Claims
1. A device for control of controller equipment (FDV) intended for serial connection between a first and a second connection point (N

_{i}, N

_{j} respectively) in a power line (L) in an electric power network for alternating current, wherein the controller equipment is adapted, in dependence on a supplied control order (SO, SO'), to serially generate in the power line (L) a voltage (

_{s}) for the purpose of damping power oscillations in the power line,

characterized in that it comprises a controlling equipment (CE) which, in dependence on quantities (Yn) sensed in the power network, forms a phase deviation (θ

_{i} - θ

_{j}) as the difference (θ

_{i} - θ

_{j}) of the phase angle (θ

_{i}) for the voltage (

_{i} ) at the first connection point and the phase angle (θ

_{j}) for the voltage (

_{j}) at the second connection point, and, in dependence on the time rate of change of a function of the phase deviation, forms the control order such that it brings the controller equipment to generate a voltage in the power line, which feeds therein an active power counteracting changes in active power in the power line which are associated with the power oscillations.

2. A device according to claim 1, wherein the voltage generated by the controller equipment is controllable with respect to its amplitude in relation to a first upper limiting value (V

_{smax}) and with respect to its phase angle (γ) relative to the voltage at the first connection point,

characterized in that the controlling equipment, if the absolute value of the time rate of change of the phase deviation (

(θ

_{i} - θ

_{j}) ) exceeds a chosen rate of change limit value (D), forms a control order which causes the controller equipment to generate a voltage, the amplitude of which is formed in dependence on the first upper limiting value and the phase angle (γ) of which is equal to the difference of -π/2 and the phase deviation if the time rate of change of the phase deviation is smaller than zero, and equal to the difference of π/2 and the phase deviation if the time rate of change of the phase deviation is greater than zero.

3. A device according to claim 1, wherein the voltage generated by the controller equipment is controllable with respect to its amplitude in relation to a first upper limiting value (V

_{smax}) and its phase angle (γ) relative to the voltage at the first connection point may assume either of the values -π/2 and π/2,

characterized in that the controlling equipment, if the absolute value of the time rate of change of the phase deviation (

(θ

_{i} - θ

_{j}) ) exceeds a chosen rate of change limit value (D), forms a control order which causes the controller equipment to generate a voltage, the amplitude of which is formed in dependence on the first upper limiting value and the phase angle (γ) of which is equal to -π/2 if the time rate of change of the sine function for the phase deviation (

[sin(θ

_{i} - θ

_{j})]) is smaller than zero, and equal to π/2 if the time rate of change of the sine function of the phase deviation is greater than zero.

4. A device according to any of claims 2-3,

characterized in that the controlling equipment forms a control order which causes the controller equipment to generate a voltage, the amplitude of which is formed in dependence on a product of the first upper limiting value and the absolute value of the time rate of change of the phase deviation.

5. A device according to any of claims 2-3,

characterized in that the controlling equipment forms a control order which causes the controller equipment to generate a voltage with the amplitude equal to said first upper limiting value.

6. A device according to claim 1, wherein the voltage generated by the controller equipment, at a given current (

_{ij}) through the power line (L) at the first connection point, may assume a first control value, the phase angle (β) of which has the value -π/2 relative to said current, and a second control value, the phase angle (β) of which has the value π/2 relative to said current, and the amplitudes of which are controllable in relation to a first upper limiting value (V

_{smaxc}) and to a second upper limiting value (V

_{smaxi}) respectively,

characterized in that the controlling equipment, if the absolute value of the time rate of change of the phase deviation (

(θ

_{i} - θ

_{j}) ) exceeds a chosen rate of change limit value (D), forms a control order which causes the controller equipment to generate a voltage equal to the first control value, with an amplitude formed in dependence on the first upper limiting value, if the time rate of change of the cosine function (

[cos(θ

_{i} - θ

_{j})]) for the phase deviation is smaller than zero, and equal to the second control value, with an amplitude formed in dependence on the second limiting value if the time rate of change of the cosine function of the phase deviation is greater than zero.

7. A device according to claim 6,

characterized in that the amplitude of the first control value is formed in dependence on a product of the first upper limiting value and the absolute value of the time rate of change of the phase deviation, and the amplitude of the second control value is formed in dependence on a product of the second upper limiting value and the absolute value of the time rate of change of the phase deviation.

8. A device according to any of claims 6-7,

characterized in that the first upper limiting value is a function of said current, a chosen reactance value (X

_{FC}) and a first amplification factor (K

_{c}), and the second upper limiting value is a function of said current, the chosen reactance value and a second amplification factor (Ki).

