EP3931955A1

EP3931955A1 - Ac-to-ac mmc with reduced number of converter arms

Info

Publication number: EP3931955A1
Application number: EP19708287.8A
Authority: EP
Inventors: Thomas SCHAAD; Michail VASILADIOTIS; Georgios STAMATIOU; Jan Svensson
Original assignee: ABB Power Grids Switzerland AG
Current assignee: Hitachi Energy Ltd
Priority date: 2019-02-27
Filing date: 2019-02-27
Publication date: 2022-01-05
Also published as: WO2020173563A1

Abstract

The present disclosure relates to an AC-to-AC MMC (2) configured to be connected between a first AC system (10a) and a second AC system (10b). The MMC comprises a plurality of parallel connected phase legs (4), each phase leg comprising an upper converter arm (5a) between a terminal (x/y) for a phase line of one of the first or second AC system (loa/lob) and an upper DC link (9a), and a lower converter arm (5b) between said terminal and a lower DC link (9b). Each arm comprises a plurality of series-connected converter cells (7). The MMC also comprises an intermediary leg (n), connected to the upper DC link and the lower DC link and across the phase legs, the intermediary leg comprising upper (8a) and lower (8b) series connected capacitor arrangements. The intermediary leg is configured to be connected to one phase line of each of the first and the second AC systems between the upper and lower capacitor arrangements.

Description

AC-TO-AC MMC WITH REDUCED NUMBER OF CONVERTER

ARMS

TECHNICAL FIELD

The present disclosure relates to an MMC connected between a first AC system and a second AC system.

BACKGROUND

Modular Multilevel Converters (MMCs) are used for medium voltage (MV) and high-voltage (HV) converter applications. An MMC comprises converter arms of series-connected (also called cascaded) converter cells, each cell comprising an energy storage (typically capacitor) and a plurality of semiconductor valves forming a full-bridge (also called H-bridge or bi-polar) or half-bridge (also called unipolar) topology of the cell.

A direct converter in a double-star (also called double-Y or double-wye) topology is illustrated in figure ta, which is e.g. used in a railway intertie for three-phase alternative current (AC), 3AC, to single-phase AC (lAC) conversion, e.g. 3AC 50 Hz to lAC 16.7 Hz (or 60 Hz-to-25 Hz) conversion, for low-frequency catenary systems, but is not the most appropriate solution when the input and the output frequencies are equal, e.g. 50 Hz-to-50 Hz. In such a case, an undesirable cross-interaction of voltage and current components with the same frequency may occur between the two converter sides, which creates a constant-power infeed to the cell capacitors causing them to diverge. This problem can be mitigated using controlled injection of certain common mode voltage and circulating current harmonic components through the converter arms that will redistribute the charge and restore the balance. However, these methods require significant overrating of the converter devices to be able to run the increased voltage and current harmonics. Without the use of harmonic injection, the converter can perform same frequency conversion but only under a unique choice of power factors at its input and output, which is undesirable. An alternative can be an indirect conversion through a direct current (DC) link in accordance with figure lb, where an intermediate conversion step decouples the two AC systems, so that the capacitors of the same cells will not see the same frequency components. However, this solution with two separate converters (one three-phase and one single-phase) back-to-back with a DC link between requires the use of ten converter arms of a total of five phase-legs (one phase- leg consists of two converter arms, one upper converter arm and one lower converter arm). Each converter cell has a DC-capacitor. However, to increase the value of the total DC capacitor, a DC-link capacitor can be connected between the three-phase and the single-phase converter.

Extending the concept of utilizing a DC-link to couple converters connecting different three-phase grids for different applications, leads to the well-known back-to-back configuration of figure lc. Although this is an effective topology for the task, the solution requires twelve converter arms (six phase-legs) and becomes expensive. Also, for this topology it is possible to increase the total DC-capacitor value by adding extra capacitors on the DC-link between the two three-phase converters.

