The main function of the structure of solar grid connected inverters is to convert DC voltage into AC voltage suitable for the grid and feed it into the grid. For multi-stage solar inverters with DC/AC solar inverters in the later stage, the main purposes of the latter stage DC/AC solar inverters are:
(1) Maintain stable intermediate excessive DC bus voltage.
(2) Control the power factor of solar inverters.
Therefore, there are different requirements for the switch control strategies of different solar inverters. According to the different control variables, the classification of control strategies for solar grid connected inverters is shown in the table.
| Voltage oriented control strategy | Voltage Oriented Vector Control (VOC) | Based on current closed-loop control |
| Voltage oriented control strategy | Voltage Oriented Direct Power Control (V-DPC) | Based on power closed-loop control |
| Control strategy based on virtual magnetic flux orientation | Virtual flux oriented vector control (VFOC) | Based on current closed-loop control |
| Control strategy based on virtual magnetic flux orientation | Direct Power Control (VF-DPC) Based on Virtual Flux Orientation | Based on power closed-loop control |
Taking the ideal single-stage three-phase solar grid connected inverter as an example, its main circuit is shown in Figure 1.
By controlling the current vector output by the solar inverter, active and reactive power control of the solar inverter can be achieved. The vector diagrams under four special operating states are shown in Figure 2.
Among them, the four states a, b, c, and d are pure inductor characteristic operation, unit power factor inverter operation, pure capacitor operation, and unit power factor rectifier operation, respectively. As shown in the figure, the following formula is used.
Therefore, by controlling the reference value of the current vector and using the following formula.
The voltage vector control objective can be determined, so by controlling the voltage vector, the output current vector of the solar inverter can be indirectly controlled to achieve different working states of the solar inverter.
But this indirect control method has the following problems:
(1) Sensitive to system parameters.
(2) Based on fuzzy control strategy, the dynamic response speed is slow.
(3) Poor output current quality.
Therefore, in order to improve the robustness of the inverter system, a direct current control scheme based on current closed-loop is proposed. On this basis, the vector of the output current of the solar inverter is directly controlled, thereby indirectly controlling the operating state of the solar inverter, which is called voltage oriented vector control. If the active and reactive power of the solar inverter is directly controlled, it is called voltage oriented direct power control.
However, in order to achieve the accuracy of the two control strategies mentioned above, it is necessary to accurately obtain the vector of the grid voltage and require the grid voltage to be an ideal sine wave. To reduce the impact of harmonics in the grid voltage, a virtual magnetic flux orientation control strategy is proposed. Similarly, for the virtual magnetic flux control strategy, there are slight differences in current direct control and power direct control.