WO2017084716A1

WO2017084716A1 - A converter arrangement using converter modules

Info

Publication number: WO2017084716A1
Application number: PCT/EP2015/077141
Authority: WO
Inventors: Alireza NAMI; Kalle ILVES; Mats Berglund
Original assignee: Abb Schweiz Ag
Priority date: 2015-11-19
Filing date: 2015-11-19
Publication date: 2017-05-26
Also published as: CN108292844B; CN108292844A

Abstract

A converter arrangement (10) converting between AC and DC comprises first and second DC connectors (DC1, DC2), AC connectors (ACA, ACB ACC) and a set of converter modules (12, 14), each comprising at least one transformer comprising a set of primary winding groups, each comprising at least two primary windings, and a set of secondary winding groups, each comprising at least one secondary winding magnetically coupled to the primary windings of a corresponding primary winding group and connected to a corresponding AC connector, and at least two converter blocks converting between AC and DC, where each converter block has a DC side connected to the DC connectors (DC1, DC2) and an AC side connected to a corresponding primary winding of each primary winding group. The converter module comprises at least one n-level converter block providing n output voltage levels based on (n-1) series connected DC link capacitors, where n≥ 2.

Description

A CONVERTER ARRANGEMENT USING CONVERTER MODULES

FIELD OF INVENTION The present invention generally relates to converters that convert between AC and DC. More particularly the present invention relates to a converter arrangement for converting between alternating current (AC) and direct current (DC). BACKGROUND

In power transmission systems the modular multilevel converter (MMC) has become of interest to use in many high voltage direct current (HVDC) applications.

One problem with the MMC converter is that every capacitor within it is subject to a fundamental and second-order current component which leads to large low-frequency voltage-deviations in each capacitor. Thereby the capacitors are bulky and also the converter becomes bulky

It would therefore be of interest to obtain a converter configuration where the overall size may be reduced.

If basing a converter configuration on n-level converters, such as two-level or neutral-point clamped three-level converters, there is a possibility to lower the required converter size. However, with such a converter there is plenty of lower order harmonics that would require extensive filtering.

US 2008/0137382 discloses a converter configuration that is used for supplying power to an Alternating Current (AC) grid. The document describes the use of DC-to-AC power converter modules coupled in series to a DC link, where the modules comprise an inverter coupled to a transformer. There is also a use of half -bridges and bypass switches in order to obtain a bypass of power converter modules. The inverters are connected to a primary winding of the transformers, the secondary sides of which are connected in series with each other. The document is fairly silent about how the output voltages supplied to the grid are generated, but makes reference to US 2006/0126242 and US 2006/0227578. US 2006/0126242 and US 2006/0227578 both describe that the inverters (there named converters or bridges) may be switched with a phase shift in order to cancel low order harmonics or that such a shifting may be obtained through the way that the transformers are connected.

It would in view of what has been mentioned above be of interest to obtain an improvement with regard to the reduction of harmonics as well as with regard to the converter size.

It may also be of interest of obtain a more flexible use of converter modules. The present invention addresses some or all of the above mentioned problems.

SUMMARY OF THE INVENTION The present invention is directed towards providing a converter arrangement that provides an improvement with regard to at least some of the above-mentioned problems.

This object is according to a first aspect achieved through a converter arrangement for conversion between alternating current (AC) and Direct

Current (DC). The arrangement comprises a first and a second DC connector, a number of AC connectors for a number of AC phases and at least one first set of converter modules, where each converter module comprises

at least one transformer comprising

a set of primary winding groups, each comprising at least two primary windings, and

a set of secondary winding groups, each comprising at least one secondary winding magnetically coupled to the primary windings of a corresponding primary winding group and connected to a corresponding AC connector, and at least two converter blocks converting between AC and DC, where each converter block has a DC side connected to the two DC connectors and an

AC side connected to a corresponding primary winding of each primary winding group,

wherein the converter module comprises at least one n-level converter block providing n output voltage levels based on (n-i) series connected DC link capacitors, where n> 2.

The invention has a number of advantages. The use of n-level converter modules allows a reduction of the size of the converter arrangement. The transformer realization also enables a reduction of the required external transformer connections.

