A universal power flow controller for a high-power ac system and a high-power ac system

Abstract

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Description

[0001] A UNIVERSAL POWER FLOW CONTROLLER FOR A HIGH-POWER AC SYSTEM AND A HIGH-POWER AC SYSTEM

[0002] Field of the disclosure

[0003] The invention is in the field of high-power AC systems, particularly in the field of AC electric arc furnaces. Embodiments of the present application relate to a universal power flow controller, UPFC, for a high-power AC system and a high-power AC system.

[0004] Technical Background

[0005] Power flow controllers are needed in a high-power AC system to control a power and to improve a power quality in the high-power AC system. Known power flow controllers are, for example, based on a reactor connected in parallel with back-to-back thyristors to keep a load of the high-power AC system, for example, an AC electric arc furnace, running with a stable constant current. Known power flow controllers have the drawback that they have a slow response time because of the use of thyristors as the switching devices. This limits the benefits to Flicker mitigation that can be achieved. In addition, the known power flow controllers are typically combined with a power quality system (PQS), either a static var compensator (SVC) or STATCOM, to supply the reactive power to the load. Further, a passive lower order harmonic filter is typically provided to meet grid codes.

[0006] There is need for a power flow controller that overcomes the limitations described above.

[0007] Summary of the disclosure

[0008] Reference will now be made in detail to the various embodiments of the present disclosure, one or more examples of which are illustrated in the figures. Generally, only the differences with respect to individual embodiments are described. Each example is provided by way of explanation and is not meant as a limitation of the present disclosure. Features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the description includes such modifications and variations.

[0009] In light of the above, a universal power flow controller, UPFC, for a high-power AC system according to claim 1 and a high-power AC system according to claim 16 is provided. Further aspects, advantages, and features are apparent from the dependent claims, the description, and the accompanying drawings. Throughout this application, the term “directly connected” has the meaning that two elements that are directly connected are electrically connected to each other, without any further electrical components therebetween. The term “indirectly connected” has the meaning that two components that are indirectly connected are electrically connected to each other through additional electrical components. For example, two components may be indirectly connected through a transformer. That is, a first component may be connected to a primary side of the transformer and a second component may be connected to a secondary side of the transformer, such that the first component and the second component are electrically connected through the transformer. It is understood that the following description of the universal power flow controller and the features thereof applies to a multiphase AC-sy stem, particularly to a 3 -phase AC system. Throughout this application, power electronic elements, such as inverters and converters, are generally described for one direction of current flow. It is understood, that unless otherwise indicated, these power electronic elements are bi-directional to allow for bidirectional current flow.

[0010] According to an aspect, a universal power flow controller, UPFC, for a high-power AC system having a power supply line for providing power from a utility supply to a load is provided. The UPFC comprises an input coupling system configured to be connected to the power supply line for receiving an input electric power from the power supply line, a plurality of UPFC converter modules connected to the input coupling system, the plurality of UPFC converter modules provided in parallel to each other, and an output coupling system configured to be connected to the power supply line for providing an output electric power to the power supply line, the output coupling system comprising a plurality of output coupling devices connectable to the power supply line, wherein for each UPFC converter module of the plurality of UPFC converter modules, a respective output coupling device of the plurality of output coupling devices is connected to the UPFC converter module for providing an electric power to the power supply line.

[0011] The high-power AC system has a power supply line for providing power from a utility supply to a load. Particularly, for providing power from a utility supply to a load for powering the load. The utility supply may be an AC grid. For example, the utility supply may be a low voltage (LV) grid, a medium voltage (MV) grid, a high voltage (HV) grid, or an extra high voltage grid. The high-power AC system may be connected directly to the utility supply, particularly, the utility supply being an AC grid. The high-power AC system may be connected indirectly to the utility supply, for example, may be connected through a grid transformer to the utility supply, particularly to the utility supply being a low voltage (LV) grid, a medium voltage (MV) grid, a high voltage (HV) grid, or an extra high voltage grid.

[0012] The high-power AC system has a load. In some embodiments, the high-power AC system may be an AC electric arc furnace. A load of the AC electric arc furnace may be electrodes of the AC electric arc furnace, particularly the load may be an electric arc provided by the electrodes. A power may be provided to the electrodes of the AC electric arc furnace for operating the AC electric arc furnace, e.g. for melting metal containing materials. The power supply line may be an electric connection in the high-power AC system. The power supply line may be an electric connection in the high-power AC system that is connected directly or indirectly to the load. The power supply line may be configured for providing a power to the load. Particularly, the power supply line may be configured for providing most or all of the power provided to the load. For example, in an AC arc furnace the power supply line may be a connection between the utility grid and an AC arc furnace transformer, wherein the AC arc furnace transformer is connected to the electrodes of the AC arc furnace.

[0013] The UPFC comprises a plurality of UPFC converter modules. The UPFC comprises two or more UPFC converter modules. In some embodiments, the UPFC converter modules comprise switch mode semiconductors. Switch mode semiconductors have a fast response time. Particularly, such that the response time of the UPFC converter module, particularly of the UPFC, can be much faster than in known solutions. Advantageously, the faster response time results in the flicker and harmonic distortion being reduced. In some embodiments, which are described in more detail below, the UPFC converter module may comprise an active front-end, such that an SVC or STATCOM would not be needed as well. The active front-end may include or correspond to an active rectifier. The UPFC converter modules may be configured to receive an AC current from the input coupling system and to provide an AC current to the output coupling system. Particularly, the UPFC converter modules may be configured to receive an AC current from the input coupling devices and to provide an AC current to the output coupling devices.

[0014] The UPFC according to the present invention allows for compensation of transient load, compensation of reactive power, elimination of harmonics, improvement of power quality and / or flicker mitigation. In high-power AC systems, particularly in AC electric arc furnaces, typically the transient load due to variations of the arc is less than the full power load of the furnace. Advantageously, the installed power of the UPFC converter is less than when using a full converter in series with the power supply line. The UPFC according to the present invention can supply active and reactive power to supply active power to the power supply line if needed and also compensate the reactive power needed by the load, similar to a STATCOM. The output coupling system, particularly the output coupling device, injects a voltage to control a current in the power supply line. For example, to control a current in a power supply line of an AC electric arc furnace. The UPFC allows to control the current externally. Advantageously, the arc voltages can be independently controlled by adjusting the heights of the electrodes. The UPFC according to the present invention comprises a plurality of UPFC converter modules. Advantageously, a total number of UPFC converter modules can be matched to the needed active and reactive power provided to the power supply line.

[0015] The plurality of UPFC converter modules allows to eliminate harmonics drawn by the load connected to the power supply line. For example, an UPFC according to embodiments described herein having two UPFC converter modules allows to eliminate the 5thand 7thharmonic drawn by the load connected to the power supply line, particularly the load of an AC electric arc furnace. Advantageously, the need for passive filtering may be reduced. For example, an UPFC according to embodiments herein having four UPFC converter modules allows to eliminate 5th, 7th, 11th, 13thharmonics. A higher number of UPFC converter modules allows to eliminate harmonics up to higher orders.

