Electrical Insulator

What is an Electrical Insulator?

An electrical insulator (also referred to as an insulator) is used in an electrical system to prevent unwanted flow of current to the earth from its supporting points. The insulator plays a vital role in the electrical system. An electrical insulator is a very high resistive path through which practically no current can flow.

In transmission and distribution systems, the overhead conductors are generally supported by supporting towers or poles. The towers and poles both are properly grounded. So there must be an insulator between tower or pole body and current-carrying conductors to prevent the flow of current from conductor to earth through the grounded supporting towers or poles.

Insulating Material

The main cause of failure of the overhead line insulator is flashover, which occurs between line and earth during abnormal overvoltage in the system. During this flashover, the huge heat produced by arcing causes puncher in the insulator body. Viewing this phenomenon the materials used for electrical insulator has to possess some specific properties.

Properties of Insulating Material

The materials generally used for the insulating purpose is called insulating material. For successful utilization, this material should have some specific properties as listed below-

It must be mechanically strong enough to carry the tension and weight of conductors.It must have a very high dielectric strength to withstand the voltage stresses in High Voltage transmission systems. It must possess high Insulation Resistance to prevent leakage current to the earth.The insulating material must be free from unwanted impurities.It should not be porous.There must not be any entrance on the surface of the electrical insulator so that the moisture or gases can enter it.There physical as well as electrical properties must be less effected by changing temperature.

Porcelain Insulator

Porcelain is the most commonly used material for overhead insulators in the present day. The porcelain is aluminum silicate. The aluminum silicate is mixed with plastic kaolin, feldspar, and quartz to obtain final hard and glazed porcelain insulator material.

The surface of the insulator should be glazed enough so that water should not be traced on it. Porcelain also should be free from porosity since porosity is the main cause of deterioration of its dielectric property. It must also be free from any impurity and air bubble inside the material which may affect the insulator properties.

Properties of Porcelain Insulator

Property        Value(Approximate)
Dielectric Strength     60 kV / cm
Compressive Strength 70,000 Kg / cm²
Tensile Strength 500 Kg / cm²

Glass Insulator

Nowadays glass insulators have become popular in transmission and distribution systems. Annealed tough glass is used for the insulating purpose. Glass insulator has numbers of advantages over conventional porcelain insulator

Advantages of Glass Insulator

It has a very high dielectric strength compared to porcelain.Its resistivity is also very high.It has a low coefficient of thermal expansion.It has a higher tensile strength compared to porcelain insulator.As it is transparent in nature the is not heated up in sunlight as porcelain.The impurities and air bubbles can be easily detected inside the glass insulator body because of its transparency.Glass has a very long service life because the mechanical and electrical properties of glass do not be affected by aging.After all, glass is cheaper than porcelain.

Disadvantages of Glass Insulator

Moisture can easily be condensed on the glass surface and hence air dust will be deposited on the wed glass surface which will provide a path to the leakage current of the system.For higher Voltage glass can not be cast in irregular shapes since due to irregular cooling internal strains are caused.

Properties of Glass Insulator

Property    Value(Approximate)
Dielectric Strength 140 kV / cm
Compressive Strength 10,000 Kg / cm²
Tensile Strength 35,000 Kg / cm²

Polymer Insulator

A polymer insulator has two parts, one is glass fiber reinforced epoxy resin rod-shaped core and the other is silicone rubber or EPDM (Ethylene Propylene Diene Monomer) made weather sheds. The rod-shaped core is covered by weather sheds. Weather sheds protect the insulator core from the outside environment. As it is made of two parts, core and weather sheds, polymer insulator is also called a composite insulator. The rod-shaped core is fixed with Hop dip galvanized cast steel made end fittings on both sides.

Advantages of Polymer Insulator

It is very lightweight compared to porcelain and glass insulator.As the composite insulator is flexible the chance of breakage becomes minimum.Because of lighter in weight and smaller in size, this insulator has lower installation costs.It has a higher tensile strength compared to a porcelain insulator.Its performance is better, particularly in polluted areas.Due to lighter weight polymer insulator imposes less load to the supporting structure.Less cleaning is required due to the hydrophobic nature of the insulator.

Disadvantages of Polymer Insulator

Moisture may enter in the core if there is any unwanted gap between core and weather sheds. This may cause the electrical failure of the insulator.Over crimping in end fittings may result in cracks in the core which leads to mechanical failure of polymer insulator.

In addition to this, some other disadvantages might be experienced. Let us give a practical example where many difficulties are faced in maintaining a distribution network in Victoria Australia due to polymeric insulators.

