SE1750290A1 - Interface arrangement between a first and a second power system - Google Patents

Abstract

An interface arrangement () for connection between a first power system and a second power system () comprises a transformer () having a primary winding PW and a secondary winding SW, where the secondary winding is connected to a converter () connected to the second power system (), a string of cascaded converter blocks (A,B,C,D,E), where the string is connected in parallel with the primary winding (PW) of the transformer (), each converter block comprising a converter module () for connection to a corresponding power entity () of the first power system, an energy storage element (8) and a waveshape contributing module (6), and a control unit () controlling the waveshape contributing modules of the converter blocks of a string to form a first voltage as a quasi two-level waveshape.Fig.i

Description

lO INTERFACE ARRANGEMENT BETWEEN A FIRST AND A SECONDPOWER SYSTEM FIELD OF INVENTION The present invention relates to an interface arrangement connectedbetween a first power system and a second power system as well as to a method of controlling such an interface arrangement.

BACKGROUND An upcoming trend for power converters for (direct-drive) wind generatorsis the application of modular designs. The "conventional" modularmultilevel converter (MMC) is however not well suited, as the alternatingcurrent (AC) frequency of the generator is low. A promising topology hasbeen described in EP2096732. The stacked-polyphase bridge topology canbe used in relation to direct current (DC) and allows generation of a DCvoltage in the medium voltage (MV) range, i.e. of an MVDC voltage, andthe fact that polyphase bridges are used (instead of single-phase) allows touse small DC-link capacitors. Three-phase modules are standard buildingblocks, which have a further positive impact on the cost. The modularity,and in particular if bypass switches are added, allows for high systemavailability, as operation with a failed module may be possible (provided for appropriate generator design).

However, MVDC is not suited for transmission of the power of e.g. anoffshore wind farm to land - high voltage direct current (HVDC) would beneeded. Thus, typically, a separate DC/ DC converter would need to beadded to convert power from medium voltage to high voltage level.Although direct generation of HVDC is possible with the existing topology,it would require high-voltage DC insulation inside the machine. As of today, this appears to be not feasible. lO Instead of a direct series connection of DC-link-capacitors of the polyphasebridges, one could add a full-bridge, allowing the capacitors to be insertedin either direction (or not at all) in a series connection. This would allow(single-phase) MVAC to be generated. This, by itself, is a well-knownsolution: the cascaded multi-level topology is being used for generation ofgrid-frequency MVAC, for connection of wind generators to an MVAC grid.However, this low AC frequency requires a significant increase in capacitorsize, and this solution still requires a bulky transformer and rectifier ifHVDC output is desired.

The use of a (3-phase) cascaded multi-level converter in combination withsingle or multi-phase bridge rectifiers for a wind generator is for exampledescribed in “A transformerless modular permanent magnet windgenerator system with minimum generator coils”, X. Yuan, Y. Li, J. Chai,APEC 2o1o.

Moreover, it is also possible that the transformer design requires that theMVAC frequency used is fairly low, such as below 1 kHz, which also has an impact on the capacitor size requirement.

There is in view of the above-mentioned prior art still a need forimprovements in relation to transmission of power to or from a powerentity in a first power system such as a to a windfarm generator in a power generating system.

SUMMARY OF THE INVENTION The present invention is directed towards providing improvements in the interface between a first power system and a second power system.

This object is addressed through the independent claims. lO BRIEF DESCRIPTION OF THE DRAWINGS The present invention will in the following be described with reference being made to the accompanying drawings, where f1g. 1 schematically shows a first realization of an interface arrangementconnected between a power generating system and a DC powertransmission system, f1g. 2 schematically shows a second realization of the interfacearrangement, f1g. 3 schematically shows a third realization of the interface arrangement,and f1g. 4 schematically shows a fourth realization of the interface arrangement.

DETAILED DESCRIPTION OF THE INVENTION In the following, a detailed description of preferred embodiments of the invention will be given.

