WO2019068311A1

WO2019068311A1 - Coordinating current stabilizing control with tap changer control

Info

Publication number: WO2019068311A1
Application number: PCT/EP2017/075024
Authority: WO
Inventors: Paulo TOLEDO
Original assignee: Abb Schweiz Ag
Priority date: 2017-10-03
Filing date: 2017-10-03
Publication date: 2019-04-11
Also published as: GB202005787D0; GB2581079B; GB2581079A

Abstract

In a power transmission system comprising two parallel converters operating as inverters (101a, 101b) at a first end of a dc link (116a, 116b) control units (108, 110a, 110b, 112a, 112b) determine that a first inverter (101a) is to control the dc link voltage,controls the extinction angle of the inverters via tap changer control, controls, based on the extinction angle being in the extinction angle window, the dc voltage of the first inverter, controls the second inverter to follow the voltage of the first inverter, and applies a current stabilizing control scheme on the inverters based on the inverters operating in the extinction angle window.

Description

COORDINATING CURRENT STABILIZING CONTROL WITH TAP

CHANGER CONTROL

FIELD OF INVENTION

The present invention relates to a power transmission system as well as to a method, converter control system and a computer program product for such a power transmission system. BACKGROUND

In power systems such as power transmission systems, for example so called High Voltage Direct Current (HVDC) power transmission systems, the voltages used in the power transmission are becoming increasingly higher. Voltage levels of about 8oo kV are used today, and in some scenarios, voltage levels of up to ιοοο kV or even 1200 kV or higher are expected to be used in the future. Converters in HVDC power transmission systems convert between a relatively high alternating current (AC) voltage and such high level direct current (DC) voltages.

WO 2014/044293 describes a HVDC power transmission system where it is known to interconnect two parallel rectifiers with two parallel inverters via a direct current (dc) link and to perform current when controlling the extinction angle of the rectifying converters. This firing angle control can also be combined with tap changer control for keeping the dc voltage to within a dc voltage window and the extinction angle to within an extinction angle window.

Another type of control is described in US 5,627,734. Here a number of controllable quantities being actuated are provided as a single controllable variable termed a multidimensional vector and used by a vector controller for controlling a converter. There is a need to improve on the combining the firing angle control with tap changer control in a system of the type described in WO 2014/044293.

The present invention is directed towards such an improvement. The present invention is directed towards coordinating tap changer and current stabilization control.

SUMMARY One object is therefore to provide a method in a power transmission system that coordinates tap changer and current stabilization control.

This object is according to a first aspect achieved through a method in a power transmission system comprising at least two inverting converters connected in parallel at a first end of a direct current, dc, link, wherein a first alternating current, ac, power line is connected to the first end of the dc link via a first and a second transformer and the inverting converters, 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, the characteristic comprising a first section where the current is constant for different values of the voltage and a second section where the dc voltage varies with the dc current;

the method being performed in at least one control unit for controlling operation of the dc link and comprising:

determining that a first of the inverting converters is to control the voltage of the dc link and the other to follow the voltage of the first inverting converter;

setting an extinction angle window for the inverting converters,

controlling the extinction angle of the inverting converters via tap changer control; controlling, 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 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, the current stabilizing control scheme comprising controlling the extinction angle of the inverting converters in the second section of the characteristic.

Another object is to provide a power transmission system that coordinates tap changer and current stabilization control. This object is according to a second aspect achieved through a power transmission system comprising:

a direct current, dc, link,

at least one control unit for controlling operation of the dc link,

a first and second transformer, and

at least two inverting converters , connected in parallel at a first end of the direct current, dc, link,

wherein a first alternating current, ac, power line is connected to the first end of the dc link via the first and the second transformer and the inverting converters, 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, the characteristic comprising a first section where the current is constant for different values of the voltage and a second section where the dc voltage varies with the dc current, wherein the at least one control unit is configured to:

determine that a first of the inverting converters is to control the voltage of the dc link and the other to follow the voltage of the first inverting converter; set an extinction angle window for the inverting converters,

control the extinction angle of the inverting converters via tap changer control;

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, the current stabilizing control scheme comprising controlling the extinction angle of the inverting converters in the second section of the characteristic.

Yet another object is to provide a converter control system for a power transmission system that coordinates tap changer and current stabilization control.

This object is according to a third aspect achieved through a converter control system for a power transmission system comprising at least two inverting converters connected in parallel at a first end of a direct current, dc, link, wherein a first alternating current, ac, power line is connected to the first end of the dc link via a first and a second transformer and the inverting converters, 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, the characteristic comprising a first section where the current is constant for different values of the voltage and a second section where the dc voltage varies with the dc current;

the converter control system comprising at least one control unit 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

Another object is to provide a computer program product for such converter control system that coordinates tap changer and current stabilization control system.

This object is according to a fourth aspect achieved through a computer program product adapted to be executed in a converter control system in a power transmission system comprising at least two inverting converters connected in parallel at a first end of a direct current, dc, link, wherein a first alternating current, ac, power line is connected to the first end of the dc link via a first and a second transformer and the inverting converters, 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, the characteristic comprising a first section where the current is constant for different values of the voltage and a second section where the dc voltage varies with the dc current;

the computer program product comprising computer-readable means carrying computer program code configured to, when executed in the converter control system, cause the converter control system to: determine that a first of the inverting converters is to control the voltage of the dc link and the other to follow the voltage of the first inverting converter;

set an extinction angle window for the inverting converters;

control the extinction angle of inverting converters via tap changer control; 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

