  High voltage direct current links based on voltage source converters
(VSC-HVDC) embedded in alternating current (AC) systems are receiving a great
deal of attention recently because they can contribute positively to the
flexibilisation of modern power systems. Among several possibilities,
AC-line-emulation control has been highlighted as an simple-but-useful
alternative for these type of systems. With this strategy, the power flow
through the link is controlled proportionally to the angle difference between
its two AC terminals and this provides self-adaptation of the power flow in
case of contingencies in the parallel AC lines, naturally. Although this
controller is mainly concerned with steady state, it can also have an impact on
the dynamic behaviour of the system, which has not been sufficiently analysed.
Along this line, this paper analyses the impact of AC-line-emulation
controllers of VSC-HVDC on power system transient stability. Nonlinear
time-domain simulations were carried out by using PSS/E on a small test system
with an embedded point-to-point VSC-HVDC link. The critical clearing time (CCT)
of a test fault has been used to assess transient-stability margins of the
whole system. The paper provides recommendations for the design of
AC-line-emulation controllers in order to ensure that transient stability is
not jeopardised.
