Under normal power frequency voltage, especially during prolonged high-temperature, high-humidity weather or pollution, flashover may occur on HTV silicone rubber composite insulators. The discharge mechanism of hydrophobic composite insulators differs from that of hydrophilic porcelain and glass insulators. When the creepage distance is the same and the hydrophobicity is good, the effective creepage distance of composite insulators is greater than that of porcelain and glass insulators. When the hydrophobicity of the composite insulator drops to HC5 or above, its effective creepage distance can become smaller than that of porcelain and glass insulators. The deterioration of HTV silicone rubber sheds poses a risk of flashover in composite insulators after several years of operation.

The hydrophobicity of HTV silicone rubber can reduce the humidity effect on composite insulators by 10% to 30%, and under operating frequency, the pollution withstand voltage decreases by 30% to 40%. A significant drop in power frequency strength is also one of the causes of flashover. If the hydrophobicity is partially or completely lost, and the shed diameter is small or the shed pitch is narrow, short arcing across the sheds can easily occur under high-humidity atmospheric conditions, reducing the electrical strength. This is one of the main causes of flashover in composite insulators and should draw sufficient attention from manufacturers and operating departments.

For 220 kV lines, when no high-voltage equalizing device is installed at the end of the composite insulator, or when equalizing devices are installed separately on the high-voltage and low-voltage sides, the potential distribution on the sheds at the high-voltage side accounts for 17.2%, 12.4%, and 10.1% of the operating voltage, respectively. Installing an equalizing device significantly reduces the voltage at the attachment point on the high-voltage side, greatly improving the power frequency electrical performance.

After installing ring-shaped equalizing devices with protection layer thicknesses of 40 mm and 75 mm, the potential distribution on the second shed at the high-voltage side accounts for 21.3% and 12.2% of the operating voltage, respectively. The potential distribution curve of the 75 mm device is more uniform than that of the 40 mm device. The depth of the equalizing device has a greater influence on the impulse potential distribution. When an equalizing device is installed, the potential distribution at the high-voltage side accounts for 19% of the operating voltage, which is 1.8% higher than when no equalizing device is installed. This indicates that proper installation of equalizing devices has a significant impact on the electrical performance of composite insulators.

The shed shape is optimized based on aerodynamic principles and external insulation requirements. The large sheds have a diameter of 200–250 mm, and the spacing between large sheds is 130–150 mm. This improves the effective creepage distance per shed and greatly reduces the power frequency flashover voltage of silicone rubber insulators under high-humidity conditions during normal operation. That is, under high-humidity conditions, the hydrophobicity of the composite insulator drops to a level comparable to that of porcelain and glass insulators. This is equivalent to the operation of XP-160 disc-type suspension porcelain and glass insulators.