Research On Small Signal Impedance Modeling And Control Strategy Of Series-connected MMC In Hvdc Transmission System

With the development of the regional economy and the large-scale commissioning of the MMC HVDC projects,there are noticeable potential needs for some areas alongside the existing HVDC transmission corridor to send or receive power from it.Under the above background,compared with parallel-connected MMC（PC-MMC）,three-phase series-connected MMC（SC-MMC）is more beneficial to reduce construction costs and the station footprint,which is more suitable for the HVDC tapping station.To further explore the feasibility of SC-MMC,it is essential to study the small-signal dynamics of it and analyze its stability issue when interconnected with the power grids.At the same time,the feasibility of the series-connected topology and its operating performance under fault conditions need to be verified by experiments.This dissertation begins with the overview of structure and operating principles of SC-MMC.Then the harmonic function model of SC-MMC considering multiple harmonic interaction coupling is established,and the sequence and the mode of internal harmonics in it under different perturbation are analyzed,which lays the foundation for its ac side impedance modeling.Based on above analysis,the ac side impedance model of SC-MMC is developed by multi-harmonic linearization method,which including the phase lock loop（PLL）,the inner loop and outer loop controllers,and the proportional-resonant（PR）controller to suppress 3^rd harmonic components.In order to verify the accuracy of impedance model developed in this paper,the calculated analytical values are compared with the measured values of detailed SC-MMC circuit built in simulation software.Based on the established model,the impedance characteristics of SC-MMC and its main influencing factors are discussed.After considering the frequency coupling effect between SC-MMC and the power grid,the stability of the interconnected system composed of them is further analyzed.The case study highlights the necessity of considering frequency coupling effect in the study of grid-connection stability.Further research shows that when the interconnected system oscillates,the resonance phenomenon can be suppressed through the optimization design of the controller parameters of SC-MMC.Finally,the lower arm of each phase of SC-MMC adopts full-bridge sub-modules to form series-connected asymmetric hybrid MMC（SCAH-MMC）so that it has dc fault ride-through capability.And the control strategies of SCAH-MMC under both dc and ac fault conditions are introduced in detail.Theoretical calculation method are used to analyze the capacitor voltages,which explains the mechanism of the uneven ac current distribution between upper and lower arms of the same phase under reduced dc bus voltage condition.Furthermore,the experiments are carried out on SCAH-MMC experimental platform to verify the feasibility of the series topology and the effectiveness of the proposed control strategy under fault conditions.