| The increasing penetration level of renewable power generation has caused a series of frequency stability problems due to its inertia-and damping-less characteristics.Voltage source converter based high voltage direct current(VSCHVDC)transmission system has been widely constructed with its advantages of high controllability and good adaptability,but it also has the characteristics of inertia-and damping-less characteristics.Therefore,this thesis focuses on how to enhance the frequency adjustment ability and improve the frequency dynamic response of low inertia systems based on the VSC-HVDC system.The main contents are as follows:(1)This thesis sorts out the development and main application scenarios of VSCHVDC system,and analyzes the main differences between the VSC-HVDC system and conventional line commuted converter based HVDC systems.The advantages and disadvantages of the existing inertia and damping emulation control schemes are analyzed;(2)The topology of VSC-HVDC systems is briefly introduced.And the mathematical model of the VSC-HVDC system in the dq axis is derived including the converter and its control system models,grid-connected line model,DC network model and AC network model.The small-signal model of the aforementioned VSCHVDC system is established and its accuracy is verified through the time-domain simulation results;(3)To address the problem of lack of flexibility of the existing inertia emulation control strategies,a variable-inertia emulation control(VIEC)scheme for VSCHVDC transmission systems is proposed in this thesis,in which the emulated inertia time constant can be flexibly adjusted according to the rate of change of frequency and frequency deviation.The root locus is used to analyze the influence of the emulated inertia time constant and the DC capacitance on the system stability and optimize the controller parameters.The time-domain simulation results verify that the proposed VIEC scheme can effectively enhance the frequency support capability of the power system and shorten the recovery time;(4)To solve the problem of the lack of damping emulation control for DC voltage regulation converter,a coordinated flexible damping mechanism(CFDM)with inertia emulation capability for modular multilevel converter(MMC)based multi-terminal DC transmission systems is proposed in this thesis,which uses the energy of capacitors in MMC sub-modules(SM)and the power of remote MMCs stations to emulate the inertial and damping response of SG respectively.Under the proposed VIEC scheme,the communication system is avoided to improve the reliability and the transmission losses are taken into account to increase the accuracy.The root locus is used to optimize the controller parameters,and analyze the stability influence of the SM’s DC capacitance.The effectiveness of the proposed CFDM scheme is verified by the time-domain simulation results,which shows the CFDM scheme can effectively suppress the system power oscillation and improve the stability of the power grid;(5)To overcome the drawbacks of the existing inertia and damping emulation control schemes which usually can only provide the frequency support to one side,a bilateral inertia and damping emulation(BIDE)control scheme of VSC-HVDC transmission systems is proposed in this thesis.The proposed BIDE scheme can emulate the SG’s inertia and damping for both the both interconnected AC grids.And the inertial and damping power for one AC gird comes from the augmented DC link capacitance and the other AC grids.The root locus of the BIDE’s scheme is carried out to analyze the influence of the emulated inertia time constant,damping factor,DC capacitance,and communication delay.Various simulation studies verify that the VSC-HVDC system under the proposed BIDE scheme is capable of providing the inertial and damping response to the bilateral power system,suppressing the power oscillation and improve the system stability. |
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