在“西电东送”大背景下，我国南方电网目前有大量传统直流（line commutated converter based high voltage direct current，LCC-HVDC）馈入交流电网，然而不同传统直流之间的相互影响导致传统直流的安全稳定运行问题日益严重，交流电网对传统直流电力的接纳能力趋于饱和。而柔性直流（voltage source converter based high voltage direct current，VSC-HVDC）凭借其不依赖电网换相、输出有功无功解耦、能够提供瞬态无功支撑等优点，近年来得到快速发展，从而形成了传统直流和柔性直流在不同落点馈入同一交流电网的混合多馈入的电网结构。那么柔性直流馈入交流电网后会对传统直流的受端电网强度产生怎样的影响，如何定量评价这一影响，柔性直流是否可以无限制的接入交流电网，对于这些问题的研究具有很强的理论价值和工程意义。本论文首先介绍了传统单馈入系统中受端电网强度的评价指标有效短路比，并从静态电压稳定性的角度对临界有效短路比进行了理论上的推导。接着在含有柔性直流和传统直流的混合双馈入系统中，以功率传输曲线近似相同为原则将混合双馈入系统等效为传统单馈入系统，分析了等效系统的参数计算方法，并且借鉴传统单馈入系统中ESCR和CESCR的概念和计算方法，定义了混合双馈入和多馈入临界等效有效短路比。然后将所提MEESCR指标扩展到混合多馈入系统中去，分析了目前工程上普遍使用的多馈入有效短路比指标和本文所提指标MEESCR的关系。机理分析和PSCAD仿真结果表明MIESCR是MEESCR在特定工况下做一定近似之后的特例，MEESCR指标能够准确地反应混合多馈入系统中柔性直流对传统直流受端电网强度的影响。最后本论文根据所提MEESCR指标，在混合多馈入系统中探讨了柔性直流的容量、直流的接入形式、交流侧阻抗参数的变化对柔性直流提升系统外区电力接纳能力的影响。本文在混合多馈入系统中提出了计及柔性直流影响的传统直流的受端电网强度的评价指标，分析了柔性直流对外区电力接纳能力的提升作用，本文的研究对混合多馈入系统中直流落点的选择、直流的容量设计、交流电网的结构改善等规划问题有重要意义。

There are a large number of LCC-HVDC fed into ac grids in China Southern Power Grid currently under the background of “West-East electricity transmission project”. The interaction between different LCC-HVDC leads to the increasingly serious problem of the safe and stable operation of LCC-HVDC and the AC grid's acceptance of LCC-HVDC power tends to be saturated. VSC-HVDC has developed rapidly in recent years due to its advantages of independent of grid commutation, decoupling of active and reactive power output and providing transient reactive power support, thus forming a hybrid multi-infeed power grid structure of LCC-HVDC and VSC-HVDC feeding into the same ac grid at different landing points. Then what is the influence on the strength of LCC-HVDC receiving power grid when VSC-HVDC fed into ac grid, how to quantitatively evaluate the influence and whether VSC-HVDC can be fed into ac grid without limit? The research on these issues has strong theoretical value and engineering significance.Firstly, this paper introduces the effective short circuit ratio(ESCR), which is the evaluation index of the receiving power grid strength in single-infeed LCC-HVDC system, and deduces the critical effective short circuit ratio(CESCR) theoretically from the perspective of static voltage stability. Secondly, in the hybrid dual-infeed system with VSC-HVDC and LCC-HVDC, the hybrid dual-infeed system is equivalent to a single-infeed LCC-HVDC system on the principle that the max power transmission curve is approximately the same. The calculation method of the parameters of the equivalent system is analyzed. Based on the concept and calculation method of ESCR and CES CR in single-infeed LCC-HVDC system, the evaluation index of the receiving power grid strength in the hybrid dual-infeed system: multi-infeed equivalent effective short circuit ratio(MEESCR) and multi-infeed critical equivalent effective short circuit ratio(MCEESCR) are defined. Then the proposed MEESCR index is extended to the hybrid multi-infeed system, and the relationship between the multi-infeed effective short circuit ratio (MIESCR) which is commonly used in engineering and the MEESCR index mentioned in this paper is analyzed. The mechanism analysis and PSCAD simulation results show that MIESCR is a special case of MEESCR after a certain approximation under a specific operating condition and the MEESCR index can accurately reflect the impact of VSC-HVDC on the LCC-HVDC receiving power grid strength in hybrid multi-INfeed system. Finally, according to the proposed MEESCR index, this paper discusses the influence of VSC-HVDC capacity, the access form of HVDC and the change of impedance of ac grid on the external power admission capability VSC-HVDC promote in hybrid multi-infeed system.In this paper, an evaluation index of LCC-HVDC receiving power grid strength considering the influence of VSC-HVDC in hybrid multi-infeed system is proposed, and the enhancement effect of VSC-HVDC on the power admission ability for external area is analyzed. The research in this paper is of great significance to the selection of HVDC landing point, the design of HVDC capacity and the improvement of AC network structure in hybrid multi-infeed system.