The current stabilization control may therefore only be active for steady state and low frequency dynamic oscillations if the control conditions are fulfilled. The invention has a number of advantages. It opens an operating window where current stabilization control can be performed, which in turn improves the steady-state, dynamic and transient stability of parallel inverters. It thus coordinates the current stabilizing control with the operation of the tap changers thereby enhancing the efficiency of the current stabilization. Furthermore, the current stabilization has the advantage of more quickly reaching steady state operation after a previous disturbance. Another advantage is that the rating and costs of the converters may be optimized. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the invention will be described below with reference to the accompanying drawings, in which:

Fig. l is a schematic view of a part of a power transmission system in accordance with an embodiment of the present invention comprising parallel poles;

Fig. 2 is a schematic flowchart of a method of coordinating current stabilization control with tap changer control in the power transmission system in fig. l;

Fig. 3 schematically shows the setting of operating points in direct current U/I characteristics of inverters and rectifiers in the system in fig. l in a typical application of parallel poles,

Fig. 4a shows the control of a tap changer in order keep the firing angle of a rectifier within a firing angle window;

Fig. 4b shows the change of the U/I characteristic of a rectifier caused by the rectifier tap changer control of the rectifier;

Fig. 5 shows the control of a tap changer in order keep the extinction angle of an inverter within an extinction angle window;

Fig. 6a shows the control of an inverter tap changer in order keep the dc voltage within a dc voltage control window;

Fig. 6b shows the change of the U/I characteristic caused by the dc voltage control in the inverter;

Fig. 7 schematically shows the U/I characteristic of a pair of inverters for use in current stabilization control in the inverters;

Fig. 8 shows a block schematic of a control loop used for controlling the firing angle of an inverting converter set to control the dc voltage; and Fig. 9 shows a computer program product comprising computer program code means for implementing at least some of the control system. DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying

embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the invention to those skilled in the art. Furthermore, like numbers refer to like or similar elements or components throughout. The steps of any method described herein do not have to be performed in the exact order as described, unless specifically stated. Referring now to Fig. l, there is shown a schematic view of a power transmission system. The power transmission system may be a High Voltage Direct Current (HVDC) power transmission system and comprises a dc link. The dc link includes a first and a second conductor n6a and n6b interconnecting two converter stations, where each converter station comprises two converters. There are two inverting converters, a first inverting converter Ii 101a and a second inverting converter l2 101b connected in parallel with each other. In a typical topology of parallel poles there are also two rectifying converters 102a, 102b, a first rectifying converter Ri 102a and a second rectifying converter R2 102b, connected in parallel with each other . The inverting converters may be provided in one converter station and the rectifying converters may be provided in another converter station. The first and second inverting converters 101a and 101b are thereby connected between a first end of the first dc conductor 116a and a first end of the second dc conductor 116b, while the first and second rectifying converters 102a and 102b are connected between a second end of the first dc conductor 116a and a second end of the second dc conductor n6b.

It is noted that many variations of the particular arrangement of the other elements in the power transmission system relatively to each other can be contemplated. For instance, the inverting converters may be provided in a common converter station and the rectifying converts in separate converter stations. It is also possible that the rectifying converters are provided in a common converter station and the inverting converts in separate converter stations. Another possible variation is that all converters are provided in separate converter stations.

The rectifying converters 102a, 102b and the inverting converters 101a, 101b may for example be thyristor-based.

The inverting converters 101a, 101b and the rectifying converters 102a, 102b are connected to first and second alternating current (ac) lines schematically indicated by reference numerals 104 and 103 via first and second transformers 106, 106b of the inverting converters 101a, 101b and via third and fourth transformers 105a, 105b of the rectifying converters 102a, 102b, respectively. Thus, the first ac power line 104 is connected to the dc link at a first end thereof, and the second ac power line 103 is connected to the dc link at a second end thereof. Each of the inverting converters 101a, 101b is adapted to convert dc power to ac power. The respective outputs from the inverting converters 101a, 101b are outcoupled from the dc link at the first end thereof.

The power transmission system comprises a superior control unit in the form of a controller 108 for controlling operation of the DC link.

The power transmission system also comprises converter control units 110a, 110b. According to the embodiment depicted in Fig. 1, a first converter control unit 110a is adapted to control operation of the first inverting converter 101a, and a second converter control unit nob is adapted to control operation of the second inverting converter 101b. It should be realized that similar converter control units may be provided for the rectifying converters. However, these have been omitted, due to the fact that the control of these rectifying converters is conventional and not directly related to the current stabilization control.

One or both of the inverting converters 101a, 101b may be configured to control the dc line voltage, i.e. the dc voltage over the dc link. This may be achieved for example by means of varying or adjusting an extinction angle reference value for the one or both inverting converters 101a, 101b, which extinction angle reference value for example is supplied from the controller 108, thereby controlling the DC voltage across the inverting converter 101a, 101b.

The power transmission system in Fig. l also comprises tap changer control units 112a, 112b, 114a and 114b for the transformers 105a, 105b, 106a, 106b. The tap changer control units 112a, 112b are configured to control a tap changer mechanism in the transformers 106a, 106b of the inverting converters 101a, 101b, respectively, while the tap changer control units 114a, 114b are configured to control a tap changer mechanism in the transformers 105a, 105b of the rectifying converters 102a, 102b, respectively. The tap changer mechanism can change the position of the transformer tap or tapping connection, which is a connection point along a transformer winding that allows a certain number of turns to be selected, thereby enabling voltage regulation of the outputs of the respective transformers 105a, 105b, 106a, 106b. The tap changer mechanism may for example comprise on-load tap changers that can be used for changing the position of the tapping connection of energized transformer windings. Thereby the turns ratio of the transformers is controllable via tap changer control performed by the tap changer control units. The tap changer control units 112a, 112b are configured to control the tap changer mechanisms in the transformers 106a, 106b of the inverting converters 101a, 101b, respectively, in order to regulate the extinction angle of one or both inverting converters 101a, 101b so as to keep the extinction angle or angles within an extinction angle interval about an extinction angle reference value that for example may be received from the controller 108. One of the converter tap changer control units also controls the voltage of the dc line. As is illustrated by two-way arrows in fig. 1, the tap changer control units 112a, 112b, 114a and 114b are coupled with the controller 108 so as to be able to transmit signals to and receive signals from the controller 108.

