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

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.
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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
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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.
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What Are Frequency Inverters?

Frequency inverters are used to adapt the power coming from source into a piece of equipment like an electric motor. There are two types of current – direct current (DC) and alternating current (AC) and both work differently. Inverters were invented to allow Alternating Currents or Direct Currents to be used together, even though they are very different, to produce the energy needed to power items.
Lenze SMD inverter

AC and DC current

Direct Current is the simplest one to explain. The current all runs in the same direction, making a ‘chain’ or ‘circuit’ of power. Direct Current is used in standard circuit products like torches where the batteries are inserted and complete the circuit allowing the product to work.
Alternating Current works differently. This power supply changes direction about 50 to 60 times a second. This can also be written as 50 – 60 Hz a second. When the two types are brought together the electrons in both the currents are combined so the Alternating Current is continually ‘hit’ by the direct current and ultimately makes it stand still within the cables, forcing the energy through from source to equipment.

Electric Motors

Different motors need different currents to work but the electricity supplied is usually Alternating Current. This means, an inverter needs to break the current between the Alternating Current and the Direct Current and mix it up, otherwise the motor will just not work. The inverters job is to convert the power to what is needed. It works like a switch, alternating the supply on a regular basis to about 50 – 60 Hz a second, to produce the energy needed to power the motor.
There are DC motors and AC motors, but both need both types of current to work completely. When a motor is first switched on, the power generated can start off at speed with no way of controlling it. Introducing the opposite current via an inverter, allows the user to vary and control the speed, making the equipment more useable. Electric motors need inverters to adjust the current being supplied so the user can control it, making the motor more effective in its running and usage.

Selecting a Supplier

As you can tell, inverters are complicated pieces of equipment and as such, you need to find a supplier who actually understands them. Find a company, such as Bearing Boys, who can offer the best in the market, built from reliable companies such as the German firm Lenze.
The inverters need to be able to convert what you need them to while also offering options of having different phases. Inverters can come in all shapes and sizes with single-phase input, 3-phase input and 3-phase output. Having variations in the range mean you can discuss options to suit your needs.
Variations do not stop at outputs; you may need one that can withstand moisture in the air or work areas that are washed down on a regular basis. Keep an eye out for these products if that is what you need. No point spending the money before discovering it will blow the first time you clean the area. Ultimately, you need the products to be affordable, so value for money is top of the list.
Purchase from a company such as Bearing Boys that has the knowledge and support to help you make the right choices for you and not for them.
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Insulators used in overhead power lines

Overhead line insulators

It is obvious that if overhead power lines are not properly insulated from their support poles/towers, the current will flow towards the ground through the poles/towers which also become hazardous. Of course, the power line won't even work in that case! Hence, overhead power lines are always supported on insulators mounted on their support poles/towers.
Overhead line insulators should have the following properties:
  • high mechanical strength in order to withstand the conductor load, wind load etc.
  • high electrical resistance in order to minimize the leakage currents
  • high relative permittivity of insulating material so that the dielectric strength is high
  • high ratio of puncture strength to flashover
Most commonly used material for overhead line insulators is porcelain. But glass, steatite and some other special composite material may also be used sometimes.

Types of insulators used in overhead power lines

For the successful operation of power lines, proper selection of insulators is very essential. There are several types of overhead line insulators. Most commonly used types are
  • Pin type insulators
  • Suspension type insulators
  • Strain insulators
  • Shackle insulators

Pin type insulators

Pin type insulators or pin insulators are popularly used in electric distribution systems up to 33 kV voltage level. They are secured on the cross arms of the pole to carry power lines. There is a groove on the upper end of a pin insulator for housing the conductor. Conductor wire is passed through this groove and secured by binding with the same wire as of conductor.

pin insulator
A pin insulator is usually made from porcelain, but glass or plastic may also be used in some cases. As pin insulators are almost always employed in open air, proper insulation while raining is also an important consideration. A wet pin insulator may provide a path for current to flow towards the pole. To overcome this problem, pin insulators are designed with rain sheds or petticoats. Beyond operating voltage of 33kV, pin insulators become too bulky and uneconomical.

Insulation failure

An insulator must be properly designed so as to withstand mechanical as well as electrical stresses. Electrical stress on insulator depends on the line voltage, and hence, proper insulators must be used according to the line voltage. Excess electrical stress can break-down the insulator either by flash-over or puncture.
  • Flash-over: In insulator flash-over, electrical discharge occurs by forming an arc between the line conductor and the insulator pin (which is connected to the cross-arm). The discharge jumps through the air surrounding the insulator following the shortest distance. In case of a flash-over, the insulator continues to act according to its designed capacity unless it gets destroyed due to the excess heat.
  • Puncture: In case of insulator puncture, electrical discharge occurs from conductor to pin through the body of the insulator. Sufficient thickness of porcelain (or the insulator material) must be provided to avoid a puncture breakdown. When such breakdown is involved, the insulator is permanently damaged.
  • Safety factor of insulator: The ratio of puncture strength to flash over voltage is called as safety factor. It is desirable to have high value of safety factor so that a flash-over takes place before the insulator gets punctured. For pin type insulators, the value of safety factor is about 10.

Suspension insulators

As it is already mentioned above, pin insulators become too bulky an uneconomical beyond 33 kV. So, for voltages higher than 33 kV, suspension insulators are used. A suspension insulator consists of a number of porcelain discs connected to each other with metal links in the form of a string. Line conductor is suspended at the bottom end of the suspension string which is secured to cross-arm of the tower. Each disc in a suspension insulator string is designed for a low voltage, say 11 kV. The number of discs in a string depends on the working voltage.

suspension string insulator

Advantages of suspension insulators

  • Each unit of disc is designed for a low voltage, say 11 kV. Hence, depending upon the working voltage, desired number of discs can be connected in series to form an insulator string suitable for particular voltage.
  • If any of the discs in insulator string is damaged, it can be replaced easily. Replacement of the whole string is not required.
  • In case of increased demand on the line, the line voltage can be increased and the additional insulation required for the raised voltage can be easily provided by adding the desired number of discs in the insulator strings.
  • As the line conductors are suspended by suspension strings, they run below the earthed cross-arms of the towers. This arrangement provides partial protection from lightning.
  • The suspension arrangement provides greater flexibility to the line. Suspension insulators are allowed to swing so that they can take up the position where mechanical stresses are minimum.

Strain insulators

strain insulator

At a dead end of a transmission line or at a corner or sharp curve, the transmission line is subjected to a great tensile load. In order to sustain this great tension, strain insulators are used at dead ends or sharp corners. For high voltage transmission lines, stain insulator consists of an assembly of suspension insulators. In this case, the suspension string is arranged horizontally and the insulator discs are in vertical plane. Two or more suspension strings can be assembled in parallel to sustain greater tensions. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators.

Shackle insulators

shackle insulator
Shackle insulators are used in low voltage distribution lines as strain insulators. A shackle insulator can be used vertically as well as horizontally and it can be directly fixed to a pole with a bolt or to the cross arm. However, the use of such insulators is decreasing after increasing the use of underground cables for distribution purpose.
Some additional types of insulators are: post insulators, glass insulators, polymer insulators, long-rod insulators, stay insulators etc. Read about these additional types of insulators here.
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