THYRISTOR VALVE FOR A CONVERTER INTENDED FOR USE IN POWER TRANSMISSION BY MEANS OF HIGH-VOLTAGE DIRECT CURRENT

Abstract: A thyristor valve for a converter for power transmission by means of high-voltage direct current has a number of preferably series-connected thyristors (T1). Each thyristor has means (NV4, BV2, PG1) adapted to sense when a conduction interval of the thyristor is completed and, during a time interval thereafter, to generate a control signal (RP, RP', RP1) which is supplied to a blocking input (BG) of the thyristor for blocking the firing of the thyristor during said time interval.

Description: Thvristor valve for a converter intended for use in power transmission bv means of hiσh-voltaσe direct current TECHNICAL FIELD The present invention relates to a thyristor valve for a converter intended for use in power transmission by means of high-voltage direct current. The valve comprises at least one thyristor as well as means adapted to sense when a conduction interval of the thyristor is completed and, during a time interval thereafter, to generate a control signal for influencing the firing of the thyristor when an off-state voltage starts growing during the recovery interval of the thyristor. BACKGROUND ART Converters for power transmission by means of high-voltage direct current - HVDC converters - are usually composed of three-phase bridges, where each bridge has six thyristor valves. Each valve comprises a number of series-connected thyristors with associated voltage dividers, control circuits, etc. Such valves are described, for example, in the compendium "High Power Electronics, HVDC and SVC", Ake Ekstrόm, Stockholm, June 1990, pages 9-1 through 9-25. Further, they are described in the customer information NLAS 284b from ABE Power Systems AB, Ludvika, rev. 7, Dec. 1990, in US patent specification 4,377,835, and in European patent specification EP-Bl 0 028 784. In such a valve, each thyristor has a thyristor control unit which is arranged at the potential of the thyristor and which is fed with power by means of the voltage divider of the valve. The thyristor control units receive firing pulses from a valve control unit common to all the thyristors of the valve, and the thyristor control units deliver indicating pulses to the valve control unit for monitoring the thyristors and for controlling the supply of firing pulses from the valve control unit. Each valve control unit supplies a firing pulse to its thyristor when a firing pulse is received from the valve control unit, provided that, at the same time, an off-state voltage of sufficient magnitude (e.g. about 30-40 V) lies across the thyristor and that sufficient firing energy is available in the thyristor unit. For protection of the thyristor against overvoltage in the forward direction, there is a circuit which triggers protec- tive firing of the thyristor if the off-state voltage reaches a predetermined level, for example 7 kV. It should be noted that all information as to levels m this application is based on a dimensioning thyristor off-state voltage immediately above 7 kV. Other dimensioning off-state voltages, of course, give other levels. During the recovery interval of a thyristor after a conduction interval, the thyristor has not had time to regain its full capacity to withstand voltage in the forward direction without self-firing. A returning off-state voltage during this inter¬ val can, therefore, cause the thyristor to self-fire. The thyristor can then, without being damaged, self-fire or be protectively fired at low off-state voltages, but if self- firing (or protective firing) is performed at higher off-state voltages, the result may be that the thyristor is damaged or destroyed. To protect the thyristors against this risk, each thyristor control unit in the known valves described here is designed such that, during the recovery interval, it lowers the level, at which protective firing is made, to a relatively low value, for example about 1 kV. This is achieved m these known valves by a circuit sensing when, after a conduction interval, a reverse blocking voltage arises across the thyristor. This circuit triggers a timing circuit which delivers a so-called RP pulse (RP = Recovery Protection) of a predetermined αuration, for example abut 1 ms . If, during this pulse, an off-state voltage amounting to the lower value just mentioned arises across the thyristor, a firing pulse for protective firing of the thyristor is delivered. In this way, the risk of damage by self-firing from too high a voltage is avoided. These known valves, including the control units, are of a thoroughly tested design and have given good operating experience In controllable converters, it must always be ensured that the extinction angle γ, that is, the time expressed in electrical degrees which remains between the calculated time of completed commutation and the zero crossing of the commutating voltage, does not fall below a predetermined minimum value, the so- called minimum extinction angle. This value must be chos n so large that it comprises both the recovery time of the thyris¬ tor and margins for calculating errors, for unforeseen changes of the voltage and for variations in thyristor properties. In known thyristor valves it usually amounts to about 15 to 20 degrees. This corresponds in time to about 