WO2018145748A1 - Parallel connecting of cell modules in a modular multilevel converter by means of interphase transformers - Google Patents

Abstract

The present disclosure relates to a Modular Multilevel Converter (MMC) 40 comprising at least one phase leg 41 comprising a plurality of cascaded converter cells 42. Each converter cell comprises a number of N parallel connected Power Electronic Building Blocks (PEBB) 43 each PEBB comprising an energy storage 44, a plurality of semiconductor switching devices 45, a first conductor terminal A connecting a first conductor a and a second conductor terminal B connecting a second conductor b. Each converter cell also comprises an inductor arrangement 20 comprising a number of N inductors 2, each inductor comprising two conductor windings on the same core, a first conductor winding of one of the first and second conductors a or b of one of the N PEBB and a second conductor winding of one of the first and second conductors a or b of another of the N PEBB. For each of the N PEBB, the first and second conductors a and b comprise two conductor windings, one of the two conductor windings of each of two of the N inductors.

Description

PARALLEL CONNECTING OF CELL MODULES IN A MODULAR MULTILEVEL CONVERTER BY MEANS OF INTERPHASE TRANSFORMERS

TECHNICAL FIELD

The present disclosure relates to parallel-connected single-phase power converters.

BACKGROUND

Figure ι shows a standard parallel connection of a plurality of single-phase power converters Bia-e. The converters Bia-e are connected together to a common output via inductors B2a-e. In this way, a multilevel waveform can be obtained at the output. If any number n of 2-level converters Bia-n are connected in parallel, n+i levels can be obtained at the output.

The parallel connection in figure ι may however be problematic since the full load current of each converter must go through its corresponding inductor. Accordingly, in order for the solution to function well, large inductors are required.

SUMMARY

It is an objective of the present invention to provide an improved way of parallel connecting single-phase Power Electronic Building Blocks (PEBB), e.g. in a Modular Multilevel Converter (MMC). It is proposed that

magnetically coupled inductors are arranged such that the flux generated by the load current is cancelled, which will allow for a more compact inductor design.

According to an aspect of the present invention, there is provided an MMC 40 comprising at least one phase leg comprising a plurality of cascaded converter cells. Each converter cell comprises a number of N parallel connected PEBB, each PEBB comprising an energy storage, a plurality of semiconductor switching devices, a first conductor terminal connecting a first conductor and a second conductor terminal connecting a second conductor. Each converter cell also comprises an inductor arrangement comprising a number of N inductors, each inductor comprising two conductor windings on the same core, a first conductor winding of one of the first and second conductors of one of the N PEBB and a second conductor winding of one of the first and second conductors of another of the N PEBB. For each of the N PEBB, the first and second conductors comprise two conductor windings, one of the two conductor windings of each of two of the N inductors.

According to another aspect of the present invention, there is provided a converter cell for an MMC. The converter cell comprises a number of N parallel connected PEBB, each PEBB comprising an energy storage, a plurality of semiconductor switching devices, a first conductor terminal connecting a first conductor and a second conductor terminal connecting a second conductor. The converter cell also comprises an inductor arrangement comprising a number of N inductors, each inductor comprising two conductor windings on the same core, a first conductor winding of one of the first and second conductors of one of the N PEBB and a second conductor winding of one of the first and second conductors of another of the N PEBB. For each of the N PEBB, the first and second conductors comprise two conductor windings, one of the two conductor windings of each of two of the N inductors. N is an integer of at least two and is used for both the number of PEBB and the number of inductors, indicating that the number of PEBB and inductors in the converter cell is the same.

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:

Fig l is a schematic circuit diagram of parallel connected single-phase converters in accordance with prior art. Fig 2 is a schematic circuit diagram of parallel connected single-phase converters by means of an embodiment of an inductor arrangement in accordance with the present invention.

Fig 3 is a schematic illustration of an embodiment of an inductor of an inductor arrangement in accordance with the present invention. Fig 4 is a schematic circuit diagram of an embodiment of parallel connected PEBB in cells of an MMC, in accordance with the present invention.

Fig 5 is a schematic circuit diagram of an embodiment of parallel connected PEBB by means of an inductor arrangement in an MMC cell, in accordance with the present invention. Fig 6 is a schematic circuit diagram of another embodiment of parallel connected PEBB by means of an inductor arrangement in an MMC cell, in accordance with 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.

