WO2018041352A1

WO2018041352A1 - Direct current handling in an interface arrangement

Info

Publication number: WO2018041352A1
Application number: PCT/EP2016/070607
Authority: WO
Inventors: Sri Ramya KALLURI; Gaurav-Kumar KASAL
Original assignee: Abb Schweiz Ag
Priority date: 2016-09-01
Filing date: 2016-09-01
Publication date: 2018-03-08

Abstract

An interface arrangement for connection between a first Alternating Current (AC) power system (ACS1) and a second power system comprises a first converter (14), converting between alternating current (AC)and direct current (DC),and having an AC side and a DC side, where the DC side is connected to a DC network (DCN),an AC line (L1) connecting the first power system to the AC side of the first converter (14) and at least one current sensor (20) sensing the currents of the AC line (L1). The first converter (14) comprises a set of converter valves and a control unit operative to control the set of converter valves to generate at least one AC waveform on the AC line (L1) and to remove DC current components of the currents (iva1, ivb1, ivc1) being sensed by the at least one current sensor(20) in the AC line.

Description

DIRECT CURRENT HANDLING IN AN INTERFACE ARRANGEMENT

FIELD OF INVENTION The present invention generally relates to voltage source converters. More particularly the present invention relates to an interface arrangement, as well as to a method and computer program product for controlling a converter of the interface arrangement. BACKGROUND

Converters are often used for converting between alternating current (AC) and Direct Current (DC), such as between three-phase AC and DC. A converter is then typically provided in an interface arrangement, such as a converter station, and made up of a number of phase legs. Typically there is thus one phase leg for each phase and each phase leg comprises two phase arms, an upper and a lower phase arm connected to an AC line. Such a converter station may be a High Voltage Direct Current (HVDC) converter station.

It is in some power transmission situations of interest to remove the transformer in the converter station with which the voltage source converter is connected to an AC power system, often denoted an AC grid. There may exist many reasons for this. One reason may be that the converter already converts to the voltage used by the AC power system.

There may thus be no need for any further adaption. An additional transformation may therefore be unnecessary from a voltage level adaption point of view. Furthermore, a transformer may in these cases also be big, bulky and expensive and it would therefore also be of interest to remove it for this reason. Another reason for removing the transformer is for limiting losses.

There may thus exist several reasons for removing a transformer. A converter station comprising a converter that is in this way connected to an AC system is often denoted a transformerless converter station. One example of a transformerless converter station is given in WO

However, the removal of the transformer may lead to other problems. One problem that may occur is the risk of introducing DC currents on the AC side of the converter.

A DC current can appear in the converter phases due to many reasons. Some of the important causes are:

• If AC and DC lines are running in parallel, induction from the nearby AC lines may cause an AC voltage to appear on the DC side of the converter, which in turn appears as a DC current in the converter phases.

Mismatch of circuit parameters between upper and lower arms of the converter.

Solar storms leading to disturbances in magnetic field of the Earth and which induce DC currents, called Geo-magnetically Induced Currents (GIC) in the converter phases.

DC pole voltage imbalance or pole-ground faults.

• Measurement errors leading to DC offset in the AC converter voltages etc. DC currents may thus occur for various reasons.

These DC currents are problematic in transformerless interface

arrangements in that they are passed on to the AC system, where they appear as unwanted harmonics and saturate nearby transformers. Hence, these DC currents have to be handled and prevented from entering into the

AC system. However, they may also be problematic in interface arrangements comprising transformers. In an HVDC interface arrangement comprising a transformer, the DC current generated or induced in the converter phases, is blocked by the transformer. However, if the value is above a limit, the transformer core may saturate, which can lead to problems such as excessive noise, increase in losses, large magnetizing currents etc.

A few methods of preventing dc current flow in the converter phases have been proposed. WO 2013/044940 does for instance suggest the use of series capacitors for blocking DC currents.

Other known suggestions include the use of bypass devices such as zigzag transformers or star reactors to bypass the DC current. However, series capacitors are found to create stability issues and ferro-resonance issues in the interface arrangement. Also, zig-zag transformers are not

commercially available for use in transmission level voltages and hence other means are preferred.

Some control schemes have been devised in order to remove DC offsets in magnetizing currents in a transformer. Examples of this can be found in US 6577111 and WO 2015/139743.