The rectifying converters 102a, 102b may be configured to control the currents there through, i.e. each of the rectifying converters 102a, 102b is controlling the current through it.

Each of the converter control units 110a, nob is adapted to receive a current reference value, or current order, for the desired current through the inverting converters 101a and 101b, respectively, e.g. from the controller 108 as illustrated in Fig. 1.

As illustrated in Fig. 1 by two-way arrows, the converter control units 110a, nob are coupled with the controller 108 so as to be able to transmit signals to and receive signals from the controller 108. In another embodiment, the converter control units 110a, 110b are coupled with the controller 108 so as to be able to receive signals from the controller 108, but not necessarily so as to be able to transmit signals to the controller 108. As further illustrated in Fig. 1 by two-way arrows, the converter control units 110a, 110b are coupled with the inverting converters 101a and 101b, respectively, so as to be able to transmit signals to and receive signals from the inverting converters 101a and 101b, respectively. The converter control units noa, nob are intercoupled so as to be able to transmit signals there between.

Transmission of signals between different elements in the power transmission system as discussed in the foregoing and in the following may be effected by means of communication means which may be wired and/or wireless. For example, transmission of signals between the converter control units noa, nob may be effected by means of a wireless communication link as known in the art.

Generally in the drawings, two-way arrows between two elements indicate two-way communication capability between the respective elements, but do not necessarily imply necessity of two-way communication; one-way communication may be contemplated.

Each of the converter control units noa, nob is adapted to cause the inverting converter 101a and loib, respectively, to measure current JDC,H, DC,I2 through the respective inverting converter 101a, loib, and to determine a difference value between the measured current and the current reference value. Alternatively, the inverting converters 101a, loib, after having measured the current through the respective inverting converter 101a, 101b, transmits the respective measured current values to the converter control units noa and nob, respectively, at which the determination of the difference value is performed.

According to the embodiment depicted in Fig. l, the difference values for the respective inverting converters 101a, 101b are exchanged between the converter control units noa, nob. The converter control units noa, nob may be adapted to measure or determine the DC voltage UDC,n, UDC,i₂ over the inverting converter 101a and loib, respectively. However, as will be evident later on, only one of the converters 101a and 101b actually controls the voltage. By allowing a variation of the extinction angle within a selected angle interval, no or less 'hunting' of the tap changer mechanism may occur. The regulation of the extinction angle of one or both inverting converters 101a, loib may be performed over a time period so as to maintain the extinction angle or angles within the selected angle interval during that time period.

In a similar manner the tap changer control units may be configured to control the tap changer mechanism in the transformers of the inverting converters in order to regulate dc voltage of an inverting converter so as to keep the dc voltage within a dc voltage control window.

The dc voltage over the dc link may be controlled via one of the inverting converters 101a, 101b. For example, the inverting converter 101a may be configured to control the dc voltage over the dc link, whereas the inverting converter 101b may be set to follow the inverting converter 101b. The dc voltage control is in this example implemented in the tap changer control unit ii2a. The dc voltage over the inverting converter 101a is determined or measured at the inverting converter 101a. On a condition that there is a difference between the dc voltage over the inverting converter 101a and a dc voltage reference value, which for example may be established by the controller 108, the dc voltage over the inverting converter 101a may be adjusted so as decrease the difference between the dc voltage over the inverting converter 101a and the dc voltage reference value. The

adjustment of the dc voltage over the inverting converter 101a is thus effected by means of adjusting the tap positions of the transformer 106a of the inverting converter 101a controlled by the converter transformer tap changer control unit 112a. The above-mentioned dc voltage control is conditional on the extinction angle of an inverting converter being in an extinction angle window that may be set through a minimum extinction angle ymin and a maximum extinction angle ymax .The dc voltage control may also be conditional on the firing angle of rectifying converters being within a firing angle window, set through a minimum firing angle a_min and a maximum firing angle a_max.

A no-load control function, which for example may be implemented in the converter transformer tap changer control units 112a, 112b, may control the tap changer mechanism of the inverting converter transformers 106a, 106b when the inverting converters 101a, 110b are in a blocked state so as to establish a so called no-load DC voltage ZTdio level corresponding to the steady state requirement at a minimum current level. Thereby, when the inverting converters 101a, 101b are brought out of the blocked state, no immediate stepping would be required by the tap changer mechanism.

A Z dio limitation function, which for example may be implemented in the converter transformer tap changer control units 112a, 112b, may monitor, e.g. repeatedly measure, the voltage in the inverting converters 101a, 101b, and override the tap changer mechanisms in the transformers 106a, 106b of the inverting converters 101a, 101b, respectively, in case a no-load DC voltage level is detected that exceeds a predefined ZTdio threshold value. The Z/dio threshold value may for example be received from the controller 108. The Z/dio limitation function may regulate or step down the voltage and/or inhibit or reduce the tap changer mechanism operability or functionality in case a no-load DC voltage level is detected that exceeds the predefined Z/dio threshold value. The Z/dio limitation function may protect the valves of the inverting converters 101a, 101b from excessive voltage stress.

Similar to the "classical" scheme, the control scheme to be described makes use of the U/I characteristic of the converters, which is a curve defining the relationship between the dc voltage and the dc current of a converter. The U/I characteristic is a curve having a first section where the current is constant for different values of the voltage and a second section where the voltage varies with the current. The first section may be seen as being vertical and the second section as being essentially horizontal. While in the "classical" scheme all, except one, converter are operating in the first section of their characteristics and just one operating in the second section (this specific converter will be defining the operating voltage), in the present scheme only the rectifier converters are operating in the first section of their characteristic where the dc current is constant for different values of the dc voltage. All the inverting converters are operating in the second essentially horizontal section of the U/I characteristic, where the dc voltage varies slowly for different values of the dc current.

