Patent ID: 8553433

Claim:
A method to control a multi-terminal HVDC power transmission network comprising at least two transmission lines, wherein at least one of the transmission lines has a length of approximately or more than 500 km, wherein the DC voltage level of the transmission lines is maintained to be within a predefined voltage range by controlling an active voltage source device which is connected in series to one of the transmission lines to inject a DC voltage in series with the one of the transmission lines, wherein the active voltage source device is arranged to be power supplied from the one of the transmission lines and comprises a first converter in series connection with the one of the transmission lines and a second converter in shunt connection with the one of the transmission lines, the method comprising the steps of: converting, by the second converter working as an inverter, DC power from the one of the transmission lines into AC power, supplying, by said second converter, the first converter with said AC power, and providing, by the first converter working as a rectifier, the DC voltage to be injected into the one of the transmission lines.