In order to allow a more compact inductor design, an inductor arrangement 2 in accordance with figure 2 may be used. The load current through each inductor should preferably only generate a low flux in the core 3 of the inductor by winding the first and second windings 4a and 4b of each of the inductors 2 such that their respective currents are generating inducted currents in the core which substantially cancel each other out. Figure 3 illustrates this in an embodiment of the inductor 2ab. By means of the inductors, the differential current between the converters 1 may be limited by connecting two windings 4a and 4b to the same core 3. Using such winding arrangement, N cores 3 may be used to interconnect N single-phase converters 1 as illustrated in figure 2. The output of the N converters may be connected in a daisy-chain pattern by the inverters 2.

If the same carrier and reference waveforms are used for all converter modules 1, there will, ideally, not be any flux in the magnetic cores 3. A flux would only be generated in the case of a fault in one of the converter modules 1 which would cause a differential mode current to flow through the coupled inductors 2. However, in order to improve the harmonic performance, phase- shifted carrier modulation may be used for the different converter modules 1. Although this may increase the differential mode current during nominal operation, significant improvements may be achieved in terms of harmonic performance. Accordingly, there may be a trade-off between harmonic performance and required size of the magnetic components. How this tradeoff is best performed may differ from one application to another. Figure 2 illustrates an embodiment of an Alternating Current (AC) to Direct Current (DC) converter 1 comprising a plurality of converter modules la-e, and an inductor arrangement 20, on the AC side of the converter 1, comprising a plurality of inductors 2, each inductor comprising two conductor windings 4 on the same core 3, a first conductor winding 4a of a conductor a or b of one of the converter modules and a second conductor winding 4b of one of a conductor a or b of another (typically adjacent) converter module. The number of inductors 2 is the same as the number of converter modules, why conductors connected to each converter modules comprise two conductor windings in two different inductors, since each inductor has two conductor windings 4 (from different converter modules) on the same core, as shown in figure 3. In figure 2, there are five converter modules la-e and thus also five inductors 2ab, 2bc, 2cd, 2de and 2ea.

The present invention relates to using this way of parallel connecting single- phase converter modules to parallel connect a plurality of PEBB 43 in a cell 42 of an MMC 40. The discussion above relating to converter modules 1 and inductor arrangement 20 is thus also relevant to the PEBB 43 and inductor arrangement 20 discussed herein.

Figure 4 illustrates a part of an MMC 40, e.g. a STATCOM, comprising at least one phase leg 41. Each phase leg comprises a plurality of cascaded (i.e. series connected) converter cells 42, here a first cell 42a and a second cell 42b are shown. Each cell 42 comprises a plurality of PEBB 43, each PEBB comprising an energy storage 44 (e.g. a capacitor or battery arrangement) and a plurality of semiconductor switching devices 45, e.g. MOSFET. Any number of PEBB may be parallel connected in the cell. In the example of figure 4, there are four parallel connected PEBB 43a-d in each cell 42a-b. The PEBB are parallel connected to each other by means of the inductor arrangement 20. However, it is noted that the PEBB are parallel connected in such a way that the respective energy storages 44 of the PEBB are not directly parallel connected with each other by virtue of the semiconductor switching devices 45. The semiconductor switching devices may form any convenient topology, such as the half-bridge topology of the example of figure 4 in which each PEBB 43 comprises a first semiconductor switching device 45a and a second semiconductor switching device 45b. Alternatively, the semiconductor switching devices may form e.g. a full-bridge topology (in which each PEBB comprises four semiconductor switching devices 45) or a three-level topology.

Figures 5 and 6 illustrates different embodiments of the inductor

arrangement 20 which may be used, here with only three parallel connected PEBB 43a-c, to simplify the figures. The inductor arrangement embodiment of figure 5 may e.g. be used as the inductor arrangement 20 illustrated more generally in figure 4, while the embodiment of the inductor arrangement 20 shown in figure 6 differs from that of figure 4. Each PEBB 43 comprises a first conductor terminal A connecting a first electrical conductor a, and a second conductor terminal B connecting a second electrical conductor b. The paralleling of the PEBB includes that the first conductors a of each PEBB are connected at a first Point of Common Coupling (PCC) PCCa, and the second conductors b of each PEBB are connected at a second Point of Common Coupling (PCC) PCCb, from each of which PCCa and PCCb the cell 42 is connected to its neighbouring cells of the cascaded cells at references X and Y, respectively. In the embodiment of figure 5, the inductor arrangement 20 is similar to that of figure 2, in that it is only the first conductor a of each PEBB which comprises the conductor windings 4, and thus comprises both of the two conductor windings 4a and 4b of each PEBB. The asterisks * indicate the magnetic coupling direction in each of the inductors 2ab, 2bc and 2ca, which implies that the respective winding directions of the first and second conductor windings 4a and 4b are anti-parallel (as also shown in figure 3) since all windings 4 are by the first conductors a (all conducting either positive or negative current).

