WO2025153166A1

WO2025153166A1 - Power conversion system

Info

Publication number: WO2025153166A1
Application number: PCT/EP2024/050814
Authority: WO
Inventors: Frederick Kieferndorf; Francisco Canales; Raeto STADLER; Nandhakumar VIJAYAKUMAR; Daniel ROTHMUND
Original assignee: ABB Schweiz AG
Current assignee: ABB Schweiz AG
Priority date: 2024-01-15
Filing date: 2024-01-15
Publication date: 2025-07-24
Anticipated expiration: 2026-07-15

Abstract

A power conversion system (1) comprises a line interphase transformer (10), LIT, configured for phase-shifting between a medium AC voltage from a medium voltage AC grid (50) and a phase-shifted AC voltage; a medium voltage, MV, distribution bus (20); a rectifier circuit (11) for converting the phase-shifted AC voltage to a distribution bus voltage of the distribution bus (20); a plurality of electrical subsystems (100, 200, 300, 400) connected to the distribution bus (20), wherein each electrical subsystem includes an electrical appliance (110, 210, 310, 410) serving as an electric power load or an electric power source at an appliance voltage and an appliance intermediate converter (120, 220, 320, 420) configured to convert between the appliance voltage and an intermediate voltage; and a solid state transformer, SST, converter (30, 130, 230, 330, 430) configured to convert between the intermediate voltage and the distribution bus voltage.

Description

POWER CONVERSION SYSTEM

TECHNICAL FIELD

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

BACKGROUND

In electrical power conversion and distribution appliances, interconnecting electrical appliances, such as electrical sources and electrical loads, having different voltage levels and voltage types (AC or DC) is challenging. Conventionally, the appliance voltage is converted via a low frequency transformer to the voltage of a 50 or 60 Hertz medium voltage three-phase grid. When the appliance voltage is a DC voltage, a DC-to-AC converter is additionally provided. In particular when renewable energy sources and traditional generator-based voltage supplies are used, this results in a relatively complex and expensive system to account for the varying output voltages of the sources and the varying input voltages of the loads. Typically, the comparatively large number of conversion steps leads to a poor efficiency. Consequently, there is a demand for a simpler power conversion system that has an improved efficiency.

SUMMARY OF THE INVENTION

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.

According to an aspect of the present disclosure, a power conversion system includes a medium voltage (MV) distribution bus, a grid-side converter circuit, a plurality of electrical subsystems, and a solid state transformer (SST). In one example, a grid voltage is an AC voltage and is considered to be in the medium-voltage range. In another example, the grid voltage is a HVDC voltage. The grid-side converter circuit is configured for converting the grid voltage to a distribution bus voltage on the DC distribution bus. Each of the electrical subsystems includes an electrical appliance and an appliance intermediate converter. The electrical appliance serves as an electric power load or as an electric power source at an appliance voltage. The appliance intermediate converter is configured for converting between the appliance voltage and an intermediate voltage. The SST is configured for converting between the intermediate voltage and the distribution bus voltage. According to another aspect of the present disclosure, a power conversion system includes a medium voltage (MV) distribution bus, a plurality of electrical subsystems, and a solid state transformer (SST). A supply voltage is an MVDC voltage, and the MV distribution bus is connected, without any step-up or step-down voltage conversion, to the supply voltage. Each of the electrical subsystems includes an electrical appliance and an appliance intermediate converter. The electrical appliance serves as an electric power load or as an electric power source at an appliance voltage. The appliance intermediate converter is configured for converting between the appliance voltage and an intermediate voltage. The SST is configured for converting between the intermediate voltage and the distribution bus voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic block diagram showing a power conversion system according to an embodiment.

Fig. 2 is a schematic block diagram showing a power conversion system according to another embodiment.

Fig. 3 is a schematic block diagram showing a power conversion system according to yet another embodiment.

Fig. 4 is a schematic block diagram showing a power conversion system according to yet another embodiment.

Fig. 5 is a schematic block diagram of a configuration of a SST unit consisting of one SST section used in some embodiments.