[0016] The UPFC converter modules may be based on switching mode semiconductors. The switching mode semiconductors may be, for example, IGBT, IGCT, SIC MOSFET, or any other power electronic semiconductor device. The switching mode semiconductors may have a switching frequency. The UPFC converter module may have a switching frequency. The switching frequency of the UPFC converter module may depend on the switching frequency of the switching mode semiconductors, particularly correspond to the switching frequency of the switching mode semiconductors. The UPFC converter modules may by controlled in an interleaved manner. In some embodiments, which can be combined with further embodiments described herein, the UPFC converter modules are configured to be interleaved for interleaved switching of the switch mode semiconductors. As an example of interleaved switching, two parallel connected UPFC converter modules, particularly rectifiers and / or inverters of the UPFC converter modules, may receive the same set of switching commands. A second UPFC converter module may receive the switching commands delayed by half a switching period, with respect to the switching commands received by a first UPFC converter module, such that the combined switching pulses of the two UPFC converter modules, and / or the effective pulse frequency is double that of an individual UPFC converter module pulse frequency. The interleaving results in that an effective switching frequency of the interleaved controlled UPFC converter modules is increased. The effective switching frequency may be about an integer multiple of a switching frequency of an individual UPFC converter module. As a further example, 4 output coupling devices, for example series transformers, with series connected voltages of the output coupling devices, can effectively have 4 times as many steps as in the individual injected voltages, equivalent to a switching frequency, with 4 times the bandwidth of one UPFC converter. An effective switching frequency may be doubled for two interleaved UPFC converters, compared to a switching frequency of a single UPFC converter Providing further interleaved UPFC converters further increases an effective switching frequency. For example, having four UPFC converter modules may result in that an effective switching frequency may be quadrupled, compared to a switching frequency of a single UPFC converter. Advantageously, the isolated inputs and outputs of the parallel / series connected UPFC converters allows interleaving of the switching waveforms resulting in higher control bandwidth meaning better overall control performance.

[0017] The output coupling system is configured to provide the output electric power to the power supply line. The output electric power may have an active portion, i.e. an active output electric power, and a reactive portion, i.e. a reactive output electric power. The UPFC converter modules provide a converter power which may be provided to the power supply line. The plurality of UPFC converter modules may provide the output electric power. The converter power may have an active portion, i.e. an active converter power, and a reactive portion, i.e. a reactive converter power. Particularly, the sum of the converter power provided by each UPFC converter module of the plurality of UPFC converter modules may correspond to the output electric power. The output coupling system may be configured to provide, for each UPFC converter module of the plurality of UPFC converter modules, the converter power to the power supply line. A respective output coupling device of the plurality of output coupling devices is connected to the UPFC converter module for providing an electric power to the power supply line, particularly for providing an electric power from the UPFC converter module to the power supply line. The output coupling system may be configured to receive an AC current from the plurality of UPFC converter modules and to provide an AC current to the power supply line. Particularly, the output coupling devices may be configured to receive an AC current from the UPFC converter modules and to provide an AC current to the power supply line.

[0018] The output coupling system comprises a plurality of output coupling devices connectable to the power supply line. For each UPFC converter module of the plurality of UPFC converter modules, a respective output coupling device of the plurality of output coupling devices is connected to the UPFC converter module for providing an electric power to the power supply line. Particularly for providing the converter power to the power supply line.

[0019] Each UPFC converter module is connected to a respective one of the output coupling devices. The further description will be with respect to a one pair of UPFC converter module and output coupling device connected to each other. It is understood, that unless otherwise indicated, the further described features for the one pair of UPFC converter module and output coupling device apply to each pair of UPFC converter module and associated output coupling device.

[0020] The UPFC converter according to the present application may compensate disturbances originating in the load of the high-power AC system, for example in the load of the AC electric arc furnace. The disturbances originating in the load of the high-power AC system may couple back to the power supply line. The disturbances originating in the load of the high-power AC system may, for example, include power fluctuations, transients, harmonics, and / or possibly resulting in a reduction of a power quality of the supplied power. The UPFC converter according to the present application may allow to compensate power fluctuations in the power supply line, for example to compensate power fluctuations, transients, and / or harmonics in a power supply line and / or to improve a power quality. To compensate power fluctuations, the UPFC converter provides an output electric power. The output electric power is large enough to compensate the fluctuations in the power supply line. For example, the UPFC controller may provide an output electric power of up to 10 MVA, up to 30 MV A, up to 60 MV A, or up to 100 MV A, or even up to 300 MVA. The UPFC controller may provide about 10% to 100%, particularly 30% to 80%, particularly 30% to 60%, particularly 40% to 6 % of a total power provided to the load of the high-power AC system.

[0021] In some embodiments, the UPFC may be configured to control a power in the power supply line and / or improve a power quality in the power supply line. The UPFC may provide the output electric power to control a power in the power supply line and / or improve a power quality in the power supply line. Particularly, the UPFC converter modules may provide a converter power to control the power in the power supply line and / or improve a power quality in the power supply line. A power quality may be improved by compensating a reactive power in the power supply line, particularly in a power provided to the load. A power quality may be improved, for example, by eliminating and / or reducing harmonics and transients.

[0022] In some embodiments, the UPFC may be configured to control an active power and / or a reactive power in the power supply line. The UPFC may provide the output electric power to control the active power and / or the reactive power in the power supply line. The output electric power may have an active output electric power and a reactive output electric power. The active output electric power and the reactive output electric power may be provided by the UPFC, particularly by the plurality of UPFC converter modules. In some embodiments, the UPFC may provide the reactive output electric power through the input side to the power supply line. The UPFC may provide the reactive output electric power through the input side and through the output side to the power supply line. The reactive output electric power may flow from the power supply line through the input coupling system and / or through the output coupling system to the plurality of UPFC converter modules. The reactive output electric power may be fed from the plurality of UPFC converter module through the input coupling system and / or through the output coupling system to the power supply line. The UPFC may provide a reactive power, particularly the reactive output electric power to compensate a reactive power drawn by the load. An effective reactive power seen by the utility supply may be reduced or even eliminated. Particularly, such that the power supply line is effectively only provided with active power from the utility supply. The active output electric power may be fed from the power supply line through the input coupling system to the plurality of UPFC converter modules. The active output electric power may be fed from the UPFC converter modules through the output coupling system to the power supply line. Advantageously, the UPFC according to the present invention allows to control a reactive power and an active electric power in the power supply line, particularly in the high- power AC system.

[0023] The UPFC converter according to the present application is provided with a plurality of UPFC converter modules. The UPFC converter modules of the plurality of UPFC converter modules may be provided in parallel to each other between the input coupling system and the output coupling system. Each of the UPFC converter modules may provide a converter power. The converter power may have an active converter power and a reactive converter power. The sum of converter powers provided by each of the UPFC converter modules may correspond to the output electric power. The UPFC converter modules of the plurality of UPFC converter modules may be identical to each other. Each UPFC converter module of the plurality of UPFC converter modules may provide a same converter power. Advantageously, by having a plurality of UPFC converter modules, each UPFC converter module only provides a fraction of the output electric power. For example, for a UPFC having two UPFC converter modules, each UPFC converter modules provides one half of the output electric power. For a UPFC having four UPFC converter modules, each UPFC converter module provides one fourth of the output electric power. The UPFC converter modules of the plurality of UPFC converter modules may have different power ratings to each other. The UPFC converter modules of the plurality of UPFC converter modules may have the same power ratings.

[0024] The output coupling devices are connected to the UPFC converter modules. The output coupling device may be connected to a power supply line for providing the converter power to the power supply line. Advantageously, the converter power that the output coupling devices receive from the UPFC converter modules and provide to the power supply line can be reduced. For example, for a UPFC having two UPFC converter modules, each coupling device provides one half of the output electric power to the power supply line. For a UPFC having four UPFC converter modules, each output coupling device provides one fourth of the output electric power to the power supply line. Advantageously, a stress applied to the output coupling devices is reduced, a lifetime of the output coupling devices can be increased. The output coupling devices may be operated more efficiently over their entire lifetime. Further, a size of the output coupling devices may be reduced, requiring less material for manufacturing the output coupling devices and / or allowing for easier manufacturing.

