1. Introduction: With the development of power electronics technology, using high-pressure technology for mass resonant electrical equipment frequency voltage withstand test has become possible, it has been widely used in a power device having a large-capacity cable, capacitor, generators, etc. Exchange test. The principle is to adjust the air gap length of the core magnetic circuit to obtain a continuously varying inductance L to resonate with the capacitance C of the test object to the ground. In this paper, a 150kVA test device is used as a model to illustrate the principle of high-voltage resonant transformer and the calculation of related parameters.
2. Principle of resonant transformer
2.1 Structure: resonant transformer core can be made two different configurations: core and shell type. The core-type iron core transformer is not as good as the shell-type iron core transformer in a series of main indexes, and its weight and shape are large, and the transmission mechanism for adjusting the air gap is complicated. To this end, the test device we developed has a shell structure with the resonant transformer windings placed outside the movable center column.
2.2 characteristic curve: characteristic resonance transformer shown in Fig. It can be seen from Fig. 2 that the volt-ampere characteristics of the resonant transformer have a good linear relationship at different air gap lengths δ, and the inductance L is independent of the voltage value on the transformer. Therefore, when used in an AC resonance test, the resonant transformer can be tuned under low voltage conditions (by changing the length of the air gap between the moving iron core and the lower yoke core), and when tuning to resonance, the test is further increased. Voltage, system tuning is very convenient.
2.3 The relationship between the loop inductance L and the core air gap length δ: The air gap adjustable resonant transformer, whether it is series or parallel type, changes the inductance of the core by adjusting the length of the air gap of the core, so that the resonant transformer resonates. This is the mechanism by which the resonant transformer resonates by changing the length of the core air gap for a tester with a certain capacitance to ground. However, it should be noted that the length of the air gap should not be too large, and the excessive resonance will destroy the established resonance conditions.
2.4 Tuning principle: (1) Series tuning: The equivalent circuit of the series resonant transformer is shown in Figure 3. When a power frequency voltage of US=220V, f=50Hz is applied to the resonant transformer, by manual or automatic adjustment, when ωL=1/ωC, ie XL=XC, the series resonance occurs in the loop, where the loop current IS is max=Us/ (RL+RC) Because RC>>RL, there is Is≈US/Rc (1) The voltage UC on the test object and the voltage UL on the tuned reactor are: Uc=(1/ωc)Is=XcIs UL =ωLIs=XLIs When tuning to resonance Uc=UL=ω0LIs=(ω0L/Rc)Us (2) In the equation (2), the ratio ω0L/Rc=(the root of the L/C 2)/Rc=Q ( 3) ω0 is the resonant angle frequency Q is called the quality factor of the series resonant tank. Because (root number L / C 2nd power) >> RC, so Q>>1. Thus, the power supply capacity Ps=UsIs=(Uc/Q)Is=Pc/Q (4) It can be known from equation (4) that when the resonant transformer is tuned to resonance, the power supply voltage and capacity are the corresponding voltage and capacity of the test product. 1/Q. Therefore, compared with the general test transformer, the resonant transformer has the advantages of light weight and small volume. (2) The equivalent circuit of the parallel-tuned parallel resonant transformer is shown in Fig. 4. When RL≤ωL, Rc≤1/ωc, the resonant frequency fo of the parallel resonance is: the quality factor Q of the parallel loop is: Q=(ω0L)/(RL+Rc)=1(RL+Rc)ω0C (6) RL, RC - equivalent series resistance (Ω) of inductor and capacitor L - tuning reactor inductance (H) C - sample capacitance to ground (F) when applying 50Hz AC voltage to the parallel resonant transformer As the voltage increases, forced oscillations will occur in the loop. When the oscillation frequency of the loop is equal to the external power supply frequency, the impedance of the loop is the largest (and purely resistive), so the loop current is the smallest, but the currents IL and IC on L and C are Q times the loop current I, ie IL. =IC=QI.
