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I. Principle of Scaling

The ratio-reduced model of power transformer is different from similar scale model. It is necessary to determine the ratio relation of all physical quantities. The key to determining the ratio principle lies in the selection of ratio-scaling coefficients. The dielectric constant, conductivity, permeability and resistivity are consistent with the original model, namely, ratio-reduced coefficients of all physical quantities are 1.

2. Design of Ratio-reduced Model of Power Transformer

The design of power transformer’s ratio-reduced model is mainly composed of iron core structure, winding number and tap structure. The capacity of original model of power transformer used here is 80MVA while that of ratio-reduced model is 10kVA. According to the scale norms, the iron core of final transformer’s ratio-reduced model is made of laminated silicon steel sheets and rectangle-shaped structure; the upper and lower columns are square while right and left ones are round. The windings are outside the iron core, the diameter of which is 105mm. HV/LV windings are 116mm away from the diameter, and epoxy cloth and tubes are in the middle. There are 960 turns of HV windings in all, which are equally divided into two parts and at right and left round columns. 48 turns of windings are taken as one layer and around the iron core. Moreover, 48 turns lead out a tap and there are 20 taps altogether. There are 176 turns of LV windings, evenly distributed at right and left round columns.

3. Calculation of Electromagnetic Distribution of Ratio-reduced Model

To compare electric field and magnetic distribution of original transformer with that of ratio-reduced model, infinite element simulation calculation is made on electromagnetic distribution of both according to the principle of similarity and similar boundary conditions are also applied. When the electric field and magnetic field jointly act on the transformers, distribution results indicate that the distribution of potential and magnetic flux of ratio-reduced model is consistent with that of original model.

4. Distribution of Winding Impulse Voltage of Ratio-reduced Model

Based on the ratio-reduced model of power transformer, a test platform is established to measure the impulse voltage distribution of HV windings. The voltages with different amplitudes are respectively applied on the HV side of winding, serial LV winding and parallel-connected LV winding. The winding voltage distribution at different applied voltages is achieved by measuring voltage waveforms of HV winding taps. Test results are given as follows: under the influence of impulse voltage, the winding voltage is unevenly distributed; the distribution trend of voltage is basically the same; the voltage bore by HV winding head terminal is higher.

5. Conclusions

Given that the skin depth is the same, the frequency of ratio-reduced model is 400 times the original model and the flux density B is 1/20 of original model. When the impulse voltage is applied, the distribution of winding voltage is uneven and the voltage borne by the head end is very high. The principle of scaling proposed here can be applied to measure the voltage distribution of transformer winding.

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