If you can't explain it simply, then you don't know it well enough. — Albert Einstein

Corona rings and Grading rings

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.

How 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.

corona discharge on a corona ring
Corona discharge on a corona ring
Image source: Wikimedia commons

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 rings and grading rings

Testing of overhead line insulators

Proper operation of a transmission or distribution line is highly dependent upon the proper working of insulators. A good insulator should have a good mechanical strength to withstand the mechanical load and stresses. It should have a high dielectric strength to withstand operating and flashover voltages. Also, an insulator must be free from pores or voids, which may damage it. Therefore, to ensure desired performance of insulators, each insulator has to undergo various tests.

testing of overhead line insulators

Testing of insulators

Following are the different types of tests that are carried out on overhead line insulators.
  1. Flashover tests
  2. Performance tests
  3. Routine tests

Flashover tests of insulators

Three types of flashover tests are conducted before the insulator is said to have passed the flashover test.
  1. Power frequency dry flashover test
  2. Power frequency wet flashover test
  3. Impulse frequency flashover test

Power frequency dry flashover test

The insulator to be tested is mounted in the same manner in which it is to be used. Then, a variable voltage source of power frequency is connected between the electrodes of the insulator. The voltage is gradually increased up to the specified voltage. This specified voltage is less than the minimum flashover voltage. The voltage at which surrounding air of the insulator breaks down and become conductive is known as flashover voltage. The insulator must be capable of withstanding the specified voltage for one minute without flashover.

Power frequency wet flashover test (Rain test)

In this test also, the insulator to be tested is mounted in the same manner in which it is to be used. Similar to the above test, a variable voltage source of power frequency is connected between the electrodes. Additionally, in this test, the insulator is sprayed with water at an angle of 45° in such a manner that its precipitation should not be more than 5.08 mm/min. The voltage is then gradually increased up to the specified voltage. The voltage is maintained at the specified value for 30 seconds or one minute and the insulator is observed for puncture or breakdown. If the voltage is maintained for one minute, this test is also called as one-minute rain test.

Impulse frequency flashover test

This test is to ensure that the insulator is capable of sustaining high voltage surges caused by lightning. The insulator under test is mounted in the same manner as in above tests. An impulse voltage generator which generates a very high voltage at a frequency of several hundred kilohertz is connected to the insulator. This voltage is applied to the insulator and spark-over voltage is noted. The ratio of impulse spark-over voltage to spark-over voltage at power frequency is called as the impulse ratio. This ratio should be approximately 1.4 for pin type insulators and 1.3 for suspension type insulators.
[Also read: String efficiency of suspension insulators]

Performance tests of insulators

  1. Temperature cycle test
  2. Puncture voltage test
  3. Mechanical strength test
  4. Electro-mechanical test
  5. Porosity test

Temperature cycle test

In this test, the insulator under test is first heated in water at 70° for one hour. Then the insulator is immediately cooled at 7° for another hour. This cycle is repeated three times. Then the insulator is dried and its glazing is thoroughly observed for any damages or deterioration.

Puncture voltage test

The purpose of this test is to determine the puncture voltage. The insulator to be tested is suspended in insulating oil. A voltage is applied and increased gradually until the puncture takes place. The voltage at which insulator starts to puncture is called as puncture voltage. This voltage is usually 30% higher than that of the dry flash-over voltage for a suspension type insulators.

Mechanical strength test

In this test, the insulator under test is applied by 250% of the maximum working load for one minute. This test is conducted to determine the ultimate mechanical strength of the insulator.

Electro-mechanical test

This test is conducted only for suspension type insulators. In this test, a tensile stress of 250% of maximum working tensile stress is applied to the insulator. After this, the insulator is tested for 75% of dry spark-over voltage.

Porosity test

In this test, a freshly manufactured insulator sample is broken into pieces. These pieces are then immersed into a 0.5% to 1% alcohol solution fuchsine dye under pressure of 150 kg/cm2 for several hours (say 24 hours). After that, the pieces are removed from the solution and examined for the penetration of the dye into it. This test indicates the degree of porosity.

Routine tests of insulators

  1. High voltage test
  2. Proof load test
  3. Corrosion test

High voltage test

This test is usually carried out for pin insulators. In this test, the insulator is inverted and placed into the water up to the neck. The spindle hole is also filled with water and a high voltage is applied for 5 minutes. The insulator should remain undamaged after this test.

