WE CLAIM:
1. A method in a power transmission system comprising at least
two inverting converters (101a, 101b) connected in parallel at a first end of
) a direct current, dc, link (116a, 116b) and at least two rectifying converters (102a, 102b) connected in parallel at a second end of the dc link, wherein a first alternating current, ac, power line (104) is connected to the first end of the dc link via a first and a second transformer (106a, 106b) and the inverting converters (101a, 101b), where the turns ratio of the first and
) second transformers are controllable via tap changer control and the inverting converters are controlled using a characteristic of the relationship between the dc voltage and the dc current, said characteristic (U/I) comprising a first section (Si) where the current is constant for different values of the voltage and a second section (S2) where the dc
> voltage varies with the dc current and a second alternating, ac, power line
(103) is connected to the second end of the dc link (116a, 116b) via a third
and a fourth transformer (105a, 105b) and the rectifying converters (102a,
102b), and the turns ratio of the third and fourth transformers are
controllable via tap changer control;
) the method being performed in at least one control unit (108,110a, 110b, 112a, 112b, 114a, 114b) for controlling operation of the dc link and comprising:
determining (201), in a superior control unit (108), that a first of the inverting converters (101a) is to control the voltage of the dc link and the
> other (101b) to follow the voltage of the first inverting converter;
setting (203), in the superior control unit (108), an extinction angle
window (yiow - Yhigh) for the inverting converters and a firing angle window
(aiow - ahigh) for the rectifying converters (102a, 102b);
controlling (205), in tap changer control units (112a, 112b), the extinction ) angle of the inverting converters via tap changer control of a tap changer mechanism in the first and second transformers,

controlling, in tap changer control units (114a, 114b), the firing angle of the
rectifying converters via tap changer control of a tap changer mechanism
in the third and fourth transformers;
controlling (207), based on the extinction angle of the inverting converters
being in the extinction angle window, the dc voltage of the first inverting
converter;
controlling the second inverting converter to follow the voltage of the first
inverting converter; and
applying (209) a current stabilizing control scheme on the inverting
converters based on a fulfillment of a control condition that the inverting
converters operate in the extinction angle window and based on a
fulfillment of a control condition that the rectifying converters operate in
the firing angle window, said current stabilizing control scheme
comprising controlling the extinction angle of the inverting converters in
the second section (S2) of said characteristic.
2. A method according to claim 1, further comprising setting (204) a voltage control window (Udciow - Udchigh), wherein the control of the dc voltage is made using tap changer control and the current stabilizing control scheme is applied on the inverting converters also based on a fulfilment of a control condition that the dc voltage is in the voltage control window.
3. A method according to claim 1 or 2, wherein the second section of the characteristic comprises a positive slope segment (PS) and a negative slope segment (NS) where the current stabilizing control scheme is applied in the positive slope segment and the negative slope segment is used for constant commutation margin control.
4. A method according to any previous claim, wherein the current stabilization control in an inverting converter comprises adjusting the current through the inverting converter based on a difference between a current error in said converter and a current error in the parallel

converter, where a current error is the difference between a current order and a measured current.
5. A method according to claim 4, wherein the adjusting of the current comprises applying proportional control on the difference between the current errors.
6. A method according to claim 5, wherein the adjustment is made

7. A method according to claim 6, wherein the adjusting of the current comprises applying integrating control on the difference between the current errors in the inverting converter that controls the dc voltage.
8. A power transmission system comprising: a direct current, dc, link (116a, 116b),
at least one control unit (108,110a, 110b, 112a, 112b, 114a, 114b) for controlling operation of the dc link,
a first, second, third and a fourth transformer (105a, 105b, 106a, 106b), at least two inverting converters (101a, 101b) connected in parallel at a first end of the direct current, dc, link (116a, 116b); and
and at least two rectifying converters (102a, 102b) connected in parallel at a second end of the dc link,
wherein a first alternating current, ac, power line (104) is connected to the first end of the dc link via the first and the second transformer (106a, 106b) and the inverting converters (101a, 101b), where the turns ratio of the first and second transformers are controllable via tap changer control and the inverting converters are controlled using a characteristic of the relationship between the dc voltage and the dc current, said characteristic (U/I) comprising a first section (Si) where the current is constant for different values of the voltage and a second section (S2) where the dc voltage varies with the dc current

