history of hvdc transmission

8/3/2019 History of HVDC Transmission

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History of HVDC transmission

Schematic diagram of a Thury HVDC transmission system

HVDC in 1971: this 150 kVmercury arc valveconverted AC hydropowervoltage for transmission to distant cities

fromManitoba Hydrogenerators.
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Bipolar system pylons of theBaltic-Cable-HVDC in Sweden

The first long-distance transmission of electric power was demonstrated using direct current in

1882 at the Miesbach-Munich Power Transmission, but only 2.5 kW was transmitted. An early

method of high-voltage DC transmission was developed by the Swiss engineerRen Thury[5] and

his method was put into practice by 1889 inItaly by theAcquedotto De Ferrari-Galliera company.

This system used series-connectedmotor-generatorsets to increase voltage. Each set was

insulated from ground and driven by insulated shafts from a prime mover. The line was operated

in constant current mode, with up to 5,000 volts on each machine, some machines having

double commutators to reduce the voltage on each commutator. This system transmitted 630 kW

at 14 kV DC over a distance of 120 km.[6][7] The Moutiers-Lyonsystem transmitted 8,600 kW of

hydroelectric power a distance of 124 miles, including 6 miles of underground cable. The system

used eight series-connected generators with dual commutators for a total voltage of 150,000 volts

between the poles, and ran from about 1906 until 1936. Fifteen Thury systems were in operationby 1913[8] Other Thury systems operating at up to 100 kV DC operated up to the 1930s, but the

rotating machinery required high maintenance and had high energy loss. Various

otherelectromechanical devices were tested during the first half of the 20th century with little

commercial success.[9]

One conversion technique attempted for conversion of direct current from a high transmission

voltage to lower utilization voltage was to charge series-connected batteries, then connect the

batteries in parallel to serve distribution loads.[10]While at least two commercial installations were

tried around the turn of the 20th century, the technique was not generally useful owing to the

limited capacity of batteries, difficulties in switching between series and parallel connections, and

the inherent energy inefficiency of a battery charge/discharge cycle.

The grid controlledmercury arc valvebecame available for power transmission during the period

1920 to 1940. Starting in 1932,General Electric tested mercury-vapor valves and a 12 kV DC

transmission line, which also served to convert 40 Hz generation to serve 60 Hz loads,
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atMechanicville, New York. In 1941, a 60 MW, +/-200 kV, 115 km buried cable link was designed

for the city ofBerlin using mercury arc valves (Elbe-Project), but owing to the collapse of the

German government in 1945 the project was never completed.[11]The nominal justification for the

project was that, during wartime, a buried cable would be less conspicuous as a bombing target.

The equipment was moved to theSoviet Unionand was put into service there.[12]Introduction of

the fully static mercury arc valve to commercial service in 1954 marked the beginning of the

modern era of HVDC transmission. A HVDC-connection was constructed byASEA between the

mainland of Sweden and the island Gotland.

Mercury arc valves were common in systems designed up to 1975, but since then, new HVDC

systems have used onlysolid-state devices. On March 15, 1979, a thyristorbased direct current

connection between Cabora Bassa and Johannesburg (1410 km, 533 kV, 1920 MW) was turned

on. Though the electronics were built in 1974 by AEG, and BBC (Brown Boveri Company) and

Siemens were partners in the project, the late turn on was a result of the civil war.[citation needed]

After 1975 mercury valves in HVDC began to be replaced by solid state valves, and as of 2011

the Inter-Island HVDC (high voltage direct current) link between the North and South Islands

ofNew Zealand is the last major operating mercury arc HVDC not yet replaced with a solid state

system (this is being planned for 2012).

From 1975 to 2000 arc valves were replaced by so-called line-commutated converters (LCC)

using simple thyristor valves with gates activated by line voltage. According to Sood,[13] the next

25 years may well be dominated by "force commutated converters" (i.e., thyristor or thyristor-like

semiconductors with gates that are actively controlled by separate switching circuitry, for

smoother switching response). This era has already begun with "capacitor commutated

converters" (CCC), which are simple thyristor networks with gates operated from an external

capacitive circuit, drawn from the AC line.

Such externally-controlled thyristor-based circuits are expected to eventually be replaced by "self-

commutated converters" based around more complex semiconductor switching devices. These

"self-commutating converters" will finally largely supplant today's externally-commutated systems

entirely, after self-commutating solid-state devices in the required power ranges become

economically viable.[13] Such self-commutated devices include the insulated gate bipolar

transistors (IGBT) and variant thyristors called integrated gate-commutated thyristors (IGCT),

andgate turn-off thyristors (GTO). All these devices are used now in medium power high-voltage

DC systems, and are capable of being scaled-up in power to the point that they (or other similar

variants of multilayer solid-state high-power devices) will probably eventually replace all simple

thyristor-based systems now in use, even for very highest power transmission DC applications.[4]
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Since thyristor-based switches (i.e., solid-state rectifiers) were incorporated into them, hundreds

of HVDC sea cables have been laid, and have worked with high reliability, usually better than

96% of the time.

[edit]Advantages of HVDC over AC transmission

The advantage of HVDC is the ability to transmit large amounts of power over long distances with

lower capital costs and with lower losses than AC. Depending on voltage level and construction

details, losses are quoted as about 3% per 1,000 km.[14]High-voltage direct current transmission

allows efficient use of energy sources, remote from load centers.

In a number of applications HVDC is more effective than AC transmission. Examples include:

Undersea cables, where high capacitance causes additional AC losses. (e.g.,

250 km Baltic Cable betweenSweden and Germany,[15]the 600 km NorNed cable between

Norway and the Netherlands, and 290 kmBasslink between the Australian mainland and

Tasmania[16])

Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for

example, in remote areas

Increasing the capacity of an existing power gridin situations where additional wires are

difficult or expensive to install

Power transmission and stabilization between unsynchronised AC distribution systems

Connecting a remote generating plant to the distribution grid, for example Nelson River

Bipole

Stabilizing a predominantly AC power-grid, without increasingprospective short circuit

current

Reducing line cost. HVDC needs fewer conductors as there is no need to support

multiple phases. Also, thinner conductors can be used since HVDC does not suffer from

the skin effect

Facilitate power transmission between different countries that use AC at differing

voltages and/or frequencies

Synchronize AC produced by renewable energy sources

Long undersea / undergroundhigh voltage cableshave a high electricalcapacitance, since

the conductors are surrounded by a relatively thin layer of insulation and a metal sheath

while the extensive length of the cable multiplies the area between the conductors. The

geometry is that of a long co-axialcapacitor. Where alternating current is used for cable
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transmission, this capacitance appears in parallel with load. Additional current must flow

in the cable to charge the cable capacitance, which generates additional losses in the

conductors of the cable. Additionally, there is adielectricloss component in the material

of the cable insulation, which consumes power.
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