The technology disclosed herein relates generally to the field of power converters, and in particular to a modular multilevel converter for use in a high voltage traction system, and different arrangements comprising such modular multilevel converter.

Background

A traction power network (or simply traction network) is an electricity grid for the power supply of electrified railway networks. A separate traction network is generally installed only for the case that the railway in question uses alternating current (AC) with a frequency lower than that of a national utility grid. In other instances, the three-phase alternating current of the national utility grid can be converted in substations by rotary transformers or static inverters into the voltage and type of current required by the trains. For railways which run on direct current (DC), this method is always used, as well as for railways which run on single-phase AC of lower frequency. The converter stations were originally rotating converters, but have gradually been replaced by static converters.

Figure l illustrates known solutions for a railway power supply system. In a decentralized solution (left-hand part of below figure 1), a national public power grid loo and an overhead contact line no (also denoted catenary) of the railway grid are interconnected by means of a number of converter stations 120a located

approximately 100 km apart. In a centralized solution (right-hand part of figure 1), the overhead contact line 110 feeding the train is interconnected to a (approximately) 132 kV transmission line 130 via step-up single-phase transformers (three illustrated in the box of figure 1). The step-up single-phase transformers are used to reach the high voltage of the public grid 100. In this case, the overhead contact line 110 is fed by step-down transformers 140 located approximately 50 km apart. The converter stations 120b might then be located as far as 200 km from each other. Similar topologies are valid in other 15 kV, 16.7 Hz railway power supply systems. In some countries, the traction energy is converted from the 50 Hz three-phase utility grid by means of converter stations or generated by special one-phase generators and transmitted over the 110 kV railway grid to the substations which supply the trains at The recent conversion stages are based on modular multilevel converter (MMC) topologies which offer high level of modularity, high waveform quality and lower loses compared to conventional monolithic converters. The ac/ac frequency converters topologies are either based on the Back-To-Back (BTB) or Double-Star Bridge-Cells (DSBC) structures. In common for these known solutions is their big volume, which is due to the size of the cell capacitors, and that both of these methods (BTB, DSBC) require a large low-frequency transformer in order to adjust their output single-phase voltage for I32kv railway grids (as shown in Figure l). Such low- frequency transformer of about 25-30MW at 16.7Hz is heavy and bulky and also reduce the efficiency of the system.

Summary

An objective of the present invention is to address and improve various aspects for high voltage railway grids. A particular objective is to provide a modular multilevel converter for a high voltage traction system enabling the omission of transformers. Another particular objective is to provide higher flexibility to converters of high voltage railway applications. This objective and others are achieved by a modular multilevel converter according to the appended independent claim, and by the embodiments according to the dependent claims.

The objective is according to an aspect achieved by a modular multilevel converter for use in a high voltage ac/ac traction system. The modular multilevel converter 10 comprises three phases, each phase comprising a number of series-connected cascaded converter cells. In the modular multilevel converter, the three phases are series-connected and arranged to provide a single-phase alternating current to the high voltage traction system.

The modular multilevel converter according to the invention provides a number of advantages. For instance, by series-connecting the phases a higher design flexibility is obtained, e.g. in view of number of converter cells to use. Further, each phase may have a lower rating compared to the conventional converters that have parallel- connected phases. Further, the converter according to the invention has high DC fault tolerance. Still further, the invention provides a compact converter for high voltage applications, in particular for use in high voltage railway grids, e.g. since no bulky transformers are required owing to the series-connecting of the phases. The reduction of footprint enabled by the invention is particularly important in urban environments, wherein available land is scarce and expensive. The design flexibility allows omission of transformers for tapping down/tapping up voltage to required levels. More generally, the suggested converter gives increased flexibility for converter design for high voltage applications.

In an embodiment, the single-phase alternating current side of the modular multilevel converter is connected directly to a high voltage low frequency side of the high voltage traction system. The low-frequency single-phase transformers that are currently used between the converters with parallel-connected phases and the traction systems can be omitted by means of the invention.

In some embodiments, each phase is connected to a respective alternating current phase conductor of a high voltage utility grid on a three-phase side of the modular multilevel converter. The phases may be connected to the high voltage utility grid via some additional devices (e.g. transformer or filter) on the three-phase utility side.

In some embodiments, the converter cells are full-bridge converter cells.

The objective is according to an aspect achieved by use of the modular multilevel converter as above in a high voltage traction system, wherein the modular multilevel converter is connected to at its single-phase alternating current side to the traction system and at its three-phase alternating current side to a public utility grid.

The objective is according to an aspect achieved by an arrangement for use in a high voltage traction system, the arrangement comprising a first and a second modular multilevel converter as above, wherein the first and the second modular multilevel converters are series-connected.

The objective is according to an aspect achieved by an arrangement for use in a high voltage traction system, the arrangement comprising a first and a second modular multilevel converter as above, wherein the first and the second modular multilevel converters are connected in a back-to-back structure.

Further features and advantages of the embodiments of the present invention will become clear upon reading the following description and the accompanying drawings. Brief description of the drawings

Figure l illustrates a known decentralized solution and a known centralized solution.

Figure 2 illustrates a basic circuit structure for a series-connected modular multilevel converter according to an embodiment of the invention.

Figure 3 illustrates a 130 kV system using two series-connected modular multilevel converter arms according to an embodiment of the invention in series in a bipolar fashion.

