VOLTAGE EQUIPMENT

FIELD OF INVENTION

The present invention generally relates to high voltage equipment. More particularly the present invention relates to a switching block for use in the forming of high voltage equipment as well as to a modular multilevel converter with submodules of which at least one has been formed through such a switching block.

BACKGROUND

The performance of state of the art high power high voltage (HV) switches, such as HV Insulated-Gate Bipolar Transistors (IGBTs), is limited by the overall stray inductance and the available electrical contact area.

This has led to the development of new power switch packages for high power IGBTs consisting of half-bridge configurations. Examples on these packages are LinPak packages, flexible High-power Platform packages and Next High Power Density Dual packages.

A package with switches may then be used to construct a unipolar submodule for a Modular Multilevel Converter (MMC). Compared to the configuration with standard HV IGBTs which contains only one switch and its antiparallel free-wheeling diode, for instance the LinPak includes already the two switches with their freewheeling diode needed for the unipolar submodule. The packages may thus be used a building blocks for forming the submodules of an MMC.

However, there exists a problem with a modular multilevel converter being constructed using such unipolar submodules. The problem is that the loss distribution between the two switches of the package/submodule is not equal. The output power of the converter is therefore limited either by the diode or the transistor of one of the switches in dependence on the operating point of the converter. The invention addresses this problem.

SUMMARY OF THE INVENTION

The present invention is directed towards providing a more even power loss distribution of a switching block used for forming high voltage equipment, such as a modular multilevel converter.

This object is according to a first aspect achieved through a switching block for use in forming high voltage equipment, where the switching block comprises a first type of switch in series with a second type of switch and the first type of switch comprises a transistor with anti-parallel diode, while the second type of switch comprises a reverse conducting power transistor. This object is according to a second aspect achieved through a modular multilevel converter comprising at least one phase leg comprising a number of submodules connected in cascade, wherein at least one of the submodules is a submodule formed based on a switching block according to the first aspect.

The invention has a number of advantages. It reduces the power loss imbalance between the switches of the switching block. Thereby it is also possible to increase the output power and reduce the number of

components of a converter that has been formed using such switching blocks. 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 modular multilevel converter connected between a pole and ground,

fig. 2 schematically shows a unipolar submodule used in the converter, and fig. 3 schematically shows a power switch package used to form the submodule.

DETAILED DESCRIPTION OF THE INVENTION

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

Fig. l shows one variation of voltage source converter that is a modular multilevel converter (MMC) io. The converter operates to convert between alternating current (AC) and direct current (DC). The converter io in fig. l comprises a three-phase bridge made up of a number of phase legs. There are in this case three phase legs. It should however be realized that as an alternative there may be for instance only two phase legs. There is thus a first phase leg PLi, a second phase leg PL2 and a third phase leg PL3. The phase legs are more particularly connected between a first DC pole Pi and ground, where the mid points of the phase legs are connected to

corresponding alternating current terminals ACAl, ACBi, ACCi. A phase leg is in this example divided into two halves, a first upper half and a second lower half, where such a half is also termed a phase arm. The first DC pole Pi furthermore has a first potential Vp that may be positive. The first pole Pi may therefore also be termed a positive pole. The pole may furthermore be part of a DC power transmission system such as a High Voltage Direct Current (HVDC) power transmission system. The AC terminals ACAi, ACBi, ACCi is in turn connected to an AC system, such as a flexible alternating current transmission system (FACTS), for instance via a transformer. A phase arm between the first pole Pi and a first AC terminal ACAi, ACBi and ACCi may be termed a first phase arm or an upper phase arm, while a phase arm between the first AC terminal and ground may be termed a second phase arm or a lower phase arm.

As mentioned above, the type of voltage source converter shown in fig. l is only one example of a modular multilevel converter where the invention may be used. It is for instance possible to provide a series modular multilevel converter comprising three converter modules, one for each phase, stacked upon each other in series between the two poles. It is also possible to provide the converter as a Static VAR compensator (SVC) in an AC system.

Moreover, the voltage source converter depicted in fig. ι has an

asymmetric monopole configuration. It is thus connected between a pole and ground. As an alternative it may be connected in a symmetric bipole configuration or symmetric monopole configuration.

