Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1999-12-09
2000-10-31
Sterrett, Jeffrey
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 51, H02J 336
Patent
active
061412260
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a plant for transmitting electric power comprising a direct voltage network for High Voltage Direct Current (HVDC) having at least one pole conductor extending between and interconnecting at least two stations with, for the pole conductor, a line commutated valve with thyristors as rectifying semiconductor components for transmitting electric power between the direct voltage network and alternating voltage networks connected to the stations, said pole conductor being at each station connected to an earth electrode.
The invention is in particular directed to the case of bipolar direct voltage networks having two pole conductors, and as a consequence thereof that case will be discussed more thoroughly hereinafter, but this is only done for illuminating the invention and the problems to be solved thereby and is not at all to be interpreted as limiting the invention, but this relates also to the case of a plant having only one pole conductor.
A plant already known having a direct voltage network 1 with two pole conductors 2, 3 extending between two stations 4, 5 and which are at each station provided with a valve each 6, 7 and 8, 9, respectively, for transmitting electric power between an alternating voltage network 14 and 15, respectively, connected to each station through transformers 10, 11 and 12, 13, respectively, is illustrated in FIG. 1. The two pole conductors are at each station connected to a common earth electrode 16 and 17, respectively. The valves are line commutated and have conventional thyristors with only one conducting direction as rectifying semiconductor components, which means that the current through each valve and by that in a given pole conductor only may assume a determined direction, so that the direction of the feeding of power between the stations only may be changed by changing the polarity of the pole conductors. Would now one of the pole conductors drop out, the plant would then become a plant having a unipolar direct voltage network with one pole conductor and an earth return circuit, in which in the case of the drop out of the pole conductor 3 the earth electrode 16 will become anode and the earth electrode 17 cathode, while on drop out of the pole conductor 2 the earth electrode 16 will become cathode and the earth electrode 17 anode. This means that the two earth electrodes have to be able to function both as anodes and cathodes should a plant be able to function after drop outs, since the earth electrodes on drop out of one pole conductor may assume one or the other type depending upon which of the poles drops out. However, the earth electrode is in certain cases depending upon the character of the ground not at all allowed to assume an optional polarity, which means that it will be necessary to either move the earth electrode in question or restrict the operation to only certain configurations. An earth electrode located in clay ground is for example not allowed to function as anode, since there is then a risk of explosions as a consequence of osmosis and gas formation, while it doesn't matter if a cathode is arranged in clay ground. However, an arrangement of an electrode in such a ground may not be considered for the plant already known, since it has to be able to assume an optional polarity. This means that it is necessary to locate the electrodes on such places that unnecessarily long lines or conductors are required. Furthermore, certain materials may not be used for the earth electrodes, such as copper, which is only suited as material for a cathode.
It may also be mentioned that in the case of a plant having from start only one pole conductor and an earth return circuit, in which accordingly each earth electrode is determined to always be either cathode or anode, would it be possible to later on discover that it would as a consequence of the ground conditions prevailing be advantageous that the earth electrodes had the opposite polarity, but it would then be necessary to move the earth electrodes
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Halvarsson Bruno
Hermansson Lars
ABB AB
Sterrett Jeffrey
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