WO2014005634A1

WO2014005634A1 - Three-level submodule for a voltage source converter

Info

Publication number: WO2014005634A1
Application number: PCT/EP2012/063106
Authority: WO
Inventors: Kalle ILVES
Original assignee: Abb Ab; Norrga, Staffan
Priority date: 2012-07-05
Filing date: 2012-07-05
Publication date: 2014-01-09

Abstract

The invention concerns a voltage source converter including submodules and a submodule in such a voltage source converter. The submodule (C1u1) comprises a connection terminals(TE1, TE2),each providing a connection for the submodule to the phase leg,a first and a second energy storage element (C1, C2) and a number of switching units (S1, S2, S3, S4, S5, S6, S7, S8) configured to selectively connect the first and second energy storage element (C1, C2) between the connection terminals (TE1, TE2) for obtaining at least a first voltage level, a second higher voltage level and a third intermediate voltage level (Ud) between the connection terminals, where the switching units comprise switching units (S1, S2, S3, S4, S5, S6, S7, S8) switchable to connect the first and second energy storage elements in parallel between the connection terminals (TE1, TE2) for providing the third intermediate voltage level (U_d).

Description

THREE-LEVEL SUBMODULE FOR A VOLTAGE SOURCE CONVERTER

TECHNICAL FIELD The present invention generally relates to voltage source converters. More particularly the present invention relates to submodule for use in series with other submodules in a voltage source converter as well as to a voltage source converter including such a submodule.

BACKGROUND

There have recently evolved submodules or cells that are used as building blocks in voltage source

converters .

These submodules have traditionally been provided as two-level units or half bridge circuits comprising two switching units and a single capacitor providing a voltage. The submodule then gives a voltage

contribution to the conversion, typically used for forming an AC voltage, which contribution is either the capacitor voltage or a zero voltage.

It is also known to provide a full bridge submodule comprising four switching units and a capacitor. A full bridge will then provide three levels: the positive voltage of the capacitor, the negative voltage of the capacitor and a zero voltage.

There has also recently been introduced submodules with two capacitors. WO 2012/040257 describes two submodule variations where two capacitors are used. In a first two-level

variation, where either a zero or a capacitor voltage is provided, the capacitors are connected in parallel when a capacitor voltage is provided and in series with each other when a zero voltage is provided. In a second three-level variation of the submodule, the capacitors have different voltages, where a first has a voltage that is double the voltage of a second. This second submodule variation provides zero voltage, the second capacitor voltage, the difference between the two capacitor voltages or the voltage of the first

capacitor .

Another two capacitor three-level submodule realization is described in US 2008/0198630 in which the three levels are provided through a zero voltage, the sum of the voltages of two capacitors and the voltage of a single one of the two capacitors.

There is a problem when using submodules in voltage source converters and that is that there exists a differential ripple between submodules. This voltage ripple is due to the submodules being switched

differently. The ripple is considerable at low

switching frequencies. One way to reduce the ripple is consequently to raise the switching frequency. However, it is desirable to employ low switching frequencies because then the switching losses are low. This means that some other way than raising of the frequency is needed to lower the submodule voltage ripple. SUMMARY

One object of the present invention is to provide a submodule with energy storage elements, which submodule allows the ripple on the energy storage elements to be lowered without the need to raise the switching

frequency .

This object is according to a first aspect of the present invention solved through a submodule for connection in series with other submodules in a phase leg of a voltage source converter and comprising:

a first and a second connection terminal, each providing a connection for the submodule to the phase leg,

a first and a second energy storage element, and a number of switching units configured to

selectively connect the first and second energy storage element between the first and second

connection terminals for obtaining at least a first voltage level, a second higher voltage level and a third intermediate voltage level between the

connection terminals,

wherein the switching units comprise switching units switchable to connect the first and second energy storage elements in parallel between the first and second connection terminals for providing the third intermediate voltage level. Another object of the present invention is to provide a voltage source converter having a number of submodules with energy storage elements, which submodules allows a lowering of the ripple on the energy storage elements without the need to raise the switching frequency.

This object is according to a second aspect of the present invention solved through a voltage source converter including at least one phase leg, where each phase leg includes a number of submodules and at least one submodule comprises:

a first and a second connection terminal, each providing a connection for the submodule to the corresponding phase leg,

a first and a second energy storage element, and a number of switching units) configured to

selectively connect the first and second energy storage element between the first and second connection terminals for obtaining at least a first voltage level, a second higher voltage level and a third intermediate voltage level between the connection terminals,

wherein the switching units comprises switching units switchable to connect the first and second energy storage elements in parallel between the first and second connection terminals for providing the second intermediate voltage level.

