电弧炉变压器选择

Transformer Capacity (MVA) 10 12 15 18 20 22 25 27 30 35 40 45 50 60 70 80 100 120 150 Max. (rated) Voltage (V) 50 Hz 290 320 350 380 400 420 440 460 480 500 530 570 610 650 700 750 800 850 900 60 Hz 320 350 380 400 420 440 460 480 500 530 560 600 650 700 750 800 850 900 950 Min. full-capacity Voltage (V) 50 Hz 240 260 280 300 320 340 360 380 400 420 440 460 480 500 550 600 650 700 750 60 Hz 260 280 300 320 340 360 380 400 420 440 460 480 500 550 600 650 700 750 800 Min. voltage (V) 50 Hz 110 115 120 130 135 140 150 160 170 180 190 200 210 230 250 270 300 340 380 60 Hz 110 120 130 140 145 150 160 170 180 190 200 210 220 240 260 280 320 360 400
P2
Melting period Refining period Shutdown
Time
Fig.1
Heat profile of an arc furnace
P-1/5
(2) Secondary voltage Secondary voltage is categorized into maximum voltage, minimum full-capacity voltage, and minimum voltage, all three selected by voltage taps. The maximum voltage is usually selected for the rated voltage, which provides a basis for all calculations. The profile of transformer operation under the rated voltage is shown in Fig. 100(%)
P = 3 ⋅U ⋅ I
Where,
X : Total reactance in furnace circuitry ( Ω ) I : Secondary current of transformer (A) U : Secondary voltage of transformer (V) P : Transformer capacity (kVA)
Designation of Transformer Ratings
A given production plant determines the capacity and specifications of the furnace, which in turn determines the furnace transformer capacity and the furnace current. The furnace transformer represents the core of the entire installation since it plays the important role of supplying energy to the AC arc furnace. Utmost care should be taken in determining its specifications. (1) Load cycle and transformer capacity As shown in Fig.1, scrap melting arc furnace operations are roughly divided into three periods: (1) melting, (2) refining, and (3) shutdown for furnace maintenance, and the furnace load differs according to these three periods. To select the most suitable transformer, the heat profile must first be clarified. Life of a transformer is generally governed by 6 principle; i.e. the service life of a transformer is reduced to half when the maximum winding temperature is raised 6 above allowable limit. Since the service life of a transformer is governed by temperature, the heat profile of an electric furnace should be precisely projected. Transformer capacity can be calculated from a given operation schedule. Assuming:Furnace capacity (scrap charge): M(t) Melting time: T(h) Specific rate of power consumption: (kWh/t) Transformer utilization factor: Mean power factor: PF Then, the required transformer capacity P is obtained by the equation:-
P-2/5
Minimum voltage
Minimum fullCapacity voltage
Secondary voltage
The %IX is usually required to exceed 40% in order to attain stabilized arcs. On the other hand, the %IX is obtained from the required arc length for furnace operation. Table 1 Transformer capacity and secondary voltages (for reference only)
Capacity Secondary current (A) Capacity (kVA) Secondary current
Maximum voltage
Constant capacity
Reduced capacity with Constant secondary current
Fig.2 Transformer output characteristics as viewed from secondary voltage The maximum voltage and the minimum full-capacity voltage are selected based on operating modes in the melting stage (long-arc mode, short-arc mode, and so on). The voltage in a reduced-capacity range and the minimum voltage are calculated based on power requirement during the refining period (or occasionally for drying the bricks). The maximum voltage and the minimum full-capacity voltage are determined based on furnace circuit constants – particularly the reactance. They are to be selected to meet furnace operations rather than being determined only by transformer capacity. For reference, the general relationship between the capacity and the secondary voltage of a transformer is represented in Table 1. Given the total reactance of the furnace circuitry (furnace transformer, bus bars, flexible cables, bus tubes, and electrodes), the rated secondary voltage is determined by the equations:-
Calculation example:-
X = 3.3(mΩ) P = 35,000(kVA)
The %IX at the operating voltage is assumed to be 45%. Then, the required voltage U for furnace operation is calculated by rewriting the foregoing equation as –