| Recently, electrical power systems are evolving with the increased deployment and advancement of renewable energy resources, which are nonlinear in nature, to limit the use of fossil fuels and supply the demand of ever increasing load. This promotes offshore wind farms(WF); as they have more capacity to generate electricity because of more favorable wind conditions and less public interest conflict. Wind power resources are largely distributed in northeast, China while the main power consumption groups are located in south-eastern part. Integration of large scale offshore power plants with the existing power systems posing substantial techno-economic challenges such as long transmission lines under the sea and varying wind power. To meet the needs, multiterminal voltage source converter(VSC) based high voltage direct current(M-HVDC) is a promising anticipation to transmit and integrate large offshore wind energy to onshore grids.In the past, a lot of work on M-HVDC has been done, mainly on control aspects that usually describe the operation of M-HVDC system qualitatively. In this thesis, both simulation and experimental approaches are used to expedite the coordinated control, operation and analysis of M-HVDC systems. Comparative study of M-HVDC circuit topologies is also carried out through dynamic simulations and topological evaluation standard is presented. The research work can be put into the following parts:(i) investigation of the control strategies for M-HVDC systems(ii) operational characteristics of VSCs with designed control schemes and analysis under different steady state and transient conditions(iii) comparison of various M-HVDC system topologies is conducted based on certain techno-economic factors and a novel topology for M-HVDC transmission systems is proposed.In the first case, master-slave, voltage margin and dc voltage droop control are described, which are typical control strategies for multi-terminal VSC based HVDC systems(M-VSC-HVDC). The decoupled mathematical model of VSC is developed, divided into upper and lower control loops which are decoupled from each other. Protection standard of M-VSC-HVDC system are outlined. Local control of VSC station is strongly depends on the nature of connected ac network. Operating principal of MHVDC systems is described along with the control of every VSC to accredit the solemn working and power sharing between various ac connected networks and tested with the dynamic simulation model. A control scheme is implemented using dual hierarchical control structure and verified through experiments to stabilize the dc link voltage by shifting fixed dc voltage to droop control in case of failure of master controlled terminal and vice versa.In the area of M-HVDC system analysis and operational characteristics, three MHVDC configurations are studied, each with different control strategies both for steady state and dynamic situation. The voltage-current(V-I) characteristics of VSCs are presented and VSC converter operation with various output powers from offshore WFs is assessed. A generalized droop control strategy and proportional droop control are mainly used to realize autonomous coordination among converters without the need of communication. Operation of three configurations with respect to their control system is analyzed through simulations and experimentation. Dynamic simulations are developed by using PSCAD/EMTDC. A four terminal experimental rig is established to analyze the system behavior under steady state and dynamic system condition for developed four different control schemes. Results show the control performance during normal operation of M-HVDC, wind power change, eventual permanent VSC disconnection, and change in power demand from the ac grid side converter. Simulation results and experimental measurements show very good agreement.During the third part of this research work, multi-terminal VSC topologies for HVDC transmission systems for the integrating large offshore wind farms, are scrutinized. A novel two wind farms with one platform(2WF-1PF) topology is proposed. M-HVDC system standards are defined along with control schemes. HVDC switch gear and dc link circuit topologies are reviewed, considering the need of extra offshore platforms and HVDC circuit breakers. Numerous M-HVDC link topologies are analyzed and compared based on certain techno-economic criteria: length, capacity and number of HVDC circuits, cost, flexibility, stability, number of offshore substations and HVDC circuit breakers. Operation of various topologies with respect to the designed control system are analyzed and compared through PSCAD/EMTDC simulations, considering two different tests: permanent VSC disconnection and a dc line to line fault. Results show 2WF-1PF is a promising topology for future M-HVDC grids.Additionally, the control of the HVDC link voltage is a priority, in order to guarantee overall system stability. Thus a ±5% dc link voltage tolerance limit is considered as a stability evaluation in this thesis for various representative case models. |
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