Embodiments of the present invention have a number of advantages. Some embodiments provide a three-level submodule for a voltage source converter where the third intermediate voltage level can be provided with the energy storage elements connected in parallel. Through this measure the voltage ripple of the

submodule can be reduced at low switching frequencies. The conduction losses, voltage ratings and cost of semiconductors compared to conventional two-level modules are furthermore not negatively influenced through the more complex structure of embodiments described herein. The current rating is in some

variations lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be

described with reference being made to the accompanying drawings, where fig. 1 schematically shows a voltage source converter having a number of parallel branches in the form of phase legs each provided with a number of voltage source converter submodules,

fig. 2 schematically shows the structure of a voltage source converter submodule according to a first embodiment,

fig. 3 schematically shows the structure of a voltage source converter submodule according to a second embodiment,

fig. 4 schematically shows the structure of a voltage source converter submodule according to a third

embodiment

fig. 5 schematically shows the structure of a voltage source converter submodule according to a fourth embodiment,

fig. 6 schematically shows the structure of a voltage source converter submodule according to a fifth

embodiment, and fig. 7 schematically shows the structure of a voltage source converter submodule according to a sixth

embodiment . DETAILED DESCRIPTION

In the following, a detailed description of preferred embodiments of voltage source converter submodules and voltage source converters including voltage source converter submodules will be given.

Voltage source converters can be used in many types of electrical power systems, such as high-voltage power transmission systems. Examples on systems are direct current power transmission systems like HVDC (High

Voltage Direct Current) , HVDC back-to-back systems and FACTS (Flexible Alternating Current Transmission

System) . In such systems voltage source converters are used as for instance rectifiers and inverters. However, they may also be used as SVCs (Static VAr compensator) .

Fig. 1 shows a block schematic outlining an example of a voltage source converter 10, which may be provided as either a rectifier or an inverter in a power

transmission system. The voltage source converter 10 here includes a group of branches in the form of phase legs connected in parallel between two DC terminals DC+ and DC- for connection to a DC transmission system. In the example given here there are three such branches or phase legs PL1, PL2 and PL3 in order to enable

connection to a three-phase AC transmission system. It should however be realized that as an alternative there may be for instance two phase legs or even one phase leg. Each phase leg PLl, PL2, PL3 has a first and second end point. In a converter of the type depicted in fig. 1 the first end points of all the phase legs PLl, PL2 and PL3 are connected to a first DC terminal DC+ while the second end points are connected to a second DC terminal DC- .

Each phase leg PLl, PL2, PL3 of this first type of voltage source converter 10 further includes a lower and upper phase leg half and at the junction where the halves of a leg meet, there is provided an AC terminal. In the exemplifying voltage source converter 10 there is here a first phase leg PLl having an upper half and a lower half, a second phase leg PL2 having an upper half and a lower half and a third phase leg PL3 having an upper half and a lower half. At the junction between the upper and lower halves of the first phase leg PLl there is provided a first AC terminal AC1, at the junction between the upper and lower halves of the second phase leg PL2 there is provided a second AC terminal AC2 and at the junction between the upper and lower halves of the third phase leg PL3 there is provided a third AC terminal AC3. Each AC terminal AC1, AC2, AC3 is here connected to the corresponding phase leg via a respective inductor LAC1, LAC2, LAC3. Here each half furthermore includes one current limiting inductor Lul, Lu2, Lu3, Lll, L12, and L13 connected to the corresponding DC terminal DC+ and DC-. It should be realized that the phase inductors can be split in half, where one half is provided in one phase leg half and the other in the other phase leg half of a phase leg. It is also possible to completely omit the phase inductors and/or the current limiting inductors described above.

The phase legs all comprise submodules that are used for forming the AC voltages. The submodules are

typically connected in series or in cascade in the phase legs .

In the present example there are three submodules in each phase leg half. Thus the upper half of the first phase leg PLl includes three submodules Clul, C2ul, C3ul, while the lower half of the first phase leg PLl includes three submodules Clll, C211, and C311. In a similar fashion the upper half of the second phase leg PL2 includes three submodules Clu2, C2u2, C3u2, while the lower half of the second phase leg PL2 includes three submodules C112, C212, and C312. Finally the upper half of the third phase leg PL3 includes three submodules Clu3, C2u3, C3u3, while the lower half of the third phase leg PL3 includes three submodules C113, C213, and C313. The numbers are here only chosen for exemplifying a possible layout of a voltage soruce converter. It is often favourable to have many more submodules in each phase leg, especially in HVDC applications. It can also be seen that the submodules of a phase leg are with advantage provided

symmetrically around the AC terminal.

There is furthermore a control unit 12 which is set to control the submodules. Control of each submodule in a phase leg half is normally done through providing the submodule with control signals that control the contribution of that submodule to an AC waveform provided by converter 10.

Here the common control unit 12 controls the submodules for converting AC power to DC power or vice versa. The submodules further provide a voltage based on energy stored in energy storage elements.

In the exemplifying converter 10 the submodules in the upper half of a phase leg, such as the submodules Clul, C2ul and C3ul of the upper half of the first phase leg PL1, are typically controlled so that they provide a DC component corresponding to a positive DC voltage of the first DC terminal DC+ and an AC component corresponding to the full AC voltage of a corresponding AC terminal AC1, AC2 or AC3, while the submodules of the

corresponding lower half of the phase leg, such as the submodules Clll, C211 and C311 of the first phase leg PL1, typically provide a DC component corresponding to a negative DC voltage of the second DC terminal DC- and an AC component corresponding to the full AC voltage of the corresponding AC terminal AC1, AC2 or AC3. The instantaneous AC voltage values provided by the

submodules on opposite sides of an AC terminal of a phase leg here typically have opposite polarities.

