Research On Oscillation Mechanism In Low-Frequency Range Of Hybrid Dual-infeed HVDC System Under Weak AC Grid Condition

Line Commutated Converter based High Voltage Direct Current(LCC-HVDC)has been widely applied in long-distance and large-capacity transmission projects,and in recent years,Modular Multilevel Converter based HVDC(MMC-HVDC)has been more and more widely used in engineering.When the locating stations of MMC-HVDC projects are close to that of LCC-HVDC projects which have been put into operation,the hybrid multi-infeed HVDC system is formed.The hybrid multi-infeed HVDC system contains many types of equipment,such as LCC converter,MMC converter and generator.And due to the wide bandwidth range of LCC and MMC controllers,they are very likely to interact with generator control system in low-frequency range,which increases the risk of low-frequency range oscillation in hybrid multi-infeed HVDC system under weak AC grid condition.Taking the typical hybrid dual-infeed HVDC system as the research object,this paper establishes the small-signal model of the hybrid dual-infeed HVDC system considering the dynamic characteristics of generator,studies the oscillation mechanism in low-frequency range of the hybrid dual-infeed HVDC system under weak AC grid condition,and proposes an additional damping control to suppress the low-frequency range oscillation of the system.Firstly,the state space models of generator,LCC converter station,MMC converter station,load and tie line in hybrid dual-infeed HVDC system are established,and through the connection of each part of the state space models,the state space model of the hybrid dual-infeed HVDC system considering the dynamic characteristics of generator is obtained.The small-signal model of the system is obtained by linearizing the state space model at the steady operating point of the system,and the accuracy of the small-signal model is verified by comparing the step response characteristics of the small-signal model and the PSCAD/EMTDC simulation model.Secondly,based on the established small-signal model,the low-frequency weak damping mode of the system is identified by eigenvalue method,and the low-frequency weak damping mode of the system is analyzed by participation factor and sensitivity analysis method.Then the effects of AC system inertia and AC system short-circuit ratio on the low-frequency range oscillation of hybrid dual-infeed HVDC system are studied.Based on the transmission path of system frequency disturbance signal,the low-frequency range oscillation mechanism of hybrid dual-infeed HVDC system under weak AC grid condition is revealed.Finally,based on the low-frequency range oscillation mechanism of hybrid dual-infeed HVDC system under weak AC grid condition,an additional damping control for suppressing low-frequency range oscillation of the system is proposed.The structure of the additional damping controller is given,and the key parameters of the additional damping controller are designed by pole assignment method.The effectiveness of the proposed additional damping controller in suppressing the low-frequency range oscillation of hybrid dual-infeed HVDC system under weak AC grid condition is verified by simulation on PSCAD/EMTDC.