The phase arms of the voltage source converter 10 in the example in fig. ι comprise submodules. A submodule is a unit that may be switched for providing a voltage contribution to the voltage on the corresponding AC terminal. A submodule then comprises one or more energy storage elements, for instance in the form of capacitors, and the submodule may be switched to provide a voltage contribution corresponding to the voltage of the energy storage element or a zero voltage contribution. If more than one energy storage element is included in a submodule it is possible with even further voltage contributions.

The submodules are with advantage connected in series or in cascade in a phase arm. In the example given in fig. 1 there are five series-connected or cascaded submodules in each phase arm. Thus the upper phase arm of the first phase leg PLi includes five submodules SMipi, SM2pi, SM3pi, SM4pi and SM5pi, while the lower phase arm of the first phase leg PLi includes five submodules SMini, SM2ni, SM3ni, SM-jni and SMsni. As the upper phase arm is connected to the first pole Pi it may also be considered to be a positive phase arm. Consequently the lower phase arm may be considered to be a negative phase arm. The upper phase arm is furthermore joined to the AC terminal ACAl via a first or upper arm reactor Laarmi, while the lower phase arm is joined to the same AC terminal ACAl via a second or lower arm reactor Laarm2. In a similar fashion the upper phase arm of the second phase leg PL2 includes five submodules SMip2, SM2p2, SM3p2, SM4p2 and SMsp2 while the lower phase arm of the second phase leg PL2 includes five submodules SMm2, SM2n2, SM3n2, SM4n2 and SMsn2. Finally the upper phase arm of the third phase leg PL3 includes five submodules SMip3, SM2p3, SM3P3, SM4P3 and SM5P3 while the lower phase arm of the third phase leg PL3 includes five submodules SMm3, SM2n3, SM3n3, SM4n3 and SMsn3. The upper phase arms are

furthermore joined to the corresponding AC terminals ACBi and ACCi via corresponding first or upper arm reactors Lbarmi and Lcarmi,

respectively, while the lower phase arms are joined to the same AC terminal ACBi and ACCi via corresponding second or lower arm reactors Lbarm2 and Lcarm2, respectively. The number of submodules provided in fig. 1 is only an example. It therefore has to be stressed that the number of submodules in a phase arm may vary. It is often favorable to have many more submodules in each phase arm, especially in HVDC applications. A phase arm may for instance comprise hundreds of submodules. There may however also be fewer.

Control of each submodule in a phase arm is normally done through providing the submodule with a control signal directed towards controlling the contribution of that submodule to meeting a reference voltage. The reference voltage may be provided for obtaining a waveform on the AC terminal of a phase leg, for instance a sine wave. In order to control the cells there is therefore a control unit 12. The control unit 12 is provided for controlling all the phase arms of the converter. However, in order to simplify the figure only the control of the upper phase arm of the first phase leg PL is indicated in fig. 1.

One submodule that is frequently used is the unipolar or half-bridge submodule that is formed through a string of switches connected in parallel with an energy storage element, for instance in the form of a capacitor.

Both the switches of such a submodule are typically realized using the same type of switch, for instance as a transistor with anti-parallel diode, where the transistor may be an Insulated-Gate Bipolar Transistor (IGBT).

There exist a problem with this type of realization and that is that the loss distribution between the two switches may be unequal. This may for instance be the case for the converter shown in fig. 1.

Aspects of the invention address this problem.

Fig. 2 schematically shows a unipolar submodule or half-bridge submodule HBSM according to a first variation of the invention.

The submodule HBSM in fig. 2 is thus a half-bridge submodule and includes an energy storage element, here in the form of a capacitor C, which is connected in parallel with a first string of switches Si and S2. The energy storage element C provides a voltage Udm, and therefore has a first, positive and second, negative end, where the first, positive end has a higher potential than the second, negative end. The switches Si and S2 in the first string are connected in series with each other, where the first switch may be realized in the form of a switching element that may be an Insulated Gate Bipolar Transistor (IGBT) together with an anti-parallel unidirectional conducting element that is here a diode. The second switch S2 is in turn realized as a reverse conducting power transistor, like a reverse conducting IGBT. It may more particularly be a Bi-mode Insulated Gate Transistor (BIGT). The second switch S2 is thus a power

semiconductor device. In fig. 2 the first switch Si, which is also an upper switch, has a first transistor Ti with a first anti-parallel diode Di, where the first diode Di is connected between the emitter and collector of the transistor Ti and has a direction of conductivity from the emitter to the collector as well as towards the positive end of the energy storage element C. Thereby the collector of the switching element Ti is also connected to the first end of the energy storage element C. The second switch S2, which in turn is a lower switch, comprises a second transistor T2 having an emitter connected to the second end of the energy storage element.

