Apparatus for the supply of electric energy to an electric furnace, electric furnace and use of an apparatus

Abstract

The invention relates to an apparatus (10) for the supply of an electric furnace (90) with electric energy, wherein the apparatus (10) is electrically connectable to a power network (100), preferably a three phase power network and at least one electrode (91) of the electric furnace (90); wherein the apparatus (10) comprises at least one power module (20), wherein the power module (20) comprises a rectifier stage (30), configured to be electrically connected to less than three phases, particularly to less than three different phases of the power network (100); an inverter stage (40), electrically connectable to at least one electrode of the electric furnace (90); and a DC bus (50), wherein the DC bus (50) electrically connects the rectifier stage (30) to the inverter stage (40) of the power module (20).

Description

[0001] Seite 1 / 34

[0002] Anmelder : SMS Group S . p .A.

[0003] Unser Zeichen : P81086WO

[0004] 18 . December 2024

[0005] Apparatus for the supply of electric energy to an electric furnace , Electric furnace and Use of an apparatus

[0006] The present invention relates to an apparatus for the supply of an electric furnace with electric energy, an electric furnace comprising said apparatus and a use of such an apparatus for the supply of an electric furnace with electric energy .

[0007] Metals , especially steel , and metal ores , are regularly melted and heated by an electric arc in an electric furnace . These electric furnaces are operated with direct current ( DC ) , alternating current (AC ) or multiphase alternating current . Usually, at least one electrode is used for this purpose , which proj ects through the furnace roof into the furnace vessel , while the other electrodes are provided corresponding to the first electrode or are arranged in the bottom of the furnace vessel .

[0008] Electric furnaces are typically operated with dedicated power supply systems in order to both supply the required amount of electrical energy as well as to mitigate the highly nonlinear loads during operation of an electric furnace on the electrical power network . In order to mitigate these nonlinear loads to the electric power network, power supply systems for electric furnaces are typically designed in order to create at least a partial separation between the furnace and the supplying electric power network in order to reduce undesirable electrical network distortions , in particular flicker, higher harmonic currents , minimi ze reactive power inj ected and the like , on the electrical power network . Seite 2 / 34

[0009] P81086WO The invention is based on the task of providing the state of the art with an improvement .

[0010] According to a first aspect of the invention, the task is solved by an apparatus for the supply of an electric furnace with electric energy, wherein the apparatus is electrical ly connectable to a power network, preferably a three phase power network and at least one electrode of the electric furnace ; wherein the apparatus comprises at least one power module , wherein the power module comprises a recti fier stage , configured to be electrically connected to less than three phases , particularly to less than three di f ferent phases of the power network; an inverter stage , electrically connectable to at least one electrode of the electric furnace ; and a DC bus , wherein the DC bus electrically connects the recti fier stage to the inverter stage of the power module .

[0011] In the following the term " intended use of the apparatus" is used to describe the apparatus being used to supply electric energy to an electric furnace during operation of the electric furnace .

[0012] An apparatus designed in such a way has the advantage of an increased resilience against fluctuations in electric power consumption during operation of an electric furnace resulting in an increased availability of the electric furnace . In the intended use of the apparatus , an electric furnace produces high power fluctuations due to arc instabilities during the melting process . These arc instabilities often involve more than one electrode of an electric furnace . In case of an electric arc break between one electrode and the metal material , the ef fect on electric arcs of further electrodes is reduced due to the fact , that each arc is supplied by at least one di f ferent phase of the power network . Further, an apparatus designed in such a way Seite 3 / 34

[0013] P81086WO requires less switching devices , particularly compared to a conventional three phase power apparatus for the supply of an electric furnace . In this way, a control complexity of the apparatus may be reduced even further during electric energy supply to an electric furnace .

[0014] A power network preferably is a poly phase system used for electricity generation, transmission and distribution . A power network preferably provides alternating current (AC) . The power network may be a three phase power network providing three alternating currents , wherein each alternating current has a + 120 degree phase di f ference to one of the two other alternating currents and a - 120 degree phase di f ference to the respective other alternating current .

[0015] The power network may be a high-voltage power network or a medium-voltage power network or a low- voltage power network .

[0016] High-voltage can be greater than or equal to 36 kV, preferably greater than or equal to 60 kV, and particularly preferably greater than or equal to 100 kV . Further advantageously, high-voltage can be greater than or equal to 150 kV, preferably greater than or equal to 200 kV, and particularly preferably greater than or equal to 300 kV . Further advantageously, high-voltage can be greater than or equal to 400 kV, preferably greater than or equal to 700 kV, and particularly preferably greater than or equal to 1100 kV .

[0017] Medium- voltage can be less than or equal to 36 kV . Further advantageously, medium- voltage can be less than or equal to 30 kV, preferably less than or equal to 20 kV, and particularly preferably less than or equal to 15 kV .

[0018] Medium- voltage can be greater than or equal to 1 kV, preferably greater than or equal to 2 kV, and particularly preferably Seite 4 / 34

[0019] P81086WO greater than or equal to 10 kV . Further advantageously, medium-voltage can be greater than or equal to 15 kV, preferably greater than or equal to 20 kV, and particularly preferably greater than or equal to 30 kV .

[0020] Low-voltage can be greater than or equal to 50 V, preferably greater than or equal to 60 V, and particularly preferably greater than or equal to 100 V . Further advantageously, low voltage can be greater than or equal to 120 V, preferably greater than or equal to 220 V, and particularly preferably greater than or equal to 240 V .

[0021] Low-voltage can be less than or equal to 1 . 000 V, and particularly preferably less than or equal to 900 V . Further advantageously, low-voltage can be less than or equal to 600 V, preferably less than or equal to 240 V, and particularly preferably less than or equal to 220 V .

