Research On Operation Control Strategy Of VSC-HVDC System In Different Application Scenarios

With the increasing requirement of a clean and low-carbon modern energy system construction and the continuous development of novel transmission technologies,the traditional power system is shifting away from AC transmission to DC transmission then towards hybrid AC/DC transmission.The High voltage DC(HVDC)transmission technology has attracted much attention for its reliable power supply quality and excellent power supply capacity.With the development of the voltage source converter based HVDC(VSC-HVDC)transmission technology,the VSC-HVDC transmission system has good application prospects in asynchronous power grid interconnection,renewable energies integration,and urban power system.However,compared to AC transmission technology,VSC-HVDC technology is a new type of transmission technology,which lacks sufficient experience on its control,operation,and scheduling.Therefore,in this thesis,a specific research is carried out on the operating characteristic,control strategies and dispatching methods of the VSC-HVDC system.The thesis is organized as follows:(1)A system frequency support&recovery control of VSC-HVDC system for asynchronous power grid interconnection.The VSC-HVDC system could realize the independent control of active power and reactive power,in addition,the VSC-HVDC system could provide a fast response to the various disturbances.Due to the insensitivity of the VSC-HVDC system to the frequency oscillation,the VSC-HVDC system for asynchronous power grid interconnection could prevent the cascading failure and improve the robustness of the inteconnections.With the controllability of the VSC-HVDC system,a system frequency support&recovery control is proposed for the VSC-HVDC system connected with asynchronous power grid interconnections.The system frequency support&recovery control could realize the frequency response sharing between the asynchronous power grid interconnections so that increasing the financial benefit.When a disturbance occurs in one interconnection,the system frequency support&recovery control could provide fast frequency response to increase the frequency nadir.After the contingency,the system frequency support&recovery control could also participate into frequency recover process.Simulation results show that the proposed control could particiapte the VSC-HVDC system into the system primary frequency response and secondary frequency response so that improving the operating reliability and economically.(2)VSC-HVDC system integrating large-scale offshore wind farms based optimal distribution method.VSC-HVDC system has been regarded as one of the effective solutions to integrate offshore wind farms for its controllability.An optimal distribution method is proposed for the VSC-MTDC system integrating offshore wind farms to increase the financial benefit of the wind producers by optimizing the distribution of the wind power output deviation.With the case study,the proposed optimal distribution method is proved that it can bring more benefit to wind producers.(3)An Optimal combined operation scheme for pumped storage and hybrid wind-photovoltaic complementary power generation system.VSC-HVDC system is a prospective solution for the integration of large-scale wind,solar and energy storage into the grid.The control strategies of the VSC-HVDC system for pumped storage hydro and hybrid wind-photovoltaic complementary power system is studied.The controls strategy aims to use the DC voltage to control the pumped storage hydro operation.The generating/pumping rates and the capacity of the pumped storage hydro are controlled following the DC voltage oscillation.An optimal combined operation scheme is proposed for pumped storage hydro and hybrid wind-photovoltaic complementary power generation system interconnected by a VSC-HVDC system,in order to suppress the power oscillation before the renewable energies integration into the grid.(4)Operation modes and combination control for urban multi-voltage-level VSC-HVDC system.With the development of the urban economy and the growing requirement of electricity,the operating of the urban power grid is facing its transmission stress.On the other hand,the tight of land resource and increasing land cost makes expansions requiring new circuits and new substations very difficult to implement.VSC-HVDC system is a good option for the urban power grid for its high transmission capability,fully controlled power flow,and,no short-circuit current limitation.However,the urban power grid upgrading with the VSC-HVDC technology has not received too much attention.A multi-voltage-level VSC-HVDC system is modelled for the urban power grid upgrading.Secondly,three operating modes:normal operating mode,power limited operating mode and stand-alone operation mode are proposed based on the model in order to enhance the reliability of urban power grid in various operating conditions.(5)Coordination control for multi-voltage-level VSC-HVDC sytem based on the multiple dc-dc converters.As necessary devices for transferring energy between different voltage levels,dc-dc converters play a pivotal role in the coming dc system.A switch mode control and a dc-dc coordination control are developed for dc-dc converters'coordinated operation in the multi-voltage-level dc grid.The control strategies could coordinate operation of the multiple dc-dc converters in order to enhance the realiable operation of the multi-voltage-level VSC-HVDC sytem.In addition,when a serious disturbance occurs in the system,the unbalanced power from one VSC-HVDC system could be optimally distributed to other systems,in order to avoid serious power fluctuation events.