Using Corona Cameras as Reliable Tools to Diagnose Compromised Insulators Prior to Failure
Corona discharge is a major threat to the integrity of non-ceramic polymer insulators (NCI). It produces Ozone, nitric oxides and emits Ultra Violet leading to erosion and decomposition of the insulators' and affecting their mechanical and electrical properties.
Ongoing research is performed by utilities and high voltage laboratories to investigate the endurance of insulators to various environmental conditions and to their aging patterns. One of the studies conducted in Mexico aimed at correlating between findings by a corona camera which is nondestructive optical device and an E-filed measurement equipment. The use of corona camera can alleviate the task of preventive maintenance and streamline the maintenance processes provided its output is indeed reliable.
A study was performed in Mexico on 230kV transmission line NCI insulators that lasted 3 years. During the study 3 types of insulators (A, B, C) were monitored with a DayCor® II corona camera and a partial discharge measuring technique. The following results were noted:
2003 inspection – initial state:
- No corona detected on any of the insulators
- E-field measurements did not detect any defective insulator
- Insulators were in good shape and operation conditions
- A bare eye inspection of insulator type "A" (8 years in service) detected small signs of erosions on the surface (1.5 cm long and 1.0mm deep) the result of an earlier dry band arcing activity
- It was concluded that degradation was not risky and periodic monitor is recommended
2004 inspection – a year later
- Corona activity was observed on the energized end of all insulators
- E-field values closer to the grounded end were slightly higher
- E-field values closer to the energized end stayed the same as last year
- The erosion signs of insulator type "A" grew (2 cm long 1.5 mm deep)
- It was concluded that degradation was still not risky and periodic monitoring is recommended
2006 inspection – three years later:
- High corona activity was observed around the energized end of all insulators
- The E-field presented major distortions along the insulator strings resulted from severe damage of the insulators
- A bare eye inspection on all 3 insulators showed severe degradation
- On insulators type "B" and "C" after 6 years of operation the rods were exposed near the energized ends
- Insulator type "A", after 10 years of operation showed severe erosion of the sheath close to the energized end
- It was concluded that all 3 types of insulators reached their end of life and were removed and replaced
The study lead investigators to conclude that there was a true match between findings by e-field measurements and corona camera readings, hence both techniques revealed degradations such as cracking erosion or tracking on insulators. This particular study was conducted in a polluted area where the lifetime of polymeric insulators is diminished due to the sever conditions. In such areas using corona cameras to periodically test the conditions of insulators is of high importance in order to maintain the reliability of the power line.
The study verified that daytime corona cameras should be used as predictive maintenance tools and their return on investment is clear since they reduce the cost of maintaining the lines and the number of maintenance works that would have been performed otherwise.