Partial discharge is a phenomenon where local and repetitive breakdown and extinction occur within the insulation of electrical equipment under certain applied voltages due to weaknesses existing within the insulation. As local partial discharge occurs within the insulation, it will be accompanied by phenomena such as light, heat, noise, electrical pulses, increased dielectric loss, and electromagnetic wave radiation. This discharge can occur in voids within solid insulation, bubbles in liquid insulation, between different dielectric properties of insulation layers, or at the edge sharp corners of metal surfaces. Therefore, classified by discharge type, it can be roughly divided into internal discharge within the insulation material, surface discharge, and corona discharge.
Internal discharge
In the insulation system of electrical equipment, the electric field intensities at different parts are often unequal. When the electric field intensity in a local area reaches the breakdown field strength of the dielectric, discharge will occur in that area. However, this discharge does not extend between the two conductors to which the voltage is applied, meaning that the entire insulation system has not been broken down and still maintains its insulation performance. The discharge that occurs within the insulating material is called internal partial discharge.
When local discharge occurs within the insulating medium, even if the externally applied voltage is lower than the initial voltage, the discharge can still continue. In theory, this voltage can be half of the initial voltage, that is, the voltage between the two ends of the insulating medium is only half of the initial voltage. The voltage maintained until the discharge disappears is called the discharge extinction voltage. However, the actual situation differs from the theoretical analysis. In solid insulators, the extinction voltage is approximately 5% to 20% lower than the initial voltage.
In oil-impregnated paper insulation, due to the rapid formation of bubbles caused by partial discharge, the extinguishing voltage is much lower. This also indicates that in certain situations, if electrical equipment has local defects but is operating normally, the partial discharge quantity is relatively small, meaning that the operating voltage is not sufficient to trigger a discharge with a large amount of energy. When there is a transient overvoltage disturbance in the system, it triggers a large amplitude local discharge and if the discharge continues after the overvoltage disappears, it will eventually lead to accelerated deterioration and damage of the insulation.
Surface discharge
If there is a component of electric field parallel to the surface in the insulating medium of an electric field, and when this component reaches the breakdown electric field strength, then surface discharge may occur. This situation may occur at the flange of the bushing, at the cable terminal, or at the surface of the angle between the conductor and the insulator. At the edge between the inner medium and the electrode, there is a component of the electric field parallel to the surface of the medium at point r, and when the electric field is strong enough, surface discharge occurs. In some cases, the starting discharge voltage in the air can be calculated.
The waveform of surface partial discharge is related to the shape of the electrode. If the electrode is asymmetrical, the amplitudes of partial discharge in the positive and negative half cycles are not equal. When the electrode that generates surface discharge is at a high potential, the discharge pulses that occur in the negative half cycle are larger and sparser, while those in the positive half cycle are denser but have smaller amplitudes. At this time, if the high-voltage end and the low-voltage end are reversed, the discharge pattern will also be opposite.
Corona discharge
Corona discharge occurs when the electric field near the surface of a conductor reaches the breakdown field strength of the gas under extremely uneven electric field conditions. At the edges of high-voltage electrodes, and around the tips, corona discharge may occur due to the concentration of electric fields. Corona discharge is more likely to occur in negative polarity, that is, in alternating current, they may only appear in the negative half-cycle. Corona discharge is a self-sustaining discharge form. When corona occurs, a large amount of space charge appears near the electrode, and streamer discharge forms near the electrode. As an example, let’s consider a rod-plate electrode. In the case of negative corona, if positive ions appear near the rod electrode, the electric field attracts and moves the ions towards the negative electrode, releasing a large number of electrons and forming a positive ion cloud near the tip. Negative electrons move towards the positive electrode, and then the ion region expands. A relatively concentrated positive space charge appears near the rod electrode, while the negative space charge near the electrode is more dispersed. This positive space charge distorts the electric field. Therefore, the electric field near the negative rod becomes stronger, making it easier to form.
Under an alternating voltage, when there is a sharp tip at the high-voltage electrode and the electric field intensity is concentrated, corona discharge usually occurs in the negative half-cycle. Or when the grounded electrode also has a sharp point, there will be a discharge with a larger amplitude in the negative half-cycle and a smaller amplitude in the positive half-cycle.
Post time: Jul-16-2026