1 ms, of which typically approximately half consists of the above-mentioned margins and half of the recovery time of the thyristor. The necessity of maintaining such a large margin of commutating results in an unwanted high consumption of reactive power as well as in the maximally available direct voltage in inverter operation being lower than the theoretically possible one. Further, a large value of the minimum extinction angle provides a higher generation of harmonics on the direct- and alternating-voltage sides of the converter. Another disadvantage of the known valves described above is that a self-firing or protective firing during the recovery- interval may result in a commutating error, which entails an unwanted disturbance of the operation of the converter. SUMMARY OF THE INVENTION The object of the invention is to provide a thyristor valve of the kind described m the introduction, which - compared with what has been possible in converters with valves of a pre¬ viously known kind - is able to operate with a lower extinct¬ ion angle, which entails lower voltage requirements on the valve and on other apparatuses in the installation, and which entails lower harmonic generation. Another object of the invention is to provide a thyristor valve in which the risk of self-firings, and the requirement for protective firings, and hence of commutating errors and operational disturbances, can be reduced compared with prior art valves. Still another object of the invention is to make it possible to achieve these advantages with the aid of well-tested and reliable thyristor control units of a previously known kind, in which only insignificant changes can be made. What characterizes a thyristor valve according to the lnven- tion will become clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail m the following with reference to the accompanying Figures 1-5. Figure 1 shows a thyristor valve according to the invention. Figure 2 shows one of the thyristor control units included in the valve. Figure 3 schematically shows part of a section through one of the thyristors of the valve. Figure 4 shows, as a function of time, the voltage across and the current through the thyristor at the termination of a conduction interval. Figure 5a shows an alternative method of detecting the start of the recovery interval by sensing the zero crossing of the thyristor current, and Figure 5b shows the thyristor current and the measurement signal as functions of the time. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 schematically shows a thyristor valve V according to the invention. The valve has a number ( ) of series-connected thyristors Tl ... Tn ... Tm with associated thyristor control units TCUl ...TCUn ... TCU . From a valve control device (not shown) at ground potential, the thyristor control units receive control pulses CPl ... CPn ... CPm for firing of the thyristors of the valve and deliver to the valve control device indicating pulses IPl ... IPn ... IPm. The valve con¬ trol device is arranged at ground potential, and the control and indicating pulses are transferred between the valve and thyristor control units with the aid of light guides. Each thyristor control unit is connected to the anode and cathode of the corresponding thyristor and delivers to its thyristor a firing pulse FPl ... FPn ... as well as a blocking pulse RPl ... RPn ... RPm. In the valve according to the this embodiment of the inven- tion, the thyristors are of the type which has MOS transistor- controlled emitter shorts. Such thyristors are previously known, for example from Swedish published patent application 392 783, and Stoisiek, Patalong: "Power Devices with MOS- controlled Emitter Shorts", Siemens Forsch. u. Entwickl . -Ber. , Bd. 14 (1985) , No. 2, pages 45-59. These publications describe the structure of such thyristors as well as the use of the controllable shorts for blocking the firing of the thyristor and/or for turning off the thyristor. In the latter of the above-mentioned publications, the shorts are proposed to be constantly activated and are disconnected during the firing process and the conduction interval of the thyristor. Each thyristor (e.g. Tl) thus has a control electrode, which is supplied with a firing pulse (FPl) for firing the thyris- tor, and a control electrode for the MOS shorts, which elec¬ trode is supplied with a blocking pulse (RPl) . A thyristor of the kind referred to here will be described below with reference to Figure 3. Otherwise, the valve may be composed in the previously known way, and for the sake of clarity no further parts of the valve, such as, for example, voltage dividers, are shown. Figure 2 shows the fundamental structure of the thyristor control unit TCUl. Like prior art control units, it has a function for normal firing of the thyristor, a function for protective firing of the thyristor, and a monitoring function. The control unit is connected to the anode and the cathode of the thyristor Tl . With the aid of a power supply circuit PC, an energy magazine in the form of a capacitor C2 is charged, the voltage of which constitutes the supply voltage for the circuits included m the control unit. The power supply circuit PC is connected to the cathode and to the anode of the thyristor, the connection to the latter being made via a voltage divider connection