[0025] In some embodiments, which can be combined with further embodiments described herein, the output coupling devices are connected in series to each other. The output coupling devices may be configured to be connected in series to the power supply line. By providing the output coupling devices in series to each other, an output voltage provided to the power supply line may be increased. The output voltage may be the sum of voltages provided by each of the output coupling devices. Advantageously, a voltage and current provided by the output coupling devices can be made to better match the rating of the UPFC converter modules. A total voltage applied by the UPFC to the power supply line may be a sum of voltages provided by the series connected output coupling devices. Advantageously, the total voltage supplied by the UPFC and / or seen by the UPFC may be distributed over the output coupling devices, particularly over the UPFC converter modules.

[0026] In some embodiments, which can be combined with further embodiments described herein, at least one output coupling device is a series transformer for providing the output electric power on a secondary side of the series transformer. In some embodiments, which can be combined with further embodiments described herein, the output coupling devices are series transformers for providing the output electric power on a secondary side of the series transformers.

[0027] In the following, an example series transformer and features thereof are described. It is understood, that this applies to all series transformers. That is, some or all of the output coupling devices may be series transformers as described below. The series transformer comprises a primary side and a secondary side. The primary side may be connected to the UPFC converter module. The primary side may be a low voltage side or a MV voltage side. The secondary side may be configured to be connected to the power supply line. The secondary side may be a MV voltage side or a high voltage side. An input voltage on the primary side of the series transformers may be substantially equal to an output voltage on the secondary side of the series transformer. The series transformer may have a winding ratio of about 1 : 1. The series transformers may have a winding ratio of about 1 : 1.1, 1 : 1.2, 1 : 1.5, 1 :2, 1.1 : 1, 1.2: 1, 1.5: 1 or 2: 1. The series transformers may have high isolation on the windings. The currents may have an inverse ratio as compared to a ratio of the voltages. A plurality of series transformers may be connected in series. Particularly, a plurality of series transformers may be connected on the secondary side in series to each other. The plurality of series transformers may be connected to the power supply line on the secondary side. The plurality of series transformers may be connected on the secondary side in series to each other and connected to the power supply line. Particularly, connected to the power supply line such that the plurality of series transformers feed an output electric power to the power supply line.

[0028] A size of the series transformers may be decreased when increasing a number of UPFC converter modules. Advantageously, less material for manufacturing the series transformers is required and / or easier manufacturing is achieved.

[0029] In some embodiments, which can be combined with further embodiments described herein, the series transformers are connected to the UPFC on their primary side.

[0030] In some embodiments, the input coupling system comprises for each UPFC converter module of the plurality of UPFC converter modules a supply terminal connected to the UPFC converter module. That is, each UPFC converter module may have an associated supply terminal, such that there is a 1 : 1 association of the UPFC converter modules and the associated supply terminal. The supply terminal may provide a current and / or power to the UFPC converter module. The input coupling device is configured for receiving an input electric power from the power supply line. The supply terminals may provide the input electric power to the plurality of UPFC converter modules. Each supply terminal may provide a fraction of the input electric power to the associated UPFC converter module. Particularly, the input electric power may be evenly distributed over all supply terminals, such that each supply terminal provides a same power to the UPFC converter module. The input coupling system may be configured to receive an AC current from the power supply line and to provide an AC current to the plurality of UPFC converter modules. Particularly, the input coupling device may be configured to receive an AC current from the power supply line and to provide an AC current to the plurality of UPFC converter modules.

[0031] In some embodiments, which can be combined with further embodiments described herein, the input coupling system comprises at least one input coupling device connectable to the power supply line, the at least one input coupling device connected to the plurality of UPFC converter modules for providing an input electric power to the plurality of UPFC converters. The at least one input coupling device may be connected to the power supply terminals.

[0032] In some embodiments, the at least one input coupling device comprises a parallel transformer that has a single input on the primary side for connecting to the power supply line and a single output on the secondary side connected to a UPFC converter module. In some embodiments, that can be combined with further embodiments described herein, the at least one input coupling device comprises a parallel transformer that has a single input on the primary side for connecting to the power supply line and a plurality of outputs on the secondary side connected to a plurality of supply terminals. Two or more parallel transformers may be phase shifted with respect to each other. Beneficially, the phase shifted two or more parallel transformers compensate harmonics.

[0033] The plurality of outputs correspond to a plurality of secondary sides. That is, the parallel transformer having a plurality of outputs may have a plurality of separate windings on the secondary side for providing a plurality of outputs. For example, the at least one input coupling device may comprise a parallel transformer having a single input and two outputs connected to two UPFC converter modules. The transformer may have a DELTA / star / delta configuration (12-pulse active front end (AFE) / passive front end (PFE)). The active front end may be, for example, an active rectifier of the UPFC converter module. That is the primary side of the transformer may be in a delta configuration, the first output on the secondary side may be in a star configuration and the second output on the secondary side may be in a delta configuration. The DELTA / star / delta transformer (12-pulse AFE / PFE) may eliminate the 5th and 7th harmonic in the power supply line, particularly drawn by the load, thus reducing the needed passive filtering. In another example, the input coupling system may comprise two DELTA / star / delta transformers connected to four UPFC converter modules. The two DELTA / star / delta transformers may be phase shifted to each other. Additional benefits of the phase shifted 2 DELTA / star / delta transformers (24-pulse AFE / PFE) are the elimination of 5th, 7th, 11th, and 13thharmonics, requiring only a higher order harmonic filter which is much smaller and simpler than known solution. In another example, the input coupling system includes a parallel transformer having a single input and four outputs connected to four UPFC converter modules. Advantageously, in addition to the advantages described above with respect to the DELTA / star / delta transformers, a size of each of the parallel connected DELTA / star / delta transformers can be reduced in size and / or rating as compared to a single DELTA / star / delta transformer. In addition, increasing the number of UPFC converter modules, and consequently, the output coupling devices, for example being series transformers, can be reduced in size and / or rating. That is, a physical size of the output coupling devices, for example being series transformers, can be reduced.

[0034] In some embodiments, which can be combined with further embodiments described herein at least one UPFC converter module of the plurality of UPFC converter modules includes afront end having a rectifier, an inverter, and a DC link having a DC buffer, the DC link being provided between the rectifier and the inverter. The front end may be an active front end. The front end may be a passive front end. The rectifier may be an active rectifier. The rectifier may be a passive rectifier. The front end, particularly the rectifier may be connected to the input coupling system. The front end, particularly the rectifier, may be connected to the supply terminals. The front end, particularly the rectifier, may be connected to the parallel transformers. The inverter may be connected to the output coupling system. The inverter may be connected to the output coupling devices, particularly to the primary side of the series transformer. The DC buffer may be configured to store and release an electric energy. The DC buffer may be configured to store and release an electric energy for compensating power fluctuations in the power supply line. The DC buffer may be configured to output the converter power. The UPFC converter module may comprise one or more rectifiers. The one or more rectifiers may be active rectifiers and / or passive rectifiers. The UPFC converter module may comprise active rectifiers and / or passive rectifiers. The one or more rectifiers may be comprised in the front end. The one or more rectifiers may have a same input connected to the input coupling system, particularly to a same supply terminal, particularly to a same input coupling device. The one or more rectifiers may be connected to a same DC link. The UPFC converter module may comprise one or more inverters. The one or more inverters may be connected to a same DC link. The one or more rectifiers and the one or more inverters may be connected for providing the DC link therebetween. The one or more inverters may have a same output connected to the output coupling system, particularly to a same output coupling device. As illustrative examples, the UPFC converter module may comprise two rectifiers and one inverter, or comprise two rectifiers and two inverters, or comprise one rectifier and two inverters, or comprise three rectifiers and two inverters. In some embodiments, that can be combined with other embodiments described herein, the input coupling system includes for each UPFC converter module of the plurality of UPFC converter modules a supply terminal connected to the UPFC converter module; the input coupling system includes at least one input coupling device connectable to the power supply line, the at least one input coupling device connected to the plurality of UPFC converter modules for providing an input electric power to the plurality of UPFC converter; the at least one input coupling device includes a parallel transformer that has a single input on the primary side for connecting to the power supply line and a plurality of outputs on the secondary side connected to a plurality of supply terminals; and at least one UPFC converter module of the plurality of UPFC converter modules includes a front end having a rectifier, an inverter, and a DC link having a DC buffer, the DC link being provided between the rectifier and the inverter, the rectifier being connected to the supply terminal.