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 first realization of a converter arrangement comprising two DC connectors, a number of AC connectors and a set of converter modules,

fig. 2 schematically shows the realization of one exemplifying converter module comprising a first and a second converter block, fig. 3 schematically shows one realization of a switching stage of a converter module,

fig. 4 schematically shows the operation of the switching stage for connecting the converter blocks of the converter module in series between the two DC connectors,

fig. 5 schematically shows the operation of the switching stage for connecting the converter blocks in parallel between the two DC

connectors,

fig. 6 schematically shows the operation of the switching stage for bypassing the first converter block,

fig. 7 schematically shows the operation of the switching stage for bypassing the second converter block,

fig. 8 schematically shows a second converter arrangement realization, fig. 9 schematically shows a variation of the second converter arrangement realization connected to a bipole DC link,

fig. 10 schematically shows the variation of the second converter arrangement realization connected to a monopole DC link, and

fig. 11 schematically shows a third converter arrangement realization. DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given. Fig. l shows a converter arrangement 10 for conversion between alternating current (AC) and direct current (DC). The arrangement 10 is for this reason arranged to be connected to poles Pi and P2 of a DC power transmission system and to the phases of an AC power transmission system, which AC power transmission system with advantage is a three- phase system.

For this reason the converter arrangement 10 comprises a first and second DC connector DCi and DC2, where the first DC connector DCi may be connected to a first DC pole Pi of a DC link, and the second DC connector DC2 may be connected to a second DC pole P2 of the DC link which DC link may be a power transmission system. As an alternative, the second DC connector DC2 may be connected to ground. The converter

arrangement also comprises a number of AC connectors ACA, ACB and ACC for connection to a number of corresponding AC phases. There is a first AC connector ACA, a second AC connector ACB and a third AC connector ACC. The converter arrangement 10 also comprises a number of converter modules, where fig. 1 shows a first converter module CMi 12 and a second converter module CM2 14. The converter modules 12 and 14 are connected in series between the two DC connectors as well as in series between the AC connectors and ground. The converter modules are thus connected in series with each other. However the string of converter modules is also connected in shunt in relation to the DC link. In relation to the connection between the AC connectors ACA, ACB and ACC and ground, there are more particularly three strings connected between the AC connectors and ground and the converter modules each have module elements connected in theses strings. Although only two converter modules are shown, it should be realized that it is possible with more converter modules connected between the two DC connectors and in the strings between the AC connectors and ground. There is also a control unit CU 15 controlling the two converter modules.

The realization of a first type of converter module 12A is shown in more detail in fig. 2. The converter module 12A comprises a switching stage 20 with two terminals for connection in the series connection between the DC connectors. One connection terminal of a converter module may thus be connected to the connection terminal of another converter module or to a DC connector of the converter arrangement. The switching stage 20 more particularly comprises two full-bridge switching arrangements, where a first full-bridge arrangement provided in relation to a first converter block 16 is for connecting to the first DC connector and a second full-bridge arrangement provided for a second converter block 18 is for connection to the second DC connector. The first full-bridge arrangement is more particularly connected across a DC input or DC side of the first converter block 16 and the second full-bridge arrangement is connected across a DC input or DC side of the second converter block 18. Thereby the DC sides are connected to the DC connectors via the switching stage 20. Each such DC input has an upper DC terminal for connection to the first DC connector and a lower DC terminal for connection to the second DC connector. It should be realized that there may be several switching stages of other converter modules connected between such a DC terminal and the corresponding DC connector. Each converter block also has a

corresponding DC link capacitor Ci and C2 connected across its DC input or across its DC side. The first converter block 16 thus has a first DC link capacitor Ci connected across its DC input and the second converter block 18 has a second DC link capacitor C2 connected across its DC input.

Each converter block 16 and 18 converts between DC and AC and therefore it also has an AC side with a number of phase legs or AC links, where there is one such phase leg or AC link for each phase. The midpoint of such a phase leg provides a corresponding AC terminal with an output voltage. The first converter block 16 thus has a first AC terminal on which it provides a first voltage vai, a second AC terminal on which it provides a second voltage vbi and a third AC terminal on which it provides a third voltage vci. The second converter block 18 has a first AC terminal on which it provides a first voltage va2, a second AC terminal on which it provides a second voltage vb2 and a third AC terminal on which it provides a third voltage vc2. The first AC terminal of the first converter block 16 is connected to a first primary winding PWAi of a transformer 15 for a first transforming function. The first primary winding PWAi is also provided for a first AC phase. The first AC terminal of the second converter block 18 is connected to a second primary winding PWA2 of the transformer 15 for a second transforming function. The second primary winding PWA2 is provided for the same AC phase as the first primary winding PWAi.

