# Three-phase photovoltaic grid-connected inverter control system

The control structure diagram of the three-phase photovoltaic grid-connected inverter system is shown in Figure 1. The control system mainly has three parts: current Pl regulator, voltage feedforward, and repetitive control unit.

In order to realize the decoupling of the three-phase current, it is necessary to transform the currents Ia, Ib, Ic in the three-phase static ABC coordinate system to the two-phase synchronous rotating d-q coordinate system. The relationship between the stationary ABC coordinate system and the synchronously rotating d-q coordinate system is shown in Figure 2.

In order to track the current to the grid voltage, the grid voltage needs to be phase-locked. Due to the limited processing capacity of the DSP used, in order to reduce the processing burden of the DSP, this book adopts a simple phase-locking method: capture the zero-crossing points of the line voltage Uab and Ubc through the hardware circuit, and determine the power grid through the relationship between the two zero-crossing points. The direction of rotation of the voltage is oriented when the Ua phase voltage reaches its peak value. In the next cycle, the angle θ between the d-axis and the A-axis increases with the grid angular velocity ω. This method avoids the calculation of division and arctangent, saves processor resources, and is proven to be feasible.

(1) PI regulator design
After 3s/2r conversion, the three-phase AC component becomes a two-phase DC component to facilitate the design and control of the PI regulator. The phase-locked loop orients the voltage space composite vector on the d-axis, and the flux linkage composite vector on the q-axis. In this way, the decoupling of power is realized, which can realize the independent adjustment of active and reactive power. The d-axis is the active component and the q-axis is the reactive component. The goal of control is to realize the tracking of the command current Id, Iq by the sampling current Id *and Iq* so that the current steady-state error is close to zero. Figure 3 shows the control block diagram of the d-axis.

in:
Gdi(s)=(KpS＋Ki)/S
Pdi(s)=1/(LS+R)
Since the switching frequency is much higher than the power grid’s whisker rate, the Kpwm link is equivalent to a proportional link K in order to facilitate the analysis and ignore the influence of the switching action on the system.
Then the open-loop transfer function of the system is:
Gd(s)=Gdi(s)*K*Pdi(s)
In the actual system, L=4.5mH, R=0.2Ω, K=0.77, Kpd=0.05, Kid=10, draw the Bode diagram of the d-axis open-loop transfer function as shown in Figure 4. It can be seen from the figure that the phase angle margin at the shear frequency is 60°, and the amplitude margin is also large enough. In the debugging process, the adjustable range of the proportional integral coefficient is very large, which shows that the control system has good stability. The design of the PI regulator of the q-axis is basically the same as that of the d-axis, as long as the command value is changed to Iq* that is, the q-axis current is not separately adjusted during the test adjustment process, and the parameters are the same as the d-axis. When the inverter is generating full power to the grid, set Iq*to zero.