DayCor® Camera Technology and Methodology
This paper was presented at the Electrical Engineering Conference Eilat 2012.
The paper describes the use of Ofil's solar blind UV-Visible Bi-spectral corona cameras for detection of partial and full discharge in day light. Recently there is a widespread upsurge in interest of utilizing corona partial discharge detection as yet another of the various diagnostic tools available to monitor the condition of critical components in an electrical network. Based upon data gathered by Ofil throughout eleven years of global operation, a methodology for detecting corona and arcing and utilizing it for diagnostics and for predictive maintenance was constructed. This methodology will be discussed in correlation to corona concerns.
INTRODCUTIONCorona is a luminous phenomenon and an artifact of partial discharge (PD). Corona is triggered by localized high electric fields that exceed a certain critical value which ionize air and lead to discharge activity. Under normal atmospheric conditions the critical value is in the range of 20-30 kV/cm, this value will be reduced under conditions of low air pressure or high relative humidity. Partial discharge generates audio noise, corrosive materials like ozone and nitrogen oxides, radio and TV reception interferences and emits light, mainly in the UV spectral range.
During the PD process nitrogen molecules in the air are excited and emit ultraviolet radiation, generally in the 200-405 nm wavelength range. To detect corona, there is a need to look for the UV radiation it emits. One of the primary problems associated with observing UV radiation during daytime is that the wavelength of the emitted radiation corresponds directly to that of background solar radiation and therefore is basically blocked from normal view.
To see and capture corona during daytime Ofil developed the "DayCor®" technology. DayCor® technology is now being embedded within all of Ofil's video cameras and allows pinpointing of both the UV radiation and its emitting sources.
DETECTION IN THE SOLAR BLIND UVOfil employs its proprietary "DayCor®" Solar Blind UV technology that uses special optical filters to totally block out background solar radiation and view only radiation emanating from the corona sources, in full daylight. Ofil's filtering technology utilizes the fact that in the solar blind UV band (240 - 280 nm) all UV radiation is absorbed by the Ozone layer surrounding the earth. Ofil deploys a bi-spectral approach where one channel in the camera is dedicated to the visual spectrum allowing seeing the inspected object while another channel is dedicated to transmitting the UV radiation emanating from corona discharge. The signals from the two channels are overlapped, generating a composite image of the test object and the corona. This proprietary Bi-spectral approach allows exact location of the corona.
Although the emission of corona discharge in the Solar Blind UV range is much weaker as compared to the emission in the rest of the UV spectrum (see fig.1), by using an optical filter that transmits only in the SBUV spectral range and effectively blocks all sun radiation from reaching the UV camera, corona can be imaged and its strength indicated by Ofil's DayCor® cameras in full daytime with high sensitivity, without interference from solar radiation.
CORONA INSPECTION – CHARACTERISTICSUV inspection is designed to detect mainly external phenomena on the surface of the equipment being monitored and is voltage dependent only. While ambient humidity is advantageous, inspections can be performed under virtually any weather conditions (not including rain). Such inspections allow for detecting potential problems at their very earliest 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 to current, corona discharge has a flashing periodic appearance that correspond the sinusoidal pattern of the line. To be able to perceive and capture corona for documentation purposes the phenomena must be recorded as video clips.
Once detected, deciding upon the relative severity of the findings depends on validating where the observed corona is taking place and on the substance of the emitting component. Often high definition visual inspection with good zooming complements the detection findings and assist obtaining a comprehensive picture of the condition.
As an example, corrosion and erosion due to ozone and Nitrogen Oxides created by the corona are processes that co-exist with corona discharge. Therefore, corona on a cap & pin type porcelain insulator will cause corrosion of the pin and may pit the cement, leading to pin separation (as can be seen in fig 2). Composite insulators wear out under UV radiation and their hydrophobicity is compromised in response to corona activity (as can be seen in fig. 3). While one of the main applications of the DayCor® technology is to detect corona activity on polymer insulators, still audio noise or radio or TV interferences are commonly perceived as high priority issues among utilities hence these involve immediate customers' complaints.
CORONA AN ENVIRONMENTAL AND SAFETY CONCERN
Partial discharge, as mentioned, is a source of audio noise (AN) or radio or TV interferences (RI or TVI), leading to complaints from the public; it should therefore be eliminated even if it has no immediate effect on the line reliability. The first action to be taken is to exactly locate the PD source by looking for corona. PD emanating from a tie wire where corona was picked disturbed the AM radio communication of an airport control tower. The local utility was asked by the FAA to immediately remove the radio interference. Inspection of the line with Ofil's camera quickly located the corona source. Similar interference problems may occur from substations located in populated areas.
Often utilities confront annoying noise generated by at night when the humidity is high. During one of 115kV line inspections Ofil's camera clearly showed corona discharge on all of the screws that were used to fix conductors to the insulators (Fig. 4) and on many broken conductor strands. The line was poorly designed and not adequately maintained.
Residents next to an existing OHT line in OR USA complained of unbearable noise. Using Ofil's camera reviled considerable corona activity. The growing demand for power in the area led the local utility perform changes in the line that did not correspond to the original design of the poles' clearance and height resulting in unbearable audio noise and in complaints.
