Patent ID: 8830712

Claim:
A method of controlling a capacitor commutated inverter device being connected to an AC voltage bus of an AC power system and converting between DC power and AC power, the inverter device including a number of converter valves in a 6-pulse valve bridge, and the method comprising: receiving measurements of a measured AC voltage (U AC ) at the AC voltage bus; applying the measured AC voltage in a proportional control branch for obtaining an AC voltage dependent control factor (α MAX — UAC ) that increases or decreases a maximum limitation signal (α MAX ) in order to provide a constant AC voltage; determining a commutation margin based control factor (α MAX — γ′ ), based on a commutation margin, as a firing angle calculated for a given commutation margin reference (γ ref ); combining the AC voltage dependent control factor and the commutation margin based control factor through selecting the lower of the commutation margin based control factor and the AC voltage dependent control factor as the maximum limitation signal (α MAX ); and controlling the inverter device to provide a constant AC voltage on the AC voltage bus based on the received measurements through: providing the maximum limitation signal to a maximum output limitation terminal (max lim) of a DC current controller amplifier that determines a fire angle order based on a direct current control and being forced against a maximum limitation; and controlling the firing angle (α) of the converter valves using the fire angle order (α order ) at an output of the DC current control amplifier for generating reactive power to or absorbing reactive power from the AC voltage bus.