[0035] In some embodiments the input coupling system comprises at least one converter having a line interphase transformer for receiving the input electric power, a rectifier, and a solid-state transformer connected to a UPFC converter module of the plurality of UPFC converter modules. The line interphase transformer (LIT) may be configured to be connected to the power supply line.

[0036] A LIT may be a phase coupled magnetic component. The LIT may combine 3- or more input voltages of 3- or more different input phases to produce 3 -or more output voltages, resulting in 3- or more output phases. The 3 or more output phases may have different group phase shifts relative to the 3- or more input phases. The LITs, a combination of active and passive rectifiers and solid state transformers (SSTs) may be provided as an input coupling device. This arrangement can provide all the benefits mentioned previously with respect to the input coupling device, particularly with respect to the parallel transformer. Additionally, new benefits include a significant reduction of size and volume of the input transformers by using MFTs in the SSTs. Using LITs, a combination of AFEs / passive rectifiers and SSTs further reduces the size of the input parallel transformers significantly. Further, SSTs use dry transformers that have no oil, resulting in simpler maintenance, are less bulky and lighter, and have reduced fire and environmental hazards. The SSTs may be more readily available and may be more cost effective.

[0037] In some embodiments, for each UPFC converter module of the plurality of UPFC converter modules the input coupling system comprises a converter of the at least one converter, the converter having a line interphase transformer for receiving the input electric power, a rectifier, and a solid-state transformer connected to the UPFC converter. Each LIT, may be configured to be connected to the power supply line.

[0038] In some embodiments, at least one UPFC converter module of the plurality of UPFC converter modules comprises a front end, an inverter, and a DC link having a DC buffer, the DC link provided between the front end and the inverter. The DC link comprises an output of the solid- state transformer. The front end may be an active front end. The front end may be a passive front end. The front end may be connected to the solid-state transformer. The inverter may be connected to the output coupling system. The inverter may be connected to the output coupling devices, particularly to the primary side of the series transformer.

[0039] In some embodiments, which can be combined with further embodiments described herein, a renewable energy source is connected to the DC buffer for providing electric power from the renewable energy source to the DC buffer. The renewable energy source may be connected directly to the DC buffer. The renewable energy source may be connected indirectly to the DC buffer. In further embodiments, the DC buffer includes an element selected from the group consisting of capacitor, supercapacitor, battery, flywheel, gravity energy storage, compressed- air energy storage (CAES), any other storage element which can be interfaced to a DC voltage, and a combination thereof. The DC buffer, particularly the elements selected from the group above may be for storing and releasing an electrical energy.

[0040] The renewable energy source may provide at least a part of the stored energy stored in the DC buffer. The renewable energy source may charge the DC buffer. The renewable energy source may be a solar energy source, a wind energy source, a geothermal energy source, a hydropower energy source, an ocean energy source, a bioenergy source, or any combination thereof.

[0041] In the following, the plurality of UPFC converter modules is referred to as the plurality of first UFPC converter modules. The output coupling devices may be low frequency transformers and / or output wirings connecting the UPFC converter modules in series to the power supply line. An output wiring may connect an AC port of a UPFC converter module in series to the power supply line for providing the converter power to the power supply line. A plurality of output wirings may connect the plurality of UPFC converter modules in series to the power supply line for providing the output electric power from the plurality of UPFC converter modules to the power supply line. The power supply line may have a utility line and a load line connected by at least one output wiring. In some embodiments, which can be combined with other embodiments described herein, at least one output coupling device may be an output wiring connecting in series the utility line, the UPFC converter module, and the load line for providing an electric power to the power supply line from the plurality of first UPFC converter modules. In some embodiments, which can be combined with further embodiments described herein, at least one output coupling device is a series transformer for providing the output electric power on a secondary side of the series transformer. Advantageously, a total number of low frequency transformers in the UPFC may be reduced.

[0042] The power supply line may have a utility line and a load line connected by a plurality of output wirings including the first output wirings. The electrically conducting power supply line may consist of the supply line and the load line, which are electrically connected by the plurality of output wirings including the first output wirings. The utility line may be connected to the utility supply for receiving a power from the utility supply. The load line may be connected to the load for providing a power to the load. The load line may receive a power from the utility line through the output coupling system. The load line may receive a power from the utility line through the first output wirings connecting in series the utility line, the plurality of first UPFC converter modules, and the load line. The load line may receive a power from the UPFC converter module through the output coupling system. The load line may receive a power from the UPFC converter module through the first output wirings connecting in series the utility line, the plurality of first UPFC converter modules, and the load line. Each of the first output wirings may be rated for transmitting a power for operating the high-power AC system at the rated power of the high-power AC system. It is understood that the power-flow can be bi-directional. The utility line may receive a power from the load line and / or the UPFC converter modules through the first output wirings. Each first UPFC converter module may receive a power from the utility line and / or the load line through the corresponding first output wiring.

[0043] The load line may provide substantially all of the power provided to the load. The utility line may be configured for providing most or all of the power provided to the load line. The plurality of UPFC converter modules may provide the output electric power to the load line. The output electric power provided by the plurality of UPFC converter modules to the load line may be small compared to the power provided by the utility line. The utility line may provide more than 50%, more than 60%, more than 65%, more than 70%, more than 75%, more than 80%, more than 85%, more than 90%, or more than 95% of the total power provided to the load line, particularly of the total power provided to the load. The plurality of UPFC converter modules according to the present application may provide less than 50%, less than 45%, less than 40%, less than 35%, less than 30%, less than 25%, less than 20%, less than 15%, less than 10%, or less than 5% of the total power provided to the load line, particularly of the power provided to the load. The output electric power may be between 50% to 5% of the power provided to the load, particularly between 40% to 10% of the power provided to the load, particularly between 25% to 15% of the power provided to the load. Only a part of the total power provided to the load line, particularly of the power provided to the load, may be provided by the plurality of UPFC converter modules. The plurality of UPFC converter modules may not provide the total power provided to the load line, particularly the power provided to the load.

[0044] Advantageously, having at least one output device being an output wiring allows to reduce a number of low frequency transformers.

[0045] In the following a single first UPFC converter module with a corresponding first output wiring is described. It is understood, that the following description applies to one or more, particularly all, first UPFC converter modules and corresponding first output wirings.

[0046] The first UPFC converter module may have an AC port. The AC port of the first UPFC converter module may be configured for transmitting an AC current from the first UPFC converter module to the first output wiring. The first UPFC converter module may be configured for providing an AC current, having a frequency matching a frequency of an AC current provided by the utility supply, to the AC port. The first output wiring may be connected to the AC port of the first UPFC converter module. The first output wiring may connect the AC port of the first UPFC converter module to the utility line, the load line, and / or an output wiring connected to an AC port of a further first UPFC converter module. AC ports of the plurality of first UPFC converter modules may be connected in series by first output wirings. The first output wirings may connect in series the utility line, the AC port of each of the plurality of first UPFC converter modules, and the load line.

