Power conversion system

Abstract

A power conversion system (1) comprises a rectifier circuit (11) for converting an AC voltage to a distribution bus voltage; a solid state transformer, SST, (30) having an input side DC link (20), an internal medium frequency AC link (502), and an output side DC link (535 ), the SST (30) being configured for converting between the distribution bus voltage on the input side (501) and a DC target voltage on the output side (503), the SST (30) including at least one medium frequency transformer, MFT (511), in the AC link (502) for galvanic isolation, wherein the SST (30) includes multiple solid state transformer, SST, cells interconnected on the input side (501), the interconnection including a parallel connection, a series connection, or a combination thereof.

Description

[0001] POWER CONVERSION SYSTEM

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to a power conversion system for electrical power.

[0004] BACKGROUND

[0005] In electrical power conversion and distribution appliances, interconnecting electrical appliances, such as electrical sources and electrical loads, having different voltage levels is challenging. Conventionally, low frequency transformers (LFTs) are used to step down industrial medium voltage levels of e.g. 20-36 kV, to lower voltage levels, e.g. 500 V to 5 kV). Those LFTs are bulky, heavy, and contain flammable oil that is hazardous to the environment. Consequently, there is a demand for a simpler power conversion system that is downsized and imposes a reduced fire hazard.

[0006] SUMMARY OF THE INVENTION

[0007] According to an aspect, a power conversion system as defined in the independent claim is provided. Further aspects, features, effects and advantages can be derived from the dependent claims.

[0008] According to an aspect of the present disclosure, a power conversion system includes an input converter, and a solid state transformer (SST). The input converter is configured for converting an input voltage to a distribution bus voltage on a distribution bus. In one example, the input voltage is an AC voltage, such as a medium voltage (MV) AC, the distribution bus voltage is a medium voltage (MV) distribution bus voltage, and the distribution bus is an MV distribution bus. The SST has an input side DC link, one or more internal medium frequency AC links, and an output side DC link. The SST is configured for converting between the distribution bus voltage on the input side and an output DC voltage on the output side. The SST includes at least one medium frequency transformer (MFT) in the AC link for galvanic isolation. The SST includes multiple SST cells interconnected on the input side, the input-side interconnection including a parallel connection, a series connection, or a combination thereof. The multiple SST cells are interconnected on the output side, the output-side interconnection including a parallel connection, a series connection, or a combination thereof.

[0009] According to another aspect of the present disclosure, a power conversion system, as disclosed herein, is used for feeding power to an electrical appliance, the electrical appliance including one or more selected from the group consisting of: electric arc furnace, aluminum smelting electrolysis apparatus, chemical electrolysis apparatus, graphitization apparatus, electroplating apparatus, molten oxide electrolysis apparatus, hydrogen electrolysis, electrowinning apparatus, smelting furnace, ladle furnace, induction furnace, arc heater, plasma torch, high power magnet.

[0010] BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1 is a schematic circuit diagram showing a power conversion system according to an embodiment.

[0012] Fig. 2 is a schematic circuit diagram showing a power conversion system according to another embodiment.

[0013] Fig. 3 is a schematic circuit diagram showing a power conversion system according to yet another embodiment.

[0014] Fig. 4 is a schematic circuit diagram showing a power conversion system according to yet another embodiment.

[0015] DETAILED DESCRIPTION

[0016] For the sake of better understanding the new and useful features as disclosed herein, the technical background will be discussed hereinbelow in more detail.

[0017] High-power, high current converter applications in the industrial sector include feeding of industrial plants such as electric arc furnaces, aluminum smelting electrolysis apparatuses, chemical electrolysis apparatuses, graphitization apparatuses, electroplating apparatuses, molten oxide electrolysis apparatuses, hydrogen electrolysis, electrowinning apparatuses, smelting furnaces, ladle furnaces, induction furnaces, arc heaters, plasma torches, or high power magnets. Electric arc furnaces in the steel industry are one example and can be either AC furnaces powered by inverters or DC furnaces powered by DC choppers. The chopper solution can also be used for hydrogen production by electrolysis or new technologies related to steel production, i.e. high temperature molten oxide electrolysis, etc. High current, as used herein, may refer to a current equal to or greater than IkA, such as 5kA or more, 20kA or more, or 500kA or more. A conventional system for supplying a high power electric arc furnace (EAF) with power includes multiple low frequency transformers (LFT) at 50 or 60 Hz, each with multiple secondary winding sets supplying diode bridges (12 or 18 -pulse) which create LV DC links to supply either parallel choppers or inverters.