Moreover the emitter of the first switching element Ti is also connected to the collector of the second switching element T2.

This half-bridge submodule HBSM comprises a first connection terminal TEi and a second connection terminal TE2 in order to connect the submodule in cascade with other submodules of a phase leg or phase arm. The junction between the first and second switches Si and S2 therefore provides the first connection terminal TEi, while the junction between the second switch S2 and the second end of the energy storage element provides a second connection terminal TE2. These connection terminals

TEi and TE2 thus provide points where the submodule HBSM as an example can be connected to the upper phase arm of the first phase leg. The first connection terminal TEi thereby joins the upper phase arm with the connection point or junction between two switches Si and S2 of the string, while the second connection terminal TE2 joins the upper phase arm with a connection point between the second switch S2 and the second end of the energy storage element C. It can also be observed that the second type of switch S2 is connected between the two connection terminals TEl and TE2. Thereby it will provide the zero voltage submodule contribution when being turned on. Fig. 3 schematically shows a power switch package PSP that may be used in the forming of a submodule. The power switch package PSP may be a so-called LinPak power switch package comprising the first and second switches Si and S2 described above as well as the first connection terminal TEl. However it does not comprise the energy storage element or the second connection terminal. The above-described power switch package has the advantage of providing a building block that can be used for forming unipolar or half-bridge submodules as well as bipolar or full- bridge submodules. Another advantage of such a power switch package is that it has a low stray inductance.

As was mentioned above, there may be a problem of uneven power losses when a submodule is used in which both switches are of the same type, such as IGBTs. This would in the case of the submodule in fig. 2 mean that the loss distribution between the upper switch and the lower switch would not be equal and the output power would be limited either by the diode or the transistor of the lower switch.

The invention proposes a switching block for use in forming high voltage equipment, where the high voltage equipment may be an MMC, such as the MMC in fig. 1. This switching block comprises a first type of switch Si in series with a second type of switch S2, wherein the first type of switch comprises a transistor with anti-parallel diode and the second type of switch comprises a reverse conducting power transistor like a bi-mode insulated gate transistor. In one variation of the invention the switching block is the power switch package of fig. 3 and in another variation it is the submodule in fig. 3. In the latter case, the submodule may be formed using the power switch package. In the latter case the switching block that forms the submodule may thus comprise the power switch package. Through providing a hybrid submodule based on a power switch package, such as the ABB LinPak, where the hybrid submodule comprises a conventional IGBT/diode pair together with a reverse conducting power transistor chip set, such as a BiGT chip set, the previously described unequal loss distribution between the upper and lower switches is eliminated or reduced. This type of structure will therefore be

advantageous to use for forming a unipolar submodule of a MMC due to the fact that it eliminates the drawback of the unequal loss distribution using IGBT/diode pairs.

As can be seen in fig.2, the lower switch S2, i.e. the switch connected between the connection terminal TEl and TE2, is a reverse conducting IGBT or BiGT.

Through using a BiGT for the lower switch S2 the problem of unequal loss distribution is thus addressed.

With the proposed power switch package it is possible to built a standard submodule suitable for MMC purposes. Due to the easy scalability of the power switch package it can be used as a base for a wide range of MMC applications.

Moreover, since the output power may be increased, it is also possible to reduce the number of components. Thereby also the stray inductance and footprint can be reduced compared with a conventional submodule and package construction.

As indicated above it is possible to use power module packages also to construct bipolar or full-bridge submodules. It should be realized that it is possible to construct also such submodules through a mixture of the two types of switches. Moreover, in a variation of the unipolar submodule, it is possible to place the second connection terminal at the junction between the first switch and the first end of the energy storage element. It is possible to have the two different types of switches also in this variation of the unipolar submodule. In this case the first switch may be the second type of switch and the second switch may be the first type of switch, where in this case the first switch is connected between the two connection terminals.

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.