Fig. 6 is a schematic block diagram of another configuration of a SST unit consisting of one SST section used in some embodiments.

Fig. 7 is a schematic block diagram of yet another configuration of a SST unit consisting of multiple SST sections used in some embodiments.

Fig. 8 is a schematic block diagram showing a power conversion system according to another embodiment.

Fig. 9 is a schematic block diagram showing a power conversion system according to another embodiment. DETAILED DESCRIPTION

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

There is an increasing number of renewable power sources, such as wind turbines and photovoltaic units, that need to be connected to existing medium voltage (MV) grids. Buffering of their energy is increasingly becoming a key factor in stabilizing these MV grids. For example, battery units are connected to the MV grid that altematingly serve as loads (when being charged, e.g. by the renewable power sources) and sources (when being discharged). High-power loads such as electric arc furnaces connected to the MV grid impose fluctuations on the grid that need to be handled. The common feature of the appliances in these examples is the variability of the power flow to and from the grid. Furthermore, these appliances typically operate at different and sometimes variable voltage levels.

In existing systems, the interconnection of sources with different voltage levels requires inverters and rectifiers and then through low frequency transformers (LFTs) to a 50 or 60 Hz medium voltage three-phase grid. The LFTs, used to step down medium voltage to low voltage, to step up low voltage to medium voltage, or possible for doing a 1 : 1 coupling, are bulky and expensive. This results in a relatively complex and expensive system to provide both renewable and traditional generator-based voltage supplies with varying output voltages. In addition, the efficiency is typically quite poor because of the number of conversion steps in the overall system. In typical renewable systems such as a wind generator the converter is connected to the three-phase generator and has a DC link which is then connected to the three-phase AC grid with an active rectifier and LFT. In the case of a photovoltaic solar system the DC output of the interconnected solar cells is also connected to the three-phase grid with an active rectifier and LFT to bring the voltage up to MV.

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. An SST has a DC primary side and a DC secondary side. The DC primary side is on the input level, and the DC secondary side is on the output level. The SST converts the voltages between the DC primary side and the DC secondary side.

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.

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.

Figs. 1 through 4 each show a block diagram of a power conversion system 1 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.

Note that the utility supply AC grid, e.g. an MV grid, denoted with 50 is not considered to be part of the power conversion system 1. In general, the supply AC grid is present and operational, but it may be non-operational, leading to an island operation of the power conversion system.

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.

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 voltage” 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. 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 secondary 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.

Electrical subsystems 100, 200, 300, 400 are connected to the distribution bus 20. That is, the electrical subsystems 100, 200, 300, 400 each interface with the distribution bus 20. In the embodiment shown in Fig. 1, each electrical subsystem 100, 200, 300, 400 includes an electrical appliance 110, 210, 310, 410, an appliance intermediate converter 120, 220, 230, 420, and a solid state transformer 130, 230, 330, 430.

The electrical appliance 110, 210, 310, 410, as used herein, is generally an apparatus driven by or providing electrical energy. Examples of an electrical appliance include a solar energy source, a wind energy source, a battery storage system, an electric arc furnace, an aluminum smelting electrolysis apparatus, a chemical electrolysis apparatus, a graphitization apparatus, an electroplating apparatus, a molten oxide electrolysis apparatus, a hydrogen electrolysis, an electrowinning apparatus, a smelting furnace, a ladle furnace, an induction furnace, an arc heater, a plasma torch, a high power magnet. In the examples shown in Figs. 1 through 4, without limitation and by way of example, electrical appliance 110 is an arc furnace, electrical appliance 210 is a storage battery system, electrical appliance 310 is a solar energy source, and electrical appliance 410 is a wind energy source. Typically, the electrical appliances 110, 210, 310, 410 are at least partially different from another. The difference may be the type of the electrical appliance 110, 210, 310, 410, the voltage level (the appliance voltage) of the electrical appliance 110, 210, 310, 410, the rated or actual power of the electrical appliance 110, 210, 310, 410, or combinations thereof.

