Mitigating the presence and impact of corona on overhead lines and at substations is among the important tasks confronting power system operators. The reason for this is that, if left unchecked, corona can lead to a variety of problems, including progressive degradation and eventual failure of certain types of insulation. In 2015, INMR traveled to Israel to meet a supplier of specialized cameras that help maintenance personnel monitor their networks for presence of corona. Ofil Systems has been offering such equipment since the mid 1990s but expanded its product range in recent years to meet different inspection protocols and user requirements.
Corona in Power Networks
Corona is caused by ionization of gas and subsequent release of light when electrons that have gained energy from a high electric field revert to their original stable state. Since the discharge does not completely bridge the electrodes, it is sometimes referred to as partial discharge.
The reason that the glowing effect is concentrated around the corona source is that insulation provides an effective barrier to further ionization. Moreover, intensity of the electric field decays rapidly with greater distance and eventually field strength becomes unable to sustain continued ionization. Indeed, one of the requirements for build-up of sufficient ionization to trigger the UV emissions associated with corona is some critical level of localized electric field. If this minimum value is not reached, corona will essentially be absent.
Corona can affect a range of network hardware, equipment and components – from switchgear to capacitors, from conductors to busbars, from generators to breakers. When it comes to insulators, corona can originate from end fittings, contamination or from protruding elements such as ice sickles and water drops.
Polymeric materials are more susceptible to degradation from corona due to corrosive substances created when corona is emitted. Corona ruptures stable oxygen molecules in air to create radicals. These then combine with molecules to form ozone (O3), which attacks unsaturated bond sites in elastomeric materials such as silicone or EPDM. The end result is usually cracking. Even tiny amounts of ozone, e.g. in the ppm range, are sufficient to initiate cracking, however the time required for this to occur will depend on material composition.
Ozone also combines with nitrogen in air to produce nitric oxide gas, a major greenhouse contributor. While most elastomers used these days in electrical insulation are formulated to resist ozone attack, they may eventually succumb should its concentration become sufficiently high
Corona produces oxalic and nitric acids in the presence of water from ambient humidity such as dew, fog or any surface moisture. Depending on the pH of these acids, polymers can then undergo additional degradation. Corona has even been known to ‘drill’ holes in a material, suggesting that degradation is not due solely to chemical attack. For example, temperatures at the tip of a corona discharge have been found to be high enough to cause ‘evaporation’ of even inorganic materials. There is also danger of mechanical attack to an elastomeric material, much like sandblasting, due to the impact of repeated discharges.
Since degradation due to corona is initiated at the molecular level, inorganic dielectrics with strong chemical bonds – such as porcelain and glass – have much higher resistance to it than do polymeric materials. But this does not mean that these insulators are immune to all possible negative effects, which can include acid and heat attack on the cement used to attach fittings.
Among the questions often asked by maintenance personnel who are responsible for inspecting lines and substations relates to the relative seriousness of any corona activity that may be observed. Is it reason to take immediate action or perhaps only a transient phenomenon posing no long-term threat? The answer depends largely on where the observed corona is taking place.
For example, corona on insulator hardware due to a rough surface may be relatively benign. However corona directly on the insulator itself is a warning that degradation processes may soon be underway.
Another question typically raised is how inspecting for corona relates to thermal analysis using infrared cameras, a well-established complementary inspection technology. According to industry experts, IR cameras find most benefit detecting internal phenomena such as hotspots that are typically current dependent. These suggest a progressive existing problem that needs to be remedied quickly.
By contrast, UV inspection for corona is voltage dependent and designed to detect mainly external phenomena affecting the surface of what is being monitored. Such predictive inspections can be performed under virtually any weather conditions, apart from rain, and can help detect problems at an early stage – before they have progressed to the point of serious deterioration.
In spite of the many problems linked to corona, it is not necessarily a given that it will always shorten the expected service life of composite insulators installed on high voltage lines. This is because corona can be greatly mitigated and even eliminated through good insulator design and manufacturing. At the same time, need for a corona/field-grading ring will depend not only on voltage level but also on location. For example, insulators operating in areas of heavy contamination, at high altitudes or under prolonged heavy wetting must always be equipped with properlydesigned and correctly-installed corona rings at the line end – ideally even at voltages below 230 kV. That is because if sustained corona activity takes place near a composite insulator’s polymeric housing, its normal life expectancy will almost certainly be reduced.
Detecting & Assessing Corona Using Cameras
For most practical purposes, corona cannot be seen or heard without the use of specialized equipment. Therefore, it can go unnoticed and cause progressive degradation of insulation up to the point that failure occurs. “That,” says Ofil Systems CEO, Moshe Goldbaum, “is why the name of the game is not only to detect corona but ideally to do it when still at an early stage.”
