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Electrical Engineering

High Voltage Direct Current

Historically, electric power systems had to be either alternating current or direct current. In this article we explain the difference between the two, and how these systems can now be combined.

“transformers are responsible for the more extensive use of alternating current than direct current.” – Basic Electricity

Direct current (DC) is a steady stream of electricity. Alternating current (AC), on the other hand, fluctuates rapidly. That makes AC inefficient for many uses. The most important way to make AC less inefficient is to use three phase power instead of single phase.

Alternating current (AC) is generated in sine waves, because of the circular motion of generators. If one (single phase) generator is used, the AC power has particular instants of time in which there is no current, when the current is changing directions:

But if three generators are used that are slightly out of phase, or if a three-phase generator is used (with three separate windings that are slightly out of phase with each other), then there is always current between different conductors:

That is how you want to use and transmit alternating current, and is why long distance AC transmission lines have multiples of three lines.

Another problem with AC is increased line loss. Compared to DC, when you transmit AC on the same type of wire (conductor) you actually lose more of the electricity. The longer the distance, the worse AC is. Some of those losses are due to extra resistance that is caused by the AC fluctuations:

“when a conductor is transmitting alternating current, the current-density across the conductor cross section is nonuniform and is a function of the ac frequency. This phenomenon, known as the skin effect, causes the ac resistance to be greater than the dc resistance.” [ 1 ]

Resistive line losses depend on voltage, current and distance. Increasing (stepping up) the voltage reduces current and line losses. At the other end, the voltage can be reduced (stepped down) after the higher voltage had been used to reduce line losses.

The important benefit of AC is that it can easily step up and step down voltage, with transformers. Transformers do not work on DC. Historically this gave AC a major advantage. While AC has more line losses than DC at a given voltage, an AC system can raise and later lower the voltage more easily, and therefore experience less loss system-wide. Voltages could be high for transmission, and stepped down right before use, while for DC the voltage would need to be kept low all the time which results in greater line losses.

The reason why transformers only work on AC is because they rely on the inductance of the AC fluctuations. Coils are placed near each other, and one induces voltage in the other depending on intersections of the magnetic field changes as the current fluctuates rapdidly.

“the electromagnetic field surrounding a coil expands, collapses, and reverses as the current increases, decreases, and reverses.” [ 2 ]

Since DC does not fluctuate, it cannot induce such voltage. For decades AC was used instead of DC because of the ease of stepping up and stepping down voltage with transformers. It was not practical to convert between AC and DC, so only one could be used, and AC was widely chosen.

Recent advances in electrical engineering now make it cost-effective to convert between AC and DC at large scales. The primary use of this technology is to transport huge amounts of electricity using DC with high voltage. An example is the DC transmission link between the eastern and western United States. Both ends of the country operate AC grids, but send electricity to each other on efficient DC lines. The stepping up and stepping down of voltage is done on the AC grids.

Besides transporting electricity over very long distances much more efficiently than AC, another use for high voltage DC is to link AC grids that are different frequencies, which is now done in Japan, and to connect steady AC grids with AC grids that have variable frequencies, which is now done in Germany.

High voltage DC is especially well suited for transporting very large amounts of electricity very long distances, such as for use with solar thermal electric plants. For other uses, such as wind farms, sometimes AC is better.

For more information see electrical power engineering books.

“The use of d.c. in modern power systems is often confusing to those who remember that d.c. was historically associated with excessive line losses… [that was] because there was no practical way to step d.c. voltage up and down between transmission and end users… By contrast, modern d.c. lines, conected to the high-voltage a.c. system with advanced solid-state technology, carry very high voltages and are therefore highly efficient.” [ 3 ]

“The insulation required in a dc line is equivalent only to that required for the peak voltage in an ac system, and lower towers can therefore be used with considerable cost savings.” [ 4 ]

“The converters at each end can operate either as a rectifier or as an inverter, which allows power flow in either direction.” [ 5 ]

References

1.	Syed A. Nasar, Electric Power Systems, p. 26.
2.	Thomas L. Floyd, Electronics Fundamentals 5th ed., p. 648.
3.	Alexandra von Meier, Electric Power Systems: A Conceptual Introduction, p. 167.
4.	D. F. Warne, Newnes Electric Power Engineer's Handbook 2nd ed., p. 381.
5.	Gilbert M. Masters, Renewable and Efficient Electric Power Systems, p. 146-147.

Thursday, 28-Aug-2008 04:05:54 GMT