The invention addresses the problem of providing power connectivitysolutions for power entities, such as power sources like windfarm gen GFEIÉOTS .

For offshore wind, but also for other types of (renewable) generator inremote locations, high-voltage direct current (HVDC) is a practical meansfor transporting the energy. Typical voltage levels could be from 100 to500 kV, depending on the size of the plant. Generators do however not produce energy at HVDC levels.

Electromagnetic (rotating or linear) generators, such as used for wind orhydro (e.g. wave), generate alternating current (AC) waveforms, such as AC voltages. Although generation at high voltage is possible (e.g. lO windformer, or designs using an iron-less stator), the required insulationmakes the generators bulky, costly and inefficient. Typically, peak (AC orDC offset) voltage is therefore limited to a maximum in the range of 10-20kV.

The AC voltages produced by these generators typically have a lowfrequency, e.g. up to 10 Hz - since for reliability reasons the use ofgearboxes is undesirable. Transforming these voltages to higher voltages istherefore not feasible, and even the choice of converter topologies islimited.

It may be desirable to use a modular converter design to connect thegenerator to an HVDC level. The main reasons for modularity are: - cost: the use of relatively standard, high-volume low voltage (LV)components or component assemblies (modules, drives) is cheaper thanthe use of (low-volume, special) high-voltage devices; - availability: the system can be designed to allow continued operation in case of failure of a single module.

For HVDC, Flexible Alternating Current Transmission Systems (FACTS),etc. and even certain drive applications, the Modular Multilevel Converter(MMC) is becoming the preferred choice. However, this topology is notwell suited for operation at low frequencies, as huge capacitors would beneeded. Furthermore, this topology does not extend the modularity intothe machine - a single failure in the generator would stop the complete wind mill.

An attractive modular converter is disclosed in EP2096732. Two versionsare proposed, one where a single machine winding is connected to a singlerectifier and DC capacitor, and another where multiple windings aretogether connected to a rectifier and DC capacitor. The latter solution maybe preferred - whereas a single winding produces a pulsating power, multiple windings combined can produce a constant power, which allows lO the capacitor to be dimensioned depending on the used switchingfrequency. As switching frequencies increase (introduction of wide band-gap devices), capacitor size is reduced. The use of 3-phase rectifiers isespecially advantageous, since 3-phase modules and drives are the industry standard.

The series connection of the DC-link capacitors of a number of modules, asin EP2096732, results in a total DC voltage of up to ca. 20 kV - due to thepreviously mentioned practical limit in machine winding insulation. Uponparallel connection of several wind generators (of e.g. 5-20 MW each), theresulting DC current would be too high (for equipment and efficiencyreasons). WOo1/52379A2 describes the related issues well, and proposes series connection of wind generators with their associated converters.

This however requires each of these to withstand the HVDC voltage, eitherin the machine windings or in transformer windings. The proposed use ofHV cables for the windings is not very attractive, and also leaves thequestion how safe maintenance in a single wind mill is possibly withoutpowering off all others. Series connection to reach HVDC is thus not an attractive solution.

Thus, each wind generator (or group of them, in a parallel connection) will require a converter to reach HVDC level.

At this point it may be noted that similar considerations apply not only forother generator systems with electromagnetic generators, but also forphoto voltaic (PV) plants, where a number of panels (possible withconverters) can be combined to generate power at a DC voltage of ca. 20kV but probably not much higher.

A possible solution is the use of a DC/ DC converter for each generator (or group of parallel connected generators). A conversion ratio of 5-25 would lO be needed (from ca. 20 kV to ca. 100-500 kV), which favors solutions containing a transformer (or multiple transformers).

Known solutions include the MMC and the dual active bridge, see forinstance “Isolated DC/ DC structure based on modular multilevelconverter”, S. Kenzelmann et al., IEEE Trans. Pow. El., Vol 30 No 1, Jan2015 and “Comparison of the modular multilevel DC converter and thedual-active bridge converter for power conversion in HVDC and MVDC grids”, S. Engel et al., IEEE Trans. Pow. El., Vol. 30 No 1, Jan 2015.