[0047] The first UPFC converter module may have a rectifier connected to the input coupling system and a full bridge (FB) converter. The rectifier and the FB converter may be connected to form a DC link. The FB converter may have a DC buffer and a DC / AC converter. The FB converter may have the AC port of the first UPFC converter module.

[0048] The input coupling system may include at least one first input coupling device connectable to the power supply line, the at least one first input coupling device connected to the plurality of first UPFC converter modules for providing an electric power to the plurality of first UPFC converter modules. The at least one first input coupling device may provide some or all of the input electric power to the plurality of first UPFC converter modules. The at least one first input coupling device may include for each first UPFC converter module of the plurality of first UPFC converter modules a supply terminal connected to the first UPFC converter module. The at least one first input coupling device may include a parallel transformer that has a single input on the primary side for connecting to the power supply line and a single output on the secondary side connected to a single supply terminal. The at least one first input coupling device may include a parallel transformer that has a single input on the primary side for connecting to the power supply line and a plurality of outputs on the secondary side connected to a plurality of supply terminals. Two or more parallel transformers may be phase shifted with respect to each other. Beneficially, the phase shifted two or more parallel transformers compensate harmonics.

[0049] According to one aspect, a universal power flow controller, UPFC, for a high-power AC system having a power supply line for providing power from a utility supply to a load, the power supply line having a utility line connected to the utility supply and a load line connected to the load, is provided. The UPFC includes an input coupling system configured to be connected to the utility line for receiving an input electric power from the utility line, a plurality of first UPFC converter modules connected to the input coupling system, the plurality of first UPFC converter modules provided in parallel to each other, and an output coupling system connecting the utility line to the load line for providing an output electric power to the power supply line. The output coupling system includes a first output wiring for each first UPFC converter module of the plurality of first UPFC converter modules, the first output wirings connecting in series the utility line, the plurality of first UPFC converter modules, and the load line for providing an electric power to the power supply line from the plurality of first UPFC converter modules.

[0050] In some embodiments, which can be combined with further embodiments described herein, the UPFC may include a plurality of second UPFC converter modules. The plurality of second UPFC converter modules may be provided in parallel to each other. The plurality of second UPFC converter modules may be provided in parallel to the plurality of first UPFC converter modules. The output coupling system may include a second output wiring for each second UPFC converter module of the plurality of second UPFC converter modules. The second output wirings may connect the plurality of second UPFC converter modules in series to each other. The first output wirings and the second output wirings may connect in series the utility line, first UPFC converter modules of the plurality of first UPFC converter modules, second UPFC converter modules of the plurality of second UPFC converter modules, and the load line, for providing an electric power to the power supply line from the first UPFC converter modules and the second UPFC converter modules.

[0051] The plurality of output wirings may include the first output wirings and the second output wirings. The second output wirings may be configured similar to the first output wirings. In the following a single second UPFC converter module with a corresponding second output wiring is described. It is understood, that the following description applies to one or more, particularly all, second UPFC converter modules and corresponding output wirings.

[0052] The second UPFC converter module may have an AC port. The AC port of the second UPFC converter module may be configured for transmitting an AC current from the second UPFC converter module to the second output wiring. The second UPFC converter module may be configured for providing an AC current, having a frequency matching a frequency of an AC current provided by the utility supply, to the AC port. The second output wiring may be connected to the AC port of the second UPFC converter module. The second output wiring may connect the AC port of the second UPFC converter module to the utility line, the load line, a first output wiring connected to an AC port of a first UPFC converter module, and / or a further second output wiring connected to an AC port of a further second UPFC converter module. AC ports of the plurality of second UPFC converter modules may be connected in series by second output wirings.

[0053] The second UPFC converter module may include a transformer having a primary winding and a secondary winding, a rectifier connected the secondary winding of the transformer, and a full bridge (FB) converter. The rectifier and the FB converter may be connected to form a DC link. The FB converter may have a DC buffer and a DC / AC converter. The FB converter may have the AC port of the second UPFC converter module. The transformer may be a low frequency transformer or a medium frequency transformer. Preferably the second UPFC converter module includes a medium frequency transformer. The transformer may be configured to operate at a frequency between 50 Hz and 100 kHz. A low frequency transformer may operate at a frequency of 50 Hz to 60 Hz. A medium frequency transformer may operate at frequencies of 60 Hz to 100 kHz, particularly 1 kHz to 50 kHz, more particularly 10 kHz to 20 kHz. The rectifier may be configured to receive an AC current from the secondary winding of the transformer and to provide a DC current to the DC link. The FB converter may be configured to receive the DC current from the DC link and to provide an AC current with a frequency matching a frequency of an AC current provided by the utility supply to the AC port.

[0054] The input coupling system may include at least one second input coupling device connectable to the power supply line, the at least one second input coupling device connected to the plurality of second UPFC converter modules for providing an electric power to the plurality of second UPFC converter modules. The at least one second input coupling device may provide some or all of the input electric power to the plurality of second UPFC converter modules. The at least one second input coupling device may include a supply terminal connected to the second UPFC converter module. The input coupling system may include a same number of second input coupling devices as the number of second UPFC converter modules of the UPFC. The at least one second input coupling device may include for each second UPFC converter module of the plurality of second UPFC converter modules a supply terminal connected to the second UPFC converter module. The at least one second input coupling device may include a full bridge (FB) converter and a inverter connectable to a primary winding of the transformer. The rectifier and the FB converter may be connected to form a DC link. The FB converter may have a DC buffer and an AC / DC converter. The FB converter of the second input coupling devices may be connectable to the power supply line.

[0055] The following is with respect to one second input coupling device and one second UPFC converter modules. It is understood, that this applies to some or all second input coupling devices and second UPFC converter modules. The inverter of the second input coupling device may be connected to the primary winding of the transformer of the second UPFC converter module. The secondary winding of the transformer of the second UPFC converter module may be connected to rectifier of the second UPFC converter module. The FB converter of the second input coupling device may receive an AC current from the power supply line, particularly the utility line. The FB converter of the second input coupling device may provide a DC current to the DC link. The inverter of the second input coupling device may receive the DC current from the DC link and provide an AC current to the primary winding of the transformer of the second UPFC converter module at an operating frequency of the transformer of the second UPFC converter module. The inverter of the second input coupling device may provide a low frequency AC current to the transformer of the second UPFC converter module or provide a medium frequency AC current to the transformer of the second UPFC converter module. A low frequency AC current is understood as having a frequency of 50 Hz to 60 Hz. A medium frequency current is understood as having a frequency of 60 Hz to 100 kHz, particularly 1 kHz to 50 kHz, more particularly 10 kHz to 20 kHz.

[0056] In some embodiments, the second UPFC converter modules may include a medium frequency transformer. The at least one second input coupling device may be configured to provide a medium frequency AC current to the second UPFC converter modules. Advantageously, the at least one second input coupling device, the second UPFC converter modules, and the second output wirings may be provided without a low frequency transformer. Thereby, a number of low frequency transformers can further be reduced without reducing a power rating of the UPFC. According to a further aspect, a universal power flow controller, UPFC, for a high-power AC system having a power supply line for providing power from a utility supply to a load, the power supply line having a utility line connected to the utility supply and a load line connected to the load, is provided. The UPFC includes an input coupling system configured to be connected to the utility line for receiving an input electric power from the utility line, a plurality of second UPFC converter modules connected to the input coupling system, the plurality of second UPFC converter modules provided in parallel to each other, and an output coupling system connecting the utility line to the load line for providing an output electric power to the power supply line. The output coupling system includes a second output wiring for each second UPFC converter module of the plurality of second UPFC converter modules, the second output wirings connecting in series the utility line, the plurality of second UPFC converter modules, and the load line for providing an electric power to the power supply line from the plurality of second UPFC converter modules.

