In electrical engineering, 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 solid insulation system are not visible.
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 breakdown of insulation.
PD usually begins within voids, cracks, or inclusions within a solid dielectric, at conductor-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid dielectrics. Since discharges are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials.
Partial discharges within an insulating material are usually initiated within gas-filled voids within the dielectric. Because the dielectric constant of the void is considerably less than the surrounding dielectric, the electric field across the void is significantly higher than across an equivalent distance of dielectric. If the voltage stress across the void is increased above the corona inception voltage (CIV) for the gas within the void, then PD activity will start within the void.
PD can also occur along the surface of solid insulating materials if the surface tangential electric field is high enough to cause a breakdown along the insulator surface. This phenomenon commonly manifests itself on overhead line insulators, particularly on contaminated insulators during days of high humidity. Overhead line insulators use air as their insulation medium. Partial discharge equivalent circuit
The equivalent circuit of a dielectric incorporating a cavity can be modeled as a capacitive voltage divider in parallel with another capacitor. The upper capacitor of the divider represents the parallel combination of the capacitances in series with the void and the lower capacitor represents the capacitance of the void. The parallel capacitor represents the remaining unvoided capacitance of the sample.
Partial discharge currents
When partial discharge is initiated, high frequency transient current pulses will appear and persist for nano-seconds to a micro-second, then disappear and reappear repeatedly. PD currents are difficult to measure because of their small magnitude and short duration. The event may be detected as a very small change in the current drawn by the sample under test. One method of measuring these currents is to put a small current-measuring resistor in series with the sample and then view the generated voltage on an oscilloscope via a matched coaxial cable.
When PD occurs, electromagnetic waves propagate away from the discharge site in all directions. Detection of the high-frequency pulses can identify the existence and location of partial discharge.
Discharge detection and measuring systems
With the partial discharge measurement, the dielectric condition of high voltage equipment can be evaluated, and treeing in the insulation can be detected.
Whilst tan delta measurement allows detection of water trees, partial discharge measurement is suitable for detection and location of electrical trees.
Data collected during the procedure is compared to measurement values of the same cable gathered during the acceptance-test.
This allows simple and quick classification of the dielectric condition (new, strongly aged, faulty) of the device under test and appropriate maintenance and repair measures may be planned and organized in advance. Partial discharge measurement is applicable to cables and accessories with various insulation materials, such as polyethylene or lead-covered paper-insulated cable. Partial discharge measurement is routinely carried out to assess the condition of the insulation system of rotating machines (motors and generators), transformers and gas-insulated switchgear.
This new product line includes several teams Amperis for detecting partial discharges. Teams detect partial discharges associated with the presence of insulation faults on underground cable accessories and other cables, in order to verify the safety and reliability of electricity networks. All our products are accurate, great to be part of a preventive maintenance program.
Our products detect with great accuracy the partial discharge type cable joints XLPE / EPR, circuit breakers, Elbows, finishing and distribution transformers. In addition, the ULD 40 allows the detection of ultrasound generated by the corona effect and the arches on the components of overhead power networks.