This approach follows the traditional scheme of a two terminal

transmission system, where the rectifier converter is set to control the injected current into the dc line, and in the other terminal the inverter converter receives the current and this converter is set to define the operating voltage of the transmission.

It should be noted that in the case of a topology with parallel converters as presented in Figure l, it requires that all the inverters should have the second section in their Udc/Idc characteristics be positioned at the same level, and this is achieved by appropriate action from the tap changer control units, with appropriate coordination between the inverters. In addition, the injected current from the rectifier converters is distributed among the inverters, following a current order established by a high level controller (Power Control in case of parallel poles or Current Order

Coordinator in case of parallel converters in a multi -terminal system).

The operation of the system in fig. l will now be described in some more detail with reference also being made to fig. 2, 3, 4a, 4b, 5, 6a, 6b and 7, where fig. 2 is a flowchart of a method of coordinating current stabilizing control with tap changer control under steady-state and small-signal stabilization conditions, fig. 3 schematically shows the setting of operating points in the U/I characteristics of the inverters and rectifiers, fig. 4a shows the control of a rectifier tap changer in order keep a used firing angle a within a firing angle window, fig. 4b shows the change of the U/I characteristics caused by the rectifier tap changer control, fig. 5 shows the control of an inverter tap changer in order keep an extinction angle γ within the extinction angle window, fig. 6a shows the control of an inverter tap changer in order keep the dc voltage within a dc voltage control window, fig. 6b shows the change of the U/I characteristic caused by the inverter tap changer dc voltage control and fig. 7 schematically shows the U/I characteristics of the inverters for use in the current stabilization.

The operation may be started by the control unit 108 determining or selecting which inverting converter is to be controlling the voltage, step 201. As one alternative it is possible that the converter in a Parallel Poles topology that uses its own line is selected. As an example the first inverting converter Ii may be selected by_the control unit 108. Thereby the second inverting converter is to follow the voltage of the first inverting converter.

Thereafter operating positions OPRi, OPR2, OPIi and OPI2 of the first rectifying converter, the second rectifying converter, the first inverting converter and the second inverting converter are determined, step 202.

Such determination may be based on a desired amount of power to be transmitted through the dc system and using the U/I characteristics of the respective converters.

The second section of the UOC-IOC characteristic corresponds to a negative slope segment where the voltage decreases with increasing current levels. This segment can be described by the relationship: UOC = i7dio-COS Y - (dxN-drN)-(iJdioN / -TDCNMDC,

where: ί/dio-is the no-load direct voltage,

t dioN is the no-load direct voltage at nominal converter transformer tap position and nominal AC voltage,

yis the extinction angle for the inverting converter or converter valves of the inverting converter,

dxN is the relative inductive voltage drop at rated direct current and at rated no-load direct voltage,

drN is the resistive voltage drop at rated operation,

/DCN is the rated current, and

IOC is the direct current.

A rectifying converter can be described with a UOC-IOC characteristic similar to that above for an inverting converter. As will be seen later the second section of the U/I characteristic where the voltage varies slowly with the current may in the inverting converters 10a and loib also have a positive slope segment where the voltage increases with increasing current levels. Four operating points OPRi, 0PR2, OPI2 and OPIi that have been determined are schematically shown in fig. 3. There is here a first rectifier operating point OPRI determined for the first rectifier 102a, a second rectifier operation point OPR₂, determined for the second rectifier 102b, a first inverter operating point OPn determined for the first inverter 101a and a second inverter operating point OP12 determined for the second inverter 101b. In fig. 3 for the sake of simplicity the operating points are shown as being the same in both the rectifier and inverters. In the determination typically both rectifiers have the same operating point and the inverting converters have the same operating point. The operating points of the inverting converters also correspond to the operating points of the rectifying converters and are typically lower by a current margin order, where the lower point corresponds to the transmitted power.

After having determined operating points OPn, OPi₂, OPRI, OPR₂ for the various converters, the control unit 108 determines or sets an extinction angle window within which the extinction angle γ used by the inverting converters is allowed to vary and a firing angle window in which the firing angles a of the rectifying converters are allowed to vary, step 203. The firing angle a may here be set to be allowed to vary between a low firing angle limit ai_ow and a high firing angle limit cihigh that thus define the firing angle window, while the extinction angle γ may be set to be allowed to vary between a low extinction angle limit yiow and a high extinction angle limit Yhigh, which limits thus define the extinction angle window. Moreover the control unit 108 may also determine or set a voltage control window for the control voltage, i.e. a voltage within which the dc voltage is allowed to vary, step 204, which is the dc voltage at the inverter side. The dc voltage may in this case be set to vary between a low dc voltage limit Udciow and a high dc voltage limit Udchigh that thus define the dc voltage window.

Thereafter the inverter tap changer control units 112a and 112b are informed about the extinction angle window and the rectifier tap changer control units 114a and 114b are informed about the firing angle window. The first tap changer control unit 112a of the first inverter 101a that controls the voltage is also informed about the dc voltage control window.

