What is claimed is:1. A method, for use in a plant for transmission of high voltage direct current, which plant is coupled to an alternating current network with a fundamental frequency, and which plant has a control equipment and a direct current link, the method being for damping an oscillation in a current in the direct current link at a frequency equal to the fundamental frequency, and having the steps of:generating reference signals oscillating with an influencable reference frequency:multiplying the reference signals by a measured current value of the current in the direct current link;forming an actuating signal, which oscillates with the influenceable reference frequency and has a phase position equal to the sum of an adjustable phase angle displacement and a phase position of the oscillation in the current, in relation to a voltage of the alternating current network; andforming a control signal in dependence on the actuating signal, and supplying the control signal to the control equipment, and wherein the step of generating reference signals comprises the steps of:forming a synchronizing signal representing the fundamental frequency, and;influencing the reference frequency to vary in dependence on said synchronizing signal so that the reference frequency coincides with the fundamental frequency.2. A method according to claim 1, wherein the step of forming the control signal comprises the steps of:forming an amplitude signal representing the amplitude of the oscillation;forming an amplification signal in dependence on said amplitude signal; andforming a product of said amplification signal and the actuating signal, and forming the control signal in dependence on said product.3. A method according to claim 2, wherein said amplifcation signal is formed in dependence on said amplitude signal so that it increases with increasing amplitude of the oscillation.4. A method according to claim 2, wherein the step of supplying the control signal to the control equipment comprises the steps of:comparing said amplitude signal with a pre-selected level reference signal; andsupplying the control signal to the control equipment in dependence on said comparison.5. A method according to claim 1, wherein an order value for the control angle for a converter in the plant is generated in dependence on an output signal of a controller comprised in the control equipment, and wherein the step of supplying the control signal to the control equipment comprises the step of forming the order value for the control angle in dependence on a sum of the output signal and the control signal.6. A device for use in a plant for transmission of high voltage direct current, which plant is coupled to an alternating current network with a fundamental frequency, and which has a control equipment and a direct current link, the device being for damping an oscillation in a current in the direct current link at a frequency equal to the fundamental frequency, the device including:means for generating reference signals oscillating with an influenceable reference frequency;means for multiplying the reference signals with a measured current value of the current in the direct current link;means for forming an actuating signal, which oscillates with the reference frequency and has a phase position equal to the sum of an adjustable phase angle displacement and a phase position of the oscillation in the current, in relation to a voltage of the alternating current network; andmeans for forming a control signal in dependence on the actuating signal, which control signal is supplied to the control equipment, and wherein the device is supplied with a synchronizing signal representing the fundamental frequency, which synchronizing signal influences the means for generating reference signals to vary the reference frequency in dependence on said synchronizing signal so that the reference frequency coincides with the fundamental frequency.7. A device according to claim 6, further comprising:means for forming an amplitude signal representing the amplitude of the oscillation;means for forming an amplification signal in dependence on said amplitude signal; andmeans for forming a product of said amplification signal and the actuating signal, and wherein the control signal is formed in dependence on said product.8. A device according to claim 7, wherein said means for forming an amplification signal comprises means for forming said amplification signal in dependence on said amplitude signal so that said amplification signal increases with increasing amplitude of the oscillation.9. A device according to claim 7, further comprising:means for comparing said amplitude signal with a pre-selected level reference signal;means for forming a selector signal in dependence on the result of the comparison; andmeans for supplying the control signal to the control equipment in dependence on said selector signal.10. A control equipment for a plant for transmission of high voltage direct current having a controller, wherein an order value for the control angle for a converter in the plant is generated in dependence on an output signal of the controller, and wherein the control equipment is supplied with a control signal formed by a device according to claim 6, and wherein the control equipment comprises means for forming the order value for the control angle in dependence on a sum of the output signal and the control signal.