There are also schemes for reducing GIC currents, see for instance WO

Common for these schemes for removing DC offsets in magnetizing currents and reducing GIC currents is that they are all closely linked to the transformer either in respect of the current being measured or sensed, through the control being performed in the transformer or both.

It would therefore be of interest to obtain a DC current control that allows a higher degree of freedom from the type of interface arrangement used, i.e. that can be used for both transformer less or transformer based interface arrangements and that does not require any additional components.

The present invention is directed towards this type of DC current removal.

SUMMARY OF THE INVENTION

The present invention is directed towards the removal of DC currents in an AC line of an interface arrangement.

This object is according to a first aspect achieved through an interface arrangement for connection between a first and a second power system, where the first power system is a first Alternating Current (AC) system, the interface arrangement comprising:

a first converter, converting between alternating current (AC) and direct current (DC), and having an AC side and a DC side, the DC side being adapted to be connected to a DC network,

an AC line adapted to connect the first power system to the AC side of the first converter, and

at least one current sensor sensing the currents of the AC line,

the first converter comprising a set of converter valves and a control unit operative to control the set of converter valves to generate at least one AC waveform on the AC line and to remove DC current components of the currents being sensed by the at least one current sensor in the AC line.

This object is according to a second aspect achieved through a method of controlling a converter of an interface arrangement interconnecting two power systems, where the first power system is a first alternating current (AC) power system and the converter comprises a set of converter valves, converts between alternating current (AC) and direct current (DC) and has an AC side and a DC side, the AC side being connected to the first power system via an AC line and the DC side being connected to a DC network, the method being performed in a control unit of the converter and comprising:

controlling the set of converter valves to generate at least one AC

waveform on the AC line and to remove DC current components of currents being sensed in the AC line by at least one current sensor.

The object is according to a third aspect achieved through a computer program product for controlling a converter of an interface arrangement interconnecting two power systems, where the first power system is a first alternating current (AC) power system and the converter comprises a set of converter valves, converts between alternating current (AC) and direct current (DC) and has an AC side and a DC side, the AC side being connected to the first power system via an AC line and the DC side being connected to a DC network, the computer program product comprising a data carrier with computer program code configured to cause a control unit of the converter to

control the set of converter valves to generate an AC waveform on the galvanic connection and to remove DC current components of currents being sensed in the AC line by at least one current sensor.

The present invention has a number of advantages. It allows the DC components to be completely eliminated. It does not create any stability problems. Furthermore, the control does not depend on the use of a transformer and is therefore flexible. It can thus be used for transformer- based as well as transformer less interface arrangements. There is no need for any additional components and the control can be easily integrated into existing control system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where fig. l schematically shows a DC network comprising a DC power line connected between two converter stations both being transformerless and each connected to a corresponding AC power network,

fig. 2 schematically shows a first converter of the first converter station, fig. 3 schematically shows a control unit for controlling the first converter, fig. 4 schematically shows a first part of a dc current reduction control module in the control unit,

fig. 5 schematically shows a second part of the DC current reduction module, and

fig. 6 schematically shows a computer program product in the form of a data carrier comprising computer program code for implementing the control unit.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given.

Fig. l shows a single line diagram of a first converter station 10 and a second converter station 12 interconnected by a direct current network DCN comprising a Direct Current (DC) line 18, where the direct current network DCN may be a High Voltage Direct Current (HVDC) network. Thereby the first and second converter stations may also be HVDC converter stations. The first converter station 10 comprises a first

Alternating Current (AC) line Li and a first converter 14 for converting between AC and DC, where the first converter 14 comprises an AC side and a DC side. In a similar manner the second converter station 12 comprises a second AC line L2 and a second converter 16 for converting between AC and DC, where the second converter 16 also comprises an AC side and a DC side. Thereby a first end of the direct current network DCN, which in this case is also a first end of the DC line 18, is connected to the DC side of the first converter 14 and a second end of the DC network, also here in the form of the DC line 18, is connected to the DC side of the second converter 16. It can thereby be seen that the DC side of the first converter is adapted to be connected to the DC network DCN. The AC side of the first converter 14 is furthermore connected to a first AC system ACSi via the first AC line Li, while the AC side of the second converter 16 is connected to a second AC system ACS2 via the second AC line L2. It can thereby be seen that the first AC line Li is adapted to connect the first AC system ACSi to the AC side of the first converter 14 and that the AC side of the second converter 16 is adapted to be connected the second AC system ACS2 via the second AC line L2.

