Study On Control Strategy Of Grid Side Bi-directional Converter In Microgrid System With AC And DC Bus

With the continuous development of modern industrial, energy shortage problem hasbecome increasingly prominent, and the wind and solar complementary generatingelectricity system is a typical example of new energy. Because of the flexibility of energyin microgrid system with AC and DC bus, it received extensive attention, and in which thegrid-connected bi-directional inverter play a key role. On the one hand, the main fuctionof grid-connected bi-directional inverter is achieve two-way flow of energy between ACbus and DC bus. On the other hand, when disconnected or islanded from the main grid, thebi-directional inverter also has the responsibility to set the nominal voltage and frequencyfor the entire microgrid. To achivev highefficiency, appropriate control strategy forbi-directional inverter is essential. In this paper, for the bi-directional inverter in the ACand DC bus structure microgrid, the strategy for the grid connected mode and island modeare investigated.Firstly, the structural characterisitics of AC and DC bus microgrid and the gridconfiguration mode is discussed, and on this basis, two modes of operation of theconverter is summarized—grid connected mode and island mode. The continuous-timedynamic model of these two modes is established, and the control system of gridconnected mode is tuned as an example, designing the paramenters of the PI regulator.Secondly, the strategy of the bi-directional converter for the grid connected mode andisland mode are investigated. For the grid connected mode, aα-βstationary framecontrol scheme using PR regulator under unbalanced voltage conditions is proposed. Themethod for getting the sequence components of the source voltage and selecting theparamenters of PR regulator is also discussed, and they are all realized in the digitalsystem. For the island mode, on the basis of the tradition P-U droop control for weak lowvoltage (LV) network, this paper improve the second quadrant of the P-U droopcharacteristics so as it is more suitable for bi-directional control of the converter. Theimproved scheme can not only achieve two-way flow of energy between AC bus and DCbus, but also set the nominal voltage and frequency for the entire microgrid. Finally, by using MATLAB, the simulation of system control strategy is validated,and the experiments are implemented through TMS320F2818 DSP with C programlanguage. The result of simulation and experiments prove that, for the grid connectedmode control scheme, the negative sequence component of the AC currents is eliminated,and the proposed scheme is more simple then the traditional strategy; for the island modecontrol sheme, the improved scheme can not only achieve two-way flow of energybetween AC bus and DC bus, but also set the nominal voltage and frequency for the entiremicrogrid.