Small Signal Model And Stability Analysis For LCC-MMC Series Connected Hybrid HVDC System

The series hybrid DC transmission system adopts the structure of Line Commutated Converter(LCC)and Modular Multilevel Converter(MMC)in series,LCC on the high voltage side and MMC on the low voltage side.It can integrate the advantages of both LCC and MMC,reduce manufacturing costs and operating losses,possessing high engineering application value.However,this topology also has some problems,such as the coupling effect between LCC and MMC control system,small signal instability when connected weak receiving AC system and so on.Therefore,the small signal stability analysis of the series hybrid DC transmission system has high theoretical value and engineering reference significance.First of all,in order to provide a theoretical analysis basis for the study of this paper,a small signal model of the series hybrid DC transmission system is established.By combing the electrical and control signal transfer relationship between the AC and DC sides of the LCC and MMC,deriving the interface models between converters,AC and DC systems,a small-signal model is developed.The entire system contains LCC,MMC with detailed internal dynamic characteristics,DC transmission line and AC system.The dynamic performances comparisons between small-signal model and detailed electromagnetic transient simulation in PSCAD/EMTDC are conducted.The results validate the accuracy of the derived small-signal model.Then,based on the established small-signal model,the influence of the receiving AC system strength on the small-signal stability is studied,and the electrical/control links which have a higher impact on stability when connecting weak AC system is explored.Then the coupling effect between LCC and MMC control system is analyzed,and obtained the control link with obvious coupling effect,and the influence of the receiving AC system strength on the parameters coupling effect between the control systems is further analyzed.Finally,a quadratic index that can quantitatively describe the small signal stability is derived,and combined with the damping ratio characteristic,an objective function is developed.Then,the key control parameters with higher sensitivity are identified.Finally,an optimization approach for control parameters is presented by the utilization of Monte Carlo method.The eigenvalue analysis and the electromagnetic transient simulation results show that with the optimized control parameters,the small signal stability and the dynamic responses of the system are effectively improved.