The converter 10 may here be operated in two

directions. If three-phase AC voltages are applied on the AC terminals AC1, AC2 and AC3 a DC voltage is generated, while if a DC voltage is applied between the DC terminals DC+ and DC-, a three-phase AC voltage is generated on terminals AC1, AC2 and AC3. The control furtmermore typically involves generating control signals by the control unit 12 in known fashion based on PWM modulation.

Fig. 2 schematically shows a submodule Clul according to a first embodiment of the present invention that may be used in the converter of fig. 1.

The submodule Clul comprises a first and a second connection terminal TE1 and TE2, each providing a connection for the submodule to a phase leg of the converter. The first connection terminal TE1 may be a positive connection terminal and the second connection terminal TE2 may be a negative connection terminal, which means that a positive voltage is provided between the first and second connection terminals TE1 and TE2. The submodule Clul also comprises a first and a second energy storage element CI and C2, which may be provided in the form of capacitors. It is sometimes possible with batteries instead.

The submodule Clul illustrated in fig. 2 also comprises a number of switching units SI, S2, S3, S4, S5, S6, S7 and S8 that are configured to selectively connect the first and second energy storage elements CI and C2 between the first and second connection terminals TE1 and TE2.

Through this selective connection it is possible to provide three different voltage levels between the first and the second terminal TE1 and TE2. It is possible to provide a first voltage level, a second higher voltage level and a third intermediate voltage level. The first voltage level may be a zero voltage level, the second voltage level may be the sum of the voltages U_d of the two energy storage elements CI and C2, while the third voltage level may be the voltage level U_d of the first energy storage element CI or the second energy storage element C2. All these voltage levels may furthermore be positive voltage levels.

According to example embodiments of the invention the third intermediate voltage level may also be the voltage obtained through connecting the first energy storage element CI in parallel with the second energy storage element C2. The voltages U_d of the two energy storage elements CI and C2 are furthermore with

advantage essentially equal. However, they may at times differ somewhat because of the operation of the

submodule .

It can thus be seen that the submodule Clul comprises switching units switchable to connect the first and second energy storage elements CI and C2 in parallel between the first and second connection terminals for providing the second intermediate voltage level.

As can also be seen in fig. 2 the first energy storage element CI has two ends where a first end is coupled to the first connection terminal TE1 and a second end is coupled to the second connection terminal TE2 via a first branch BR1. The first branch comprises a first and a second switching unit SI and S2. Also the second energy storage element C2 has two ends, where a first end is coupled to the first connection terminal TE1 via a second branch BR2 and the second end is coupled to the second connection terminal TE2. The second branch BR1 comprises a third and a fourth switching unit S3 and S4.

In the submodule there is also a fifth switching unit S5 connected between the first end of the second energy storage element C2 and the junction between the first and second switching units SI and S2 of the first branch BR1 and a sixth switching unit S6 connected between the first connection terminal TE1 and the junction between the first and second switching units SI and S2 of the first branch BR1. There is also a seventh switching unit S7 connected between the second end of the first energy storage element CI and the junction between the third and fourth units S3 and S4 of the second branch BR2. There is finally an eighth switching unit S8 connected between the second

connection terminal TE2 and the junction between the third and fourth switching units S3 and S4 of the second branch BR2.

The switching units of the submodule may be provided as switching elements with anti-parallel diodes.

These switching elements may be realized in the form of IGBTs (Insulated Gate Bipolar Transistor) . As can be seen in fig. 2, the conductivity of the diode of the first switching unit SI is towards the first connection terminal TE2. The direction of conductivity of the diode of the second switching unit S2 is towards the second connection terminal TE2. The direction of conductivity of the diode of the third switching unit S3 is towards the first connection terminal TE1 and the conductivity of the diode of the fourth switching unit S4 is towards the second connection terminal TE2.

The direction of conductivity of the diode of the fifth switching unit S5 is towards the first end of the second energy storage element C2, while the direction of conductivity of the diode of the sixth switching unit S6 is towards the first connection terminal TE1. The direction of conductivity of the diode of the seventh switching unit S7 is away from the second end of the first energy storage element CI and the

direction of conductivity of the diode of the eighth switching unit S8 is away from the second connection terminal TE2.

Submodules have traditionally been provided as two- level or half bridge circuits comprising two switching units and a single energy storage element in the form of a capacitor providing a voltage. Such a traditional submodule then gives a voltage contribution to the conversion, which is either the capacitor voltage or a zero voltage.

The energy storage elements of the submodules thus act as voltage sources that can be either inserted or bypassed in a chain of series connected submodules. When the submodules are provided in a voltage source converter, such as that shown in fig. 1, the power exchange between the DC link and the submodules is carried out by the current that is flowing between the DC terminals through each phase leg. This current is often referred to as a circulating current. Similarly, a power exchange between the AC side and the submodules is obtained as a consequence of the alternating current that is injected at the AC terminal. If the converter is controlled in such a way that the difference of the input and output power corresponds to the losses in the converter, the stored energy in each energy storage element can be kept approximately constant.

