How to Improve Reliability of NCI With the Help of Corona Rings
"Detecting problems early enough enables a less costly intervention and avoids a potential failure of the polymer string" Corona Rings: Are they needed? By Raymond Ferraro (PSE&G), Emilie Dohleman (PNM) and Andrew Phillips (EPRI)
Corona is a well known phenomenon to those working with high-voltage circuits, grids or components. In the past 2 decades, many utilities worldwide have been experiencing an increased number of polymer insulator failures due to corona and partial discharge.
The use of polymer insulators became very popular during the 80's. They consist of a fiberglass rod attached to 2 metal end fittings covered by a rubber weathershed system. Their advantages over conventional glass and ceramic insulators are their light weight, resistance to vandalism, small profile and improved contamination resistance. They are also cost saving comparing to glass or porcelain units. On the other hand they are very susceptible and sensitive to the corona phenomenon.
Corona activity causes polymer insulator failure The root cause is aging of the rubber material. One of the factors that accelerate insulators' aging is corona due to high electrical field close to the energized end.
Corona activity is usually found on the metallic end fitting and nearby the insulator's surface. This corona erodes the insulators housing and degrades the end fitting seal. Over time, this may expose the fiberglass rod to the environment causing brittle fractures and internal flashovers.
Under wet conditions such as fog, mist or rain, droplets and water patches accumulates on the insulator's surface changing the electrical field's distribution and thus corona become more intense and its damaging effect is accelerated.
Daytime corona camera such as DayCor® has shown that insulators near the live (energized) end share the biggest amount of the line voltage therefore, more susceptible to failures caused by corona. The use of corona rings reduces sharply the electrical field on the live end and improves the voltage distribution along the insulator and thus, improves performances and reliability of the units.According to a study "Effect of corona ring design on electric field intensity and potential distribution along an insulator string" performed by Mr. Suat Ilhan & Mr. Aydogan Ozdemir there are several parameters for choosing an optimal ring to a certain insulator:
- Diameter - As the corona diameter increases, the voltage on the units near the energized end decreases but near the dead end increases
- Positioning - A vertical positioning of the corona ring will get the electrical field on the live end to its minimum value
- Radius - Inner and outer radiuses of the corona ring are also an important parameter, that is, maximum electrical field on the outer radius of the ring and minimum on the inner radius ring
The use of corona ring in an insulators string will significantly decrease the voltage percentage on the lowermost units and will slightly increase the voltage of the uppermost units helping the electrical field distribution to be more uniform.
To conclude, corona rings can prevent defects and failures in polymer insulators. Moreover, corona ring increase the reliability of the insulators as well as their lifetime, thus reducing major unexpected failures, costs and maintenance expenses.
"Condition assessments of corona made on an existing in service population of polymer insulators are a vital element for utilities to assess the degree of deterioration and their individual level of risks". Corona Rings: Are they needed? By Raymond Ferraro (PSE&G), Emilie Dohleman (PNM) and Andrew Phillips (EPRI).