[0022] Preferably, voltage levels can be defined according to IEC 60038 . Particularly preferably, voltage levels can be defined according to Table 1 , Table 3 and Table 4 of TEC 60038 .

[0023] An electric furnace can be an electric arc furnace , an electric reduction furnace , a submerged arc-resistance furnace and / or any other electric furnace suitable to melt metal or non-metal materials .

[0024] An electrode is an electrical conductor used to make contact with a part of a circuit , particularly an electric furnace , in particular a nonmetallic part of a circuit . The nonmetallic part of the circuit can correspond to the atmosphere in the electric furnace .

[0025] An electrode can be produced from high density graphite and / or Wol fram . An electrode may be designed to trans fer electrical Seite 5 / 34

[0026] P81086WO energy forming arcs between tip and charge material . An electrode can be a prebaked electrode or a sel f-baking electrode ( Soeder- berg electrode ) and / or an extrusion / composite electrode , which is a combination of a Soederberg electrode with a prebaked electrode as a core and / or a hollow electrode system, which allows charging of fines via the center hole (prebaked, sel f-baking) , whereby the selection of the type of electrode can depend on : si ze of the electrode , produced material / metallurgy, and economic aspects such as operational costs .

[0027] An electrode of an electric furnace may be located at the top of an electric furnace . Preferably, an electrode located at the top is connected to a height adj ustment means , whereby the distance of the electrode to a metal material located in a furnace vessel of the electric furnace and / or a molten metal material located in a furnace vessel of the electric furnace can be varied . Such variation can be controlled and / or regulated by an electrode regulator .

[0028] A second electrode and / or a third electrode may be disposed in a furnace vessel of an electric furnace . The second electrode and / or the third electrode may also be arranged at the top of the electric furnace and preferably also be connected to a height adj ustment means . An electric furnace may have four or more electrodes . Each electrode may be connected to a height adj ustment means .

[0029] An electric furnace can be operated by means of alternating current (AC ) , particularly by means of a single phase alternating current and / or by means of a poly phase alternating current , preferably by means of a three phase alternating current .

[0030] Preferably the apparatus is electrically connected to the power network and / or to the at least one electrode of the electric furnace . Seite 6 / 34

[0031] P81086WO

[0032] A power module in the context of this invention preferably is a functional unit, comprised of a rectifier stage, an inverter stage and a DC bus electrically connecting the rectifier stage to the inverter stage.

[0033] A rectifier stage configured to be electrically connected to less than three phases, particularly less than three different phases of the power network, can be understood as a rectifier stage, that cannot be electrically connected to three or more different phases of the power network.

[0034] The rectifier stage may be configured to rectify an alternating current, particularly two different alternating currents, into a direct current. The rectifier stage may be configured to rectify an alternating voltage, particularly two different alternating voltages, into a direct voltage.

[0035] Preferably the rectifier stage is electrically connected to less than three phases, particularly less than three different phases of the power network.

[0036] An inverter stage may be electrically connected to the at least one electrode of the electric furnace.

[0037] The DC bus may comprise one or more than one capacitor and / or coil connected in series and / or in parallel to each other to store energy and create an electrical separation between the rectifier stage and the inverter stage. An electrical separation may be achieved through electrical energy stored in the DC bus and / or in its capacitors. An electrical separation may be achieved through magnetic energy stored in coils of the DC bus. The electrical separation stored can absorb fluctuations, particularly instantaneous fluctuations, in electrical energy supply and / or demand. Seite 7 / 34

[0038] P81086WO

[0039] The DC bus may comprise or consist of power cables and / or bus bars . The DC bus may comprise an alloy comprising copper, silver and / or another metal with a high electric conductivity . The DC bus may consist of copper and / or silver . The DC bus may comprise at least one section comprising a superconducting material .

[0040] A superconducting material in the context of the invention may be a semi-ceramic, or ceramic, material defined as HTS (High Temperature Superconductivity) , or a metallic material , defined as LTS ( Low Temperature Superconductivity) . These materials , i f taken to a critical temperature , speci fic for each one of them, have the characteristic of having substantially zero resistance to the passage of current . In particular, the superconductor material in question may be defined as such according to the ceramic-based, or metal-based, or salt-based BCS (Bardeen- Cooper-Schrief f er ) theory of superconductivity .

[0041] The apparatus may be electrically connectable to one or more than one additional energy source , wherein the additional energy source may be electrically connectable to the DC bus , particularly directly connectable to the DC bus . The apparatus may be electrically connected to one or more than one additional energy source , wherein the additional energy source may be electrically connected to the DC bus , particularly directly connected to the DC bus .

[0042] The additional energy source may comprise a renewable energy source , particularly a photovoltaic based energy source , a wind based energy source , a hydropower based energy source or the like . The additional energy source may comprise a fossil fuel based energy source , particularly a gas powered power plant , coal powered power plant or the like . The additional energy source may provide electric energy in direct current ( DC ) or in Seite 8 / 34

[0043] P81086WO alternating current (AC ) . Particularly the additional energy source may comprise one or more than one battery .

[0044] The additional energy source may be connected via a connection circuit to the DC bus . The connection circuit may comprise at least one converter, configured to provide a voltage level at its output that is substantially equal to the voltage level of the DC bus . The converter may comprise a recti fier circuit and / or a DC / DC converter circuit .

[0045] A power module in the context of the invention may be understood as a functional unit that receives and alternating current with an alternating voltage and a frequency at its input side and provides an alternating current with an alternating voltage and a frequency at its output side , wherein the alternating current , the alternating voltage and / or the frequency at the output side may be substantially independent of the alternating current , the alternating voltage and / or the frequency at the input side .

