#### Typical three-phase L-filter grid-connected inverter current control

The overall control block diagram of the three-phase L-filter grid-connected inverter is shown in Figure 1. Among them, e_{abc} and i_{abc} represent the three-phase grid voltage and grid current, U_{dc} is the DC bus voltage, and L_{dbc} is the grid-side inductance., i_{αβ} is the component of the three-phase grid side current in the two-phase static αβ coordinate system, i_{d}*, i_{q}* are the current reference values, θ_{u}, ω are the phases of the grid voltage.

The current loop is the core part of the entire grid-connected power generation control system, mainly controlling the amplitude and phase of the grid-side current. The grid-connected current control of the three-phase inverter can be divided into three categories: control in three-phase abc coordinate system, two-phase static αβ coordinate system and two-phase dq rotating coordinate system control according to whether or not to perform coordinate transformation and which coordinate transformation to implement. The current loop under the three-phase abc coordinate system is to independently control the three-phase currents of a, b, and c, and its structural block diagram is shown in Figure 2. At this time, the configuration of the three-phase current controller needs to consider the structure of the three-phase circuit, such as whether the circuit is a three-phase three-wire system or a three-phase four-wire system, and whether it is a star connection or a delta connection. In different circuit structures, the settings of the regulator are not the same, and it is not universal, so the current control in this coordinate system is generally not used.

The current loop under the two-phase static αβ coordinate system realizes the control of the grid-side current under the two-phase static aB coordinate system, and its structural block diagram is shown in Figure 3. Because the control variable is an alternating sinusoidal signal, the steady-state zero error cannot be achieved by PI regulation, but the proportional resonance (PR) regulation and selection of appropriate regulator parameters can ensure that the current loop can achieve zero error regulation.

The current loop under the two-phase dq rotating coordinate system controls the grid-connected current under the dq synchronous rotating coordinate system, and its structural block diagram is shown in Figure 4. The grid voltage and grid-side current are respectively transformed by abc/dq to obtain the components i_{d}, i_{q}, e_{d} and e_{q} in the dq synchronous rotating coordinate system. The angle of the d-axis of the synchronous rotating coordinate system is the phase signal θ locked by the phase-locked loop, and the rotating angular frequency of the rotating coordinate system is consistent with the angular frequency of the fundamental wave of the power grid. At this time, the three-phase AC quantities are equivalent to the DC quantities in the synchronous rotating coordinate system, and the zero-error control of the current can be realized by adjusting Pl.

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