Discharge at the End Windings of High-Voltage Motors and Anti-Corona Treatment

I. Causes and Hazards of End Winding Discharge

The end windings of high-voltage motor stator coils are prone to partial discharge due to highly non-uniform electric field distribution. When the electric field strength exceeds the breakdown strength of air (approximately 3 kV/mm), corona discharge occurs, characterized by blue fluorescence and the generation of ozone and nitrogen oxides. The primary causes include:

1. Electric Field Concentration: The highest field strength occurs at the slot exit. A single coil can produce corona at 4 kV, sliding discharge at 20 kV, and flashover at 40 kV.
2. Insulation Defects: Manufacturing or operational flaws such as voids, delamination, or burrs exacerbate electric field distortion.
3. Environmental Factors: A 10% increase in humidity reduces the corona inception voltage by 10%, while contaminants (e.g., dust, oil) degrade gas insulation performance.

Hazards:

• Thermal effects cause carbonization of insulating materials (e.g., adhesives, mica), leading to whitening, loosening, or short circuits of strand insulation.
• Electromagnetic vibrations induce spark discharge in slot gaps, eroding insulation surfaces.
• Prolonged operation allows tracking discharge to penetrate the main insulation, resulting in breakdown.

II. Fundamental Principles of Anti-Corona Treatment

The core of anti-corona technology lies in electric field uniformization to prevent gas ionization, achieved through:

1. Resistivity Gradient Design:
   • The resistivity of the anti-corona layer increases gradually from the slot exit to the end winding, ensuring linear voltage decay and avoiding abrupt field strength changes.
   • Examples include three-stage transitions using low-resistance (10³–10⁵ Ω), medium-resistance (10⁹–10¹¹ Ω), and high-resistance semiconductor paints, or nonlinear resistivity characteristics of silicon carbide (lower resistivity under higher field strength).
2. Capacitive Voltage Division:
   • Internal shielding structures insert electrodes within the coil insulation, forming a bushing-type configuration for capacitive voltage division.
   • Suitable for motors above 24 kV but involves complex processes and higher costs.

III. Mainstream Anti-Corona Technologies

Anti-corona treatments are categorized based on voltage levels and applications:

Typical Process Flow (Brushed-Wrapped Type):

1. Apply low-resistance semiconductor paint (e.g., 5150 epoxy resin paint) to the straight portion, extending 25 mm beyond each side of the iron core.
2. Apply high-resistance semiconductor paint (e.g., 5145 alkyd paint) over a 200–250 mm range from the slot exit to the end winding, overlapping 10–15 mm with the low-resistance paint.
3. Wrap with 0.1 mm-thick dewaxed glass tape in a half-lap pattern.
4. Apply additional low- and high-resistance semiconductor paints over the glass tape for multi-stage protection.

IV. Key Parameter Control in Anti-Corona Treatment

1. Resistivity Selection:
   • The surface resistivity (ρs) of the anti-corona layer must match the voltage distribution: excessive ρs causes steep voltage gradients and premature corona at the starting end, while insufficient ρs leads to corona at the trailing end.
   • Recommended range: 5×10⁹–10¹⁰ Ω (single-stage), ≤10⁵ Ω (low-resistance section), ≥10⁹ Ω (high-resistance section).
2. Environmental Adaptability:
   • Corona inception voltage decreases by 1% per 100 m increase in altitude, necessitating parameter adjustments for high-altitude applications.
   • Motors operating in harsh environments (e.g., high humidity, pollution) may require anti-corona treatment even at 3 kV.
3. Process Quality Control:
   • Paint films must be uniform, continuous, and smooth with strong adhesion to avoid field concentration due to uneven thickness.
   • Semiconductor paint drying temperatures (e.g., 180–220°C for dewaxing) must be strictly controlled to prevent performance degradation.

V. Technological Trends

1. Nonlinear Resistive Materials: Silicon carbide anti-corona layers dominate due to their self-adjusting resistivity, significantly enhancing performance.
2. Nanocomposite Materials: Research focuses on incorporating nanoparticles (e.g., SiO₂, TiO₂) into anti-corona paints to improve corona resistance and mechanical strength.
3. Smart Monitoring: Integration with online partial discharge monitoring enables real-time assessment of anti-corona layer conditions for predictive maintenance.