Therefore, in operation of the dc system, the tap changer control units 114a and 114b of the rectifiers 102a and 102b control the firing angles using tap changer control and the tap changer control units 110a and 110b of the inverters 101a and 101b control the extinction angles also using tap changer control. The first tap changer control unit 112a may also control the dc voltage. Moreover, the tap changer control units continuously investigate the control angles used, i.e. the firing angles and extinction angles, as well as the dc voltage. The tap changer control units more particularly investigate if the control angles that they have been informed about are within the control angle windows. This means that the first and second rectifier tap changer control units 114a and 114b investigate if the firing angle a is within the firing angle window and the first and second inverter tap changer control units 112a and 112b investigate if the extinction angle γ is within the extinction angle window. In case any control angle is outside of its corresponding window, step 205, then the discrepancy is handled, step 206, and thereafter a new

investigation is made.

The investigation of if the firing angle is outside of a firing angle window and the handling of the discrepancy, i.e. the control performed by a rectifying tap changer control unit if the firing angle is outside of the firing angle window may then be performed in the way shown in fig. 4a.

The strategy applied for the rectifier converters when in parallel operation of converters is to use the tap-changer control unit on the converter transformers in the rectifier in a kind of higher level feedback a-control with the intention to keep the firing angle a as close as possible to a nominal value OLN despite variations in ac voltage. If the ac voltage on the rectifier commutation bus increases, i.e. the ac voltage between a transformer and the corresponding rectifier, the dc voltage in the rectifier and by that the dc current will increase and the tap changer control unit must increase the firing angle a to restore the current. Vice versa, if the ac bus voltage drops, the firing angle a is decreased by the current control. In Figure 4a, it can be seen that if the firing angle a exceeds the upper window limitation cihigh, then the tap change position is changed, here shown as a decrease of the amount of turns by a unitary step size of -1, in order to decrease the ac bus voltage. If in a similar manner the firing angle a falls below the lower window limitation ai_ow, then the tap change position is changed, here shown as an increase of the amount of turns by a unitary step size of +1, in order to increase the ac bus voltage. It can be seen that through this type of control the firing angle is brought back to within the firing angle window.

As can thus be seen in fig. 4a the order of stepping is given by: a < aiow or a > cihigh The above mentioned tap change operation also leads to the no-load direct voltage Udio being influenced, where a turns increase on the secondary side leads to an increase of Udio and a decrease leads to a decrease of Udio. Figure 4b illustrates the influence of the ac voltage on the dc voltage Udc in the U/I characteristic, where the solid curve corresponds to the control of the firing angle in the firing angle window, and the dashed curve corresponds to the situation when Uac at the rectifier has increased so that the tap changer control unit has forced a to a high value. In this situation the tap changer control unit has increased a to a > cihigh after which the tap changer control unit reduces Udio and moves a back into the window. Such a tap change may also lead to the determination of a new operating point by the controller 108. The control of the extinction angle is analogous.

The investigation of if the extinction angle γ is outside of the extinction angle window and the handling of the discrepancy, i.e. the control performed by an inverting tap changer control unit if the extinction angle is outside of the window, may then be performed in the way shown in fig. 5· The strategy applied for the inverter converters when in parallel operation of converters is also here to use the tap-changer control units on the converter transformers in the inverters in a kind of higher level feedback γ- control with the intention to keep the extinction angle γ as close as possible to a nominal value YN despite variations in ac voltage.

If the ac voltage on the inverter commutation bus increases, i.e. the ac voltage between a transformer and the corresponding inverter, the dc voltage in the inverter and by that the dc current will increase and the tap changer control unit must increase γ to restore the current. Vice versa, if the ac bus voltage drops, γ is decreased.

The tap changer control function mentioned above is indicated in Figure 5, where it can be seen that if the extinction angle γ exceeds the upper window limitation yhigh, then the tap change position is changed, here shown as a decrease of the amount of turns by a unitary step size of -1, in order to decrease the ac bus voltage. If in a similar manner the extinction angle γ falls below the lower window limitation YI_ow, then the tap change position is changed, here shown as an increase of the amount of turns by a unitary step size of +1 in order to increase the ac bus voltage. It can be seen that through this type of control the extinction angle is brought back to within the extinction angle window. The extinction angle is thus kept close the nominal value YN. If the ac voltage on the commutation bus increases, the direct voltage in that inverter tends to increase resulting increase of operating extinction angle. In this case, the tap changer control unit will operate to restore the angle within the normal operating window.

As can thus be seen in fig. 5 the order of stepping is given by: γ < Yiow or γ > Yhigh

The above mentioned tap change operation likewise leads to the no-load direct voltage Udio to be influenced, where a turns increase on the secondary side leads to an increase of Udio and a decrease leads to a decrease of Udio. Therefore a tap change may also lead to the

determination of a new operating point.

If the control angles are inside their respective windows, step 205, then the tap changer control of the inverter set to control the voltage may

investigate if the dc voltage is within the voltage control window.

If the voltage is outside of the voltage control window, step 207, then the discrepancy is handled, step 208, in order to bring the voltage back to within the window followed by a possible new investigation of if the voltage is within the window.

The investigation of if the dc voltage is outside of the voltage control window and the handling of the discrepancy, i.e. the control performed if it is outside of the window may then be performed in the way shown in fig. 6a.

One strategy that can be applied for the inverter converters when in parallel operation of converters is to use the tap-changer control on the converter transformers in the inverter in a kind of higher level feedback dc voltage control with the intention to keep the dc voltage as close as possible to a reference voltage Udcref despite variations in ac voltage.

As we have stated before, an increase of the ac voltage on the inverter commutation bus will lead to an increase of the ac voltage in the inverter and correspondingly the dc voltage. Because of this ac voltage increase, the dc voltage may go outside the voltage control window. If it does then the first tap changer control unit 112a will have to control the tap changer to bring the dc voltage to within the window.