However, these solutions require capacitors in each of their modules,which increases the size and cost of the solution. Other solutions, which donot require these additional capacitors, are on the other hand not modular,see for example “High-power-density power conversion systems forHVDC-connected offshore wind farms”, A. Parastar, J -K Seok, Journal of Power Electronics, Vol 13 No 5, Sept. 2013.

It is therefore proposed to use an interface arrangement based on thecascaded multi-level topology for generation of medium-voltage quasi-twolevel AC, for instance in a frequency above 100 Hz but below 1 kHz, whichthen through a transformer is made into high-voltage AC, which with any known type of rectifier (e.g. MMC-based) can be transformed into HVDC.

The interface arrangement may either include single-phase or multiple-phase (3-phase) AC where all modules are composed of standard (LV)components or modules. Capacitor sizes are small- although if integratedenergy storage (such as for grid-fault ride through) is desired they may beincreased in size. The included transformer is small, since an AC frequencyin the range of 100 - 1 kHz and preferably 200 - 400 Hz is used. Themulti-level nature of the converter allows for generation of e.g. an arbitraryAC waveform, a sinusoidal or quasi two-level waveform while switchingonly at the fundamental AC frequency, so that both converter switching losses and transformer losses are kept low. lO The interface arrangement is thus expected to have low cost, high availability and high efficiency.

As an alternative, it is thus proposed the use of the cascaded multi-leveltopology to generate a medium-voltage sinusoidal or quasi two-levelwaveform as a first step for eventually reaching high-voltage DC, where asecond step is the conversion of the medium voltage to high voltage AC(HVAC) and a third and final step is the conversion of HVAC to HVDC.

An interface arrangement is furthermore connected between a first powersystem and a second power system, where the first power system may be apower generating system comprising at least one power entity such as abattery or an electric machine (or a winding group of an electric machine)like a windfarm generator and the second power system may be a powertransmission system. Thereby a power generating system may be a systembased on windmills and/ or solar panels. The power transmission systemmay in turn be a DC power transmission system such as an HVDC powertransmission system. As an alternative the first power system may be anelectric drive system where the power entity is an electric machine in the form of an electric motor.

An interface arrangement of this type is schematically shown in fig. 1.

The interface arrangement 10 may be considered to be an AC interfacearrangement 10 and may thus be provided for connection between electricmachines 11 of a power generating/ drive system and at least one converter 24 of a power transmission system 25.

The interface arrangement comprises a transforming element 22 fortransforming between a first and a second voltage and having at least oneprimary winding PW and at least one secondary winding SW, where thesecondary winding SW is connected to the converter 24. The interface arrangement 10 may essentially operate with AC voltages in the medium lO voltage range. In the example of fig. 1 the transforming element is a single phase transformer 22 that transforms between medium voltage AC (MVAC) such as in the range of 2 - 35 kV and high Voltage AC (HVAC), i.e. a voltage at or above 110 kV. The converter 24 being used may withadvantage be a converter converting between AC and DC and thereby theconverter 24 may be a part of the DC transmission system, which may be aHVDC system, for instance operating in a range of 1oo - 5oo kV. As analternative it is possible that the converter 24 is a part of the interfacearrangement. The converter 24 may be any type of suitable converter, suchas a current source converter (CSC) or a voltage source converter (VSC), for instance in the form of an MMC.

The interface arrangement 10 also comprises a least one string of cascadedconverter blocks 12A, 12B, 12C, 12D, 12E connected in parallel with acorresponding primary winding PW of the transformer 22. In the examplein fig. 1 the transformer 22 is a single-phase transformer only comprisingone primary winding PW. Therefore there is also only one string and this string is connected with the primary winding PW.