Although the energy that is stored in the capacitors forming energy storage elements can be kept constant over time, the voltage in each capacitor will oscillate around its mean value. This is easily explained by considering a three-level submodule where three

different voltage levels can be generated. When the intermediate voltage level is requested, one of the capacitors will be inserted whereas the other capacitor is bypassed. Consequently, the charge that is stored in the capacitors as a result from the current flowing through the submodules will not be split evenly between the two capacitors.

In order to avoid an unnecessarily large voltage ripple across the capacitors, the maximum charge in each period should be distributed as evenly as possible between all of the series connected submodules.

Typically, this is in a two-level submodule solved by increasing the switching frequency and thus increasing the rate at which submodules are inserted and bypassed. In this way, one of the capacitors can be inserted for a shorter period of time, and then be replaced by another capacitor that was previously bypassed. In high-voltage applications an increased switching frequency is, however, associated with a significant increase of the total losses. Therefore, a submodule design that can improve the sharing of charge between different submodule capacitors at low switching

frequencies would be advantageous. The traditional way of using a submodule when providing a voltage contribution to a converter, such as the converter in fig. 1 is to apply the voltage of one capacitor between the connection terminals. The inventors have discovered that by connecting submodule energy storage elements, e.g. capacitors, in parallel with each other, the above-mentioned ripple is considerably reduced. In operation the control unit 12 of the converter 10 controls the switching units of the submodules, e.g. of the type depicted in fig. 2, of a phase leg to provide a voltage contribution of 0, U_d or 2U_d, which voltage contribution is used to form an AC voltage on an AC terminal of the phase leg.

As can be seen in fig. 2, the first branch BR1

comprises the first switching unit SI, while the second branch BR2 compromises the third switching unit S3, which are both primary switching units. The second switching unit S2 is a secondary switching unit. Also the fourth switching unit S4 is a secondary switching unit. One of the branches is furthermore a primary branch, while the other is a secondary branch. The fifth switching unit S5 is connected between the junction joining the switching units of the primary branch and the point where the secondary branch is connected to its associated energy storage element. The sixth switching unit S6 is connected between the same junction of the primary branch and the connection terminal TE1 to which the energy storage element associated with the primary branch is connected. In the example given in fig. 2, it can furthermore be seen that the first branch BRl is a primary branch, while the second branch BR2 is a secondary branch. In other versions of the invention the second branch BR2 is a primary branch, while the first branch is a secondary branch.

In this case the control unit 12 selectively controls the switching units of the first branch BRl to connect the first energy storage element CI between the two connection terminals for providing the third voltage level U_d, selectively controls the switching units of the second branch BR2 to connect the second energy storage element C2 between the two connection terminals for providing the third voltage level U_d, selectively controls the sixth and primary switching unit of the primary branch for interconnecting the two connection terminals TE1 and TE2 for providing the first voltage level 0 and selectively controls the fifth and

secondary switching unit of the primary branch for connecting the first and second energy storage elements CI and C2 in series between the connection terminals for providing the second voltage level 2U_d-

With regard to the submodule according to the first embodiment, the control unit 12 thus selectively controls the switching units SI and S2 of the first branch BRl to connect the first energy storage element CI between the two connection terminals TE1 and TE2 for providing the third voltage level ¾, selectively controls the switching units S3 and S4 of the second branch BR2 to connect the second energy storage element C2 between the two connection terminals TE1 and TE2 for providing the third voltage level ¾, selectively controls the first and sixth switching units SI and S6 for interconnecting the two connection terminals for providing the first voltage level OV and selectively controls the second and fifth switching units S2 and S5 for connecting the first and second energy storage elements CI and C2 in series between the connection terminals for providing the second voltage level 2¾.

In the first embodiment the control unit 12 is

furthermore configured to selectively control the fifth and the sixth switching units S5 and S6 to connect the second energy storage element C2 between the two connection terminals for providing the third voltage level U_d, to selectively control the third and eighth switching units S3 and S8 for interconnecting or short- circuiting the two connection terminals TE1 and TE2 for providing the first voltage level 0V, to selectively control the fourth and seventh switching units S4 and S7 for connecting the first and second energy storage elements CI and C2 in series between the connection terminals TE1 and TE2 for providing the second voltage level 2U_d and to selectively control the seventh and eighth switching units S7 and S8 for connecting the first energy storage element CI between the two

connection terminals for providing the third voltage level Ud. Some embodiments of the invention provide an

alternative submodule design that can improve the energy storage element voltage balancing process at low switching frequencies. One variation of the proposed submodule design comprises 8 unidirectional switches with antiparallel diodes and two DC capacitors.

The functionality of this proposed submodule design is in some respects similar to two conventional series connected submodules. The main difference is that when the intermediate voltage level is used, the two energy storage elements can be connected in parallel. By connecting the energy storage elements in parallel when the third intermediate voltage level is used, the charge is always distributed evenly between the two energy storage elements. Consequently surge currents are avoided when the capacitors are connected in parallel since they always hold the same charge. The submodule shown in fig. 2 has six allowable

switching states that are outlined in table I below.

