Abstract
The industry of wind energy has rapidly increased in the last decade, especially in countries in Europe and East Asia. Offshore wind energy is expected to be one of the major power generation sources in the near future. However, the integrating of offshore wind plants into onshore ac grids remains a challenge in both technical and economic terms.
Offshore wind plants can be connected to ac grids using ac or dc transmission systems. HVDC transmission is favored over HVAC transmission as the transmission distance increases. Recently, with the improvement in the field of semiconductor, it became possible to build a VSC-based HVDC system providing many benefits including the ability to independently control active and reactive power transfer. VSC-based Multiterminal HVDC (MTDC) transmission systems allows the interconnection of multiple offshore wind farms with multiple onshore ac grids. The challenges of building MTDC systems has been a hot topic in the past years and it appears this will continue into the future. Different control schemes based on droop characteristics have been presented as a means to regulate the dc voltage and control the power flow in MTDC transmission systems.
In this thesis, the control and steady state operation of VSC-based MTDC systems is studied. A generalized control scheme based on droop characteristics is presented. The control methodology is based on the design of the droop constants to control the grid side VSCs for three different operational modes taking into consideration the varying nature of wind power. The three operational modes differ in how the generated wind power is distributed between the ac grids in the MTDC system.
The difficulties and challenges of adding tap stations on HVDC lines in MTDC systems is also discussed in the thesis. A new control scheme approach for adding multiple taps on HVDC lines in MTDC systems is presented.
A switching VSC models of the MTDC systems discussed throughout the thesis are built in Matlab/Simulink software. The models are simulated for different case scenarios to verify the control and normal operation of the systems.