which comprises one branch with a resistor Rl and a capacitor Cl, and one purely resistive branch with the resistors R2 and R3. From the latter branch, a signal VD IS obtained which is proportional to the voltage across the thyristor Tl . The normal firing of the thyristor is triggered by a control pulse CPl from the valve control unit. This is detected m an amplifier AMI and is supplied to an input of an AND circuit AG1. The signal VD IS supplied to a level detector NVl, which emits a signal if the off-state voltage across the thyristor is sufficiently high to perform reliable firing, for example higher than 30 V. This signal is supplied to an input of a second AND circuit AG2. A second input of this circuit is supplied with the output signal from a level detector NV2. This detector senses whether the voltage of the capacitor C2 amounts to a predetermined minimum value, which is adapted such that the output signal from the level detector indicates that sufficient firing energy is available for firing of the thyristor. Thus, if sufficient firing energy is stored m the control unit and off-state voltage of a magnitude sufficient for firing prevails across the thyristor, the AND circuit AG2 delivers a signal to a second input of the AND circuit AGl. Normally, this signal is delivered at the beginning of the off-state voltage interval of the thyristor, and when the control pulse CPl arrives, a signal NFP for normal firing of the thyristor is supplied from the circuit AGl. This signal is supplied to an OR circuit 0G1, which then, via an amplifier AM2 for adaptation of the power and voltage levels of the firing pulse, delivers a firing pulse FPl to the firing gate FG for firing the thyristor. Protective firing of the thyristor is triggered by a leveL detector NV3 , which delivers a signal PFPl at a predetermined value, for example about 7 kv, of the off-state voltage of the thyristor. The thyristor is thus protected against overvoltage in the blocking direction. The level detector delivers the signal PFPl to a second input of the OR circuit OG1, which then delivers a firing signal to the thyristor via the ampli- fier AM2. The monitoring function is obtained with the aid of the output signal from the AND circuit AG2. This signal is supplied to the valve control unit via an amplifier AM3 and a light guide in the form of an indicating signal IPl for monitoring of the state and mode of operation of the thyristor. In a previously known way, the control unit further comprises means for generation of the RP pulse mentioned in the intro- ductory part of the description. These means consist of a level detector NV4 , which delivers a signal when the thyristor voltage at the end of the conduction interval of the thyristor becomes negative - the thyristor takes up reverse blocking voltage. This signal sets a bistable circuit BV2, which circuit, in its turn, triggers a pulse-generating circuit PGl, which delivers a pulse RP of a predetermined length. The length of the pulse may, for example, be about 1000 μs. The bistable circuit BV2 is reset upon firing of the thyristor by the firing pulse FPl. In control units of the prior art kind described above, the RP pulse was used for activating a circuit which during the pulse, that is, during the recovery interval of the thyristor, provides protective firing of the thyristor at a considerably lower voltage than otherwise. According to the invention, the RP pulse instead controls the thyristor by means of its block- ing gate (a firing-preventing control electrode) BG in such a way that the emitter shorts of the thyristor are activated during the RP pulse for preventing firing of the thyristor. The RP pulse from the circuit PGl is supplied to the blocking gate BG of the thyristor via a bistable circuit BVl and an amplifier AM4. The RP pulse is supplied to a dynamic S input of the circuit BVl, which is thus set at the start of the RP pulse. Further, the RP pulse is supplied to a negated dynamic R input Rl of the circuit BVl, which is thus reset at the end of the RP pulse. The output signal RP' from the circuit BVl will thus (l ke the signal RPl) m the normal case coincide with the RP pulse, and it is supplied to the blocking gate BG of the thyristor via the amplifier AM4 and causes the emitter shorts of the thyristor to be activated during the RP pulse If, for some reason, firing of the thyristor is ordered during the RP pulse, the activation of the emitter shorts should be cancelled to make firing possible. This case may occur either by the voltage across the thyristor becoming so high that a signal PFPl for protective firing is obtained from the flip- flop NV3 , or by a control pulse CPl being obtained from the valve control device. In these cases, the amplifier AM2 delivers a firing signal to the thyristor , and this signal is supplied to a dynamic R input R2 of the circuit BVl, which is then reset, thus cancelling the activation of the emitter shorts. The bistable flip-flop BV2 is reset by the signal FPl upon firing of the thyristor. In this way, the RP pulse is caused to be supplied only at the first zero crossing of the thyristor voltage, which prevents unjustified RP pulses to be supplied to the thyristor at such repeated zero crossings as may occur in case of network disturbances . It may be desirable, during the recovery