For instance, the increased electrification of railway locomotives worldwide has been a driving force for the development and installation of efficient power-electronic converters for handling the railway power supply. In many cases, other than a typical three-phase supply, locomotives require single- phase voltage supply, of the same or different frequency than the main three- phase grid. Specialized power-electronic topologies are thus employed to perform the necessary voltage/power transformation between the main grid and the railway supply system, that are supposed to be decoupled from each other. Moreover, to increase the performance of pumped hydro storage, full power converters are introduced to form a three-phase back-to-back converter system in which one side is connected to the grid and the other side is connected to a synchronous machine. The synchronous machine operates at a variable speed and acts both as a motor to pump up water and as a generator to tap the water storage to produce electricity. SUMMARY

It is an objective of the present invention to provide an AC-to-AC MMC which reduced complexity and cost.

According to an aspect of the present invention, there is provided an AC-to- AC MMC configured to be connected between a first AC system and a second AC system. The MMC comprises a plurality of parallel connected phase legs, each phase leg comprising an upper converter arm between an AC terminal for a phase line of one of the first or second AC systems and an upper DC link, and a lower converter arm between said terminal and a lower DC link. Each arm comprises a plurality of series-connected converter cells. The MMC also comprises an intermediary leg, connected to the upper DC link and the lower DC link and across the phase legs. The intermediary leg comprises upper and lower series connected capacitor arrangements. The intermediary leg is configured to be connected via at least one AC terminal to one phase line each of the first and the second AC systems between the upper and lower capacitor arrangements.

According to another aspect of the present invention, there is provided an MMC arrangement comprising an embodiment of the MMC of the present disclosure, connected between the first and second AC systems, and the AC terminals connecting the MMC to the first and second AC systems,

respectively.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of “first”,“second” etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Figures la, lb and IC are schematic illustrations of different MMC topologies of the prior art.

Figure 2 is a schematic circuit diagram of an MMC topology for a three-phase to one-phase converter, in accordance with embodiments of the present invention.

Figure 3 is a schematic circuit diagram of an MMC topology for a three-phase to three-phase converter, in accordance with embodiments of the present invention. Figure 4 is a schematic circuit diagram of an MMC topology for a three-phase to three-phase converter for a synchronous machine, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

Figure 2 illustrates an embodiment of an MMC arrangement 1 comprising a three-to-single phase AC MMC 2. The MMC 2 is connected to and between a first AC system 10a, in this case a three-phase system, and a second AC system 10b, in this case a single-phase system e.g. a railway system for powering trains 20. Conventionally, the three-phase system of the first AC system 10a connects to the MMC 2 via three phase lines, one per phase, which each connects at respective input/output terminals xi, x2 and xy of the MMC 2, typically via (an optional) first transformer 3a, or a line reactor, functioning as a galvanic insulation between the three-phase system 10a and the single-phase system 10b. Similarly, the single-phase system of the second AC system 10b connects to the MMC 2 via two phase lines, one for the single phase and one grounded and/or for closing the circuit of the single phase, optionally via a second transformer 3b or line reactor. If a second

transformer 3b is not used, the grounded phase line also grounds the intermediary leg 11 by its connection to the xy terminal thereon. The first AC terminals, connecting the MMC 2 to the first AC system 10a, are herein denoted x, while the second AC terminals, connecting the MMC to the second AC system 10b are herein denoted y. In case a communal terminal is used on the intermediary leg 11, it is denoted xy.

The MMC 2 has a double-star topology, with upper and lower DC links 9a and 9b between the at least one phase leg 4a and 4b connected to the first AC system 10a and the at least one phase leg 4c connected to the second AC system 10b. Each phase leg 4 comprises an upper converter arm 5a and a lower converter arm 5b, typically each in series with an arm reactor 6. The upper end of each upper arm 5a is connected to an upper DC link 9a, and the lower end of each lower arm 5b is connected to a lower DC link 9b. Each converter arm 5 comprises a plurality of series-connected converter cells 7. Each cell 7 comprises an energy storage (typically a capacitor arrangement comprising at least one capacitor or supercapacitor) and a plurality of semiconductor valves forming a full-bridge (also called H-bridge or bi-polar) or half-bridge (also called unipolar) topology of the cell. Any suitable valve configuration may be used, e.g. a one-directional semiconductor switch connected across an anti-parallel diode, wherein the semiconductor switch e.g. may comprise an Insulated-Gate Bipolar Transistor (IGBT), an