[0046] A power module may comprise a power module housing that limits a power module housing volume at least partially . The power module housing may comprise or be one or more than one cabinet , particularly a switching cabinet .

[0047] A housing in the context of this invention is designed to protect components arranged inside the housing volume from external influences , in particular from mechanical and / or electrical influences and to provide a defined air condition in the housing volume , particularly a defined temperature and humidity in the housing volume . A housing may comprise a bottom part , a top part and at least one side part , preferably four side parts . The bottom part , the top part and at least one side part limit the housing volume at least partially . The bottom part , the top part and at least one side part may be connected to each other forming an integral component . Furthermore , a housing can be provided Seite 9 / 34

[0048] P81086WO with an electrical ground connection so that the housing can increase the safety for personnel in the vicinity of the electrical components enclosed by the housing in a designated manner .

[0049] Two parts forming an integral component in the context of this invention are interconnected with each other by means of at least one mechanical connection . In other words , two parts forming an integral component change their relative spatial position to each other within the limits of their mechanical connection when the integral component is moved from one spatial position to another spatial position .

[0050] In a preferred embodiment of the invention, the mechanical connection of an integral component is fixed . In other words , the relative position of two parts forming an integral component , wherein the mechanical connection between the two parts is fixed, is constant during change of a spatial position of the integral component .

[0051] The recti fier stage and / or the inverter stage and / or the DC bus may be arranged in the power module housing volume .

[0052] The apparatus may comprise an electronic control unit . An electronic control unit is any electronic system, which is adapted to receive signals and / or to store signals and / or to process signals and / or to control or regulate an apparatus in dependence of at least one signal . The apparatus may comprise one or more than one sensor to provide information about harmonic distortions and / or flicker and / or a ratio of active power flow and reactive power flow in the power network . The electronic control unit can be operatively connected to one or more than one of such a sensor and can receive sensor signals , process them, and use them to control and / or regulate the apparatus . Seite 10 / 34

[0053] P81086WO An electronic control unit may be adapted to control and / or regulate a recti fier stage , particular a recti fier circuit , in particular to reduce or prevent harmonic distortions and / or flicker, in particular to mitigate flicker, and / or to reduce reactive power in the power network . An electronic control unit may be adapted to control and / or regulate an inverter stage , particularly an inverter circuit , in particular to reduce or prevent harmonic distortions and / or flicker, in particular to mitigate flicker, in the power network and / or to optimi ze the ratio of active power flow and reactive power flow in the power network, particularly to minimi ze the reactive power flow into the power network . An electronic control unit may be adapted to control and / or regulate an inverter circuit , in particular to reduce or prevent harmonic distortions and / or fl icker, in par- ticular to mitigate flicker, in the power network, in particular preferably by applying a pulse-width modulation strategy algorithm .

[0054] The apparatus may comprise a cooling system . The cooling system may be configured to provide cooling power for the apparatus to dissipate heat losses , particularly for the one or more than one power module of the apparatus . The cooling system may be arranged in the power module housing . Alternatively, the cooling system may be arranged in a separate cooling system housing .

[0055] The cooling system may comprise a cooling circuit configured to guide a cooling fluid to and away from the power module .

[0056] An apparatus designed in such a way has the advantage of an improved cooling capability . In the intended use of the apparatus , it is now possible to provide cooling power to electrical components , particularly to switching devices , individually . In this way, heat losses can be removed with a further increased ef ficiency . Seite 11 / 34

[0057] P81086WO

[0058] The cooling system may comprise a first cooling fluid pump configured to transport the cooling fluid through the cooling circuit . The cooling system may comprise a second cooling fluid pump . The second cooling fluid pump may be a reserve cooling fluid pump, configured to automatically turn on, in case the first cooling fluid pump has a defect and / or mal function . The first cooling fluid pump and the second cooling fluid pump may be the same type of cooling fluid pump . The cooling fluid pump may be arranged in the cooling system housing .

[0059] The cooling system may comprise a filtration system, preferably a de-ioni zing system, configured to regulate and / or control the electrical conductivity of the cooling fluid . The filtration system may be configured to regulate and / or control the electrical conductivity of the cooling fluid to an electrical conductivity within a range of 1 1 pS / cm and < 50 pS / cm, preferably within a range of > 0 , 5 pS / cm and < 1 pS / cm and preferred within a range of > 0 , 1 pS / cm and < 0 , 5 pS / cm . Particularly preferred, the filtration system may be configured to regulate and / or control the electrical conductivity of the cooling fluid to an electrical conductivity within a range of > 0 , 055 pS / cm and < 0 , 1 pS / cm . An apparatus designed in this way has the advantage of an increased protection against unwanted electrical currents and / or electrical voltages in the cooling circuit .

[0060] The cooling system may comprise at least one air conditioning system . The air conditioning system may be configured to supply cooling power to the power module . Particularly, the air conditioning system may be configured to drain heat and humidity from air of the power module housing volume .

[0061] The cooling system may be configured to provide cooling power through the cooling circuit and the air conditioning system to the power module . The cooling power can be provided in such a way, that one part of the cooling power is provided by the Seite 12 / 34

[0062] P81086WO cooling circuit and the respective other part is provided by the air conditioning system . Particularly, the part of cooling power provided by the cooling circuit can be h 70 % of the total cooling power and the part of cooling power provided by the air conditioning system can by < 30 % of the total cooling power . The part of cooling power provided by the cooling circuit can be > 80 % of the total cooling power and the part of cooling power provided by the air conditioning system can by < 20 % of the total cooling power . According to a preferred embodiment , the part of cooling power provided by the cooling circuit can be > 90 % of the total cooling power and the part of cooling power provided by the air conditioning system can by < 10 % of the total cooling power . The total cooling power is the amount of cooling power provided by the cooling system to the power module at any given time .

