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

Abstract

A universal power flow controller, UPFC, for a high-power AC system having a multi-phase power supply line for providing power from a utility supply to a load, the multi-phase power supply line having at least a first to third phase, the UPFC comprising: a parallel coupling system configured to be connected to the power supply line in parallel for transmitting an input electric power between the parallel coupling system and the power supply line; a series coupling system comprising at least one first full-bridge, FB, converter configured to be connected in series to the first phase for transmitting a first output electric power between the at least one first FB converter and the first phase, at least one second FB converter configured to be connected in series to the second phase for transmitting a second output electric power between the at least one second FB converter and the second phase, and at least one third FB converter configured to be connected in series to the third phase for transmitting a third output electric power between the at least one third FB converter and the third phase; and an interconnection system comprising at least one first solid state transformer, SST, connecting the parallel coupling system to the at least one first FB converter, at least one second SST connecting the parallel coupling system to the at least one second FB converter, and at least one third SST connecting the parallel coupling system to the at least one third FB converter.

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. Such power flow controllers have a slow response time, are limited to compensating lagging reactive power, have limited flicker compensation and require additional power factor correction and reactive power correction, for example, through a static var compensator (SVC) or a STATCOM. Power flow controllers may have a parallel unit and a series unit made up of series connected (FB) full bridge converters. In this case the range of operation is limited because the individual series strings can only generate reactive power while maintaining their capacitors at the rated DC link voltage of each cell. These power flow controllers have very limited to substantially no active power control. Particularly, the active power control is limited in that too much active power control would discharge the DC links of the full bridge converters. In addition, these power flow controllers have limited flicker compensation and may require higher order passive filters. Power flow controllers based on low frequency transformers (LFTs) have the drawbacks that the LFTs are large in size, expensive, not readily available, prone to saturation, and sensitive to harmonics from the power flow controller.

[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] UPFCs that are based on low frequency transformers, and / or UPFC having a parallel unit and a series unit that are independent from each other have a limited range of operation. The parallel unit and the series unit may respectively be made up of series connected full bridge (FB) converters. The individual series strings can only generate reactive power while maintaining their capacitors at the rated DC link voltage of each cell. The parallel connected FB converters may act as a STATCOM (static synchronous compensator) and the series connected FB converters act as a SSSC (static series synchronous compensator) or DVR (dynamic voltage restorer).

[0010] Low frequency transformers (LFTs) have several drawbacks which include high cost, up to 40- 50% of the total cost of the UPFC, large size, and / or limited availability. Beneficially, the UPFC according to the present application uses solid state transformers in the interconnection system and full bridge converters in the series coupling system, thereby beneficially allowing to reduce an amount of LFTs. In addition, the parallel coupling system can be implemented without low frequency transformers, particularly with FB converters or line interface transformers. Further, the direct connection of series FB converters in the series coupling system, as well as in the parallel coupling system, has the additional benefit of allowing a more compact design of each series FB converter. That is, the power ratings of each series FB converter can be reduced. Additionally, a total series impedance can be reduced by removal of some or all of the LFTs, which may increase the dynamic performance as well as power capability. The LFTs may be replaced by a series inductor, such as an air core series inductor, which has no saturation issue like the LFT. The series inductor may be small in size, compared to the LFT. Replacing low frequency transformers with solid state transformers as described herein, particularly SSTs having a medium frequency transformers (MFTs), reduce the dependency on traditional transformer suppliers. Beneficially, the SSTs may be more readily available, faster delivery times may be achieved, and / or total costs of the UPFC may be reduced.

[0011] In view of the above it is beneficial to have a UPFC with increased range of operation. Particularly a UPFC that has active power compensation, reactive power compensation and load current control. Further, it is beneficial to reduce a dependency on low frequency transformers. The UPFC described herein may beneficially be implemented without low frequency transformers. 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 20 is provided. Further aspects, advantages, and features are apparent from the dependent claims, the description, and the accompanying drawings. The solution of the present invention is based on connecting the isolated DC links of the FB converters, particularly the FB converters in the series coupling system and the parallel coupling system, using solid state transformers (SST).

[0012] 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-system, particularly to a 3-phase AC system.

[0013] According to an aspect, a universal power flow controller, UPFC, for a high-power AC system having a multi-phase power supply line for providing power from a utility supply to a load is provided. The multi-phase power supply line has at least a first to third phase. The UPFC includes a parallel coupling system configured to be connected to the power supply line in parallel for transmitting an input electric power between the parallel coupling system and the power supply line, a series coupling system including at least one first full-bridge, FB, converter configured to be connected in series to the first phase for transmitting a first output electric power between the at least one first FB converter and the first phase, at least one second FB converter configured to be connected in series to the second phase for transmitting a second output electric power between the at least one second FB converter and the second phase, and at least one third FB converter configured to be connected in series to the third phase for transmitting a third output electric power between the at least one third FB converter and the third phase, and an interconnection system including at least one first solid state transformer, SST, connecting the parallel coupling system to the at least one first FB converter, at least one second SST connecting the parallel coupling system to the at least one second FB converter, and at least one third SST connecting the parallel coupling system to the at least one third FB converter.

[0014] 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.

[0015] The utility supply may include a renewable energy source. The utility supply may be directly or indirectly connected to a renewable energy source and / or may, at least partly, be powered by a renewable energy source. The utility supply may include an energy storage system. The energy storage system may be connected to a renewable energy source. 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.

[0016] The high-power AC system has a load. The load may be a multi-phase 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. The AC electric arc furnace may have a plurality of electrodes, such as 3 electrodes, 4 electrodes, 5 electrodes, or more than 5 electrodes. The plurality of electrodes may form the multi-phase load. Particularly, each electrode may be connected to, i.e. be powered by, one phase of the multi-phase power supply line. 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.

[0017] The high-power AC system has a multi-phase power supply line. The multi-phase power supply line has at least a first to third phase. The multi-phase power supply line may have more than three phases. 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 have a utility supply line and a load line, connected to each other via the series coupling system. The utility supply 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 series coupling system may be connected to the utility supply line. The series coupling system may be connected to the load line. The series coupling system may connect the utility supply line and the load line in a series connection. The parallel coupling system may be connected to the utility supply line. A power may flow between the utility supply line and the load line through the series coupling system. A power may flow between the UPFC and the load line through the series coupling system. A power may flow between the utility supply line and the parallel coupling system.

[0018] The one or more first FB converters may separate the first phase into a first phase of the utility supply line and a first phase of the load line. The one or more first FB converters may be connected in series to provide a series connection between the first phase of the utility supply line and the first phase of the load line. The one or more first FB converters may be connected in series at their respective AC sides to each other and to the first phase of the utility supply line and the first phase of the load line. The one or more second FB converters may separate the second phase into a second phase of the utility supply line and a second phase of the load line. The one or more second FB converters may be connected in series to provide a series connection between the second phase of the utility supply line and the second phase of the load line. The one or more second FB converters may be connected in series at their respective AC sides to each other and to the second phase of the utility supply line and the second phase of the load line. The one or more third FB converters may separate the third phase into a third phase of the utility supply line and a third phase of the load line. The one or more third FB converters may be connected in series to provide a series connection between the third phase of the utility supply line and the third phase of the load line. The one or more third FB converters may be connected in series at their respective AC sides to each other and to the third phase of the utility supply line and the third phase of the load line. The same applies for further phases of the multiphase power supply line as described herein.

