Artisan Industry：
Transmission Line high voltage Corona Rings Manufacturer
Price Inquiry：lilychin@vip.163.com
WhatsApp：+86 150 5061 5346
Corona rings are toroidal shaped metallic rings which are fixed at the end of bushings and insulator strings. They are also called as anti-corona rings. Corona rings are used to prevent corona discharge that occurs in high-voltage power lines. Corona discharge or corona loss is a significant issue with very high voltage power lines which causes power loss. One way to reduce corona discharge is using corona rings.
The components of EHV/UHV lines and substations can produce significant corona. To limit the consequent Radio Interference and Audible Noise on these systems, suitable corona control rings are employed. The shapes of these rings could vary from circular to rectangular with smooth bends. Many manufacturers seem to adopt trial and error method for arriving at the final design. As such neither the present testing standard nor the final design adopted consider the practical scenario like corona produced by deposition of dirt, bird droppings, etc. The present work aims to make a first step in addressing this practically important problem. This requires an accurate evaluation of the electric field and a reliable method for the evaluation of corona inception. Based on a thorough survey of pertinent literature, the critical avalanche criteria as applicable to large electrodes, has been adopted. Taking the rain drop on the surface as the biggest protrusion, conducting protrusions modeled as semi-ellipsoid is considered as representative for deposition of dust or the boundary of bird droppings etc. Through examples of 400 kV and 765 kV class toroidal corona rings, the proposed method is demonstrated. This work is believed to be useful to corona ring manufacturers for EHV/UHV systems.
How does Corona Rings Work?
Corona discharge occurs when the electric field (potential gradient) at the surface of conductors exceed a critical value, called as critical disruptive voltage. The value of critical disruptive voltage varies with atmospheric condition. Its value is roughly 30 kV/cm. The electric field greatest where the curvature is sharpest. Therefore, corona discharge occurs first at the points where the curvature is greatest - i.e. at suspension points, corners and edges. Corona rings are installed at these points to prevent corona formation.
What's corona discharge on a corona ring?
A corona ring is electrically connected to the conductor, encircling the points where corona discharge may occur. Therefore, the corona ring distributes the electric field (or charge) across a wider area, because of its smooth round shape. Hence, it reduces the potential gradient below the critical disruptive value.
Manufacturers usually recommend aluminum corona rings to be installed at the conductor end of the string insulators for lines above 230 kV and on both ends of the insulator for 500 kV.
What Are Grading Rings?
Grading rings are very similar to corona rings. In fact, one can say, grading ring and corona ring are two different names for the same device. The difference is due to their main purpose of use and their placement. Grading rings encircle insulators rather than conductors. Their main purpose is to reduce the potential gradient along the insulator. Grading rings help in equalizing the potential distribution over a string of suspension insulator. Hence, grading rings improve the string efficiency and prevent insulation breakdown. Grading rings also serve the purpose of corona rings to some extent.
Corona Definition | Avoid Corona | Purpose of Corona Ring | Advantage
Corona:
When the potential difference increase in the transmission conductor according to that the potential gradient also increases. When the potential gradient increase above 30kV/cm, the air surrounding to the line conductor starts ionizing due to electron present in the free space. This phenomenon causes faint violet glow, hissing noise and production of ozone gas. This is called corona. It is the self-sustained electric discharge in which the field intensified ionization is localized only over a portion of the distance between the electrodes.
Corona Formation:
When the potential gradient is less than 30kV per centimeter, the corona will not occur. But even you can see the road side distribution transformer the little visible glow. That’s also corona, because of that electric discharge at sharps is high. However, in high voltage transmission, the potential gradient is higher than 30 kV/cm, the ions attain a sufficiently high velocity and strike each other and other neutral molecule, dislodge one or more electrons from the neutral molecule. This action creates new electron and proton and both collide with other molecule. This causes complete electric breakdown occurs and an arc produced between the electrodes.
For AC voltage the corona appearance of the conductor is same because of the current flow changes its polarity, but for DC the current is in uniform, so that in positive conductor, corona seems white bluish and in negative conductor seems reddish color.
If the voltage exceeds more than 100kV, the corona power losses occurs. So that, neglecting necessary action, the corona damages electrical equipment especially insulators.
Corona Power Loss:
In order to Maintaining the motion of charges produced by electric field, the energy required which will be drawn from transmission line. This is called corona power loss. Let’s see the mathematical expression, Pc….
The above equation is called peak’s formula and it is applicable only, where the conductor radius is greater than 0.25cm, frequency between 25Hz to 120Hz and the ratio of Vp/Vo > 1.8
Where the Vp/Vo < 1.8 then Peterson’s formula applicable.
Corona will be higher when the weather condition is Poor.
Factor Affecting Corona: Atmospheric condition:
Temperature: Temperature increases the corona also increases
Pressure: atmospheric pressure increases the density of the air increases which reduces the corona.
Rain, Snow Fog, Dust and Dirt: All are opposite side of electrical stability, so ultimately all increases the corona in the transmission line.
Factors Affecting corona:
Frequency: the corona power losses Pc is directly proportional to the frequency. Therefore, corona power losses less where the system frequency is less. That’s why in DC transmission line has less corona losses as compared with AC transmission line.
The corona power loss is high when the voltage is high.
Method of reducing Corona:
Increasing the distance between the conductor
Increasing the diameter of the conductor.
Using hallow conductor. That’s why in Switchyard, you can see that mostly use hallow conductor.
Shape pf the conductor: Round or Solid has less corona loss but flat conductor has high corona losses.
Surface condition: Polished Surface where the corona loss is less and Rough surface has corona loss is more.
Using Bundled conductor: In EHV lines, bundled conductors are used, So effective radius increases. Vc increases and corona loss reduces.
Temperature of the conductor: Decreasing the temperature of the conductor, reduces the corona loss.
Using corona discharge ring on high voltage. The corona ring is electrically connected to the high voltage conductor, encircling the points where corona would form. Since the ring is at the same potential as the conductor, the presence of the ring reduces the potential gradient at the surface of the conductor greatly, below the disruptive potential gradient, so corona does not form on the metal points.
Advantage of corona loss:
- It reduces the Voltage fronted wave steeps during the lightening or switching which are dissipated in terms of corona loss.
- It is used in many applications such as Van de craft generators, ESP, electro-printing etc.
Disadvantage:
- However, corona leads to Power loss.
- Due to the oxygen formation corona creates correction on the electrical material.
- Reduces efficiency of the line.
- Triple frequency current causes electromagnetic interference on the communication line.
Artisan Industry：
Transmission Line Corona Rings Manufacturer
Price Inquiry：lilychin@vip.163.com
WhatsApp：+86 150 5061 5346
Product Name: High Voltage Corona Rings
Size: As Drawing Request
Surface: Polishing Silvery Bright, Ra3.2
High Voltage Corona Rings Specification
High voltage corona rings are widely used on the end fittings of high voltage bushings and other high voltage testing equipments. High voltage can result in unwanted noise and corona, so corona rings are needed.
High voltage corona rings are suitable for the voltage form of alternating current (AC), which can distribute the high voltage evenly around the object to ensure that there is no potential difference (PD) between the various parts of the ring.
The high voltage corona ring are used in high voltage test equipment and power systems (substations, high voltage transmission line, AC voltage system, etc.) and are mainly made of aluminum alloy. Generally the corona ring needs to be polished bright to achieve more smooth and no burr surface finish to ensure better corona control effect.
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