In contrast, in the embodiment of figure 6, the first and second conductor a and b, of each PEBB 43, each comprises one conductor winding in an inductor 2. The total number of conductor windings 4 provided in total by the first and second conductors a and b of each PEBB is still two, one in each of two different inductors 2 (as in the embodiment of figure 5 where both windings are provided by the same conductor). As is shown by the asterisks *, the magnetic coupling orientations of the firs and second conductor windings 4a and 4b relative each other are also different compared with the

embodiment of figure 5, a result of each inductor 2 in the embodiment of figure 6 having one conductor winding of a first conductor a and one conductor winding of a second conductor b.

Thus, embodiments of the present invention may be used to realize a cell of a cascaded (also called chain-link) multilevel converter by paralleling half- bridge PEBB 43 as shown in figures 4-6. In this way, the required energy storage in each cell 42 may be split into several smaller capacitors which are not directly parallel connected. This may provide advantages for fault handling in cells where the stored energy is difficult to handle and may even result in an explosion in the event of a fault.

Below follow some more specific embodiments or examples of the present invention.

In some embodiments, the two conductor windings 4a and 4b comprised in the first and second conductors are comprised in one of said first and second conductors a or b (for example as in the embodiment of figure 5).

In some other embodiments, one of the conductor windings 4a or 4b is comprised in the first conductor a and the other of the conductor windings 4a or 4b is comprised in the second conductor b (for example as in the

embodiment of figure 6). In some embodiments, the semiconductor switching devices 45 form a half- bridge in each of the N PEBB 43, e.g. in an AC-DC MMC.

In some other embodiments, the semiconductor switching devices 45 form a full-bridge in each of the N PEBB 43, e.g. in a STATCOM.

In some embodiments, the MMC 40 is an AC-AC converter, e.g. a STATCOM. In some other embodiments, he MMC 40 is an AC-DC converter.

In some embodiments, the energy storage 44 comprises a capacitor arrangement comprising one or more capacitor elements, and/or the energy storage 44 comprises a battery.

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. A Modular Multilevel Converter, MMC, (40) comprising at least one phase leg (41) comprising a plurality of cascaded converter cells (42), each converter cell comprising: a number of N parallel connected Power Electronic Building Blocks, PEBB, (43), each PEBB comprising an energy storage (44), a plurality of

semiconductor switching devices (45), a first conductor terminal (A) connecting a first conductor (a) and a second conductor terminal (B) connecting a second conductor (b); and an inductor arrangement (20) comprising a number of N inductors (2), each inductor comprising two conductor windings (4) on the same core (3), a first conductor winding (4a) of one of the first and second conductors (a/b) of one of the N PEBB and a second conductor winding (4b) of one of the first and second conductors (a/b) of another of the N PEBB; such that, for each of the N PEBB, the first and second conductors (a, b) comprise two conductor windings, one of the two conductor windings (4a, 4b) of each of two of the N inductors.

2. The MMC of claim 1, wherein the two conductor windings (4a, 4b) comprised in the first and second conductors (a, b) are comprised in one of said first and second conductors.

3. The MMC of claim 1, wherein of the two conductor windings (4a, 4b) comprised in the first and second conductors (a, b), one of the conductor windings (4a/4b) is comprised in the first conductor (a) and the other of the conductor windings (4a/4b) is comprised in the second conductor (b).

4. The MMC of any preceding claim, wherein the semiconductor switching devices (45) form a half-bridge in each of the N PEBB (43).

5. The MMC of any claim 1-3, wherein the semiconductor switching devices (45) form a full-bridge in each of the N PEBB (43).

6. The MMC of any preceding claim, wherein the MMC (40) is an AC-AC converter, e.g. a Static Synchronous Compensator, STATCOM.

7. The MMC of any claim 1-5, wherein the MMC (40) is an AC-DC converter.

8. The MMC of any preceding claim, wherein the energy storage (44) comprises one or more capacitor elements.

9. A converter cell (42) for a Modular Multilevel Converter, MMC, (40), the converter cell comprising: a number of N parallel connected Power Electronic Building Blocks, PEBB, (43), each PEBB comprising an energy storage (44), a plurality of

semiconductor switching devices (45), a first conductor terminal (A) connecting a first conductor (a) and a second conductor terminal (B) connecting a second conductor (b); and an inductor arrangement (20) comprising a number of N inductors (2), each inductor comprising two conductor windings (4) on the same core (3), a first conductor winding (4a) of one of the first and second conductors (a/b) of one of the N PEBB and a second conductor winding (4b) of one of the first and second conductors (a/b) of another of the N PEBB; such that, for each of the N PEBB, the first and second conductors (a, b) comprise two conductor windings, one of the two conductor windings (4a, 4b) of each of two of the N inductors.