Each converter block 12B comprises a converter module 14, such as an LVrectifier module comprising at least one rectifying element RE1, RE2, RE3,RE4, RE5 and RE6, an energy storage element 18 and a waveshapecontributing module 16, where the waveshape contributing module may bea dual voltage polarity contributing module, such as a full-bridge moduleFB, sometimes also called H-bridge module. As an alternative thewaveshape contributing module may be a half-bridge (HB), which issometimes also called a half H-bridge module. In case the first powersystem is a power drive system, the converter module may be an inverter module comprising inverting elements.

In the example in fig. 1 the converter module 14 of a certain converterblock 12A is a rectifier bridge and the power entity is a power source and therefore there is a number of rectifier legs each comprising two rectifying lO elements, and where at least a part of a power source element is connectedto the midpoint of such a rectifier leg. If the power source element is anelectric machine, then an inductive element of this electric machine isconnected to the midpoint of the rectifier leg. In the example in fig. 1, theelectric machine is a three-phase machine and because of this there arethree rectifier legs, where a corresponding inductor of the machine, suchas a stator or rotor, is connected to the midpoint of a correspondingrectifier leg. The energy storage element 18, which in the example in fig. 1is a capacitor, is in turn connected across the output of the rectifier whichin this case is in parallel with the three rectifying legs. As an alternative the energy storage element is a battery.

The fiill-bridge module 16 is on the other hand made up of two strings ofswitching elements, where both strings are connected in parallel with theenergy storage element 18. The first string comprises a first and a secondswitching element S1 and S2, while the second string comprises a third anda fourth switching element S3 and S4. As can be seen the fiill bridgemodule 16 comprises two connection terminals TE1, TE2, where a firstconnection terminal TE1 is provided at a junction between the first andsecond switching elements S1 and S2 and a second connection terminal isprovided at a junction between the third and fourth switching elements S3and S4. Furthermore, the first connection terminal TE1 is connected to arespective connection terminal of a first neighboring converter block 12Aand the second connection terminal TE2 is connected to a respectiveconnection terminal of a second neighboring converter block 12C, whereboth the neighboring converter blocks 12A and 12C have the same configuration as the described converter block 12B.

In a half-bridge module there would only be one string of switchingelements, for instance the first string. In this case the second connectionterminal would be placed at a junction between a switching element of this string and the energy storage element. lO lO There is also a control unit 20 that is set to control the different conversionblocks 12A, 12B, 12C, 12D and 12E. This control more particularlycomprises controlling the switching elements of the full bridge modules.Depending on how the rectifier module is realized the control may also comprise a control of rectifying elements.

In fig. 1 both the rectifying elements and full-bridge switching elements arerealized in the form of switches with anti-parallel diodes, where a switchmay be a transistor such as an insulated-gate bipolar transistor (IGBT), ametal oxide semiconductor field effect transistor (MOSFET) or a similarelement such as an Integrated Gate-Commutated Thyristor (IGCT) usingeither silicon or other materials such as SiC, GaN. It may here be realizedthat as an alternative the rectifying elements may be realized only using diodes or thyristors.

The capacitors in each of the full-bridges of the cascaded converter blockscoincide with the DC-link capacitors of the stacked-polyphase bridgeconverter, so that no additional capacitors are needed. The rectifiermodule 14 and full-bridge module 16 thus use the same capacitor 18. Thefull-bridges of the converter blocks can be constructed using LVcomponents, similar or identical to those used in connection with the generator, which assures low cost.

Each fiill-bridge can assume four states, connecting the capacitor in eitherpositive or negative direction in series with others, or bypassing thecapacitor (in two possible ways, depending on the direction of current). Afull-bridge thus provides a voltage contribution that may be the positivecapacitor voltage, the negative capacitor voltage or a zero voltage. Thevoltage contribution thus has dual voltage polarities. A half-bridge does onthe other hand only have two states, connecting the capacitor in onedirection, such as positive, and bypass. Thus, for a half-bridge there areonly two states and one voltage polarity. In either case it is possible for the control unit 20 to create a sinusoidal AC waveform based on the capacitor lO ll voltages of the modules using the different switching elements.Furthermore, the waveform created in this way may have a highfundamental frequency for instance above 100 Hz. It would be of interestto use a fiindamental frequency that is as high as 1 kHz, i.e. significantlyhigher than the conventional frequency of 10 Hz used in windfarmapplications or the frequency of 50 Hz used in AC transmission systems.

