Recently, HVDC based on VSC technology has become an area of growing interest because of its suitability in forming a transmission link for transmitting large amounts of power. M-VSC-HVDC has the possibility of being an attractive alternative to AC transmission in city centres, where underground cable transmission is preferred for safety and environmental reasons. Multi-terminal DC grids based on VSC-HVDC could be a competitive and attractive option, for many applications such as the integration of renewable energy and oil/gas platforms into the onshore grid system for supplying power to large metropolitan areas. Therefore, this thesis focuses on the control of M-VSC-HVDC and DC grids based on VSC. Firstly, a detailed non-linear model on a Power System Computer Aided Design/ElectroMagnetic Transients including Direct Current (PSCAD/EMTDC) simulation software for a 2-terminal HVDC based on VSC is presented in chapter 3. In the context of what is a complicated controller analysis and design task, the detailed analytical linear small signal state-space VSC-HVDC test system is modelled in MATLAB and is presented in chapter 3. The model should have good accuracy within the frequency range for the main HVDC control loop i.e. below 100Hz. Secondly, an eigenvalue stability study for control gains optimization is presented in chapter 4, with the use of the root locus technique. Very good matching accuracy is established in chapter 4 for the linear analytical model when compared with the detailed non-linear PSCAD test system models. A detailed comparison of the outer-loop control performance at the receiving end is presented in chapter 5. PID control (inner-loop) with d-axis current control and the DC voltage droop control (outer-loop) is confirmed to be adequate for advanced control design for an M-VSC-HVDC system and DC grid network. A 121st order MIMO small signal linearized dynamic model of a 5-terminal DC network is presented in chapter 6. The model accuracy is verified using detailed non-linear PSCAD simulation. The model has been used to study the effects of the DC voltage droop control on the dynamic and transient behaviour of the DC network. The work presented in this thesis therefore seeks to make a novel contribution by; presenting a detailed non-linear and linearized dynamic model of a DC grid based on a VSC test system. This model has significantly increased our confidence in the feasibility of DC grid networks. A higher order MIMO small signal linearized dynamic model of a 5-terminal DC network and an M-VSC-HVDC has been developed. They are the most detailed analytical models currently available. These models can be used for larger DC grids of any complexity. This thesis applies modeling knowledge boundaries to the automated building of an analytical model of a DC system and could be adapted for a very complex DC system. Two main issues regarding the implementation of the droop scheme have been investigated systematically by using the developed small signal model. Namely, the impacts and the selections of the DC droop gain and the cutoff frequency of the DC voltage droop filter. A systematic design of DC droop gains for DC grids has been presented. This thesis resolves a number of issues with developing DC grids and increases our confidence in building future complex DC transmission systems.The work presented in this thesis therefore seeks to make a novel contribution by; presenting a detailed non-linear and linearized dynamic model of a DC grid based on a VSC test system.
|Title||:||Dynamics and Control of High Voltage DC Grids|
|Author||:||Aleisawee M. Alsseid|