The DC network DCN may be a transmission network covering long distances, for instance in order to transfer power over these long distances. However, it may also be small and provided for back-to-back

communication between the two converters 14 and 16, in which latter case the two converter stations may form a joint converter station also comprising the DC network DCN.

Aspects of the invention are directed towards an interface arrangement between two power systems of which the first AC network is a first power system. In case of the DC network DCN being a DC transmission network, for instance covering long distances, then the first converter station 10 comprising the first AC line Li and the first converter 14 in essence make up the interface arrangement, with the DC network DCN is the second power system.

However, in case the converters are used in a back-to-back configuration, the second AC system ACS2 is the second power system. Furthermore, then the interface arrangement comprises the first AC line Li, the first converter 14, the second converter 16, the second AC line L2 and the DC network DCN. The interface arrangement is in this case thus made up of the first and second converter stations 10 and 12 together with the DC network DCN. Moreover, it is possible that either the first, the second or both converter stations are transformerless, i.e. lack transformers, or are equipped with transformers. In case of transformerless converter stations, then the first AC line Li is galvanically connected to the first AC system ACSi and the second AC line L2 is galvanically connected to the second AC system ACS2, while if there are transformers, then the first AC line Li would be connected to a secondary side of a first transformer, the primary side of which would be connected to the first AC system ACSi. In a similar manner the second AC line L2 would be connected to a secondary side of a second transformer, the primary side of which would be connected to the second AC system ACS 2. Here it may be mentioned that it is also possible that one of the converter stations is transformerless, while the other is not.

Furthermore, an interface arrangement also comprises at leads one current sensor 20 in the first AC line Li for sensing at least one current in this first AC line Li, i.e. for sensing the currents of the first AC Line Li, where such a sensor may be a Hall sensor or an optical current

transformer (DCOCT). It is possible that also the second AC line L2 comprises at least one current sensor. Furthermore the AC systems may be three-phase systems, in which case the first AC line Li would comprise three phases and the sensor 20 could be arranged to sense currents in reach of the phases. The currents ivai, ivbi and ivci of the different phases in the first AC line Li being sensed by the at least one current sensor 20 are also reported to the first converter 14. The AC lines may also be considered to form links between the converters and the corresponding AC systems. When the converters are

transformerless, the first and second AC lines Li and L2 may therefore be galvanic links. A galvanic link is a link in which, at least during steady state operation, all elements are electrically and physically in contact with each other. Each element also has a physical and electrical path within itself. Thereby there are no gaps in the electrical path, such as magnetic or capacitive gaps. However, the galvanic link may comprise a circuit breaker. This may be opened in case of faults for instance in the DC system or the first converter. However, during steady state operation such a circuit breaker is closed.

As can be seen above, there is no transformer connected to the first and second AC lines Li and L2 when there are galvanic links.

In the following the first converter 14 and the first AC line Li will be described. It should however be realized that this description may just as well be provided for the second converter 16 and the second AC line L2.

Fig, 2 shows one way of realizing the first converter 14. The first converter 14 may be a three-phase voltage source converter for converting between AC and DC. The first converter 14 therefore comprises three phase legs PLi, PL2 and PL3, for instance connected in parallel between a first and a second DC terminal DCi and DC2, where the first DC terminal DCi may be connected to a first pole of the DC network and the second DC terminal DC2 may be connected to a second pole of the DC network or to ground. Each phase leg furthermore comprises a set of converter valves, which in this example is a pair of converter valves. The first phase leg PLi therefore comprises a first and a second converter valve CVAi and CVA2, the second phase leg comprises a first and a second converter valve CVBi and CVB2 and the third phase leg PL3 comprises a first and a second converter valve CVCi and CVC2. The mid points of the phase legs are connected to corresponding AC terminals ACi, AC2, AC3, where each AC terminal is connected to a corresponding phase of the first AC line. A phase leg is in this example divided into two halves, a first upper half and a second lower half, where such a half is also termed a phase arm. The first DC pole furthermore has a first potential that may be positive. The first pole may therefore also be termed a positive pole. A phase arm between the first DC terminal DCi and a first, second or third AC terminal ACi, AC2 and AC3 may be termed a first phase arm or an upper phase arm, while a phase arm between the first, second or third AC terminal ACi, AC2 and AC3 and the second DC terminal DC2 may be termed a second phase arm or a lower phase arm. The phase arm mid points are

furthermore connected to the AC terminals via phase reactors LACi, LAC2 and LAC3.

