Industrial furnace transformers

     Operating modes and features of technical requirements for electric furnace transformers

 Electric furnace transformers (EPT) are part of electrothermal installations (ETU) - installations of electric furnaces and electric heating devices used to obtain ferrous, non-ferrous and rare metals and their alloys with desired properties, as well as ore-smelting furnaces.
 Features of operation, modes and technical requirements distinguish EPT in a separate class of power transformers.
 The most significant of these features are as follows:

  1.  Power supply of electric drives, the power of which reaches 100 MB-A, is carried out with a voltage from several to hundreds of V, therefore, the currents of low-voltage electric motors can be many tens of thousands of amperes.
  2.  The voltage supplying the electric drive should vary over a wide range with their ratio reaching 5: 1 or more. Voltage changes must be ensured by an EPT with fine-stage control under load (OLTC) or with a transformer disconnected from the network (PBV).
  3.  The reactive resistance of the EPT should be less than the resistance of the short network and the furnace, so as not to significantly reduce the energy consumption of the EPT, i.e., the short-circuit voltage of the EPT should be minimal.
  4.  Numerous ignition and arc breaks on the electrodes in arc electric drives cause sharp changes in the current in the electric drives, which leads to electrodynamic effects and overvoltages in the windings and imposes additional requirements on the design of transformers.
  5. Frequent switching by operating switches on the HV side of the EPT, especially with vacuum interrupter chambers, are also sources of overvoltage, including of a resonant nature in the regulating windings of the EPT.

These features are most pronounced in EPFs feeding steel arc furnaces (EAF).

     Steel Arc Furnace Transformers

  EAF are direct-acting arc furnaces, the operation of which is accompanied by a sharply variable load, especially in the initial period of melting. The change in the EAF load in time for the melting cycle is set by the so-called directive charts, depending on the furnace capacity, steel grade, quality and characteristics of the charge, etc. p. [1].
  In fig. 28.1 shows a typical directive load schedule for a 5-ton chipboard during steel melting. The melting cycle in EAF is characterized by three periods with different electrical loads [2]: melting, oxidation, refining, and the fourth period, when the EAF is turned off, and the metal is tapped and the furnace is reloaded. The power of the furnace, and, consequently, of the EPF feeding it, changes during the melting cycle. The greatest power is consumed during the melting period, when the arc is unstable, short, and the voltage must be increased to increase the power. The duration of this period is 50-60 % of the total melting duration, increasing in powerful high-performance EAF to 60-70%.

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    During the period of oxidation and especially refining, the power of the EAF should decrease. The reduction in power is achieved by reducing the secondary voltage of the EPT using a step voltage regulator. For EAF transformers with a capacity of up to 12 t (EPT power up to 8 MB • A), regulators with no-load voltage switching (PBV) are used, for large-capacity EPTs - under load (OLTC). The depth (D) of voltage regulation, i.e. the ratio of the highest secondary voltage to the lowest reaches 2.0-2.5:
  where U2Art. max ~~ secondary voltage at the maximum voltage stage, V; U2st.min ~~ secondary voltage at the undervoltage stage, V.
The operation of the EPT is accompanied by frequent shutdowns of the EAF and no-current pauses of a technological nature. Such a sharply variable EPT load is determined by fluctuations in the current of electric arcs:
    1) regular, cyclic frequency 2-8 Hz within 15-40% rated load current I2ohm
     2) irregular frequency up to 1 Hz caused by short circuits of the furnace electrodes with a charge, called operational short circuits (SC). 
 At the same time, in accordance with [1], the coefficient K of the multiplicity of the operational short-circuit current / kse, defined as the ratio / kse / 72 nom, is different for EAF of different capacities (Table 28.1).
The short circuit is reduced in furnaces with a larger capacity. Operational short-circuits cause mechanical stress on the transformer windings, and special measures are required to ensure resistance against these influences. One of such measures is the inclusion of a current-limiting reactor with adjustable inductance in the HV EPT winding circuit. The reactor is built into a common tank with a transformer (for electric drives with a capacity of 0.5-H2 t) or installed separately.

