Partial discharges: understand what they are, how they are formed, and what causes them. A partial electrical discharge is one that only partially breaks the insulation between conductors. It is emphasized, then, that PDs do not travel the entire path between the conductors.
The ionization processes in a gaseous environment within dielectric materials, caused by an intense and localized electric field, result in micro-electrical discharges in areas where the electric field intensity is higher or where the dielectric strength is lower.
Various physical phenomena are generated in this specific location, such as:
- Conducted and radiated electromagnetic pulses;
- Luminosity;
- Acoustic noise;
- Temperature increase;
- Chemical reactions.
Partial discharges can be classified into three types:
1. Surface discharges:
Occur in gases or liquids on the surface of a dielectric material from the electrode to the surface. This type of discharge usually begins when the tangential component of the electric field on the surface exceeds the insulation threshold. They can be responsible for causing surface tracking phenomena, leading to material breakdown;
2. External discharges:
Usually occur at tip-type electrodes and cause harmful chemical processes to the insulation;
3. Internal discharges:
Occur in cavities or voids in solid dielectrics, causing strong disturbances in the insulation.
The main physical manifestations and measurement methodologies can be summarized as follows:
- Electric pulses detected through high-voltage capacitors. The capacitor has a high impedance for the voltage at industrial frequency but a low impedance for high-frequency PD voltage pulses. The capacitor's output is voltage pulses that can be measured with an oscilloscope, spectrum analyzer, or pulse intensity analyzer.
- Radio frequency (RF) pulses. Through an RF directional antenna, the environment of PD activity inside an electrical machine can be located; PDs have RF frequencies between 100 kHz and hundreds of megahertz.
- Acoustic pulses. Caused by a small “shockwave” resulting from the rapid increase in gas temperature in its immediate vicinity. This small shockwave consequently creates acoustic noise. When many PD pulses are occurring on the surface of transformer coils, there is a detectable sound. Therefore, directional microphones can be used to measure the sound level of the discharge, as well as to locate where surface PD may be occurring.
- Light and chemical reactions in cooling gases. These can be air or hydrogen. As the formation and concentration of ozone occur with substantial surface PD activity, there are several ways to measure ozone concentration, including electronic sensors. It is worth noting that the concentration is affected by temperature, ambient humidity, and airflow, and may also be related to machine load and power factor.
- Power factor slope, also known as delta tangent. In each PD event, the insulation absorbs a certain amount of energy, which is dissipated or supplied by some source, leading to an increase in dielectric loss in the transformer winding.
Therefore, an indirect means of measuring the total discharge activity in a coil or winding is to measure the dissipation factor or insulation power factor at low voltage, below the PD initiation voltage, and at high voltage, when the presence of any PD will increase dielectric losses. A steep slope of the power factor or dissipation factor may indicate severe PD activity in the winding.
Thus, measuring and quantifying the number of discharge occurrences is of great importance as they are a continuous source of insulation material deterioration, altering its dielectric properties. Depending on the intensity of the partial discharges, the material's useful life may be drastically reduced.