However, this frequency may also be problematic.

The power flowing through the (single-phase) transformer will thereby bepulsating between zero and a maximum at twice the AC frequency. It isthis pulsation that determines the dimensioning of the capacitors in the converter modules.

Especially if transformer design constraints and/ or converter switchingfrequency constraints would limit the frequency to less than 1 kHz, such as200-400 Hz, this power pulsation requires a significant capacitor size -larger (measured in J /W) than what is common in standard three-phase COIIVCITCTS .

Because of this the modulation technique used by the control unit in thecontrol of the full-bridge modules may be a quasi two level modulationQ2L technique, where the modules are switched during a dwell time Ta.Thereby a total transition time TIT between the maximum and minimumvalues of the waveshape would be TIT = 2*(N-1)*Td, when the modules arehalf-bridge modules or TIT = (N-1)*Td when the modules are full-bridgemodules used for forming both positive and negative halves of thewaveshape. In both cases N is the number of converter modules. If thefundamental period T is much longer than this total transition time TIT,such as two, five or ten times longer, then a waveshape resembling a square wave is essentially obtained. lO 12 This type of control will result in a significant reduction of the requiredcapacitor size, which will also reduce the cost of the proposed converter topology.

It can thereby be seen that the interface arrangement 10 receives andstores energy related to the power entities (to be supplied to or receivedfrom the power entities) in the energy storage elements of the converterblocks of the strings, and that the control unit 20 controls the waveshapecontributing modules of the converter blocks of a string to form the firstvoltage as a quasi two-level waveshape having a fundamental frequency above 100 Hz and below 1 kHz, for instance in the range of 200 - 400 Hz.

The bypass functionality is useful also for fault cases, and eliminates theneed for a separate bypass switch. The control unit 20 therefore controlsthe waveshape contributing switching elements of a converter block to bypass this converter block.

The converter control may need to be equipped with a capacitor-voltagebalancing method. To a lesser degree, balancing can be done by controlling the power flow to or from the electric machine.

However, typically one would want all winding groups of the electricmachine to provide or receive at all times the same power. Balancing,which is also performed by the control unit 20, can thereforeadvantageously be done by choosing the switching instants of the full-bridges, and/ or by choosing which fiill-bridge is switched at a moment a switching is needed.

The fiill-bridges may be switched at the fundamental frequency of thehigh-frequency AC voltage, which means that switching losses can be keptlow. Additional switchings might be added by the control unit 20 in the part of the period where ac voltage and current (typically almost in phase) lO 13 are low, to reduce the required capacitor size. Such additional switchings, since occurring at low current, would not add much switching losses.

The relatively high switching frequency of the full-bridges, i.e. thefundamental AC frequency, allows to use small capacitors and a small transformer.

Fig. 2 schematically shows an alternative realization of an interfacearrangement. There is in this case a first number of machines connected toa first string of converter blocks 12A, 12B and 12C in parallel with theprimary winding of a first transformer 22A, the secondary winding ofwhich is connected to a DC transmission system 25 via a first converter24A. Furthermore, there is also a second number of machines connected toa second string of converter blocks 12D, 12E and 12F in parallel with theprimary winding of a second transformer 22B, the secondary winding ofwhich is connected to the same DC system 25 via a second converter 24B.The converters 24A, 24B may in this case be connected in parallel betweentwo poles of the DC system 25 or a pole and ground of the DC system 25.Furthermore each converter 24A, 24B may be connected to the DC system25 via a disconnector 26A and 26B. Thereby the use of a DC breaker may be avoided.