  In addition to operational short circuits, EPHs are exposed to emergency currents caused by short circuits in the sections of the short network between the furnace and the EPT outputs. The closer to the terminals the location of the short circuit, the greater the emergency current / kzav. With short-circuits at the terminals of the transformer, the current / kz av reaches the highest value, since it is limited only by the resistance of the transformer itself and the power of the short-circuit of the power system at the power point of the EPT. In this case, it is not always possible to ensure the electrodynamic stability of the EPT.
  The sharply uneven load curve of the EPT for the EAF makes it impractical to select its power according to the maximum load in the melting cycle, since the rest of the cycle time the transformer would remain underloaded. Therefore, the nominal power of the EPT is usually chosen less than the maximum, determined by the load graph, allowing for a certain overload during the melting period.

      Transformers for ore smelting furnaces

Unlike EAF, ore thermal furnaces (RTP) refer to resistance arc furnaces operating on a mixed principle, when energy is released both in the arc and in the thickness of the charge and slag. Ore-thermal furnaces are very diverse in purpose and features of technological processes. At the same time, the operating mode of most RTPs is quite calm: the power consumption per melting cycle remains practically unchanged, and operational short circuits are almost completely absent. Therefore, transformers for ore-smelting furnaces do not require additional current-limiting resistances (reactors). The recovery processes taking place in the RTP require low voltages and high currents of the EPT. This imposes special requirements on the design of the secondary windings and LV transformer terminals. When switching to another alloy, raw materials, etc., it is necessary to change the operating modes of the furnace, that is, to change the voltage and current supplied to it within wide limits. The depth of regulation of the secondary voltage for most EPTs for RTP is in the range of 1.54-2.0. However, for some technological processes, a larger voltage range is required, and the depth of regulation for some EPT reaches      
  EPT with on-load tap-changer is usually used for electric furnaces of medium and high power, in which each disconnection is accompanied by voltage fluctuations in the supply network, and therefore it is desirable to reduce the number of switching on and off of the furnaces to a minimum. The use of on-load tap-changers is also necessary in furnaces where work is carried out with a fixed electrode, and regulation of the furnace operation is achieved by changing the voltage on the electrodes. Powerful RTPs also make specific requirements for EPTs associated with the measurement of secondary currents. The fact is that the design of the short network and the values of the currents for which there are no measuring transformers do not allow measurements to be made directly on the side of the LV EPT. At the same time, measuring the current on the side of the primary voltage of the EPT does not make it possible to correctly judge the LV current. This is explained by the fact that for most RTPs, constant LV power is required at a certain part of the range of the secondary voltage of the EPT. As a result, during load fluctuations, the primary EPT current remains unchanged within this range and cannot be used to measure the LV current. In this case, EPTs should be built with circuit solutions that would have auxiliary circuits with a relatively small current that varies strictly in proportion to the LV current at all positions of the PU. Instrument transformers are built into these auxiliary circuits.

      Transformers for electroslag remelting plants

    Electroslag remelting units (ESR) are adjacent to arc resistance furnaces. In ESR furnaces, electrodes are remelted from special steels, obtained, for example, in arc steel-making furnaces; The ingot purified in the remelting process is formed in a water-cooled crystallizer. The arc process in ESR furnaces occurs only when the furnace is started up, when a slag bath is created from electrically conductive and working flux. Subsequently, melting occurs as an arcless process, the operating current heats the electrode and maintains the slag in a molten state.
Transformers for ESR furnaces are produced in a single-phase design in accordance with three main power supply schemes: single-electrode furnaces with one consumable electrode; two-electrode single-phase with two electrodes and three-phase with three consumable electrodes (Fig. 28.2 a, b, c). In the latter case, three single-phase EPTs feed three consumable electrodes placed in a common mold and located at the vertices of the triangle. In the latter case, three single-phase EPTs feed three consumable electrodes placed in a common mold and located at the vertices of the triangle. In the latter case, three single-phase EPTs feed three consumable electrodes placed in a common mold and located at the vertices of the triangle.
Throughout the entire melting, the EPT must ensure the continuity of the furnace operation.
  In fig. 28.3 shows the load graph of a 1000 kV transformer * Ad for ESHP-2.5 furnaces. In the first melting period, the furnace consumes maximum power, the flux melts and the arc process takes place. Further, an electric current, passing through the electrode, maintains the slag in a molten state; the end of the electrode lowered into the slag begins to melt, its length and resistance decrease. To maintain the stability of the process, it is necessary to reduce the secondary voltage, and, consequently, the power of the EPT. The depth of LV regulation for most EPTs for ESR furnaces should be Г = 3.5-4.0, and the voltage drop of adjacent steps should be from 2.0-2.5 V at the first to 0.2-0.3 V at the last steps secondary voltage. To ensure such discreteness, modern EPTs are equipped with built-in PUs, allowing to obtain up to 90 LV stages. A feature of the ESR process is the need for currents reaching tens of kA, which requires special design solutions for the ESR. EPTs for ESRs must have a certain versatility in order to provide remelting of ingots from different steels and different weights. For this purpose, EPTs have the ability to operate with constant (highest rated) power over a significant part of the LV range. 

