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High Voltage Direct Current (HVDC)

Page 2: Introduction to Electricity

Electrons

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 outer 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 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 corresponds to how much force is pushing/pulling the electrons, and current corresponds to intensity (how many electrons are flowing, kind of like how many photons are flowing in a beam of light, with brighter light meaning more photons – higher intensity).

Voltage is measured in Volts (V), current is measured in Amperes or Amps (A), and power is measured in Watts (W) or volt-amperes (VA).

Voltage is also referred to as potential difference or electromotive force (emf or E).


Induced EMF

Current flowing in a conductor creates magnetism.

Figure 2: Current in a wire conductor creates a magnetic field around the wire. [WECC]

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.

Figure 3: Moving a conductor across magnetic field lines. [WECC]

Basic Electricity (p. 32, see References below) 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 conductor’s 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.

Generators and motors work on this principle, by rotating wires relative to magnets, or rotating magnets relative to wires.


Resistance

Reistance is the opposition to current flow. The more current that flows, the more resistance there is.

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 reduce the current by half and double the voltage, which by reducing current reduces line loss yet still provides the same power.

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 (increasing current) 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:

Figure 4: Filament light bulb connected to a battery (left), and schematic diagram of the circuit (right). [NE1]

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:

Figure 5: Battery and resistor circuit. [NE1]

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:

Figure 6: DC voltage is constant (blue line).

In this graph, 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 (and current) reversing direction many times per second. That is illustrated in the following voltage graph:

Figure 7: AC voltage oscillates (red line). [NE1]

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 8: Basic 4-pole AC generator. [BE]

The generator in this example 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

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 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.

“the electromagnetic field surrounding a coil expands, collapses, and reverses as the current increases, decreases, and reverses.”
— 
Thomas L. Floyd, Electronics Fundamentals 5th ed., p. 648

References for This Page

 1.  Fundamentals of Electricity, Western Electric Coordinating Council, 2015. [WECC]

(6.7 MB)

 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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