Thereby the first and second primary windings PWAi and PWA2 are primary windings in a first primary winding group in a set of primary winding groups. Both these primary windings are magnetically coupled to a secondary winding SWA of the first transformer, which in turn is connected to the first AC connector ACA. Thereby the first secondary winding SWA is a winding in a first secondary winding group in a set of secondary winding groups, which first secondary winding group is linked or corresponds to the first primary winding group. The transformer 15 thus performs a first and a second transforming function, where the first transforming function comprises the transforming of the voltage on the first primary winding PWAi and the second transforming function comprises the transforming of the voltage on the second primary winding PWA2. Thereby the secondary winding SWA provides a first phase voltage contribution that is based on the first and second voltages vai and va2. There is thus a transformation of the sum of the two voltages vai and va2 that is used as contribution from the first converter module to the first phase voltage on first AC connector ACA.

In a similar manner the second AC terminal of the first converter block 16 is connected to a first primary winding PWBi of the transformer 15 for the first transforming function and a second phase. The second AC terminal of the second converter block 18 is connected to a second primary winding PWB2 of the transformer 15 for the second transforming function and the second AC phase. Thereby the first and second primary windings PWBi and PWB2 are primary windings in a second primary winding group in the set of primary winding groups. Both these primary windings PWBi and PWB2 are magnetically coupled to a second secondary winding SWB, which in turn is connected to the second AC connector ACB. Thereby the second secondary winding SWB is a winding in a second secondary winding group in the set of secondary winding groups, which second secondary winding group is linked or corresponds to the second primary winding group. It can in this case be seen that the first transforming function also comprises the transforming of the voltage on the first primary winding PWBi and the second transforming function comprises the transforming of the voltage on the second primary winding PWB2. Thereby the secondary winding SWB provides a second phase voltage contribution that is based on the first and second voltages vbi and vb2. There is thus a transformation of the sum of the two voltages vbi and vb2 that is used as contribution from the first converter module to the second phase voltage on the second AC connector ACB.

The third AC terminal of the first converter block 16 is connected to a first primary winding PWCi of the transformer 15 for the first transformer function and a third phase and the third AC terminal of the second converter block 18 is connected to a second primary winding PWC2 of the transformer for the second transformer function and the same phase. Thereby the first and second primary windings PWCi and PWC2 are primary windings in a third primary winding group in the set of primary winding groups. Both these primary windings PWCi and PWC2 are magnetically coupled to a third secondary winding SWC, which in turn is connected to the third AC connector ACC. Thereby the third secondary winding SWC is a winding in a third secondary winding group in the set of secondary winding groups, which group is linked or corresponds to the third primary winding group. It can in this case be seen that the first transforming function also comprises the transforming of the voltage on the first primary winding PWCi and the second transforming function comprises the transforming of the voltage on the second primary winding PWC2. Thereby the secondary winding SWC provides a third phase voltage contribution that is based on the first and second voltages vci and vc2. There is thus a transformation of the sum of the two voltages vci and vc2 that is used as contribution from the first converter module to the third phase voltage on third AC connector ACC. The second converter module 14 has its terminals connected in the same way between the various connectors and thereby it can be seen that the voltages on the three AC connectors are each based on a sum of the individual AC terminal voltages from the converter blocks of the converter modules.

For the primary winding, it is important to note that the voltage of the AC terminals of each converter block may have a direct as well as an alternating voltage. Therefore, the structure of the primary windings may be such that the direct-voltage component of the phase legs is not able to draw any direct current.

The transformer 15 may be placed or encapsulated in a transformer tank through which each AC connector and AC terminal is connected to the transformer via a corresponding bushing. Due to the fact that there is a single secondary winding magnetically coupled to two primary windings in each phase, i.e. that each group of secondary windings only comprises one secondary winding being magnetically coupled to all primary windings of the corresponding primary winding group, the number of bushings will be lowered compared with a case when each converter block is connected to a corresponding transformer.

As an alternative, it is also possible that the converter module comprises at least two transformers, one for each converter block, where the secondary windings of these transformers are connected in series. In this case each group of secondary windings comprise an equal number of windings to the number in the corresponding group of primary windings and each secondary winding is magnetically coupled to a corresponding primary winding in the corresponding primary winding group. As can be seen this interconnection will avoid two bushings through the transformer tank as any series-connection made within a winding group, such as the secondary winding groups, is made internally in the tank. It can thus be seen that a converter module provides two transforming functions, one for each converter block, where the transforming functions of a converter module may be implemented using a single transformer having at least two primary windings magnetically coupled to a single secondary winding or at least two primary wings, each magnetically coupled to a corresponding secondary winding and where any connection between two windings of a group is made internally in the tank. It can also be seen that all the primary windings in a primary winding group are wound around the same transformer core as all the secondary windings of the corresponding secondary winding group.

