  Reducing the current is a common method to ensure the synchronous stability
of a modular multilevel converter (MMC) when there is a short-circuit fault at
its AC side. However, the uncertainty of the fault location of the AC system
leads to a significant difference in the maximum allowable stable operating
current during the fault. This paper proposes an adaptive MMC fault-current
control method using local phasor measurement unit (PMU) measurements. Based on
the estimated Thevenin equivalent (TE) parameters of the system, the current
can be directly calculated to ensure the maximum output power of the MMC during
the fault. This control method does not rely on off-line simulation and adapts
itself to various fault conditions. The effective measurements are firstly
selected by the voltage threshold and parameter constraints, which allow us to
handle the error due to the change on the system-side. The proposed TE
estimation method can fast track the change of the system impedance without
depending on the initial value and can deal with the TE potential changes after
a large disturbance. The simulation shows that the TE estimation can accurately
track the TE parameters after the fault, and the current control instruction
during an MMC fault can ensure the maximum output power of the MMC.