Proof load test

In this test, each insulator is applied with 20% in excess of working mechanical load (say tensile load) for one minute. The insulator should remain undamaged after this test.

Corrosion test

In this test, the insulator with its metal fitting is suspended into a copper sulfate solution for one minute. Then the insulator is removed from the solution and wiped and cleaned. This procedure is repeated for four times. Then the insulator is examined for any metal deposits on it. There should be zero metal deposits on the insulator.

String efficiency of suspension insulators and methods of improving it

A suspension type string insulator consists of a number of porcelain discs connected in series through metallic links. Suspension insulators or string insulators are very widely used in electrical overhead transmission system. However, there is a significant thing to be considered in case of these string insulators, known as string efficiency.

Potential distribution over a suspension insulator string

The figure below shows a 3-disc string of suspension insulator. As each porcelain disc lies in between two metal links, it forms a capacitor. This capacitance is known as self-capacitance or mutual capacitance. Moreover, air capacitance is also present between metal links and the earthed tower. This is known as shunt capacitance. The figure below illustrates the equivalent circuit of a 3-disc suspension insulator (assuming that shunt capacitance is some fraction of self-capacitance i.e shunt capacitance = k * self-capacitance).

string efficiency

If there were only mutual capacitances, then the charging current would have been the same through all the discs. In this case, the voltage would have been uniformly distributed across the string, i.e. voltage across each disc would have been the same. But, due to the shut capacitances, charging current is not the same through all the discs.

From the above equivalent circuit, applying Kirchoff's current law to node A,
I2 = I1 + i1
V2ωC = V1ωC + V1ωkC
V2 = V1 + V1k
V2 = (1 + k)V1              ...... eq.(i)

applying Kirchoff's current law to node B,
I3 = I2 + i2
V3ωC = V2ωC + (V2 + V1)ωkC
V3 = V2 + (V1 + V2)k
V3 = kV1 + (1 + k) V2
V3 = kV1 + (1 + k)2 V1       ...... from eq.(i)
V3 = V1 [k + (1 + k)2]
V3 = V1 [k + 1 + 2k + k2]
V3 = V1 (1 + 3k + k2)              ...... eq.(ii)

Now, voltage between the conductor and the earther tower is,
V = V1 + V2 + V3
V = V1 + (1 + k)V1 + V1 (1 + 3k + k2)
V = V1 (3 + 4k + k2)               ...... eq.(iii)

from the above equations (i), (ii) & (iii), it is clear that the voltage across the top disc is minimum while voltage across the disc nearest to the conductor is maximum, i.e. V3 = V1 (1 + 3k + k2). As we move towards the cross arm, voltage across the disc goes on decreasing. Due to this non-uniform voltage distribution across the string, the unit nearest to the conductor is under maximum electrical stress and is likely to be punctured.

String efficiency

As explained above, voltage is not uniformly distributed over a suspension insulator string. The disc nearest to the conductor has maximum voltage across it and, hence, it will be under maximum electrical stress. Due to this, the disc nearest to the conductor is likely to be punctured and subsequently, other discs may puncture successively. Therefore, this unequal voltage distribution is undesirable and usually expressed in terms of string efficiency.
The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is called as string efficiency
String efficiency = Voltage across the string / (number of discs X voltage across the disc nearest to the conductor).
Greater the string efficiency, more uniform is the voltage distribution. String efficiency becomes 100% if the voltage across each disc is exactly the same, but this is an ideal case and impossible in practical scenario. However, for DC voltages, insulator capacitances are ineffective and voltage across each unit would be the same. This is why string efficiency for DC system is 100%.
Inequality in voltage distribution increases with the increase in the number of discs in a string. Therefore, shorter strings are more efficient than longer string insulators.

Methods of improving string efficiency

(i) Using longer cross arms

It is clear from the above mathematical expression of string efficiency that the value of string efficiency depends upon the value of k. Lesser the value of k, the greater is the string efficiency. As the value of k approaches to zero, the string efficiency approaches to 100%. The value of k can be decreased by reducing the shunt capacitance. In order to decrease the shunt capacitance, the distance between the insulator string and the tower should be increased, i.e. longer cross-arms should be used. However, there is a limit in increasing the length of cross-arms due to economic considerations.