Fig. 2 shows a 3-phase transformer with 2 open secondary windings per AC phase or limb which has been proposed to get an equal impedance. For example a 333 MVA transformer can be made by having two three-phase transforming functions in one tank, represented by the 6 valve side bushings through which AC voltages and ground are provided.

Note that, the internal series connection can theoretically be extended to more than two windings. However, this may cause some asymmetric flux in the core and thus increase the losses.

Fig. 3 show a realization of the switching stage 20 in more detail. The switching stage 20 is controllable, i.e. may be controlled by the control unit 15, to connect the converter blocks in series or in parallel between the DC connectors. It is also controllable to bypass any of the converter blocks. As can be seen there are two full-bridge arrangements, a first full-bridge arrangement SAi connected across the DC input of the first converter block and a second full-bridge arrangement SA2 connected across the DC input of the second converter block.

The first full-bridge arrangement SAi comprises two strings with series connected switches Si, S2, S3 and S4, which strings are both connected in parallel across the DC input of the first converter block 16A. The second full-bridge arrangement SA2 likewise comprises two strings with series connected switches S5, S6, S7 and S8, which strings are both connected in parallel across the DC input of the second converter block 18A. In the first full-bridge arrangement SAi, the first string comprises a first and a second switch Si and S2 and the second string comprises a third and a fourth switch S3 and S4. The midpoint of the first string is connected to the first DC connector of the converter arrangement and the midpoint of the second string is connected to a junction between the two strings of the second full-bridge switching arrangement SA2. It is more particularly connected to an upper of these two junctions of the second switching arrangement SA2. The second string midpoint of the first full-bridge arrangement SAi is also connected to the upper DC terminal of the DC input of the second converter block 18. The first string of the second full- bridge arrangement SA2 comprises a fifth and a sixth switch S5 and S6 and the second string of the second full-bridge arrangement SA2 comprises a seventh and an eighth switch S7 and S8. The two strings of series connected switches S5, S6, S7 and S8 of the second switching arrangement SA2 are thus connected in parallel across the DC input of the second converter block 18, where the midpoint of the first string is connected to the second DC connector of the converter arrangement and the midpoint of the second string is connected to a junction between the two strings of the first full-bridge switching arrangement. It is more particularly connected to a lower of these two junctions. The second string midpoint of the second full-bridge arrangement SA2 is also connected to a lower DC terminal of the DC input of the first converter block 16.

Each switch may be realized as a semiconductor switch that may be implemented as a transistor or similar switching element with anti-parallel diode or similar unidirectional conduction element. Such a switching element may be an Insulated Gate Bipolar Transistor (IGBT), a metal oxide semiconductor field effect transistors (MOSFET) or a gate turn-off thyristor (GTO). The switches may furthermore be Silicon (Si) or Silicon

Carbide (SiC) switches. Also the switches of the converter blocks may be Si or SiC switches. As an alternative the switching stage may be realized through distributed solid state, passive mechanical or hybrid switches, where a hybrid switch may be realized as a string of solid state switches in parallel with a surge arrester and optionally also with a disconnecting string, where the disconnecting string comprises a disconnector such as a mechanical switch in series with a low rate electronic switching element such as a lowrate IGBT.

Another possible variation of the switching stage is to include an inductor in the connection of a string midpoint of one converter arrangement to the junction between the two strings of the other switching arrangement. Thereby the inductor is connected between a midpoint of a switching arrangement and one end of the DC link capacitor across which the other switching arrangement is connected. This means that a first inductor may be connected between the midpoint of the second string of the first switching arrangement SAi and an upper or positive end of the second DC link capacitor C2 and/or a second inductor may be connected between the midpoint of the second string of the second switching arrangement SA2 and an lower or negative end of the first DC link capacitor Ci. Such inductors are beneficial in case of a connection of the DC link capacitors in parallel with each other.

The converter blocks are, which may be seen in fig. 5 - 8, with advantage n-level converter blocks, where n is an integer > 2. A converter block furthermore provides n output voltage levels based on (n-i) series connected DC link capacitors through the operation of switches. Such a converter block thus uses DC link capacitors, i.e. capacitors connected between the two DC connectors in order to form AC voltages. A converter block may thus be a 2-level converter block, a three-level converter block, a four level converter block etc., where one example on a three-level converter block is a neutral-point clamped 3-level converter block. In fig. l it can be seen that a converter arrangement may be formed through two converter modules, where each converter module comprises two n-level converter blocks. In the example of two-level converters, this means that there are four two-level converter blocks used for forming an AC voltage.