This tap changer control function is indicated in Figure 6a, where it can be seen that if the dc voltage Udc exceeds the upper window limitation Udchigh, then the tap change position is changed, here shown as a decrease of the amount of turns by a unitary step size of -1, in order to decrease the ac bus voltage. If in a similar manner the dc voltage falls below the lower window limitation Udciow, then the tap change position is changed, here shown as an increase of the amount of turns by a unitary step size of +1, in order to increase the ac bus voltage. It can be seen that through this type of control the dc voltage is brought back to within the dc voltage control window. Also this type of tap change may lead to the determination of a new operating point.

As can thus be seen in fig. 5a the order of stepping is given by:

Udc < Udciow Or Udc > Udchigh The above mentioned tap change operation also leads to the no-load direct voltage Udio being influenced, where a turns increase on the secondary side leads to an increase of Udio and a decrease leads to a decrease of Udio. Figure 6b illustrates the influence of the ac voltage on the dc voltage Udc in the U/I characteristic, where the solid curve corresponds to operation within the voltage control window Udciow < Udc < Udchigh and the dashed curve corresponds to the dc voltage falling below the lower voltage limit Udciow and an ensuing control of the tap changer to lower Udio in order to bring the voltage back into the voltage control window.

As can be seen in fig. 6b the positive slope segments of the second section of the U/I characteristic have been omitted for simplification purposes. It can be seen that with the dc voltage control function in the tap changer control unit 112a will control the dc voltage in the first inverter 101a selected to control the dc voltage, and this will establish the overall operating dc voltage of the transmission system.

With the inverter converters running in parallel connection, all the inverters receive influence from each other when establishing the operating dc voltage. The inverter that is designed to control the dc voltage may end-up in a condition that the operating point occurs at a higher extinction angle γ than the extinction angle reference YN. Because of this the extinction angle control function of both of the tap changer control untis 112a and 112b of both inverters 101a and 101b also have to be activated to maintain the operating extinction angle γ close to the extinction angle reference YN. In an analogous manner also the rectifying converters may have to operate within the firing angle window and the firing angle control function of both the tap changer control units 114a and 114b accordingly have to keep the firing angle a close to the firing angle reference OLN. Hence, the dc voltage control function, extinction angle control function and the firing angle control function must be coordinated. The dc voltage control function can thus only operate if the measured extinction angle γ is within the extinction angle window in all inverter converters and the firing angle is within the firing angle window in all rectifying converters. That means that the dc voltage control has lower priority than the extinction angle control and the firing angle control, and a way to establish this is the introduction of Control Permit logic as follows:

To allow stepping of the tap by the dc voltage control function it is required that the extinction angle control function on all parallel inverters should be satisfied. This means that all parallel inverters are operating with an extinction angle close to its reference value. • Once this condition of measured extinction angle is fulfilled there will be a permit condition to the dc voltage control function that is activated in that inverter that has been selected as the one to establish the operating dc voltage.

· If the dc voltage control function orders a step of the tap, this order is sent to all parallel inverters, which means that it is sent to the tap changer control units 112a and 112b of the parallel inverters 101a and 101b.

The dc voltage control function operates in a window, necessary to avoid hunting of the tap changer control.

· Also, in order to avoid hunting of the tap changers, the extinction angle control function includes a window for the measured extinction angle.

If all control angles lie within their respective control angle windows, step 205, and the dc voltage lies within the dc voltage control windows, step 207, then the converter control units of the inverter perform current stabilizing control during normal steady state operation or during small signal dynamic oscillation, step 209. As long as three control conditions are fulfilled in the dc system, then a current stabilization control scheme is carried out in the two inverting converters in the second section of the U/I characteristics. A first condition is here that the tap changer control units 114a and 114b of the rectifying converters 102a and 102b are able to keep the firing angle within the firing angle window, a second condition is that the tap changer control units 112a and 112 b of the inverting converters 102a and 102b are able to keep the extinction angle γ within the extinction angle window and a third condition is that the tap changer control unit 112a of the first inverting converter 101a is able to keep the dc voltage within the dc voltage control window.

In an analogous manner in case any of the three conditions is not fulfilled or unfulfilled, the current stabilization control cannot perform its stabilization functionality and inverter control is instead performed in the first section of the U/I characteristic.

In the current stabilization control scheme the inverting converters operate in the second section of U/I characteristics where the dc voltage varies slowly with dc current changes. The current stabilization control scheme is more particularly carried out in the positive slope section.

Fig. 7 shows the U/I characteristic of the two inverting converters comprising the first and second sections Si and S2, where the second section S2 comprises both the positive and negative slope segments PS and NS and the two operating points OPn and OPi₂. Here it may be mentioned that the negative slope segment NS is used for the conventional inverter control. The second section S2 of the first inverting converter is also shown as being bold. This is done in order to indicate that this converter is used for controlling the voltage.

The current stabilization function performed by the converter control units noa and nob provides a control contribution and this control

contribution is influenced by the dc current error between current order and a measured current for each paralleled inverter, where the current error may be defined as a difference between the current order and a momentaneous current through the respective inverting converter 101a, io lb monitored over a period. The resulted value is multiplied by a gain Gi and this will affect the extinction angle γ of the valve - see formula below, applied for two or more parallel inverter:

⁾1⁰¹ The proportional gain may be time dependent or predefined or constant.

The control contribution also corresponds to the positive slope section of the U/I characteristic. The above shown control contribution AY_Bal_order is furthermore added to a control signal used for the regular extinction angle control, i.e. used for keeping a constant extinction angle γ or for constant commutation margin control. This control signal may be expressed as:

which corresponds to the positive slope segment of the U/I characteristic.

The current stabilization control thus corresponds to a positive slope segment PS that has been introduced into the second section of the inverter U/I characteristic, while the constant extinction angle control corresponds to the negative slope section NS defined by (d_XN - drN)*(UdioN/IdcN).

The second section of the U/I characteristic thereby includes a negative slope segment for constant γ = γο and a positive slope segment for γ > γο. Moreover, the positive slope section affects not only the steady state current through the converter, but also the dynamic behavior of the converter.

