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Voltage grading rings on Lightning arrresters

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Voltage grading rings on Lightning arrresters

Voltage grading rings on Lightning arrresters

prc

(Electrical)

(OP)

19 Dec 16 06:07

LAs are provided with metallic rings at the top terminal end (one small ring and below it another larger ring,reaching down to 30-40 % of LA height)These are not for mitigating the corona, but for distributing the power frequency electric field uniformly in the axial direction. In the days of gapped, silicon carbide arresters( before 1980s) voltage grading rings were required to prevent the gap spark over during wet conditions. With gapless MOV arresters of today, the dia of rings has come down (as axial height of LA came down. With out these grading rings, upper sections of Zn oxide blocks will be heated more due to non uniform, axial voltage sharing between sections of LA.

RE: Voltage grading rings on Lightning arrresters

electricpete

(Electrical)

19 Dec 16 16:08

=====================================
(2B)+(2B)' ?

good info. I've heard them [incorrectly] called corona rings and never thought about it much. But I see now from your discussion and from here that corona ring and grading rings are different things with different functions. And I guess there are multiple variations within the category of grading rings.=====================================(2B)+(2B)' ?

RE: Voltage grading rings on Lightning arrresters

prc

(Electrical)

(OP)

21 Dec 16 06:54

Thank you pete, for that link. Wiki is marvelous-crisp, to the point. I was also having the same impression as you, but corrected by the writings of Jonathan J Woodworth - www.arresterworks.com is a great link for Application issues on Surge Arresters.

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News

Grading rings are often mistaken for corona rings. They indeed look similar and they both are designed to affect the electric field in the near vicinity. However, unlike corona rings, reducing partial discharge is not a function of grading rings. The purpose of grading rings on arresters is to significantly modify the voltage distribution along the vertical axis of the arrester. This voltage distribution allows the arrester to operate without an increase in its temperature near the top of the arrester. The lower temperature level allows the arrester to absorb more energy during a switching surge event. The grading ring in effect increases the energy handling capability of the arrester.

It should be noted that grading rings are designed for a specific arrester and are not interchangeable.

Grading rings are typically 5% to 30% of the arresters total height. The longer length is necessary of 500kV & 800kV systems.

The latest generation of high voltage surge arresters - ZnO surge arresters - have been under focused recently. Optimized distribution of electrical field intensity leads to less stress exerted on ZnO disks of the mentioned arresters and finally lengthens the longevity of high voltage arresters. Grading rings are considered as the main element of smoothing electrical field in arresters. In this literature, the effect of existence of grading rings, installation height, diameter of grading rings and radius of grading rings pipe on distribution of 230kV electrical field of Zno arresters are investigated using finite element method and two-dimensional simulation in COMSOL.

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