Winding impulse voltage distribution based on transformer scale ratio model

As the core equipment of the power grid, the power transformer is often required to withstand the impact voltages such as lightning and operation in actual operation. Calculating and measuring the winding voltage distribution of power transformer under the impact voltage has important practical significance and engineering application value for designing and manufacturing transformer insulation structure. However, there is no measuring tap on the actual running power transformer winding, and it is difficult to measure the voltage distribution directly on the winding of the actual transformer. Therefore, by designing a transformer scale model with similar electrical characteristics and measuring taps, the measurement of winding voltage distribution is carried out on the basis of this, and the results can not only be effectively reversed to the prototype transformer, but also the winding voltage distribution under the impact voltage. The mechanism analysis provides the underlying data.

Transformer reduction ratio criterion

The transformer scale model is different from the simple scale model and needs to determine the shrinkage relationship of each physical quantity. The key to establishing the scale-down criterion is the selection of the scale factor. In principle, the scale-down model is the same as the original model. The dielectric constant, electrical conductivity, magnetic permeability and electrical resistivity ρ of the shrinkage ratio model are consistent with the original model, that is, the reduction ratio coefficients of these physical quantities are all 1. At the same time, it is assumed that the electric field strength E remains unchanged, that is, the reduction ratio coefficient kE is 1. According to the constitutive relationship between the current density J and the electric field strength E, kJ=kE=1 is obtained. It is stated that the winding current density of the scaled model and the original model remains unchanged. When the scaled model is produced, the length, the width, and the radius are reduced in proportion to the reduction factor k, so that the scale factor of the two-dimensional parameters such as the area S of the rectangle and the circle is k2. The scaling factor kt of time t is 1. The scaling factor of parameters such as voltage and current can be determined by the relationship between physical quantities.

Transformer reduction ratio model design

The design of the transformer scale model mainly includes the design of transformer core structure, the design of transformer winding turns and the design of winding tap structure. The original model capacity of the power transformer used in this paper is 80 MVA, and the shrinkage model capacity is 10 kVA. According to the reduction ratio criterion, the final designed transformer scale model core is made of silicon steel sheet laminated, and is a mouth-shaped structure, iron. The upper and lower columns of the core are square columns, and the left and right columns are approximately cylindrical. The winding is located outside the core, the core diameter is 105 mm, the transformer high and low voltage windings are located outside the diameter of 116 mm, and the middle is epoxy cloth and epoxy tube. The transformer high-voltage winding has a total of 960 匝, divided into two parts, each part 480 匝, located in the left and right two cylinders. The windings are wound as a layer every 48 turns and are continuously wound around the core. And each 48 匝 leads a tap with a total of 20 taps. The low-voltage winding of the transformer has a total of 176 turns, which are evenly distributed on the two cylinders on the left and right sides of the core.

Electromagnetic field distribution calculation of transformer scale model

In order to compare the electric field distribution and magnetic field distribution of the original transformer and the scaled model, the electric field distribution and magnetic field distribution of the original model and the scaled model are simulated by the similarity principle. The original model and the scaled model apply similar boundary conditions. When the electric field and the magnetic field act together on the transformer, the electromagnetic field distribution results of the original model and the scaled model of the transformer show that the potential distribution and flux density distribution of the scaled model are consistent with the original model.

Based on the transformer scale model, a test platform is built to measure the surge voltage distribution of the transformer high voltage winding. Based on the transformer scale-down model impulse voltage distribution test platform, the pulse width is 1 400 ns generated by the nanosecond pulse generator, and the voltages of different voltage amplitudes are respectively applied to the high voltage side of the winding, the low voltage winding connected in series, and the low voltage winding connected in parallel. The winding voltage distribution under different applied voltages is obtained by measuring the voltage waveforms of the taps of the high voltage windings. It can be seen from the test results that the voltage distribution of the winding under the impact voltage is extremely uneven, the voltage distribution trend under different voltages is basically the same, and the voltage at the head end of the high voltage winding is relatively large. Mainly because of the existence of stray capacitance between the windings, the winding voltage distribution is extremely uneven, and the shunting action makes the voltage distribution of the high voltage winding extremely uneven and the high voltage winding head end is subjected to a large voltage.

in conclusion

1) Considering the same skin depth, the frequency of the reduction model is 400 times that of the original model, and the magnetic flux density B is 1/20 of the original model.

2) Based on the design of the transformer scale model, the design parameters are designed according to the requirements of the scale reduction criterion. The original model and the scale model of the transformer are simulated by electromagnetic field, and the correctness of the scale reduction criterion is verified.

3) When the surge voltage is applied, the winding voltage distribution is extremely uneven, and the voltage at the head end is large. The shrinkage criterion proposed in this paper can be applied in the test of measuring the transformer winding voltage distribution.