Fig. 3 shows another alternative realization of the interface arrangement.In this case the transformer 22 has two primary windings magneticallycoupled to a single secondary winding, which is connected to the DCsystem 25 via a converter 24. In this case a first number of machines isconnected to a first string of converter blocks 12A, 12B and 12C in parallelwith the first primary winding of the transformer 22, while a secondnumber of machines are connected to a second string of converter blocks12D, 12E and 12F in parallel with the second primary winding of thetransformer 22. In this case it is possible that a disconnector 26A, 26B isconnected in parallel with a string of converter blocks, i.e. on the MV side of the transformer 22. lO 14 Fig. 4 shows a further realization of the interface arrangement where theMV system is a three-phase system and the transformer 22 is a three-phase transformer. In this case the transformer 22 has three primarywindings, each magnetically coupled to a corresponding secondarywinding which in turn is connected to the DC system 25 via the converter24 and typically via a phase leg of the converter. In this case a first numberof machines is connected to a first string of converter blocks 12A, 12B and12C in parallel with the first primary winding of the transformer 22, asecond number of machines is connected to a second string of converterblocks 12D, 12E and 12F in parallel with the second primary winding of thetransformer 22 and a third number of machines is connected to a thirdstring of converter blocks 12G, 12H and 12I in parallel with the thirdprimary winding of the transformer 22. The transformer may be Y-Aconnected, although other connections are possible such as Y-Y, A-Y andA-A.

As can be seen above, the proposed interface arrangement can either beconstructed as one or several single-phase AC converters, as shown in fig. 1and 2, or as one or several multi-phase AC converters, where the case ofone 3-phase AC converter is shown in fig. 4. Paralleling of severalgenerators can either be done at HVDC side as shown in fig. 2, or by meansof a multi-winding transformer with several MVAC windings for severalgenerators, as shown in fig. 3. In addition to generators, also energy-storage units may be added. Generators and storage modules may all havedifferent power and/ or voltage and/ or current ratings, as long as they are compatible at the point of parallel connection.

The use of a cascaded multi-level converter is also a known solution forgenerating a (3-phase) AC voltage from a battery, possible combined withcapacitors, see “Investigation of a multilevel inverter for electric vehicleapplications”, O. Josefsson, master thesis, Chalmers Univ. of Technology, 2015. However, also in this case the AC voltage has a low frequency and is lO not intended for interfacing to HVDC. The interface arrangement might beapplied to interface batteries (combined with capacitors) or super-capacitors with HVDC.

If needed for fault ride through or for combination of renewablegeneration with storage, the capacitors in either the cascaded multi-level converter or the HVDC-side converter may be increased in size.

To keep cable connections limited, the converter blocks (rectifier,capacitor, full-bridge) are advantageously placed close to the machine, orintegrated with it. The machine may be a segmented machine, and one or several converter blocks may be integrated with each segment.

The MVAC to HVAC transforming element may either be a fully isolatedtransformer or an autotransformer. The conversion from MVAC to HVDCmay be realized using a transformer plus a modular converter, or by other IIICEIIIS.

The system may be complemented with other types of switches such asdisconnectors (for safety during maintenance) as shown in fig. 2 and 3 or bypass switches for blocks, or fuses.

Control may be implemented at a central level as shown in fig. 1 ordistributed among blocks. In a distributed scenario each converter blockmay be equipped with is own control unit. A distributed control may offeradvantages with regard to availability. Communication (uni-directional orbi-directional) between distributed parts of the control shall be foreseen tocommunicated measures values and command values, such as to controlgenerator (section / segment/ module) power, such as to balance capacitor voltages and such as to generate a desired AC waveform. lO 16 The control unit 20 may be implemented through a computer or aprocessor with associated program memory or dedicated circuit suchField-Programmable Gate Arrays (FPGAs).

The control unit may be realized in the form of discrete components, suchas FPGAs. However, it may also be implemented in the form of a processorwith accompanying program memory comprising computer program codethat performs the desired control functionality when being run on theprocessor. A computer program product carrying this code can beprovided as a data carrier such as one or more CD ROM discs or one ormore memory sticks carrying the computer program code, which performsthe above-described control functionality when being loaded into a control unit of a voltage source converter.