[0019] The load line may provide substantially all of the power provided to the load. A power may be provided to the load line through the utility supply line. A power may be provided to the load line through the UPFC, particularly through the series coupling system. The utility supply line may be configured for providing most or all of the power provided to the load line. The UPFC according to the present application may provide a power to the load line. The power provided by the UPFC to the load line may be small compared to the power provided by the utility supply line. The utility supply 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 to the load. The UPFC according to the present application may provide less than 50%, less than 45%, less than 40%, less then 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 to the load. Only a part of the total power provided to the load line, particularly to the load, may be provided by the UPFC. The UPFC may not provide the total power provided to the load line, particularly to the load.

[0020] The high-power AC system may be an AC arc furnace and the power supply line may be a connection between the utility supply and an AC arc furnace transformer. The AC arc furnace transformer may be connected to the electrodes of the AC arc furnace. The main part of the power to the furnace may flow from the multi-phase power supply line to the load. A part of the power may flow via the parallel coupling system through the interconnection system to the series coupling system. This may include buffering of power in DC link storage devices, such as capacitors.

[0021] The UPFC includes a parallel coupling system. The parallel coupling system is configured to be connected to the power supply line in parallel for transmitting an input electric power between the parallel coupling system and the power supply line, particularly the utility supply line. A power flow between the parallel coupling system and the power supply line, particularly the utility supply line, may be bi-directional. A power may flow from the power supply line, particularly the utility supply line, to the parallel coupling system. A power may flow from the parallel coupling system to the power supply line, particularly the utility supply line. Throughout this description, a power flowing from the power supply line, particularly the utility supply line, to the parallel coupling system is defined as a positive power. A power flowing from the parallel coupling system to the power supply line, particularly the utility supply line, is defined as a negative power. That is, an input electric power is positive if power flows from the power supply line, particularly the utility supply line, to the parallel coupling system and the input electric power is negative if power flows from the parallel coupling system to the power supply line, particularly the utility supply line.

[0022] The parallel coupling system may be configured to be connected to each phase of the multiphase power supply line, particularly each phase of the utility supply line. For example, the multi-phase power supply line may have three phases and the parallel coupling system may be connected to each of the three phases. The parallel coupling system may be configured to be connected to the first phase of the power supply line for transmitting a first input electric power between the parallel coupling system and the first phase of the power supply line, particularly the first phase of the utility supply line. The parallel coupling system may be configured to be connected to the second phase of the power supply line for transmitting a second input electric power between the parallel coupling system and the second phase of the power supply line, particularly the second phase of the utility supply line. The parallel coupling system may be configured to be connected to the third phase of the power supply line for transmitting a third input electric power between the parallel coupling system and the third phase of the power supply line, particularly the third phase of the utility supply line.

[0023] The input electric power may have an active part, also referred to as active input electric power, and a reactive part, also referred to as reactive input electric power. According to an embodiment, at least one of the first input electric power, the second input electric power, and the third input electric power includes an active electric power and a reactive electric power. The parallel coupling system may be configured for compensating reactive power fluctuations in the multi-phase power supply line. Particularly, the parallel coupling system may be configured for compensating reactive power fluctuations in the multi-phase power supply line without interactions with the interconnection system and / or the series coupling system.

[0024] The UPFC includes a series coupling system. The series coupling system may be configured to be connected to the power supply line in series for transmitting an output electric power between the series coupling system and the power supply line, particularly between series coupling system and the utility supply line and / or the load line. The series coupling system includes at least one first full-bridge (FB) converter configured to be connected in series to the first phase for transmitting a first output electric power between the at least one first FB converter and the first phase, particularly the first phase of the utility supply line and / or the first phase of the load line. The series coupling system includes at least one second FB converter configured to be connected in series to the second phase for transmitting a second output electric power between the at least one second FB converter and the second phase, particularly the second phase of the utility supply line and / or the second phase of the load line. The series coupling system includes at least one third FB converter configured to be connected in series to the third phase for transmitting a third output electric power between the at least one third FB converter and the third phase, particularly the third phase of the utility supply line and / or the third phase of the load line. A power flow between the series coupling system and the power supply line, particularly the utility supply line and / or the load line, may be bi-directional. A power may flow from the power supply line, particularly the utility supply line and / or the load line, to the series coupling system. A power may flow from the series coupling system to the power supply line, particularly the utility supply line and / or the load line. Throughout this description, a power flowing from the series coupling system to the power supply line, particularly the load line, is defined as a positive power. A power flowing from the power supply line, particularly the utility supply line, to the series coupling system is defined as a negative power. That is, an output electric power is positive if power flows from the series coupling system to the power supply line, particularly the load line, and the output electric power is negative if power flows from the power supply line, particularly the utility supply line, to the series coupling system.

[0025] The output electric power may have an active part, also referred to as active output electric power, and a reactive part, also referred to as reactive output electric power. According to an embodiment, at least one of the first output electric power, the second output electric power, and the third output electric power includes an active electric power and a reactive electric power. The series coupling system may be configured for compensating active power fluctuations in the multi-phase power supply line. The series coupling system may be configured for compensating reactive power fluctuations in the multi-phase power supply line. Particularly, the series coupling system may be configured for compensating reactive power fluctuations in the multi-phase power supply line without interactions with the interconnection system and / or the parallel coupling system.

[0026] The UPFC includes an interconnection system. The interconnect system includes at least one first solid state transformer (SST) connecting the parallel coupling system to the at least one first FB converter. The interconnection system includes at least one second SST connecting the parallel coupling system to the at least one second FB converter. The interconnection system include at least one third SST connecting the parallel coupling system to the at least one third FB converter.

[0027] The interconnection system may be configured for transmitting an electric power between the parallel coupling system and the series coupling system. The interconnection system may be configured for transmitting an electric power between the parallel coupling system and the at least one first full bridge converter. The interconnection system may be configured for transmitting an electric power between the parallel coupling system and the at least one second full bridge converter. The interconnection system may be configured for transmitting an electric power between the parallel coupling system and the at least one third full bridge converter. The at least one first SST may be configured for providing an electric power from the parallel coupling system to the at least one first FB converter. The at least one second SST and the at least one third SST may be configured for transmitting an electric power between the parallel coupling system to the at least one second FB converter and the at least one third FB converter.

[0028] A power flow between the series coupling system and the parallel coupling system through the interconnection system may be bi-directional. A power may flow from the parallel coupling system through the interconnection system to the series coupling system. A power may flow from the series coupling system through the interconnection system to the parallel coupling system. Throughout this description, a power flowing from the parallel coupling system through the interconnection system to the series coupling system is defined as a positive power. A power flowing from the series coupling system through the interconnection system to the parallel coupling system is defined as a negative power.

[0029] A power transmitted between the parallel coupling system and the series coupling system, through the interconnection system, may have an active part, i.e. an active power, and a reactive part, i.e. a reactive power. The parallel coupling system may absorb an active power from the power supply line, particularly from the utility supply line. The interconnection system may transmit the absorbed active power from the parallel coupling system to the series coupling system. The series coupling system may provide, at least parts, of the absorbed active power to the power supply line, particularly to the utility supply line and / or the load line. This power flow is bi-directional. That is, the series coupling system may absorb the active power and the parallel coupling system may supply, at least parts, of the absorbed active power to the power supply line, particularly the utility supply line. The series coupling system, the parallel coupling system, and the interconnection system may absorb and supply the active power through a DC link, for example a DC link formed between a FB converter and a SST.

