Transient Characteristic Analysis And Control Strategy Research Of Power-controlled Inverter

Power-controlled inverter has the advantage of directly controlling the output power,and is widely used in photovoltaic,wind turbine and other clean energy grid-connected occasions.When grid fault occurs,the operation condition of power-controlled inverter deteriorates rapidly,resulting in over-current,power oscillation,voltage imbalance and other problems,which can easily cause the protection action of the device.At the same time,the grid requires the inverter to output more reactive power to support the grid during the fault,which can help the grid voltage recover quickly after the fault is cleared.With the increasing proportion of clean energy in microgrid,the problem of fault ride through and voltage support of inverter become more and more important.In this paper,the power-controlled inverter is taken as the research object.On the basis of revealing the fault transient operation mechanism,the multi-objective fault ride through and voltage support of inverter are achieved,which can provide theoretical guidance for the relay protection design of active distribution network and microgrid.This paper mainly includes the following aspects:(1)Aiming at the problem that the transient characteristics of power-controlled inverter are not clear during grid fault,this paper fully considers the influence of micro source on DC side,obtains the analytical expressions of transient characteristics of inverter under symmetrical and asymmetrical fault through simultaneous control equation and circuit equation,and then analyzes the influence of important parameters on transient characteristics.The response characteristics and parameter effects of output current,output power and DC side voltage during inverter fault are deeply revealed,which provides guidance for subsequent fault ride through strategy.(2)Aiming at the problem that it is difficult to coordinate the multi-objective fault ride through of power-controlled inverter in case of grid fault,this paper proposes a current limiting and power oscillation suppression method considering dynamic positive-and negativesequence reactive current injection.Firstly,three fault ride through objectives of powercontrolled inverter,namely current limiting,power oscillation suppression and reactive power injection,are introduced.Secondly,the coordination method of positive-and negative-sequence current reference is proposed,and the typical control objectives are achieved on this basis.Then,considering the requirement of dynamic positive-and negative-sequence reactive current injection,the current limiting and active power oscillation suppression are realized simultaneously by using the flexibility of reference current.Finally,the influence of micro source on the strategy is considered,and the strategy of reactive power first is adjusted to active power first,so as to realize fault ride through in multiple scenes.(3)Aiming at the problem of weak fault voltage support of microgrid with powercontrolled inverter,this paper proposes the optimal support strategy of PCC voltage based on impedance matching and the elimination strategy of overvoltage and undervoltage considering the influence of zero-sequence voltage respectively for different fault degrees.Firstly,the principle of positive-and negative-sequence active and reactive current injection to support the PCC voltage is introduced.Then,for serious fault scenarios,an optimal support strategy based on impedance matching is proposed to support the voltage to the maximum extent within the capacity limit of the inverter while taking into account the inhibition of active power oscillation.Then,a calculation method of phase voltage amplitude considering the influence of zerosequence voltage is proposed for slight fault scenarios.On this basis,an elimination strategy of overvoltage and undervoltage is proposed to support the PCC voltage within the normal range.(4)The proposed transient characteristic calculation method and control strategy were theoretically fitted and simulated in MATLAB/Simulink simulation software,and the actual microgrid model of Dongao Island was built using RT-Lab platform,and the proposed control strategies were verified experimentally in the actual microgrid model.