The SST 30, 130, 230, 330, 430 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 usually not the grid voltage itself, and particularly, the medium voltage may be 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”.

Each appliance intermediate converter 120, 220, 320, 420 converts between the respective appliance voltage and the respective intermediate voltage. That is, the appliance intermediate converter 120 converts between the appliance voltage of the electrical appliance 110 and a voltage to be provided to the SST 130; the appliance intermediate converter 220 converts between the appliance voltage of the electrical appliance 210 and a voltage to be provided to the SST 230; the appliance intermediate converter 320 converts between the appliance voltage of the electrical appliance 310 and a voltage to be provided to the SST 330; and the appliance intermediate converter 420 converts between the appliance voltage of the electrical appliance 410 and a voltage to be provided to the SST 430. Each appliance intermediate converter 120, 220, 320, 420 may convert from DC on the SST side to DC on the appliance side, or may convert from DC on the SST side to one or more AC phases on the appliance side, as need be. In the exemplary embodiment of Fig. 1, appliance intermediate converter 120 is a DC/AC converter, appliance intermediate converter 220 is a DC/DC converter, appliance intermediate converter 320 is a DC/DC converter, and appliance intermediate converter 420 is an AC/DC converter.

Each SST 130, 230, 330, 430 converts between the intermediate voltage from/to the respective appliance intermediate converter 120, 220, 320, 420 and the (common) distribution bus voltage on the distribution bus 20. That is, the distribution bus 20 has its own specified DC level voltage.

The optional rectifier circuit 11 may include 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 the type of semiconductor switches is not particularly limited, and other types, such as, but not limited to, SiC-based MOSFETs may be employed. The semiconductor switches are ON/OFF controlled such that the primary-side voltage (e.g. the MV grid 50 voltage, possibly phase-shifted by the LIT 10) is rectified. A multipulse AFE or PFE rectifier may contribute to 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 can freely exchange active and reactive power with the grid 50, the load disturbances to the grid (flicker) can be reduced. In addition, an AFE can provide better harmonic performance by increasing the losses by switching faster. By using an LIT with an 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.

Alternatively, the rectifier circuit 11 may include a diode-based rectifier. As a further alternative, the rectifier circuit 11 may include a thyristor-based rectifier.

In the embodiment shown in Fig. 1, each of the electrical subsystems 100, 200, 300, 400 includes one SST converter 130, 230, 330, 430. That is, each electrical appliance 110, 210, 310, 410 is assigned, via the corresponding appliance intermediate converter 120, 220, 320, 420, one dedicated corresponding SST converter 130, 230, 330, 430. Note that the intermediate voltages in each electrical subsystem may differ from each other in the embodiment of Fig. 1.

In the embodiment shown in Fig. 2, one common SST converter 30 is provided for the electrical subsystems 100, 200, 300, 400. That is, each electrical appliance 110, 210, 310, 410 is assigned, via the corresponding appliance intermediate converter 120, 220, 320, 420, to one single common SST converter 30. In the embodiment of Fig. 2, the appliance intermediate converters 120, 220, 320, 420 convert between the voltage of the respective appliance 110, 210, 310, 410 and a common intermediate voltage 121. The common intermediate voltage 121 may be chosen such that the current based losses are reduced. For example, the common intermediate voltage 121 is chosen to be higher in the case of a source that has a larger physical distance to a load.

In the embodiment shown in Fig. 3, one common SST converter 130 is provided for the electrical subsystems 100, 200, 300, and another SST converter 430 is provided for the electrical subsystem 400. That is, each electrical appliance 110, 210, 310 is assigned, via the corresponding appliance intermediate converter 120, 220, 320, to one single common SST converter 130. The appliance intermediate converters 120, 220, 320 convert to one appliance intermediate voltage 121 common for the electrical subsystems 100, 200, 300. The electrical appliance 410 is assigned, via the appliance intermediate converter 420, to the dedicated corresponding SST converter 430. The appliance intermediate converter 420 converts to one appliance intermediate voltage 421. Note that the appliance intermediate voltages 121, 421 may differ from each other in the embodiment of Fig. 3. For example, the wind generator as the appliance 410 is located at a notable physical distance to e.g. the load 110, e.g. an arc furnace. The battery storage system as the appliance 210 and the solar array as the appliance 310 are located closer to the arc furnace 110, and that the appliance intermediate voltage 121 may be different from the appliance intermediate voltage 421.