[0018] In the present application, Solid State Transformers (SSTs) are used. Preferably, in the configurations according to the present application, no LFTs are used. An SST, as used herein, is a type of a power electronic based converter which behaves as a DC to DC transformer. An SST is configured to convert voltages between a specific input level, for example a medium voltage (MV) DC level, and a specific output level, for example a low voltage (LV) DC level.

[0019] An SST has a primary side and a DC secondary side. The primary side is on the input level, and the DC secondary side is on the output level. The SST converts the voltages between the primary side and the DC secondary side. For example, an input converter provides a DC voltage on the primary side, in which case the SST has a DC primary side and the DC secondary side.

[0020] An SST may comprise a plurality of SST cells. An SST cell, as used herein, is a single unit (a single smallest unit) having one or more SST input converter(s) (HV-side converters) on the DC primary side, one or more SST output converter(s) (LV-side converters) on the DC secondary side, and one or more Medium Frequency Transform er(s), MFT, to connect between the SST input converter(s) and the SST output converter(s). In general, an SST cell connects between the input and the output side.

[0021] Examples of an SST input converter include a 3 -level converter, a 2-level converter, a multilevel converter, but are not limited thereto. Examples of an SST output converter include a unidirectional converter and a bidirectional converter, but are not limited thereto.

[0022] Figs. 1 through 4 each show a schematic circuit diagram of embodiments of a power conversion system according to a respective embodiment of the present disclosure. Unless otherwise specified, the details of the embodiments according to Figs. 1 through 4 are described in common in order to avoid unnecessary repetitions.

[0023] Note that the utility supply AC grid, or MV grid, denoted with 50, or a load such as DC / DC converters 550-1, 550-2 feeding a DC electric arc furnace 110 (Fig. 1) or DC / AC inverters 560-1, 560-2, 560-3 feeding an AC electric arc furnace 110 (Fig. 3) is not considered to be part of the power conversion system 1. A low voltage, as used herein, may refer to a voltage above 200 Volt (V), such as a voltage between 200 V - 1 kV, or even 200 V - 1.5 kV. Non-limiting examples include 1000 VAC, or 1500 VDC. A medium voltage, as used herein, may refer to a voltage higher than the low voltage, such as a voltage of above 1 kV, or even of above 1.5 kV, such as a voltage between 1 kV - 52 kV or 1.5 kV - 52 kV, particularly between 1 kV - 30 kV or 1.5 kV - 30 kV, and preferably 20 - 36 kV. For example, a medium voltage may be a voltage received or delivered from or to the MV grid 50, which is for example a 4.16 kV grid, a 10 kV grid, a 13.8 kV grid, a 15 kV grid, a 20 kV grid, a 25 kV grid, a 30 kV grid, or even a 50 kV grid. Another example is a level of 66 kV, but there is no particular limitation. The medium voltage grid 50 may be e.g. a 50 Hz grid or a 60 Hz grid.

[0024] A line interphase transformer (LIT) 10 of the power conversion system, if present, performs an operation such as, but not limited to, a phase-shifting operation between the medium AC voltage on the grid 50 and a secondary-side voltage which will be referred to as a “phase-shifted AC voltages” in the following. In the present embodiment, the LIT 10 forms a unit with a rectifier circuit 11, but this is merely an example, and the LIT 10 and the rectifier circuit 11 may also be provided as separate units. Instead of the exemplary rectifier circuit 11, another kind of input converter (not shown in the drawings) may be used, such as, without limitation, an input-side SST (to be distinguished from SST 30) for converting between the input voltage and the distribution bus voltage. Any such input-side SST may, for example, be configured to step up or step down the input voltage.