[0030] The interconnection system may receive the input electric power from the parallel coupling system. Particularly, the interconnection system may receive the first input electric power, the second input electric power, and the third input electric power from the parallel coupling system. The series coupling system may receive the output electric power from the interconnection system. The series coupling system may receive the first output electric power, the second output electric power and the third output electric power from the interconnection system. The output electric power may substantially correspond to the input electric power. Particularly, neglecting losses, the output electric power may substantially correspond to the input electric power.

[0031] The parallel coupling system may receive the input electric power from the power supply line, particularly a first input electric power from the first phase, a second input electric power from the second phase, and a third input electric power from the third phase. The interconnection system may receive the input electric power from the parallel coupling system, particularly the first input electric power, the second input electric power and the third input electric power. The interconnection system may transmit the input electric power to the series coupling system. The series coupling system may receive the output electric power from the interconnection system. Particularly, the at least one first FB converter may receive the first output electric power from the interconnection system, the at least one second FB converter may receive the second output electric power from the interconnection system, and the at least one third FB converter may receive the third output electric power from the interconnection system. The series coupling system may provide the output power to the power supply line. Particularly, the at least one first FB converter may provide the first output power to the first phase of the power supply line, the at least one second FB converter may provide the second output power to the second phase of the power supply line, and the at least one third FB converter may provide the third output power to the third power supply line.

[0032] A power provided from the utility supply for powering the load may have power quality distortions. The Power quality distortion may result, for example, from fluctuations in the reactive power, i.e. AQ, or fluctuation in the active power, i.e. AP, harmonics, and / or voltage unbalance. Further, fluctuations in reactive power (AQ) and active power (AP) may result in what is called flicker. Particularly, flicker describes a long-term integration of AQ and / or AP. Wherein long-term is with respect to the time scale of AQ and / or AP. Voltage variations can be caused by AQ and AP, which can result in flicker. By reducing AQ and AP, the voltage variations can be reduced, particularly the flicker can be reduced. Advantageously, the present invention allows to improve power quality parameters, such as flicker, harmonics, active power factor, and / or voltage unbalance, resulting in an overall improvement in power quality. Advantageously, more power quality control objectives can be addressed. It is possible to focus on a single power quality parameter or on various power quality parameters to improve the overall power quality of the load power. Also, the priorities between the control objectives can be set depending on the needs. According to an embodiment the UPFC may control a power in the power supply line and / or a power quality in the power supply line by at least one of the first output electric power, the second output electric power, and the third output electric power. Further, the UPFC may control a power in the power supply line and / or a power quality in the power supply line by at least the input electric power, particularly one of the first input electric power, the second input electric power, and the third input electric power. According to an embodiment the UPFC is configured for providing a reactive electric power through the series coupling system, particularly through the series coupling system and through the parallel coupling system.

[0033] The UPFC according to the present application is for a high-power AC system having a multiphase power supply line. The multi-phase power supply line has at least three phases. The multiphase power supply line may have four, five, six, or more phases. The AC currents in the phases of the multi-phase power supply line may have an equidistant phase shift to each other. The multi -phase power supply line may have a fourth phase, a fifth phase, and / or a sixth phase.

[0034] The parallel coupling system may be configured for to be connected to a fourth phase of the power supply line, a fifth phase of the power supply line, and / or a sixth phase of the power supply line for transmitting a fourth input electric power between the parallel coupling system and the fourth phase of the power supply line, a fifth input electric power between the parallel coupling system and the fifth phase of the power supply line, and / or a sixth input electric power between the parallel coupling system and the sixth phase of the power supply line respectively.

[0035] The UPFC may include one or more further full-bridge converters configured to be connected in series to further phases of the multi-phase power supply line. The UFPC may include at least one seventh FB converter configured to be connected in series to the fourth phase for transmitting a fourth output electric power between the at least one seventh FB converter and the fourth phase. Particularly, the UPFC may include at least one eighth FB converter configured to be connected in series to the fifth phase for transmitting a fifth output electric power between the at least one eighth FB converter and the fifth phase. More particularly, the UPFC may include at least one ninth FB converter configured to be connected in series to the sixth phase for transmitting a sixth output electric power between the at least one ninth FB converter and the sixth phase.

[0036] The interconnection system may include at least one fourth SST connecting the parallel coupling system to the at least one seventh FB converter. Particularly, the interconnection system may include at least one fifth SST connecting the parallel coupling system to the at least one eighth FB converter. More particularly, the interconnection system may include at least one sixth SST connecting the parallel coupling system to the at least one ninth FB converter. The at least one fourth SST, at least one fifth SST, and at least one sixth SST may be configured for transmitting an electric power between the parallel coupling system and the at least one seventh FB converter, at least one eighth FB converter, and at least one ninth FB converter respectively.

[0037] The series coupling system includes FB converters, particularly the at least one first FB converter, the at least one second FB converter, and the at least one third FB converter. The parallel coupling system may include FB converters. The FB may be a main power electronic building block of the UPFC according to the present application. The full bridge (FB) converters described herein may have an AC side and a DC side. The full bridge converter may be a 2-level converter, a 3 -level converter, or an n-level converter. Particularly, n being a natural number in this case, such as 2, 3, 4, 5, 6, 7, 8, 9, 10 and so on.

[0038] Throughout this description features of the at least one first FB converter are described. It is understood, that the same applies mutatis mutandis to the at least one second FB converter and the at least one third FB converter, particularly the at least one seventh FB converter, the at least one eight FB converter, the at least on ninth FB converter, and / or further similar FB converter. That is, the at least one second FB converter and the at least one third FB converter, particularly the at least one seventh FB converter, the at least one eight FB converter, the at least one ninth FB converter, and / or further similar FB converter may be similar and or identical to the at least one first FB converter.

[0039] The at least one first FB converter may be connected at their AC side to the first phase of the multi-phase power supply line. Particularly, the at least one first FB may be connected at their AC side in series to the first phase of the multi-phase power supply line. The at least one first FB converter may be connected in series to each other at their respective AC sides.

[0040] The at least one first FB converter may be a first plurality of FB converters configured to be connected in series to the first phase. The first plurality of FB converters may form a first chain of series connected FB converters, particularly series connected at their AC sides. Beneficially, connecting the FB converters in series allows to reduce a voltage rating requirement of the respective FB converters. One or more parallel FB converters may be provided in parallel to a FB converter in the first chain of series connected FB converters, beneficially allowing to reduce a current rating requirement of the respective FB converters. For connecting the one or more parallel FB converters in the first chain to the interconnection system, the one or more parallel FB converters are considered as a single FB converter. Beneficially, multiple first FB converter units can be connected in parallel and / or series to meet the voltage and / or current needs of the load.

[0041] The first chain of series connected FB converters may be connected in series to the first phase of the multi-phase power supply line. According to an embodiment, that can be combined with other embodiments described herein, at least one of the at least one first FB converter, the at least one second FB converter, and the at least one third FB converter is a respective plurality of FB converters configured to be connected in series to the corresponding phase.

[0042] The UPFC includes an interconnection system. The interconnect system includes at least one first solid state transformer (SST) connecting the parallel coupling system to the at least one first FB converter. The interconnection system includes at least one second SST connecting the parallel coupling system to the at least one second FB converter. The interconnection system include at least one third SST connecting the parallel coupling system to the at least one third FB converter.

[0043] Throughout this description features of the at least one first SST are described. It is understood, that the same applies mutatis mutandis to the at least one second SST and the at least one third SST, particularly the at least one fourth SST, the at least one fifth SST, the at least on sixth SST, and / or further similar SSTs. That is, the at least one second SST and the at least one third SST, particularly the at least one fourth SST, the at least one fifth SST, the at least one sixth SST, and / or further similar SSTs may be similar and or identical to the at least one first SST.

