Discussion on the Prevention and Treatment of Corona Corrosion of High Voltage Motor Windings

The most direct difference between high-voltage motors and low-voltage motors lies in the manufacturing of motor windings. The insulation of high-voltage coils must first be of sufficient thickness, and secondly, anti-corona measures such as anti-corona paint and resistance band wrapping must be taken. The varnishing process is generally VPI vacuum pressure varnishing to ensure the safety and reliability of high-voltage motor coils during operation.

During the operation of high-voltage motors, the winding insulation is subjected to a high electric field strength. If the anti-corona layer on the insulation surface is in poor contact with the stator core, the coil will vibrate in the slot, the anti-corona layer on the insulation surface will be damaged, and severe corona discharge will occur in the worn parts, resulting in local instantaneous high temperature and burning of the coil insulation. In addition to the high temperature effect, corona discharge will also produce highly destructive chemical reactions, seriously corroding the insulation and coil wires. In order to avoid such problems, standardized motor manufacturers will strictly follow the operating specifications in the wire embedding process while controlling the manufacturing quality of high-voltage motor windings to ensure that the anti-corona layer of the winding is not damaged. At the same time, by adding flexible insulating materials, the gap between the coil and the core is reduced to enhance the immersion and curing effect.

What is corona discharge?

The formation mechanism of corona discharge varies with the polarity of the tip electrode, which is mainly caused by the different accumulation and distribution of space charge during corona discharge. Under the action of DC voltage, negative polarity corona or positive polarity corona accumulates space charge near the tip electrode.

In the negative polarity corona, after electrons cause collision ionization, the electrons are driven to the space far away from the tip electrode and form negative ions, while positive ions gather near the electrode surface. When the electric field continues to strengthen, positive ions are sucked into the electrode, and a pulse corona current appears, while negative ions diffuse into the gap space. After that, the next ionization and charged particle movement process is repeated. This cycle results in many pulsed corona currents.

The phenomenon of corona current was discovered by GW Tritchel in 1938 and is called Tritchel pulse. If the voltage continues to rise, the pulse frequency and amplitude of the corona current increase, and it turns into negative glow discharge. When the voltage rises further, negative streamer discharge appears, which is also called feather discharge or brush discharge because of its shape. When the negative streamer discharge continues to develop to the opposite electrode, it causes spark discharge, which causes the entire gap to break down. Positive polarity corona also has positive ions distributed near the tip electrode, but they are constantly pushed into the gap space, while electrons are sucked into the electrode, also forming a repetitive pulse corona current. When the voltage continues to rise, streamer discharge appears, which can cause gap breakdown.

The discharge process of power frequency AC corona in the positive and negative half cycles is basically the same as that of DC positive and negative corona. The power frequency corona current and voltage are in phase, reflecting the corona power loss. In engineering applications, the relationship between the applied voltage and the corona charge is often used to represent the corona characteristics, which is called the corona volt-cubic characteristic.