[0057] According to an aspect, a high-power AC system is provided. The high-power AC system including a power supply line for providing power from a utility supply to a load, and a universal power flow controller according to embodiments described herein.

[0058] In some embodiments, the high-power AC system is an AC electric arc furnace, the AC electric arc furnace further comprising a load.

[0059] Further advantages of the UPFC according to the present invention are as follows. By providing a plurality of UPFC converter modules, each UPFC converter can be designed smaller. Advantageously, smaller UPFC converter modules can be fabricated easier and with less material. Use of SSTs with MFTs may reduce the dependence on traditional transformer suppliers and may allow for faster delivery time. Smooth current control of the series injection transformers on the load side benefits the process by reducing excessive stresses on the system, thus enabling a higher system availability. Specific stress reduction examples include: the electrodes (graphite or Soderberg type) in EAFs or OBFs can easily break causing disruption of the process; reduced electrode consumption; reduced refractory wear; independent control of the electrode voltage; reduced wear and tear of the high current cables. Independent voltage control by electrode height adjustment typically allows higher power input to the furnace. Improved flicker performance and elimination or reduction of need for STATCOM and filtering can be achieved. The parallel connected AFE rectifiers can fully compensate the reactive and active power and the need for a STATCOM can be eliminated. One or more units can be connected in parallel on the AFE side and in series on the converter out-put side to create high pulse number converters (24-pulse and above) with very low distortion seen by the grid resulting in reduced filtering requirements. One or more units can be connected in parallel / series on the output side to meet the voltage / current needs of the load. The number of rectifiers and the number of inverters can be different for each isolated unit, i.e. for each UPFC converter module. The UPFC may comprise at least one active front end and may allow bidirectional power flow.

[0060] Those skilled in the art will recognise additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

[0061] Brief description of the drawings

[0062] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments of the disclosure and are described in the following:

[0063] Fig. 1 shows a schematic diagram of a high-power AC system having an universal power flow controller according to embodiments described herein.

[0064] Fig. 2 shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0065] Fig. 3 shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0066] Fig. 4 shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0067] Fig. 5 shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0068] Fig. 6 shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein. Detailed description of the drawings

[0069] Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations.

[0070] Within the following description of the drawings, the same reference numbers refer to the same or to similar components. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment can apply to a corresponding part or aspect in another embodiment as well.

[0071] Fig. 1 shows a schematic diagram of a high-power AC system 10 including a universal power flow controller, UPFC, 100 according to embodiments described herein. The high-power AC system 10 includes a power supply line 140 for providing a power from a utility supply 160 to a load 150. The load 150 may be a load of the high-power AC system 10. The utility supply may be an AC grid. For example, the utility supply may be a low voltage (LV) grid, a medium voltage (MV) grid, a high voltage (HV) grid, or an extra high voltage grid. The high-power AC system may be connected directly to the utility supply, particularly, the utility supply being a LV grid or a MV grid. The high-power AC system may be connected indirectly to the utility supply, for example, may be connected through a grid transformer to the utility supply, particularly to the utility supply being a low voltage (LV) grid, a medium voltage (MV) grid, a high voltage (HV) grid, or an extra high voltage grid.

[0072] The high-power AC system may be any type of high-power AC system. Particularly, any type of high-power AC system for which a power is provided to a load. A power provided to the load may be controlled and / or a power quality may be improved, e.g. by compensating power fluctuations, transients and / or harmonics in the power supply line and / or the load. For example, the high-power AC system may be an AC electric arc furnace. The further embodiments shown in Figs. 2 through 5 show the high-power AC system as an AC electric arc furnace. It is understood, that the description of the embodiments shown in Figs. 2 through 5, particularly the description of the universal power flow controller 100 applies to any suitable high-power AC system. The load 150 may be any suitable type of load that can be powered with an AC current. In the further embodiments shown in Figs. 2 through 5, the load may be a load of the AC electric arc furnace. For example, the load may be electrodes of the AC electric arc furnace to which an electric power is provided for operation of the AC electric arc furnace. Particularly, electrodes to which an electric power is provided for melting metal containing materials in a furnace of the AC electric arc furnace. Particularly, by an electric arc produced by the electrodes.

[0073] The UPFC 100 comprises an input coupling system 110, a plurality of UPFC converter modules 120i through 120N, and an output coupling system 130, having a plurality of output coupling devices 135xthrough 135N. The plurality of UPFC converter modules are provided between the input coupling system 110 and the output coupling system 130. The plurality of UPFC converters 120i through 120Nare provided in parallel to each other.

[0074] The input coupling system 110 is configured to be connected to the power supply line 140. In Fig. 1 the input coupling system 110 is shown connected to the power supply line 140. The input coupling system 110 may receive an input electric power from the power supply line. The input coupling system 110 may be connected to the plurality of UPFC converter modules 120i through 120Nfor providing the input electric power to the plurality of UPFC converter modules 120i through 120N. As is shown in Fig. 1, the input coupling system may be connected to each UPFC converter module of the plurality of UPFC converter modules 120i through 120N.

[0075] The plurality UPFC converter modules 120i through 120Nmay be provided in parallel to each other. The plurality of UPFC converter modules 120xthrough 120Nmay be identical to each other. The plurality of UPFC converter modules 120xthrough 120Nmay be connected to the output coupling system 130. In Fig. 1, the plurality of UPFC converter modules are connected to the output coupling system 130. Particularly, each UPFC converter module of the plurality of UPFC converter modules is connected to an associated output coupling device 1351 through 135N. The plurality of UPFC converter modules may be connected to the plurality of output coupling devices 1351 through 135Nin a 1 : 1 association. That is, each UPFC converter module is connected to exactly one output coupling device and each output coupling device is connected to exactly one UPFC converter module. In the following description, features are described with respect to a single converter module 120 connected to a single output coupling device 130, forming a single pair of the UPFC converter module 120 connected to the associated output coupling device 135. It is understood, that any features described apply to any UPFC converter module of the plurality of UPFC converter modules 120i through 120N, and any output coupling device 135 of the plurality of output coupling devices 1351 through 135N. The output coupling system 130 is configured to be connected to the power supply line 140. In Fig. 1 the output coupling system 130 is shown to be connected to the power supply line. The output coupling system 130 may provide an output electric power to the power supply line 140. The output coupling devices 135 may receive a converter power from the UPFC converter module 120 and provide the converter power to the power supply line 140. The UPFC converter module 120 may provide the converter power to the output coupling device 135. The output coupling device may provide the converter power to the power supply line 140. The sum of the converter powers provided by each UPFC converter module of the plurality of UPFC converter modules may correspond to the output electric power. The plurality of output coupling devices 135i through 135Nare configured to be connected to the power supply line. The plurality of output coupling devices 135xthrough 135Nare connected in series to the power supply line. The plurality of output coupling devices 135i through 135Nmay provide the output electric power to the power supply line 140.