According to some variations of the invention, these n-level converter blocks are combined in such a way that the voltages of the n-level converter blocks are used for forming a stepped AC voltage. This is done through using pulse width modulation (PWM) control for instance using a triangular or saw-tooth carrier. According to one specific variation of the invention, the waveform used may be phase-shifted or delayed between the different n-level converter modules. Such PWM control is typically made by the control unit 15.

In order to synthesize the multilevel output, each n-level converter block may be modulated separately via a triangular PWM carrier. The triangular carriers may then be phase shifted so that for k stacked blocks in the converter arrangement, each two adjacent blocks will have two PWM

360

carriers which are ( ° ) out of phase. The control unit 15 thus controls k

each control block with a PWM signal, where the PWM signal of one n- level converter block is phase shifted in relation to the PWM signals of the

360 other n-level converter blocks. If the phase shift of all carriers is °

v k from one carrier to another while having the same frequency, the output voltage of each converter block will be as follows: ( — +— M cos(e¾i - φ) +— ^-Υ TT —J„ (m—M)sm([m + n]—)cos(m(oj_ct - 0) + η(ω₀ί - φ))

2 2 "^~ °° fn 2 2

(0 = — +—M cos(e¾i - φ) + —J_n (m— M) sin([m + «]— ) cos(m(ojj - °) + η(ω_αί - φ))

2 2 ΐτι 2 2

(t) = — +—Μ cos(<¾i - φ) + y^™ y^™ — J_n (m— M ) sin([m + «]— ) cos(m(ojj - 2 * °) + η(ω_αί - φ)) 2 2 π *—"·—" _m " 2 2 k

— +—M cos(e¾i - φ) + -^∑°_m°-_i )^sin([^m + n]—)cos(m(o^j _ct - (k - 1) * ^^°) + η(ω₀ί - φ)) Here t is time, va the pole voltage level, M the degree of modulation, k the number of converter blocks, m and n the summation indices, J_n the Jacobian function, ω₀ the fundamental frequency, ooc the carrier frequency and φ the load angle.

It can be observed in the equation above that when summing up all cell voltages of one phase in the converter arrangement, the k first carrier harmonics as well as their side bands will be removed. If the converter blocks are two-level converter blocks, it can then be seen that the combination of four two-level converter blocks provides an output voltage waveform that has 7 different voltage levels. Thereby the filtering requirements are relaxed in that the lower order frequencies do not need to be filtered. As this is obtained using two-level converters instead of multilevel converter blocks, the use of large cell capacitors is also avoided, which thus also allows the size of the converter arrangement to be reduced.

It can also be seen that the switching frequency can become very low.

Generally speaking, the higher the number of converter blocks is, the lower is the switching frequency. A proper converter arrangement and switching frequency may therefore be chosen based on the number of converter blocks and the quality of the output waveforms.

In the operation there may furthermore be a need to bypass faulty converter blocks, to adapt the converter arrangement to different DC voltage levels as well as to obtain fault current limiting.

The switching stage of the converter modules may be used for obtaining such flexibility and fault handling capability.

Examples of this will now be described with reference being made to fig. 4 - 7, which figures show the switching of the switching stage in the first converter module having two three-phase two-level converter blocks 16A and i8A. The switching of the switches in a switching stage may likewise be performed under the control of the control unit 15.

In steady stage operation, when the DC voltage levels of the DC

transmission system are high, it may be of advantage to have the converter blocks of one or more converter modules to be connected in series between the two DC connectors DCi and Dc2. As can be seen in fig. 4, this series- connection is obtained through having the first switch Si of the first string and the fourth switch S4 of the second string in the first switching arrangement SAi turned on or conducting and having the sixth switch S6 of the first string and the seventh switch S7 of the second string of the second switching arrangement SA2 turned on or conducting. It can thus be seen that the second switch S2 of the first string and the third switch S3 of the second string in the first switching arrangement SAi, the fifth switch S5 of the first string and the eighth switch S8 of the second string in the second switching arrangement SA2 are turned off or non-conducting. Thereby the first switch Si connects the first DC connector with the upper DC terminal of the first converter block 16A, while the fourth and seventh switches S4 and S7 connects the lower DC terminal of the first converter block 16A with the upper converter terminal of the second converter block 18A. Thereby there are two parallel current paths between the two DC link capacitors. Furthermore, the sixth switch S6 connects the lower DC terminal of the second converter block 18A with thee second DC connector and it can thereby be seen that the two Dc inputs are connected in series between the two DC connectors.