wherein a second alternating, ac, power line (103) is connected to the
second end of the dc link (116a, 116b) via the third and fourth transformers
(105a, 105b) and the rectifying converters (102a, 102b), where the turns
ratio of the third and fourth transformers are controllable via tap changer
control;
said at least one control unit (108,110a, 110b, 112a, 112b) comprising:
a superior control unit (108) configured to
determine that a first of the inverting converters (101a) is to
control the voltage of the dc link and the other (101b) to follow
the voltage of the first inverting converter;
set an extinction angle window (yiow - Yhigh) for the inverting
converters, and
set a firing angle window (aiow - ahigh) for the rectifying
converters (102a, 102b); tap changer control units (112a, 112b) configured to control the extinction angle of the inverting converters via tap changer control of a tap changer mechanism in the first and second transformers ;tap changer control units (114a, 114b) configured to control the firing angle of the rectifying converters via tap changer control of a tap changer mechanism in the third and fourth transformers;
wherein said at least one control unit (108,110a, 110b, 112a, 112b) is further configured to:
control, based on the extinction angle of the inverting converters being in the extinction angle window, the dc voltage of the first inverting converter; control the second inverting converter to follow the voltage of the first inverting converter; and
apply a current stabilizing control scheme on the inverting converters based on a fulfilment of a control condition that the inverting converters operate in the extinction angle window, said current stabilizing control scheme comprising controlling the extinction angle of the inverting converters in the second section (S2) of said characteristic,

where the applying of the current stabilization control scheme is based on a fulfilment of a control condition that the rectifying converters operate in the firing angle window.
9. The power transmission system according to claim 8, wherein said at least one control unit is further configured to set a voltage control window (Udciow - Udchigh) for the control voltage, wherein the control of the dc voltage is made using tap changer control and the current stabilizing control scheme is applied on the inverting converters also based on a fulfilment of a control condition that the dc voltage is in the voltage control window.
10. The power transmission system according to claim 8 or 9, wherein the second section of the characteristic comprises a positive slope segment (PS) and a negative slope segment (NS) where the current stabilizing control scheme is applied in the positive slope segment and the negative slope segment is used for constant commutation margin control.
11. The power transmission system according to any of claims 8 -10, wherein the current stabilization control in an inverting converter comprises adjusting the current through the inverting converter based on a difference between a current error in said converter and a current error in the parallel converter, where a current error is the difference between a current order and a measured current.
12. A converter control system for a power transmission system comprising at least two inverting converters (101a, 101b) connected in parallel at a first end of a direct current, dc, link (116a, 116b) and at least two rectifying converters (102a, 102b) connected in parallel at a second end of the dc link, wherein a first alternating current, ac, power line (104) is connected to the first end of the dc link via a first and a second transformer (106a, 106b) and the inverting converters (101a, 101b), where the turns ratio of the first and second transformers are controllable via tap

changer control and the inverting converters are controlled using a
characteristic of the relationship between the dc voltage and the dc
current, said characteristic (U/I) comprising a first section (Si) where the
current is constant for different values of the voltage and a second section
(S2) where the dc voltage varies with the dc current;
wherein a second alternating, ac, power line (103) is connected to the
second end of the delink (116a, 116b) via a third and a fourth transformer
(105a, 105b) and the rectifying converters (102a, 102b), and the turns ratio
of the third and fourth transformers are controllable via tap changer
control;
the converter control system comprising at least one control unit (108,
110a, nob, 112a, 112b) comprising
a superior control unit (108) configured to:
determine that a first of the inverting converters (101a) is to
control the voltage of the dc link and the other (101b) to follow
the voltage of the first inverting converter;
set a firing angle window (cu0w - cihigh) for the rectifying
converters (102a, 102b); and
set an extinction angle window (yiow - Yt»gh) for the inverting
converters, tap changer control units (112a, 112b) configured to control the extinction angle of the inverting converters via tap changer control of a tap changer mechanism in the first and second transformers; and tap changer control units (112a, 112b) configured to control the firing angle of the rectifying converters via tap changer control of a tap changer mechanism in the third and fourth transformers; said at least one control unit (108,110a, nob, 112a, 112b) being further configured to control, based on the extinction angle of the inverting converters being in the extinction angle window, the dc voltage of the first inverting converter;
control the second inverting converter to follow the voltage of the first inverting converter via tap changer control; and

apply a current stabilizing control scheme on the inverting converters based on a fulfilment of a control condition that the inverting converters operate in the extinction angle window and based on a fulfilment of a control condition that the rectifying converters operate in the firing angle window, said current stabilizing control scheme comprising controlling the extinction angle of the inverting converters in the second section (S2) of said characteristic.
13. A computer program product comprising computer-readable
means carrying computer program code operable to implement the method steps of any of claims 1 - 7 in a converter control system (108, 110a, 110b, 112a, 112b, 114a, 114b) according to claim 12.