As long as all conditions are fulfilled, i.e. that the dc voltage is within the dc voltage control window, the extinction angle is within the extinction angle window and the firing angle is within the firing angle window, the rectifying converters are controlled along the first section of the U/I characteristic where the current is constant and the inverting converters are controlled along the second section of the DC U/I Characteristic, where the dc voltage varies slowly with the dc current.

However, if the above mentioned conditions are not fulfilled then the inverting converters are controlled along the first section of the U/I characteristic instead of the second section. Thus, as long as the control angles and the dc voltage are within the windows, then the rectifying converters control the current using phase angle control and the first section of the U/characteristic, while the phase angle of the rectifying converters is controlled with current stabilizing control using the second section of the U/I characteristic.

Having only a proportional loop included in the current stabilization control may lead to a permanent residual error. Therefore a second path in the control may be added with integration characteristic. This integral term included in the current stabilization control is introduced in the control in just one of the parallel inverting converters and with advantage in the first inverting converter.

Fig. 8 shows one example of a realization of the control scheme used in the current control unit noa of the first inverting converter 101a, where voltage control is performed.

The converter control unit noa comprises a first subtracting unit 801 having a first negative input terminal on which it receives an own current order current Ioown and a second positive terminal on which it receives and own detected current Idown, where the detected current is the detected current Idc,i in fig. l and the own current order is the current order for reaching a desired current in the first inverting converter 101a, typically a current corresponding to a constant extinction angle. There is likewise a second subtracting unit 802 having a first negative input terminal on which it receives a parallel current order Iopar and a second positive terminal on which it receives a detected parallel current Idpar, where the detected current is the detected current Idc,2 in the second inverter 101b in fig. 1 and the parallel current order is the current order for reaching a desired current in this second inverting converter 101b. The first subtracting unit 801 forms a first current error as the difference between the two own currents and supplies this first current error to a first positive input of a third subtracting unit 803. The second subtracting unit 802 forms a second current error as the difference between the two parallel currents and supplies this second current error to a second negative input of the third subtracting unit 803, which in turns forms a difference signal as a difference between the first and second current errors and supplies it to a proportional control unit 804 and to an integrating control unit 805. It can thereby be seen that the converter control unit 110a uses a difference between a current error in the own first inverting converter and a current error in the parallel inverting converter. It can here also be seen that the first difference signal is supplied to a current firing control unit CFC 807.

The proportional control unit 804 performs proportional control on the third difference signal and supplies the result to a first positive terminal of a first adding unit 806, the integrating control unit 805 performs integrating control on the third difference signal and supplies the result of the processing to a second positive terminal of the first adding unit 806. The current firing control unit 807 in turn performs conventional control on the first difference signal and supplies the result to a third positive input of the adding unit 806. Finally the adding unit 806 adds the signals it has received to each other in order to obtain a firing control signal that is to be applied on the first inverting converter 101a. It can thereby be seen that the current through the first inverting converter is adjusted based on the above-mentioned difference between the current errors.

It should here also be realized that the actual firing angle used may be obtained from the known expression α+μ+γ= π, where μ is the overlap angle. A similar control function is typically provided in the second inverting converter 101b. However, in this case the integrating control unit is typically removed. A small filter time constant may be included in the measured dc current path in the controller. This filter has the purpose to reduce the level of ripple introduced in the control system. As the control loop uses the current order and current response from all parallel inverter converters as the input variables, these variable signals have to be exchanged between the converters.

When exchanging the signals between converters it becomes natural that there might be latency in the time from sending of a signal from the source to the destination. The latency generally depends on the characteristics of the communication means or equipment used for transmission of signals between different elements in the power transmission system. In the context of the present application, latency may be between 4-10 ms or less. Latency may introduce restrictions on gain used in the control loop.

Simulations have shown that it may be desired or required to reduce gain when taking into account effects of the latency in the control loop. To this end, a parametric analysis of the gain, including the effect of latency, may be performed.

The functioning of the current stabilization control can be described in the following way:

The main objective is to guarantee that the measured dc current is equal to its current order and to stabilize the inverter after disturbances.

All the inverters will be operating near their minimum extinction angle YN, and the control will be functioning around this value of y(with purpose to consume reduced reactive power and for keeping the dc voltage close to the desired dc voltage.

Let consider the inverters operating close to YN, following its horizontal static characteristic - see above Figure 7. With the current stabilization control function the firing angle of the converter valves are dynamically changed creating a transient/dynamic positive resistance characteristic.

The rate of the slope of the positive slope segment is given by the proportional gain of the regulator. If the firing is dynamically advancing, the converter tends to reduce the excess of current. In the opposite way, if the firing is dynamically retarding, the converter tends to increase its current that is lacking in relation to its current order. In this way it tends to operate in a more stable condition, achieving the desired current defined by its current order.

The action of the function is limited. When advancing the firing, this is allowed to only a few degrees, up to a minimum extinction angle γο (γο is the minimum value to prevent commutation failure).

