Causes and prevention of corona at the end of the stator winding end of high voltage motor
Electric field characteristics and anti-corona measures at the motor slot
The electric field distribution at the motor slot is very similar to the voltage distribution on the transformer winding at high frequencies (overvoltage). When the notch is not treated with anti-halation treatment, the leakage charge at the end is mostly concentrated on the capacitance near the slot, so the electric field at that place is concentrated, which is easy to generate corona. The constant resistivity or non-linear resistivity antihalation layer and the inner screen antihalation layer can make the electric field at the notch uniform and prevent the end corona.
When the slot is treated with a semiconductor resistor lacquer (or strip), the voltage distribution is related to its surface resistivity ρs. The larger ρs is, the steeper the voltage distribution is, the first end is halo; the ρs is too low, the outer side of the antihalation layer is First dizzy. The upper limit of ρs is generally 5 × 109 ~ 1010 ohms. The lower limit of ρs is generally (1~5)×107 ohms.
The electric field distribution of the primary constant resistivity antihalation layer is still not uniform enough. If the resistivity is decreased toward the core, a more uniform electric field distribution can be obtained. For high voltage motors with higher rated voltages, a secondary (or multi-level) resistivity anti-halation layer can be used. Generally, the high resistivity ρs2 is higher than the medium resistivity ρs1 by about the square.
When the end insulation surface adopts the silicon carbide antihalation layer, it can automatically adjust the resistivity with the applied field strength to make the voltage distribution uniform. When the inner screen antihalation layer is used, when the coil insulation is packaged to a certain number of layers, a semiconductor layer is coated, and the first semiconductor layer is the longest, and is gradually shortened later. This makes it possible to form a semiconductor shield similar to a capacitive bushing. This method is very stable and reliable for improving the distribution of electric fields. However, the structural process is complicated and the cost is high, and it is only used on a high-voltage generator with a particularly large capacity.
The anti-corona paint used for the epoxy powder mica insulation coil is generally prepared by adding an appropriate amount of conductive material such as carbon black, graphite or silicon carbide to the epoxy polyester paint. The conductive material in the paint is easy to precipitate and must be stirred evenly during use.
Semiconductor lacquers are generally hand-coated with a brush, according to the anti-corona structure and requirements of the coil. When the main insulation has been solidified, if a first-order constant-resistance anti-halation structure is used, the straight-line part is first brushed with a low-resistance semiconductor lacquer, and then a layer of an alkali-free glass cloth tape or an asbestos tape is applied. After wrapping, apply a layer of low-resistance semiconductor lacquer to infiltrate the glass tape or asbestos tape and dry at room temperature. The end is brushed in the middle of the semiconductor lacquer, the two semiconductor lacquers must be lapped according to the regulations, then a layer of glass cloth tape is half-stacked, and then the semiconductor lacquer is applied twice, and the special electrode is used to check the coil after drying. Surface resistance.
When the primary anti-halation process is used, after the main insulating layer is wrapped, a low-resistance semiconductor strip is half-stacked in the straight portion, and then the semiconductor strip is blocked in the end half-packed two layers, and the two semiconductor strips are specified. Lap is good. After the semiconductor tape is wrapped, a layer of polyester film is wrapped and then hot pressed.
When using the internal screen anti-halation structure, it must be strictly guaranteed that its position and ruler are in compliance with the regulations.
Prevention of corona between different phases and between fixtures
The end bevel gap should ensure that corona does not occur under the working voltage, and a corona-resistant compliant material can be used to fill the gap between the fixing members. The conformable material can be cured at room temperature or high temperature, and the high temperature curing glue is used, and it needs to be baked and heat cured.