interval, to reduce to a certain extent the voltage level at which protective firing occurs. To achieve this, in the thyristor control unit shown in Figure 2, an additional level flip-flop NV5 is adapted to be supplied with the voltage signal VD and to deliver a signal PFP2 wen the thyristor voltage exceeds a predetermined level. This level is then chosen somewhat lower - for example 5 kV - than that level - for example 7 kV - at which the protective firing (via the flip-flop NV3 ) , described with reference to Figure 2, occurs. The output signal of the flip-flop is then supplied to the thyristor for firing there¬ of, by being supplied to a further input of the OR circuit OG1. To achieve activation of the lower protective firing level only during the recovery interval of the thyristor, the output signal PFP2 of the level flip-flop is supplied to the OR circuit via an AND circuit AG3 which is controlled by the RP pulse. The other thyristor control units are identical, in construc¬ tion and function, with that shown in Figure 2 and described above. Figure 3 schematically shows part of a section through one of the thyristors of the valve. The thyristor has an anode emitter layer 1, an n-base layer 2, a p-base layer or control layer 3 as well as a cathode emitter layer 4 consisting of the portions 4a, 4b etc., which may be parts of a coherent pattern or constitute separate parts. The thyristor has an anode con¬ tact AE and a cathode contact KE with the parts KEa, KEb etc. , which contact the emitter portions 4a, 4b and so on. Between the cathode emitter portions 4a and 4b, the control layer 3 is contacted by a short-circuit contact SE, below which a p + - doped region 5 is arranged to provide a low-resistance ohmic contact. On both sides of the region 5, n + -doped regions 5a and 5b are arranged in the control layer and in contact with the short-circuit contact SE. The region 5a and the emitter portion 4a constitute source and drain regions of an n channel MOS transistor with the control electrode GEa arranged on the control oxide layer SOa. In the same way, the region 5b and the emitter portion 4b constitute source and drain regions of a transistor with the control electrode GEb arranged on the control oxide layer SOb. The control electrodes of the MOS transistors are connected to the blocking gate BG of the thyristor. A firing gate FG for firing the thyristor in con- ventional manner is connected to the control layer 3. The emitter layer 4 with the cathode contact KE and the MOS transistor structures is suitably arranged in a manner known per se as such a finely branched pattern that a sufficiently efficient short-circuit of the cathode emitter junction may be obtained. To obtain good resistance to self-firing (a high VBO and a good dvn/dt resistance) , there is suitably arranged, in addition thereto, a pattern of permanent emitter shorts, for example in the form of short-circuit holes 6a, 6b, 7a, 7b in the emitter layer, through which the control layer reaches up to and is contacted by the cathode contact KE.. The MOS transistors are non-conducting in the absence of a control signal on the electrode BG. A current from the control layer 3 to the emitter layer 4 normally flows directly to the cathode emitter portions 4a, 4b etc. and causes an injection at the emitter junction. The thyristor can then be set in a conducting state - by controlled firing or self-firing - or remain in a conducting state, and carry current with a low voltage drop. A positive signal to the electrode BG causes the formation of an n-conducting channel below the gate of each MOS transistor. A current from the control layer 3 to the WO«-.,3_ .. PC__.E9U0. 4 emitter layer 4 will then flow from the control layer 3 through the region 5, the contact SE, the regions 5a and 5b and the n channels to the cathode emitter. The emitter junct¬ ion may in this way be efficiently short-circuited and the injection at the emitter junction hence be reduced to such an extent that firing of the thyristor is prevented. Figure 4 shows the mode of operation of the valve, described above, in connection with the completion of a conduction interval of a thyristor, for example the thyristor Tl in Figure 2. The voltage "u" and the current "i" of the thyristor are positive in the forward blocking/conducting direction of the thyristor. Prior to time t=0, the thyristor carries current at low on-state voltage. The current then decreases during the continuing commutating process at a rate determined by the commutating voltage and the commutating inductance. At t=0, the current passes through zero, and immediately there¬ after, at t=tl, the thyristor starts taking up reverse block¬ ing voltage. The level flip-flop NV4 then delivers a signal which triggers the pulse generator PGl, which causes the generator to deliver the RP pulse. This pulse is supplied to the blocking gate BG of the thyristor and controls the emitter-shorting MOS transistors to a conducting state, where¬ by, as described above, the injection at the cathode emitter is reduced and firing prevented. The RP pulse lasts from t=tl to t=t3, and the pulse length (t3-tl) may be, for example, 1000 μs . According to the invention, it is chosen at least so long that it corresponds to the recovery time of the thyris¬ tor, suitably with a certain margin to take into account