Figure 4 illustrates embodiments of a series connection of phases used in back-to- back structure.

Detailed description

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description with unnecessary detail. Same reference numerals refer to same or similar elements throughout the description.

Briefly, the invention addresses the problems mentioned in the background section by suggesting use of cascaded converters in a series connection for ac/ac high voltage railway applications. In particular, the present invention provides a design wherein phases of a modular multilevel converter are series-connected and used for ac/ac railway systems, in particular a traction system thereof. The modular multilevel converter according to the invention offers a high degree of flexibility for ac/ac conversion to reach the high voltage railway grid. This enables the currently used low-frequency single-phase transformer to be removed, which thus saves on cost as well as footprint. In addition, the invention requires a lower number of cells compared to the conventional MMC solutions with the phases parallel-connected.

Figure 2 shows the basic circuit structure for a series-connected MMC 10 according to the present invention. The illustrated MMC 10 comprises twelve identical arms which form the three-stacked single phase converters. Each arm is formed by a stack of series-connected full-bridge converter cells. Each full-bridge converter cell 12, often denoted full-bridge submodule, comprises four switching devices Si, S2, S3, S4, a (anti-parallel) diode and a capacitor. The switching devices Si, S2, S3, S4 may , for instance, be insulated gate bipolar transistors (IGBTs). The full-bridge converter cells 12 are controllable by a control device (not illustrated), which may transmit control signals for changing state of the full-bridge converter cells 12, block the full-bridge converter cells 12 etc.

As a note on vocabulary, in the MMC, a number of converter cells are series- connected with an inductor to form a converter arm. Two converter arms are used to build a converter leg, and each leg is then one phase of the MMC.

In figure 2, Uac indicates the single-phase input voltage,/ac is the frequency of Uac, lac is the input current, Ua, Ub and Uc are the three-phase output voltages of the series-connected MMC 10, and/is the frequency of the three-phase voltages. In this case,/ac cannot be equal to f for proper operation. UaP and UaN are the arm voltages, which each include the two frequency components, fac and/. The positive and negative arms are connected to the upper and lower terminals of an inductor LA, LB, LC, respectively, and the center terminal produces an ac voltage with a frequency/ at each converter leg.

In Figure 2, iaP and iaN are the positive and negative arm currents, respectively, ia is the output current, and iz is the circulating current in the u-phase loop. The following equations hold for the four currents:

It is noted that iz is not a dc quantity, but an ac quantity with a dominant frequency being frequency component fac. Instantaneous voltages UaP and UaN, shown in figure 2, are given by:

1 u. c 1 u.

u. a U, ac

aP a ' u. aN + U, a

2 3 2 3

The series-connected MMC 10 is, as mentioned earlier, controlled by a control device. The control of the series-connected MMC 10 may entail use of control loops controlling voltages. According to the invention, the described MMC 10 with series- connected phases may be used in an electric traction system 20 which provides electric power to the trains of a railway system.

Figure 3 illustrates a 130 kV system using two series-connected MMC 10a, 10b in series in a bipolar fashion. Each series-connected MMC 10a, 10b may provide a respective single phase ac to the electric tractions system 20, e.g. 65 kV, 16,7 Hz. A first series-connected MMC 10a is connected to a grounding pole, as is a second series-connected MMC 10b. By means of this arrangement 30, the added voltages of the first and second series-connected MMCs 10a, 10b can be reached. For instance, if each series-connected MMC 10a, 10b is rated for 65 kV, then 130 kV can be reached. The arrangement 30 may be used for the electric traction system 20, in particular connected between a national utility grid and the traction system 20 without any transformers being needed, or with a highly reduced number of transformers.

Figure 4 illustrates a series connection of phases used in a back-to-back structure. That is, a first and a second series-connected MMC 10a, 10b are connected in a back- to-back arrangement. A conventional back-to-back structure using a MMC is an indirect ac-to-ac conversion, in particular, an ac-to-dc-to-ac conversion. The series- connected MMC 10a, 10b according to the invention may replace such AC/DC converter on the three-phase side. This arrangement 40 of the first and second series- connected MMC 10a, 10b connected in a back-to-back structure reduces the losses, cost and footprint compared to the known parallel MMC.

At the upper-most part of figure 4, the converter cells 12 are indicated as all being full-bridge converter cells. In the lower-most part of figure 4 a variation is illustrated. In particular, in the back-to-back structure of the series-connected MMCs shown at the lower-most part of figure 4, one converter leg (left-most) comprises one-phase converter arms. That is, the converter cells of one converter arm may be half-bridge cells, and the other converter arm comprises full-bridge cells. It is noted that still further variations are conceivable. In essence, the phases to be series-connected may be designed differently, using different converter cells.

The series-connection of the phases according to the invention provides several advantages. For instance, in the known MMC, wherein the phases are connected in parallel, each phase needs to be able to block the total output voltage. In contrast to this, in the series-connected MMC according to the present invention, each phase needs only be able to block a third of the output voltage. Another advantage is that the footprint of the converter can be reduced compared to the known MMC

structures, since series-connection of the phases can be done in a more space efficient way. Further still, the invention enables a high design flexibility as has been described e.g. in relation to figure 4.

The invention has mainly been described herein with reference to a few

embodiments. However, as is appreciated by a person skilled in the art, other embodiments than the particular ones disclosed herein are equally possible within the scope of the invention, as defined by the appended patent claims.