Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
1999-12-08
2001-03-06
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
active
06198641
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a plant for transmitting electric power. A direct voltage network for High Voltage Direct Current (HVDC) and at least two stations connected thereto have line commutated valves with thyristors as rectifying semiconductor components for transmitting electric power between the direct voltage network and alternating voltage networks connected to the stations. The plant further comprises at least one arrangement along the direct voltage network for draining electric power from or feeding electric power into the direct voltage network independently of the polarity thereof and by that the power feeding direction between the different stations of the plant. The arrangement comprises at least one valve having thyristors as rectifying components connected between the direct voltage network and an alternating voltage network adapted to receive electric power drained from the direct voltage network.
In a plant for transmitting electric power through a direct voltage network for High Voltage Direct Current the direct voltage network may extend over very long distances, well in the order of hundreds of kilometres or more, between stations belonging thereto. Sometimes the need arises for feeding or draining electric power from the direct voltage network at certain locations therealong. For example, as a consequence of the discovery of a local gas well or the like. It is then important to ensure that when there is a need, electric power may be drained or fed through the arrangement independently of the power feeding direction on the direct voltage network between the stations, i.e. independently of the polarity of the direct voltage network.
FIG. 1
shows a known plant for a unipolar direct voltage network
1
for High Voltage Direct Current. Two stations
2
,
3
are each connected to the direct voltage network and an alternating voltage network
7
through respective valves
4
,
5
and a transformers
6
. The plant also has an arrangement
8
having only one valve
9
for draining electric power from the direct voltage network. The direction of the current through valve
9
, determines at a given polarity of the direct voltage network, i.e. at a given direction of the feeding of power between the stations
2
and
3
, if electric power is really drained or tapped through the arrangement
8
. Thus, the current direction of the arrangement
8
has to be the same as in the one of the stations
2
and
3
which, in a given instant, functions as inverter, i.e. to which power is fed from the direct voltage network. The arrangement
8
has a high voltage switch gear
10
with two breakers
11
,
12
and a low voltage switch gear
13
with two breakers
14
,
15
. When the station
2
receives electric power at a negative high voltage polarity on
1
, i.e. the valve
4
is in inverter operation, the breakers
11
and
15
are closed. Likewise, breakers
12
and
14
are closed when the station
3
receives electric power and the polarity on
1
is positive. This solution is complicated and costly as a consequence of the requirement for the two switch gears, as well as the increased demands on insulation distances.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a plant having a simple construction with considerably lower costs than arrangements of the prior art discussed above.
The object is obtained by employing a valve having bidirectionally controlled thyristors (BCT) with ability to conduct current in both directions, and controlling the valve so that the current direction between the direct voltage network and the valve is such that power is fed either the direct voltage network to the alternating voltage network connected to the valve or in the opposite direction.
By arranging such bidirectionally controlled thyristors in the valve of the arrangement the current direction between the valve and the direct voltage network may be freely chosen without requiring more than one valve per pole of the direct voltage network. This means that the current direction may always be adjusted so that it corresponds to the current direction of the station receiving electric power and may thus drain (or feed) electric power from the direct voltage network independently of the polarity of the direct voltage network without any complicated switch gears or double valves. The bidirectionally controlled thyristors are of the type described in for example DE 44 39 012 A1 and are sometimes also called two-directions thyristors.
Thus, the preferred arrangement of a unipolar network requires only one valve per pole without any costly surrounding equipment of the direct voltage network
Further advantages of the invention will appear from the following description.
REFERENCES:
patent: 4689733 (1987-08-01), Guth et al.
patent: 5187651 (1993-02-01), Ekstrom
patent: 5867375 (1999-02-01), Svensson et al.
patent: 6067238 (2000-05-01), Halvarsson et al.
Åström Urban
Halvarsson Bruno
Hermansson Lars
ABB AB
Gossett PLLC Dykema
Laxton Gary L.
Riley Shawn
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