In the other direction, when retarding the firing, the limitation is also applied to only few degrees to avoid the inverter taking over the current control. The integrating control is added into the current stabilization control in only in one of the inverters, and it will eliminate any residual error existent in the difference in the inverter converters. This path offset the second section in the U/I characteristic of this converter. The indication can be interpreted as a slow move upwards or downwards of the second section of the Udc/Idc characteristic of that particular converter, as shown in Figure

7·

The complete control scheme of coordinating current stabilization control with tap changer control applied to parallel poles embodied by conductors n6a and 116 b is then the following:

One of the Poles, for instance the first conductor n6a

• Rectifier converter 102a in dc current control • Inverter converter 101a in constant γ (Minimum γ) control Inverter Current Stabilization control: Prop-part + Int-part are activated

Converter transformer tap change control:

o Rectifier 102a: keeps aclose to OLN

o Inverter: keeps dc voltage close to Udcref reference; in addition monitors the operating γ and keep it close to YN

The second Pole formed by the second conductor 116b

· Rectifier 102b converter in dc current control

• Inverter converter 101b in constant γ (Minimum γ) control

Inverter Current Stabilization control: only Prop-part is activated

Converter tap transformer change control:

o Rectifier 102b: keep aclose to OLN

o Inverter: monitors the operating γ measured and keep it close to YN. It is assumed that dc voltage control is performed by the other Pole

Latency time is an important parameter to be considered while tuning the control. Reasonable latency time seem to be not greater than 5 ms for a proper adjusting of the control.

Having the new control loop activated it introduces a dynamic

compounding into the second section of the Udc/Idc characteristic, leading to a stable point of crossing between the rectifiers and inverters characteristics. This dynamic compounding is created along the second section and is influenced by the error between measured dc current and current order made of every inverting converter. The cascade principle is also applied in this approach of parallel inverters.

The dynamic compounding is effective only when the inverters are operating along the second section of the U/I characteristic. If, for any reason, an inverter is switched to current control mode (during disturbances or after sudden change in the ac voltage level, resulting the point of crossing between converter characteristics is the vertical line) the control loop forming the current stabilization control function becomes inactive.

It can be seen that an operating window is opened where current stabilization control can be performed. By adjustment of the currents in this operating window, the current flow between the inverting converters loia, loib may become more balanced, or the difference between the currents through the inverting converters 101a, 101b may become less, as compared to using a control scheme for operation of the inverting converters 101a, 101b. Thus, by adjustment of the currents through the inverting converters 101a, 101b, occurrence of imbalance between the currents through the respective inverting converters 101a, 101b may be reduced or even eliminated. This control thus improves the steady-state, dynamic and transient stability of parallel inverters. .

When parallel rectifiers are controlling the injected current into the dc line (operating firing angle close to normal operating range - operation in the vertical line of their static characteristic) and the parallel inverters are operating close to normal extinction angle range (operating along the normal negative slope characteristic, resulting that the inverters are receiving the injected current from the dc line), the inverter Stabilization Control on each inverter will be able to perform its functionality, and will able to distribute the current among the inverters as well as stabilizing the operation of the individual inverter. Another advantage is that the rating and costs of the converters may be optimized.

In the example given above, a voltage control window and tap changer control to keep the dc voltage within the voltage control widow was described. It should however be realized that as an alternative voltage control can be implemented using firing angle control, which in the example given above would be a part of the Converter Firing Control iioa, where in this case, it actuates direct in firing angle of the control of converter valve 101a. This means that, the tap changer control 112a would only operate to keep the extinction angle in the converter 101a within an operating range that is made up by the extinction angle window, which is established during the design of the converter.

One or more of the above described control units including the superior control unit may together form a converter control system. The control units may be realized in the form of one or more discrete components. However, they may also be implemented in the form of one or more processors with accompanying computer readable means, such as a computer program memory, comprising computer program code that performs the desired functionality when being run on the one or more processors. A computer program product comprising a computer-readable means carrying such code can be provided as a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into one or more control units of the converter control system. One such data carrier in the form of a CD Rom disk 901 carrying computer program code 902 is shown in fig. 9.

While the present invention has been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed

embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.

Claims

CLAIMS l. 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 (n6a, n6b), 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;

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) 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) an extinction angle window (yiow - Yhigh) for the inverting converters;

controlling (205) the extinction angle of the inverting converters via tap changer control;

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;

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, 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 l, 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 one of claims 1-3, wherein the power transmission system comprises at least two rectifying converters (102a, 102b) connected in parallel at a second end of the dc link, 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 and further comprising setting (203) a firing angle window (ai_ow - ahigh) for the rectifying converters (102a, 102b and controlling (205) the firing angle of the rectifying converters via tap changer control, wherein the applying of the current stabilization control scheme is based on a fulfillment of a control condition that the rectifying converters operate in the firing angle window.

5. 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.

6. A method according to claim 5, wherein the adjusting of the current comprises applying proportional control on the difference between the current errors.

7. A method according to claim 6, wherein the adjustment is made based on Ay _ Bal_order = i_y^l l^Id _own ^~ I _own ) -∑ (ld_par - Io_par )] Gl .

8. A method according to claim 7, 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.

9. 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 and a second transformer (106a, 106b), and

at least two inverting converters (101a, 101b) connected in parallel at a first end of the direct current, dc, link (116a, 116b);

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 said at least one control unit (108, 110a, 110b, 112a, 112b) is 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, control the extinction angle of the inverting converters via tap changer control;

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.

10. The power transmission system according to claim 9, 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.

11. The power transmission system according to claim 9 or 10, 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.

12. The power transmission system according to any one of claims 9

- 11, further comprising a third and a fourth transformer (105a, 105b) and at least two rectifying converters (102a, 102b) connected in parallel at a second end of the dc link, 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) , the turns ratio of the third and fourth transformers are controllable via tap changer control, said at least one control unit is further configured to set a firing angle window (ai_ow - ahigh) for the rectifying converters (102a, 102b) and control the firing angle of the rectifying converters via tap changer control and 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.

13. The power transmission system according to any of claims 9 - 12, 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.

14. 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), 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;

the converter control system comprising at least one control unit (108, 110a, 110b, 112a, 112b) 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;

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

15. A computer program product adapted to be executed in a converter control system(io8, 110a, 110b, 112a, 112b, 114a, 114b) 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), 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;

the computer program product comprising computer-readable means carrying computer program code configured to, when executed in the converter control system, cause the converter control system 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;