  The small-signal stability of multi-terminal HVDC systems, which is related
to the dynamic interactions among different VSCs through the coupling of DC and
AC networks, has become one of the important issues for the safety and stable
operation of modern power systems. On the other hand, the robust stability
theory with {\nu}-gap metric is an effective tool for the stability analysis
and synthesis of uncertain feedback systems. In this paper, we combine it with
the self-/en-stabilizing coefficients method to measure the relative stability
and analyze the stability influence of different paths of interactions in an
MTDC system. The stability index is defined to represent the stability margin
with respect to different paths of interactions in an MTDC system. A method for
calculating the range of uncertain parameters preserving the stability is
presented based on the stability criterion. The influence of control parameters
on robust stability through interactions among VSCs can be analyzed
quantitatively. Extensive examples are given to demonstrate the application and
the effectiveness of the proposed method.
