Control and design of Quasi-Z-Source Inverter (qZSI) for grid connected Photovoltaic (PV) arrays
Examensarbete för masterexamen
Electric power engineering (MPEPO), MSc
The renewable energy sources are greener and more environment friendly than nonrenewable energy sources. They are used for the sustainable power production. Solar energy is abundant in a lot of places and can be tapped to satisfy energy needs. The Photovoltaic(PV) arrays can be connected to an inverter to convert DC output of PV cells to AC supply for the grid. A type of converter that we can use for this application is Quasi-Z-Source Inverter(qZSI). In this project, one of the task has been to calculate and verify the value of elements used in the qZSI. The verification was done by pole-zero maps, bode plots and through optimization via simulations of the model. This was followed by designing a controller for the DC side of the qZSI. Since it was for DC side, it is called the DC side controller. Its design involved verification of the transfer functions derived from the signal flow graph using Mason’s gain formula. The signal flow graph comes from the dynamic model of the qZSI. The block diagram from the transfer function was then simplified so as to apply the Internal model control(IMC) principle to obtain the proportional and integral constants(for the inner and outer loop) of the controller. The inner loop is for the inductor current while the outer loop is for the DC link voltage. These were later fine-tuned to another set of proportional and integral controller constants when both loops were cascaded. Later, comparison of the output of qZSI’s Simulink model was done with the Fronius IG 15 inverter installed in Chalmers Grundkurs lab. It was also compared with a reference model*(already available in Simulink library which was optimized for comparison with qZSI) which is Simulink model of a VSI connected with a boost converter supplying the grid. The input for it is DC supply from PV arrays. For comparison it was important that the DC input remained same for both the models; the MPPT voltage input to VSI was sampled and used as input for qZSI model. The fast fourier transform(FFT) and total harmonic distortion(THD) measurement of the output of all three inverters was compared. It was found that the Fronius inverter had the least THD followed by VSI and then the qZSI. In all cases, THD was well within the limit as per IEEE standard STD 519-2014. The FFT showed presence of lower odd harmonics in all three inverters. The magnitude of odd harmonics upon FFT followed the same suit as the THD. All in all, a much more fine tuned controller and better filters can be designed for the qZSI to decrease the harmonics content. A better switching strategy can also be used to eliminate the harmonics. Considering that qZSI has a single stage unlike the reference model* with cascaded boost and VSI inverter, it is cost effective to use qZSI plus it can perform both the buck and boost operations. This makes it an attractive replacement option of VSI for usage in applications like PV array and fuel cells.
Elkraftteknik , Electric power engineering