In the embodiment shown in Fig. 4, each of the electrical subsystems 100, 200, 300, 400 includes one SST 130, 230, 330, 430. That is, each electrical appliance 110, 210, 310, 410 is assigned, via the corresponding appliance intermediate converter 120, 220, 320, 420, to one dedicated corresponding SST 130, 230, 330, 430. Note that the intermediate voltages in each electrical subsystem may differ from each other in the embodiment of Fig. 4. The difference from the embodiment shown in Fig. 1 is that in the embodiment shown in Fig. 4, the arc furnace 110 is DC driven, and consequently, the appliance intermediate converter 120 converts from DC to DC.

In the following, possible configurations of the SST 30, 130, 230, 330, 430 are described. In some embodiments, the SST 30, 130, 230, 330, 430 includes one converter section, or SST unit,

500, exemplary embodiments of which are shown in Figs. 5 and 6. In some embodiments, the SSTs 30, 130, 230, 330, 430 includes one converter section, or SST unit, 500, wherein an exemplary embodiment is shown in Fig. 7.

Fig. 5 is a schematic block diagram of a configuration of a SST unit 500 used in some embodiments. The SST unit 500 has a bus side 501 and an appliance side 503. On the bus side

501, the SST unit 500 is connected to the MV distribution bus 20 via bus-side terminals 505. On the appliance side 503, the SST unit 500 is connected to the appliance intermediate converter 120, 220, 320, 420 via appliance-side terminals 535.

In Fig. 5, the bus side 501 is composed of a plurality of bus-side SST converters 601 . . .610 that are combined to form a series connection 520. The appliance side 503 is composed of a plurality of appliance-side SST output converters 611...620 that are combined to form a parallel connection 530.

Each bus-side SST input converter 601...610 has a corresponding appliance-side SST output converter 611...620. 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...610 and the respective counterpart appliance-side SST output converter 611 . . .620, an internal AC link 502 is present. For example, each internal AC link 502 includes at least one Medium Frequency Transformer, MFT 515. Each combination of SST input converter 601 . . .610, SST output converter 611 . . .620, and the corresponding MFT forms one SST cell. The SST unit 500 is configured for conversion between the distribution bus voltage and the appliance voltage.

As used herein, an 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 515 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 mediumfrequency low voltage AC into a medium-frequency low voltage AC. Note that at least one MFT 515 is employed per SST cell, and that for the sake of better overview, only one MFT 515 (the MFT of the first SST cell) may be assigned a reference numeral in the drawings.

In the embodiment shown in Fig. 5, the converter section includes 10 SST cells. Note that the number of respectively SST cells is not particularly limited, and may be any number equal or greater than 2.

Fig. 6 is a schematic block diagram of a configuration of another SST unit 500 used in some embodiments. The SST unit 500 has a bus side 501 and an appliance side 503. On the bus side 501, the SST unit 500 is connected to the MV distribution bus 20 via bus-side terminals 505. On the appliance side 503, the SST unit 500 is connected to the appliance intermediate converter 120, 220, 320, 420 via appliance-side terminals 535.

In Fig. 6, the bus side 501 is composed of a plurality of bus-side SST input converters 601...606 that are connected in series to form a series connection. The appliance side 503 is composed of a plurality of appliance-side SST output converters 611 . . .616 that are connected in parallel 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. For example, each internal AC link 502 includes a Medium Frequency Transformer, MFT. The converter section 500 is configured for conversion between the distribution bus voltage and the appliance voltage. Note that, for the sake of better overview, not all reference numerals are repeated in all the drawings, and reference is made to the drawings in which the respective components are assigned a reference numeral as to the functions and the features of corresponding components in the other drawings.