      Induction Furnace Transformers

Induction crucible and channel furnaces are designed: for melting ferrous and non-ferrous metals and their alloys; for overheating the metal before casting and leveling its chemical composition; for alloying and maintaining constant temperatures during casting (mixers). Induction furnaces are fairly quiet energy consumers, using constant or slowly increasing power for melting (aluminum melting furnaces). after a long stoppage of the furnace or to dry the crucible after repair, the EPT must provide a reduced supply voltage and power consumption. EPTs for induction furnaces are a lot like general purpose power transformers. However, to meet all the requirements of induction electric drives, EPTs are built with built-in PUs and a large depth of regulation Г = 5-6. In this case, an EPT with a capacity of 1000 kV * A and less is usually performed with an off-circuit tap-changer and remote control, a higher power - with an on-load tap-changer.
To maintain the performance of the induction electric drive, the EPT must provide several values of secondary currents and voltages at maximum power, i.e., have a constant power range. The range covers secondary voltages within five positions of the PU. Starting from the 6th position, a decrease in LV occurs simultaneously and proportionally to a decrease in the power of the EPT. A feature of this unit with induction electric drives with a capacity of more than 10 tons are inrush current surges, which are accompanied by each turn on of the transformer. These inrush currents of high magnification impose additional requirements on the mechanical strength of the windings and the design of EPBs for induction EPs of large capacity. 

   Resistance furnace transformers

 Electric resistance furnaces of indirect and direct action are widespread and various in purpose. Combine their principle of operation, as well as power sources - single or three-phase dry transformers with HV 220 or 380 V and various ranges of secondary voltages. The choice of the required secondary voltage supplying the furnace is ensured by the design of the EPT: the output from the transformer of the branches of the HV and HV windings, which can be connected in a certain way. The connection is made using jumpers (HV and LV side) or blade contacts (HV side). Some of the transformers are produced in protective casings, the majority - in an open, unprotected design.

