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High Voltage Direct Current (HVDC)
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AC and DC

Alternating current (AC) is electricity that rapidly fluctuates, reversing direction 50 or 60 times per second. The rapid reversals of current direction create magnetic disturbances around the conductors (wires) that are carrying the AC electricity.

The rapidly changing magnetic distrubances induce electricity in nearby wires (conductors), with the induced voltage depending on the configuration of the conductors. A transformer (Figure 1) uses this phenomenon to transfer electricity from one set of conductors to another set of conductors while changing the voltage.


Figure 1.  Transformer
(AC only).

Direct current (DC) does not create these magnetic disturbances, and therefore cannot have transformers to change voltage. DC voltage is fixed and cannot be changed electrically.

Electricity transmission is much more efficient (with less line losses) at higher voltage. AC is widely used because the voltage can be changed easily with transformers. Transformers step the voltage up and down as needed:


Figure 2. AC transmission and distribution (1 kV = 1000 Volts).

Large transformers step up and step down the voltage in electrical substations. For a residential customer, the last transformer is pole mounted for overhead lines, or pad mounted for underground service.

Long distance DC transmission is much more efficient than long distance AC transmission, with less radiation, smaller towers, less wires, and greatly reduced line losses. But DC does not have any way to step voltage up and down.

The best solution would be to use both AC and DC. Until recently, that was not possible. Now it is possible, in certain circumstances.

An important breakthrough was achieved in 1939 with the development of the mercury-arc valve, by Uno Lamm of ASEA (now called ABB after merging with BBC), to convert between high voltage AC and high voltage DC. More recently, the thyristor (silicon controlled rectifier) has replaced the mercury-arc valve.

“Modern HVDC systems use thyristors as converter valves. Thyristors are usually rated 3–5 kV in voltage 2.5–3 kA in current. Due to limited ratings, converters usually consist of one or more converter bridges connected in series or parallel.”
— 
Xi-Fan Wang, Yonghua Song, and Malcolm Irving, Modern Power Systems Analysis (Springer 2009), p. 259


Figure 3.  BBC 1100 A 500 kV thyristor-based converter used in 1989 to convert AC to DC and to convert DC to AC. NREL ]

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