Research On Safe Operation Area Of Three-Phase Grid-Connected Inverters Under Unbalanced Grid-Voltage Conditions

With the increase in the proportion of new energy power generation capacity, the impact of new energy power stations on the power grid is becoming deeper and deeper. In order to ensure steady operation of the power grid, grid-connected inverters are required to achieve fault ride-through capacity and reactive power support capacity. The fault ride-through capability and the reactive power support capability are mutual restriction relations. In the premise of safe operation, it has the significant research values to analysis the maximum power output capacity and power adjustment range of grid connected inverter during grid fault. This paper takes three-phase grid connected inverters as the research object, the impacts of each component of the instantaneous power and safe operation area analysis under unbalanced grid voltage were investigatedThe impacts of each power component of the instantaneous power on both ac and dc side of inverter under unbalanced grid voltage was analyzed firstly. Under unbalanced grid voltage conditions, the instantaneous power is composed of positive and negative DC components and double frequency AC component. The characteristics of different power components were specified through the derivation of the expression of instantaneous power in this study. In particular, it was found that one of the two mutually orthogonal second order AC components is determined by the positive and negative sequence active power and the other one is determined by the positive and negative sequence reactive power. This characteristic facilitates the following safe operation area analysis. By analyzing the impacts of each component of the instantaneous power on the dc and ac parts of the inverter, the impact of each power component is defined. The results showed that the oscillation component of instantaneous active power cause dc bus voltage ripple while the oscillation component of instantaneous reactive power have no impact on both ac and dc side. Besides, the positive sequence reactive power can compensate for the positive sequence voltage and the negative sequence reactive power can reduce the negative sequence voltage. The correctness of the above analysis was verified by both simulation and experiments.For the safe operating area analysis of each power component of the instantaneous power under unbalanced grid voltage this article presented a priority analysis method. The voltage, current and oscillation of instantaneous active power, which are related to the safe operation of grid-connected inverters, were fully considered in this paper, and the relationship between these three influence factors and each power component was analyzed. Every kind of the influence factors is associated with the four power components. In the premise of safe operation,any power component of the output power changes,the safe operation area of other power components will also change. So it is meaningless to analyze the safe operation area of one power component independently by setting other components to zero. However, it is difficult to analyze the safe operation area of the four component in one time. A novel safe operation area analysis method by setting the priority was proposed in this paper. The safe operation area of the highest priority power components were first analyzed and the outputs of the highest priority power components were chosen within the safe operation area. Then, based on the chosen powers, the safe operation area of the next priority power components were investigated. According to the priority analysis method, it can be sure that all the selections of output power orders are conform to the safe operation limitation. The analysis also can be used to judge whether the power command from upper control system is reasonable or not and provides help for the adjustment scheme if the command is unreasonable.According to the priority analysis method, two priority setting ways, by setting reactive as the highest priority power and by setting active as the highest priority power, are applied to analysis the safe operation area of strong power grid and the weak power grid. And the accuracy of the analysis results was verified by simulation and experiments.