There are many Cockatoos, Galahs, and Parrots in that area of Australia, which loves to chew on polymeric strain insulators. Here, the 22 kV network has many polymeric strain insulators installed, and now after a few years of installing polymeric strain insulators, the authority is now replacing many of them back with Glass disc insulators.

Another disadvantage is that they have had post type polymeric insulators melt and bend in bush fire areas. They have a concrete pole and a steel cross arm that survives a bush fire, however, the polymers in some cases fail. This would not be the case with glass or porcelain insulators.

They have also had polymeric insulators fail in areas close to the ocean coastline where there are high salt levels in the air.

Subject to bird attack by Parrots, Cockatoos, and Galahs.Not resilient to bushfire temperatures.Not recommended for a location near surf beaches due to salt spray.

The information is contributed by Robert Lancaster of Australian Electricity Supply Industry

Types of Insulators

There are mainly three types of insulator likewise

Pin InsulatorSuspension InsulatorStray Insulator

In addition to that, there are other two types of electrical insulators available mainly for low voltage applications, i.e. stay insulator and shackle insulator. Read more about the various types of insulators used in transmission lines.

Corona

This is electrical Corona 

A corona discharge is an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged. Spontaneous corona discharges occur naturally in high-voltage systems unless care is taken to limit the electric field strength. A corona will occur when the strength of the electric field (potential gradient) around a conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same property as a gas discharge lamp.

In many high voltage applications, corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO2), and thus nitric acid (HNO3) if water vapor is present. These gases are corrosive and can degrade and embrittles nearby materials, and are also toxic to humans and the environment.

Corona discharges can often be suppressed by improved insulation, corona ring, and making high voltage electrodes in smooth rounded shapes. However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.

Cooling water

Correct statement about cooling water used in condenser of steam power plant:
1. The colder the water the more is the efficiency
2. The warmer the water the more is the efficiency
3. Water temperature has no effect on efficiency
4. The cooling water should be used in very small quantity
Answer: 2. The warmer the water the more is the efficiency

Answers to Questions 1-7

1. A voltmeter connects in:
1. Series
2. Parallel
3. Any of these
4. None of these
Tag your electrical engineer friends
#eee-af
The correct answer is 2.

2. The form factor of sinusoidal alternating current is:
1. 0.97
2. 1.11
3. 3.54
4. 7.48
Tag your electrical engineer friends
#eee-af
The correct answer is 2.

3. If the span is increased, the sag will:
1. Increase
2. Decrease
3. Remain constant
4. None of these
Tag your electrical engineer friends
#eee-af
The correct answer is 1.

4. Voltage regulation of an alternator when connected to inductive load:
1. Positive
2. Negative
3. Zero
4. None of these
Tag your electrical engineer friends
#eee-af
The correct answer is 1.

5. Country with the highest uranium resources:
1. Australia
2. Canada
3. Namibia
4. Nigeria
Tag your electrical engineer friends
#eee-af
The correct answer is 1.

6. Which one of the following is not a part of DC Machines:
1. Commutator
2. Yoke
3. Damping winding
4. Field winding
Tag your electrical engineer friends
#eee-af
The correct answer is 3.

7. An alternator converts ____________ to ______________:
1. Alternating current, Direct current
2. Direct current, Alternating current
3. Electrical Energy, Mechanical Energy
4. Mechanical energy, Electrical Energy
Tag your electrical engineer friends
#eee-af
The correct answer is 4.

How electricity is delivered to our homes?

I work for an electric utility and this is how we do it.

It all starts at the generating station. In this case, It is coal fired. The generators generate electric at 13.8 thousand volts (13.8kv). As the power leaves the building, it goes in to a transformer that steps up the voltage up to 138 thousand volts (138kv). It then goes to a substation where it is sent out over several lines that take it to other substations. Some of the power in that first substation is also put through another transformer that steps up the voltage to 345 thousand volts (345kv) for long distance transmission. Higher voltages are used for longer distance transmission since you can get by with smaller wires.

A large 345kv to 138kv transformer. I am standing next to it to give an idea of scale. I am 6'-2" (188cm)

These substations contain breakers just like you have in your house only much larger to protect the system in case of a problem. Each circuit coming in and going out of any substation will have at least one breaker on it for protection.

Here is a 345kv breaker. Just to the right is the stand that it will be mounted on. Perhaps I should have posed like a spokesmodel for these pictures.

So, the electric leaves the generation station substation and travels down the wires at 138kv. It enters another substation where it goes in to a transformer where it is stepped down to 69kv and is divided in to several circuits.

