Electrical Partial Discharge (PD)
Electrical partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors. While a corona discharge is usually revealed by a relatively steady glow or brush discharge in air, partial discharges within a solid insulation system are invisible.
PD can occur in a gaseous, liquid, or solid insulating medium. It often starts within gas voids, such as voids in solid epoxy insulation or bubbles in transformer oil. Protracted partial discharge can erode solid insulation and eventually lead to a breakdown of insulation.
PD causes progressive deterioration of insulating materials, ultimately leading to electrical breakdown. The effects of PD within high voltage cables and equipment can be very serious, ultimately leading to complete failure. The cumulative effect of partial discharges within solid dielectrics is the formation of numerous branching, partially conducting discharge channels, a process called treeing. Repetitive discharge events cause irreversible mechanical and chemical deterioration of the insulating material. Damage is caused by the energy dissipated by high-energy electrons or ions, ultraviolet light from the discharges, ozone attacking the void walls, and cracking as the chemical breakdown processes liberate gasses at high pressure. The chemical transformation of the dielectric also tends to increase the electrical conductivity of the dielectric material surrounding the voids. This increases the electrical stress in the (thus far) unaffected gap region, accelerating the breakdown process. Several inorganic dielectrics, including glass, porcelain, and mica, are significantly more resistant to PD damage than organic and polymer dielectrics.
Testing PD in the field
Field measurements preclude using a Faraday cage, and the energizing supply can also be a compromise from the ideal. Therefore, Field measurements are prone to noise and may be less sensitive.
Factory-quality PD tests in the field require equipment that may not be readily available. Therefore, other methods have been developed for field measurement, which, while not as sensitive or accurate as standardized measurements, are substantially more convenient. By necessity, field measurements have to be quick, safe, and simple if they are to be widely applied by owners and operators of MV and HV assets.
1) Ultrasound measurement relies on the fact that the partial discharge will emit sound waves. The frequency for emissions is "white" noise in nature, producing ultrasonic structure waves through the solid or liquid-filled electrical component. Using a structure-borne ultrasonic sensor on the exterior of the item under examination, internal partial discharge can be detected and located when the sensor is closest to the source. Very cost-effective method for early detection of PD
2) Transient Earth Voltages (TEVs) are induced voltage spikes on the surface of the surrounding metalwork. TEVs occur because the partial discharge creates current spikes in the conductor and, hence, in the earthed metal surrounding the conductor. TEVs are a very convenient phenomenon for measuring and detecting partial discharges as they can be detected without making an electrical connection or removing any panels. While this method may help detect some issues in switchgear and surface tracking on internal components, the sensitivity is not likely to be sufficient to detect issues within solid dielectric cable systems.
3) HFCT Method This method is ideal for detecting and determining the severity of the PD by burst interval measurement. The closer the bursts get to "zero voltage crossing," the more severe and critical the PD fault is. The location of the fault area is accomplished using the AE described above.
4) Electro Magnetic Field detection picks up the radio waves generated by the partial discharge. As noted before, the radio waves can generate TEVs in the surrounding metalwork. More sensitive measurement, particularly at higher voltages, can be achieved using built UHF antennas or external antennas mounted on insulating spacers in the surrounding metalwork.
5) Directional Coupler detection picks up the signals from a partial discharge. This method is ideal for joints and accessories, with the sensors located on the Semicon layers at the joint or accessory.