[0025] Reverting to the example of the input converter being a rectifier circuit, as in the embodiments shown in Figs. 1 through 4, the rectifier circuit 11 is provided between the LIT 10 and an MV distribution bus (20). The voltage on the MV distribution bus 20 is referred to as the distribution bus voltage. The rectifier circuit 11 converts the phase-shifted AC voltage to the distribution bus voltage on the output side thereof. Note that the LIT 10 is not mandatory, and in the case without a LIT, an example of a 6-pulse system may be employed. When a LIT 10 is present, it may be adapted for 12-pulse or more and include inductors feeding another 3-phase converter. In general, one 3-phase system has no harmonic cancellation, 2 or more 3-phase systems (phase- shifted) provide increasing amounts of harmonic cancellation.

[0026] A solid state transformer (SST) 30 has a DC link 20 on the input side 501 (hereinafter, referred to as the input-side DC link 20), one or more internal medium frequency AC links 502, and a DC link 525 on the output side 503 (hereinafter, referred to as the output-side DC link 525). The SST 30 is configured for converting between the MV DC distribution bus voltage on the input side, or DC primary side, 501 and a DC target voltage on the output side, or DC output side, 503. The input side 501 may also be referred to as the “bus side”, and the output side 503 may also be referred to as the “appliance side”. The SST 30 includes one or more medium frequency transformers (MFT) 5 I lin the AC link 502 for galvanic isolation. In Fig. 1, for the sake of intelligibility, only the uppermost MFT 511 between SST input converter 601 and SST output converter 611 is designated a reference numeral; however, also the remaining SST input and output converters 602. . .606, 612. . .616 each include an MFT. The SST 30 includes one or more SST cells, in the examples shown, multiple SST cells are interconnected on the input side 501 and on the output side 503. Note that the number of SST cells, as well as the cell design, is not particularly limited and may be adapted, e.g. to the voltage levels on the DC primary and secondary sides etc. The interconnections may include parallel connections or series connections or combinations thereof. Note that in the example shown in Fig. 1, a first SST cell is formed by SST input converter 601, MFT 511, and SST output converter 611. Likewise, a second through sixth SST cell is each formed by a respective SST input converter 602. . .606, a corresponding MFT, and a respective SST output converter 612. . .616. For better overview in the Figures, the MFTs of the second through sixth SST cells are not assigned a reference numeral. The MFTs each provide the internal AC link (e.g., in the case of the first SST cell, the internal AC link 502), of the corresponding SST cell.

[0027] The SST 30, as used herein, generally refers to a technology that is capable of directly interfacing a medium voltage on the MV grid with power electronic converter stages. Note that the medium voltage that the SST interfaces is not necessarily the grid voltage itself, and particularly, an AC medium voltage is rectified and may be phase-shifted before the rectification. In the embodiments shown in Figs. 1 through 4, the medium voltage is the distribution bus voltage on the distribution bus 20, and it is both phase-shifted by the LIT 10, and rectified by the rectifier circuit 11. For the sake of convenience of the description, and not for limitation, the medium voltage that the SST interfaces may be referred to as a “MV level”. Note that it is also possible to establish a direct connection between the input voltage and the distribution bus 20, for example in the case that the input voltage is a voltage of an MVDC grid.

[0028] A load, or electrical appliance 110, is connected to the output side 503 of the SST 30, optionally via one or more further converters, such as DC / DC converters 550-1, 550-2 or DC / AC converters 560-1 through 560-3. Note that the further converters are not mandatory, and the electrical appliance 110 may be connected directly to the output side 503 of the SST, i.e. the output-side DC link 525. The electrical appliance 110 is, for example, a DC electric arc furnace (EAF) 110 (see Fig. 1) or an AC electric arc furnace (EAF) 110 (see Fig. 3), but not limited thereto. In the embodiment shown in Fig. 1, the rectifier circuit 11 includes an active front end (AFE) rectifier. In an AFE rectifier, ON / OFF controllable semiconductor switches, such as insulated gate bipolar transistors (IGBTs) are used instead of diodes. Note that IGBTs are merely an example, and other types of semiconductor switches may be employed. The semiconductor switches are ON / OFF controlled such that the primary-side voltage (e.g. the MV grid 50 voltage phase-shifted by the LIT 10) is rectified. A multi-pulse rectifier may contribute in reducing harmonics, particularly low-order harmonics, and may reduce the total harmonic distortion. While some of the beneficial effects are common to diode rectifiers and AFE rectifiers, AFE rectifiers provide a trade-off between a good harmonic performance and the losses involved. By using an LIT AFE, the system may have a very low switching frequency and still achieve a harmonic cancellation, but with lower losses. When many LITs are connected in parallel, such a harmonic cancellation is even more significant.