Integrated Gate-Commutated Thyristor (IGCT) or a Metal-Oxide- Semiconductor Field-Effect Transistor (MOSFET). If power is only transferred from the first AC system toa to the second AC system tob, as in the example with the second AC system being a railway system of figure 2, and only DC current being circulated over the DC links 9, it may be convenient to reduce cost by using only half-bridge cells 7. Otherwise, full- bridge cells or a combination of half-bridge and full-bridge cells may be used in each arm 5. It should be noted when the terms“upper” and“lower” are used herein, it is only to easily distinguish different parts of the MMC in relation to how the MMC circuit is drawn in the figures, and does not necessarily indicate an actual orientation of the MMC.

An intermediary leg 11 is connected to the upper DC link 9a and the lower DC link 9b, thus being connected across each of the parallel phase legs 4. The intermediary leg comprises an upper capacitor arrangement 8a in series with a lower capacitor arrangement 8b, each of the upper and lower capacitor arrangements comprising at least one capacitor or supercapacitor. Between the upper and lower capacitor arrangements 8a and 8b, a phase line of the first AC system 10a is connected as well as a phase line of the second AC system. In figure 2, the phase lines of both the first and second AC systems are connected to the same terminal xy, but alternatively they can be connected to different x and y terminals between the upper and lower capacitor arrangements 8a and 8b. By one phase line of each of the first and second AC systems 10a and 10b being connected to the intermediary leg 11, which comprises capacitor arrangements 8a and 8b but no converter cells 7, the number of respective phase legs 4 with converter arms 5 to which the remaining phase lines are connected can be reduced while retaining the advantages of a back-to-back MMC (cf. figures lb and tc) in contrast to a direct MMC (cf. figure la). Generally, each of the phase lines which are not the one connected to the intermediary leg n is connected to a respective phase leg 4. In case of a three- phase to single-phase MMC 2, as in figure 2, a first phase line of the three- phase system 10a is connected to a terminal xi of the first phase leg 4a, between the upper and lower arms 5a and 5b thereof, and a second phase line of the three-phase system 10a is connected to a terminal x2 of the second phase leg 4b, between the upper and lower arms 5a and 5b thereof, the third phase line of the three-phase system 10a being connected to the terminal xy of the intermediary leg 11, while a first phase line of the single-phase system 10b is connected to a terminal yi of the third phase leg 4c, between the upper and lower arms 5a and 5b thereof, the second phase line of the single-phase system 10b being connected to the terminal xy of the intermediary leg 11.

Thus, a three-to-single phase MMC 2 in accordance with the present invention only requires three phase legs 4, with a total of six converter arms 5, compared with the five phase legs required according to prior art (see figure lb). Similarly, a single-to-single phase MMC 2 in accordance with the present invention only requires two phase legs 4, with a total of four converter arms 5, compared with the four phase legs required according to prior art, and a three-to-three phase MMC 2 in accordance with the present invention only requires four phase legs 4, with a total of six converter arms 5, (see figures 3 and 4) compared with the six phase legs required according to prior art (see figure tc).

In the example of figure 2, it is assumed that power flows from the first AC system 10a to the second AC system 10b, since power is consumed by the trains 20 connected in the second AC system 10b, whereby the phase legs 4a and 4b connected to the first AC system 10a operates as a rectifier and the phase leg 4c connected to the second AC system 10b operates as an inverter. However, embodiments of the invention may also be designed for power flowing from the second AC system 10b to the first AC system 10a, or for alternatingly flowing from either of the AC systems to the other one, e.g. a train that supports regenerative breaking and can send power back to the main grid. Figure 3 illustrates embodiments of an MMC arrangement 1 in case of a 3AC- to-3AC MMC 2. The MMC is as discussed in relation to figure 2, with the exception that here two phase legs 4c and 4d are connected to phase lines of the second AC system 10b. A first phase line of the second AC system 10b is connected to the terminal yi of the third phase leg 4c, between the upper and lower arms 5a and 5b thereof, a second phase line of the second AC system 10b is connected to the terminal y2 of the fourth phase leg 4d, between the upper and lower arms 5a and 5b thereof, and a third phase line of the second AC system 10b is connected to a terminal xy of the intermediary leg 11 as discussed before.

