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.
Why does a lighting arrester have a ring on its top?
These rings are mainly used to fight against corona. At sharp points (like junction) the electric field is high which leads to corona. Corona ring ( a metallic ring) is connected at this points. The ring maintains same potential as the point where it is connected. So, Electric field gradient reduces and corona does not occurs（Replied from Pradeep V, ABB Global Industries and Services Pvt Ltd）
Single point and double point arcing horns in galvanized mild steel flat type upto 69 kV, round ball ended type upto 138 kV and Pipe Type upto 230 kV are available.
For voltage of 380 kV or above corona rings are supplied normally made of aluminiun alloy.
The values for radio interference voltage , corona protections are always kept below the normal internationally specified limits.
Power Transmission Lines should be provided with maximum possible service safety to avoid possible damages to insulators and conductors.
Flashover is often the cause of outages due to high arc temperature which may damage the conductors and destroy the insulators. Arcing protections an important feature of equipment design. They consists of horns, Grading rings, the shapes and sizes of which are in accordance with system voltage and standardised by High Voltage Electrical Tests. Aluminium Corona Control Rings are supplied for High Voltage applications above 330 KV. A properly designed protection device will avoid the flashover damaging the insulator string. It will make the arc start away from the line conductor and ensure that during all the flashover time the arc is kept away from the insulator string. The ring shape design give a more linear voltage distribution all along insulator string, thus avoiding bottom insulator from Electrical over stress. While for system voltage upto 220KV conventional flat type, ball and rod type and tubular racquet type arcing horns are adequate for UHV lines i.e. 400KV and above special designs are developed.
Corona Rings
For voltage of 380 kV or above corona rings are supplied normally made of aluminiun alloy.
The values for radio interference voltage and corona requirements of the protections are always kept below the normal internationally specified limits.