| Nowadays,in order to deal with the contradiction between economic development,environmental protection and resource shortage,renewable energy is vigorously developed.Flexible HVDC has attracted much attention and favor from academia and engineering circles for its advantages of independent power control,flexible regulation,low harmonic level,no reactive power compensation and no commutation failure,especially for grid-connected renewable energy generation.The application of modular multilevel converter(MMC)makes the voltage level and transmission capacity of flexible HVDC system further improved,and the application field of flexible HVDC is further broadened.As a flexible and controllable advanced transmission technology,flexible HVDC plays an increasingly important role in the power grid.In this paper,the transient characteristics of AC and DC side faults in MMC-HVDC system are studied.Aiming at the adaptability of transient characteristic analysis of MMC-HVDC system,the control system of MMC adapting to asymmetric fault analysis and symmetric fault analysis is established.The control links and basic working principles of MMC control system are analyzed in detail,and the common groundino modes of MMC-HVDC system are introduced.Aiming at the transient characteristics of AC side faults in MMC-HVDC system,the response of MMC controller to faults is analyzed in detail with asymmetric faults near converter station as an example.After the controller responds,the short-circuit current provided by MMC includes two processes:transient state and steady state;after the fault,MMC is represented as a current source on the AC side,and its magnitude and phase can be calculated by the inverse Park transformation of the reference value of the inner current;the maximum steady-state short-circuit current provided by MMC is limited by the current limiting link of the controller.A method for calculating steady-state short-circuit current is proposed based on the characteristics of short-circuit current provided by MMC.Finally,a two-terminal MMC-HVDC system based on practical engineering parameters is built under PSCAD/EMTDC and verified by simulation.Aiming at the transient characteristics analysis of DC side faults in MMC-HVDC system,three typical DC side fault mechanisms are analyzed in detail,the generation mechanism and characteristics of voltage and current in transient process are analyzed,and the corresponding mathematical formulas are deduced.The single-pole grounding fault mainly leads to the overvoltage of the non-fault pole bus and AC side equipment of converter.The single-pole disconnection fault leads to the overvoltage of non-fault pole and fault pole bus.and the fault pole bus is more serious.The pole-to-pole short circuit fault mainly leads to the overcurrent of bridge arm devices and DC lines.Finally,a two-terminal MMC-HVDC system is built under PSCAD/EMTDC and verified by Simulation. |
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