[0029] An AFE may switch at a low switching frequency, thus reducing losses and taking advantage of the LIT 10 in high pulse configurations, e.g. 12 pulses or more, or 18 pulses or more, as detailed below. In this way, harmonic pollution may be further reduced. The AFE may help to control the power factor, which may eliminate the need for an additional STATCOM or SVC or any other reactive power compensator. As active and reactive power can be freely exchanged with the grid 50, the load disturbances to the grid (flicker) can be reduced. Each of the switches in 11 shown in Figs. 1 through 4 may include a plurality of series and / or parallel connected semiconductors to act as a single switch, as described e.g. in US20230049615A1, US20230049948A1, or US20230048596A1.

[0030] Alternatively, the rectifier circuit 11 may include a diode-based rectifier.

[0031] As used herein, the MFT refers to a transformer configured for transforming a medium frequency AC voltage. The medium frequency AC voltage may have a medium frequency. A medium frequency, according to embodiments described herein, may be understood as a frequency at or above 400 Hertz (Hz), at or above 600 Hz, at or above 800 Hz, at or above 1 kHz, at or above 2 kHz, at or above 5 kHz, at or above 10 kHz, at or above 20 kHz, at or above 50 kHz, or even at or above 100 kHz. The medium frequency transformer 511 may be a medium frequency transformer as described in document WO2021115966A1, which is incorporated herein in its entirety, and / or particularly to the extent of the description of a medium frequency transformer in the document. The medium frequency transformer may be configured for transforming the medium-frequency medium voltage AC voltage into a medium-frequency low voltage AC voltage or vice versa. The MFT may also be configured to transform a medium- frequency low voltage AC into a medium-frequency low voltage AC. Note that at least one MFT 511 is employed per SST cell, and that for the sake of better overview, only one MFT 511 (the MFT of the first SST cell) may be assigned a reference numeral in Figs. 1 through 4.

[0032] In the embodiment shown in Fig. 1, in the SST 30, the bus side 501 is composed of a plurality of bus-side SST input converters 601 . . .606 that are combined to form a series connection. The appliance side 503 is composed of a plurality of appliance-side SST output converters 611 . . .616 that are combined to form a 2-series / 3 -parallel connection. Each bus-side SST input converter 601...606 has a corresponding appliance-side SST output converter 611...616. For example, bus-side SST input converter 601 has a counterpart in appliance-side SST output converter 611; bus-side SST input converter 602 has a counterpart in appliance-side SST output converter 612, and so on. Between each bus-side SST input converter 601...606 and the respective counterpart appliance-side SST output converter 611 .. .616, an internal AC link 502 is present. Herein, each internal AC link 502 includes at least one Medium Frequency Transformer, MFT 511. Each combination of SST input converter 601...606, SST output converter 611...616, and the corresponding MFT forms one SST cell. The SST 30 is configured for conversion between the distribution bus DC voltage and the appliance DC voltage. In the embodiment shown in Fig. 1, the SST 30 includes 6 SST cells each including a corresponding SST input converter 601...606 that are interconnected in series to form the series connection and each including a corresponding SST output converter 611 ...616 interconnected to form the 2-series / 3 -parallel connection. Note that the number of respectively interconnected SST cells is not particularly limited, and may be any number greater than 2.

[0033] Furthermore, in the embodiment shown in Fig. 1, the SST 30 includes multiple SST units, or converter units 701, 702, 703, each configured in the same manner. The SST units, or converter units, 701, 702, 703 have a bus side 501 and an appliance side 503. The converter units 701, 702, 703 are connected, on the distribution bus side, in parallel, and this parallel connection is connected to the MV distribution bus via bus-side terminals. Moreover, the converter units 701, 702, 703 are connected, on the appliance side, in parallel. Note that this parallel connection may be connected to further converters, such as, but not limited to, further DC / DC converters. However, any such additional converters are not mandatory.

