"A plant for transmitting electric power"is to be given a very broad sense and is intended to comprise in principle all equip- ment and the like in which electric power is fed in a conductor.

However, the invention is particularly well applicable to the case of transmitting high electric powers through high voltage, in which high voltage is typically a voltage between 2 and 400 kV, and then primarily in connection with stations provided with con- verters for converting the voltage with respect to level and/or type (direct voltage, alternating voltage), and for that reason the particular case of a plant having a VSC-converter for converting direct voltage into alternating voltage or direct voltage and con- versely will hereinafter be described for illuminating, but accord- ingly not in any way restricting, the invention and the problem forming the basis therefor.

A plant of the type defined in the introduction is schematically illustrated in appended Fig 1 and 2, said plant having a VSC (Voltage Source Converter)-converter, which here is constituted by a so called 6-pulse bridge having three phase legs 1-3, each consisting of two current valves 4-9 connected in series, which in their turn are formed by a first power semiconductor device 10 of turn-off type and a rectifying member 11 in the form of a so called free-wheeling diode connected in anti-parallel therewith. Each valve comprises in practise a comparatively high number of power semiconductor devices of turn-off type connected in series and free-wheeling diodes for enabling the valve to take the high voltages it has to take in the blocking state. The mid- points between the two valves of the respective phase leg are at a phase output 12 connected through a phase reactor 13 to a phase each of a three phase alternating voltage network 14. The phase legs are at the ends thereof connected to a pole conduc- tor 15,16 each of a direct voltage network 17, the voltage of which is defined through two capacitors 18,19 connected in se- ries and having a grounded midpoint 20.

The power semiconductor devices 10 of turn-off type are con- trolled in a way known per se through a control arrangement 21 according to a determined pulse width modulation pattern (PWM) for using the direct voltage over the DC-capacitors 18, 19 to generate a voltage on the phase output 12, the funda- mental tone component of which constitutes an alternating volt- age having a desired amplitude, frequency and phase position. The switching frequency of the power semiconductor devices 10 is then usually 1-3 kHz, but the frequency of the alternating volt- age will be 50 or 60 Hz. The plant has also a breaker 33 adapted to close and open, respectively, the connection be- tween the alternating voltage network and the converter, and which is closed when a voltage is supplied to the converter after interruption of the operation.

The principle of a VSC-converter already known and just de- scribed implies that the direct voltage is higher than the peak value of the alternating voltage, i. e. the diodes 11 connected in anti-parallel are reversed biased when the converter is blocked.

When a ground fault occurs on the direct voltage side, which is schematically indicated at 22, this means that the diodes 11 will be excerted to a short circuit current having a peak value being approximately 2,5 times higher than a symmetrical short circuit current until the breaker 33 therebehind has opened, which takes place after 2-3 periods. The blocking of the 6-pulse bridge does not prevent the fault current as schematically indicated at 23. This means in the practise that the users require that the di- odes have to be able to take said current therethrough during a considerably longer time, maybe 12-20 periods, for having a sufficient safety margin, which means that unreasonable de- mands are put on the diodes, which have to be heavily over-di- mensioned for satisfying this and they will by that be very costly.

Another problem adhered to a converter of this type concerns the putting of the converter under voltage after an interruption of the operation. When the alternating voltage breaker in such a case is closed the DC-capacitors 18,19 are charged through the diodes 11 in the valves. Energy stored in the phase reactor 13 when the capacitors are charged will raise the direct voltage further, in the worst case to a duplicating of the direct voltage.

This problem may be solved by using a so called switching-in- resistance, but that solution is costly and it is desired to improve the result thereof.

It is shown in Fig 2 how a so called quick disconnector 24 is ar- ranged between the direct voltage network 17 and the converter for the respective pole conductor. The direct voltage network has in this case a so called meshed fashion with a system of a plurality of stations with converters connected in parallel to the direct voltage network. The quick disconnector 24 is formed by a power semiconductor device 25 of turn-off type and a diode 26

connected in anti-parallel therewith and has the object to isolate the converter at a ground fault on the direct voltage network 17, so that the converter will not feed a large current directly into such a ground fault. However, the presence of the diode 26 means that the quick disconnector 24 will be conducting in one direction. This results in its turn in the fact that if a ground fault 27 schematically indicated occurs within the converter, all the converters connected to the system will feed current into the fault, such as indicated through the dashed lines 28-30. If all the stations of the system, as is the case in Fig 2, are provided with quick disconnectors of their own they will open as a conse- quence of the overcurrent and this means that all transmission on the direct voltage network will be interrupted as a conse- quence of an internal fault in one of the stations, which of course is unacceptable.

SUMMARY OF THE INVENTION The object of the present invention is to provide a plant for transmitting electric power of the type defined in the introduc- tion, which has a function improved in certain respects with re- spect to such plants already known and enables a reduction of the inconveniences discussed above.