Moreover, the upper phase arms are joined to the first DC terminal DCi via a corresponding first or upper arm reactor LAi, LBi and LCi, while the lower phase arms are joined to the second DC terminal DC2 via a second or lower arm reactor LA2, LB2 and LC2.

The first voltage source converter 14 may be a two-level converter, where each converter valve is made up of a number of series connected switching units. Alternatively the converter may be a modular multilevel converter where each converter valve is formed through a series-connection of a number of cells, where a cell may be a half-bridge cell or a full-bridge cell. A cell then comprises one or two strings of series connected switching units in parallel with an energy storage element like a capacitor. A switching unit may be realized in the form a transistor with anti-parallel diode. However, it is also known to be realized using other types of semiconducting units.

It should here also be realized that there exist countless variations of voltage source converters, where a converter may for instance be an n-level converter, such as a neutral point clamped three-level converter. Also a modular multilevel converter may be made up of a number of different types of cells. There may also exist hybrid converters that use cells in an n- level environment. As an alternative to a voltage source converter, the converter may also be a current source converter.

There is finally a control unit 22, which controls the operation of the converter 14 and more particularly controls each converter valve. In this control the measured currents are used. The control unit 22 is provided for controlling all the phase arms of the converter. However, in order to simplify the figure only the control of the first convert valve CVAl of the upper phase arm of the first phase leg PL is indicated. This control is performed using the current ivai measured in the first phase of first AC line and therefore only this current is shown as well. The current ivai is also used in the control of the second converter valve CVA2 of the lower phase arm (not shown). It should be realized that all converter valves are controlled by the control unit 22. The control unit 22 may be implemented through a computer or a processor with associated program memory or dedicated circuit such Field-Programmable Gate Arrays (FPGAs).

Fig. 3 shows a block schematic of one way of realizing the control unit 22. The control unit 22 comprises a waveform control module WFC 24 and a DC current reduction module DCCR 26.

The phase arms of a phase leg generate a waveform on the corresponding AC terminal, which waveform provides an AC voltage that is supplied to the first AC system ACSi. This waveform is generated through the control of the waveform control unit 24.

As mentioned earlier the first converter station 10 may be transformerless. There may thus be no transformer between the first AC system ACSi and the first converter 14.The first AC line Li may therefore be galvanically connected to the first AC system ACSi. This has a number of advantages.

However, it may also give rise to some problems. One problem being addressed here is that it is possible that DC currents may be present on the first AC line. A DC current may have several different causes, some of which are mentioned below.

• If AC and DC lines are running in parallel, the AC lines may induce AC voltages in the DC Line, which in turn may cause as DC currents to appear in the converter phases.

Mismatch of circuit parameters between upper and lower arms of the converter.

DC pole voltage imbalance or pole-ground faults.

• Measurement errors leading to DC offset in the AC converter voltages etc.

If there is a transformer in the first converter station 10, the DC current generated or induced in the converter phases, may be blocked by this transformer. However, if the value of the current is above a limit, the transformer core may saturate, which can lead to problems such as excessive noise, increase in losses, large magnetizing currents etc.

In a transformer less system, the DC current is passed on to the first AC system ACSi, which leads to unwanted harmonics and saturates nearby transformers. Hence, this DC current has to be controlled by some means and prevented from entering in to the AC system ACSi.

Both problems, transformer core saturation and DC current injection, may be addressed through reducing the DC components of the currents on the first AC line Li.

Aspects of the invention are therefore directed towards controlling the converter so that such DC components of the currents on the first AC line Li are reduced or even eliminated, How this may be done will now be described.

