The Control Strategy Study Of Grid-connected Inverter Under Unbalanced Voltage Conditions

Currently, the power system has developed into a single centralized supply system,large units, large power grids, and high-voltage are its main features. However, with thegrowing of the grid’s scale, the standards in today’s society for the quality of electricalenergy supply, safety and reliability have become much higher. Moreover, the seriousdrawbacks of the large power grids are also increasingly prominent. For example, runningthe ultra large scale power systems is very difficult, the investment of power gridconstruction is higher, and it also causes serious environmental pollution, and so on. On theother hand, the global environmental crisis and environmental crisis make the developmentand use of new energy sources a urgent demand. With the continuous growth in the installedcapacity of distributed power generation, the impacts of distributed generation systemcan’t be ignored, when it is connected with the grids. Against this backdrop, taking grid-connected power generation system to the network as the research object, this paper made adetailed analysis and study of the control strategy of its grid-connected inverter in theasymmetrical grid voltage conditions.This paper studied the synchronization problem of the grid-connected inverter firstly. Itanalyzed the working principles of traditional three-phase locked loop based onsynchronous coordinate system, and simulated it in the asymmetrical grid voltage conditions.By analyzing the results of simulation, it showed the drawbacks of traditional three-phaselocked loop, and then it proposed the decoupled double synchronous reference frame phase-locked loop expanded the locked loop on this basis to expand the traditional three-phaselocked loop. To validate the new scheme, it was also simulated in the same grids voltageconditions.In symmetrical grid voltage conditions, the dual-loop control strategies constituted bythe voltage outer and current inner works well with good efficiency, but when the gridvoltage is asymmetrical, the efficiency becomes low, because the current inner has nosuitable negative sequence current control loop to control the negative sequence current. Inthis paper, the study focused on how to achieve the control of negative sequence current.For this reason, we constructed a dual PI current controller based on the measured currentand the error signal decoupling, and then studied the calculation method for its referencecurrent. Finally, we verified that the theoretical analysis is correct and feasibility bymodeling and simulation. The results of simulation showed that the proposed decoupling dual synchronousreference frame phase-locked loop has better dynamic performance. It can track the voltagevariables quickly and accurately in the asymmetrical grid voltage conditions; the dual PIcurrent controller based on the measured current and the error signal decoupling constructedin this paper can quickly track the reference current without steady-state error.; In theclosed-loop conditions, the grid-connected inverter obtained the value of reference currentby positive and negative sequence control algorithm, and kept a good balance between thetwo control objectives,eliminating the oscillation in the active power and the sinusoidal ofcurrent. And the negative sequence current was controlled well in the same grids voltageconditions.