(ii) Grading of insulator discs

In this method, voltage across each disc can be equailized by using discs with different capacitances. For equalizing the voltage distribution, the top unit of the string must have minimum capacitance, while the disc nearest to the conductor must have maximum capacitance. The insulator discs of different dimensions are so chosen that the each disc has a different capacitance. They are arranged in such a way that the capacitance increases progressively towards the bottom. As voltage is inversely proportional to capacitance, this method tends to equalize the voltage distribution across each disc.

(iii) By using a guard or grading ring

A guard ring or grading ring is basically a metal ring which is electrically connected to the conductor surrounding the bottom unit of the string insulator. The guard ring introduces capacitance between metal links and the line conductor which tends to cancel out the shunt capacitances. As a result, nearly same charging current flows through each disc and, hence, improving the string efficiecy. Grading rings are sometimes similar to corona rings, but they encircle insulators rather than conductors.

Electrical Insulators - insulating materials and different types

Purpose of an insulator is to prevent the unwanted flow of current from the energized conductor or conducting parts. Electrical insulation plays a vital role in every electrical system. An electrical insulator provides very high resistance so that practically no current can flow through it.

Insulating Materials

Basically, an insulating material or insulator contains a very small amount of free electrons (charge carriers) and, hence, could not carry electrical current. But, a perfect insulator does not exist, because even insulators contain a small number of charge carriers which may carry leakage current (negligibly small). In addition, all insulators become conductive when sufficiently large voltage is applied. This phenomenon is called as insulation breakdown and the corresponding voltage is called as breakdown voltage.
An insulating material must have high resistivity and high dielectric strength. Additional desirable properties of insulating material depend on the type of applications. Insulating material used for manufacturing insulated cables/wires must be flexible such as rubber or PVC. On the other hand, insulator used to support overhead power lines must be mechanically strong, such as porcelain or glass insulators.

Important properties of insulating materials

  • Resistivity (specific resistance) is the property of a material that quantifies how strongly the material opposes to flow of electric current. Resistivity of a good insulator is very high.
  • Dielectric strength of a material is the ability to withstand electric stresses without breaking down. Dielectric strength is usually quoted in kilovolts per millimeter (kV/mm).
  • Relative permittivity (or dielectric constant) is the ratio of the electric flux density produced in the material to that produced in vacuum.
  • Electrical dissipation factor (dielectric loss) is the ratio of the power lost in the material to the total power transmitted through it. It is given by the tangent of the loss angle and, hence, also known as tan delta
Some of the commonly used electrical insulating materials are paper, mica, teflon, rubber, plastic, polyvinyl chloride (PVC), glass, ceramic, porcelain etc.

Types of electrical insulators

  • Pin insulators
  • Suspension insulators
  • Strain insulators
  • Shackle insulators
The above types of insulators are commonly used in overhead power lines. You can read more about these overhead line insulators by clicking this link.
Some more types of insulators are as follows.

Post insulators

A post insulator is more or less similar to a pin insulator. It has relatively higher number of petticoats and rain sheds. Post type insulators are mostly used in substations, but in some cases, they can be used for overhead lines also. Thus, there are two types of post insulators: (i) Station post insulators and (ii) Line post insulators.
post insulators
Image source: Wikimedia commons
A line post insulator can be used for voltages up to 132 kV (pin insulators are used for up to 33 kV). Station post insulators are used in substations for low as well as very high voltages. For higher voltage levels, multiple station post insulators are stacked together.

[Also read: Basics of power transmission system]

Glass insulators

Pin type glass insulators were earlier used in the 18th century primarily for telegraph/telephone lines. Use of ceramic and porcelain insulators spread in the 19th century. They proved superior protective properties than glass and become widely used. However, use of toughened glass insulators is becoming popular today. Unlike porcelain or non-ceramic materials, toughened glass never ages, and thus, offers a longer lifespan. Toughened glass insulator discs can be used in suspension insulators.
glass insulator string
Glass insulator string

Polymer insulators

Polymer insulators are composed of a fibre glass rod covered by polymer weather sheds. Polymer weather sheds are generally made from sillicon rubber. Few other materials may also be used for weather sheds, such as polytetrafluorethylene (PTFE or Teflon), EPM, EPDM etc. Polymer insulator are sometimes also called as composite insulators or silicon rubber insulators. They are almost 90% lighter than porcelain insulators and still offer almost equal or better strength.
polymer insulators
Polymer insulators