The waveform control module 24 of the control unit 22 controls the converter valves so that an AC waveform is generated on each AC terminal ACi, AC2, AC3, where the waveform on an AC terminal may in a known fashion be separated by 120 degrees from the waveforms on the other AC terminals. In this control it is possible that the waveform control module 24 performs pulse width modulation control, such as Sinusoidal Pulse Width Modulation (SPWM) or 3PWM, where 3PWM is typically not used in a transformerless interface arrangement. In doing this a converter valve may receive a control signal representing a voltage level that it is desired to output by the converter valve. However, as was mentioned above, it is also possible that the currents on the first AC line Li include DC components that may be desirable to cancel out. The DC current reduction module 26 is provided for such DC component reduction. The waveform control module 24 thus controls the set of converter valves of each phase leg to generate an AC waveform on the phases of the first AC line Li, while the DC current reduction module 26 removes at least one DC current component of a phase current appearing on the same phase. It thus removes the DC current components of the currents sensed by the current sensor.

Now one way of reducing the DC components of the different phases in the first AC line Li will be described with reference being made also to fig. 4 and 5, where fig. 4 schematically shows a first part 26A of the DC current reduction module 26 and fig. 5 schematically shows a second part 26B of the DC current reduction module 26. The operation starts by measuring the currents in the first AC line Li, such as in the individual phases or galvanic connections of the first AC line Li. The current may as an example be measured at the interface between the first AC line ACi and the first AC terminal ACi of the converter 14 or at the interface at which the first AC line Li is connected to the first AC system ACSi (such as at a circuit breaker interconnecting the interface

arrangement with the first AC system ACSi).The measurements are made by the sensor 20 and provided to the first part 26A of the DC current reduction module 26. In this way a first phase current ivai, a second phase current ivb and a third phase current ivc are being obtained by the DC current reduction module 26. Each current is then low pass filtered in a corresponding low pass filter in order to filter out any AC components and to obtain a low pass filtered current only comprising DC components. The filter may therefore be set to a frequency below the fundamental frequency of the AC line Li in order to guarantee such low pass filtering and a filter may be set to the fundamental frequency divided by 5. This would lead to a cut-off frequency of 10 Hz for a fundamental frequency of 50 Hz. It can be seen in fig. 4 that a first phase current ivai is lowpass filtered by a first lowpass filter LPF 28A in order to obtain the DC current component ivaoi of the first phase. In a similar manner a second phase current ivbi is lowpass filtered by a second lowpass filter LPF 28B in order to obtain the DC current component ivboi of the second phase and a third phase current ivci is lowpass filtered by a third lowpass filter LPF 28C in order to obtain the DC current component ivcoi of the third phase.

The DC current components ivaoi, ivboi and ivcoi are then to be compensated, which takes place in the second part 26B of the DC current reduction module 26. As can be seen the DC component ivaoi of the first phase is provided to a negative input terminal of a first subtracting element 30A. The first subtracting element 30A also has a positive terminal on which it receives a desired first phase DC component, which in this case is set to zero since it is desirable to eliminate the DC component. The first subtracting element 30A subtracts the sensed DC current component ivaoi from the desired DC current component and provides the difference to a first controller 32A, which is a PI controller performing proportional and integrating control with respect to the difference in order to provide a current removing control signal rca for the first phase. The first controller 32A thus applies proportional and integrating control on the DC current component ivaoi in order to obtain the current removing control signal rca.

In a similar manner the DC component ivboi of the second phase is provided to a negative input terminal of a second subtracting element 30B. The second subtracting element 30B also has a positive terminal on which it receives a desired second phase DC component, which in this case is also set to zero. The second subtracting element 30B subtracts the sensed DC current component ivboi from the desired DC current component and provides the difference to a second controller 32B, which is also a PI controller performing proportional and integrating control with respect to the difference in order to provide a current removing control signal rcb for the second phase. The second controller 32B thus applies proportional and integrating control on the DC current component ivboi in order to obtain the current removing control signal rcb.

Also the DC component ivcoi of the third phase is provided to a negative input terminal of a third subtracting element 30C. The third subtracting element 30C also has a positive terminal on which it receives a desired third phase DC component, which in this case is likewise set to zero. The third subtracting element 30C subtracts the sensed DC current

component ivcoi from the desired DC current component and provides the difference to a third controller 32C, which is also a PI controller

performing proportional and integrating control with respect to the difference in order to provide a current removing control signal rcc for the third phase. The third controller 32C thus applies proportional and integrating control on the DC current component ivcoi in order to obtain the current removing control signal rcc. The control signals are thus signals that remove the DC components on all phases of the first AC line Li. Furthermore, although PI controllers are used in the described embodiment, it should be realized that it is just as well possible to use P controllers, i.e. controllers only employing

proportional control.

