Small Signal Stability Analysis And Controller Parameter Optimization For Multi-Terminal DC Transmission System

As the most potential way to realize grid integration of renewable energy,multiterminal direct current(MTDC)transmission is the main trend of the development of high voltage direct current(HVDC).For MTDC transmission systems with increasing structural complexity,many key problems have not yet been solved reasonably.On one hand,the expansion of the scale of the system makes the coordinated control strategy between converter stations more complex.On the other hand,low-frequency oscillations frequently occur in engineering due to under-damped conditions.Therefore,this paper focuses on the MTDC transmission system and the specific work carried out is listed as follows.(1)Based on the topology and mathematical model of modular multilevel converter(MMC),this paper studies the method of average modeling,and verifies the validity of average modeling of two-terminal flexible HVDC based on MMC.Based on the traditional master-slave control and voltage droop control,a hybrid inter-station control method is designed.While stabilizing the DC voltage,the power shortage can be shared for the main converter station when the power flow changes abruptly.The effectiveness of the hybrid control strategy is verified by a four-terminal simulation model.(2)In order to build the small signal model of multi-terminal DC system,the system is divided into several subsystems: main circuit,control system and DC network.Considering the electrical characteristics of each converter station and cascade control loop,a small signal model of four-terminal MTDC system with typical network topology is established.In order to maintain the fidelity of DC network modeling,the dynamic components and losses of DC network are also considered.A time domain simulation model is established in Simulink to verify the accuracy of the small signal model.(3)Aiming at the problem of voltage stability in MTDC network,a method for selecting parameters of voltage controller is proposed.The correlation between oscillation modes and state variables is analyzed by calculating eigenvalues and participation factors.Through the root locus of variable parameters,the damping characteristics and response speed of the system under different combinations of controller parameters are analyzed,and the optimal combination of parameters is selected.The simulation results in the time domain model verify the correctness and optimization effect of the proposed method.Focusing on the coordinated control strategy and controller parameter design of multi-terminal flexible HVDC transmission system,this paper studies the method of small-signal modeling.The research results have an active reference significance for solving the problem of low-frequency oscillation caused by unreasonable controller parameter design.