[0044] The interconnection system includes solid state transformers (SST). An SST as described herein is generally understood as being configured for converting an input DC level to an output DC level. Particularly, an SST may be defined as the electric components needed to convert the input DC level to the output DC level. Either one can be low voltage (LV), medium voltage (MV), and / or high voltage (HV). An SST may have one or more first terminals configured to be connected to the parallel coupling system and one or more second terminals configured to be connected to the series coupling system. Each of the one or more first terminals may be connected to a single FB converter, particularly to a single FB converter in the parallel coupling system. Each of the one or more second terminals may be connected to a single FB converter, particularly to a single FB converter in the series coupling system.

[0045] An SST may have one or more SST cells. If an SST has exactly one SST cell the one SST cell may correspond to the SST. Each SST cell may include a DC / AC converter, a type of transformer, such as a medium frequency transformer (MFT) or any other suitable transformer, and an AC / DC converter. The transformer may be provided in an AC link formed between the DC / AC converter and the AC / DC converter. Each SST cell may have one first terminal and one second terminals Two or more first terminals of two or more SST cells of an SST may be connected in series, in parallel, and / or in any combination thereof for connecting to a single FB converter. It is understood that connecting to a single FB also includes connecting to two or more FB converters that are provided in parallel to each other, for example, two or more fourth FB converters that are provide in parallel to each other. Two or more second terminals of two or more SST cells of an SST may be connected in series, in parallel, and / or in any combination thereof for connecting to a single FB converter. It is understood that connecting to a single FB also includes connecting to two or more FB converters that are provided in parallel to each other, for example, two or more first FB converters that are provide in parallel to each other.

[0046] An SST can be connected to one or more FB converters on the output side. The number of FB converters that are connected to the SST can be customized by the number of output terminals. The number of output terminals can be customized by the number of SST cells included in the SST and / or by connecting one or more second terminals in series. Beneficially, the power requirements of the FB converters can be more easily satisfied. For example, the number of output terminals of an SST can be increased such that more FB converters can be connected to the SST. Thereby, the number of components, such as LITs, AFEs, SSTs, and the like, connected at a first side of the SST, e.g. an input side of the SST, can be reduced while meeting the power requirements of the FB converters connected to a second side of the SST, e.g. an output side of the SST.

[0047] The SSTs have a type of transformer, such as a medium frequency transformer. The transformers of the SST, for example the MFTs, are isolating elements in the UPFC as described herein. Each SST may be configured to meet the isolation requirement of withstanding the full grid voltage. This may be true also if the SST only sees a fraction of the full grid voltage during operation.

[0048] According to an embodiment, two or more first FB converters are connected in parallel to one first SST, two or more second FB converters are connected to one second SST, two or more third FB converters are connected in parallel to one third SST. Particularly, the one first SST, the one second SST, and / or the one third SST may include a plurality of SST cells, each SST cell having first terminals for connecting to the parallel coupling system and second terminals for connecting to the series coupling system, wherein for a respective SST two or more first terminals of the plurality of SST cells are connected in series, in parallel, or in a combination thereof, and / or two or more second terminals of the plurality of SST cells are each connected in series, in parallel, or in a combination thereof.

[0049] The at least one first SST may be a first plurality of solid state transformers (SSTs). Each first SST of the first plurality of SSTs may be connecting the parallel coupling system to a first FB converter, particularly to an FB converter of the first plurality of FB converters. In some embodiments, that can be combined with other embodiments described herein, the at least one first SST, the at least one second SST, and the at least one third SST is a respective plurality of solid state transformers.

[0050] The first plurality of SSTs may have a same number of solid state transformers as a number of FB converters in the first plurality of FB converters. Particularly, the first plurality of SSTs may have a same number of output terminals as a number of FB converters in the first plurality of FB converters. The at least one first FB converter and the at least one first SST may be connected in a pairwise manner. A pairwise manner is understood as a one-to-one relation, particularly as an exclusive one-to-one relation. That is, each SST of the at least one first SST may be connected to an individual FB converter of the at least one first FB converter. More particularly, each SST of the at least one first SST is connected to exactly one FB converter of the at least one first FB converter and each FB converter of the at least one first FB converter is connected to exactly one SST of the at least one first SST. According to an embodiment, that can be combined with other embodiments described herein, the at least one first FB converter and the at least one first SST are connected in a pairwise manner; the at least one second FB converter and the at least one second SST are connected in a pairwise manner; and / or the at least one third FB converter and the at least one third SST are connected in a pairwise manner.

[0051] An operation of the UPFC may be controlled by controlling the solid state transformers. Particularly, a power provided by the UPFC to the multi-phase power supply line may be controlled by a power flow through the SSTs. The control of the SST may be dependent on the type of SST. The SST may be a dual active bridge (DAB) type SST. For the DAB type SST the AC / DC and DC / AC converters may consist of full bridge converters. The DAB control may consist of applying a first duty cycle to the FB converters in the parallel coupling system and a second duty cycle to the FB converters in the series coupling system. The first duty cycle and the second duty cycle may be the same. The first duty cycle and the second duty cycle may be phase shifted to each other. The power flow through the SST may be controlled by the first duty cycle and / or the second duty cycle. The SST may be a resonant type SST. For the resonant SST the AC / DC and DC / AC converters may consist of half bridge converters with a series capacitor to provide resonance. The control may consist of applying a fixed duty cycle of 50% in constant frequency with no adjustments. For the resonant type SST, the power flow through the SST may be passively controlled by the FB converters in the parallel side and the FB converters in the series side full bridge converters.

[0052] The first plurality of SSTs may have number of solid state transformers that is more than a number of FB converters in the first plurality of FB converters. Two or more first SSTs may be connected to a single first FB converter. Particularly, two or more first SSTs may be connected in series to a single FB converter. The two or more SSTs may be connected in series at the respective output terminals of the two or more SSTs. According to an embodiment, that can be combined with other embodiments described herein, two or more of the first SSTs are connected in parallel to one first FB converter, two or more of the second SSTs are connected in parallel to one second FB converter, and / or two or more of the third SSTs are connected in parallel to one third FB converter.

[0053] The parallel coupling system may include a parallel unit for each phase of the multi-phase power supply line. The parallel coupling system may have a first parallel unit associated with the first phase, particularly the first phase and the second phase. The parallel coupling system may have a second parallel unit associated with the second phase, particularly the second phase and the third phase. The parallel coupling system may have a third parallel unit associated with the third phase, particularly the third phase and the first phase.

[0054] The first parallel unit, the second parallel unit, and the third parallel unit may be connected in a wye configuration. The first parallel unit, the second parallel unit, and the third parallel unit may be connected in a delta configuration.

[0055] According to an embodiment, that can be combined with other embodiments described herein, the parallel coupling system includes a first parallel unit connected to the first phase for transmitting a first input electric power between the first parallel unit and the first phase, a second parallel unit connected to the second phase for transmitting a second input electric power between the second parallel unit and the second phase, and a third parallel unit connected to the third phase for transmitting a third input electric power between the third parallel unit and the third phase. Particularly, the first parallel unit may be connected to the second phase for transmitting a fourth input electric power between the first parallel unit and the second phase, the second parallel unit may be connected to the third phase for transmitting a fifth input electric power between the second parallel unit and the third phase, and the third parallel unit may be connected to the first phase for transmitting a sixth input electric power between the third parallel unit and the first phase.