From the foregoing discussion it is evident that the present invention canbe 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 (15)

1. An interface arrangement (10) for connection of a first powersystem to a second power system (25), the interface arrangement (10)comprising: a transforming element (22; 22A) for transforming between a first and asecond voltage and having at least one primary winding (PW) and at leastone secondary winding (SW), where the secondary winding is connected toat least one converter (24; 24A, 24B) providing a voltage of the secondpower system (25), at least one string of cascaded converter blocks (12A, 12B, 12C, 12D, 12E;12A, 12B, 12C), which string is connected in parallel with a correspondingprimary winding (PW) of the transforming element (22; 22A), each converter block comprising a converter module (14) for connection toa corresponding power entity (11) of the first power system, an energystorage element (18) for storing energy of the power entity and awaveshape contributing module (16), andat least one control unit (20) operative to control the waveshapecontributing modules of the converter blocks of a string to form the first voltage as a quasi two-level waveshape .

2. The interface arrangement (10) according to claim 1, whereinthe control unit (20) is operative to control the waveshape contributingmodule of the converter blocks to form the quasi two level waveshape with a fundamental frequency above 1oo Hz and below 1 kHz.

3. The interface arrangement (10) according to claim 1 or 2,wherein the control unit (20) is operative to control the waveshapecontributing module of at least one converter block to bypass the converter block in case of a fault.

4. The interface arrangement (10) according to claim 1 or 2, wherein the control unit is operative to control the waveshape contributing lO 18 modules of the converter blocks to balance the voltages of the energy storage elements during the forming of the first voltage.

5. The interface arrangement according to any previous claim,wherein the control unit (20) is further operative to switch the waveshapecontributing module of at least one converter block at the fundamental frequency of the first AC voltage.

6. The interface arrangement (10) according to claim 5, whereinthe control unit (20) is operative to perform further switching at low voltage and current levels.

7. The interface arrangement (10) according to any previousclaim, wherein there is only one control unit controlling all converter blocks of a string.

8. The interface arrangement (10) according to any of claims 1 - 6, wherein there are several control units, one for each converter block.

9. The interface arrangement (10) according to any previousclaim, wherein the power entities are electric machines each connected to a separate converter block.

10. The interface arrangement (10) according to any previousclaim, wherein the transformer comprises more than one primary winding, each connected to a corresponding string of converter blocks.

11. The interface arrangement (10) according to claim 10, whereinthe primary windings are magnetically coupled to a common secondary winding. lO 19

12. The interface arrangement (10) according to claim 10, whereineach primary winding is magnetically coupled to a corresponding secondary winding.

13. The interface arrangement (10) according to any previousclaim, wherein in case there is more than one string of converter blocksfurther comprising at least two disconnectors (26A, 26B), each associatedwith a separate string and being operable to disconnect the associated string from delivering power to the power transmission network.

14. The interface arrangement (10) according to any previousclaim, wherein the energy storage element (18) of a converter block is shared by the rectifier module and the waveshape contributing module.

15. A method of controlling an interface arrangement (10)connected between a first power system and a second power system (25),the interface arrangement comprising a transforming element (22; 22A)for transforming between a first and a second voltage and having at leastone primary winding (PW) and at least one secondary winding (SW),where the secondary winding (SW) is connected to at least one converter(24; 24A, 24B) providing a voltage of the second power system (25) and atleast one string of cascaded converter blocks (12A, 12B, 12C, 12D, 12E; 12A,12B, 12C), where the string is connected in parallel with a correspondingprimary winding (PW) of the transforming element (22; 22A), where eachconverter block comprises a converter module (14) connected to acorresponding power entity (11) of the first power system, an energystorage element (18) and a waveshape contributing module (16), themethod comprising: receiving and storing energy of the power entities in the energy storageelements of the converter blocks of a string, and controlling, by a control unit (20), the waveshape contributing modules ofthe converter blocks of said string to form the first voltage as a quasi two- level waveshape.