TABLE I As can be seen in Table I, there are four different possible combinations of switching states for the intermediate voltage level U_d, two of which are to be used during nominal operation of the submodule. The switching states that results in only one inserted energy storage element should only be used in

exceptional cases. Since there are two possible

combinations of switching states that result in a parallel connection of the energy storage elements, there is redundancy in the switching states during nominal operation. This redundancy can be used to improve the loss distribution between the

semiconductors, i.e. the switching units. An example of a switching sequence using both of the two possible switching state combinations for the intermediate voltage level is shown in Table II.

TABLE II

It can be seen that during nominal operation, all of the switching units have the possibility to operate at fundamental switching frequency.

By considering the switching states given in Table I and the schematic of the submodule in Fig. 2 it can be seen that the maximum voltage each switching unit must be able to block is the voltage U_d of one energy storage element. Consequently, the voltage rating of each switching unit in the proposed submodule design is the same as the voltage rating of the switching units of two-level submodules. It can also be observed that for every combination of switching states in Table I, the current flowing through the submodule is conducted through two parallel paths. This means that each switching unit only conducts half of the current that flows through the submodule. Consequently, the current rating of the switching units in the proposed submodule design is half of the current rating required for the switching units in traditional half-bridge submodules. Therefore, it can be concluded that the combined power rating of the semiconductors is the same as in the traditional submodule realization.

By comparing the proposed circuit in Fig. 2 with the known combination of two cascaded half-bridge

submodules it can be observed that for all relevant switching state combinations, the number of series connected diodes and series connected switching

elements are the same in all cases. Hence it can be concluded that the conduction losses will be the same in the two cases.

Various advantages associated with embodiments of the invention can be summarized as follows:

Some embodiments of the invention provide a 3-level converter cell with two voltage sources or energy storage elements. When the third intermediate voltage level U_d is used, the energy storage element can be connected in parallel. An advantage with this new feature is that submodule voltage-ripple can be reduced at low switching frequencies.

Considering losses, voltage ratings, and cost of the switching units compared to the known combination of two cascaded half-bridge submodules:

• Conduction losses: The proposed converter cell has the same conduction losses.

• Voltage blocking capability: The required voltage blocking capability of each switching unit is the same.

• Current rating: The current rating of each switching unit is half of the current rating of the switching used in the known combination of two cascaded half- bridge .

• Rated power of semiconductors: In total, the combined power rating of all semiconductors is the same as with the known combination of two cascaded half-bridge submodules.

As was mentioned above, the nominal operation is to insert two energy storage elements, e.g. capacitors, in parallel. However, if by any reason there is a

significant difference between the two energy storage element voltages, it is also possible to insert only one of the two energy storage elements. In this way, the voltages can be controlled individually in such a way that the imbalance in the voltages of the energy storage elements is removed.

A submodule according to a second embodiment is shown in fig. 3. This embodiment differs from the first embodiment in that the seventh and eighth switching units have been removed. Through the removal of the seventh switching unit there is now no connection between the second end of the first energy storage element CI and the second branch BR2 and through the removal of the eighth switching unit there is no connection between the second branch BR2 and the second end of the second energy storage element C2. These two latter connections have thus been removed.

The switching states for this second embodiment are the following :

TABLE III The number of switching states are in this case reduced compared with the first embodiment. There is for instance only one state where there is a parallel connection . As the switches in the submodule presented in Fig. 2 are unidirectional, a negative voltage across the terminals will result in a short circuit through the diodes in S3, S4, S6 and S8. That is, the submodule cannot block negative currents or voltages. The ability to block negative currents or voltages can, however, be included in the submodule design at the cost of a few extra switching units. Negative current or voltages may typically occur if there is a ground fault on the DC side of the converter 10. When a negative current is to be blocked, the current cannot be turned off instantaneously since there may be energy stored in the converter inductors, i.e. the inductors in the phase legs and/or the AC connections. This can be solved by redirecting the current during the fault in such a way that it is charging the submodule energy storage elements. In this way, the stored energy in the inductors is moved to the submodule energy storage elements. As the energy in the inductors is removed, the current drops to zero and the submodule can completely block both positive and negative voltages.

A third embodiment of a submodule with a reverse voltage blocking functionality is shown in Fig. 4. This submodule is a variation of the submodule of the first embodiment, i.e. with all the previously described eight switching units SI - S8. In this embodiment there is a ninth switching unit S9 between the sixth

switching unit S6 and the first connection terminal TE1, where the ninth switching unit S9 has a diode with a direction of conductivity away from the first

connection terminal TE1. There is also a tenth

switching unit S10 between the second end of the second energy storage element C2 and the second connection terminal TE2, where the tenth switching unit S10 has a diode with a direction of conductivity towards the second connection terminal TE2. In fig. 4 there is furthermore an optional first diode Dl connected between the second connection terminal TE2 and the junction between the first and the second switching units SI and S2 and having a direction of conductivity away from the second connection terminal TE2. There is also an optional second diode D2

connected between the first connection terminal TE1 and the junction between the third and the fourth switching units S3 and S4 and having a direction of conductivity towards the first connection terminal TE1.