[0056] The parallel coupling system may have fourth, fifth, sixth, and / or further parallel units associated with fourth, fifth, sixth, and / or further phases. The parallel coupling system may include a fourth parallel unit, particularly a fifth parallel unit, more particularly a sixth parallel unit. The fourth parallel unit may be connected to a fourth phase of the multi-phase power supply line for transmitting a seventh input electric power between the fourth parallel unit and the fourth phase. The fifth parallel unit may be connected to a fifth phase of the multi-phase power supply line for transmitting an eighth input electric power between the fifth parallel unit and the fifth phase. The sixth parallel unit may be connected the sixth phase of the multi-phase power supply line for transmitting a ninth input electric power between the sixth parallel unit and the sixth phase. The at least one fourth SST, at least one fifth SST, and at least one sixth SST may be configured for transmitting an electric power between the fourth parallel unit and the at least one seventh FB converter, between the fifth parallel unit and the at least one eighth FB converter, and / or between the sixth parallel unit and the at least one ninth FB converter respectively.

[0057] The number of parallel units may be the same as the number of phases in the multi-phase power supply line. The parallel coupling system having fourth, fifth, sixth, and / or further parallel units can be provided in a wye or delta configuration. In a wye configuration the parallel units may each be connected to a different phase. The phase shift between the different phases may be equidistant. For example, for a 3, 4, 5, and 6-phase power supply line the phase shift may be 120°, 90°, 72°, and 60° respectively. In the case of a delta configuration, the shape of the connection changes to a square for 4 parallel units, a pentagon for 5 parallel units, and a hexagon for 6 parallel units. The phase shift in the delta configuration may be the same as in the respective wye configuration. That is, for a 3, 4, 5, and 6-phase power supply line, the phase shift in the delta configuration may be 120°, 90°, 72°, and 60° respectively. For example, for 4 phases the first and second parallel unit may be connected to the power supply line as in a 3- phase system. The third parallel unit may be connected to the third phase and to the fourth phase. The fourth parallel unit may be connected to the fourth phase and the first phase. The connection is similarly adapted for for 5-phases, 6-phases, or more than 6 phases.

[0058] The first parallel unit may be associated with the at least one first SST and the at least one first FB converter. The second parallel unit may be associated with the at least one second SST and the at least one second FB converter. The third parallel unit may be associated with the at least one third SST and the at least one third FB converter. According to an embodiment, the at least one first SST connects the first parallel unit to the at least one first FB converter, the at least one second SST connects the second parallel unit to the at least one second FB converter, and / or the at least one third SST connects the third parallel unit to the at least one third FB converter.

[0059] The parallel units may include at least one FB converter. The first parallel unit may include at least one fourth FB converter. The second parallel unit may include at least one fifth FB converter. The third parallel unit may include at least one sixth FB converter. The fourth parallel unit may include at least one tenth FB converter. The fifth parallel unit may include at least one eleventh FB converter. The sixth parallel unit may include at least one twelfth FB converter. The at least one first SST may connect the at least one fourth FB converter to the at least one first FB converter. The at least one second SST may connect the at least one fifth FB converter to the at least one second FB converter. The at least one third SST may connect the sixth FB to the at least one third FB converter. An SST may provide a one-to-one connection, for example, connecting one first FB converter to one fourth FB converter. An SST may have two or more SST cells and have two or more first terminals and / or two or more second terminals for providing a connection different to a one-to-one connection. For example, an SST may connect two first FB converters to one fourth FB converter, or an SST may connect two fourth FB converters to one first FB converter. Particularly, the at least one fourth SST may connect the at least one tenth FB converter to the at least one seventh FB converter. The at least one fifth SST may connect the at least one eleventh FB converter to the at least one eighth FB converter. The at least one sixth SST may connect the twelfth FB to the at least one ninth FB converter.

[0060] Throughout this description features of the at least on fourth FB converter are described. It is understood, that the same applies mutatis mutandis to the at least one fifth FB converter and the at least one sixth FB converter, particularly the at least one tenth FB converter, the at least one eleventh FB converter, the at least on twelfth FB converter, and / or further similar FB converter. That is, the at least one fifth FB converter and the at least one sixth FB converter, particularly the at least one tenth FB converter, the at least one eleventh FB converter, the at least one twelfth FB converter, and / or further similar FB converter may be similar and or identical to the at least one fourth FB converter.

[0061] The at least one fourth FB converter may be connected at their AC side to the first phase of the multi -phase power supply line. Particularly, the at least one fourth FB may be connected at their AC side in parallel to the first phase of the multi-phase power supply line. According to an embodiment the first parallel unit has at least one fourth FB converter, the second parallel unit has at least one fifth FB converter, and / or the third parallel unit has at least one sixth FB converter.

[0062] The at least one fourth FB converter may be a fourth plurality of FB converters. The fourth plurality of FB converters may form a fourth chain of series connected FB converters, particularly series connected at their AC sides. Beneficially, connecting the FB converters in series allows to reduce a voltage rating requirement of the respective FB converters. One or more parallel FB converter may be provided in parallel to a FB converter in the fourth chain of series connected FB converters, beneficially allowing to reduce a current rating requirement of the respective FB converters. Beneficially, multiple fourth FB converters units can be connected in parallel and / or series to meet the voltage and / or current needs of the load.

[0063] The fourth chain of series connected FB converters may be connected in parallel to the first phase of the multi-phase power supply line. According to an embodiment at least one of the at least one fourth FB converter, the at least one fifth FB converter, and the at least one sixth FB converter is a respective plurality of FB converters that are connected in series.

[0064] The parallel coupling system may include a line interface transformer (LIT) connected to an active front end (AFE). The line interface transformer connected to the active front end may form a rectifier. The rectifier, i.e. the LIT connected to the AFE, transforms the three phase AC current to a DC current. The LIT may be connected to the multi-phase power supply line. Particularly, the LIT may be connected to all phases of the multi-phase power supply line. The LIT receives the AC current from the power supply line and splits and phase shifts the AC current received from the power supply line to multiple multi-phase AC current systems. Each multi-phase AC current system of the multiple multi-phase AC current systems may have 2, 3, 4, 5, or more than 5 phases. Each multi-phase AC current system provided by the LIT may have a same number of phases as the AC current in the power supply line. The currents of the multiphase AC current systems may be phase-shifted, particularly have equidistant phase shifts. The multiple multi-phase AC systems may be phase shifted to each other. The active front end may have an input for each of the multi-phase AC current systems. The active front end may receive the currents from each of the multi-phase AC current systems provided by the LIT and transform the currents from each of the multi-phase AC current systems to a single DC current.

[0065] According to an embodiment, the parallel coupling system includes at least one line interface transformer (LIT) connected to an active front end (AFE). Beneficially, the combination of LITs connected to an AFE and / or passive rectifiers in the parallel coupling system and SSTs in the interconnection system allows to eliminate a low frequency transformer and replace the low frequency transformer with a smaller magnetic component, particularly the magnetic component being the LIT. Particularly, when compared to typical low frequency transformers. An improved flicker performance and elimination or reduction of the 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 could be eliminated.

[0066] The parallel coupling system may include a plurality of LITs each connected to an AFE. Each LIT connected to an AFE may form a rectifier. The parallel coupling system may include a rectifier for each phase of the multi-phase power supply line. The parallel coupling system may include two or more rectifiers for each phase of the multi-phase power supply line. Each rectifier, particularly the respective AFE of the rectifier, may be connected to at least one of the at least one first SST, the at least one second SST, and the at least one third SST. According to an embodiment, that can be combined with other embodiments described herein, the parallel coupling system includes a plurality of LITs, each LIT of the plurality of LITs being connected to an AFE, and the plurality of LITs being provided in parallel to each other.