[0063] The cooling system may comprise a temperature and humidity control unit configured to regulate and / or control the temperature of the power module and / or a humidity of a power module housing volume . A temperature and humidity control unit is any electronic system, which is adapted to receive signals and / or to store signals and / or to process signals and / or to control and / or regulate the temperature of the power module and / or the humidity of the power module housing volume of the apparatus in dependence of at least one signal .

[0064] The apparatus may comprise one or more than one sensor to provide information about a temperature of the power module and / or a humidity of the power module housing volume . The temperature and humidity control unit can be operatively connected to one or more than one of said sensor and can receive sensor signals , process them, and use them to control and / or regulate the temperature of the power module and / or the humidity of the power module housing volume . The temperature and humidity control unit Seite 13 / 34

[0065] P81086WO may be operatively connected to the cooling system, particularly to the cooling fluid pump and the air conditioning system .

[0066] The sensor may be a temperature and / or a humidity sensor . The sensors may be arranged at the recti fier stage , at the DC bus , at the inverter stage and / or in the power module housing volume .

[0067] The temperature and humidity control unit may be adapted to regulate and / or control the temperature of the recti fier stage and / or the DC bus and / or the inverter stage individually . Particularly, the temperature and humidity control unit may be adapted to regulate and / or control the temperature and / or the humidity of the power module housing volume .

[0068] The temperature and humidity control unit may be adapted to regulate and / or control the temperature of the recti fier stage and / or the DC bus and / or the inverter stage and / or the temperature and humidity of the power module housing volume in dependence of the control and / or regulation of the electronic control unit . In case a high amount of electrical energy during the intended use of the apparatus is required, for instance at the beginning of and during a melting process in an electric furnace , a higher amount of cooling power is required . The temperature and humidity control unit is thus configured to receive at least one sensor signal , process them, and use them to control and / or regulate the cooling system to provide a high amount of cooling power to the power module housing volume , the recti fier stage , the DC bus and / or the inverter stage , particularly by increasing the cooling fluid volume flow and / or by reducing the cooling fluid temperature of the at least one cooling circuit and / or by increasing the air volume flow and / or reducing the air temperature and / or by reducing a water content of the air volume flow of the air conditioning system . Seite 14 / 34

[0069] P81086WO Preferably the power module comprises exactly one DC bus , that electrically connects the recti fier stage to the inverter stage of the power module .

[0070] An apparatus designed in such a way has the advantage that in the intended use of the apparatus , a separation between the power network and the electric furnace is created with a reduced amount of DC busses resulting in a reduced overall cost of the apparatus . Further, a risk of a current unbalance between multiple DC busses that can arise when using power modules with a plurality of DC busses can be mitigated .

[0071] The power module may comprise more than one DC bus , wherein the DC busses are connected in parallel to each other . In this way, an increased amount of electric energy may be trans ferred between the recti fier stage and the inverter stage .

[0072] Preferably the apparatus comprises less than six power modules .

[0073] An apparatus designed in such a way has the advantage that in the intended use of the apparatus , an electric furnace can be supplied with poly phase current , particularly with a three phase current , using a reduced amount of power modules , thereby reducing overall cost of the apparatus . Further, using less than six power modules reduces complexity of the overall apparatus , particularly control complexity during the intended use of the apparatus . Thus , the reliability of the apparatus can be increased .

[0074] According to a preferred embodiment , the apparatus comprises exactly three power modules .

[0075] Preferably the recti fier stage comprises one or more than one recti fier circuit , wherein the recti fier circuit comprises electrical input terminals for less than three di f ferent phases of Seite 15 / 34

[0076] P81086WO an alternating current supplied at an input side of the rectifier circuit .

[0077] An apparatus designed in such a way has the advantage that in the intended use of the apparatus, the effect of an electric arc break between one electrode and the metal material on electric arcs of further electrodes is reduced due to the fact, that each arc is supplied by at least one different phase of the power network .

[0078] An input side of a rectifier circuit is the side of the rectifier circuit an alternating current is electrically connected to, in order to be rectified by the rectifier circuit.

[0079] A rectifier circuit comprising input terminals for less than three different phases of an alternating current supplied at an input side of the rectifier circuit cannot be electrically connected to three or more than three different phase of an alternating current.

[0080] The rectifier circuit may be configured to convert an alternating current and an alternating voltage into a direct current and a direct voltage. Preferably, the rectifier circuit is configured to convert two different phases of a poly phase alternating current into a direct current.

[0081] In a preferred embodiment the rectifier circuits are connected in parallel and / or in series to each other.

[0082] A rectifier circuit may comprise an uncontrolled diode bridge. A rectifier stage comprising rectifier circuits designed in such a way can also be named as Diode Front End (DEE) . A DEE is a unidirectional converter. Seite 16 / 34

[0083] P81086WO A unidirectional converter allows electric energy to flow in one direction only and blocks electric energy in the opposite direction . That means , electric energy can flow from a power network to a load, preferably an electric furnace , and not flow from a load to a power network . Alternatively, electric energy can flow from a load, preferably an electric furnace , to a power network and not flow from a power network to a load .

