How is repetitive control of the current inside a PV system realized? In layman's terms, don't say it's some kind of inner membrane principle or something.

In order to make the inverter output a good grid-connected current waveform, the output grid-connected current of the inverter must be closed-loop controlled. The presence of dead zones, asymmetric factors within the inverter, disturbances such as dc-side voltage and the grid can make the inverter output grid-connected current waveform distorted. When the traditional PI control is used to track the sinusoidal given signals, there are some limitations as follows: 1) When the tracking signal is a rapidly varying sinusoidal waveform, theoretically, the whole system is an aberration system, and it is impossible to do static-aberration-free tracking; 2) While the steady state error can be reduced by increasing the proportionality coefficient, however, an increase in the proportionality coefficient will lead to a decrease in the control accuracy and even cause the system to oscillate; in addition, increasing the proportionality coefficient may also amplify the noise signal at the same time, so the proportionality coefficient is impossible to obtain too large. It can be seen that the traditional PI control in this system does not realize the static-free tracking of the system, while the repetitive control based on the principle of internal mode proposed in recent years not only realizes the static-free tracking of the system, but also suppresses the cyclic disturbances of the load and effectively reduces the THD of the grid-connected current waveform.