At nominal operation, the switching elements of the switching units S6 and S10 remain closed at all times. When a fault occurs, such as a ground fault on the DC side of the converter, and the submodule is set to block the current caused by the fault, all of the switching elements of the switching units SI through S10 are turned off. Any current that is forced through the submodule due to the stored energy in the inductors will then be redirected through the diodes in such a way that the current is charging the energy storage elements CI and C2. When the energy in the inductors is removed and the current drops to zero, the submodule will block voltages and currents in both directions.

The circuit design also has a possibility to insert a negative voltage level by closing the switching

elements of the switching units SI, S3, S5, S6, S7 and S8 while the switching elements of the remaining switching units are open. This would, however, increase the switching losses and the power rating of the switching units SI and S3. There are alternative ways for blocking negative voltages and currents, where the most straightforward way is to use full bridges instead of half bridges. Another way to block negative voltages and current is the double-clamped submodule, which is described by for instance by R Marquardt in "Modular Multilevel

Converter Topologies with DC-Short Circuit Current Limitation, ICPE, ECE Asia 2011. Both these latter alternatives increase the total silicon area (or combined power rating of the semiconductors) .

Blocking a current by moving the stored energy in the inductors to the submodule energy storage elements requires more switching units, which increases the conduction losses. An alternative solution is therefore to bypass the current via a thyristor which may give lower conduction losses during nominal operation. Two examples of how this can be done are shown in Fig. 5 and Fig. 6. In both figures there is a thyristor Tl connected between the first and second connection terminals TEl and TE2 and having a direction of

conductivity towards the first connection terminal TEl.

In a fourth embodiment of the invention shown in fig. 5 there is an eleventh switching unit Sll having the same function as the ninth switching unit in fig. 4 and a twelfth switching unit S12 having the same function as the tenth switching unit in fig. 4. The eleventh switching unit Sll is connected between the sixth switching unit S6 and the junction between the first and second switching units SI and S2, where the

eleventh switching unit Sll has a diode with a

direction of conductivity away from the first connection terminal TEl. The twelfth switching unit S12 is connected between the eight switching unit S8 and the second connection terminal TE2 and has a diode with a direction of conductivity towards the second

connection terminal TE2.

In the same manner in a fifth embodiment of the

invention that is also based on the first embodiment and being shown in fig. 6, there is a thirteenth switching unit S13 having the same function as the ninth switching unit in fig. 4 and a fourteenth

switching unit S14 having the same function as the tenth switching unit in fig. 4. The thirteenth

switching unit S13 is connected in the first branch BR1 between the second connection terminal TE2 and the first switching unit SI and has a diode with a

direction of conductivity towards the second connection terminal TE2, while the fourteenth switching unit S14 is connected in the second branch BR2 between the first connection terminal TEl and the third switching unit S3 and has a diode with a direction of conductivity away from the first connection terminal TEl.

The switching elements of the extra switching units Sll and S12 and S13 and S14, respectively, do not switch during nominal operation. Furthermore, the voltage and current ratings of Sll and S12, S13 and S14,

respectively, are the same as for the other switching units in the submodule, meaning that, in total, they correspond to a 25% increase of the combined power rating of the switching units in the converter. An advantage with these latter submodules is that it is possible to block a reverse voltage without increasing the conduction losses when the energy storage elements CI and C2 are connected in series or in parallel. The additional switching units only conduct current when the submodule is fully bypassed or when one of the two possible switching states for the intermediate voltage is used. The circuit in Fig. 5 does not give any additional conduction losses when the energy storage elements are connected in parallel by closing the switching elements of the switching units SI, S2, S3, and S4. The circuit in Fig. 6 does not give any

additional conduction losses when the energy storage elements are connected in parallel by closing the switching elements of the switching units S5, S6, S7, and S8. That is, for both submodule variations, there exist a combination of switching states that does not increase the conduction losses for the intermediate and the second voltage level. When the submodules are fully bypassed, the current must, however, always be

conducted through three series connected switching units .

When the energy storage elements are connected in parallel, a small difference in the energy storage element voltages will result in a surge current

balancing the energy storage element voltages. In order to limit this surge current, small inductors can be included in the circuit as shown in Fig. 7, which shows a sixth embodiment of the invention that is based on the first embodiment. In this sixth embodiment the submodule comprises a first inductor LI connected between the first energy storage element CI and the first connection terminal TE1 and a second inductor L2 connected between the second energy storage element C2 and the second connection terminal TE2. Ideally, the current in the first inductor LI is always the same as the current in the second inductor L2. This means that if the inductors LI and L2 are magnetically coupled, the magnetic coupling only affects the

differential mode component. That is, the voltage difference between the first and the second energy storage elements CI and C2 when these are connected in parallel. Consequently, by having magnetically coupled inductors, the surge current can be reduced even further .

As mentioned earlier the second branch could be a primary branch and the first branch could be a

secondary branch. In a variation of the second

embodiment, this means that the sixth switching unit would be connected between the second connection terminal TE2 and the junction between the third and fourth switching units in the same way as the eight switching unit shown in fig. 2. Consequently the fifth switching unit would in this situation be connected between the second end of the first energy storage element and the above mentioned junction just as the seventh switching units as shown in fig. 2. In the same way there would be no switching unit connected from the first branch to the first connection terminal or to the first end of the second energy storage element.