This object is according to the invention obtained by providing such a plant, in which said member is formed by a controllable second power semiconductor device having the conducting di- rection opposite to that of said first power semiconductor device of turn-off type.

Through this totally new approach to use a controllable power semiconductor device as a member connected in anti-parallel with a power semiconductor device of turn-off type a number of advantages in different operation situations are obtained. By ar- ranging the controllable second power semiconductor device the flexibility is considerably increased, since it will be possible to

design the unit formed by the two power semiconductor devices to either block when a voltage is applied over the power semi- conductor device of turn-off type in the blocking direction or conduct by controlling the second power semiconductor device to conduct. Thus, an optimum adaption to the operation situa- tions prevailing may be obtained.

According to a preferred embodiment of the invention, which is applicable to a plant comprising a VSC-converter for converting direct voltage into alternating voltage or direct voltage and con- versely and which has at least one phase leg with two current valves connected in series, and in which a point of the phase leg between said valves is intended to be connected to a phase of a direct or alternating voltage network and the opposite ends of the phase leg are intended to be connected to a pole conductor each of a direct voltage network, the valves are formed by at least one said first power semiconductor device and a second power semiconductor device connected in anti-parallel there- with. This means that in the case discussed above of a ground fault on the direct voltage side the short circuit current may through the converter to the ground fault be eliminated very rapidly by controlling the second power semiconductor device in a corresponding way. This means that in the case that this power semiconductor device is a controllable power semicon- ductor device being not of turn-off type, such as a thyristor, the current could be eliminated after a maximum of one period, which may be compared with the considerably longer time it will take before the so called back-up-breaker located therebehind between the converter and the alternating voltage network may break or interrupt. Would this second power semiconductor de- vice be of turn-off type the fault current could be eliminated even more rapid.

According to another preferred embodiment of the invention the second power semiconductor device of the valve is controllable but not of turn-off type. An advantage of using such a power

semiconductor device, which preferably is a thyristor, in the valve instead of a power semiconductor device of turn-off type is that it is not required that the energy stored in inductances, the alternating voltage reactor or transformers has to be dealt with by for example capacitors between the phases.

According to another preferred embodiment of the invention the plant comprises a switch having the first and the second power semiconductor device, and the first power semiconductor device of turn-off type is adapted to conduct current during normal function of the plant and to be controlled to be turned off for ful- filling a disconnector function when this is desired. By arranging a first power semiconductor device of turn-off type and a con- trollable second power semiconductor device connected in anti- parallel therewith in this way for fulfilling said disconnector function the problems discussed above at ground faults within a converter can be solved in a satisfying way. Thus, by controlling the second power semiconductor device to the blocking state si- multaneously as the first power semiconductor device is turned off in the disconnector arranged at the converter in question it may be prevented that current is fed into the fault in question from the direct voltage network and from possibly other convert- ers. This means in its turn that no over-current will occur in the other converters in the case of a multiple station system, so that the transmission on the rest of the network will continue as if the fault has never occurred. This also constitutes other preferred embodiments of the invention.

According to another preferred embodiment of the invention the second power semiconductor device is of turn-off type, which is particularly advantageous in the case when the two power semi- conductor devices belong to a switch having a disconnector function, since the disconnecting may then take place very rap- idly and the two components of turn-off type may then be con- trolled by the same order, which simplifies the control electronic.

According to another preferred embodiment of the invention, which relates to a plant according to above having a VSC-con- verter, the plant comprises an arrangement adapted to control the turning on of the second power semiconductor device when a voltage is applied to the converter and by that the voltage in- crease on the direct voltage side. It will by this be possible to manage without any switching-in-resistance mentioned above, and the voltage increase may nevertheless be kept within ac- ceptable limits by controlling the turning on, i. e. the control an- gle of the power semiconductor device, preferably a thyristor.

According to another preferred embodiment of the invention the controllable second power semiconductor device is adapted to be in the conducting state at normal operation of the plant. This means that this power semiconductor device at normal operation of the plant will in principle function as a free-wheeling diode, but when desired it may be controlled into the blocking state, i. e. so that it does not conduct in the forward biased direction for avoiding the disadvantages of a rectifying diode in such fault states.

According to another preferred embodiment of the invention the plant comprises an arrangement adapted to control the first and second power semiconductor devices through a light conductor in common by coded orders. A degree of complicity of the con- trol electronic of the plant being essentially unchanged will by this be obtained in spite of the fact that in such a plant the sec- ond power semiconductor device is controllable contrary to the case of such plants already known.

According to a preferred embodiment of the invention the con- trollable second power semiconductor device is a thyristor, in which this may be only controllable and not of turn-off type, or a GTO of turn-off type. The second power semiconductor device could also be constituted by an IGBT.

The first power semiconductor device is preferably an IGBT.

Further advantages as well as advantageous features of the in- vention will appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a spe- cific description of a preferred embodiment of the invention cited as an example.

