High Voltage Direct Current (HVDC)
Page 2: Introduction to Electricity
According to the electron theory of electricity, electricity is the flow of electrons in a conductor.
A conductor is a material in which electrons flow producing electricity. An insulator is a material that prevents electrons from flowing in it. A semiconductor is a material that is sometimes a conductor, sometimes an insulator, depending on conditions.
A conductor is a material composed of atoms that allow electrons to break off an atom and join another atom. Electron flow in a conductor is the transfer of an electron from one atom to another atom, then the transfer of an electron from that other atom to yet another atom, etc.
Figure 2.1 Electron flow from atom to atom. [WECC]
Current, Voltage and Power
Considering electricity as the flow of electrons in a conductor, the number of electrons flowing past a certain point is called current, and the pressure that is pushing the electrons to flow is called voltage.
Power is voltage times current:
P = V × I
where P denotes power, V denotes voltage, and I denotes current.
Voltage is measured in Volts, current is measured in Amperes (Amps), and power is measured in Watts or volt-amperes (VA).
Voltage is also referred to as potential difference or electromotive force (emf or E).
Current flowing in a conductor creates magnetism.
Whenever current flows through a conductor, a magnetic field is created around the conductor. Conversely, by moving a conductor across a magnetic field, electricity is created in the conductor.
Basic Electricity (see References below), on page 31, lists the following three conditions for creating a voltage from magnetism:
1. There must be a conductor, in which the voltage will be induced.
2. There must be a magnetic field in the conductors vicinity.
3. There must be relative motion between the field and the conductor. The conductor must be moved so as to cut across the magnetic lines of force, or the field must be moved so that the lines of force are cut by the conductor.
All generators and motors work on this principle, by rotating wires relative to magnets, or rotating magnets relative to wires.
Reistance is the opposition to current flow. Conductors can have high resistance to current flow if there is too much current for the conductor to handle.
If the conductor is a wire, resistance is an important kind of line loss. If the current is too high for the wire, some of the electricity is lost, for example as heat.
Generally, we are interested having a wire deliver power. Recall the formula for power:
P = V × I
Notice how, according to that formula, for a given level of power, you get the same amount of power if you cut the current in half and double the voltage.
For example, say the current is 10 amps (A) at a voltage of 400 volts (V). We could achieve the same power using 5 A at 800 V (half the current at twice the voltage). This is the idea behind transformers: to step up the voltage so that less current can be used to transport electricity with less line losses, and then to later step down the voltage for use in homes.
Transformers are only possible with AC. Before discussing transformers, we first review the difference between AC and DC.
Direct Current and Alternating Current
Direct current, abbreviated DC, is the flow of electric current in one direction, for example a simple circuit connected to a battery as illustrated in Figure 2.4:
The battery is a voltage source, and the filament of the light bulb causes resistance (converting some of the electricity to heat and light). Following is a schematic diagram of a light bulb as a resistor.
In this case, the battery (voltage source E) is a multi-cell battery (or multiple single cell batteries) on the left, and the light bulb is illustrated as a resistor (R) on the right.
The current and voltage in this type of circuit is constant, as illustrated in the following graph:
In Figure 2.6, the vertical axis measures voltage, and the horizontal axis measures time. As time goes on, the voltage stays the same. This is for direct current (DC).
Alternating current (AC) on the other hand oscillates, with the voltage reversing direction many times per second. That is illustrated in the following voltage graph:
This graph shows two systems. One system is direct current (DC), with constant voltage of 200 V. The other system is alternating current (AC) that oscillates between positive 200 V and negative 200 V, which correspond to opposite directions of pushing current.
That's right, for AC the direction of current generally reverses each oscillation. Imagine what that does to the magnetic field around a conductor!
For DC, the surrounding magnetic field is constant, but for AC the surrounding magnetic field collapses and regenerates every oscillation (many times per second).
This illustration shows how a generator can produce AC electricity:
Figure 2.8 Basic 4-pole AC generator. [BE]
The generator of Figure 2.8 rotates a wire that is bent at right angles and positioned between four poles of magnets. The wire is attached to slip rings. A rotating shaft (not shown) rotates the slip rings and wire counter-clockwise, generating alternating current (I) in the clockwise direction. Metal brushes transfer the electric power to an AC circuit.
Each revolution of that generator induces two cycles of alternating current electricity. For this to generate 60 cycles per second (hertz) of AC electricity, the shaft would need to rotate 30 times per second (1800 revolutions per minute RPM).
Modern generators (and motors) are more advanced, now using wire windings instead of a single wire loop, to generate more electricity from generators (or more horsepower from motors).
Transformers allow the voltage of AC lines to be stepped up or stepped down to a different voltage, resulting in a corresponding change in current since power stays the same (other than a small loss of power to the transformer).
This is accomplished by positioning two coils of wires near each other so that magnetic fields caused by electricity in one coil induces electricity in the other coil. The amount of stepping up or stepping down of the voltage depends on the number of windings of the wires of each coil. The ratio of the voltage change is the ratio of number of windings.
Figure 2.9 Transformer. [WECC]
Transformers do not work with DC because fluctuating magnetic fields are needed for induced EMF. Recall the three conditions needed for induced EMF. The first condition is to have a conductor, and the second condition is to have a magnetic field. The third condition is that the conductor must be moved so as to cut across the magnetic lines of force, or the field must be moved so that the lines of force are cut by the conductor. DC cannot do that, because DC has a constant magnetic field. Alternating current does meet the condition that the field must be moved so that the lines of force are cut by the conductor, by oscillating current.
References for this page:
1. Western Electric Coordinating Council, [WECC]
2. Basic Electricity, Army Field Manual FM 55-506-1, developed by the U.S. Navy to provide basic electrical fundamentals for Army watercraft engineers. [BE]
3. Chapter 1 Basic Electrical Theory and Mathematics, Navy electronics training course, NAVEDTRA 14026A. [NE1]
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