With reference being made to fig. 2, it also has to be mentioned that it should be realized that the series connection of the seventh and eight switching units S7 and S8 may be considered to form a first branch and the series connection of the fifth and sixth switching units S5 and S6 may be considered to form a second branch that may both be varied along the principles outlined above and described in more detail in relation to the second embodiment.

The switching elements used in the switching units of the submodules have been described as being IGBTs. It should be realized that other types of switching elements may be used, such as elements based on

thyristors, MOSFET transistors, GTOs (Gate Turn-Off Thyristor) , IGCTs (Integrated Gate Commuted Thyristor) and mercury arc valves.

The control unit need not be provided as a part of a voltage source converter. It can be provided as a separate device that provides control signals to the voltage source converter. This control unit may

furthermore be realized 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.

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. A submodule (Clul) for connection in series with other submodules in a phase leg (PL1, PL2, PL3) of a voltage source converter (10) and comprising:

a first and a second connection terminal (TE1, TE2) each providing a connection for the submodule to the phase leg,

a first and a second energy storage element (CI, C2), and

a number of switching units (SI, S2, S3, S4, S5, S6, S7, S8) configured to selectively connect the first and second energy storage element (CI, C2) between the first and second connection terminals (TE1, TE2) for obtaining at least a first voltage level (0V), a second higher voltage level (2U_d) and a third intermediate voltage level (U_d) between the

connection terminals,

wherein the switching units comprise switching units (SI, S2, S3, S4, S5, S6, S7, S8) switchable to connect the first and second energy storage elements in parallel between the first and second connection terminals (TE1, TE2) for providing the third

intermediate voltage level (U_d) .

2. The submodule according to claim 1, wherein the first energy storage element (CI) has two ends, where a first end is coupled to the first connection terminal (TE1) and a second end is coupled to the second

connection terminal (TE2) via a first branch (BR1) comprising a first primary switching and a second secondary switching unit (SI, S2), the second energy storage element (C2) has two ends, where a first end is coupled to the first connection terminal (TE1) via a second branch (BR2) comprising a third primary and a fourth secondary switching unit (S3, S4) and a second end is coupled to the second connection terminal (TE2), one of the branches is a primary branch and the other is a secondary branch, the submodule further comprising a fifth switching unit (S5) connected between the junction joining the switching units of the primary branch and the point where the secondary branch is connected to its corresponding energy storage element as well as a sixth switching unit (S6) connected between said junction of the primary branch and the connection terminal (TE1) to which the energy storage element associated with the primary branch is

connected.

3. The submodule according to claim 2, wherein the switching units of the first branch (BR1) are

selectively controllable to connect the first energy storage element (CI) between the two connection

terminals (TE1, TE2) for providing the third voltage level ( U_d ) , the switching units of the second branch (BR2) are selectively controllable to connect the second energy storage element (C2) between the two connection terminals (TE1, TE2) for providing the third voltage level ( U_d ) , the sixth switching unit (S6) and primary switching unit (SI) of the primary branch are selectively controllable for interconnecting the two connection terminals (TE1, TE2) for providing the first voltage level (0V) and the fifth switching unit (S5) and secondary switching unit (S2) of the primary branch are selectively controllable for connecting the first and second energy storage elements (CI, C2) in series between the connection terminals TE1, TE2) for providing the second voltage level (2U_d) ·

4. The submodule according to claim 3, wherein also the sixth and the fifth switching units (S6, S5) are selectively controllable to connect the second energy storage element (C2) between the two connection terminals for providing the third voltage level.

5. The submodule according to any of claim 2 - 4, wherein the primary branch is the first branch and the secondary branch is the second branch with the sixth switching unit (S6) connected between the first

connection terminal (TE1) and the junction between the first and second switching units (SI, S2), with the fifth switching unit (S5) being connected between the same junction and the first end of the second energy storage element (C2), and the submodule further

comprises a seventh switching unit (S7) connected between the second end of the first energy storage element (CI) and the junction between the third and fourth switching units (S3, S4) of the second branch (BR2) and an eighth switching unit (S8) connected between the second connection terminal (TE2) and the junction between the third and fourth switching units (S3, S4) of the second branch (BR2) .

6. A submodule according to claim 5, wherein the third and eighth switching units (S3, S8) are

selectively controllable for interconnecting the two connection terminals (TE1, TE2) for providing the first voltage level, the fourth and seventh switching units (S4, S7) are selectively controllable for connecting the first and second energy storage elements (CI, C2) in series between the connection terminals (TE1, TE2) for providing the second voltage level and the seventh and eighth switching units (S7, S8) are selectively controllable for connecting the first energy storage element (CI) between the two connection terminals (TE1, TE2) for providing the third voltage level.