[0076] Fig. 2 shows a schematic diagram of an UPFC 100 according to embodiments described herein. The three parallel lines crossing the power supply line 140 indicate a 3-phase AC system. The UPFC 100 shown in the figures are for a 3-phase AC system. The UPFC 100 shown in the figures is shown for one phase, it is understood that analogous UPFCs can be provided to each phase of the 3-phase AC system The embodiments may also be provided for a multi-phase AC system, having for example 4 or 5 phases. The UPFC 100 has two UPFC converter modules 120i, 1202connected to two output coupling devices 135b1352. The input coupling system 110 includes an input coupling device that is a parallel transformer 115. The AC electric arc furnace 10 includes a filtering unit 180 for filtering harmonics in the power supply line, particularly harmonics drawn by the load 150. The filtering unit may eliminate harmonics, such as the 5th, 7th, 11thand 13thharmonic. The AC electric arc furnace includes an AC arc furnace transformer 170. The AC arc furnace transformer may be provided between the utility supply 160 and the load 150, particularly the electrodes of the AC electric arc furnace. The AC arc furnace transformer may be connected to the power supply line on a high voltage, low current side and connected to the load on a low voltage, high current side.

[0077] The parallel transformer 115 has a single input on a primary side and two outputs on a secondary side. The primary side of the parallel transformer 115 is connected to the power supply line for receiving an electrical input power. The parallel transformer has two outputs on a secondary side. Particularly, the parallel transformer has a first secondary side having a first output and a second secondary side having a second output. The two outputs of the secondary side provide two different electric outputs, for example for providing a first electric power and a second electric power. The sum of the first electric power and the second electric power may correspond to the input electric power. The two outputs are connected to the plurality of UPFC converter modulesl20i, 1202. The first output may be connected to the first UPFC converter module 120i for providing the first electric power to the first UPFC converter. The second output may be connected to the second UPFC converter module 1202for providing the second electric power to the second UPFC converter module 1202.

[0078] The parallel transformer 115 has a DELTA / star / delta configuration. The primary side of the parallel transformer 115 has a delta configuration. The first secondary side of the parallel transformer 115 has a star configuration. The second secondary side of the parallel transformer has a delta configuration. The DELTA / star / delta configuration (12-pulse AFE / PFE) of the parallel transformer 115 eliminates the 5thand 7thharmonic in the power supply line 140, particularly harmonics drawn by the load 150. A passive filtering may be reduced. Particularly, such that the filtering unit only eliminates higher order harmonics, such as 11thand 13thorder harmonic.

[0079] As shown in the enlarged view of the UPFC converter module 120 in Fig. 2, the UPFC converter module includes switch mode semiconductors. For example, switch mode semiconductors may be IGBT, IGCT, SIC MOSFET, or similar semiconductor devices. The UPFC converter has a switching frequency. The plurality of UPFC converter modules may be interleaved for interleaved switching of the UPFC converter modules. The interleaving results in a multiplication of the switching frequency of the individual UPFC converter modules. For example, 4 UPFC converter modules, with series connected voltages, can effectively have 4 times as many steps in the injected voltage with 4 times the bandwidth of one UPFC converter module.

[0080] The UPFC converter module 120 has a front end having a rectifier 122, a DC link having one or more DC buffer 124, and an inverter 126. The DC link is provided between the rectifier 122 and the inverter 126. The at least one DC buffer 124 is provided in the DC link. The front end may be an active front end. The front end may be a passive front end. The rectifier 122 may be an active rectifier. The rectifier 122 may be a passive rectifier. For example, the DC link may comprise two DC buffers 124 for providing a three-level output. The rectifier 122 may be one or more rectifiers provided in parallel to each other. The one or more rectifiers may be connected to a same input connected to the input coupling system. The inverter 126 may be one or more inverters provided in parallel to each other. The one or more inverters may be connected to a same output connected to the output coupling system, particularly to a same output coupling device.

[0081] The output coupling system 130 has two output coupling devices 135b1352. The two output coupling devices 135b1352are series transformers. The series transformers 135b1352may provide the output electric power on a secondary side of the series transformers. Particularly, provide the output electric power to the power supply line 140. The secondary side of the series transformers 135b1352may be a medium voltage or a high voltage side, . The primary side of the series transformers 135b1352may be low voltage or a medium voltage side. A voltage provided by the series transformersl35b1352on the secondary side may be substantially equal to a voltage provided to the series transformers 135b1352on the primary side. The series transformers 135b1352may have a winding ratio of substantially 1 : 1. The series transformers 135b1352may have high isolation on the windings. The series transformers 135b1352may be connected on a primary side to the UPFC converter modules 120b1202. A first series transformer 135xmay be connected to the first UPFC converter module. A second series transformer 1352may be connected to the second UPFC converter. The primary side of the series transformers may be a low voltage side, particularly a high current side.

[0082] The series transformers 135b1352are connected on the secondary side to each other. The series transform ersl35b1352are connected on the secondary side in series to each other. The series transformer 135b1352may provide the converter power in series to the power supply line. Advantageously, a power provided by each of the series transformer 135b1352may be reduced. The output electric power may be evenly distributed and provided by the plurality of series transformers 135b1352. Each series transformer 135b1352only sees a part of a voltage drop during power fluctuations in the power supply line. Particularly, voltage drops or peaks, or power drops or peaks, may be evenly distributed over the plurality of series transformers 135b1352. Voltage drops or peaks, or power drops or peaks, may be evenly distributed over the plurality of UPFC converter modules. Advantageously, having two UPFC converter modules reduces the power and / or voltage compensation provided by each UPFC converter module. Further, by interleaved controlled of the two UPFC converter modules an effective switching frequency of the UPFC converter modules may be doubled.

[0083] Fig. 3 shows a further embodiment of a UPFC 100 provided in an AC electric arc furnace 10. The UPFC 100 of Fig. 3 is similar to the UPFC 100 of Fig. 2 and comprises two additional UPFC converter modules 1203, 1204connected to output coupling devices 1353, 1354, that are series transformers. The above advantages described with respect to the embodiment of Fig. 2 apply and advantageously, the four UPFC converter modules further reduce the power and / or voltage compensation provided by each UPFC converter module. The input coupling system comprises a second parallel transformer 1152having a DELTA / star / delta configuration connected to the two additional UPFC converter modules 1203, 1204. Additional benefits of this configuration include, that due to the 2 DELTA / star / delta transformers (24-pulse AFE / PFE), elimination of 5th, 7th, 11thand 13thharmonics can be accomplished, requiring only a high order harmonic filter by the filtering unit 180 which can be designed much smaller and simpler than known solutions. Also, having four UPFC converter modules allows a further doubling of the effective switching frequency, resulting in a further reduction of flicker disturbances. The configuration of Fig. 3 has even more reduction in size and cost of the series connected transformers with the same benefits mentioned earlier.

[0084] Fig. 4 shows an UPFC 100 similar to the UPFC 100 shown in Fig. 3. Instead of having two parallel transformers 115i, 1152having a DELTA / star / delta configuration, a parallel transformer 215 is provided. The parallel transformer 215 has a single input and four outputs. The parallel transformer 215 is connected to the power supply line at the input for receiving an input electric power. The parallel transformer has the four outputs on a secondary side. Particularly, the parallel transformer 215 has a first output on a first secondary side, a second output on a second secondary side, a third output on a third secondary side, and a fourth output on a fourth secondary side. The four outputs are connected to the UPFC converter modules 120i, 1202, 1203, 1204. In addition to the advantageous described with respect to Fig. 3, the UPFC 100 according to Fig. 4 additional allows a further reduction in size.