Figure 4 also illustrates embodiments of an MMC arrangement 1 in case of a 3AC-to-3AC MMC 2, but with two optional modifications, which

modifications may also be applicable to other embodiments of the MMC 2. First, the second AC system 10b may comprise a synchronous machine 40, e.g. a (typically large) motor and/or generator, e.g. for pumped hydro or a drive system, depending on application, preferably pumped hydro in some embodiments. The synchronous machine may be connected directly to the MMC 2, or be connected via an optional second transformer 3b as discussed before. Second, a high-frequency AC voltage 41 may be applied to the upper and lower DC links 9, allowing said DC links to carry the high-frequency AC 41 as well as the circulating DC. In any version of the converter 2, the DC-link may be modified by having an alternating high-frequency voltage across it. This may help reducing the size of passive components in the converter 2. However, there may be a trade-off with the rating of the semiconductors and their losses. When alternating voltage 41 is used across the DC-link, the cells 7 of all arms 5 of the converter 2 typically must be full-bridge cells. The AC voltage is illustrated by a square wave in figure 4, but the wave may advantageously be sinusoidal or a mixture there between. The high-frequency AC voltage 41 may have a nominal frequency of at least too Hz or at least 300 Hz such as about 500 Hz, and/or have a nominal frequency of e.g. at most 1000 Hz. The proposed converter 2 is using a DC-link 9 to avoid the problem generated in a direct converter (cf. figure la) when the input and output systems have the same frequency, but with a reduced number of phase-legs 4. The ground connection may be used, e.g. in an existing railway system 10b e.g. as in figure 2. A galvanic insulation 3a and/or 3b may be needed between the first and second AC systems 10a and 10b, e.g. three-phase power system and railway system. A transformer 3a and/or 3b of any configuration may be used, such as a standard three-phase type or a V-V type or a Scott

transformer.

Embodiments of the converter 2 may be used with any nominal voltages of the first and/or second AC systems 10a and 10b, e.g. high-voltage of at least 80 kV, but some embodiments may be especially useful with medium voltage AC systems 10a and/or 10b having a nominal voltage within the range of 1-80 kV, e.g. within the range of 15-30 kV.

The first and second AC systems 10a and 10b may have any, same or different, nominal fundamental frequency. In case e.g. of a three-phase national distribution grid, the nominal frequency may be 50 or 60 Hz. In case e.g. of a single-phase railway system, the nominal frequency may be 25 Hz (which is standard in North America), or 16.7 or 50/3 Hz (which is standard in some European countries).

In some embodiments of the present invention, a point on the intermediary leg 11 is configured to be grounded via the phase line of the second AC system 10b to which it is configured to be connected. This may e.g. be the case when a second transformer 3b is not used.

In some embodiments of the present invention, the first AC system 10a is a three-phase system.

In some embodiments of the present invention, the second AC system 10b is a single-phase system, e.g. a railway system. In some other embodiments of the present invention, the second AC system 10b is a three-phase system, e.g. a pumped hydro system or a large drive system.

In some embodiments of the present invention, the first AC system 10a has a nominal frequency of 50 or 60 Hz, which are typical distribution network frequencies used in many countries.

In some embodiments of the present invention, the second AC system 10b has a nominal frequency of 50 or 60 Hz, which are typical distribution network frequencies used in many countries, or 25, 16.7 or 50/3 Hz which are often used frequencies for railway power systems.