variations between the individual thyristors. Since the firing of the thyristor is efficiently blocked during its recovery interval, the recovery time of the thyristor need not be taken into account when determining the minimum value of the extinc¬ tion angle γ, but only the margins for calculating errors, mentioned in the introduction, unforeseen voltage changes, variations in thyristor properties etc. need be taken into account. In a thyristor valve according to the invention, the minimum extinction angle can therefore typically be reduced from about 17° to typically 10-12°. This results in a consi¬ derable reduction of the consumption of reactive power in inverter operation and furthermore in an increase of the maxi- mally available direct voltage. A diode for reduction of unwanted hole injection from the region 5 can be arranged in a thyristor of the kind shown in Figure 3. The diode is then arranged in the current path from the short-circuit contact (SE) through the short-circuiting member (the MOS transistor) to the cathode contact (KE) . The diode should have a low on-state voltage and may, for example, consist of a so-called Schottky diode. It may consist of a separate diode or of a diode integrated into the thyristor body. According to an alternative embodiment of the invention, that voltage level at which protective firing occurs may be made time-dependent such that it has a low value at the beginning of the recovery interval, which value is successively increased and, at the end of the RP pulse, is caused to assume its full value. Further, the voltage level at which protective firing occurs may be made dependent on the rate of change of the thyristor voltage in such a way that, at a rapidly growing voltage, the protective firing occurs at a lower voltage level. In the thyristor control unit described above with reference to Figure 2, the beginning of the recovery interval is detec¬ ted - and the RP pulse initiated - by sensing when the thyris¬ tor voltage becomes negative. Alternatively, this can be done in dependence on the thyristor current. Figure 5 shows an example of how this can be done. Figure 5a shows how a current transformer PCT can be arranged in series with a thyristor Tl for sensing the thyristor current ITy. The secondary winding of the current transformer is connected to a current-measuring device CM which generates a measurement signal IPCT correspon¬ ding to the thyristor current. The signal IPCT is supplied to a level flip-flop NV6 which delivers an output signal when the signal IPCT becomes negative. The output signal from the flip- flop is supplied to the bistable circuit BV2 in Figure 2. The current transformer is preferably designed such that it becomes saturated at a relatively low current. Figure 5b shows the thyristor current ITy and the measurement signal IPCT as functions of the time t. At the start of the conduction inter- val of the thyristor, that is, at t = io n Figure 5b, a positive pulse is obtained from the measuring device CM. At the end of the conduction interval, that is, at t = tn in Figure 5b, the thyristor current passes through zero and becomes negative. The measuring device CM then delivers a negative pulse which, via the level flip-flop NV6, sets the circuit BV2. Otherwise, the function of the control circuits of the thyristor corresponds to that described with reference to Figure 2. In the foregoing, it has been described how the RP pulse is generated locally in the control device of each thyristor. Alternatively, the pulse RP' may be obtained with the aid of an extra signal from ground. In the emitter shorts with the aid of MOS transistors, described with reference to Figures 2-4, the MOS transistors are of the so-called "Normally-OFF" type, that is, the tran¬ sistors are non-conducting in the absence of a control signal and are set into conducting state when a suitable control signal is supplied. Alternatively, MOS transistors of the so- called "Normally-ON" type may be used in the thyristors in a valve according to the invention. Such transistors are con¬ ducting in the absence of a control signal and are made non¬ conducting by the addition of a suitable control signal. The control circuits are then designed such that the gates of the emitter-shorting MOS transistors of the thyristors are nor¬ mally supplied with such a control signal that they are set into a non-conducting state. During the RP pulse, the transis¬ tors are set into a conducting state, for example by inhibi¬ ting the control signal, and at the end of the RP pulse (or earlier, if a signal for firing of the thyristor is given before the end of the RP pulse) , the transistors are set into a non-conducting state by again supplying this control signal to the gates of the transistors. Such a function can be obtained by a simple modification of the control unit shown in Figure 2. As described above, a thyristor valve according to the inven¬ tion offers significant advantages in the form of reduced con¬ sumption of reactive power, lower harmonic generation, reduced frequency of such operational disturbances as are caused by protective firings, and lower voltage requirements on valves and other apparatuses in the installation. These advantages are especially great in a converter which operates near maxi¬ mum direct voltage in inverter operation.