[0085] Fig. 5 shows an UPFC 100 having four UPFC converter modules 320b3202, 3203, 3204connected to four output coupling devices 135b1352;1353, 1354that are series transformers. The connection of the UPFC converter modules to the output coupling devices is similar to the embodiment shown in Fig. 4. The UPFC converter modules 320b3202, 3203, 3204have a front end connected to the input coupling system 110, a DC link having a DC buffer 124 and an inverter 126. The front end may be an active front end. The front end may be a passive front end. The input coupling system 110 has four converters 315b3152, 3153, 3154. Each converter has a line interphase transformer, LIT, a MV rectifier, and a solid-state-transformer, SST. The line interphase transformer is connected to the power supply line 140 for receiving an electrical power. The four converters may receive the electrical input power. Particularly the LITs of the four converters may receive the electrical input power. The MV rectifier is connected between the LIT and the SST. The SST provides an electrical power. The SST is connected to a respective UPFC converter module. The DC link may comprise an output of the SST. The SST provides the electrical power to the respective UPFC converter module. In addition to the advantages described with respect to the embodiments shown in Figs. 1 through 4, the embodiment shown in Fig. 5 advantageously allows a further reduction of size and volume of input coupling system.

[0086] Fig. 6 shows a UPFC 600 for a high-power AC system having a power supply line 140 for providing a power from a utility supply 160 to a load 150. The power supply line 140 has a utility line 141 connected to the utility supply and a load line 142 connected to the load. The utility line 141 is connected to the load line 142 by a plurality of output wirings including two first output wirings 635b6352and two second output wirings 675b6752. The UPFC has two first UPFC converter modules 620b6202and two second UPFC converter modules 650b6502. The first UPFC converter modules 620b6202include a rectifier 622b6222, and a FB converter 628b6282. The FB converter has a DC buffer being a capacitor 624b6242and a DC / AC- converter 626b6262. The rectifiers 622b6222of the first UPFC converter modules 620b6202are connected to a low frequency transformer 115 of the input coupling system 110. The second UPFC converter modules 650b6502include a transformer 660b6602, an inverter and a FB converter. The inverter and the FB converter are illustrated as an AC / AC converter 670b6702. The transformer 660b6602may be a low frequency transformer or a medium frequency transformer. The transformers 660b6602are connected to second output coupling devices 640b6402of the input coupling system 110. The second output coupling devices 640b6402includes a FB converter and a rectifier.

[0087] The FB converter 628b6282may be configured as shown in the enlarged view AA. The FB converter may have the capacitor 624b6242and semiconductor switches SbS2, S3, S4. Semiconductor switches Si and S2may be provided in series to each other, and in parallel to the capacitor. Semiconductor switches S3and S4may be provided in series to each other, in parallel to semiconductor switches Si and S2;and in parallel to the capacitor. The AC output port may be formed by a first connection connected between the first switch Si and the second switch S2and a second connection connected between the third switch S3and the fourth switch S4.

[0088] In enlarged view BB a second output coupling device 640 having the FB converter 641 and the inverter 642 and the UPFC converter module 650, having the transformer 650, the inverter 671 and the FB converter 672, are shown. The input coupling system 110 is connected to the utility line 141. The UPFC includes two first output wirings 635b6352, and two second output wirings 675b675i. The output wirings connect in series the utility line 141, the two first UPFC converter modules 620b6202, the two second UPFC converter modules 650b6502, and the load line.

[0089] While the foregoing is directed to embodiments, other and further embodiments may be devised without departing from the basic scope, and the scope is determined by the claims that follow.

Claims

Claims1. A universal power flow controller, UPFC, for a high-power AC system having a power supply line for providing power from a utility supply to a load, the UPFC comprising: an input coupling system configured to be connected to the power supply line for receiving an input electric power from the power supply line; a plurality of UPFC converter modules connected to the input coupling system, the plurality of UPFC converter modules provided in parallel to each other; and an output coupling system configured to be connected to the power supply line for providing an output electric power to the power supply line, the output coupling system comprising a plurality of output coupling devices connectable to the power supply line, wherein for each UPFC converter module of the plurality of UPFC converter modules, a respective output coupling device of the plurality of output coupling devices is connected to the UPFC converter module for providing an electric power to the power supply line.

2. The UPFC according to claim 1, wherein the high-power AC system is an AC electric arc furnace.

3. The UPFC according to any one of claims 1 to 2, wherein the output coupling devices are connected in series to each other, particularly wherein the output coupling devices are configured to be connected in series to the power supply line.

4. The UPFC according to any one of claims 1 through 3, wherein the output coupling devices are series transformers for providing the output electric power on a secondary side of the series transformers.

5. The UPFC according to claim 4, wherein the series transformers are connected to the UPFC on their primary side.

6. The UPFC according to any one of the preceding claims, wherein the input coupling system comprises for each UPFC converter module of the plurality of UPFC converter modules a supply terminal connected to the UPFC converter module.

7. The UPFC according to claim 6, wherein the input coupling system comprises at least one input coupling device connectable to the power supply line, the at least one input coupling device connected to the plurality of UPFC converter modules for providing an input electric power to the plurality of UPFC converter.

8. The UPFC according to claim 7, wherein the at least one input coupling device comprises a parallel transformer that has a single input on the primary side for connecting to the power supply line and a single output on the secondary side connected to a UPFC converter.

9. The UPFC according to any one of claims 7 and 8, wherein the at least one input coupling device comprises a parallel transformer that has a single input on the primary side for connecting to the power supply line and a plurality of outputs on the secondary side connected to a plurality of supply terminals.

10. The UPFC according to any one of the preceding claims, wherein at least one UPFC converter module of the plurality of UPFC converter modules comprises a front end having a rectifier, an inverter, and a DC link having a DC buffer, the DC link being provided between the rectifier and the inverter.

11. The UPFC according to claim 10, wherein the front end is an active front end and the rectifier is an active rectifier.

12. The UPFC according to any one of claims 1 through 5, wherein the input coupling system comprises at least one converter having a line interphase transformer for receiving the input electric power, a rectifier, and a solid-state transformer connected to a UPFC converter module of the plurality of UPFC converter modules.

13. The UPFC according to claim 12, wherein for each UPFC converter module of the plurality of UPFC converter modules the input coupling system comprises a converter of the at least one converter, the converter having a line interphase transformer for receiving the input electric power, a rectifier, and a solid-state transformer connected to the UPFC converter.

14. The UPFC according to any one of claims 12 and 13, wherein at least one UPFC converter module of the plurality of UPFC converter modules comprises a front end, an inverter, and a DC link having a DC buffer, the DC link provided between the front end and the inverter, particularly wherein the DC link comprises an output of the solid-state transformer.

15. The UPFC according to any one of claims 10 and 14, wherein the DC buffer comprises an element selected from the group consisting of capacitor, supercapacitor, battery, flywheel, gravity energy storage, compressed-air energy storage (CAES), and a combination thereof.

16. The UPFC according to any one of claims 10, 14, and 15, wherein a renewable energy source is connected to the DC buffer for providing electric power from the renewable energy source to the DC buffer.

17. The UPFC according to any one of the preceding claims, wherein the plurality of UPFC converter modules comprise switch mode semiconductors.

18. The UPFC according to claim 17, wherein the plurality of UPFC converter modules are configured to be interleaved for interleaved switching of the switch mode semiconductors.

19. The UPFC according to any one of the preceding claims, wherein the UPFC may control a power in the power supply line and / or a power quality in the power supply line by the output electric power.

20. The UPFC according to claim 19, wherein the output electric power comprises an active output electric power and a reactive output electric power.

21. The UPFC according to claim 20, wherein the UPFC may provide a reactive output electric power through the input side, particularly through the input side and through the output side.

22. A high-power AC system comprising: a power supply line for providing power from a utility supply to a load; and a universal power flow controller according to any one of the preceding claims.

23. The high-power AC system according to claim 22, wherein the high-power AC system is an AC electric arc furnace, the AC electric arc furnace further comprising a load.