Abstract:
The procedure for quality control testing of external discharges on stator coils, bars, and windings of large air-cooled ac electric machines is described in this recomme...Show MoreMetadata
Abstract:
A test performed after eliminating all ambient light, specifically on energized electrical equipment, to detect external or surface discharges visible with the human eye (?naked eye?) after at least 15 min of acclimatization.
Composition materials that usually have a dielectric binder and conductive filler. For example: electrical insulation coating or compound filled with copper, silver powder, etc.
Solid materials that have a large number of free electrons that can easily be put into motion to create an electric current. For example: metal (as steel, copper) sheet; copper foil; copper, silver powder, etc.
A luminous discharge due to ionization of the air surrounding a conductor or insulated conductor caused by a voltage gradient exceeding a certain critical value.
An instrument used for the detection of corona or external surface discharges on energized test objects in ambient light and offers a visual representation of the discharge superimposed on a visible light image of the test object. Corona imaging instruments frequently use ultraviolet radiation emitted by the discharge source, but other signals such as acoustic emissions can also be used. It should be pointed out that acoustic emissions can come from internal and external discharges and depending on the detection frequency range it will be more sensitive to one or the other.
In the current document, only external discharges are of interest.
discharge extinction voltage
(ionization or corona-extinction voltage) The voltage at which discharge pulses that have been observed in an insulation system, using a discharge detector of specified sensitivity, cease to be detectable as the voltage applied to the system is decreased.
(ionization or corona inception voltage) The voltage at which discharge pulses in an insulation system become observable with a discharge detector of specified sensitivity, as the voltage applied to the system is raised.
discharge inception voltage
An external discharge is a partial discharge that occurs on the exterior of the bar/coils or other energized parts of the winding. In rotating machines, external discharges may occur on the surface of bars/coils or in any air gap present between the bar/coil surface and the stator core, or in the end-winding of the stator (between bars/coils of different phases or between bars/coils and ground, such as pressure fingers or leads from sensors or other coils near ground).
The main high-voltage electrical insulation that separates the copper conductors from the grounded stator core in motor and generator stator windings.
A test that consists of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdown of high voltage insulation system and spacing under normal conditions. Syn: high pot or hipot.
The test is used as a proof test of new apparatus, a maintenance test on older equipment, or as one method of evaluating developmental insulation systems.
A breakdown that occurs in parts of a dielectric when the electric stress in those parts exceeds a critical value without initiating a complete breakdown of the insulation system. Note: Ionization can occur on both internal and external parts of a device. It is a source of radio noise and can damage insulation.
The process by which an atom or molecule receives enough energy (by collision with electrons, photons, etc.) to split into one or more free electrons and a positive ion. Ionization is a special case of charging.
Unwanted disturbances superimposed on a useful signal that tend to obscure the signal?s information content.
Testing of the unit under test removed from its operational environment or its operational equipment. Shop testing.
An electric discharge that only partially bridges the insulation between conductors, and which may or may not occur adjacent to a conductor.
Partial discharges occur when the local electric-field intensity exceeds the dielectric strength of the dielectric involved, resulting in local ionization and breakdown.
Depending on intensity, partial discharges are often accompanied by emission of light, heat, sound, radio influence voltage (with a wide frequency range) and oxidation if PD occurs in the presence of oxygen.
?Corona? has also been used to describe partial discharges. This is a non-preferred term since it has other unrelated meanings.
partial discharge
resistance temperature detector
are composition materials, which usually have dielectric binder and semiconductive filler. For example: electrical insulation coating or compound filled with graphite, carbon black powder; SiC grains, etc.
A coating, applied on the insulation surface of the slot parts of winding. The semiconducting coating, compound or tape has a powder filler or portion of powder filler that is a semiconductive material. The surface resistivity of this coating must be large enough to prevent thermal aging and vibration sparking and low enough to prevent slot discharges. This semiconducting slot coating must have uniform tight contacts with the grounded walls of the stator slot. This coating minimizes the voltage between the surface of the coil or bar and the grounded stator core.
See Liese and Brown
Solid materials that have limited free electrons and main conduction is carried by ?electron - hole? conductivity, (n - p transition). For example: graphite, carbon black powder, SiC grains, etc.
This alternative terminology, as well as ?slot corona protection? and ?conductive armor,? of the previous definition is also used in the industry but will not be used in this document.
The paint or tape on the outside of the groundwall insulation that extends several centimeters beyond the semiconducting slot coating in high-voltage stator bars and coils. The stress control coating often contains silicon carbide particles that tend to linearize the electric field distribution along the coil or bar end turn. The stress control coating overlaps the semiconducting coating to provide electrical contact between them.
See Bui, et al.
The resistance of a thin film material measured between two opposite sides of a square and is independent of the size of the square or its dimensional units. Surface resistivity can also be measured in a concentric ring fixture. The units of surface resistivity are ohms per square.
Acoustic emissions that accompany surface PD or corona that occur in the 20?100 kHz band, which is above the frequency the human ear can detect.
In general, any radiant energy within the wavelength 10 nm to 380 nm (nanometers) is considered ultraviolet radiation. For power engineering purposes, the band of interest is the one of the emission spectrums of electrical discharges in air. The emission bands of nitrogen dominate the optical spectrum of discharges in air. Ninety percent of the total energy of the emitted optical spectrum of PD is in the ultraviolet region (280?405 nm). The main part of the emission is invisible to the human eye. A relatively weak emission around 400 nm can be observed under conditions of absolute darkness.
line-to-line voltage
line-to-ground voltage
vacuum-pressure impregnated