In other instances the DC link voltage may be low. In this case it may be of interest to connect two or more of the converter blocks in parallel with each other between the DC connectors. As can be seen in fig. 5, this is obtained through having the first switch Si of the first string and the third switch S3 of the second string in the first switching arrangement SAi turned on or conducting and having the sixth switch S6 of the first string and the eighth switch S8 of the second string of the second switching arrangement SA2 turned on or conducting. It can thus be seen that the second switch S2 of the first string and the fourth switch S4 of the second string in the first switching arrangement SAi, the fifth switch S5 of the first string and the seventh switch S7 of the second string in the second switching arrangement SA2 are turned off or non-conducting. Thereby the first switch Si connects the first DC connector with the upper DC terminal of the first converter block 16A and the sixth switch connects the second DC connector with the lower DC terminal of the second converter block 18A. The third switch S3 in turn connects the first DC connector with the upper DC terminal of the second converter block 18 and the eighth switch S8 connects the lower DC terminal of the first converter block 18 with the lower DC terminal of the second converter block 18A, thereby obtaining the parallel connection. It can be seen that the third and eighth switches S3 and S8 provide parallel current paths through the DC link capacitors.

It is furthermore possible that a converter block is faulty, in which case there may be a need to bypass it. Fig. 6 shows a bypassing of the first converter block 16A in the first converter module, while fig. 7 shows a bypassing of the second converter block 18A of the first converter module.

As can be seen in fig. 6, a bypassing of the first converter block 16A is achieved through having the second switch S2 of the first string and the fourth switch S4 of the second string in the first switching arrangement SAi turned on or conducting and having the sixth switch S6 of the first string and the seventh switch S7 of the second string of the second switching arrangement SA2 turned on or conducting. It can thus be seen that the first switch Si of the first string and the second switch S2 of the second string in the first switching arrangement SAi, the fifth switch S5 of the first string and the eighth switch S8 of the second string in the second switching arrangement SA2 are turned off or non-conducting. As the second switch connects the first DC connector with the second DC terminal of the first converter block 18A and the fourth switch S4 connects the same second DC terminal with the upper DC terminal of the second converter block 18A. This is also achieved by the seventh switch S7. The lower DC terminal of the second converter block 18A is in turn connected to the second DC connector. In this way the first converter block 18A is bypassed and only the second converter block connected between the two Dc connectors. Furthermore the two switches S4 and S7 provide two parallel current paths from the second switch S2 to the DC link capacitor of the second converter block 18A. As can be seen in fig. 7, a bypassing of the second converter block 18A is achieved through having the first switch Si of the first string and the fourth switch S4 of the second string in the first switching arrangement SAi turned on or conducting and having the fifth switch S5 of the first string and the seventh switch S7 of the second string of the second switching arrangement SA2 turned on or conducting. It can thus be seen that the second switch S2 of the first string, the third switch S3 of the second string in the first switching arrangement SAi, the sixth switch S6 of the first string and the eighth switch S8 of the second string in the second switching arrangement SA2 are turned off or non-conducting. Thereby the first switch Si connects the first DC connector with the upper DC terminal of the first converter block 16A, while the fourth and seventh switches S4 and S7 connects the lower DC terminal of the first converter block 16A with the upper converter terminal of the second converter block 18A. The upper DC terminal of the second converter block 18A is in turn connected the second DC connector via the fifth switch S5. As can be seen only the first converter block 16A is thereby connected between the DC connectors while the second converter block 18A is bypassed. It can also be seen that the fourth and seventh switches S4 and S7 provide two parallel current paths from the DC link capacitor of the first converter block 16A to the fifth switch S5. It can be seen in all of fig. 4 - 7 that the second strings of both switching arrangements are used for forming two parallel current paths through at least a part of the switching stage. It may in some cases also be of interest to limit or block a fault current running between the DC terminals. This is possible to obtain through blocking all the switches Si - S8 of the switching arrangement for connecting the DC link capacitors between the DC terminals, i.e. through removing the signal used to turn on the switching elements. The switches will thereby remain in the open state. Thereby a fault current running through the converter arrangement, for instance from the first DC terminal DCi towards the second DC terminal DC2, will be blocked by the DC link capacitors Ci and C2.

The switching stage has been proposed to regulate the DC system in normal and fault scenarios.

Some benefits of the proposed switching stage are listed below.

The switching stage is used to regulate the DC-link voltage and guarantee the converter operation during normal (series connection) and internal fault (bypass connection) operating conditions.

The proposed switching stage offers 25% less conduction loss and 12% less silicon area compared to the conventional full-bridge switching stage.

The proposed switching stage offer the parallel connection in case of reduced DC system voltage.

The proposed switching stage can block the fault-current at the DC link with the use of the DC link capacitors.

