WO2025153174A1 - A universal power flow controller for a high-power ac system and a high-power ac system - Google Patents

Abstract

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 including 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 [from the UPFC converter module] to the power supply line.

Description

A UNIVERSAL POWER FLOW CONTROLLER FOR A HIGH-POWER AC SYSTEM AND A HIGH-POWER AC SYSTEM

Field of the disclosure

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.

Technical Background

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.

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

Summary of the disclosure

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.

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.

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 [from the UPFC converter module] to the power supply line.

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. 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.

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 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.

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 5^th and 7^th harmonic 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 5^th, 7^th, 11^th, 13^th harmonics. A higher number of UPFC converter modules allows to eliminate harmonics up to higher orders.

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.

The output coupling system is configured to provide the output electric power to the power supply line. 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. 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.

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.

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.

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.

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 electric output 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.

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 electric output power to control the active power and/or the reactive power in the power supply line. The electric output power may have an active electric output power and a reactive electric output power. The active electric output power and the reactive electric output power may be provided by the UPFC, particularly by the plurality of UPFC converter modules. In some embodiments, the UPFC may provide the reactive electric output power through the input side to the power supply line. The UPFC may provide the reactive electric output power through the input side and through the output side to the power supply line. The reactive electric output 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 electric output 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 electric output 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 electric output power may be fed from the power supply line through the input coupling system to the plurality of UPFC converter modules. The active electric output 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.

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 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.

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.

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.

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.

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

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.

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.

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.

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 5^th, 7^th, 11^th, and 13^th harmonics, 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.

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 the input coupling system comprises at least one converter having a line interface 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 interface transformer, LIT, may be configured to be connected to the power supply line.

The Line Interface Transformers (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.

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 interface 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.

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.

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.

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.

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.

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

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.

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

Brief description of the drawings

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:

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

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

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

Fig. 4 shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein. Fig. 5 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

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.

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.

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.

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.

The UPFC 100 comprises an input coupling system 110, a plurality of UPFC converter modules 120i through 120_N, and an output coupling system 130, having a plurality of output coupling devices 135 _x through 135_N. 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 120_N are provided in parallel to each other.

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 electric input power from the power supply line. The input coupling system 110 may be connected to the plurality of UPFC converter modules 120i through 120_N for providing the electric input power to the plurality of UPFC converter modules 120i through 120_N. 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 120_N.

The plurality UPFC converter modules 120i through 120_N may be provided in parallel to each other. The plurality of UPFC converter modules 120_x through 120_N may be identical to each other. The plurality of UPFC converter modules 120_x through 120_N may 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 135_N. The plurality of UPFC converter modules may be connected to the plurality of output coupling devices 1351 through 135_N in 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 120_N, and any output coupling device 135 of the plurality of output coupling devices 1351 through 135_N.

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 electric output 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 model 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 electric output power. The plurality of output coupling devices 135i through 135_N are configured to be connected to the power supply line. The plurality of output coupling devices 135 _x through 135_N may provide the electric output power to the power supply line 140.

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, 120₂ connected to two output coupling devices 135 _b 135₂. 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 5^th, 7^th, 11^th and 13^th harmonic. 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.

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 120₂ for providing the second electric power to the second UPFC converter module 120₂.

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 5^th and 7^th harmonic in the power supply liner 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 11^th and 13^th order harmonic.

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.

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.

The output coupling system 130 has two output coupling devices 135_b 135₂. The two output coupling devices 135_b 135₂ are series transformers. The series transformers 135_b 135₂ may 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 135 _b 135₂ may be a medium voltage or a high voltage side, . The primary side of the series transformers 135_b 135₂ may be low voltage or a medium voltage side. A voltage provided by the series transformersl35_b 135₂ on the secondary side may be substantially equal to a voltage provided to the series transformers 135_b 135₂ on the primary side. The series transformers 135_b 135₂ may have a winding ratio of substantially 1 : 1. The series transformers 135_b 135₂ may have high isolation on the windings. The series transformers 135 _b 135₂ may be connected on a primary side to the UPFC converter modules 120_b 120₂. A first series transformer 135_x may be connected to the first UPFC converter module. A second series transformer 135₂ may 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.

The series transformers 135 _b 135₂ are connected on the secondary side to each other. The series transform ersl35_b 135₂ are connected on the secondary side in series to each other. The series transformer 135_b 135₂ may provide the converter power in series to the power supply line. Advantageously, a power provided by each of the series transformer 135 _b 135₂ may be reduced. The output electric power may be evenly distributed and provided by the plurality of series transformers 135_b 135₂. Each series transformer 135_b 135₂ only 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 135 _b 135₂. 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.

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, 120₄ connected 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 115₂ having 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 5^th, 7^th, 11^th and 13^th harmonics 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.

Fig. 4 shows an UPFC 100 similar to the UPFC 100 shown in Fig. 3. Instead of having two parallel transformers 115_b 115₂ having 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 on at the input for receiving an electric input 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.

Fig. 5 shows an UPFC 100 having four UPFC converter modules 320 320₂, 320₃, 320₄ connected to four output coupling devices 135_b 135_2; 135₃, 135₄ that 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 320_b 320₂, 320₃, 320₄ have 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 315_b 315₂, 315₃, 315₄. Each converter has a line interface transformer, LIT, 316, a MV rectifier 317, and a solid-state-transformer, SST, 318. The line interface 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 317 is connected between the LIT 316 and the SST 318. The SST 318 provides an electrical power. The SST 318 is connected to a respective UPFC converter module. The DC link may comprise an output of the SST 318. The SST 318 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.

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

Claims

1. 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 interface 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 interface 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 EAF power supply system 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 EAF power supply system 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 electric output power.

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

21. The UPFC according to claim 20, wherein the UPFC may provide a reactive electric output 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.