In the embodiment shown in Fig. 6, the converter section includes 6 SST input converters

601...606 interconnected in series to form the series connection and 6 SST output converters

611...616 interconnected in parallel to form the 2-series/3 -parallel connection. Note that the number of respectively interconnected SST input and output converters 601...606, 611...616 is not particularly limited, and may be any number equal or greater than 2.

Fig. 7 is a schematic block diagram of a configuration of a cascade of converter sections, or SST units, 701, 702, 703 used in some embodiments. In Fig. 7, the converter sections 701, 702, 703 are each configured in a same manner. The converter sections 701, 702, 703 have a bus side 501 and an appliance side 503. The converter sections 701, 702, 703 are connected, on the distribution bus side, in parallel, and this parallel connection is connected to the MV distribution bus 20 via bus-side terminals 505. Moreover, the converter sections 701, 702, 703 are connected, on the appliance side, in parallel, and this parallel connection is connected to the appliance intermediate converter 120, 220, 320, 420 via appliance-side terminals 535. That is, the SST sections 701, 702, 703 are connected in a parallel manner to form a SST unit 700. It is preferred that the SST sections 701, 702, 703 in the SST unit 700 are configured in the same manner; however, there is no limitation, and the SST sections 701, 702, 703 may be configured differently. Moreover, the number of three SST sections 701, 702, 703 is merely an example, and two or fewer, or more than three, SST sections may be employed in an SST unit.

For the sake of simplicity, SST section 701 of the SST unit 700 will be described here. In Fig. 7, the bus side 501 is composed of a plurality of bus-side SST converters 601...606 that are connected in series to form a series connection. The appliance side 503 is composed of a plurality of appliance-side SST converters 611...616 that are connected to form a 2-series/3- parallel connection. However, the configuration is not particularly limited to this combination, and e.g. a 3-series/2-parallel connection or any other combination may be configured as well. 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 is present. For example, each internal AC link 502 includes a Medium Frequency Transformer, MFT. The SST unit 700 is configured for conversion between the distribution bus voltage and the appliance voltage.

In the embodiment shown in Fig. 7, the SST section includes 6 SST cells with input converters

601...606 interconnected in series to form the series connection and output converters

611...616 interconnected to form a 2-series/3 -parallel connection. Note that the number of respectively interconnected SST cells is not particularly limited, and may be any number equal or greater than 2.

It is preferred that the SST sections 701, 702, 703 in the SST unit 700 are configured in the same manner; however, there is no limitation, and the SST sections 701, 702, 703 may be configured differently.

The configuration examples of Figs. 5 through 7 allow for a suitable connection of a particular electrical appliance 110, 210, 310, 410 to the DC level voltage on the distribution bus 20 in a modular manner. By appropriately choosing the interconnection in the SST section(s) /unit(s) 500, 701, 702, 703, adaptation to a load or source can easily be achieved. For example, if an electrical appliance (load or source) source required a 2 kV DC link and the distribution bus 20, or MV DC grid, was at 20 kV the SST making up the respective SST unit 500, 701, 702, 703 may consist of 10 or 11 cells in series on the high-voltage side of the SST to reach the desired voltage level. On the low-voltage side of the SST the cells may be connected in parallel to provide a higher current low-voltage connection. In this way the system is extremely modular and can be connected in series and/or parallel to reach any desired voltage and current level. In addition, several different standard cell voltages can be used for a standard cell design to result in a convenient and inexpensive modular way to adjust input and output voltages between the MV distribution bus 20 and any load or source.

Fig. 8 shows a block diagram of a power conversion system 1 according to another respective embodiment of the present disclosure, similar to the embodiment shown and described in Fig. 1. In order to avoid unnecessary repetitions, only the differences over Fig. 1 are discussed here. In Fig. 8, unlike the AC grid 50 of Fig. 1, the grid, or utility supply 51, is a high voltage DC (HVDC) utility supply. The grid-side converter in Fig. 8 is a DC/DC converter, such as another SST.