7. The submodule according to any of claims 2 - 6, wherein the switching units are provided as switching elements with anti-parallel diodes, where the direction of conductivity of the diode of the first switching unit (SI) is away from the second connection terminal (TE2) and the direction of conductivity of the diode of the sixth switching unit (S6) is towards the first connection terminal (TE1), the submodule further comprising a ninth switching unit (S9) between the sixth switching unit (S6) and the first connection terminal (TE1), the ninth switching unit (S9) having a diode with a direction of conductivity away from the first connection terminal (TE1).

8. The submodule according to claim 7, further comprising a tenth switching unit (S10) between the second end of the second energy storage element (C2) and the second connection terminal (TE2), the tenth switching unit (S10) having a diode with a direction of conductivity towards the second connection terminal (TE2) .

9. The submodule according to any of claims 2 - 6, wherein the switching units are provided as switching elements with anti-parallel diodes, where the direction of conductivity of the diode of the first switching unit (SI) is away from the second connection terminal (TE2) and the direction of conductivity of the diode of the sixth switching unit (S6) is towards the first connection terminal (TE1), the submodule further comprising a thyristor (Tl) connected between the first and second connection terminals (TE1, TE2), said thyristor (Tl) having a direction of conductivity towards the first connection terminal (TE1) .

10. The submodule according to claim 9, further comprising an eleventh switching unit (Sll) between the sixth switching unit (S6) and the junction between the switching units of the primary branch, the eleventh switching unit (Sll) having a diode with a direction of conductivity away from the first connection terminal (TE1) .

11. The submodule according to claim 10 as

dependent on claim 5 or 6, further comprising a twelfth switching unit (S12) between the eighth switching unit (S8) and the second connection terminal (TE2), the twelfth switching unit (S12) having a diode with a direction of conductivity towards the second connection terminal (TE2) .

12. The submodule according to claim 9, further comprising a thirteenth switching unit (S13) in the first branch (BR1), the thirteenth switching unit (S13) having a diode with a direction of conductivity towards the second connection terminal (TE2) .

13. The submodule according to claim 12, further comprising a fourteenth switching unit (S14) in the second branch (BR2), the fourteenth switching unit (S14) having a diode with a direction of conductivity away from the first connection terminal (TE1) .

14. The submodule according to any previous claim, further comprising a first inductor (LI) connected between the first energy storage element (CI) and the first connection terminal (TE1) and a second inductor (L2) connected between the second energy storage element (C2) and the second connection terminal (TE2) .

15. A voltage source converter (10) including at least one phase legs (PL1, PL2, PL3) , where each phase leg includes a number of submodules (Clul, C2u2, C3u3, Clll, C211, C311, Clu2, C2u2, C3u2, C112, C212, C312, Clu3, C2u3, C3u3, C113, C213,C313) and at least one submodule (Clul) comprises:

a first and a second connection terminal (TE1, TE2) each providing a connection for the submodule to the corresponding phase leg,

a first and a second energy storage element (CI, C2 ) , and

a number of switching units (SI, S2, S3, S4, S6, S6, S7, S8) configured to selectively connect the first and second energy storage element (CI, C2) between the first and second connection terminals (TE1, TE2) for obtaining at least a first voltage level (0V), a second higher voltage level (2U_d) and a third

intermediate voltage level (U_d) between the

connection terminals,

wherein the switching units comprises switching units (SI, S2, S3, S4, S6, S6, S7, S8) switchable to connect the first and second energy storage elements in parallel between the first and second connection terminals (TE1, TE2) for providing the second intermediate voltage level (U_d) .

16. The voltage source converter (10) according to claim 15, further comprising a control unit (12) configured to provide control signals to converter sumodules in each branch in order to control the operation of the voltage source converter.

17. A voltage source converter (10) according to claim 16, wherein the first energy storage element (CI) of the three-level submodule (Clul) has two ends, where a first end is coupled to the first connection terminal (TE1) and a second end is coupled to the second

connection terminal (TE2) via a first branch (BR1) comprising a first primary and a second secondary switching unit (SI, S2), the second energy storage element (C2) has two ends, where a first end is coupled to the first connection terminal (TE1) via a second branch (BR2) comprising a third primary and a fourth secondary switching unit (S3, S4) and a second end is coupled to the second connection terminal (TE2), where one of the branches is a primary branch and the other is a secondary branch, the submodule further comprising a fifth switching unit (S5) connected between the junction joining the switching units of the primary branch and the point where the secondary branch is connected to its corresponding energy storage element as well as a sixth switching unit (S6) connected between said junction of the primary branch and the connection terminal (TE1) to which the energy storage element associated with the primary branch is

connected .

18. The voltage source converter (10) according to claim 17, wherein the control unit (12) is configured to selectively control the switching units of the first branch (BR1) to connect the first energy storage element (CI) between the two connection terminals (TE1, TE2) for providing the third voltage level (¾) , the switching units of the second branch (BR2) to connect the second energy storage element (C2) between the two connection terminals (TE1, TE2) for providing the third voltage level (¾) , the sixth switching unit (S6) and primary switching unit (SI) of the primary branch for interconnecting the two connection terminals (TE1, TE2) for providing the first voltage level (0V) and the fifth switching unit (S5) and secondary switching unit (S2) of the primary branch for connecting the first and second energy storage elements (CI, C2) in series between the connection terminals for providing the second voltage level (2U_d) .