| Multi-Terminal Direct Current (MTDC) can realize the function of multi powersupplies and multi receiving placement by electricity. Compared with thetwo-terminal High Voltage Direct Current (HVDC) system, the MTDC system ismuch more economical and flexible, and it also has distinct advantages in gridinterconnection and new energy integration. With the development of high-powerelectronic switching device, HVDC based on Voltage Source Converter(VSC-HVDC) which is on the basis of full-controlled switching device is rapidlydeveloped. It has the advantages of adjusting active and reactive power indepentlyand supplying power to the passive network. As a new generation of the voltagesourced converter topology, Modular Multilevel Converter (MMC) is rapiddevelopment, because its switching loss is low and it also not relies on thetechnology of devices in series.The VSC-MTDC system based on MMC not only has the advantages of theMMC but also has the advantages of MTDC system such as the well flexibility,economy and realiability. However, the VSC-MTDC system based on MMC is morecomplex than the2-terminal HVDC system on simulation modeling and systemcontrol and protection strategy. And many key issues are not got reasonable solution.Therefore, the paper studies the simulation modeling and coordination control andprotection strategy. It can provide some theoretical basis for the implementation ofthe VSC-MTDC project based on the MMC.(1) VSC-MTDC model and control strategyFirst of all, it needs to study on the operating principle and control stragy ofMMC for the study of VSC-MTDC system based on MMC. Based on the operatingprinciple of MMC, this paper derived the mathematical model under dq coordinates of the three-terminal VSC-MTDC system. And based on the dq decoupling controlstrategy, the paper designed the basic MMC station-level controller and it alsodesigned the MMC modulation strategy and capacitor voltage balance controlstrategy.(2) The control strategy applied to the VSC-MTDC systemThe coordination control among all of the converter stations is the key ofVSC-MTDC system. First, the DC voltage slope control, DC voltage margin controland voltage-active regulation control method are researched and the correspondingcontrollers are designed based on MTDC system. According to the characteristic ofupper controller based on the voltage-active regulation and power controller basedon voltage-margin method, this paper put them together and propose a new controlstrategy. It can improve the stability and reliability of the VSC-MTDC. The leftconverters adjust the active power together, when one of the converters out ofcontrol. The power controller can automatic switching work mode in accordancewith the system operation, when a fault occurs in the communication line of heupper controller. The simulation verifies the correctness and applicability of the newcontrol strategy.(3) The research on the LVRT in VSC-MTDC systemIn order to solve the low voltage ride through (LVRT) problems of windgenerator, the paper design the Multiterminal VSC-HVDC system connects withwind power generation system using the dc voltage-active power controlstrategy．The dc voltage will not rise when voltage sags occurs in the ac side，because the wind power generator can dispatch the power which it generates．So thewind farm can stable operation when voltage faults and voltage recovery．The paperdesign the four-terminal system using MATLAB, the wind generator can worknormally when the voltage sags occurs respectively in the ac side of convertorstation. Simulation verified the Multiterminal VSC HVDC system is one of the mosteffective ways for solve the LVRT of wind turbine．... |
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