MMC-UPFC Internal Fault Identification And External Fault Transition Strategy

Compared with traditional control technology.flexible alternating current tr ansmission systems(FACTS)have the advantages of fast response speed,flexib le control mode and strong transmission capacity of transmission lines.The uni fled power flow controller(UPFC)is the third-generation FACTS device with s eries-parallel hybrid type,which can quickly and effectively adjust the voltage,phase angle and power flow of the system.Therefore,it is widely used in po wer systems.This paper focuses on UPFC based on modular multilevel convert er(MMC).In this paper,MMC-UPFC internal and external fault characteristics analysis,ontology internal fault identification,external transition strategy and its cooperation with line distance protection are studied in depth.First of all.this paper introduces the basic structure,working principle and equivalent mathematical model of MMC-UPFC;Then according to the MMC-U PFC structure in real project,detailed fault characteristics analysis of the intern al fault of the MMC-UPFC is carried out,and several typical fault points are considered,including the faults in the parallel transformer AC side and valve s ide,series transformer valve side,DC bus and IGBT of MMC inverter,whichl ay a good foundation for the internal fault identification and isolation of the MMC-UPFC ontology.Secondly,on the grounds of the actual engineering MMC-UPFC structure and the need of MMC-UPFC ontology protection,the protection of each area i nside the main ontology and the specific failuares reacted by these protections are all introduced in detail;Furthermore,it focuses on the analysis of the adap tability of the special series transformer protection,and proposes a new type of differential protection for the series transformer,which improves the reliability of the series transformer differential protection.At the same time,the IGBT op en circuit,short circuit identification and isolation countermeasures in the half bridge sub-module(HBSM)are discussed especially.Simulation experiments in PSCAD verify the effectiveness of the proposed fault identification and isolatio n methods.Finally,aiming at the phenomenon that the MMC of the series and paralle 1 sides are both locked after the external fault of MMC-UPFC,the cause of th e blocking is analyzed in depth,and the fault transition strategy based on the series current limiting reactor on the DC bus is put forward.The simulation e xperiment in PSCAD confirms the feasibility of the transition strategy.Meanwh ile.through the simulation experiment in PSCAD,the impact of the fault transi tion method making on the line distance protection is discussed,and different coordination schemes are come up with for unlike faults.