Abstract: A method for controlling the operating conditions of a DC transmission line includes maintaining a predetermined current value at a first substation by controlling the voltage at that substation, while the voltage at a second substation is controlled to maintain an extreme value of a specific controlled variable dependent on the operating conditions at said second substation and formed as a function of parameters which, when reaching an extreme value, ensures a maximum possible voltage at that substation, while maintaining the predetermined current value. The method is effected via a control circuit including a controlled variable former, an adder, an automatic controlled variable optimizer and a control action discriminator.