Fig. 9 shows a block diagram of a power conversion system 1 according to yet another respective embodiment of the present disclosure, similar to the embodiment shown and described in Fig. 1. In order to avoid unnecessary repetitions, only the differences over Fig. 1 are discussed here. In Fig. 9, unlike the AC grid 50 of Fig. 1, the grid, or utility supply 52, is a medium voltage DC (MVDC) utility supply. In the configuration of Fig. 9, the distribution bus 20 is directly connected, or only connected via passive elements such as a reactor, to the MVDC utility supply 52.

With the embodiments described herein, the power conversion system allows for supplying a high-power load such as an electric arc furnace 110 by power sources without a low frequency transformer. The different power sources 210, 310, 410 can have a range of DC or AC voltage levels which are transformed by the modular configuration of the MFT based SSTs described herein, to the level of the distribution bus 20 which connects to the high-power load 110 by stepping the voltage down with the SST to that needed by the load. This allows for a seamless voltage level adjustment and interconnection from the power sources 210, 310, 410 all the way to the high-power load 110 through the DC transmission system, or distribution bus 20. At the same time, the power load may be buffered, e.g. by the battery storage 210. Note that buffering may also be achieved via other buffering systems such as gravity or mechanical systems, compressed air energy storage, flywheel storage systems, or any other kind of energy storage/buffering system. That is, depending on the size of the storage device chosen, the maximum demand from the grid can be reduced, and the power consumption can be averaged out over time. This means that for example the gravity storage absorbs a large amount of the nominal furnace power from the grid or renewable sources while the furnace is off for tapping and charging. Then, the storage continuously supplies a part, as an example 15%, of the furnace power during operation, thus reducing the maximum demand of the plant by the amount, e.g. 15%, of the furnace power. This can additionally result in a more constant power consumption and more effectively utilizing the contractual power supply from the grid. Furthermore, when the energy storage systems, e.g. battery storage 210 and gravity storage, etc., are employed in combination with the variable renewable supplies, e.g. the solar array 310 and the wind turbine 410, the power flow can be controlled such that the variability of the renewable supplies as well as the variable loads can be balanced to level and/or reduce the load drawn from the grid. This eliminates the need of AC low frequency transformers to adjust the voltage levels as well as rectifiers and other additional power electronics conversion stages. The interconnection with the medium voltage DC on the distribution bus 20 also results in a smaller cable size and fewer interconnecting cables, compared to an AC distribution system of similar power, which results in lower costs and easier installations.

In other words, with the embodiments described herein, the power conversion system allows for an adjustment, or transformation, of different supply and load voltage levels efficiently and compactly, while most efficiently utilizing the available power supplies. Since (dry) MFTs are used instead of (oil-filled) low frequency transformers, maintenance is simplified, and the fire hazard and environmental hazard are reduced.

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. for an active front-end 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.

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

1. A power conversion system (1), comprising: a medium voltage, MV, distribution bus (20); a grid-side converter (11, 12) for converting a grid voltage to a distribution bus voltage of the distribution bus (20); a plurality of electrical subsystems (100, 200, 300, 400) connected to the distribution bus (20), wherein each electrical subsystem includes: an electrical appliance (110, 210, 310, 410) serving as an electric power load or an electric power source at an appliance voltage; an appliance intermediate converter (120, 220, 320, 420) configured to convert between the appliance voltage and an intermediate voltage; a solid state transformer, SST, converter (30, 130, 230, 330, 430) configured to transform between the intermediate voltage and the distribution bus voltage.

2. The power conversion system (1) of claim 1, wherein the grid voltage is an AC voltage, and wherein the grid-side converter (11, 12 ) is a rectifier circuit (11).

3. The power conversion system (1) of claim 1, wherein the grid voltage is a DC voltage, and wherein the grid-side converter (11, 12) is a DC/DC converter (12) such as a grid-side SST, configured to step up or step down the input voltage.

4. The power conversion system of claim 1 or 2, further comprising a line interphase transformer (10), LIT, configured for transforming, optionally phase-shifting, between the AC voltage from an AC grid (50) and a transformed AC voltage, optionally a phase-shifted AC voltage.