Here is a smaller 69kv to 12.5kv transformer. Yes, I'm aware that I'm dressed like a slob. They don't pay me to be pretty.

If we follow one of the 69kv circuits, it will then either go to another location within this substation or it will travel down the wires to a smaller substation. For the purposes of this explanation, we will assume that it goes to another smaller substation. When it gets there, it goes in to another transformer that steps it down to 12.5kv. That voltage is then sent to a switch gear with breakers where it is divided up in to more circuits.

This is a 69kv breaker. The legs it would be mounted on are laying across the bottom. Next time I'll hire a model to pose for these shots. Jeez, you people are so critical.

These 12.5kv circuits then go out to provide power to a neighborhood. Once it gets there, it will go to several small transformers that are either hanging on a pole with the overhead wires or a transformer on the ground if the wires are run underground. These small transformers will feed several homes and step the voltage down to 2 legs with 120 volts each for a total of 240 volts. This voltage goes into your home at the circuit breaker panel and more breakers and is distributed throughout your home.

Here is a switchgear, used to send the power out to your neighborhoods. This is the last picture of me you will have to endure.

So there it is. A quick and dirty guide to how electric gets to your house. However, I have left out a lot of details that would unnecessarily make this post a lot longer as most people would not really be interested. There is a lot of redundancy on the system in order to keep things running in the event of problems. There is a lot of technology being used for protection and such. If taken as a whole, the American electric grid is the most complicated machine that mankind has ever created. I have worked for the electric utility for 10 years and I am still awed by what it takes to keep the lights on.

کرونا CORONA

CORONA کرونا

Air is not a perfect insulator, and even under normal conditions, the air contains many free electrons and ions. When an electric field intensity establishes between the conductors, these ions and free electrons experience forced upon them. Due to this effect, the ions and free electrons get accelerated and moved in the opposite direction. The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules. Thus, the number of charged particles goes on increasing rapidly. This increase the conduction of air between the conductors and a breakdown occurs. Thus, the arc establishes between the conductors.

The undesirable effects of the corona are:

1. The glow appear across the conductor which shows the power loss occur on it.
2. The audio noise occurs because of the corona effect which causes the power loss on the conductor.
3. The vibration of conductor occurs because of corona effect.
4. The corona effect generates the ozone because of which the conductor becomes corrosive.
5. The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur in the line.
6. The corona power loss reduces the efficiency of the line.
7. The radio and TV interference occurs on the line because of corona effect.

Corona decreases the efficiency of transmission lines. Therefore, it is necessary to minimize corona.The following factors may be considered to control corona:

1. Conductor diameter – For reducing corona loss, this method of increasing conductor diameters is very effective.Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminum conductors(ACSR) conductors.
2. The voltage of the line – Voltage of transmission lines is fixed by economic considerations. To increase the disruptive voltage the spacing of the conductors is to be increased, but this method has some limitations.
3. Spacing between conductors – If the space between conductors increases, then the voltage drops between them also increases due to increase in inductive reactance.

Transposition

Transposition of conductors in Power Transmission Lines

Parameters of Transmission Line:

A transmission line has four parameters, namely resistance, inductance, capacitance and conductance. The resistance ‘R’ of a line is because of conductor resistance, series inductance ‘L’ is due to the magnetic field surrounding the conductors, shunt capacitance ‘C’ is due to the electric field between conductors, and shunt conductance, ‘G’ is because of the leakage current between phases and ground.

What is Transposition of Conductors?

The interchange of conductor positions of a transmission line at regular intervals along the route is known as Transposition of Conductors.

Why transposition is needed?

In the power transmission line when the line conductors are asymmetrically spaced i.e. not equally spaced, the inductance of each phase is different causing voltage drops of different magnitudes in the three phases even if the system is operating under balanced condition (load currents are balanced in the three phases). Also the magnetic field external to the conductors is not zero thereby inducing voltages in adjacent communication lines and causing what is known as “telecommunication interference”. This can be overcome by the interchange of conductor positions at regular intervals along the route and this practice is known as “transposition of conductors”.

How transposition is done?

In a transposed transmission line each of the three conductors occupies all the three positions relative to other conductors (position 1, position 2, and position 3) for one-third of the total length of the transmission line. Transposition also balances out the line capacitance so that electro-statically induced voltages are also balanced. Figure shows the transposition of conductors over a complete cycle.

A complete cycle of transposition of line conductors.

Complications of Conductor Transposition:

Frequent transposition usually leads to complication of support structures (as can be seen by the picture below), increase the cost because of increased number of insulator strings and total weight of supports. 

Transposition on 400 kV, double circuit transmission line, near Bhopal, M.P.