summary
The distributed power generation systems (DPGS), as shown in Fig. 1, have been widely utilized for renewable energy integration, such as solar, wind, and fuel cell, which greatly alleviate the energy crisis and environmental problems [1]–[5]. The comparison of the typical PD methods and the selection conditions of the damping resistances are presented, where ωr is the resonance angular frequency of the LCL-filter, kp is the proportional coefficient of current controller, and KPWM is the gain of the inverter bridge. Without considering the effect of control delay, the WAC of the currents can be regarded as a virtual impedance in parallel with the filter capacitor, in which case the high-pass characteristics of the virtual impedance enable a low impedance flow path for high-frequency harmonics equivalently [103], and the virtual impedance is modeled as follows:. The digital control delay caused by algorithm execution has a significant influence on the characteristics of virtual resistor
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