The Strategy Of Frequency Coordinated Control With DC Modulation

With the continuous expansion of ultra HVDC transmission projects' scale and quantity,AC and DC hybrid system increases the complexity of the interaction between DC and AC networks.The power inbalance caused by failure may easily spreads and causes chain failure,which even may lead to voltage instability,frequency instability and angle instability.The current social is the world of power generation.The risk of blackouts and economic loss caused by frequency instability is not negligible,and it plays an important role in the research on the stability of power system.So it is an hot spot that focusing on the application of the fast controllabity and short-time overloading capacity of HVDC systems,which helps AC/DC system to enhance its frequency stability and optimize its frequency dynamic recovery characteristic.This paper mainly includes the following aspects for the frequency control problems in AC and DC hybrid system:Firstly,a quasi-steady-state model of AC and DC hybrid system is established,based on which DC additional control of DC rectifier and inverter sides is designed.And by using equivalent method,AC/DC system are decoupled and analyzed.Then according to the different dominant frequency modulation,frequency dynamic response character is divided into three stages elaborated in detail respectively.Secondly,an improved frequency control process is proposed.It uses static sensitivity method to derive the static sensitivity index of key factors in AC/DC system,and the process is simplified accordingly.The proposed process focuses on the impacts of the change power of DC transmission on the AC grid structure,and has a posstive effect on the voltage recovery after any failure.On this basis,the corresponding control index is put forward by using the principle of trace sensitivity,includeing generator's index and DC controller's index.Finally,this paper proposes a strategy of frequency coordinated control with DC system,which emphasises the principle of active power distribution,and it takes a two-region interconnection system as an example.The control strategy has two stages,and the decoupling optimization is conducted for the DC coordination layer and the AC/DC coordination layer respectively.The influence of different factors on the control effects of DC additional control is analysized combined with simulation in DC coordination layer,including FM dead zone,FM effect coefficient,proportional regulation gain and DC control range.Then the adjustment of these parameters is proposed in the case of large disturbance.AC/DC coordination layer considers each FM method and designs a distribution strategy of active power by using particle swarm optimization algorithm.The effectiveness of this method is vertified by MATLAB/SIMULINK simulation.