In order to detect corona, it is logical to look for the UV radiation it emits and this is where specialized cameras play their role. Most of the light produced by corona has a wavelength shorter than 400 nanometers and therefore falls in the UV range. By contrast, solar radiation lies principally in the visible spectrum (400-700 nm), the shorter wavelengths being filtered by the earth’s ozone layer.
One of the past problems associated with monitoring for the UV from corona has been that the wavelength of the radiation emitted corresponds directly to that of background solar radiation. It is therefore blocked from view during daytime and corona camera manufacturers have had to find ways around this problem.
In the case of Ofil, a proprietary solar blind technology is used that relies on optical filters to block out background solar radiation and allow only the radiation emanating from a corona source to be observed. The technology is based on the fact that all UV radiation in the solar blind band (i.e. 240 – 280 nm) is absorbed by the earth’s ozone. It is therefore at this exact range that the solar blind filter enables detection of UV emissions due to corona, even during daylight.
To pinpoint the corona source in relation to the equipment being monitored, a bi-spectral approach is employed. One channel in the camera allows the observer to see the object being studied in the visual spectrum while another channel uses solar blind technology to view only the UV radiation emanating from a corona source. The two signals are then overlapped to generate a composite image of the object and the location of any corona that might be affecting it.
According to Goldbaum, the key factors when it comes to corona cameras are their transmittance and absorbance, two parameters which define the detector’s efficiency. A sensitive corona camera transmits as many UV photons emanating from corona as possible and blocks noise and visible light.
Sales & Marketing Director, Revital Lazimi, contrasts the benefit of data from thermal inspection using infrared cameras with ultraviolet inspection using a corona camera. “Once heat is detected inside an electrical component,” she explains, “it may be too late to save it. Replacement may then be the only option. By contrast, our cameras raise a red flag at an early enough stage that maintenance personnel still have time to investigate more closely. That type of information helps a power utility to save money.”
Lazimi goes on to explain that corona inspection does not always offer a ‘black or white’ answer as to whether or not there is a need to replace a component, such as an insulator. Rather, it becomes another factor for maintenance staff to consider when deciding what action may be needed to avoid a failure in the system. “Corona is a sign of some defect, resulting from issues such as improper installation, loose hardware, contamination, erosion or broken strands on a conductor,” she says. “Therefore, repetitive inspections allow maintenance staff not only to identify that something may be wrong but also to learn if that problem has progressed over time.”
One of the challenges faced by corona camera users is that no recognized international standard yet exists to measure corona intensity. That means users must be able to reach their own conclusions based on results of several inspections. Recognizing this, Marketing Communications Manager, Hannah Barzilay, points out that Ofil has developed its own counting technology to inform users whether the level of corona detected is low, medium or high. Says Barzilay, “a corona camera is really more indication equipment than measurement equipment. That means it is always best to compare findings against a database of accumulated information from previous inspections. Moreover, since the level of corona depends on ambient temperature and humidity, successive inspections of the same equipment should ideally be conducted under similar weather conditions.”
Barzilay also explains that knowing corona inception voltage of whatever object is being monitored also helps network operators determine if the right component has been specified for the application. “In the case of conductors, for example, there is a direct relationship between corona and bad practice,” she observes, “with the radius not matching applied voltage. We often see older transmission lines become noisy with abundant corona. In most cases, the reason is the utility’s response to a rise in demand for electricity.”
In recent years, much of the development work at Ofil seems to have been focused on modifying the range of cameras to make them more efficient or more practical for each alternative type of inspection. For example, European Sales Manager, Bar Weinstein, reports that cameras have now been developed that are specially suited for mounting in a gimbal for aerial inspection. These, he says, also combine an IR camera, from a partner company, integrated into one basic system. Other cameras have been specially adapted for inspection of railway lines and rely on internal software to process the many images so as to highlight only those locations where there is corona and possible need for maintenance. There are also cameras intended for mounting on vehicles or for use in high voltage testing laboratories.
In particular, an expanding range of portable cameras has been developed, with the focus on convenience, comfort, lightweight or lower cost. These units offer a menu of possible features, including wide field of view (ideal for monitoring substations), still images, video images and zoom capability. The most recent introduction, at only 0.85 kg is claimed to be the lightest corona camera in existence and intended mainly for use in indoor inspection. Says Lazimi, “we see our basic business as helping to improve maintenance of power infrastructure. But inspection processes and requirements change from one customer to another, so we offer users a choice of several options – all with the same basic internal technology.”