Claims: CLAIMS 1. A thyristor valve for a converter for power transmission by means of high-voltage direct current, said valve comprising at least one thyristor (e.g. Tl) as well as means (NV4 , BV2, PGl; PCT, CM, NV6) adapted to sense when a conduction interval of the thyristor is completed and, during a time interval there¬ after (tl-t3), to generate a control signal (RP) for influen¬ cing the firing of the thyristor when an off-state voltage starts growing during the recovery interval of the thyristor, characterized in that the thyristor is provided with short- circuit means (BG, GEa, SOa, 5a, 4a) for controllable short- circuiting of an emitter junction (3-4a) of the thyristor and that the control signal (RP; RPl) is adapted to be suppliad to the short-circuit means in such a way that, during the time interval, these are brought into a conducting state and counteract firing of the thyristor during said time interval. 2. A thyristor valve according to claim 1, characterized in that the time interval has a predetermined length (tl-t3) . 3. A thyristor valve according to claims 1 and 2, characterized in that the length of the time interval corresponds to the recovery time of the thyristor. 4. A thyristor valve according to any of claims 1-3, characterized in that the control signal (RP; RPl) is adapted to be supplied to the short-circuit means in such a way that, at the end of the time interval (t=t3), these are brought into a non-conducting state. 5. A thyristor valve according to any of the preceding claims, characterized in that the short-circuit means consist of MOS transistors (e.g. GEa, SOa, 5a, 4a) which are integrated with the thyristor and which are adapted to be brought into a con¬ ducting state during the time interval. 6. A thyristor valve according to any of the preceding claims, which valve has a plurality of series-connected thyristors (Tl ... Tn ... Tm) , characterized in that each one of the thyristors is provided with short-circuit means. 7. A thyristor valve according to any of the preceding claims, which comprises means (NV3) adapted to initiate protective firing of a thyristor in dependence on the voltage across the thyristor, characterized in that the valve comprises means (BVl) adapted, at a protective firing initiated during said time interval (tl-t3) , to bring the short-circuit means into a non-conducting state. 8. A thyristor valve according to any of the preceding claims, which comprises means (AMI, AGl) adapted to initiate firing of a thyristor of a thyristor upon receipt of a firing signal (CPl) for firing of all the thyristors of the valve, characterized in that the valve comprises means (BVl) adapted, at a firing signal received during said time interval (tl-t3), to bring the short-circuit means into a non¬ conducting state. 9. A thyristor valve according to any of the preceding characterized in that the means (NV4, BV2 , PGl; PCT, CM, NV6) , which are adapted to sense when a conduction interval of the thyristor is completed, comprise level-sensing means (NV4) adapted to be supplied with a signal (VD) corresponding to the thyristor voltage and to deliver a signal when the thyristor voltage at the end of a conduction interval becomes negative. 10. A thyristor valve according to any of claims 1-8, characterized in that the means (NV4, BV2 , PGl; PCT, CM, NV6) , which are adapted to sense when a conduction interval of the thyristor is completed, comprise means (PCT, CM) adapted to form a signal (IPCT) corresponding to the thyristor current (ITy) , and level-sensing means (NV6) adapted to be supplied with said signal and to deliver a signal when the thyristor current at the end of a conduction interval becomes negative.

Inventors: DANIELSSON BO, DANIELSSON, BO
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1996
Main classification: H02H7
IPC codes: H02M1, H03K17