[0067] Multiple units can be connected in parallel to create high pulse number converters (24-pulse and above) with very low distortion seen by the grid resulting in reduced filtering requirements.

[0068] According to an aspect, a high-power AC system is provided. The high-power AC system includes 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.

[0069] According to an embodiment, the high-power AC system is an AC electric arc furnace, the AC electric arc furnace further comprising electric arc furnace electrodes forming at least part of the load. Particularly, the electric arc furnace electrodes may be the loads.

[0070] Further advantages of the UPFC according to the present invention are as follows. The solid state transformers use dry transformers. Thereby, no oil is needed for operating the SSTs, resulting in simpler maintenance, more compact and lighter design, and / or reduced fire and environmental hazards. Smooth current control of the FB converters of the series coupling system on the load side beneficially allows to reduce excessive stress on the high-power AC system, thus enabling a higher system availability. Specific stress reduction examples include: preventing electrode breakdown (graphite or Soderberg type) in EAFs or OBFs, thereby preventing disruption of the process; reducing electrode consumption; reducing refractory wear; allowing independent control of the electrode voltage reducing wear and tear of the high current cables. Independent voltage control by electrode height adjustment typically allows higher power input to the furnace.

[0071] Those skilled in the art will recognise additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

[0072] Brief description of the drawings

[0073] 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:

[0074] Fig. 1 shows a schematic diagram of a high-power AC system having a universal power flow controller according to embodiments described herein.

[0075] Fig. 2A shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0076] Fig. 2B shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0077] Fig. 3 A shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0078] Fig. 3B shows a schematic diagram of an AC electric arc furnace having a universal power flow controller according to embodiments described herein.

[0079] Detailed description of the drawings

[0080] 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.

[0081] 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.

[0082] Fig. 1 shows a schematic diagram of a high-power AC system 10 having a universal power flow controller (UPFC) 100. The high-power AC system has a load 150 and a multi-phase power supply line 110 connecting the load 150 to a utility supply 105. The multi-phase power supply line 110 has a first phase 110a, a second phase 110b, and a third phase 110c. As described herein, the multi-phase power supply line 110 may have further phases, such as a fourth phase, a fifth phase, a sixth phase, and / or further phases. The multi-phase power supply line has a utility supply line 111 and a load line 112, separated by a series coupling system 140.

[0083] The UPFC 100 is according to embodiments described herein. The universal power flow controller has a parallel coupling system 120 connected to the power supply line 110 in parallel for transmitting an input electric power between the parallel coupling system and the power supply line.

[0084] The UPFC 100 has a series coupling system 140. The series coupling system has at least one first FB converter 145a connected in series to the first phase 110a for transmitting a first output electric power between the at least one first FB converter 145a and the first phase 110a. The at least one first FB converter 145a separates the first phase 110a into a first phase of the utility supply line 111 and a first phase of the load line 112. The series coupling system has at least one second FB converter 145b connected in series to the second phase 110b for transmitting a second output electric power between the at least one second FB converter 145b and the second phase 110b. The at least one second FB converter 145b separates the second phase 110b into a second phase of the utility supply line 111 and a second phase of the load line 112. The series coupling system has at least one third FB converter 145c connected in series to the third phase 110c for transmitting a first output electric power between the at least one third FB converter and the third phase 110c. The at least one third FB converter 145b separates the first phase 110c into a third phase of the utility supply line 111 and a third phase of the load line 112.

[0085] The UPFC has an interconnection system 130. The interconnection system 130 has at least one first solid state transformer (SST) 135a connecting the parallel coupling system 120 to the first FB converter 145a. The interconnection system 130 has at least one second SST 135b connecting the parallel coupling system 120 to the second FB converter 145b. The interconnection system 130 has at least one third SST 135c connecting the parallel coupling system 120 to the third FB converter 145c. Fig. 2A shows a further embodiment of a UPFC 100. The multi-phase power supply line in Fig. 2A has three phases. As described herein, the multi-phase power supply line may have more than 3 three phases.

[0086] The series coupling system has a plurality of first FB converters 145ai ... 145aj connected in series to the first phase 110a. The plurality of first full bridge converter 145ai ... 145aj are connected in series at their respective AC input sides. Particularly, the plurality of first FB converter 145ai ... 145aj form a first chain of series connected FB converters. The series coupling system has a plurality of second FB converters 145bi ... 145bmconnected in series to the second phase 110b. The plurality of second full bridge converter 145bi ... 145bmare connected in series at their respective AC input sides. Particularly, the plurality of second FB converters 145bi ... 145bmform a second chain of series connected FB converters. The series coupling system has a plurality of third FB converters 145ci ... 145cnconnected in series to the third phase 110c. The plurality of third full bridge converter 145ci ... 145cnare connected in series at their respective AC input sides. Particularly, the plurality of third FB converter 145ci ... 145cnform a third chain of series connected FB converters.

[0087] The interconnection system 130 has a plurality of first SSTs 135ai ... 135ax. The interconnection system 130 has a plurality of second SSTs 135bi ... 135by. The interconnection system 130 has a plurality of third SSTs 135ci ... 135cz. The plurality of first SSTs 135ai ... 135axis associated with the plurality of first FB converters 145ai ... 145aj. Each one of the plurality of first SSTs 135ai ... 135axis connected to a first FB of the plurality of first FB converters 145ai ... 145aj.

[0088] Each SST may have one or more first terminals 138 configured to be connected to the parallel coupling system and one or more second terminals 139 configured to be connected to the series coupling system. Each first terminal 138 and each second terminal is configured to connect to a single FB converter respectively, as exemplarily illustrated in Fig. 2A in detailed views AA and BB. The input terminals 138 are connected to DC links of the FB converters in the parallel coupling system. The output terminals 139 are connected to DC links of the FB converters in the series coupling system. The SST provides isolation between the DC links on the input side and on the output side. That is, the SST provides isolation between the DC link formed between the SST and a corresponding FB converter in the parallel coupling system and the DC link formed between the SST and a corresponding FB converter in the series coupling system.

[0089] Each SST has one or more SST cells 137. Each SST cell 137 has one input terminal 138 and one output terminal 139. If an SST has exactly one SST cell, the exactly one SST cell may correspond to the SST. An SST having two or more SST cells 137 may be connected to one or more FB converters in the series coupling system and / or one or more FB converters in the parallel coupling system. Two or more first terminals 138 of two or more SST cells 137 of an SST may be connected in series, in parallel, and / or in any combination thereof for connecting to a single FB converter. Two or more second terminals 139 of two or more SST cells 137 of an SST may be connected in series, in parallel, and / or in any combination thereof for connecting to a single FB converter. As shown in detailed view AA, third SST 135ci has two SST cells 137, two first terminals 138, and two second terminals 139. The two second terminals 139 of third SST 135ci are connected to third FB converter 145ci and third FB converter 145c2 respectively. The two first terminals 138 are connected in series to each other for connecting to sixth FB converter 125ci. As shown in detailed view BB, first SST 135axhas two SST cells 137, two first terminals 138, and two second terminals 139. First SST 135axis connected to two fourth FB converters 125a0and 125a0-i by the two first terminals 138. The two first terminals 138 of SST 135axare isolated from each other. The two output terminals 139 of the two SST cells 137 of SST 135axare connected in parallel to first FB converter 145aj. Similarly, the plurality of second SSTs 135b i ... 135byis associated with the plurality of second FB converters 145bi ... 145bm, and the plurality of third SSTs 135ci ... 135czis associated with the plurality of third FB converters 145ci ... 145cn.