In the different variations above, the converter arrangement only comprised n-level converter modules. However, in some situations it may be advantageous to comprise a limited number of modular multilevel converter (MMC) blocks in order to further relax the filtering requirements and to reduce the required number of transforming functions. The number of MMC blocks may be just one. Fig. 8 schematically shows one way in which an alternative first converter module may be realized in order to minimize the number of transformer functions. In the converter module the first converter block 16A is a two- level converter block 14A connected between a first switching arrangement SAi and the first primary windings of the three-phase transformer 15. Also the second converter block 18B is in this case connected between the second switching arrangement SA2 and the second primary windings of the three-phase transformer 15. However, in this case the second converter block is a modular multilevel converter block 18B, where each phase leg is formed through a voltage source having a DC contribution and an AC contribution, which voltage source is formed through a number of cascaded cells. Each cell comprises a cell capacitor that is used in the voltage forming process instead of the DC link capacitor. In the two-level converter the DC link capacitor is used in the voltage forming. This is indicated through the size of the DC link capacitor of the modular multilevel converter being much smaller than the size of thr DC link capacitor of the two-level converter.

In fig. 8, the two-level converter block 16A is connected to the first DC connector, while the MMC block 18B is connected to the second DC connector. It is, however, instead possible to connect the MMC to the first DC connector and the two-level converter block to the second DC connector. Since the MMC block 18B naturally operates with a low switching frequency, the 2-level converter block 16A will operate at a low switching frequency (around the fundamental frequency). The MMC block 18B will also act as an active filter for both the AC and the DC side, while at the same time transferring a part of the active power. Therefore, the switches of the 2-level converter block can be optimized for conduction losses as they switch at around the fundamental frequency. The converter arrangement shown in fig. 7 and 8 may be used in symmetrical monopole or bipolar HVDC systems.

As can be seen in fig. 9, the DC link may be a part of a bipole system, where the first DC connector is connected to a positive pole and the second DC connector is connected to a negative pole. It can also be seen that the midpoint of the series-connection between the poles is grounded. As can be seen in fig. 10 the DC link may be a part of a monopole system, where the first DC connector is connected to the pole and the second DC connector is connected to ground. As can be seen in both figures it is also possible to omit the switching stage. It should however be realized that one may also be added

Each AC terminal of the two-level converter block is connected to a corresponding first primary winding of the transformer 15, and each AC terminal of the MMC is connected to a corresponding second primary winding in the transformer 15. The windings in the transformer may be either delta- or Y-connected. However, in the case of a Y-connection the star-point cannot be grounded.

The secondary windings of transformer 15 are connected in series as illustrated in Fig. 8 - 10. In this way, the phase voltage is given by the sum of the voltages in the 2-level converter and the MMC. Accordingly, a sinusoidal output voltage can be obtained by selecting the output voltage of the MMC in such way that it cancels out the unwanted harmonics in the

2-level voltage waveform.

The latter type of converter module may with advantage be used in a back- back converter arrangement where there are two sets of converter modules connected back-to back via a DC link. This is shown in fig. 11. A first set 20 of converter modules would then be connected at one end of the DC link and a second set 22 of converter modules at the opposite end via a DC third connector DC3 connected to the first DC connector DCi and a fourth DC connector DC4 connected to the second DC connector DC2. It can here be seen that in the example in fig. 11 both the first and second sets of converter modules only comprises one converter module made up of two converter blocks. However, it should be realized that more converter blocks may be added. In this case the MMC of the first set may be connected to the first DC connector DCi, while the MMC of the second set may be connected to the fourth DC connector DC4. Alternatively, and as is shown in fig. 11, the MMC of the first set may be connected to the second DC connector DC2, while the MMC of the second set may be connected to the third DC connector DC3. Thereby the multilevel converter blocks of the two sets of converter blocks may be connected to different parts of the DC link such as to different poles or one to a pole and the other to ground. This provides further filtering relaxation in relation to the DC link. It can be seen that a switching stage is not used in fig. 11. However, it is also here possible to add one.

As described above, the control strategy using delayed PWM carriers is merely one way of obtaining a low switching frequency. It should be realized that there exists other PWM control strategies that may be used to reduce the switching frequency of the converter blocks as low as to about the fundamental frequency.

The invention has a number of advantages. The use of n-level converter modules allows a reduction of the size of the converter arrangement. The combination of two transforming functions using one secondary winding or two internally connected secondary windings reduces the number of required bushings. The use of several n-level converters relaxes the filtering requirements. The switching block comprising full-bridge switching arrangements allows a greater operational flexibility, reduced conduction losses and allows DC link fault blocking. The addition of a multilevel converter block provides further filtering requirement relaxation. The control unit 15 may be realized in the form of discrete components, such as an application-specific integrated circuit (ASIC) or a Field- Programmable Gate Array (FPGA) circuit. 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 the control unit.

