A PLANT FOR TRANSMITTING ELECTRIC POWER INCLUDING BIDIRECTIONALLY CONTROLLED THYRISTORS (BCT)

Abstract: In a plant for transmitting electric power between two stations (17, 18) through a direct voltage network (16) for High Voltage Direct Current an arrangement (23) is arranged for draining electric power from the direct voltage network. The arrangement has a valve (24) made of bidirectionally controlled thyristors (BCT) with ability to conduct current in both directions therethrough. The plant has also a unit (25) adapted to control the thyristors of the valve so that the current direction between the direct voltage network and the valve is such that electric power is fed from the direct voltage network to an alternating voltage network connected to the valve independently of the power feeding direction between said two stations (17, 18).

Description: A PLANT FOR TRANSMITTING ELECTRIC POWER INCLUDING BIDIRECTIONALLY CONTROLLED THYRISTORS (BCT). FIELD OF THE INVENTION AND PRIOR ART 10 The present invention relates to a plant for transmitting electric power comprising a direct voltage network for High Voltage Direct Current (HVDC) and at least two stations connected thereto and having line co mutated valves with thy- 15 ristors as rectifying semiconductor components for transmitting electric power between the direct voltage network and alternating voltage networks connected to the stations, in which the plant further comprises at least an arrangement arranged along the direct voltage network for draining elec- 20 trie 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, in which the arrangement comprises at least one valve having thyristors as rectifying components 25 connected between the direct voltage network and an alternating voltage network adapted to receive electric power drained from the direct voltage network. In such a plant for transmitting electric power through a 30 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, and there may be a need of connecting an arrangement mentioned above to the direct voltage 35 network for draining electric power from the direct voltage network on certain locations therealong or feeding electric power into the network as a consequence of the discovery of a local gas well or the like. It is then important to ensure that always when there is a need electric power may be drained from the direct voltage network or fed thereinto through said arrangement independently of the power feeding direction on the direct voltage network between the stations of the plant, i.e. independently of the polarity of the direct voltage network. For being able to satisfy the need last mentioned, which makes it necessary to be able to chose the current direction between the direct voltage network and the arrangement , the solution illustrated in fig. 1 has been offered in such plants already known. The plant shown there has a direct voltage network 1 for High Voltage Direct Current, which here is unipolar. Two stations 2, 3 illustrated through a valve 4, 5 each are connected to the direct voltage network for transmitting electric power between the direct voltage network and an alternating voltage network 7 connected to the respective station through a transformer 6. For the sake of simplicity the transformer and the alternating voltage network connected to the valve 5 have been omitted in the figure. The plant has also an arrangement 8 for draining electric power from the direct voltage network, in which this arrangement has only one valve 9. The direction of the current through this valve 9, to the direct voltage network far away from the direct voltage network 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 for obtaining this. When the station 2 receives electric power at a negative high voltage polarity on 1, i.e. the valve thereof is in inverter operation, the breakers 11 and 15 are closed, while the breakers 12 and 14 are closed when the station 3 receives electric power and the polarity on 1 is positive. This solution is very complicated and by that costly as a consequence of the existence of the two switch gears, as well as the increased demands on insulation distances in the valve hall, since it has to be possible to apply high potential to each connection of the valve. SUMMARY OF THE INVENTION The object of the present invention is to provide a plant of the type defined in the introduction, which has a said arrangement functioning well and having a simple construction and by that being associated with considerably lower costs than such arrangements of the prior art plants discussed above . This object is according to the invention obtained by making the thyristors of said valve of the arrangement in a plant of the type defined in the introduction to be bidirection- ally controlled thyristors (BCT) with ability to conduct current in both directions therethrough, and the unit controlling the valve of the arrangement is adapted to control the thyristors of the valve so that the current direction between the direct voltage network and this valve is such that power is fed either from 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 in every given instant be freely chosen without any requirement of more than one valve of the arrangement per pole of the direct voltage network. This means that the current direction may always be adjusted so that it corresponds to the current di- rection of the station of the plant receiving electric power and by that the arrangement may drain electric power from the direct voltage network independently of the polarity of the direct voltage network without any demand of any complicated switch gears or double valves. Is there a desire of feeding power into the direct voltage network an adjustment according to the same principle is made, but conversely. The bidirectionally controlled thyristors are thyristors of the type described in for example DE 44 39 012 Al and are sometimes also called two-directions thyristors. Thus, the arrangement requires only one valve per pole without any costly surrounding equipment of the direct voltage network, and by that for the unipolar direct voltage network only one valve, which constitutes preferred embodiments of the invention. Further advantages as well as advantageous features of the invention will appear from the following description. BRIEF DESCRIPTION OF THE DRAWING With reference to the appended drawing, below follows a description of a preferred embodiment of the invention cited as an example . In the drawing: Fig. 1 is a very simplified circuit diagram illustrating a plant already known of the type according to the in- vention, and Fig. 2 is a view corresponding to fig. 1 of a plant according to the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVEN- TION A plant according to the invention for transmitting electric power through a unipolar direct voltage network 16 between two stations 17 and 18 is illustrated in fig. 2, said sta- tions having a line commutated valve 19, 20 each with thyristors as rectifying semiconducting components, in which the respective valve may be constructed for example as a conventional 12 -pulse bridge with an amount of thyristors connected in series arranged to together take the voltage across the valve in question. This voltage may be in the order of 10-500 kV, while each thyristor can only take 1-10 kV. A direct voltage network 21 is connected to the two stations through a transformer 22 for feeding electric power between this alternating voltage network 21 and the direct voltage network 16, but here is for the sake of simplicity these components not shown for the station 18. The current direction will always be through the valve 19 to the direct voltage network 16 away to and through the valve 20. The power feeding direction between the two stations is selected through the choice of polarity of the direct voltage network 16, so that when the polarity thereof is positive, i.e. when there is a higher positive potential at the valve 19 than at the valve 20, power will be fed from the station 17 to the station 18, but at a negative polarity of the direct voltage network 16 and by that a more negative potential of the valve 20 than of the valve 19 the power will be fed in the opposite direction. The plant has an arrangement 23 for draining electric power from the direct voltage network 16 for feeding electric power thereinto to and from, respectively, an alternating voltage network connected to the arrangement and not shown in fig. 2. The arrangement 23 is very simple and has one single valve 24 constructed in the same way as the valves 19 and 20, but it has bidirectionally controlled thyristors (BCT = Bidirectionally Controlled Thyristor) instead of conventional thyristors, in which such thyristors have two halves of one and the same semiconductor disc with the conducting direction in opposite directions. The arrangement has also a unit 25 adapted to control the thyristors of the valve 24, so that the current direction between the direct voltage network 16 and this valve is such that power is fed from the direct voltage network to the alternating voltage network connected to the valve independently of the polarity of the direct voltage network 16, i.e. so that the current direction through the valve 24 with respect to the direct voltage network is the same as the current direction through the one of the valves 19 and 20 that is in inverter operation, i.e. through which power is fed from the direct volt- age network to an alternating voltage network connected to the valve. Alternatively, the unit 25 controls the thyristors so that power is always fed out to the direct voltage network independently of the polarity thereof. The invention is of course not in any way restricted to the embodiment described above, but many possibilities to modifications thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the invention such as defined in the claims. The direct voltage network could as already mentioned be bipolar, in which the draining arrangement may then has either one or two valves, depending upon the amount of power to be drained, of the type described above. Is it is possible to handle the amount of power by one single valve, this may also be made switchable between the two poles in the bipolar transmission so that a maximum of flexibility is obtained. It is of course also possible that the plant is provided with several arrangements of the type described above .