The main parameters of the calculated resonant transformer 3
3.1 Calculation of inductance L: (1) Calculation of leakage inductance LS Ls=[(4πN 2 Ss×10 -9)/Ls](H) (7) where SS is the equivalent cross-sectional area of ​​leakage flux (cm2) lS——leakage flux equivalent length (cm)N——winding turns (2) calculation of main inductance LO L0=[(4πN 2 Ss×10 -9)/δ](H) (8) ——Air gap length (cm) Sδ——Equivalent cross-sectional area of ​​gap magnetic circuit (cm2) (3) Calculation of total inductance L L=Ls+L0=[[(4πN 2 Ss×10 -9)/Ls]( H) Ss × 10 -9) / Ls] + [(4πN 2 Ss × 10 -9) / δ] = [(4πN 2(Ss / Ls + Sδ / δ) × 10 -9 (H) (9)
3.2 Calculation of core size: (1) Stepped core external diameter D = K (number 4th power) (cm) (10) where S - core single column capacity (kVA) K - proportional coefficient, 4.5-5.5 (take a small value when the cold-rolled steel sheet selection) (2) effective core cross-sectional area SGSG = (Ï€D2 / 4) KyKd (11) where Ky = 0.9-- series core using core coefficients Kd = 0.93-- Laminated system
3.3 Calculation of winding turns (12) In the formula N1 - primary coil turns U1 - primary coil voltage (supply voltage), can take 220V or 380V, f - power frequency, 50Hz B - core flux density, (1.5 ~ 1.8) × 104Gs, the secondary coil turns N2 is the same as above, only U1 in the formula can be replaced by the secondary coil voltage.
3.4 Calculation of minimum air gap δmin and maximum air gap δmax: (1) Calculation of δmin: KL in the formula – inductance adjustment coefficient, 6.5~7.0(2) δmax calculation δmax≈KδKLKδmin (14) where Kδ=2.2～ 2.5, other parameters of the resonant transformer are similar to the calculation of ordinary reactors.
4.4 150kVA transformer design resonance test
Using the above calculation formula, a power supply voltage U1=0.22kV, output voltage U2=15kV, output power P2=150kVA, and a resonant transformer with a power frequency high voltage test for a sample with a maximum calculation capacitance of 2μF can be designed. The first and second windings of the transformer moving iron core and the outer casing are shown in Fig. 5. The calculation results of the main parameters are as follows: D=12cm, the diameter of the moving iron core, D1=13.5cm, the inner diameter of the primary winding, D2=18cm , secondary winding inner diameter, D3=25.5cm, secondary winding outer diameter, H=37cm, winding height, N1=66åŒ, primary winding turns, N2=4464åŒ, secondary winding turns, resonant transformer tuning inductance parameters The calculated and measured values ​​are shown in Table 1.
Table 1 Calculated and measured values ​​of tuned inductance parameters δ(cm) 1 2 3 4 5 6 7 8 L Calculated value (H) 31.61 18.96 14.47 12.64 11.37 10.53 9.92 9.47 L measured value (H) 29.93 18.12 15.3 13.2 11.8 10.7 10.3 8.95 Error (%) 5.6 4.6 -5.4 -4.3 -3.6 -1.6 -3.7 5.8
5 Conclusion
(1) It can be seen from Table 1 that the maximum error between the calculated value and the measured value does not exceed 6%, indicating that the above calculation formula has high precision, which is sufficient to meet the requirements of engineering calculation. (2) When the test is close to the maximum capacitance calculation value of the sample, the voltage on the sample may exceed the value determined by the transformation ratio. In order to reduce the effect of voltage resonance, the leakage resistance of the secondary coil of the transformer should be as small as possible. At the same time, an overvoltage protection device should be installed in the output circuit. (3) Since the voltage is independent of the air gap δ, it should be tuned at a lower voltage first. When the resonance occurs, the output voltage is raised to the test value of the sample. (4) During the air gap adjustment process, the core and mechanical transmission mechanism of the transformer are subjected to a large electromagnetic force, resulting in strong vibration and noise, and in severe cases, the components of the resonant transformer may be damaged. Therefore the mechanical structure of such a device should be specially designed.External Rotor Motor,Outer Rotor Motor,External Rotor,External Rotor Fan
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