[0084] A recti fier circuit and / or an inverter circuit may comprise a plurality of switching devices connected in parallel and / or series to each other, wherein the switching devices may comprise semi-conductors , diodes , particularly uncontrolled diodes , thyristors such as Silicon Controlled Recti fiers ( SCR) , Gate Turn- Of f thyristors ( GTO) , Integrated Gate-Commutated Thyristors ( IGCT ) , Metal-Oxide Semiconductor Controlled Thyristors (MCT ) , transistors such as Bipolar Junction Transistors (BJT ) , Metal- Oxide Semiconductor Field-Ef fect Transistors (MOSFET ) and / or Insulated-Gate Bipolar Transistor ( IGBT ) .

[0085] A recti fier stage and / or an inverter stage may be a bidirectional converter . A bidirectional converter allows electric energy to flow essentially in both directions , that means , electric energy can flow from a power network to a load, preferably an electric furnace , and from a load, preferably an electric furnace , to a power network . A bidirectional recti fier stage may also be named as Active Front End (AFE ) .

[0086] The switching devices of a recti fier circuit may be connected in a hal f bridge or a full bridge configuration to each other . One hal f bridge and / or one full bridge may be capable of providing a direct current with a current strength of > 2500 A, particularly of > 3000 A, preferably of > 3500 A and particularly preferred of > 5000 A. Seite 17 / 34

[0087] P81086WO Preferably the recti fier circuit is a single phase recti fier circuit or a two phase recti fier circuit .

[0088] A single phase recti fier circuit is configured to recti fy a single alternating current and a single alternating voltage to a direct current . The single alternating current and the single alternating voltage may be the current and voltage of one phase of a poly phase alternating current or voltage . Particularly, a single phase recti fier cannot recti fy more than one phase of a poly phase alternating current or a poly phase alternating voltage . A single phase recti fier particularly is connected to a single alternating phase and a neutral phase of the power network . An apparatus designed in such a way has the advantage that an output voltage of the recti fier stage may be reduced . In this way, components , particularly switching devices , with a lower rated voltage may be used, thus reducing costs .

[0089] A two phase recti fier circuit is configured to recti fy two alternating currents of two di f ferent phases and two alternating voltages of two di f ferent phases of a poly phase current and voltage to a direct current and voltage . Particularly, a two phase recti fier cannot recti fy more than two phases of a poly phase alternating current or alternating voltage . An apparatus designed in such a way has the advantage that an output current of the recti fier stage may be reduced . In this way, components , particularly switching devices , with a lower rated current may be used, this reducing costs .

[0090] Preferably the recti fier circuit comprises a first electrical input terminal and a second electrical input terminal , wherein the first electrical input terminal is configured to be electrically connected to a first phase of the power network and the second electrical input terminal is configured to be electrically connected to a second phase of the power network, wherein Seite 18 / 34

[0091] P81086WO the second phase of the power network is different from the first phase of the power network.

[0092] The rectifier circuit may comprise a first plurality of first electrical input terminals. The first input terminals may be connected in parallel to each other. The rectifier circuit may comprise a second plurality of second electrical input terminals. The second input terminals may be connected in parallel to each other. In this way, a current strength of the rectifier circuit can be increased.

[0093] According to one embodiment, the rectifier circuit comprises less than three electrical input terminals, wherein one electrical input terminal is configured to be connected to exactly one phase of the power network.

[0094] The apparatus may comprise one or more than one step down transformer electrically connectable, preferably connected, with its primary winding to the power network, and electrically connected with its secondary winding to the rectifier stage, particularly to an input side of the rectifier stage, preferably to the rectifier circuit, particularly to an input side of the rectifier circuit. The step down transformer is preferably a high voltage to medium voltage transformer. In other words, the step down transformer is preferably configured to transform a voltage with a high voltage level at its primary winding to a voltage with a medium voltage level at its one or more than one secondary winding. The step down transformer is preferably an oil cooled transformer. The step down transformer may be a high voltage to low voltage transformer.

[0095] Preferably, the step down transformer is a three phase transformer that comprises a primary winding per phase and one or more secondary windings per phase. The step down transformer may be a phase-shifting transformer. Seite 19 / 34

[0096] P81086WO

[0097] A phase-shifting transformer is a specialized type of transformer, which can be configured to adjust the phase relationship between its primary windings and its secondary windings.

[0098] A phase angle between primary and secondary of a transformer is a function of a vector group of a transformer. A vector group, elsewhere defined by a connection symbol, is the International Electrotechnical Commission (IEC) method of categorizing primary winding, preferably high voltage (HV) winding, and secondary winding, preferably medium voltage (MV) or low voltage (LV) winding, configurations of three phase transformers. The vector group designation indicates the windings configurations and the difference in phase angle between them.

[0099] A vector group provides a simple way of indicating how the connections of a transformer are arranged. Different configurations are possible as to how the primary windings, and the secondary windings, are connected to each other. In particular, they can be connected to each other in a delta circuit, a star circuit or a zigzag circuit, whereby primary windings, and secondary windings, can each be connected differently, resulting in a phaseshift between the primary side and the secondary side of a phase shifting transformer.

[0100] For example, a star primary winding and a delta secondary winding may be combined to form a vector group and will result in a 30 degree phase-shift between the primary side and the secondary side .

[0101] A three phase transformer may have one or more than one set of secondary windings. If the transformer has several sets of secondary windings, the power can be divided among the existing sets of secondary windings. Between the primary windings, preferably HV windings or MV windings, and each set of secondary Seite 20 / 34

[0102] P81086WO windings , preferably MV windings or LV windings , a di f ferent vector group can be advantageously selected so that the electrical power can be transmitted with di f ferent phase of fsets .

[0103] The portion of instantaneous power that results in net trans fer of energy in one direction is known as instantaneous active power . The portion of instantaneous power that results in no net trans fer of energy but instead oscillates between the source and load in each cycle due to stored energy, is known as instantaneous reactive power .

