Inverter Control Strategy Under Unbalanced Voltage Conditions

With the increasing utilization of renewable energy sources such as photovoltaic power and wind power generation,grid-interfacing inverter,as an important part of energy conversion,has an increasingly close interaction with the grid,and has become a hotspot in academic research.As the terminal of the power system,the distribution network has a complex operating environment,which increases the possibility of asymmetrical faults such as single-phase or two-phase grounding and unbalanced voltage sag caused by high-power load switching.The accompanying problems such as power oscillation and overcurrent greatly endanger the safety and stability of the power system.Aiming at the shortcomings and deficiencies of the traditional control strategy of grid-interfacing inverters,this paper conducts researches focusing on the following aspects:Aiming at the problems of complicated operation and insignificant physical significance of the three-phase three-wire inverter’s traditional control strategy,this paper simplifies the unbalanced voltage faults by defining a "new neutral point",which can provides a more intuitive and effective way to analyze the unbalanced voltage faults.In addition,based on the thorough analysis of the power oscillation generation mechanism,this paper reveals the hidden law between power distribution and power oscillation in the three-phase three-wire system.Based on the conclusions,a novel power oscillation suppression strategy is designed.This paper explores a new idea to solve the problem of power oscillation suppression,which also has a fairly perfect control performance.Aiming at the problems of insufficient utilization of the control freedom and the unsatisfactory current reduction performance of the three-phase four-wire inverter’s traditional control strategy,a three-phase current analytical solution model is established in this paper.Through thorough analysis of the model,a series of laws of the influence of unbalanced voltage faults on the system operation are obtained.Based on the conclusions,a novel three-phase four-wire inverter power oscillation suppression and current reduction strategy is obtained.On the basis of realizing active power oscillation suppression,through zero-sequence current self-optimization,this strategy minimizes three-phase current peaks to the greatest extent while realizing that the neutral current in the system does not exceed the threshold.This strategy fully considers the effects of the neutral and phase current thresholds,which can also achieve optimal current control when the thresholds of the two are different.The numerical examples and simulation results are provided to verify the validity of the proposed control strategy under different severe perturbations.The proposal can greatly expand the system power operation boundary and the performance is fairly close to optimal.