In some embodiments of the present invention, all of the plurality of series- connected converter cells 7 are half-bridge cells.

In some other embodiments of the present invention, at least some, preferably all, of the plurality of series-connected converter cells 7 are full- bridge cells.

In some embodiments of the present invention, the MMC arrangement 1 further comprises a first power transformer 3a via which the first AC system 10a is connected to the MMC.

In some embodiments of the present invention, the MMC arrangement 1 further comprises a second power transformer 3b via which the second AC system 10b is connected to the MMC 1.

In some embodiments of the present invention, the MMC arrangement 1 further comprises a synchronous machine 40 of the second AC system 10b, e.g. for pumped hydro applications or large drive systems (preferably for pumped hydro in some embodiments).

In some embodiments of the present invention, a high-frequency AC current is applied in the upper and lower DC links 9a and 9b, e.g. having a frequency of at least 100 Hz or at least 300 Hz such as about 500 Hz. In some embodiments of the present invention, a nominal voltage at any of the terminals x, y and/or xy is within the medium voltage range, e.g. within the range of 15-30 kV.

The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

1. An AC-to-AC MMC (2) configured to be connected between a first AC system (10a) and a second AC system (10b), the MMC comprising: a plurality of parallel connected phase legs (4), each phase leg comprising an upper converter arm (5a) between a terminal (x/y) for a phase line of one of the first or second AC system (loa/iob) and an upper DC link (9a), and a lower converter arm (5b) between said terminal and a lower DC link (9b), each arm (5) comprising a plurality of series-connected converter cells (7); and an intermediary leg (11), connected to the upper DC link (9a) and the lower DC link (9b) and across the phase legs (4), the intermediary leg comprising upper (8a) and lower (8b) series connected capacitor arrangements; wherein the intermediary leg is configured to be connected via at least one terminal (xy) to one phase line each of the first and the second AC systems (10a, 10b) between the upper and lower capacitor arrangements (8).

2. The MMC of claim 1, wherein a point on the intermediary leg (11) is configured to be grounded via the phase line of the second AC system (10b) to which it is configured to be connected.

3. The MMC of any preceding claim, wherein the first AC system (10a) is a three-phase system.

4. The MMC of any preceding claim, wherein the second AC system (10b) is a single-phase system, e.g. a railway system.

5. The MMC of any claim 1-3, wherein the second AC system (10b) is a three-phase system, e.g. a pumped hydro system or a large drive system.

6. The MMC of any preceding claim, wherein the first AC system (10a) has a nominal frequency of 50 or 60 Hz. 7. The MMC of any preceding claim, wherein the second AC system (10b) has a nominal frequency of 50, 60, 25, 16.

7 or 50/3 Hz.

8. The MMC of any preceding claim, wherein all of the plurality of series- connected converter cells (7) are half-bridge cells.

9. The MMC of any claim 1-7, wherein at least some, preferably all, of the plurality of series-connected converter cells (7) are full-bridge cells.

10. An MMC arrangement (1) comprising: the MMC (2) of any preceding claim, connected between the first and second AC systems (10a, 10b); and the terminals (x/y/xy) connecting the MMC to the first and second AC systems (10a, 10b), respectively.

11. The MMC arrangement of claim 10, further comprising: a first power transformer (3a) via which the first AC system (10a) is connected to the MMC.

12. The MMC arrangement of claim 10, further comprising: a second power transformer (3b) via which the second AC system (10b) is connected to the MMC (1).

13. The MMC arrangement of any claim 10-12, further comprising: a synchronous machine (40) of the second AC system (10b), e.g. for pumped hydro applications or large drive systems.

14. The MMC arrangement of any claim 10-13, wherein a high-frequency AC current is applied in the upper and lower DC links (9a, 9b), e.g. of a frequency of at least 100 Hz or at least 300 Hz such as about 500 Hz.

15. The MMC arrangement of any claim 10-14, wherein a nominal voltage at any of the terminals (x, y, xy) is within the medium voltage range, e.g. within the range of 15-30 kV.