     Secondary voltage regulation circuits in electric furnace transformers

 The EPT regulation schemes are largely determined by the features and requirements of the technological processes in the EP. However, the choice of the scheme is influenced by other factors: parameters of the on-load tap-changer and off-load tap-changer; provision of preset values of short circuit voltages at various stages of regulation; necessary dynamic resistance and electric strength; transport restrictions; given dimensions; restrictions on economic parameters, etc. 
  The regulation schemes used in EPT can be classified according to the following main features:
1. The number of electromagnetic units:
a) one unit (fig. 28.4 and 28.5) - direct regulation;
b) two units (Fig. 28.6-28.8) - indirect regulation.
2. The value of induction in the magnetic system:
a) constant at all stages of regulation (Fig. 28.4);
b) changing depending on the stage (Fig. 28.5-28.8).
   3. Place of switching on the switching device:
a) in the LV winding circuit (Fig. 28.4);
b) in the HV winding circuit (Fig. 28.5);
c) in the intermediate circuit of the unit (Fig. 28.6-28.8).
4. Method of regulating the intermediate circuit of the unit:
a) using an autotransformer (Fig. 28.6);
b) in the secondary winding of the first electromagnetic unit of the unit (Fig. 28.7);
c) in the tertiary winding of the first electromagnetic unit of the unit using a booster transformer (Fig. 28.8).
  Direct control circuit according to Fig. 28.4 is used in EPTs with a power per rod of not more than 2500 kV - A at LV from 1000 V to 2400 V. Regulation takes place in the LV winding, which thus combines the functions of the regulating one. This is the most economical control method in which the induction in the magnetic system remains constant throughout the entire switching cycle. An additional advantage is the uniform change in LV when the same number of turns is turned off (or turned on) at all stages of the secondary winding. However, the scope of such regulation is limited by the value of the rated current of the PU, which is simultaneously the operating current of the electric drive. The schematic diagram in Fig. 28.5 is widely used in EPT for various purposes with a nominal HV of 6 or 10 kV at G from 1.5 to 4, and its modifications are in EPT with HV 35 kV.
  Regulation occurs when the number of turns in the HV winding (or RO, as part of it) changes. In this case, the induction in the magnetic system changes from the maximum value (minimum of the included turns) to the minimum when all the turns are connected to the supply voltage {Ux). At the same time, the LV also changes - from the maximum value (minimum of turns of the HV winding) to the minimum. The advantage of such regulation is the possibility of using the PU for relatively low HV currents, which significantly increases the variety of different circuit solutions to obtain the required LV ranges.

 However, this method is accompanied by ineffective use of the EPT magnetic system, which turns out to be "underloaded" throughout the entire regulation range, except for the position corresponding to the minimum of the switched on HV turns, when the induction is maximum. At all other positions of the PU, the induction decreases in proportion to the increase in the number of HV turns on.

  Another disadvantage of this method is the unevenness of the LV steps with an equal number of turns on (or off) turns of the primary winding.
When using indirect control circuits (Fig. 28.6-28.8) EPT is a unit of two transformers (or one autotransformer and a transformer), usually placed in a common tank. With these circuits, voltage regulation is carried out in the intermediate circuit between two electromagnetic units. The voltage and current of the intermediate circuit are selected in accordance with the technical capabilities of the control unit. The regulation in these schemes is carried out in different ways. So, with the scheme in Fig. 28.6 the first unit of the unit is a regulating autotransformer; The circuit is used in EPT with HV up to 35 kV inclusive at G <5. With this method of regulation, the autotransformer is switched on to the primary voltage U \ and has a constant induction in the magnetic system. The VN winding of the furnace transformer with the help of the PU is connected to the taps of the autotransformer winding. When the position of the PU changes, the supply voltage, magnetic flux, induction in the magnetic system of the furnace transformer and, consequently, its secondary voltage change.

 The advantage of such regulation is the ability to obtain practically any LV values within the range - from strictly uniform to sharply unequal. However, this method requires a significant consumption of active materials, especially steel, and, in addition, the PU must be selected for the voltage class corresponding to U \, which in many cases is uneconomical.

  The latter can be avoided if the regulation is carried out according to Fig. 28.7. It is used in EPTs with HV 35 kV and above, and the first unit of the unit is a control transformer with constant induction in the magnetic system and regulation in its secondary winding. Otherwise, this circuit does not differ from the autotransformer.
The only advantage of a circuit with a regulating transformer over a circuit with an autotransformer is the ability to set currents and voltages in the intermediate circuit that correspond to the parameters of certain PUs.
  According to the diagram in Fig. 28.8 regulation in the intermediate circuit is carried out using the tertiary RO of the main transformer with constant induction in the magnetic system; RO is connected to the primary winding of a booster transformer with variable induction; The LV windings of both transformers are connected in series inside the unit. The circuit is used in single-phase EPT with HV from 10 kV and above with a power from 25,000/3 to 80,000/3 kV • A.

 The advantage of such regulation lies in the relative reduction in the mass of the unit compared to that shown in Fig. 28.7. This is explained by the fact that the power of the booster transformer corresponds to the power of RO, which is only a part of the power of the main transformer. And since the RO is built into the intermediate circuit, the switching device can be selected for significantly lower currents and voltages than in the circuit in Fig. 28.7.

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