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Analysis On The Control Strategy Principle Of Grid - Connected Solar Inverter
- Jun 26, 2017 -

The control strategy of grid-connected solar inverter is the key to successful grid connection. The control strategy of grid-connected solar inverter includes four methods: voltage source voltage control, current source voltage control, voltage source current control and current source current control. Because the input side of the current source control, the need to series a large inductor, and this will make the system's dynamic response worse, so we generally use the input end of the control, that is, a capacitor grid. Inverter and power grid parallel operation when the output side control is divided into voltage control and current control, into the Tuen network system is generally a large AC voltage source, when the output side of the voltage control, there will be voltage and voltage source parallel operation Of the situation, this situation is prone to bad flow and other issues, and the control will be relatively complex, so the output side we use the current control method, it only needs to control the inverter output current tracking grid voltage can be, control is relatively simple. In summary, the photovoltaic grid-connected inverter control strategy generally use the voltage source input, current source output.

For the voltage source current control, it is generally two ways, that is, current hysteresis instantaneous control and fixed switching frequency control.

1) Current hysteresis instantaneous control mode

Current hysteresis momentary control is a current control for the inner ring, the voltage control for the outer ring of the double closed-loop structure, the structure shown in Figure 1. First, the difference between the outer ring voltage and the reference voltage is passed through the PI regulator to obtain the reference current value, and then the reference current value and the feedback of the actual current hysteresis comparison, so that their deviation is limited to the specified range, When the deviation is greater than the width of the hysteresis, it will produce the corresponding switching signal to control the output current of the solar inverter. So that the continuous adjustment of the hysteresis loop, so that the actual current always track the reference current value.

Current hysteresis momentary control is characterized by relatively simple control, and the hysteresis loop width and current tracking performance of a larger relationship. In addition, the switching frequency of the power tube will change with the load current, which will cause the power off the work of uncertainty, on the other hand changes in the switching frequency will cause changes in harmonic frequency, which also increased the filter circuit Design difficulty.

2) fixed frequency control mode

The structure of the fixed frequency control mode is shown in Fig. The control of the solar inverter also uses dual-loop control, that is, the outer loop voltage feedback control, the inner loop current feedback control. It can be seen from Figure 2 that the reference current value is generated by the difference between the reference voltage and the inverter output voltage, and then the reference current value and the inverter output actual current value of the error value After the ratio of amplification and triangular wave cross-cut control inverter off the break. From the above analysis we can see that the inverter power switch frequency for the triangular carrier frequency, that is, there is a fixed operating frequency. At the same time, because the control mode has a fixed frequency, the harmonics generated by the inverter are relatively fixed, so that the design of the filter circuit becomes more simple. In addition, it should be noted that, when this method with 9 photovoltaic power grid system, the outer ring that is turned into the inverter output power feedback control, more effectively reduce the network circulation.

Through the above discussion on the topological structure and control strategy of the PV grid-connected inverter, the two-stage non-isolated topology has many advantages, and the fixed-switching frequency current control is also more suitable for the photovoltaic grid-connected system.