Research On Frequency Control For MT-HVDC Systems Based On Multi-agent Coordination And RL

High Voltage Direct Current(HVDC)has emerged as an attractive technology to transmit power among large-scale alternating current(AC)areas.Multiterminal HVDC(MT-HVDC)system consists of several AC areas connected through HVDC grid.On the one hand,HVDC grid acts as a firewall against cascading failures in MT-HVDC system,on the other,HVDC grid blocks mutual support among the connected AC areas.As a result,power reserves,which are usually expensive,cannot be spontaneously shared among the connected AC areas when local power imbalances occur.Meanwhile,the practical MT-HVDC systems are complex and high-dimensional,of which the precise system models are hard to be obtained.Based on above discussion,this thesis focuses on the cooperative frequency control and model-free frequency control for MT-HVDC systems.The main contents are shown below.Firstly,considering the cooperative frequency control problem of MT-HVDC systems,this thesis proposes the consensus-based distributed frequency controllers.To study how the error integral term and state damping term affect the control effect of the controllers,distributed proportional type frequency controller,distributed proportional-integral type frequency controller and distributed proportional-integral type with damping frequency controller are presented.Compared to local primary frequency control,the proposed controllers can reduce the maximum frequency deviation of the AC area where load disturbance happens and suppress frequency fluctuation.Secondly,due to the difficulty in obtaining the system model,this thesis proposes a value iteration based model-free distributed frequency controller for the primary frequency control problem of MT-HVDC systems.Through setting the performance index,the frequency deviations of all the AC areas can reach consensus.Compared to local primary frequency control,the maximum frequency deviation of the AC area where load disturbance happens is greatly reduced and the frequencies converge to the stable state much faster under the proposed controller.Finally,this thesis proposes a model-free distributed frequency controller for the secondary frequency control problem of MT-HVDC systems.During the process of restoring the frequencies to the nominal frequency,the proposed controller makes the connected AC areas share their power reserves via HVDC grid to compensate load disturbance together.Compared to the local primary and secondary frequency control,the proposed controller downsizes the power reserve usage of the AC area where load disturbance happens and the frequency dynamic responses are greatly improved.The cases study conducted on an MT-HVDC system with 5 AC areas illustrate the effectiveness of the proposed frequency controllers.