Research On Key Technologies Of Asymmetric Fault Ride-through For Three-Phase Inverters

As the interface between the power generation/energy storage unit and the power grid/load,the three-phase inverter plays an important role in renewable energy power generation,marine power supply,microgrid,and many other fields.Therefore,it is required to have high reliability.The three-phase inverter can be divided into two types: the grid-tied inverter and the stand-alone inverter.For the grid-tied inverter,the fault ride-through usually means that when the grid voltage drops,the inverter needs to keep running without being disconnected from the grid and be able to continue to deliver power to help the grid accomplish fault recovery.For the stand-alone inverter,the fault ride-through usually means that when a short circuit fault occurs in a load branch,the inverter could keep operating without shutdown,and should output a larger current to trigger the action of breakers to clear the fault branch and restore normal power supply quickly.This thesis focuses on the phase synchronization technology and power control strategy of the grid-tied inverter when the grid suffers an asymmetrical voltage sag,and the current limiting technology and the smooth recovery method of the stand-alone inverter when an asymmetric short-circuit fault happens on the load side.The main research contents are as follows:Fast and accurate grid synchronization is the premise of the control of the grid-tied inverter.The new generation grid code requires that grid-tied inverters could inject both positive sequence and negative sequence reactive currents simultaneously under asymmetrical voltage sags.Therefore,both positive sequence and negative sequence phases should be synchronized.The dynamic performance of closed-loop phase-locked methods is difficult to improve due to the limitation of bandwidth.In this thesis,an open-loop simultaneous synchronization method of positive and negative sequence phases based on moving average filter(MAF)is proposed.The positive and negative sequence phases of grid voltage can be obtained simultaneously without separating the positive and negative sequence fundamental components of grid voltage.This proposed method not only inherits the advantages of open-loop synchronization methods,but also further improves the dynamic response speed since the window length of MAF adopted is only 1/6 of the fundamental period.The existing power control strategies in asymmetric grid are usually considered from the inverter point of view,and the actual requirements of the grid code for inverters are rarely considered.This thesis analyzes the constraints to ensure the safe operation of inverters from the two aspects of maximum phase current limit and minimum active power oscillation.Then a power control strategy for asymmetric low voltage ride-through is proposed.By optimizing the references of power components,this strategy can deliver the corresponding positive and negative sequence reactive power according to the requirements of grid codes to support the grid,also it can make full use of the remaining inverter capacity to deliver active power into the grid to avoid the fault expansion caused by a larger active power shortage.Focusing on the fault ride-through issue of the three-phase three-wire stand-alone inverter when an asymmetric short circuit fault occured on the load side,this thesis first defines the control objectives of fault ride-through process,that is: I)the Root Mean Square(RMS)value of healthy phase voltage should keep constant before and after fault and under different loads to ensure the continuous and stable power supply of healthy phase loads;II)the fault phase current should be limited and controllable to trigger the action of circuit breakers,so as to clear the fault branch and realize selective protection.Then the conclusion is drawn by the circuit modeling and analysis: the α axis and β axis reflect the information of healthy phase and fault phase respectively when an asymmetric short circuit fault occurs.Therefore,based on the above analysis conclusion,a current limiting strategy under asymmetric short circuit conditions is proposed in this thesis,which combines α-axis voltage control and β-axis current control.The control algorithm is simple and easy to implement.This proposed strategy can not only limit the output current of fault phase,but also ensure that the RMS value of fault phase voltage remains constant during the whole fault ride-through process.When the load branch with short-circuit fault is cleared by the circuit breaker connected to the fault branch,the inverter should restore the output voltage to the rated voltage as soon as possible to ensure the normal power supply of other loads,but the problem of output voltage overshoot is easy to occur in the fault recovery process.Firstly,this thesis analyzes the mechanism of voltage overshoot in the fault recovery process,and then a smooth recovery strategy of output voltage based on parallel virtual impedance is proposed.The current on the load branch is reduced through the shunting effect of parallel virtual impedance after the fault is cleared,then the overshoot phenomenon caused by the sharp rise of output voltage can be avoided,and the smooth transition of short-circuit recovery process can be achieved.In addition,the analysis results proved that the introduction of parallel virtual impedance will not affect the control of current limiting stage,and the design method and digital implementation of virtual impedance are analyzed.