Mercury arc valves were common in systems designed up to 1972, the last mercury arc HVDC system ( the Inter-Island HVDC link between the North and South Islands of New Zealand, which used them on one of its two poles.
32.
For this reason the twelve-pulse system has become standard on almost all line-commutated converter HVDC systems, although HVDC systems built with mercury arc valves usually allowed for temporary operation with one of the two six-pulse groups bypassed.
33.
The HVDC Inter-Island was also upgraded by increasing the operating voltage of the lines, replacing the original Cook Strait submarine cables and installing a new thyristor pole in parallel to the existing mercury arc valve converter equipment to double its capacity to 1240 MW.
34.
Mercury arc valves were used extensively in DC power systems on London Underground, and one was still observed to be in operation in 2004 at the disused Goodge Street shelter featuring in an early episode of Doctor Who as an alien brain, cast for its " eerie glow ".
35.
Mercury arc valves were built with ratings of up to 150 kV, 1800 A . The last ( and most powerful ) mercury arc system installed was that of the Nelson River DC Transmission System in Canada, which used six anode columns in parallel per valve and was completed in 1977.
36.
ZBD in the early 1880s only worked with AC . Not until grid controlled mercury arc valves became available for power transmission during the period 1920 to 1940 was it possible to utilize high-voltage direct current for large transmission projects, but by that time AC transmission was dominant, cheap and reliable.
37.
The controls and older converters, including the mercury arc valves, were completely replaced ( in the same valve hall ) by a single pair of 3, 100 MW 12-pulse converters built by ABB . The upgrade was made to eliminate the environmental risks of maintenance costs of the obsolete mercury arc valves.
38.
The controls and older converters, including the mercury arc valves, were completely replaced ( in the same valve hall ) by a single pair of 3, 100 MW 12-pulse converters built by ABB . The upgrade was made to eliminate the environmental risks of maintenance costs of the obsolete mercury arc valves.
39.
Mercury arc valves with grading electrodes of this type were developed up to voltage ratings of 150 kV . However, the tall porcelain column required to house the grading electrodes was more difficult to cool than the steel tank at cathode potential, so the usable current rating was limited to about 200 & ndash; 300 A per anode.
40.
The link originally was a bipolar 600 MW link with mercury arc valves, until the original equipment was paralleled onto a single pole ( Pole 1 ) in 1992, and a new thyristor-based pole ( Pole 2 ) was commissioned alongside it, increasing the link capacity to 1040 MW . Pole 1 was fully decommissioned effective 1 August 2012, and the replacement thyristor-based Pole 3 was fully commissioned on 29 May 2013, restoring the link to a bipolar 1200 MW configuration.
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