In the drawings: Fig 1 and 2 are simplified circuit diagrams illustrating plants for transmitting electric power according to the prior art, and Fig 3 is a view corresponding to these views of a plant according to a preferred embodiment of the inven- tion.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The general structure of the preferred embodiment of the inven- tion shown in Fig 3 appears from the description above of the plants already known, and parts thereof that may be found in the plant according to the preferred embodiment of the invention and have the same function as in the plants already known are in Fig 3 provided with the same reference numerals and will not be further described here.

The plant according to the invention has two considerable dif- ferences with respect to the plants already known, in which the first is constituted by the fact that the second power semicon-

ductor devices, which in the plants already known have been formed by rectifying diodes, are here replaced by a controllable power semiconductor device 31, which in the present case is a thyristor. The thyristor 31 is connected in anti-parallel with the power semiconductor device 10 of turn-off type of the valve, which preferably is an IGBT (Insulated Gate Bipolar Transistor).

The first power semiconductor device 10 of turn-off type and the controllable second power semiconductor device 31 is prefera- bly controlled by one and the same control arrangement 21, and this can take place through a light conductor in common be- tween the control arrangement on ground level and the valve on high potential level, although in the figure for the sake of clear- ness two different dashed lines are drawn to the gate of the re- spective power semiconductor device. All power semiconductor devices of the different valves are of course controlled through the control arrangement 21 in a corresponding way only illus- trated for the valve 4 for the sake of simplicity. Furthermore, the symbol for each power semiconductor device in the valves is to be interpreted as a summarising symbol for a possibly high number of such power semiconductor devices connected in se- ries.

The thyristors 31 will at normal operation of the converter be turned on, i. e. in the conducting state, so that they function as a rectifying diode, i. e. enable a flow of a current therethrough when they are forward biased, but they are blocking when they are reversed biased. However, would a ground fault occur on the direct voltage network the control arrangement 21 controls the thyristors to be turned off at zero point passage, so that the current is eliminated after a maximum of one period. This means that the heat dissipation in the thyristor 31 can be kept down on an acceptably low level and damage on the equipment may be avoided without the need of overdimensioning any components.

Thus, the first power semiconductor device has to be designed for reverse biased blocking voltage, which has not been neces-

sary in devices already known having free-wheeling diodes as second power semiconductor devices.

When the converter has to be brought under voltage after an interruption of operation by closing the breaker 21 for connect- ing the alternating voltage network 14 the voltage increase tak- ing place through charging the direct voltage capacitors 18,19 through the thyristors 31 may be controlled to take place in a well controlled way by the fact that the control arrangement 21 is adapted to regulate the control angle of the thyristors 31.

Thus, no particular arrangements of switching-in-resistances or the like are needed. Furthermore, the possibility to control the voltage increase by regulating the control angle also enables voltage testing of direct voltage switchgears and lines by a volt- age increase controlled by manpower (Open Line Test).

Another difference between the plant according to Fig 3 and the plants of this type already known consists in the fact that in the switch 24 to be acting as a disconnector the rectifying diode has been replaced by a second power semiconductor device 32 of turn-off type, which advantageously is an IGBT, but it may also be a GTO. The power semiconductor devices 25 and 32 are controlled by the control arrangement 21, and they are prefera- bly controlled simultaneously through the same order through one and the same line, even if two dashed lines are drawn from the control arrangement 21 thereto in the figure for illustrating the function. During normal operation of the plant the switch or the disconnector 25 is closed, i. e. the two power semiconductor devices of turn-off type are turned on, so that current may freely flow therethrough from the direct voltage network to the alter- nating voltage network and conversely. However, would a fault occur, for example a ground fault on the direct voltage network, the quick disconnector is then controlled to open by sending a turn-off order to the two power semiconductor devices 25 and 32. In the case that the fault is within the converter it may in this way through turning the power semiconductor device 32 off be

ensured that no fault current is fed in from the direct voltage network to the fault, so that components of the converter are protected, but it is also obtained that no overcurrents arise as a consequence of the fault in other converters of other stations connected to the same direct voltage network in the case of a multiple station system having a direct voltage network of a so called meshed fashion illustrated in Fig 2. Such overcurrents would namely mean that the quick disconnectors connected to these stations would also disconnect them, so that all transmis- sion on the direct voltage network would be interrupted. How- ever, this is avoided by turning the second power semiconductor device 32 of exactly the converter subjected to a fault off.

The invention is of course not in any way restricted to the dis- closure above, but many possibilities to modifications thereof would be apparent to a man with skill in the art, without depart- ing from the basic idea of the invention, such as defined in the appended claims.

It is of course completely within the scope of the invention to maintain the diodes in the valves in a plant of the type shown in Fig 3 but arrange a controllable power semiconductor device in the disconnectors or conversely if desired, although it would in most cases be advantageous to arrange controllable power semiconductor devices in both places. A switch of the type de- fined above could be arranged at any location of a line intended for transmitting electric power, and with respect to the converter case the converter could have another structure, for example be of NPC-type (Neutral Point Clamped).