E-Field Distribution on HV Insulators' End-Fitting
E-field plays a major role in the aging processes of insulators mainly because it is the factor that triggers partial discharges. Insulators are directly affected by E-field both in their long and short term performances due to corona and arcing. Local E-fields along insulators that exceed a certain critical threshold start ionization processes that release luminous discharges, termed corona, and ions producing nitric acids and other oxides that interact with the insulators' material and compromise them. Prolonged exposure to corona activity, regardless of the primary cause, degrades insulators and if not treated shorten their life time.
Numerous factors are involved with creating high local e-fields some of which are manageable and others are unmanageable. The presence of corona, its location and magnitude correspond to the local E-field magnitude and direction. E-fields values must be kept below the threshold value and though there is no universal accepted value available it is true that if corona appears the local E-field is above 30 [kV/cm]. The option to control E-field distribution along insulators is one of the ways to extend insulators' life span and consequently reduce maintenance cost.
Variations in any of the following factors will lead to changes in the E-field distribution:
- Applied voltage
- Design of the insulator
- Material and the electrical properties of the insulator
- Usage of corona rings and their design (Inner and outer radius, positioning, thickness)
- Hardware design
- Phase spacing
- Presence of grounding
- Weather (precipitations, moist, wind)
- Birds dropping
Selection the correct type and design of insulators is one of the major manageable factors. For example, polymeric insulators that are manufactured to endure contamination with their rubber housing are commonly used in polluted and coastal areas. Polymeric materials are badly affected by environmental stresses like UV-radiations and heat. In hot areas with high UV radiation using ceramic or glass insulators is more common. Attached hardware, such as: arcing horns and grading devices etc., have a large effect on the E-field distribution along insulators and need to be considered. For example corona rings change the distribution of E-field, reduce the maximum E-field at the end fitting nearest to the power transmission line and improve the voltage distribution along the insulator string.
There are three main regions of interest when considering corona in insulators:
- Within the insulator at the interfaces between 2 or more different materials
- On the surface and in the air surrounding the insulator and its end-fitting seal
- On the surface and in the air surrounding the metallic end fittings and attached corona rings
Fig. 1 NCI Design
As shown in Fig. 2, the E-field magnitude near either end of an insulator, energized or grounded, is higher than in the mid span with the highest values near the energized end and therefore much attention is drawn to handling properly these areas. Sealing at the end fittings, for example, should be kept corona free and intact. Compromised sealing might lead to water ingress into the rod, to brittle fracture and to erosion of the end fitting metal.
Fig. 2 It can be seen from the plot that the E-field magnitude is high at the energized end and reduces exponentially along the insulator length. The magnitude increases again at the grounded end but to a far lesser extent.
End fitting design changes the magnitude of the E-field on the surface of insulators. Large end fittings with rounded edges tend to reduce the peak magnitude of the E-field values closer to the end fittings. Examples of E-field distributions for different end fitting designs are shown in Fig. 3
Fig. 3 Examples of the E-field magnitude distribution surrounding three different designs of end-fitting
Corona rings, as mentioned, influence the E-field magnitude and distribution as shown in Fig. 4. Corona rings' parameters: inner and outer diameter, positioning, orientation, radius, etc., influence the E-field distribution and must be considered before application.
Fig. 4 shows the magnitude of the E-field along the axis of an insulator which utilizes corona rings on both the live and grounded ends. The presence of the corona rings results in a shift of the position of highest E-field to a location 3 sheds away from the live end fitting.
To conclude: Corona is a derivative of a high local E-field. Insulators that must maintain their mechanical and electrical characteristics should be tested periodically for corona activity. Polymer insulators, in particular, should be kept corona free or run the risk of accelerated aging. Measuring E-field values is not a task that can be performed regularly, therefore knowing the parameters that are involved with E-field can lead to efficient use of corona cameras and to a better control of the electrical asset.