Corona detection is being used as a diagnostic tool to monitor critical components in a power grid. Using a highly sensitive UV camera such as Amperis CoroCAM, corona and UV radiation can be recorded to assess the condition of equipment. Failures in high voltage equipment carry a very high risk, so preventive maintenance saves high costs. Partial discharges can also occur due to structural defects or insulation problems.
When partial discharge and corona phenomena occur, large amounts of ultraviolet radiation are released, which indicates the insulation conditions of the operating equipment and the detection of defects. The optical method is undoubtedly the most suitable discharge diagnosis method and the one that offers greater sensitivity and resolution in all conditions.
By using a highly sensitive ultraviolet radiation receiver, corona and ultraviolet radiation can be recorded to evaluate the condition of the equipment through processing and analysis of the obtained data.
Principle of corona discharge detection using ultraviolet imaging camera:.
When an ionization discharge occurs in high-voltage equipment, a corona, flashover, or arc may occur due to the difference in electric field strength. During this process of ionization of electrons (discharge), energy is released, light, sound, ozone, ultraviolet, etc. are emitted. Using UV imaging technology, these ultraviolet signals produced during the discharge process are detected. Using this technology, the location and intensity of the corona can be detected after data processing, images and overlay with visible light images.
Among all the frequencies of light emitted, UV waves have proven to be the only efficient way for this type of testing. The UV wavelength range is usually 40 - 400 nm. The wavelength of solar ultraviolet that reaches the Earth's surface is more than 300 nm. (due to the Earth's ozone layer, which absorbs some of the long-wave ultraviolet).
Ultraviolet light waves smaller than 280 nm are found in the solar blind area. If it can be detected, then it can only be from Earth's radiation.
The principle of the Amperis CoroCAM cameras is to use the solar blind interval thanks to the application of special filters, so the instrument can work between the wavelength of 240 to 280 nm of ultraviolet during the day (solar blind UV band), so that even during the day the corona effect can be observed. Some of the ultraviolet imaging cameras can only be used at a certain time of day or night due to interference from the sun from ultraviolet rays.
The corona effect in high-voltage equipment at the initial stage of discharge is not continuous, but fleeting. CoroCAM ultraviolet imaging cameras enable corona observation with two modes: real-time monitoring and integrated mode, which displays and retains the number of ultraviolet photons in a certain time and certain area (region can be adjusted) on the screen, and updates in real time.
That is, the corona is a light effect triggered by high localized electric fields that exceed a certain critical value that ionizes the air and discharge occurs (under normal atmospheric conditions that critical value is between 20-30 kV/cm.
During the PD process, nitrogen molecules in the air are excited and emit ultraviolet radiation, generally in the wavelength range of 200-405 nm.
Main factors affecting UV detection:.
With the corona effect camera, the number of photons is observed and this parameter is used as an intensity index in the observed object of the corona. Many factors have a direct effect and affect the photon counting such as: detection distance, humidity of the atmosphere, ambient temperature, pressure, altitude and instrument gain settings:
• - Detection distance: The detection distance has a notable effect on the results, that is, the greater the detection distance, the smaller the field of view and the less sensitivity.
• - Humidity and contamination: Humidity plays an important role and a way must be found to solve this problem to allow analysis. As the humidity increases, the conductivity of the insulator surface is improved, which is conducive to discharge. The more atmospheric humidity it absorbs, the more ultraviolet light, so that the detection of ultraviolet radiation is reduced.
• - Effects of air pressure and temperature: If the pressure increases or the temperature decreases, the intensity of the electric field of the corona increases, since the density of the air increases, so the average distance between the air molecules increases and the free path of the electrons decreases and the air is more difficult to ionize, increasing the intensity of the electric field of the corona.
• - Altitude effects: When elevation increases, pressure decreases, so the voltage that causes the initial corona decreases. That is, the higher the altitude, the lower the corona effect.
• - The influence of detector gain: The gain of the chamber is regulated, so the instrument can be sensitive to corona, while the influence of background interference can be reduced.
Characteristics of corona effect inspection.
UV inspection is designed to detect external phenomena on the surface of the equipment.
As we have seen before, humidity hinders the measurement, but inspections can be carried out under practically any weather condition (not including rain). Such inspections allow potential problems to be detected in their early stages, before they have progressed to the point of serious damage and the corresponding need for rapid replacement of the affected component. Being related to voltage rather than current, corona discharge has an intermittent periodic appearance that corresponds to the sinusoidal pattern of the line.
Corrosion and erosion due to ozone and nitrogen oxides created by corona are processes that coexist with corona discharge. Therefore, corrosion occurs in the porcelain insulator (in the figure below you can see the 2 separations formed).
Corona Effect: environmental and safety concern.
As mentioned, partial discharge such as is a source of audio noise (AN) or radio or TV interference (RI or TVI), so it should be eliminated even if it has no immediate effect on line reliability. The first thing to do is to exactly locate the source of PD by looking for the corona. With the use of Amperis' range of CoroCAM corona cameras, discharge sources are perfectly located and identified within minutes.
Corona active degradation agent.
With Amperis CoroCAMs, corona effect activities can be detected in polymer insulators. These non-ceramic insulators are susceptible to degradation due to the corona effect, as the nitric acid formed by the generated nitrogen oxide and water vapors damage the polymer casing. Additionally, a crack or puncture in the polymer casing will allow moisture to penetrate the fiberglass rod. The combination of moisture and the acidic material degrades the fiberglass rod, even causing it to fracture.
Porcelain insulators also suffer from the effects of PD. In these cases, the corona chamber indicates short or internal arches, cracked or eroded cement, rusty joint, etc.
Crown as indicator.
As we have explained, the corona effect is an indicator of faulty designs and installations, as well as poor material. The appearance of corona indicates degradation processes in progress and helps us as an alert. One of the applications for corona inspection is in the commissioning stage of any new electrical installation. The Amperis CoroCAM is a tool to allow the power system operator to verify that the installation was performed correctly. Using corona chambers during start-up results in less maintenance efforts.
Conclusion.
A corona camera that generates UV images plays an important role in detecting defects in high and medium voltage electrical constructions. Thus we detect and identify the real location of the discharge through the high resolution of the CoroCAMs.
Amperis corona detection cameras use ultraviolet imaging which is a relatively new technology, but it is easy to use, efficient, intuitive and live monitoring facilitates detection and maintenance. These cameras provide an immediate response in processes with problems that would remain undetected and without assistance. UV inspection on overhead distribution lines and substations provides us with information on the conditions of the line (early fault detection) and on the quality of the design and workmanship during the installation phase.
There are many more applications in the energy industry that can be exploited with this technology. Corona discharge detection is just one of them.