[0104] Preferably the DC bus is configured to handle a voltage level of

[0105] > 1500 VDC, particularly of > 3000 VDC and preferably of > 5000

[0106] VDC .

[0107] An apparatus des igned in such a way has the advantage of being able to supply large amounts of electric energy at comparably low current strengths .

[0108] The DC bus may be configured to handle a voltage level of > 10 kVDC, preferably of > 30 kVDC, preferred of > 60 kVDC and particularly preferred of > 90 kVDC . According to a preferred embodiment , the DC bus may be configured to handle a voltage level of > 100 kVDC .

[0109] The DC bus may comprise a cooling unit , particularly a cooling unit connected to the cooling system of the apparatus . The cooling unit may comprise a cooling j acket that surrounds the DC bus , particularly the power cables and / or the bus bars of the DC bus .

[0110] Preferably the inverter stage comprises a plurality of inverter circuits connected in parallel and / or in series to each other, wherein each inverter circuit is configured to provide a single phase alternating current . Seite 21 / 34

[0111] P81086WO

[0112] An apparatus des igned in such a way has the advantage that the complexity of the control of such an apparatus to supply electric energy to an electric furnace is reduced . An inverter stage configured to provide a single phase alternating current comprises less switching elements than an inverter stage configured to provide a poly phase alternating current thus reducing its control complexity during operation .

[0113] The inverter circuits may be connected in a star connection or in a delta connection to each other at their respective electrical output terminals . In this way, a poly phase alternating current , particularly a three phase alternating current , may be provided at the output of the inverter stage .

[0114] Preferably the inverter stage comprises a plurality of inverter circuits connected in parallel and / or in series to each other, wherein each inverter circuit is configured to provide a three phase alternating current .

[0115] An apparatus designed in such a way has the advantage that in the intended use of the apparatus , in case one inverter circuit mal functions , all electrodes of an electric furnace may still be provided with an alternating current .

[0116] The inverter circuits may be connected in parallel to each other at their respective electrical output terminals . In this way, a three phase alternating current with an increased current strength may be provided at the output of the inverter stage .

[0117] Preferably at least one power module comprises a trans former, particularly a three phase trans former, wherein a primary winding of the trans former is electrically connected to the inverter Seite 22 / 34

[0118] P81086WO stage of that power module and a secondary winding of the transformer is electrically connectable to the electric furnace, particularly to at least one electrode of the electric furnace.

[0119] An apparatus designed in such a way has the advantage that in the intended use of the apparatus high amounts of electric energy may be transferred at comparably high levels of voltage up to the transformer, resulting in comparably low levels of current strengths that need to be transferred through the rectifier stage, the DC bus and the inverter stage resulting in lower overall cost of the components of the apparatus.

[0120] A secondary winding of the transformer may be electrically connected to the electric furnace, particularly to at least one electrode of the electric furnace.

[0121] Preferably, the three phase transformer comprises a primary winding per phase and one or more secondary windings per phase. Preferably, the three phase transformer is a phase-shifting transformer .

[0122] Preferably, the transformer is a medium voltage (MV) to low voltage (LV) transformer. In other words, the transformer is preferably configured to transform a voltage with a medium voltage level at its primary winding to a voltage with a low voltage level at its one or more than one secondary winding. The transformer may be an oil cooled transformer.

[0123] According to an embodiment, the three phase transformer is a dry-type transformer. A dry-type transformer is a transformer, that does neither use a liquid as a cooling medium nor as an insulating medium for the its windings and core. Instead a dry-type transformer uses a gaseous fluid, preferably air, as cooling medium and a solid material, preferably an epoxy resin, polyester resin and / or aramid material, as an insulating medium. Seite 23 / 34

[0124] P81086WO

[0125] An apparatus designed in such a way has the advantage of facilitating initial installation as well as a reduction of fire potential during failure due to cooling medium inflammation .

[0126] Preferably the inverter stage provides an alternating voltage at its output side with a voltage level of > 1700 VAC .

[0127] An apparatus des igned in such a way has the advantage of being able to trans fer a comparably high amount of electric energy with comparably low current strengths resulting in lower overall costs for the need of copper or other conducting materials .

[0128] The inverter stage may provide an alternating voltage at its output side with a voltage level of > 10000 VAC, preferably of > 20000 VAC and particularly preferred of > 30000 VAC . According to a preferred embodiment , the inverter stage may provide an alternating voltage at its output side of > 33000 VAC, preferably of > 40000 VAC and preferred of > 66000 VAC .

[0129] An inverter circuit of the inverter stage may comprise one or more switching devices , particularly two switching devices , connected in series and / or in parallel to each other, wherein each switching device is capable of withstanding a voltage level of

[0130] > 1700 VAC, preferably of > 2000 VAC and particularly preferred of > 5000 VAC . According to a preferred embodiment , the switching device is capable of withstanding a voltage level of > 10000 VAC, preferably of > 20000 VAC, particularly preferred of

[0131] > 33000 VAC and further preferred of > 66000 VAC . The switching devices of an inverter circuit may be connected in a hal f bridge or a full bridge configuration to each other . One hal f bridge and or one full bridge may be capable of providing an alternating current with a current strength of > 2500 A, particularly of > 3000 A, preferably of > 3500 A and particularly preferred of > 5000 A. Seite 24 / 34

[0132] P81086WO Preferably two or more than two power modules are connected in parallel to each other .

[0133] An apparatus designed in such a way has the advantage that in the intended use of the apparatus , an electric furnace may be supplied with electrical energy with an increased current strength .

