Research On Control Strategy Of Cascaded Active Neutral Point Clamped Three-level Converter

In the scenario of supplying power to medium/low voltage and small capacity microgrid,cascaded neutral point clamped three-level converter(CNPC)has better economic advantages than modular multilevel converter(MMC).At the same time,its flexible control can better meet the operation needs of microgrid than Line Commutated Converter(LCC).However,there are more demands for CNPC control strategy because of the structural complexity of CNPC and microgrid,such as the steady-state control like voltage/current-balancing under different control modes,the transient control to maintain the stable operation of microgrid under different operating conditions.Taking the LCC-CNPC hybrid DC system supplying power to microgrid with high proportion of new energy as an example,this paper studies its steady-state and transient control strategies.The main contents are as follows:Firstly,according to the model and operation characteristics of CNPC,the paper lists the large signal average model of AC/DC side and the controller model under different control strategies,compares and analyzes the switching loss of CNPC,and studies the factors that affect the stable operation of CNPC through theoretical analysis and simulation,which lays a theoretical foundation for the subsequent control strategy.Secondly,aiming at the problem of voltage/current balancing on the DC side of the cascaded converter,a control strategy with voltage-balancing function that can supply power to the microgrid with 100%/high percentage of new energy is proposed in this paper.It includes voltage/frequency control with self voltage-balancing and optimal droop control with self voltage-balancing.The large signal stability of the proposed control strategy is analyzed according to the mixed potential function theory,and the parameter design requirements of the controller are given.Compared with the traditional voltage equalizing strategy,the strategy proposed in this paper can participate in the frequency control/regulation of the system,and the optimal droop control with self voltage-balancing can also avoid the data communication between sub-modules.The simulation results show that the proposed strategy can realize the voltage balancing function under steady-state and various fault conditions,and can also participate in the frequency modulation of microgrid to ensure that the system frequency does not exceed the safety threshold.Finally,aiming at the transient control of CNPC,the paper focuses on the switching between different controls.The paper quantifies the errors between different control strategies and presents an optimized method for seamless transfer,including a dq-axis phase angle calibration and a large disturbance instantaneous power compensation,implementing calibration/compensation of modulation wave;Taking a hybrid DC system as an example,the above compensation strategy is used to design an DC power supply strategy for isolated microgrid that can achieve seamless transfer of control mode.The strategy includes the seamless access of the hybrid DC system to the isolated microgrid and emergency power support strategy in case of large disturbance.The simulation results show that compared with the traditional strategy,this strategy reduces the impact caused by the switching between different control modes of the hybrid DC system,and can support the frequency and voltage of the system more effectively when there is a large disturbance in the microgrid.