9. A device according to any of claims 6-8,

characterized in that the controlling equipment forms a control order which brings the amplitude of the second control value to zero.

10. A device for control of controller equipment (FDV) intended for serial connection between a first and a second connection point (N

_{i}, N

_{j} respectively) in a power line (L) in an electric power network for alternating current, for the purpose of damping power oscillations in the power line by means of variation of the impedance of the controller equipment, wherein the controller equipment comprises a series capacitor (C4), and, connected in parallel therewith, a series connection comprising a reactor (TCL) and a thyristor switching device (TSW), whereby by phase angle control of the reactor in dependence on a control order (SO, SO') supplied thereto, the impedance (X

_{c}) of the controller equipment is controllable in relation to a first upper limiting value (Xcmaxc) of a capacitive nature and in relation to a second upper limiting value (Xcmaxi) of an inductive nature,

characterized in that it comprises controlling equipment (CE) which, in dependence on quantities (Yn) sensed in the power network, forms a phase deviation (θ

_{i} - θ

_{j}) as the difference (θ

_{i} - θ

_{j}) of the phase angle (θ

_{i}) for the voltage (Vi ) at the first connection point and the phase angle (θ

_{j}) for the voltage (

_{j} ) at the second connection point, and forms the control order such that, if the absolute value of the time rate of change of the phase deviation (

(θ

_{i} - θ

_{j}) ) exceeds a chosen rate of change limit value (D), it brings the controller equipment to exhibit an impedance of a capacitive nature, formed in dependence on the first upper limiting value, if the time rate of change of the cosine function (

[cos(θ

_{i} - θ

_{j})])of the phase deviation is smaller than zero, and to exhibit an impedance of an inductive nature, formed in dependence on the second upper limiting value, if the time rate of change of the cosine function of the phase deviation is greater than zero.

11. A device according to claim 10,

characterized in that the controlling equipment causes the controller equipment to exhibit an impedance of a capacitive nature, formed in dependence on a product of the first upper limiting value and the absolute value of the time rate of change of the phase deviation, and an impedance of an inductive nature, formed in dependence on a product of the upper limiting value and the absolute value of the time rate of change of the phase deviation, respectively.

12. A device according to any of claims 10-11,

characterized in that the first upper limiting value is a product of a chosen reactance value (X

_{FC}) and a first amplification factor (Kc), and that the second upper limiting value is a product of the chosen reactance value and a second amplification factor (Ki).

13. A device for control of controller equipment (FDV) which is connected serially between a first and a second connection point (N

_{i}, N

_{j} respectively) in a power line (L) in an electric power network for alternating current, for the purpose of damping power oscillations in the power line by means of variation of the impedance of the controller equipment, wherein the controller equipment comprises a number of mutually series-connected capacitors (C1, C2, C3, ...), each one connected in parallel with a switching member (SW1, SW2, SW3, ...), whereby, by means of, respectively, connection in the power line and bypassing of said capacitors via the switching members, in dependence on a control order (SO, SO') supplied thereto, the impedance (X

_{c}) of the controller equipment may assume an upper limiting value (Xcmaxc), which is a product of a chosen reactance value (X

_{FC}) and an amplification factor (Kc), and a lower limiting value equal to zero,

characterized in that, in dependence on quantities (Yn) sensed in the power line, a phase deviation (θ

_{i} - θ

_{j}) is formed as the difference (θ

_{i} - θ

_{j}) of the phase angle (θ

_{i}) of the voltage (

_{i} ) at the first connection point and the phase angle (θ

_{j}) of the voltage (

_{j} ) at the second connection point, and that, if the absolute value of the time rate of change of the phase deviation (

(θ

_{i} - θ

_{j}) ) exceeds a chosen rate of change limit value (D), the control order causes the controller equipment to exhibit an impedance formed in dependence on the upper limiting value, if the time rate of change of the cosine function (

[cos(θ

_{i} - θ

_{j})]) of the phase deviation is smaller than zero, and to exhibit an impedance equal to the lower limiting value if the time rate of change of the cosine function of the phase deviation is greater than zero.

14. A device according to claim 13,

characterized in that the controlling equipment forms a control order which causes the controller equipment to exhibit an impedance, formed in dependence on a product of the upper limiting value and the absolute value of the time rate of change of the phase deviation.

15. A device according to claim 14,

characterized in that the controlling equipment causes the controller equipment to exhibit an impedance which minimizes the difference between this and the impedance which is formed in dependence on the product of the upper limiting value and the absolute value of the time rate of change of the phase deviation.