#### Into the grid current control technology

In order to ensure the stable operation of the power grid and freedom from pollution, countries around the world have put forward strict requirements on the quality of the grid-connected current of photovoltaic grid-connected systems. And in terms of total harmonic content, single harmonic content and power factor, the quality of the current into the grid is strictly limited. Using a suitable grid filter can make the harmonic content of the switching frequency in the grid current meet the standard. However, in order to meet the requirements of power factor, low-order harmonics, and total spectrum wave content, suitable grid current control technology must be adopted.

**1. Fundamental tracking and harmonic suppression technology**

Proportional-Integral (PI) regulators are the most commonly used current regulators. This type of regulator can achieve no static error tracking of the direct current, but it cannot achieve no static error for the 50Hz fundamental signal. Therefore, the current control is performed in a three-phase synchronous rotating dq coordinate system, while in a single-phase grid-connected inverter or a three-phase static coordinate system, it is difficult for this type of regulator to achieve zero steady-state error. Although the fundamental gain can be greatly increased by adjusting the PI parameters to weaken the steady-state error, the adjustment of the control parameters in practical applications is greatly restricted due to the limitation of stability. The parameter design problem of the PI regulator in the LCL filter grid-connected inverter can be verified repeatedly by using the system Bode diagram, the design method of passive theory can be adopted, the design method of pole configuration can be adopted, and the PI parameter optimization design method widely used in L-type grid-connected inverters can also be used. However, no matter what design method is adopted, the relevant research literature has shown that the PI regulator can increase the open-loop cut-off frequency (the frequency corresponding to the intersection of the open-loop amplitude-frequency curve and the 0dB line) of the system to a sampling frequency close to one-tenth, which can ensure a satisfactory dynamic response.

In an ideal situation, the inverter bridge arm output voltage uinv only contains the fundamental wave and the side frequency harmonics near the switching frequency and the switching frequency multiples. However, when the actual photovoltaic grid-connected inverter is running, the dead zone effect, power tube turn-on and turn-off delay, turn-on voltage drop, and grid voltage harmonics will cause additional low-frequency harmonics. Therefore, the grid current harmonic suppression technology is the key technology to improve the grid current power factor and suppress the grid current distortion. Among them, current control technologies such as harmonic resonance control, repetitive control, predictive current control, and hysteresis control have all been applied in the harmonic suppression of photovoltaic grid-connected inverters. In particular, the proportional resonance plus harmonic resonance control technology for single-phase systems and the PI plus harmonic resonance control technology under synchronous rotating coordinates for three-phase systems have received extensive attention.

**2. Active damping technology for high-order filters**

When the grid-connected inverter adopts a single L filter, the stable operation of the grid-connected inverter can be realized by adopting the feedback control of the grid current. Although the grid-connected inverter based on the LCL filter can achieve a substantial attenuation of the current harmonics of the switching frequency, it is conducive to the miniaturization of the filter. However, the resonance poles of high-order filters cause resonance spikes and phase mutations in the input and output characteristics of the filter (see Figure 1). This spectral vibration characteristic will affect the stability of the grid-connected inverter. When single current feedback control is adopted, it is difficult to take into account the bandwidth and sub-harmonic suppression of resonance frequency, and the network current will contain large harmonics and even system instability will occur. Therefore, effectively suppressing LCL resonance is a prerequisite for current control to ensure high-quality network current. Passive damping technology using series and parallel resistors can suppress resonance, but it will cause power loss and weaken the ability to suppress high-frequency spectrum waves. Therefore, active damping technology with additional control algorithms has attracted widespread attention.

The active damping control method of adding digital low-pass, leading and notch filters to the forward path can suppress the resonance phenomenon caused by the LCL filter, but this method is more dependent on the filter parameters and may cause the system bandwidth to be greatly reduced. Adding the lead-lag damping link of the filter capacitor voltage to the grid current control algorithm, the proportional feedback link of the filter capacitor current, and the combined variable feedback link can all realize the resonance damping of the LCL filter, but it often requires additional sensors to increase the implementation cost. The estimation algorithm using the inherent relationship between current and voltage in the filter is a method to reduce additional sensors. There are many methods of active damping, but there is a lack of theoretical research related to active damping. It is necessary to form a unified theoretical understanding of active damping technology. This way, on the one hand, we can fully understand the characteristics of the existing methods, on the other hand, it is also conducive to the discovery of new active damping control methods.