Furnace Transformer PRESENTED BY PROF VG PATEL TRANSFORMER
Furnace Transformer PRESENTED BY PROF. VG PATEL
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Furnace transformers are used to feed furnaces of different types, such as arc furnace, reduction furnace, induction furnace, etc. , Base materials like Calcium carbide, Silicon carbide, Ferro-chromium, base Phosphorous, and various non-ferrous metals are extracted primarily from electric reduction furnaces. Arc furnace transformers are used generally in steel and special alloy steel making industries of large and medium size furnaces. Designing of furnace transformer is an art by itself and it differs completely from power transformers. Sunday, October 17, 2021 VG PATEL 2
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER INDUCTION FURNACE TRANSFORMER Induction Furnace transformers are used for “INDUCTION FURNACE”. An Induction Furnace works melting the scrap using medium frequency magnetic field created by a coil. The coil fed by the medium frequency AC current supplied by an inverter which is fed by a DC current converter. Sunday, October 17, 2021 VG PATEL 3
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER HARMONICS ► Harmonic is defined as a sinusoidal component of periodic wave having frequency that is an integral multiple of fundamental frequency. ► A sinusoidal voltage or current function that is dependent in time t may be represented by the following expressions: Voltage Function, v(t) = V sin (ωt) Current Function, i(t) = V sin (ωt+/-Ø) Where ω = 2 x Π x f ► Which is known as angular velocity of the periodic waveform and Ø is the difference in phase angle between the voltage and the current waveforms referred to as a common axis. ► The sign of phase angle Ø is positive if the current leads the voltage and negative if the current lags the voltage. Sunday, October 17, 2021 VG PATEL 4
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Sunday, October 17, 2021 VG PATEL 5
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER The second harmonic undergoes two complete cycles during one cycle of fundamental frequency, and the third harmonic complete three cycles during one cycle of fundamental cycle. V 1, V 2 and V 3 are the peak values of harmonic components that comprise waveform, which also has a frequency of f. In order to find the effect of a non sinusoidal voltage or current on a piece of equipment , only to determine the effect of individual harmonics and then vectorially sum the results to derive the net effect. For a periodic voltage wave with fundamental frequency of ω = 2Πf, v(t) = V 0+Σ(ak cos kωt + bk sin kωt) (For k = 1 to infinity). Sunday, October 17, 2021 VG PATEL 6
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Where ak and bk are the coefficient of the individual harmonics terms or components. If the function is an even function, meaning f(- t) = f (t) , then the sine terms vanish from the expression. If the function is odd, with f(- t) = -f( t), then the cosine terms disappear. For We will use the simplified expression involving sine term only. Because having both sine & cosine terms effects only on phase displacement angle of the harmonics components and the shape of the nonlinear wave and does not alter the principle. Sunday, October 17, 2021 VG PATEL 7
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Sunday, October 17, 2021 VG PATEL 8
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER HARMONIC CURRENT CANCELLATION ►Transformer connections employing phase shift are used to effect cancellation of harmonic currents in power system. ►Triplen harmonic (3 rd, 9 th, 15 th etc. ) currents are set of currents that can be effectively trapped using a special transformer configuration called phase shifting transformer. It is used to achieve cancellation of 5 th and 7 th harmonic. ►Using a Δ-Δ and Δ-Y transformers 5 th and 7 th harmonics are cancelled at the point of common connection. ►Cancellation of harmonics are depends on the pulses of transformer. More the pulses less the harmonics. ►Types of pulses are : 6 pulse, 12 pulse, 24 pulse, 48 pulse. ►H = kp+/-1 Where p = Pulses k = (1, 2, 3, 4, 5…. . ) & H = Harmonic Sunday, October 17, 2021 VG PATEL 9
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Sunday, October 17, 2021 VG PATEL 10
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER The important feature of a furnace transformer is that, it has very low secondary voltage variations with high currents. Secondary voltage variation of a furnace duty transformer will be in the order of 1: 2. 5 or even more. To meet this condition, HV winding has to be designed with a separate tapped winding, which is connected in series with the main winding. Off-circuit tap changer or OLTC may be employed on the primary side to get voltage variation on the secondary side. The number of voltage steps can be doubled by introducing one more winding called the coarse winding in series with the main winding and the tapped winding. Sunday, October 17, 2021 VG PATEL 11
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Sunday, October 17, 2021 VG PATEL 12
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Delta-star changeover switch may also be employed on the primary side to have more number of steps. The connection diagram of typical furnace transformer is shown in figure 1. Transformer core will have variable flux density in the case of direct method of voltage adjustments. OLTC should be carefully selected so as to withstand higher overhang voltages and short circuit currents in addition to its rated current. For very large furnace transformers of 66 k. V and above, built-inbooster transformers are employed for voltage variation purpose. The connection diagram of this transformer is shown in figure 2. Sunday, October 17, 2021 VG PATEL 13