[0034] In the example shown in Fig. 1, the converter units, or SST units, 701, 702, 703 are connected in a parallel manner to form a converter unit stack 700, or SST unit stack 700. It is preferred that the converter units 701, 702, 703 in the stack 700 are configured in the same manner; however, there is no limitation, and the converter units 701, 702, 703 may be configured differently. Moreover, the number of three converter units 701, 702, 703 is merely an example, and two or fewer, or more than three, converter units may be employed.

[0035] In the example shown, each isolated output -side DC link is connected in cascade to two parallel choppers, which also can be used to balance the currents in the three SST units. The combination of SST units plus choppers are in turn connected in parallel to the DC electrical arc furnace 110. Thus, a high power and high current can be provided. It should be noted that the number of parallel choppers is not limited. It should be noted that the output-side DC link may also supply an AC electric arc furnace, as in Fig. 3 or 4, wherein the choppers are replaced by 3 -phase inverters.

[0036] The embodiment shown in Fig. 2 is different from that in Fig. 1 in that a diode-based rectifier 11 is employed, and rectifier 11 is configured in a 12-pulse configuration. This system can easily be connected as identical parallel units to increase the power to a very high level, such as, but not limited to, more than 100 MW or more than 300 MW or more than 600 MW. Furthermore, Fig. 2 shows isolated DC links on the load side, but there is no limitation on this, and it may be configured such that e.g. a maximum of one isolated DC link per SST cell is present. The remaining details are the same as in the embodiment of Fig. 1, and thus the repeated description thereof will be omitted.

[0037] The embodiment shown in Fig. 3 is different from that in Fig. 2 in that the rectifier 11 is configured in an 18-pulse configuration (as in Fig. 1). Note that the configuration is not limited to this, and e.g. a configuration having another number than 18 pulses may be employed as well. The SST cells can be connected in any reasonable combination of series and parallel connections on the load side. For example, each SST cell can feed one isolated DC link. The remaining details are the same as in the embodiment of Fig. 1 and 2, and thus the repeated description thereof will be omitted. For the sake of better overview, not all reference numerals are repeated in Fig. 3, and reference is made to Figs. 1 and 2 as to the functions and the features of corresponding components in Fig. 3.

[0038] Fig. 4 shows an exemplary embodiment similar to that of Fig. 1. In Fig. 4, multiple LITs 10-1, 10-2, 10-3 and multiple AFE rectifiers 11-1, 11-2, 11-3 each corresponding to one LIT 10-1, 10-2, 10-3 are each connected to one of the converter units 701, 702, 703 of the SST 30. Note that the number of three LITs 10-1, 10-2, 10-3, three AFE rectifiers 11-1, 11-2, 11-3, and three converter units 701, 702, 703 is merely an example, and two or fewer, or more than three of each may be employed. In the configuration shown in Fig. 4, the power conversion system 1 is configured to balance the currents; e.g. the currents flowing in each of the units of LIT 10-1, AFE rectifier 11-1, and converter unit 701; LIT 10-2, AFE rectifier 11-2, and converter unit 702; and LIT 10-3, AFE rectifier 11-3, and converter unit 703 are balanced to be the same or approximately the same.

[0039] With the embodiments described herein, the power conversion system allows for supplying a high-power load such as an electric arc furnace 110 with SSTs without the need for low frequency transformers. With the configurations as described herein, an improved flicker performance and elimination or reduction of the need for a STATCOM and filtering can be achieved. A diode rectifier already provides a strong reduction of reactive power and filtering needs. An AFE rectifier can fully compensate the reactive power and the STATCOM could be eliminated. Furthermore, a smooth current control of the chopper or inverters on the load side benefit the process by reducing excessive stresses on the system, such as a stress on the one or more electrodes (graphite or Soderberg type) in an electric arc furnace that could easily be damaged should an undesired disruption of the process occur. Moreover, multiple units can be connected in parallel on the input side to create high pulse number rectifiers (e.g. 6, 12, 18, 24- pulse and above) with very low distortion seen by the grid 50. Also, multiple units can be connected in parallel / series on the output side to reach the desired power of the load, and the types of SST input converters and SST output converters used in the SST 30 can be adjusted to best meet the high medium voltage level of the application. The low-voltage side converter of the SST can be unidirectional or bidirectional to best serve the application. I.e. an active frontend type bidirectional operation would have the largest impact on flicker performance. Note that the semiconductor switches employed herein are not limited to a specific type, and may be, for example, IGBTs, SiC MOSFETs etc.