Long rod insulators

A long rod insulator is basically a porcelain rod with an outside weather shed and metal end fittings. The main advantage of long rod design is the elimination of metal parts between the units, thereby increasing the insulator's strength. Long rod insulators can be used at suspension locations as well as tension locations.
long rod insulator
Long rod insulator
Image source: Wikimedia commons

Stay insulators

The insulator used in a stay wire (guy wire) is called as stay insulator. It is usually made up from porcelain and is designed so that in case of breakage of the insulator, the stay wire will not fall to the ground. It is also called as egg-type strain insulator.
stay insulator
Stay Insulator
Image source: Wikimedia commons

Top 3 Online Electrical Engineering Courses

Online Degree Programs have been around for many years, however until recent years were not considered to be an equal to in the classroom learning. But, as tuition has skyrocketed making it a requirement for many to work while getting their degree, Online Degree Programs have become more acceptable and diverse; providing the flexibility many people need in order to get the education they want and still pay the bills. The most important thing is choosing the right online courses to fit your needs and educational requirement levels.
Top 3 online electrical engineering courses

Why choose to get your Degree or Certification Online?

Online Degree Programs are a great educational alternative to traditional in the classroom learning for both new students fresh out of high school, and those individuals who may have been out of school working for a while who want to get their degree. Besides being more flexible than in class degree programs as to when you are attending instruction, Online courses are a great way for someone who has been in their field for an extended period of time to refresh their knowledge and skills or acquire new certifications, as well as receive updates in regards to industry changes. This is especially so in technical fields such as Electrical Engineering.

Choosing a Program that Counts

It is important when choosing an online program to make sure the classes are accredited and will work towards your goals. Choose an accredited school and verify with the academic advisors that the classes are accredited in the accreditation program that you are interested in. Since there are more than one or two accreditation organizations out there. This will save you frustration, time, and money in the end.

[Also read: 11 of the best universities for electrical engineering]

The Top Online Electrical Engineering Degree Programs

According to the Guide for Online Schools website, the accreditation that you should be looking for in the Electrical Engineering field is the Accreditation Board for Engineering and Technology (ABET), and the top three schools that offer this are:
  • University of Delaware
  • University of Arizona – Engineering
  • Grantham University
These schools were chosen based on the type of degree programs offered, the tuition rates, and the number of Electrical Engineering Programs offered within the Engineering Departments, as well as their programs meet all of the academic requirements to be ABET accredited.

University of Delaware

Although the tuition is not the cheapest around, coming in at just under $30,000 per year at the time of this article, they do have one of the most expansive Engineering Divisions, with 18 programs and 1 being specifically in Electrical Engineering. The Electrical Engineering degree that you will receive upon completion of their program is a Master of Science in Electrical and Computer Engineering, making it a postgraduate program.

University of Arizona – Engineering

For a little less money, you can also obtain the same degree at the University of Arizona for just over $27,000 per year. However, the selection of programs is significantly less with only four Engineering programs and one of which being in the Electrical Engineering specialty. These programs are also fully accredited by ABET.

Grantham University

Grantham University comes in third simply because they do not off a Master’s program. But, if that is not a requirement for you, this is a school you should look into. The tuition runs about $6,500 per year, and they have 44 Engineering Programs with two of them being Electrical Engineering Degrees; Associate of Science in Electronics and Computer Engineering, and the Bachelors of Science in Electronics Engineering Technology.

Other things to Consider

One of the nice things about online classes is that you can live anywhere and attend that school. It is not always a good thing to choose a school out of your area though. If you need to deal with financial or administrative issues you will have to do it all either by mail, phone, or email. Additionally if you need extra help such as a writing service, or tutoring services you will have to search outside of the school since you will not be near campus to take advantage of these services if they offer them. Some universities have taken this into consideration and established satellite offices or schools to offer these services to their out of state online students as a way to entice more enrollment in their programs. This is something that you should figure into your decision making process.
Author: Elizabeth Leer is a former Children's Book Editor, with more than 3 decades of business world experience and writing for businesses, Elizabeth is able to write on just about anything.