The control signals rca, rcb and rcc may with advantage be combined with the waveform control of the phase legs and therefore the control signals may be in the form of voltages, which may then be combined with a control signal used by the waveform control module for controlling a phase arm. The control signal may as an example be added to the arm modulation indices, i.e. the different waveform control signals formed by the waveform control module 24 for use in the control of the converter valve of a phase arm.

Therefore, as can be seen in fig. 5, the first current removing control signal rca is supplied to a first positive terminal of a first adding element 34A, where the first adding element 34A has a second positive terminal on which it receives a voltage forming reference signal rpa, often termed modulation index, used in the upper phase arm of the first phase leg. The voltage forming reference signal rpa is here generated by the waveforming control module 24. The first adding element 34A then sums the two control signals in order to obtain a first waveforming and DC current removing control signal rpai for controlling the valves of the upper phase arm of the first phase leg. In a similar manner, the second current removing control signal rcb is supplied to a first positive terminal of a second adding element 34B, where the second adding element 34B has a second positive terminal on which it receives a voltage forming reference signal rpb used in the upper phase arm of the second phase leg. Also this voltage forming reference signal rpb is generated by the waveforming control module 24. The second adding element 34B sums the two control signals in order to obtain a second waveforming and DC current removing control signal rpbi for controlling the valves in the upper phase arm of the second phase leg. Finally the third current removing control signal rcc is supplied to a first positive terminal of a third adding element 34C, where the third adding element 34C has a second positive terminal on which it receives a voltage forming reference signal rpc, of the upper phase arm of the third phase leg. Also this voltage forming reference signal rpc is generated by the waveforming control module 24. The third adding element 34C sums the two control signals in order to obtain a

waveforming and DC current removing control signal rpci for controlling the valves of the upper phase arm of the third phase leg.

Fig. 4 and 5 thus show how the control signals for the upper phase arms are obtained. Control signal may be obtained for the lower phase arms in a similar way. However, in this case the first, second and third adding elements 34A, 34B and 34C are replaced by subtracting elements where the current removing control signals are subtracted from the voltage forming reference signals of the lower arms in order to obtain

waveforming and DC current removing control signals for controlling the valves of the lower phase arms.

It can in this way be seen that the DC components of the AC line currents are controlled to zero using PI control at the same time as waveforming is carried out. This has a number of advantages. Control is a good alternative to prevent

Dc current injection. It allows the DC components to be completely eliminated. The control technique is more suitable for eliminating DC injection currents than blocking devices, such as series capacitors as it does not create any stability problems. Since the control is applied separately to three phases, it also works well for unequal currents.

Furthermore, the control does not depend on the use of a transformer and is therefore flexible. It can thus be used for transformer-based as well as transformer less HVDC interface arrangements. There is no need for any additional components and the control can be easily integrated in to the existing control system.

The amplitude of the AC currents may be high, while the DC components may be fairly low. Therefore, the sensors used may need to have a high accuracy. They may need to measure a small value of DC current accurately in large AC converter currents. They may, as an example, need to be able to sense current levels that are at ι - 2% of the total current levels. Sensors that have shown to have sufficient accuracy are optical current transformer (DCOCTs) and Hall Effect sensors.

The DC current removal was described in relation to the first converter. As was mentioned earlier it is also possible that the same type of DC current removal is performed in the second converter.

The control unit may be realized in the form of discrete components, such as FPGAs. However, it may also be implemented in the form of a processor with accompanying program memory comprising computer program code that performs the desired control functionality when being run on the processor. A computer program product carrying this 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 a control unit of a voltage source converter. One such data carrier in the form of a CD Rom disk 36 carrying computer program code 38 is shown in fig. 6.

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

1. An interface arrangement for connection between a first and a second power system, where the first power system is a first Alternating Current (AC) system (ACSi), the interface arrangement comprising:

a first converter (14), converting between alternating current (AC) and direct current (DC), and having an AC side and a DC side, said DC side being adapted to be connected to a DC network (DCN),

an AC line (Li) adapted to connect the first power system to the AC side of the first converter (14), and

at least one current sensor (20) sensing the currents of the AC line (Li), the first converter (14) comprising a set of converter valves (CVAl, CVA2, CVBi, CVB2, CVCi, CVC2) and a control unit (22) operative to control the set of converter valves (CVAl, CVA2, CVBi, CVB2, CVCi, CVC2) to generate (24) at least one AC waveform on the AC line (Li) and to remove DC current components (ivaoi, ivboi, ivcoi) of the currents (ivai, ivbi, ivci) being sensed by the at least one current sensor (20) in the AC line .

