Key Techonology Of Fault Tolerance And Fault Ridethrough Control For MMC-based Power Router In AC/DC Distribution Grids

With the in-depth promotion of “carbon peak and carbon neuturalization” and the construcution of new power system,the penetration rate of distributed new energies in distrution grid will be further improved.The traditional distribution grid with unidirectional power supply is developing into AC/DC distribution grid with multidirectional power supply.Integrating the electrical physical system and information system,power routers have multiple power interfaces.Based on power routing optimization control and fault isolation function,it can efficiently consume distributed new energies.Therefore,power router is the key system equipment of the new AC/DC distribution grid.With modular mulitilevel converter(MMC),the power router can directly provide high-voltage DC port and have abililty to interconnect multi-circuit AC and DC distrubiton lines.The operation reliability of the MMC-based power router will directiely related to the power supply security of AC/DC distribution grid.How to improve the uninterrupted operation capability of the MMC-based power router under internal and external fault coditions has become the difficult problem in its engineering application.This dissertation focuses on the fault tolerance and fault ride-through control technology of MMC-based power router in AC/DC distribution grid.As high voltage stage MMC is its core conveter,the related researches have carried out in the optimization control of the current inner loop,the fault tolerant control and the AC and DC fault ride-through control.Furthermore,the topology and stability control of MMC-based power router with positive and negative low voltage DC dual ports are studied.Aiming at the problem of current waveform distortion of high voltage stage MMC caused by distribution grid background harmonics,the mechanism of output current waveform distortion of traditional arm current control based on proportional resonant controller under background harmonics is analyzed.For the requriements of fault-tolerant control and fault ride-through control,an arm current predictive control method is proposed to suppress the influence of background harmonics.Based on the discrete mathematical model of high-voltage stage MMC,two-step predictive control is adopted.The accurate tracking of multi-band composite signal reference value is realized.Taking accurate tracking of arm current reference value as the control objective,the design of weight factor is avoided.The calculation amount of the controller and the number of sub-modules in each arm are decoupled,and the influence of the inherent one-beat delay of the controller is eliminated.The effectiveness of the proposed control method is verified by simulation and experiment.The results show that the AC side current THD does not exceed 3% under the AC port voltage distortion condition.Aiming at the problems such as the arm circulation and the AC side current distortion caused by the arm asymmetry of high voltage stage MMC under the internal sub-module(SM)fault condition,taking advantage of the DC voltage margin that SMs can work properly,a fault-tolerant control strategy based on the capacitor voltage resilient control is proposed.It includes the capacitor voltage resilient control,the arm current reference values calculation method and the improved carrier phase-shifted pulse-width modulation(CPSPWM)method.The reliability analysis model of MMC is established.The analysis shows that adopting the proposed fault-tolerant control strategy,the operation reliability of the MMC without redundant SMs can be improved without increasing the cost.Based on the operation characteristics of high voltage stage MMC under SM fault conditions,the fault tolerant control mechanism of MMC is deduced.With the control obejective as that the sum of the SMs’ capacitor voltage can be put into operation in each arm is equal,the capacitor voltage resilient control is presented.The effectiveness of the proposed control method is verified by simulation and experiment.The results show that high voltage stage MMC can still transmit active power as normal under the finite SMs fault condition.Aiming at the problem of arm active power imbalance in high voltage stage MMC caused by AC and DC faults,the operation mechanism of MMC under AC and DC fault conditions of urban distribution grid adopting small resistance grounding mode is analyzed.A comprehensive AC and DC fault ride-through control method based on the individual arm capacitor voltage control is putforward.With the the control obejective as arm active power balance,an individual arm capacitor voltage control is presented.SMs capacitor voltage control in the upper and lower arm is decoupled.Besides,the improved arm current reference values calculation method is proposed.Based on the MMC different operation conditions,the arm current reference values can realize switching adaptively.The simulation and experimental results show that the proposed comprehensive AC and DC fault ride-through control strategy is suitable for MMC normal operation conditions,AC port voltage unbalanced conditions,DC fault conditions,and complex fault conditions where AC port voltage imbalance and DC fault occur simultaneously.The uninterrupted operation performance of high voltage stage MMC under AC and DC grid faults is improved.To improve the power supply reliability of low voltage DC port under high voltage DC port fault condition,a MMC-based power router with positive and negative low voltage DC dual ports is proposed.In the proposed power router,the positive pole and negative pole of high voltage stage MMC are repectively cascaded with isolated modular multilevel DC/DC converters(IMMDCs).Under high voltage DC port pole to ground fault condition,the healthy pole of the proposed power router can still transmit active power normally.Aiming at the problem of insufficient stability margin caused by the interaction of converters in the proposed power router,a stability control method based on series virtual resistor is proposed.In terms of impedance analysis method,the stability margin of the presented power router is studied.Then,the stability control method based on series virtual resistor is proposed,which improves the stability of the presented power router without increasing power loss.Combined with low voltage DC ports current feedforward control,a coordinated control method is proposed to improve the dynamic response speed.The simulation results show that the proposed power router topology and control method can improve the power supply reliability of low voltage ports under high voltage DC port pole to ground fault condition.