Claims: Claims 1. A plant for transmitting electric power comprising a direct voltage network (16) for High Voltage Direct Cur- rent (HVDC) and at least two stations (17, 18) connected thereto and having line commutated valves (19, 20) with thyristors as rectifying semiconductor components for transmitting electric power between the direct voltage network and alternating voltage networks (21) connected to the stations, in which the plant further comprises at least an arrangement (23) arranged 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, in which the arrangement comprises at least one valve (24) 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, characterized in that the thyristors of said valve (24) of the arrangement are bidirectionally controlled thyristors (BCT) with ability to conduct current in both directions therethrough, and that the unit (25) control- ling the valve of the arrangement is adapted to control the thyristors of the valve so that the current direction between the direct voltage network and this valve is such that power is fed either from the direct voltage network to the alternating voltage network connected to the valve or in the opposite direction. 2. A plant according to claim 1 , characterized in that the arrangement (23) has only one said valve (24) per pole of the direct voltage network (16) . A plant according to claim 2, characterized in that the direct voltage network (16) is unipolar.

Inventors: AASTROEM URBAN, HALVARSSON BRUNO, HERMANSSON LARS, AASTROEM, URBAN, HALVARSSON, BRUNO, HERMANSSON, LARS
Applicants: AASTROEM URBAN, ASEA BROWN BOVERI, HALVARSSON BRUNO, HERMANSSON LARS, AASTROEM, URBAN, ASEA BROWN BOVERI AB, HALVARSSON, BRUNO, HERMANSSON, LARS
Application year: 1998
Main classification: H02J3
IPC codes: H02J3, H02J5