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Sunday, October 17, 2021 VG PATEL 14
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Built-in-booster furnace transformer consists of two separate transformers mounted in one common tank viz. the main transformer and the booster transformer. Main transformer consists of three windings viz. HV main winding, LV winding and the fuse winding. Ampere turns of the HV main winding will be balanced by the fine winding and the LV winding together. Main transformer produces the constant value of basic secondary voltage. Booster transformer consists of only two windings viz. , auxiliary winding and the LV winding, with variable flux in the core. Both the LV winding of main transformer and booster transformers are connected in series to give the required output voltages. Sunday, October 17, 2021 VG PATEL 15
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Any convenient voltage and the corresponding current ratings may be chosen for the tapped fine winding in order to select the economical OLTC. Since, the secondary winding of the main transformer and the booster transformer are connected in series, the position of the reversing switch determines whether auxiliary voltage increases or decreases the basic voltage. Selection of the booster secondary voltage should be chosen to give maximum and minimum secondary basic voltages by operating main transformer fine winding along with the reversing switch. Sunday, October 17, 2021 VG PATEL 16
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Primary winding and the fine windings are assembled very close to the core concentrically one over the other. LV winding will be assembled after the fine and the main windings, so as to provide heavy current carrying bus bars to the out coming bushings which are provided on the lid cover or on the side wall of the transformer. Antimagnetic material or aluminium sheets should be employed near the LV bus bar bushings to reduce the heating of metal plate. Beginning and ending of secondary windings of each phase are brought out alternatively so as to achieve minimum magnetic stray field around the LV outlets. Sunday, October 17, 2021 VG PATEL 17
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Due to high currents on the secondary side, flat copper bus bar bushings or water-cooled tubular type bushings will be employed instead of normal porcelain bushings. Sufficient care should be taken to eliminate heating of the metal parts due to the high eddy currents. Secondary windings are generally designed with double disc type and connected in parallel with the number of copper bus bars. Copper cylindrical windings may also be employed for large current furnace transformers. Secondary windings are normally open connected and the delta will be made at the furnace point. Furnace transformers are expected to withstand overloads and frequent short circuits due to the nature of load. Windings and all the leads are to be properly clamped against the frequent short circuit forces. Sunday, October 17, 2021 VG PATEL 18
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER It is necessary to minimize the reactance of the transformer in order to obtain the maximum active part input in the electric arc of the furnace. Another important condition is that, the impedance resistance of all the three phases of the furnace supply leads should be the same. Otherwise it will cause unequal voltage drops in the corresponding phases and causes so called “wild and dead phases”. In the phase with lower impedance, the furnace arc burns with a higher active output and a greater arc length. In the field of this wild phase, the furnace wall lining and the lid are subjected to much more heat effects, in addition to excessive wear of the carbon electrodes. Sunday, October 17, 2021 VG PATEL 19
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER This shorter service life of the lining results in increased melting costs and reduces very substantially the total production capacity of steel melting furnace. If three single phase furnace transformers are employed, it is possible to set individual voltages on the furnace side for each phase separately and to compensate unequal voltage drops in the individual phases of the network, thereby uneven burning of the furnace and cover linings can be avoided. However, use of three or four such single-phase furnace transformers is more expensive and requires additional floor area. Sunday, October 17, 2021 VG PATEL 20
TRANSFORMER ENCYCLOPAEDIA FURNACE TRANSFORMER Furnace transformers are mostly installed indoor. As the floor area is restricted inside the industry, special type of cooling system should be employed so as to have effective cooling with the reduction in overall dimensions. Generally forced oil with water to oil heat exchangers are adopted with 100% spare standby cooler on the transformer tank. Due to polluted and dusty atmosphere, water-resisting paints should be used for the external tank surfaces. Sunday, October 17, 2021 VG PATEL 21
TRANSFORMER ENCYCLOPAEDIA OPEN FORUM Sunday, October 17, 2021 22
TRANSFORMER ENCYCLOPAEDIA THANQ Sunday, October 17, 2021 23
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