[0134] Preferably the power modules are connected in a star configuration or in a delta configuration to each other .

[0135] In a delta configuration an increased voltage level between the power modules can be achieved while in a star configuration an asymmetrical load can be better balanced .

[0136] Particularly the power modules are connected in a star configuration or in a delta configuration at their respective electrical output side to each other .

[0137] Preferably the apparatus comprises two or more than two power modules , wherein the recti fier stages of the two or more than two power modules each comprise a single phase recti fier circuit , wherein the power modules are connectable to , preferably connected to , di f ferent phases of the power network .

[0138] An apparatus designed in such a way has the advantage of an increased reliability and availability due to the modular topology .

[0139] Preferably the apparatus comprises two or more than two power modules , wherein at least one phase of the power network a recti fier stage of a first power module is configured to be connected to is di f ferent from at least one of the phases a rectifier stage of a second power module is configured to be connected to . Seite 25 / 34

[0140] P81086WO

[0141] An apparatus designed in such a way has the advantage of an increased reliability and availability due to the modular topology .

[0142] Preferably the apparatus comprises two or more than two power modules , wherein at least one phase of the power network a recti fier stage of a first power module is connected to is di f ferent from at least one of the phases a recti fier stage of a second power module is connected to .

[0143] According to a second aspect of the invention, the task is solved by a use of an apparatus according to the first aspect of the invention to supply electric energy to an electric furnace .

[0144] It should be noted that the features described with regard to the first aspect of the invention can be combined with the second aspect of the invention both independently and cumulatively . Thus the advantage described with regard to the first aspect of the invention also apply to the second aspect of the invention .

[0145] According to a third aspect of the invention, the task is solved by an electric furnace comprising one or more than one apparatus according to the first aspect of the invention .

[0146] An electric furnace designed in such a way has the advantage of an increased res ilience against fluctuations in electric power consumption during operation resulting in an increased availability of the electric furnace . During operation an electric furnace produces high power fluctuations due to arc instabilities during the melting process . These arc instabilities often deviate between multiple electrodes of the electric furnace . In case of an electric arc break between one electrode and the metal material , the ef fect on electric arcs of further electrodes is Seite 26 / 34

[0147] P81086WO reduced due to the fact , that each arc is supplied by at least one di f ferent phase of the power network .

[0148] It should be noted that the features described with regard to the first and second aspect of the invention can be combined with the third aspect of the invention both independently and cumulatively . Thus the advantage described with regard to the first and second aspect of the invention also apply to the third aspect of the invention .

[0149] Preferably the electric furnace comprises two or more elec- trodes , preferably exactly three electrodes .

[0150] Preferably the electric furnace is configured to operate at a voltage level of > 1200 VAC, particularly of > 1800 VAC, preferably of > 3000 VAC and particularly preferred of > 5000 VAC .

[0151] It should be noted that the features described with regard to the first , second and third aspect of the invention can be combined with the fourth aspect of the invention both independently and cumulatively . Thus the advantage described with regard to the first , second and third aspect of the invention also apply to the fourth aspect of the invention .

[0152] Further advantages , details and features of the present invention are explained in the description of the following embodiments , whereby :

[0153] Figure 1 : shows a schematic view of a first embodiment of a an apparatus ;

[0154] Figure 2 : shows a schematic view of a second embodiment of an apparatus ; and Seite 27 / 34

[0155] P81086WO Figure 3 : shows a schematic view of a third embodiment of an apparatus .

[0156] In the following description, same reference numerals describe same elements and same features , respectively, so that a description of one element conducted with reference to one figure is also valid for the other figures , so that repetition of the respective feature is omitted .

[0157] Figure 1 shows an apparatus 10 for the supply of an electric furnace 90 with electric energy according to a first embodiment of the invention, wherein the apparatus 10 is electrically connected to a three phase power network 100 and three electrodes 91 of the electric furnace 90 ; wherein the apparatus 10 comprises three power modules 20 , wherein each power module 20 comprises a recti fier stage 30 , connected to less than three di f ferent phases R, S , T of the power network 100 , particularly connected to exactly one phase of the power network 100 ; an inverter stage 40 , electrically connected to at least one electrode 91 of the electric furnace 90 ; and a DC bus 50 , wherein the DC bus 50 electrically connects the recti fier stage 30 to the inverter stage 40 of the power module 20 .

[0158] The recti fier stages 30 of the three power modules 20 each comprise a single phase recti fier circuit , wherein the power modules 20 are connected to di f ferent phases R, S , T of the power network 100 . Each single phase recti fier circuit is connected to the neutral N of the power network 100 .

[0159] The inverter stages 40 of the three power modules 20 comprises a plurality of inverter circuits connected in parallel and / or in series to each other, wherein each inverter circuit is configured to provide a three phase alternating current comprising the phases R, S , T at the output side of the inverter circuit . The Seite 28 / 34

[0160] P81086WO power modules 20 are connected in parallel to each other at their respective electrical outputs .

[0161] Figure 2 shows an apparatus 10 for the supply of an electric furnace 90 with electric energy according to a second embodiment of the invention, wherein the recti fier stages 30 of the power modules 20 each comprise a two phase recti fier circuit , wherein at least one phase of the power network 100 a recti fier stage 30 of a power module 20 is connected to is di f ferent from at least one of the phases a recti fier stage 30 of a di f ferent power module 20 is connected to .

[0162] Each power module 20 comprises a three phase trans former 60 , wherein a primary winding of the three phase trans former 60 is electrically connected to the inverter stage 40 of that power module 20 and a secondary winding of the three phase trans former 60 is electrically connected to the electrodes 91 of the electric furnace 90 .