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 converter arrangement (10) for conversion between alternating current (AC) and Direct Current (DC), the arrangement comprising a first and a second DC connector (DCi, DC2), a number of AC connectors (ACA, ACB ACC) for a number of AC phases and at least one first set (20) of converter modules (CMi, CM2), where each converter module comprises

at least one transformer (15) comprising

a set of primary winding groups, each comprising at least two primary windings (PWAi, PWA2, PWBi, PWB2, PWCi, PWC2), and

a set of secondary winding groups, each comprising at least one secondary winding (SWA, SWB, SWC) magnetically coupled to the primary windings of a corresponding primary winding group and connected to a corresponding AC connector, and

at least two converter blocks (16, 18) converting between AC and DC, where each converter block has a DC side connected to the two DC connectors (DCi, DC2) and an AC side connected to a corresponding primary winding of each primary winding group,

wherein

the converter module comprises at least one n-level converter block (16A, 18A) providing n output voltage levels based on (n-i) series connected DC link capacitors, where n> 2.

2. The converter arrangement according to claim 1, wherein the first set of converter modules comprises k n-level converter modules configured to receive a corresponding PWM carrier, where k > 2 and the PWM carriers

360 of the different n-level converter modules are shifted in phase by ° .

k

3. The converter arrangement according to any previous claim wherein each converter module comprises a switching stage (20) between the DC connectors and the converter blocks, the switching stage having switches (Si, S2, S3, S4, S5, S6, S7, S8) being controllable to connect the converter blocks (16, 18) in series or in parallel between the DC connectors.

4. The converter arrangement according to claim 3, wherein each converter block has a corresponding DC link capacitor (Ci, C2) connected across its DC side and the switches of the switching stage being

controllable to be blocked for connecting the DC link capacitors between the DC terminals for limiting a fault current.

5. The converter arrangement according to claim 3 or 4, wherein the switches of the switching stage (20) are active switches, passive switches or hybrid switches.

6. The converter arrangement according to any of claims 3 - 5, wherein the switching stage (20) comprises at least two full-bridge switching arrangements (SAi, SA2), each having two strings with series connected switches (Si, S2, S3, S4, S5, S6, S7, S8) and connected in parallel across the DC side of a corresponding converter block (16, 18), where the midpoint of a first string of a full-bridge switching arrangement is connected to a DC connector while the midpoint of the second string is connected to a junction between the two strings of the other full-bridge switching arrangement.

7. The converter arrangement according to claim 6, wherein the switches of the second strings are controllable to provide two parallel current paths through at least a part of the switching stage

8. The converter arrangement according to any previous claim, wherein one converter block is a modular multilevel converter block (18B)

9. The converter arrangement according to claim 8, further comprising a third and a fourth DC connector (DC3, DC4) and a second set (22) of converter modules () connected to the first set (20) of converter modules in a back-to-back configuration through the third DC connector being connected to the first DC connector and the fourth DC connector being connected to the second DC connector, wherein the second set of converter modules comprises at least one n-level converter block and a modular multilevel converter block, where if the modular multilevel converter block (18B) of the first set of converter modules is connected to the first DC connector (DCi), the modular multilevel converter block of the second set of converter modules is connected to the third DC connector (DC3) and if the modular multilevel converter block (18B) of the first set of converter modules is connected to the second DC connector (DC2), the modular multilevel converter block of the second set of converter modules is connected to the fourth DC connector (DC4).

10. The converter arrangement according to any previous claim, wherein the secondary winding groups each comprise secondary windings in a number corresponding to the number of primary windings in the corresponding primary winding group, where each primary winding of a primary winding group is magnetically coupled to a corresponding secondary winding of a corresponding secondary winding group.

11. The multilevel converter according to any of claims 1 - 9, wherein the secondary winding groups each comprise only one secondary winding and all primary windings of a primary winding group is magnetically coupled to the sole secondary winding of the corresponding secondary winding group.

12. The converter arrangement according to claim 10 or 11, wherein the primary windings (PWAi, PWA2) in a primary winding group and the secondary windings (SWAi) of the corresponding secondary winding group are wound around the same transformer core.

13. The converter arrangement according to any of claims 10 - 12, wherein the transformer is encapsulated in a transformer tank, and any connection between windings within a winding group is made internally in the tank.

14. The converter arrangement according to any previous claim, further comprising a control unit (32) configured to control the converter modules.