| Solid-state transformers(SSTs)are the core equipment for power conversion in the fields of smart distribution networks,data centers,fast charging stations,electrified railways,and electric ships.SSTs are an important carrier for the construction of new energy systems under the "dual carbon" goal.The cascaded converter based on three-level cell has the advantages of fewer power modules and higher power density,making it the preferred solution for the front-end of SSTs.However,the medium-voltage(MV)side of SST is not directly connected to the ac grid through the transformer,so it faces the complex disturbances of the MV ac grid.At the same time,there are numerous switching devices in SSTs,and the likelihood of device failures increases sharply,thereby reducing system reliability.Adopting fault-tolerant operation strategy is an important methods to improve the reliability of the converter systems.For the failure of the module on the MV side of SST,the existing fault-tolerant strategy directly bypass the faulty modules.However,due to the large module capacity of the three-level cell,directly bypassing the faulty module will lead to a large loss of system capacity,which can easily lead to chain problems such as converter instability and fault expansion.Utilizing the normal power devices in the faulty module to make fault-tolerant operation can effectively avoid the loss of system capacity.But currently,there is no research on fault-tolerant strategy based on this method for SSTs.In addition,there are differences in the front-end cascade modules for different SST applications,which affects the control flexibility during fault-tolerant operation,making it difficult to directly migrate fault-tolerant operation strategies.Thus,it is urgent to study the fault-tolerant operation strategies based on the special topology and fault characteristics of SSTs based on three-level cell.Based on the abovementioned challenges,this paper explores the control flexibility of faulty module reconstruction,control,and modulation for SST topologies of cascaded front-ends with various three-level cells,and establishes a fault-tolerance operation mechanism for module faults on the MV side of SST.For systems with bidirectional energy flow requirements,a fault-tolerant strategy is studied based on a fully-controlled three-level-cell cascaded front-end SST with a high control flexibility.By analyzing the combination of switch states,the common fault results of fully controlled modules under different fault types are summarized.Then,based on the redundancy characteristics of the converter itself,a fault-tolerant operation strategy based on the coordination work of faulty modules,faulty phases and non-faulty phases is proposed.This strategy avoids system capacity loss caused by direct bypass of the faulty module through the configuration of the faulty module,and realizes the voltage-balancing of the capacitor through coordinated control within the phase,and realizes the balance of the line voltage through the coordinated operation between the phase voltages.Taking an SST based on T-type-cell cascaded front-end as an example,simulation and experiments have proved that the proposed fault-tolerant strategy effectively utilizes the redundancy characteristics of the converter itself and improves the fault-tolerant operation ability of the converter.For energy unidirectional flow systems with low-cost requirements,a fault-tolerant strategy of multi-module faults is studied,based on the Vienna-cell cascaded front-end SST with low control flexibility.Due to the limited control freedom of the voltage and current of the Vienna cell,its fault-tolerant operation is difficult.This paper analyzes the fault characteristics of the Vienna cell H-bridge,configures the faulty module,and derives the capacitor voltage-balancing condition of the faulty module.Then,a fault-tolerant operation strategy suitable for SST based on Viennacell front-end is proposed.The strategy realizes the line voltage balance through improved zero sequence voltage injection,and realizes regulation of capacitor voltage of faulty module through isolation dc-dc stage power control.The configuration of faulty module is matched through the improved fault-tolerant modulation strategy based on sorting algorithm,which avoids current disturbance caused by mismatch between reference voltage and output voltage.Experiments are proved that SST based on Vienna-cell front-end can operate continuously with the proposed strategy,which effectively improves the reliability of the system.To address the limitation of fault-tolerant operation caused by the voltage and current of Vienna cell must be in the same sign,a hybrid cascaded front-end SST topology based on threelevel cell is proposed.By cascading Vienna cell H-bridges and T-type cell H-bridges at the frontend,the control flexibility of system is increased,enabling the converter to have a certain ability to operate in reverse sign of the voltage and current.Thus,the reliability of the converter is significantly improved.The proposed topology can work in both unit power factor and non-unity power factor scenarios.In addition,a fault-tolerant operation mechanism for module faults on the MV side of SST is established based on power factor,achieving fault-tolerant operation of SSTs and avoiding system capacity loss.The mechanism includes the configuration of faulty modules and their capacitor voltage-balancing condition analysis based on power factor,combined with fault-tolerant control,fault-tolerant modulation,and back-end converter collaborative power control.The effectiveness of the proposed fault-tolerance mechanism is verified based on the nonunity power factor condition.In addition,the proposed topology and mechanism effectively solve the power factor limitation of the Vienna cell,thereby significantly improving the grid-connected power quality during fault-tolerant operation.In order to solve the problem of difficult self power supply of the controller due to the MV DC voltage of three-level cell H-bridge,which leads to the reliable operation of the controller,a buck-boost high step-down converter(BHSC)is proposed as the auxiliary power supply on the MV side of SSTs.The proposed converter adopts the modular design of cascading buck-boost submodules,which has the advantages of high modularity,simple structure,and low cost.In addition,a submodule control strategy with Bang-Bang control is proposed for this circuit.There is no communication and synchronization between the submodules to achieve the voltagebalancing of the capacitor,which greatly simplifies the voltage control of the auxiliary power supply.Therefore,the proposed converter can be easily extended to MV and HV applications by increasing the number of submodules,which providing a new solution for auxiliary power design in MV systems.Overall,in this paper,for SST based on three-level-cell cascaded front-end,the fault-tolerant operation strategies for module failures under different control flexibility is studied to adapt to specific system requirements in different application scenarios.Based on the characteristics and operating conditions of SSTs with different front-end module structures,the configuration of faulty module is designed,and combined with the degrees of freedom such as control and modulation,a fault-tolerant operation mechanism for module faults on the MV side of SST is established to ensure without system capacity loss during fault-tolerant operation,and effectively improve system efficiency,economy,and reliability of SSTs. |