[0040] Although particular embodiments have been shown and described, it will be understood that it is not intended to limit the claimed inventions to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed inventions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed inventions are intended to cover alternatives, modifications, and equivalents.

Claims

CLAIMS1. A power conversion system (1), comprising: an input converter (11) for converting an input voltage to a distribution bus voltage; a solid state transformer, SST, (30) having an input side DC link (20), one or more internal medium frequency AC links (502), and an output side DC link (525), the SST (30) being configured for converting between the distribution bus voltage on the input side (501) and an output DC voltage on the output side (503), the SST (30) including at least one medium frequency transformer, MFT (511), in the AC link (502) for galvanic isolation, wherein the SST (30) includes multiple solid state transformer, SST, cells interconnected on the input side (501), the input-side interconnection including a parallel connection, a series connection, or a combination thereof, and interconnected on the output side (503), the outputside interconnection including a parallel connection, a series connection, or a combination thereof.

2. The power conversion system of claim 1, further comprising a line interphase transformer, LIT, (10) configured for transforming between the AC voltage and a transformed AC voltage, such as a phase-shifted AC voltage.

3. The power conversion system (1) of claim 1 or 2, wherein the AC voltage is a medium AC voltage, and the distribution bus voltage is a medium voltage, MV, distribution bus voltage.

4. The power conversion system (1) of any one of the preceding claims, wherein the input voltage is an AC voltage, and wherein the input converter (11) includes a rectifier circuit (H).

5. The power conversion system of claim 4, wherein the rectifier circuit (11) includes an active front end, AFE, rectifier.

6. The power conversion system of claim 5, comprising multiple LITs (10-1, 10-2, 10-3) and multiple AFE rectifiers (11-1, 11-2, 11-3) each corresponding to one of the multiple LITs(10-1, 10-2, 10-3), wherein the power conversion system is configured to balance the currents in the power conversion system (1).

7. The power conversion system of claim 4, wherein the rectifier circuit (11) includes a passive front end, PFE, for example a diode-based rectifier or wherein the rectifier circuit includes a semi-passive front end, for example a thyristor-based rectifier.

8. The power conversion system of any one of claim 1 through 3, wherein the input converter (11) includes an input-side solid state transformer, SST, configured to step up or step down the input voltage.

9. The power conversion system of any one of the preceding claims, wherein each SST cell provides one or more isolated DC links on the output side separate from any other DC link.

10. The power conversion system of any one of the preceding claims, wherein the LIT (10) and the input converter (11) are configured in a multi-pulse configuration, optionally a configuration having 6 pulses or more, optionally 12 pulses, 18 pulses, 24 pulses, or more.

11. The power conversion system of any one of the preceding claims, further comprising a DC / AC converter (560-1, 560-2, 560-3), a parallel connection of multiple DC / AC converters, a DC / DC converter (550-1, 550-2), or a parallel connection of multiple DC / DC converters connected to the output side of the SST (30).

12. The power conversion system of any one of the preceding claims, wherein the SST (30) comprises multiple SST units (701, 702, 703) connected to form a SST unit stack 700.

13. The power conversion system of any one of the preceding claims, wherein multiple SSTs (30) are connected in parallel to form an SST stack.

14. Use of a power conversion system (1) according to any one of the preceding claims for feeding power to an electrical appliance, the electrical appliance including one or more selected from the group consisting of electric arc furnace, aluminum smelting electrolysis apparatus, chemical electrolysis apparatus, graphitization apparatus, electroplating apparatus, molten oxide electrolysis apparatus, hydrogen electrolysis, electrowinning apparatus, smelting furnace, ladle furnace, induction furnace, arc heater, plasma torch, high power magnet.