[0163] Figure 3 shows an apparatus 10 for the supply of an electric furnace 90 with electric energy according to a third embodiment of the invention, wherein the inverter stage 40 of each power module 20 compri ses a plurality of inverter circuits connected in parallel and / or in series to each other, wherein each inverter circuit is configured to provide a single phase alternating current .

[0164] The power modules 20 are connected in a star configuration at their respective electrical outputs , wherein the star center is connected to a common neutral N . The electric furnace 90 further is connected to the neutral N . Seite 29 / 34

[0165] P81086WO

[0166] List of reference numerals

[0167] 10 Apparatus

[0168] 20 Power module

[0169] 30 Recti fier stage

[0170] 40 Inverter stage

[0171] 50 DC bus

[0172] 60 Trans former

[0173] 90 Electric furnace

[0174] 91 Electrode of the electric furnace

[0175] 100 Power network

[0176] R First phase

[0177] S Second phase

[0178] T Third phase

[0179] N Neutral

Claims

Seite 30 / 34P81086WOClaims1. Apparatus (10) for the supply of an electric furnace (90) with electric energy, wherein the apparatus (10) is electrically connectable to a power network (100) , preferably a three phase power network and at least one electrode (91) of the electric furnace (90) ; wherein the apparatus (10) comprises at least one power module (20) , wherein the power module (20) comprises: a rectifier stage (30) , configured to be electrically connected to less than three phases, particularly to less than three different phases of the power network (100) ; an inverter stage (40) , electrically connectable to at least one electrode of the electric furnace (90) ; and a DC bus (50) , wherein the DC bus (50) electrically connects the rectifier stage (30) to the inverter stage (40) of the power module (20) .

2. Apparatus (10) according to claim 1, characterized in that the power module (20) comprises exactly one DC bus (50) , that electrically connects the rectifier stage (30) to the inverter stage (40) of the power module (20) .

3. Apparatus (10) according to claim 1 or 2, characterized in that the apparatus comprises less than six power modules (20) .

4. Apparatus (10) according to one of the preceding claims, characterized in that the rectifier stage (30) comprises one or more than one rectifier circuit, wherein the rectifier circuit comprises electrical input terminals for less than three different phases of an alternating current supplied at an input side of the rectifier circuit.Seite 31 / 34P81086WO5. Apparatus (10) according to claim 4, characterized in that the rectifier circuit is a single phase rectifier circuit or a two phase rectifier circuit.

6. Apparatus (10) according to claim 4 or claim 5, characterized in that the rectifier circuit comprises a first electrical input terminal and a second electrical input terminal, wherein the first electrical input terminal is configured to be electrically connected to a first phase of the power network (100) and the second electrical input terminal is configured to be electrically connected to a second phase of the power network (100) , wherein the second phase of the power network (100) is different from the first phase of the power network (100) .

7. Apparatus (10) according to one of the preceding claims, characterized in that the DC bus (50) is configured to handle a voltage level of > 1500 VDC, particularly of > 3000 VDC and preferably of > 5000 VDC.

8. Apparatus (10) according to one of the preceding claims, characterized in that the inverter stage (40) comprises a plurality of inverter circuits connected in parallel and / or in series to each other, wherein each inverter circuit is configured to provide a single phase alternating current.

9. Apparatus (10) according to one of the preceding claims 1 to 7, characterized in that the inverter stage (40) comprises a plurality of inverter circuits connected in parallel and / or in series to each other, wherein each inverter circuit is configured to provide a three phase alternating current.

10. Apparatus (10) according to one of the preceding claims, characterized in that at least one power module (20) comprises a transformer (60) , preferably a three phase transformer, wherein a primary winding of the transformer (60) is electricallySeite 32 / 34P81086WO connected to the inverter stage (40) of that power module (20) and a secondary winding of the transformer (60) is electrically connectable to the electric furnace (90) , particularly to at least one electrode (91) of the electric furnace (90) .

11. Apparatus (10) according to one of the preceding claims, characterized in that the inverter stage (40) provides an alternating voltage at its output side with a voltage level of > 1700 VAC.

12. Apparatus (10) according to one of the preceding claims, characterized in that two or more than two power modules (20) are connected in parallel to each other.

13. Apparatus (10) according to one of the preceding claims, characterized in that the power modules (20) are connected in a star configuration or in a delta configuration to each other.

14. Apparatus (10) according to one of the preceding claims, characterized in that the apparatus (10) comprises two or more than two power modules (20) , wherein the rectifier stages (30) of the two or more than two power modules (20) each comprise a single phase rectifier circuit, wherein the power modules (20) are connectable to, preferably connected to, different phases of the power network (100) .

15. Apparatus (10) according to one of the preceding claims, characterized in that the apparatus (10) comprises two or more than two power modules (20) , wherein at least one phase of the power network (100) a rectifier stage (30) of a first power module (20) is configured to be connected to is different from at least one of the phases a rectifier stage (30) of a second power module (20) is configured to be connected to.Seite 33 / 34P81086WO16. Use of an apparatus (10) according to one of the preceding claims to supply electric energy to an electric furnace (90) .

17. Electric furnace (90) comprising one or more than one apparatus (10) according to one of the claims 1 - 15.

18. Electric furnace (90) according to claim 17, characterized in that the electric furnace (90) comprises two or more electrodes (91) , preferably exactly three electrodes (91) .

19. Electric furnace (90) according to claim 17 or claim 18, characterized in that the electric furnace (90) is configured to operate at a voltage level of > 1200 VAC, particularly of > 1800 VAC, preferably of > 3000 VAC and particularly preferred of > 5000 VAC.

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