Study On Z-source Inverter In Photovoltaic Grid System Control

The inverter is a device that converts DC to AC, so it's widely used in power conversion of electronic engineering.The traditional inverter includes the voltage source inverter(VSI) and the current source inverter(CSI). However, they both have barriers and deficiencies in principle, for example, high cost and low efficiency. The VSI usually performs buck conversion, so if we want to get a higher voltage, we need to add an extra DC boost converter, and this limits its output. The inverter switches in one bridge may be on shoot-through state because of electromagnetic interference (EMI). And that may destroy switches and even the whole circuit, which results in the reduction of the reliability of the circuit. To prevent shoot-through, the dead time is added in, which may lead to waveform distortion. The similar matter occurs in the CSI.The impedance inverter, named Z-source inverter too, brings a new style in structure and theory to power conversion, offsetting the shortage of the VSI and CSI. As it allows two switches in one bridge on at the same time, namely shoot-through state, the Z-source inverter can perform boost and buck conversion in one circuit. On the basis of Z-source PWM inverter, we can achieve a higher or lower output voltage through controlling the duty cycle. In the photovoltaic system, by using this we can track the maximum power point (MPPT).In the paper, the working principle and modes of Z-source inverter is analyzed in particular firstly, and different PWM control methods are simulated, testified and compared. For example, the voltage press on the switches and the voltage gain in the same modulate. In shoot-through mode and non-shoot-through mode different states of the capacitance voltage and inductance current are studied. Extensive research is done on maximum power point tracking in PV system. The model of the PV cell is simulated in Matlab simulink. The equivalent circuit and power relation in inverting mode is analyzed. A mathematical model is built to set a foundation on Z-source inverter grid interconnected system. Some control strategies about inverter are studied. And based on the forward, the control system including a capacitance voltage closed loop and an inductance current closed loop is designed. And its dynamic performance and stability are testified by simulation. Finally, the whole simulation circuit is built to prove theory analysis and control strategy truth.