[0090] The parallel coupling system 120 has a first parallel unit 120a connected to the first phase for transmitting a first input electric power between the first parallel unit 120a and the first phase 110a. The parallel coupling system 120 has a second parallel unit 120b connected to the second phase for transmitting a second input electric power between the second parallel unit 120b and the second phase 110b. The parallel coupling system 120 has a third parallel unit 120c connected to the third phase for transmitting a third input electric power between the third parallel unit 120c and the third phase 110c. The first parallel unit 120a, the second parallel unit 120b, and the third parallel unit 120c are connected in a wye configuration.

[0091] Fig. 2B shows a similar embodiment of a UPFC 100 wherein the first parallel unit 120a, the second parallel unit 120b, and the third parallel unit 120c are connected in a delta configuration. Thereby, the first parallel unit 120a is connected to the first phase for transmitting a first input electric power between the first parallel unit 120a and the first phase 110a and is connected to the second phase 110b for transmitting a fourth input electric power between the first parallel unit 120a and the second phase 110b. The second parallel unit 120b is connected to the second phase for transmitting a second input electric power between the second parallel unit 120b and the second phase 110b and is connected to the third phase 110c for transmitting a fifth input electric power between the second parallel unit 120b and the third phase 110c. The third parallel unit 120c is connected to the third phase for transmitting a third input electric power between the third parallel unit 120c and the third phase 110c and is connected to the first phase 110a for transmitting a sixth input electric power between the third parallel unit 120c and the first phase 110a.

[0092] The first parallel unit 120a includes a plurality of fourth full bridge converters (FB) 125ai ... 125a0. The plurality of fourth full bridge converters 125ai ... 125a0are connected in series at their respective AC input sides. Particularly, the plurality of fourth FB converters 125ai ... 125a0form a fourth chain of series connected FB converters. The second parallel unit 120b and the third parallel unit 120c similarly include a plurality of fifth FB converters 125bi ... 125bpand sixth FB converters 125ci ... 125cq. The plurality of fifth FB converters 125bi ... 125bpand sixth FB converters 125ci ... 125cqmay be connected similarly to the plurality of fourth FB converters 125ai ... 125a0to form a fifth and sixth chain of series connected FB converters. A FB converter may be provided in parallel to one or more FB converters of the fourth chain, the fifth chain or the sixth chain. This is illustrated in the third parallel unit 120c, having two FB converters 125cqprovided in parallel to each other. The two parallel FB converters 125cqmay be connected to the third SST 135czsimilar to the connection of a single FB converter to an SST.

[0093] The FB converters of the fourth plurality of FB converters 125ai ... 125a0, the fifth plurality of FB converters 125bi ... 125bp, and the sixth plurality of FB converters 125ci ... 125cqare connected to the SST of the first plurality of SSTs 135ai ... 135ax, the second plurality of SSTs 135bi ... 135by, and the third plurality of SSTs 135ci ... 135czrespectively. The FB converters are connected at their DC links to the input terminals 137 of the SSTs.

[0094] The UPFC 100 shown in Fig. 2A uses solid state transformers. The fourth chain of series connected FB converters, the fifth chain of series connected FB converters, and the sixth chain of series connected FB converters are connected in a wye configuration. The number of FB converters in the respective chains may be selected as needed to reach the line to neutral voltage of the grid. The solid state transformers of the interconnection system, that is the plurality of first SSTs 135ai ... 135ax. the plurality of second SSTs 135bi ... 135byand / or the plurality of third SSTs 135ci ... 135czmay have an input and output voltage with equal or different DC link voltages. For example, the SSTs may have equal input and output voltages, such as 2.5 kV input voltage and 2.5 kV output voltage, or different input and output voltages, such as 2.5 kV input voltage and 1 kV output voltage, or vice versa. Each SST cell can be designed for specified input and output DC link voltages. One SST cell per SST can be used for a pair of full bridge converters, such as a pair of first FB converter and fourth FB converter, second FB converter and fifth FB converter, and / or third FB converter and sixth FB converter. The Wye configuration in the parallel coupling system allows to provide a one-to-one correspondence between FB converters in the parallel coupling system and the series coupling system. Advantageously, this allows to compensate the full grid voltage (28.2kV peak), if the DC links are equal, when reacting to short-circuits of the load.

[0095] The series connected FB units are connected in series and can independently inject a voltage in each phase to control the current of an AC EAF through an Arc Furnace transformer. The arc voltages can be controlled independently of the current by adjusting the heights of the electrodes. This is true for all embodiments described herein, for example, the embodiments shown in Figs. 1 through 3C.

[0096] Fig. 3A shows a further embodiment of a UPFC 100. The parallel coupling system 120 has a rectifier 220 formed by a line interface transformer (LIT) 225 connected to an active front end (AFE) 227. The rectifier is connected to the three phases of the multi-phase power supply line 110. The rectifier 220 transforms the three phase AC current to a DC current. The LIT receives the AC current from the power supply line and splits and phase shifts the AC current received from the power supply line to multiple multi-phase AC current systems. Each multi-phase AC current system of the multiple multi-phase AC current systems may have 2, 3, 4, 5, or more than 5 phases. Each multi-phase AC current system provided by the LIT may have a same number of phases as the AC current in the power supply line. The currents of the multi-phase AC current systems may be phase-shifted, particularly have equidistant phase shifts. The multiple multi-phase AC systems may be phase shifted to each other. The active front end may have an input for each of the multi-phase AC current systems. The active front end may receive the currents from each of the multi-phase AC current systems provided by the LIT and transform the currents from each of the multi-phase AC current systems to a single DC current.

[0097] The interconnection system 130 has a first SST 135a connected to the rectifier 220, a second SST 135b connected to the rectifier 220, and a third SST 135c connected to the rectifier 220. Particularly, the first SST 135a, the second SST 135b, and the third SST 135c, are connected to the AFE 227. The first SST 135a connects the rectifier 220 to the plurality of first FB converters 145ai ... 145aj. The second SST 135b connects the rectifier 220 to the plurality of second FB converters 145bi ... 145bm. The third SST 135c connects the rectifier 220 to the plurality of third FB converters 145ci ... 145cn. In Figs. 3A and 3B respectively a single first SST 135a, second SST 135b, and third SST 135c is shown. It is understood, that one or more first SST, one or more second SST, and / or one or more third SST can be connected to the rectifier, similar to Figs. 2A and 2B.

[0098] Fig. 3B shows a further embodiment of a UPFC 100. The parallel coupling system 120 has a plurality of rectifiers 220i ... 220o. Each rectifier is formed by an LIT 225i ... 225Oconnected to a respective AFE 227i ... 227O. In Fig. 3B, the parallel coupling system 120 has a first rectifier 220i connected to the first SST 135a, a second rectifier 2202 connected to the second SST 135b, and a third rectifier 220s and a fourth rectifier 2204 connected in parallel to the third SST 135c. It is understood that any number of rectifiers 220i ... 220ocan be provided. Each rectifier 220i ... 220ocan be connected to one or more SSTs of the interconnection system.

