基于MMC-HVDC的風(fēng)電場接入系統(tǒng)特性研究.pdf

1、Every year the world installed wind farms are growing due the increase ofelectricity demand, the growth of greenhouse gas and the rise of the fossil energyprice.The use of DFIG wind turbine equipped with the three level

2、back to backNPC converters could be the ways to reduce the installation prices.Moreover thechoice of the MMC HVDC transmission increases the system reliability, andreduces the power losses.
　　This thesis presents the

3、simulation system to study the behaviors of the windfarm connected to the AC network using HVDC transmission system.
　　We started develop the model of wind turbine, described it control strategies,and its behaviors ha

4、ve been simulated and analyzed.The back to back three levelneutral point diode clamped converter model and it control strategies arepresented.
　　The research on the wind turbine using different LVRT techniques like th

5、ecrowbar, the BESS and the braking resistor combined with fault current limiting.The protection characteristics of the techniques are modeled and simulated inPSCAD.The results show that for small wind turbine the crowbar

6、 and batteryenergy storage system can be used to protect the wind turbine during faultconditions.When the number of wind turbines is high and for big wind turbinesthe braking resistor combined with the fault current limi

7、ter can be used.
　　The research on the HVDC transmission system using different LVRTtechniques like the emulated short circuiting and the multilevel chopper has beendescribed.The protection characteristics of the tech

8、niques are modeled andsimulated in PSCAD.The results show that the emulated short circuiting can beused to protect the HVDC transmission system during fault at onshore grid.Thissystem can be used to replace the telecommu

9、nication system.To protect theHVDC transmission system during onshore and offshore AC network fault theproposed multilevel chopper gives the best results.The impact of the dissipationof excess energy from wind farm in AC

10、 network voltage is reduced.Duringasymmetric fault the control of circulating current can be employed to dampingthe DC voltage fluctuations.
　　The wind farm using doubly fed induction generator wind turbine model isde

11、veloped and it control strategies are presented.The wind farm power flowcontrols during normal and abnormal operations are also presented.The resultsshow that the proposed control methods are suitable.
　　The MMC-HVDC

12、transmission system model is developed and the controlstrategies for onshore and offshore converters are studied.According to theresults the method used to model the MMC-HVDC converter reduces thesimulation time, especia

13、lly when the number of sub modules per ann is high.Therefore, we can assume that the model is suitable for the simulation.
　　Aggregated five wind turbines using doubly fed induction generator whichrotor is interfaced

14、by the back to back of a three level NPC converter has beenused to model the large wind farm.During the fault conditions, the wind turbinegenerator rotor current is controlled and limited to protect generator.The excessp

15、ower from wind farm is dissipated in braking resistor in order to protect theconverter against the DC link over-voltage.The HVDC system duringdisturbances at both the onshore and offshore AC networks remains connected to

16、the grid thus it capable to mitigate the DC link over-voltage.
　　The circulating current within the HVDC converters legs is controlled so thatin fault conditions will limit the DC link voltage oscillation and then red