Making Electricity

In Michael Faraday's generator, coils of copper wire rotating between the poles of a magnet produce a steady current of electricity. One way to rotate the disk is to crank it by hand, but this isn't a practical way to make electricity. Another option is to attach the shaft of the generator to a turbine and then let some other energy source power the turbine. Falling water is one such energy source, and, in fact, the first major plant ever built took advantage of the enormous kinetic energy delivered by Niagara Falls.

George Westinghouse opened that plant in 1895, but the principles of its operation haven't changed much since then. First, engineers build a dam across a river to create a reservoir of stored water. They place a water intake near the bottom of the dam wall, which allows water to flow from the reservoir and through a narrow channel called a penstock. The turbine -- imagine a huge propeller -- sits at the end of the penstock. The shaft from the turbine goes up into the generator. When the water moves across the turbine, it spins, rotating the shaft and, in turn, rotating the copper coils of the generator. As the copper coils spin within the magnets, electricity is produced. Power lines connected to the generator carry electricity from the power plant to homes and businesses. Westinghouse's Niagara Falls plant was able to transport electricity more than 200 miles (322 kilometers).

Not all power plants rely on falling water. Many take advantage of steam, which acts like a fluid and can therefore transfer energy to a turbine and, ultimately, to a generator. The most popular way to make steam is to heat water by burning coal. It's also possible to use controlled nuclear reactions to turn water into steam.

Of course, using a generator to make electricity is just the beginning. After you get your electrons moving along, you'll need an electrical circuit to do anything with it. Find out why next.

Direct Current Versus Alternating Current

Batteries, fuel cells and solar cells all produce something called direct current (DC). The positive and negative terminals of a battery are always, respectively, positive and negative. Current always flows in the same direction between those two terminals.


The power that comes from a power plant, on the other hand, is called alternating current (AC). The direction of the current reverses, or alternates, 60 times per second (in the U.S.) or 50 times per second (in Europe, for example). The power that is available at a wall socket in the United States is 120-volt, 60-cycle AC power.


The big advantage that alternating current provides for the power grid is the fact that it is relatively easy to change the voltage of the power, using a device called a transformer. Power companies save a great deal of money this way, using very high voltages to transmit power over long distances.


How does this work? Well, let's say that you have a power plant that can produce 1 million watts of power. One way to transmit that power would be to send 1 million amps at 1 volt. Another way to transmit it would be to send 1 amp at 1 million volts. Sending 1 amp requires only a thin wire, and not much of the power is lost to heat during transmission. Sending 1 million amps would require a huge wire.


So power companies convert alternating current to very high voltages for transmission (such as 1 million volts), then drop it back down to lower voltages for distribution (such as 1,000 volts), and finally down to 120 volts inside the house for safety. As you might imagine, it's a lot harder to kill someone with 120 volts than with 1 million volts (and most electrical deaths are prevented altogether today using GFCI outlets).

Electricity and Atomic Structure

Toward the end of the 19th­ century, science­ was barreling along at an impressive pace. Automobiles and aircraft were on the verge of changing the way the world moved, and electric power was steadily making its way into more and more homes. Yet even scientists of the day still viewed electricity as something vaguely mystical. It wasn't until 1897 that scientists discovered the existence of electrons -- and this is where the modern era of electricity starts.


Matter, as you probably know, is composed of atoms. Break something down to small enough pieces and you wind up with a nucleus orbited by one or more electrons, each with a negative charge. In many materials, the electrons are tightly bound to the atoms. Wood, glass, plastic, ceramic, air, cotton -- these are all examples of materials in which electrons stick with their atoms. Because these atoms are so reluctant to share electrons, these materials can't conduct electricity very well, if at all. These materials are electrical insulators.


Most metals, however, have electrons that can detach from their atoms and zip around. These are called free electrons. The loose electrons make it easy for electricity to flow through these materials, so they're known as electrical conductors. They conduct electricity. The moving electrons transmit electrical energy from one point to another.

Dogs that lived inside or within a fenced-in area, thereby keeping those pesky fleas contained, would be the equivalent of an electrical insulator. Free-roaming mutts, however, would be electrical conductors. If you had one neighborhood of indoor, pampered pugs and one neighborhood of unfenced basset hounds running wild, which group do you think could spread an outbreak of fleas the fastest?


So, electricity needs a conductor in order to move. There also has to be something to make the electricity flow from one point to another through the conductor. One way to get electricity flowing is to use a generator.

How Electricity Works

Generators

If you've ever moved paper clips around with a magnet or killed time arranging metal shavings into a beard on a "Wooly Willy" toy, then you've dabbled in the basic principles behind even the most complicated electric generators. The magnetic field responsible for lining up all those little bits of metal into a proper Mohawk haircut is due to the movement of electrons. Move a magnet toward a paper clip and you'll force the electrons in the clip to move. Similarly, if you allow electrons to move through a metal wire, a magnetic field will form around the wire.


Thanks to Wooly Willy, we can see that there's a definite link between the phenomena of electricity and magnetism. A generator is simply a device that moves a magnet near a wire to create a steady flow of electrons. The action that forces this movement varies greatly, ranging from hand cranks and steam engines to nuclear fission, but the principle remains the same.


One simple way to think about a generator is to imagine it acting like a pump pushing water through a pipe. Only instead of pushing water, a generator uses a magnet to push electrons along. This is a slight oversimplification, but it paints a helpful picture of the properties at work in a generator. A water pump moves a certain number of water molecules and applies a certain amount of pressure to them. In the same way, the magnet in a generator pushes a certain number of electrons along and applies a certain amount of "pressure" to the electrons.

