Sammendrag
It will be important for the operation of future multi-terminal
HVDC systems that each converter unit is capable of limiting
the influence of nearby AC faults on the DC grid. Thus, the
local control systems at each terminal should be able to
prevent double frequency active power oscillations caused by
unbalanced AC faults from propagating to the DC-side of the
converter. A simple approach for achieving this objective,
while ensuring that the converter currents are limited to safe
values, is therefore presented in this paper. The proposed
method is based on estimation of Positive and Negative
Sequence Virtual Flux components at the terminals of the
converter. Double frequency oscillations in the active power
flow through the converter can therefore be eliminated during
unbalanced conditions, without considering the influence of
the filter inductors. By preventing power oscillations in the
DC system, while keeping the currents within the available
rating, the proposed control system will prevent converter
tripping and unwanted interruption of system operation.
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