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.