  The adjustable capacity of a line-commutated-converter High Voltage Direct
Current (LCC-HVDC) connected to a power system, called the LCC-HVDC maximum
emergency power capability or HVDC-MC for short, plays an important role in
determining the response of that system to a large disturbance. However, it is
a challenging task to obtain an accurate HVDC-MC due to system model
uncertainties as well as to contingencies. To address this problem, this paper
proposes to estimate the HVDC-MC using a Thevenin equivalent (TE) of the system
seen at the HVDC terminal bus of connection with the power system, whose
parameters are estimated by processing positive-sequences voltages and currents
of local synchrophasor measurements. The impacts of TE potential changes on the
impedance estimation under large disturbance have been extensively investigated
and an adaptive screening process of current measurements is developed to
reduce the error of TE impedance estimation. The uncertainties of phasor
measurements have been further taken into account by resorting to the total
least square estimation method. The limitations of the HVDC control
characteristics, the voltage-dependent current order limit, the converter
capacity, and the AC voltage on HVDC-MC estimation are also considered. The
simulations show that the proposed method can accurately track the dynamics of
the TE parameters and the real-time HVDC-MC after the large disturbances.