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

6. The power conversion system of claim 4, wherein the rectifier circuit 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.

7. The power conversion system of any one of the preceding claims, wherein one or more of the electrical subsystems, optionally each of the electrical subsystems, includes one SST cell.

8. The power conversion system of any one of the preceding claims, wherein the SST includes at least one SST unit (500) having a bus side (501), one or more internal AC links (502), and an appliance side (503), the SST unit section (500) being configured for converting between the distribution bus voltage on the distribution bus side and the appliance voltage on the appliance side, the SST unit (500) including a medium frequency transformer (515), MFT, in the AC link.

9. The power conversion system of claim 8, wherein the converter section (500) includes multiple solid state transformer, SST, cells interconnected on the distribution bus side, the interconnection including a parallel connection, a series connection, or a combination thereof.

10. The power conversion system of claim 8 or 9, wherein the converter section (500) includes multiple solid state transformer, SST, cells interconnected on the appliance side, the interconnection including a parallel connection, a series connection, or a combination thereof.

11. The power conversion system of claim 6, wherein the converter section (500) includes 2 or more SST cells, optionally 5 or more SST cells and preferably 10 SST cells, interconnected in a series connection (520) on the distribution bus side and 2 or more SST cells, optionally 5 or more SST cells and preferably 10 SST cells interconnected in a parallel connection (530) on the appliance side.

12. The power conversion system of claim 6, wherein the converter section (500) includes 2 or more SST cells, optionally 3 or more SST cells and preferably 6 SST cells, interconnected in a series connection (520) on the distribution bus side and 6 SST cells interconnected in a 2- series/3 -parallels connection on the appliance side.

13. The power conversion system of any one of the preceding claims, wherein the SST (30) comprises multiple SST stacks or sections (701, 702, 703) connected to form a SST unit (700); and/or wherein multiple SSTs (30) are connected in parallel to form an SST stack.

14. The power conversion system of any one of claims 6 through 11, wherein the SST includes 2 or more converter sections (701, 702, 703) each comprising 2 or more SST cells interconnected on the distribution bus side and 2 or more SST cells interconnected on the appliance side, wherein the converter sections (701, 702, 703) are connected, on the distribution bus side, in a parallel connection, a series connection, or a combination thereof, and connected, on the appliance side, in a parallel connection, a series connection, or a combination thereof.

15. The power conversion system of any of the preceding claims, wherein at least two of the electrical subsystems are configured to operate on appliance voltages that are different from each other.

16. The power conversion system of any one of the preceding claims, wherein at least one of the electrical appliances include one or more selected from the group consisting of: solar energy source, wind energy source, battery storage system, gravity energy storage system, compressed air energy storage, flywheel storage systems, electric arc furnace, electrolysis apparatus, ladle furnace, induction furnace, arc heater, plasma torch, high power magnet.

17. The power conversion system of any one of the preceding claims, wherein at least one of the electrical appliances includes an energy storage system and at least one of the electrical appliances includes a high-power load, and wherein the energy storage system is configured to be controlled such that during a period of time in which the high-power load consumes less than a rated power thereof, stores supplied energy, and during a period of time in which the consumed power of the high-power load rises, the energy storage system continuously releases, at least partially, its stored energy, optionally continuously releases 10% or more, preferably 15% or more, and more preferably less than 50%, of the power consumed by the high-power load.

18. A power conversion system (1), comprising: a medium voltage, MV, distribution bus (20) connected, without conversion, to an MVDC utility supply; a plurality of electrical subsystems (100, 200, 300, 400) connected to the distribution bus (20), wherein each electrical subsystem includes: an electrical appliance (110, 210, 310, 410) serving as an electric power load or an electric power source at an appliance voltage; an appliance intermediate converter (120, 220, 320, 420) configured to convert between the appliance voltage and an intermediate voltage; a solid state transformer, SST, converter (30, 130, 230, 330, 430) configured to transform between the intermediate voltage and the distribution bus voltage.