2. The interface arrangement according to claim 1, the control unit (22) when controlling the set of converter valves to remove DC current components is operative to apply at least proportional control on the DC current components (ivaoi, ivboi, ivcoi) in order to obtain a current removing control signal (rca, rcb, rcc) for the valves.

3. The interface arrangement according to claim 2, wherein the control unit is further operative to perform integrating control together with the proportional control.

4. The interface arrangement according to claim 2 or 3, wherein the control unit (22) is further operative to combine the current removing control signal (rca, rcb, rcc) with a voltage forming reference signal (rpa, rpb, rpc) in order to obtain a waveforming and DC current removing control signal(rpai, rpbi, rpci).

5. The interface arrangement according to any previous claim, wherein the control unit is operative to low pass filter the sensed currents (ivai, ivbi, ivci) in order to obtain the DC current components (ivaoi, ivboi, ivcoi) .

6. The interface arrangement according to any previous claim, wherein the AC line (Li) is adapted to be galvanically connected to the first power system.

7. The interface arrangement according to any of claims 1 - 5, further comprising a transformer interconnecting the first power system with the AC line.

8. The interface arrangement according to any previous claim, wherein the DC network (DCN) is the second power system.

9. The interface arrangement according to any of claims 1 - 7, further comprising the DC network (DCN) and a second converter (16) converting between AC and DC and having an AC and a DC side, the DC network having a first end connected to the DC side of the first converter (14) and a second end connected to the DC side of the second converter (16), where the AC side of the second converter is adapted to be connected to a second AC system (ACS2), said second AC system being the second power system.

10. A method of controlling a converter (14) of an interface arrangement interconnecting two power systems, where the first power system is a first alternating current (AC) power system (ACSi) and the converter (14) comprises a set of converter valves (CVAl, CVA2, CVBi,

CVB2, CVCi, CVC2) , converts between alternating current (AC) and direct current (DC) and has an AC side and a DC side, the AC side being connected to the first power system via an AC line (Li) and the DC side being connected to a DC network (DCN), the method being performed in a control unit (22) of the converter and comprising:

controlling the set of converter valves (CVAl, CVA2, CVBi, CVB2, CVCi, CVC2) to generate at least one AC waveform on the AC line (Li) and removing DC current components (ivaoi, iboi, ivcoi) of currents (ivai, ivbi, ivci) being sensed in the AC line by at least one current sensor (20).

11. The method according to claim 10, wherein the control of the set of converter valves to remove the DC current components comprises applying (β2Α,32Β, 32C) at least proportional control on the DC current components in order to obtain a current removing control signal (rca, rcb, rcc) for the valves.

12. The method according to claim 11, further comprising performing integrating control together with the proportional control.

13. The method according to claim 11 or 12, further comprising combining (34A, 34B, 34C) the current removing control signal (rca, rcb, rcc) with a voltage forming reference signal (rpa, rpb, rpc) in order to obtain a waveforming and DC current removing control signal (rpai, rpbi, rpci) .

14. The method according to claim 11, further comprising low pass filtering (28A, 28B, 28C) the sensed currents (ivai, ivbi, ivci) in order to obtain the DC current components (ivaoi, ivboi, ivcoi).

15. A computer program product for controlling a converter (14) of an interface arrangement interconnecting two power systems, where the first power system is a first alternating current (AC) power system (ACSi) and the converter (14) comprises a set of converter valves (CVAl, CVA2,

CVBi, CVB2, CVCi, CVC2), converts between alternating current (AC) and direct current (DC) and has an AC side and a DC side, the AC side being connected to the first power system via an AC line (Li) and the DC side being connected to a DC network (DCN), the computer program product comprising a data carrier (36) with computer program code (38) configured to cause a control unit (22) of the converter (14) to

control the set of converter valves (CVAi, CVA2, CVBi, CVB2, CVCi, CVC2) to generate an AC waveform on the galvanic connection and to remove DC current components (ivaoi, ivboi, ivcoi) of currents (ivai, ivbi, ivci) being sensed in the AC line by at least one current sensor (20).