[0099] In the UPFC 100 of Fig. 3B the parallel coupling system has a plurality of rectifiers 220i ... 220orespectively formed by a line interface transformers (LITs) connected to an active front end (AFE) rectifier, also referred to as active front end, to supply active and reactive power. A rectifier, that is an LIT connected to an AFE can supply injection active power to the series coupling system, particularly to the at least one first FB converter, the at least one second FB converter, and the at least one third FB converter. The rectifier can compensate the reactive power of the grid and / or reactive power needed by the load. This functionality is similar to a STATCOM. The UPFC 100 can have a plurality of LITs connected to AFE rectifiers depending on the needed active and reactive power of the load.

[0100] 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 multi-phase power supply line (110) for providing power from a utility supply to a load, the multi-phase power supply line (110) having at least a first to third phase (110a, 110b, 110c), the UPFC comprising: a parallel coupling system (120) configured to be connected to the power supply line in parallel for transmitting an input electric power between the parallel coupling system and the power supply line; a series coupling system (140) comprising at least one first full-bridge, FB, converter (145ai, ..., 145aj) configured to be connected in series to the first phase (110a) for transmitting a first output electric power between the at least one first FB converter and the first phase (110a), at least one second FB converter (145b i, ..., 145bm) configured to be connected in series to the second phase (110b) for transmitting a second output electric power between the at least one second FB converter and the second phase, and at least one third FB converter (145ci, ..., 145cn) configured to be connected in series to the third phase (110c) for transmitting a third output electric power between the at least one third FB converter and the third phase (110c); and an interconnection system (130) comprising at least one first solid state transformer, SST, (135ai, ... , 135ax) connecting the parallel coupling system (120) to the at least one first FB converter, at least one second SST (135bi, ... , 135by) connecting the parallel coupling system (120) to the at least one second FB converter, and at least one third SST (135ci, ... , 135cz) connecting the parallel coupling system (120) to the at least one third FB converter.

2. The UPFC of claim 1, wherein at least one of the at least one first FB converter (145ai, ..., 145aj), the at least one second FB converter (145b i, ..., 145bm), and the at least one third FB converter (145ci, ..., 145cn) is a respective plurality of FB converters configured to be connected in series to the corresponding phase.

3. The UPFC of any one of claims 1 or 2, wherein the at least one first SST (135ai, ... , 135ax), the at least one second SST (135bi, ... , 135by), and the at least one third SST (135ci, ... , 135cz) is a respective plurality of solid state transformers.

4. The UPFC of claim 3, wherein the at least one first FB converter (145ai, 145aj) and the at least one first SST (135ai, 135ax) are connected in a pairwise manner; wherein the at least one second FB converter (145bi, 145bm) and the at least one second SST (135bi, ... ,135by) are connected in a pairwise manner; and / or wherein the at least one third FB converter (145ci, ..., 145cn) and the at least one third SST (135ci, ... , 135cz) are connected in a pairwise manner.

5. The UPFC of any one of claims 3 or 4, wherein two or more of the first SSTs (135ai, ... , 135ax) are connected in parallel to one first FB converter (145ai, ..., 145aj), two or more of the second SSTs (135bi, ... , 135by) are connected in parallel to one second FB converter (145bi, ..., 145bm), and / or two or more of the third SSTs ( 135ci, ... , 135cz) are connected in parallel to one third FB converter (145ci, ..., 145cn).

6. The UPFC of any one of claims 3 to 5, wherein two or more first FB converters (145ai, ..., 145aj) are connected in parallel to one first SST (135ai, ... 135ax), two or more second FB converters (145bi, ..., 145bm) are connected to one second SST (135b i, ... 135by), two or more third FB converters (145ci, ..., 145cn) are connected in parallel to one third SST (135ci, ... 135cz).

7. The UPFC of any one of claims 5 and 6, wherein the one first SST, the one second SST, and / or the one third SST comprises a plurality of SST cells (137), each SST cell having first terminals (138) for connecting to the parallel coupling system and second terminals (139) for connecting to the series coupling system, wherein for a respective SST: two or more first terminals of the plurality of SST cells are connected in series, in parallel, or in a combination thereof; and / or two or more second terminals of the plurality of SST cells are connected in series, in parallel, or in a combination thereof to each other.

8. The UPFC of any one of claims 1 to 7, wherein the parallel coupling system comprises:a first parallel unit (120a) connected to the first phase (110a) for transmitting a first input electric power between the first parallel unit (120a) and the first phase (110a); a second parallel unit (120b) connected to the second phase (110b) for transmitting a second input electric power between the second parallel unit and the second phase (110b); and a third parallel unit (120c) connected to the third phase (110c) for transmitting a third input electric power between the third parallel unit (120c) and the third phase (110c).

9. The UPFC of claim 8, wherein: the first parallel unit (120a) is connected to the second phase (110b) for transmitting a fourth input electric power between the first parallel unit (120a) and the second phase (110b); the second parallel unit (120b) is connected to the third phase (110c) for transmitting a fifth input electric power between the second parallel unit (120b) and the third phase (110c); and the third parallel unit (120c) is connected to the first phase (110a) for transmitting a sixth input electric power between the third parallel unit (120c) and the first phase (110a).

10. The UPFC of any one of claims 8 to 9, wherein the at least one first SST connects the first parallel unit (120a) to the at least one first FB converter, the at least one second SST connects the second parallel unit (120b) to the at least one second FB converter, and / or the at least one third SST connects the third parallel unit (120c) to the at least one third FB converter.

11. The UPFC of any one of claims 8 to 10, wherein the parallel units (120a, 120b, 120c) are connected in a delta configuration.

12. The UPFC of any one of claims 8 to 10, wherein the parallel units (120a, 120b, 120c) are connected in a wye configuration.

13. The UPFC of any one of claims 8 to 12, wherein the first parallel unit (120a) has at least one fourth FB converter (125ai, ..., 125a0), the second parallel unit (120b) has at least one fifthFB converter (125bi, 125bp), and / or the third parallel unit (120c) has at least one sixth FB converter (125 ci, 125cq).

14. The UPFC of claim 13, wherein at least one of the at least one fourth FB converter (125ai, ... 125a0), the at least one fifth FB converter ( 125b i, ..., 125bp), and the at least one sixth FB converter (125ci, ..., 125cq) is a respective plurality of FB converters that are connected in series.

15. The UPFC of any one of claims 1 to 2, wherein the parallel coupling system comprises at least one line interface transformer, LIT, (2251, ... 225O) connected to an active front end, AFE, rectifier (227i, ... 227O).

16. The UPFC of claim 15, wherein the parallel coupling system comprises a plurality of LITs (225i, ... 225O), each LIT of the plurality of LITs (225i, ..., 225O) being connected to an AFE (227i, ... 227O), and wherein the plurality of LITs are provided in parallel to each other.

17. The UPFC of any one of claims 1 to 16, wherein the UPFC controls a power in the power supply line and / or a power quality in the power supply line by at least one of the first output electric power, the second output electric power, and the third output electric power.

18. The UPFC of any one of claims 1 to 17, wherein the UPFC controls a power in the power supply line and / or a power quality in the power supply line by at least the input electric power.

19. The UPFC of any one of claims 1 to 18, wherein at least one of the first output electric power, the second output electric power, and the third output electric power comprises an active electric power and a reactive electric power.20 The UPFC of any one of claims 1 to 19, wherein at least the input electric power comprises an active electric power and a reactive electric power.

21. The UPFC according to any one of claims 19 and 20, wherein the UPFC is configured for providing a reactive electric power through the series coupling system, particularly through the series coupling system and through the parallel coupling system.

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 electric arc furnace electrodes forming at least part of the load.

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