Similar to other major electrical apparatuses, the reliability and stability of the DC network is becoming the most important issue when using voltage source converter based multi-terminal DC (VSC-MTDC) system for offshore wind power integration. A coordinated control strategy of VSC-MTDC named master-auxiliary is proposed by combining the advantages of the voltage margin and voltage droop control. This strategy has three advantages: the master converter station with the constant DC voltage control can provide reference to the system DC voltage and is helpful for the stabilization of DC voltage; the integrated control of the DC voltage in both master and auxiliary converter stations are helpful for providing adequate active power control and restrain large power variation; the active power control (APC) converter station can serve as a backup for the DC voltage control in abnormal conditions. In order to guarantee the reliability and stability of the system under various operating conditions, this paper introduces the priority of DC voltage control to the coordination control strategy. Moreover, a parameter optimizing method of controllers for this strategy is also proposed. Finally, the effectiveness of the master-auxiliary control is verified by simulations under normal and abnormal conditions. Index Terms-voltage source converter (VSC), DC transmission, multi-terminal DC (MTDC), coordination control, master-auxiliary control. His researchinterests include the control strategy of multi-terminal dc transmission and the control strategy of HVDC system etc. Ke-jun Li(Corresponding author) (M'07 )received the B.S. and M.S. degrees from Lee has been involved in research on renewable energy, power flow, transient and dynamic stability, voltage stability, short circuits, relay coordination, power quality analysis, demand response, on-line equipment protection, monitoring, and control system, and utility deregulation. He has served as the primary investigator (PI) or Co-PI of over seventy funded research projects. He has published more than one hundred sixty (160) journal papers and conference proceedings. He has provided on-site training courses for power engineers in Panama, China, Taiwan, Korea, Saudi Arabia, Thailand, and Singapore. He has refereed numerous technical papers for IEEE, IET, and other professional organizations. Prof. Lee is a Fellow of IEEE and registered Professional Engineer in the State of Texas. Zhao-hao Ding (S'11) received his Bachelor of Science degree in 2010 from Shandong University, Jinan, China. He is currently pursuing his Ph.D. degree at University of Texas at Arlington (UTA). He is also a research member of Energy Systems Research Center (ESRC). His area of interest includes renewable energy integration, bulk power system planning and operation, microgrid operation and control, power market and stochastic optimization.
Modular multilevel converters (MMCs) have shown great potential in the area of multimegawatt wind energy conversion system (WECS) based on permanent magnet synchronous generators (PMSGs). However, the studies in this area are few, and most of them refer to the MMC used in high-voltage direct current (HVDC) systems, and hence the characteristics of the PMSG are not considered. This paper proposes a steady-state analysis method for MMCs connected to a PMSG-based WECS. In the proposed method, only the wind speed (operating condition) is required as input, and all the electrical quantities in the MMC, including the amplitudes, phase angles and their harmonics, can be calculated step by step. The analysis method is built on the proposed d-q frame mathematical model. Interactions of electrical quantities between the MMC and PMSG are comprehensively considered. Moreover, a new way to calculate the average switching functions are adopted in order to improve the accuracy of the analysis method. Applications of the proposed method are also presented, which includes the characteristic analysis of capacitor voltage ripples and the capacitor sizing. Finally, the accuracy of the method and the correctness of the analysis are verified by simulations and experiments.Energies 2018, 11, 461 2 of 31 require a large amount of series or parallel connected power devices in order to achieve the required power, which reduces the reliability of WECS. Moreover, the traditional converters are not suitable for medium-voltage (3-33 kV) WECS, which is becoming popular for turbine power ratings greater than 3.0 MW [6,7]. These drawbacks lead to the studies of new full-scale power converters.The modular multilevel converter (MMC), which became the most common type of voltage-source converter used in the high-voltage direct current (HVDC) applications, is also a suitable converter for PMSG-based WECS. In [8,9], MMC was used for medium-voltage motor drives, which opened up the study of MMC applied to variable-speed machines. The application of MMCs in PMSG-based WECS was first studied in [10]. That paper concluded that MMC should be considered a suitable option for transformerless 10 MW PMSG-based WECS, and performs well in the full system. In [11], the authors proposed an improved online fault identification scheme for the MMC applied to WECS, which avoided the use of extra sensors by only using the already available variables in detection, and therefore the maintenance cost of the distant offshore wind farms can be reduced. In [12], the complete control scheme of MMC applied to the PMSG-based WECS was proposed. Low voltage ride-through problems and capacitor voltage ripple problems were also studied in that paper. In addition, the increasing interest in all-dc wind farms, in recent years, will also enlarge the application of MMC in WECS [13][14][15].Steady-state analysis can be used for component sizing and assessment of the impact of different parameters on the MMC performance. However, there is no reference available for the steady-state anal...
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