1. Compensation of electrostatic ring

In order to thicken the upper insulating layer, it is suggested to add a certain thickness of conformal cardboard strip to the upper insulating layer, and then wrap the insulating tape to realize the insulating layer of the electrostatic ring with T1 > T2. In the figure, h is the distance from the electrostatic ring to the pressing plate, and M is the distance between the windings. For the thickness t of the insulation layer, the metal part of the electrostatic ring should not be affected, that is, the shielding effect on the winding wires should not be reduced; Moreover, reducing the metal part of the electrostatic ring also has a certain influence on the axial short-circuit force.

2. "Oil wedge" at the end

If an electrostatic ring is set at the end of the high voltage winding, an "oil wedge" will be formed between the pad and the insulating layer of the electrostatic ring. For "oil wedge", the dielectric coefficient Z of insulating pad is greater than that of transformer oil y, that is Z > y, so the field strength of oil is greater than that of pad. From the perspective of polarization, the charge is concentrated at the sharp corner of the "oil wedge", where the electric field strength is high. When the electric field strength reaches the local electric field strength, the partial discharge begins at the sharp corner, so the discharge source is formed there. Partial discharge causes irrecoverable burning loss on insulation surface; At the same time, when the arrangement of the corner ring is not consistent with the distribution of the electric field, it may develop into surface discharge, so the "oil wedge" should be the weakness of the insulation structure.

When the applied voltage increases, the voltage UG in the "oil wedge" also increases. When UG reaches the "oil wedge" breakdown voltage UGB, partial discharge begins. At this time, the applied voltage is the initial discharge voltage UPD, where p is the distance between the insulating layer and the head end coil; G is the insulation distance of oil wedge; T2 is the dielectric coefficient of the lower insulating layer of the electrostatic ring; G is the dielectric coefficient of oil wedge.

As for the oil paper insulation treated by normal process, if the oil contains impurities and moisture, the initial voltage of partial discharge will be significantly reduced.

Therefore, measures must be taken in the end insulation structure, such as increasing the curvature radius of the electrode, the thickness of the insulation layer, and reducing the width of the pad. The latter is the most simple measure, that is, reducing the pad 1 in the figure to the point line, which has no effect on the compression of the winding, and can eliminate the "oil wedge", improve the electric field distribution, so as to increase the partial discharge voltage.

3. Design of electrostatic ring

One of the functions of the electrostatic ring at the end of the high voltage winding is to improve the distribution of lightning impulse voltage of its adjacent segments; Secondly, in order to make the electric field at the end of the winding uniform, it is very important to design the shape of the electrostatic ring reasonably.

The design of the shape of the electrostatic ring is mainly to determine the curvature radius of the electrode and the thickness t of the insulating layer. The design principle is: the determined and t must meet the electric field intensity on the metal surface and its insulating layer of the electrostatic ring, which is less than the allowable electric field intensity of the two places.

At present, the electrostatic ring is made of soft copper tape or metal braided tape with thickness of 0.5mm, which is half lapped on the paper ring, and its surface is then wound with insulating layer. After vacuum treatment, there may be small gap between the metal surface and the insulating layer, and when the conformal paper strip is used, there may be small gap at the joint. If the minimum breakdown field strength EBMin is considered as 1mm, then the allowable field strength e0x = EBMin / K, Where k is the safety factor, generally 1.40-1.45

For the end insulation structure of oil separator, when the corner ring and anti corner ring are used to separate the end into a small oil gap, the electrical strength of the most heavy oil gap will determine the electrical strength of the whole structure. Because the electric field strength appears in the oil gap on the surface of the insulating layer of the electrostatic ring, the electric strength of the end insulation is usually determined by the oil gap between the electrostatic ring and its adjacent corner ring. The size of the oil gap should be determined according to the direction of the power line. According to the size of the oil gap, the minimum breakdown field strength EBMin in [3] can also be obtained, and the allowable field strength e1x = EBMin / K can be obtained

The increase of T and t can decrease the field strength, but the good results can be obtained only when the ratio of the two is appropriate. According to the experimental results of the main insulation structure of 220kV transformer, the effect of increasing t is obvious when t is small; On the contrary, the effect of increasing t is not obvious; Similarly, when t is small, the effect of increasing is more significant; When t is large, the increase effect is not obvious.

On the surface of the insulation layer of the electrostatic ring of the high voltage winding, the electric field intensity Emax decreases with the increase of T, and the relative reduction amplitude also decreases with the increase of T, and is affected by the influence of T. when t increases, the relative reduction amplitude becomes smaller, but when t is small, the effect of increasing t is significant, and vice versa.

When selecting the size of T, we must pay attention to the shielding effect of the electrostatic ring on the line segment. At this time, t is equivalent to D in the formula and should be smaller. As t increases, the capacitance of the electrostatic ring to the wire box decreases, thus reducing the lightning impulse protection effect on the line segment. In order to increase the insulation thickness of the upper insulation layer of the electrostatic ring of the EHV power transformer, the conformal insulation paper pad is applied, so that the thickness of the lower insulation layer decreases naturally and the initial distribution is improved.

4. Conclusion

a. In order to give full play to the compensation function of the electrostatic ring, the thickness of the insulating layer should be as small as possible, such as using the upper insulating layer pad with conformal insulating paper strip pad, and then wrapping with paper tape; b. In order to eliminate the "oil wedge" at the end, the simplest measure is to shorten the width of the cushion block appropriately; c. When designing the shape of the electrostatic ring, it is mainly to determine and t. increasing and t can reduce the surface electric field strength of the insulation layer. When t is small, increasing t has obvious effect; When it is larger, the effect of increasing t is not obvious. For 220kV transformer winding, the field strength on the surface of insulation layer can be effectively reduced when t is 10-15mm when the process conditions permit.