In an electrical circuit, the number of electrons in motion is called the amperage or current, and it's measured in amps. The "pressure" pushing the electrons along is called the voltage and is measured in volts. For instance, a generator spinning at 1,000 rotations per minute might produce 1 amp at 6 volts. The 1 amp is the number of electrons moving (1 amp physically means that 6.24 x 1018 electrons move through a wire every second), and the voltage is the amount of pressure behind those electrons.


Generators form the heart of a modern power station. In the next section, we'll take a look at how one of these stations works.

DIFFERENTIAL PROTECTION IN TRANSFORMERS

The differential protection used for transformers is based on the principle of current circulation. This type of protection is mostly used for transformers as this responds not only to inter turn fault but also provides protection against phase-to-phase faults. Following are the complicated features in transformers and there remedial measures: 1. In a power transformer, the currents in primary and secondary are to be compared. As these two currents are usually different, therefore the use of identical tr4ansformers will give differential current and operate the relay even under no load conditions. The difference in magnitude of currents in primary and secondary of power transformers is compensated by different turns ratios of C.T.s. If T is the turn’s ratio of power transformer, then the differential protection used for transformers is based on the principle of current circulation. This type of protection is mostly used for transformers as this responds not only to inter turn fault but also provides protection against phase-to-phase faults. Following are the complicated features in transformers and there remedial measures:

1. In a power transformer, the currents in primary and secondary are to be compared.

As these two currents are usually different, therefore the use of identical transformers

will give differential current and operate the relay even under no load conditions. The difference in magnitude of currents in primary and secondary of power transformers is compensated by different turns ratios of C.T.s. If T is the turn’s ratio of power transformer, then the turns ration of C.T.s on lv side is made T times the turn’s ration of the C.T.s on hv side. When this condition is fulfilled the secondaries of the two C.T.s will carry same current under normal conditions. And thus no current will flow through the relay and it remains inoperative.

2. There is usually a phase difference between the primary and secondary currents of a 3-phase power transformer. Even if C.T.s of proper transformation ratios are used, a differential current will flow through the relay under normal condition and cause relay operation. The correction for phase difference is effected by appropriate connections of C.T.s. the C.T.s on one side of the power transformer are connected in such a way that the resultant current fed into the pilot wires are displaced in phase from the individual phase currents in the same direction as, and by an angle equal to, the phase shift between the power transformers primary and secondary currents. The table below shows the type of connections to be employed for C.T.s in order to compensate for the phase difference in the primary and secondary currents of power transformer

SL.NO	POWER TRANSFORMER CONNECTIONS		CURRENT TRANSFORMER CONNECTIONS
	PRIMARY	SECONDARY	PRIMARY	SECONDARY
1 2 3 4	Star with neutral earthed Delta Star Delta	Delta Delta Star with neutral earthed Star with neutral earthed	Delta Star Delta Star	Star Star Delta Delta

1. Another factor, which has to be considered, is the inrush of magnetizing current.

When the transformer is switched to supply the magnetizing current may assume very high values momentarily and may cause operation of the relay even though they are transient. This can be avoided by using relays with time delay characteristics.

Fig 8 shows the differential protection for transformer. In this the power transformer is delta- star connected. On delta side the C.T.s are connected in star and on the star side the C.T.s are connected in delta as in fig. Under normal working conditions the circulating currents caused by the primary and secondary load current in the relay circuit will balance; but under fault conditions the balance will no longer be there and the relay will be energized to trip the circuit breakers on the primary and secondary side

In order to understand the phase difference in the two sides consider fig 8. The primary is connected in delta and the set of current transformers CT1 is connected in star, while the secondary is connected in star and the set of current transformers CT2 is connected in delta. Fig 9 illustrates the vector diagram in reference to primary and secondary sides of current transformer. In fig 9.a IRP, IYP and IBP are the phase currents in the primary side, while IR is the line current on the same side in line R as shown in fig 9.a, the corresponding secondary current of current transformers CT1 on the primary side is in

phase with IR and is represented as IRS in fig 9.b. the current in the secondary side of the power transformer is represented as IR, IY and IB in fig 9.c, the phase current in the secondary winding of the current transformers CT2 is represented as I’R, I’Y and I’B in fig 9.d. the current in pilot wire of CT2 is represented as IRS. Now when we consider fig 9.b and 9.d its clear that the currents in the pilot wires are in phase.

In order to understand the phase difference in the two sides consider fig 8. The primary is connected in delta and the set of current transformers CT1 is connected in star, while the secondary is connected in star and the set of current transformers CT2 is connected in delta. Fig 9 illustrates the vector diagram in reference to primary and secondary sides of current transformer. In fig 9.a IRP, IYP and IBP are the phase currents in the primary side, while IR is the line current on the same side in line R as shown in fig 9.a, the corresponding secondary current of current transformers CT1 on the primary side is in

phase with IR and is represented as IRS in fig 9.b. the current in the secondary side of the power transformer is represented as IR, IY and IB in fig 9.c, the phase current in the secondary winding of the current transformers CT2 is represented as I’R, I’Y and I’B in fig 9.d. the current in pilot wire of CT2 is represented as IRS. Now when we consider fig 9.b and 9.d its clear that the currents in the pilot wires are in phase.