A 110 kV substation in south Bohemia – Czech Republic was asked by the Czech Radio - Telecommunication Authority to get rid of sources of TV interference (TVI) affecting a frequency band in use by a local TV station or risk a large penalty. By using Ofil's DayCor corona camera, within few minutes, three sources of corona discharge were pinpointed. The pin of a ceramic mid phase insulator was bent incorrectly towards the armature – wrongly installed - Causing the TV-Interferences (Fig 5)
CORONA AN ACTIVE AGENT OF DEGRADATION
One of the main applications of the DayCor technology is to detect corona activity on polymer insulators. Non-ceramic insulators (NCI, Polymer) are highly susceptible to corona activity. The nitric acid formed of the generated Nitrogen Oxide and water vapors damages the polymer housing leading to dry band arcing, tracking and loss of hydrophobicity. Furthermore, a crack or puncture in the polymer housing will let moisture penetrate into the fiberglass rod. The combination of moisture and the acid material degrades the fiberglass rod might lead to brittle fracture of the rod and to reduced dielectric properties of the insulator leading to eventual line drop , .
The effect of long time exposure of a polymer insulator to corona is being constantly studied by research institutions worldwide such as EPRI in the USA, STRI in Europe, Kepco in Korea. In particular utilities are following investigation on the processes and reactions that are related to corona and to:
- Corona rings – constitute standards of shape, size, installation distance, voltages etc.
- End fittings – shape, material, dimensions etc.
- Rods – brittle fracture issues, strength, flexibility etc.
- Rubber sheath – adhesives, material, applying methods
- Hydrophobicity and sustainability to contaminants in various geographical locations
- Leakage current, flashover and flash under – distances, calculations
- Holes through the sheds due to excessive discharge activity (undermining)
Porcelain insulators have been proven highly susceptible to contamination issues and to vandalism. Typically this time of insulators that are used in coastal areas or industrial areas will show much PD activity. Corona cameras will indicate cases of:
- Arcing shorts or permanent internal shorts between the cap and the pin
- Cracked bells
- Cracked or eroded cement
- Rusted ball & socket joint
In case of glass insulators there is evidence that it may result in shattering of the glass bell. Prolonged exposure to corona activity of the pins of porcelain or glass insulators will result in pin corrosion and pitting in the cement holding the pin, leading to reduced mechanical strength of the insulator string.
Ongoing studies on conditions leading to corona and arcing on porcelain and polymer insulators led to an inspection guide by EPRI. The results, obtained by using the Ofil's cameras discuss conditions leading to corona and arcing on porcelain and polymer insulators.
CORONA AS INDICATORS
Corona is an indication of faulty designs and installations as well as of poor material. Corona indicates existing on-going degrading processes and serves thereby as an alert and a pointer. One of the applications especially suitable for corona inspection is at the commissioning stage of any new electrical installation. This then serves as a tool to permit the power system operator to verify that the installation was done correctly.
As an example, an extreme corona activity on a polymer insulator was observed on the entrance to a 440kV substation. All the neighboring insulators were checked and diagnosed as not having grading rings. As can be seen in the image, on the same tower one side had corona rings and the other didn't. Visual inspection revealed progressive degradation of the polymer on the end fitting (see fig. 6). It turned out that the 2 years old insulators were visually inspected, on regular basis and though obvious – were not revealed.
In a 22kv power line seen in fig 7, the cable protrudes from the substation underground, comes up against a concrete and steel pole and becomes an overhead line. When this line was constructed, the center phase stand-off insulator was installed too close to the metal upright section of the pole. Either inclement weather or a bird has caused a flashover between the insulator and the pole, causing the line to trip out at the substation zone. Close in inspection revealed that the insulator with the corona caused the intermittent outages.
Commissioning inspections ensure that damaged components have not been used; mishandling did not compromise components; no loose metal hardware remains inside substation equipment. Using corona cameras during commissioning results in less maintenance efforts, reduces that cause high losses in revenue and refrain from possible delays of spare parts that are not immediately available.
OFIL'S DECISION AIDING CHART
Ofil recommends using a flow chart that assists making decisions and recommendations. This chart provides maintenance technicians with a straightforward systematic analysis of the data collected during inspection. The flow chart uses question-nodes that consider the major issues. Findings must be documented and kept in a repository. Using a database reporting software such as Ofil's Corona-Base is handy both to keep track of the history, but furthermore to produce automatic reports, analyzing failure trends, studying chronology of failures and being capable of performing cost effective predictive maintenance.
Vast and thorough field experience and testing gathered show that the DayCor® technology and inspection methodology are highly effective in monitoring high and medium voltage electrical constructions. Utilities worldwide are using daytime corona cameras tom monitor the condition of their system during implementing the DayCor® technology and methodology.
Corona is both an indicator and an agent and must be attended. Using a corona camera provides immediate authentic information about exiting reactions and processes that are otherwise left un-detected and un- attended. UV inspection of distribution, overhead lines and substations with a corona camera yields information on the condition of the line or substation as well as the quality of the design and workmanship during the installation phase. Inspection for corona with a corona camera is simple and easy to carry out and requires no load of the line The decision aiding chart simplifies and clarifies predictive maintenance tasks. UV inspection is cost effective to utilities since it allows early detection of faults that can cause outages at a progressive stage. Though there are still no accepted standards to be used by engineers Ofil's methodology serves as guidelines to decide what action to take after the inspection.
Ofil commits to support its customers in present and future time with technological innovations of reduced equipment size and improved ergonomics while retaining high sensitivity to UV sources.