WO1999056141A1

WO1999056141A1 - A method and a device for functional test of a semiconductor valve

Info

Publication number: WO1999056141A1
Application number: PCT/SE1999/000625
Authority: WO
Inventors: Ari SEPPÄNEN
Original assignee: Abb Ab
Priority date: 1998-04-28
Filing date: 1999-04-20
Publication date: 1999-11-04
Also published as: SE9801481D0; SE9801481L; SE511858C2

Abstract

A method and a device for functional testing of an optional semiconductor position (TS1, TS2, ...TSN) included in an electric semiconductor valve (V), which semiconductor valve comprises a plurality of semiconductor positions with mutually series-connected semiconductor devices (T1, T2, ...TN), wherein each of the semiconductor devices may be brought into a conducting state by being supplied with a firing pulse, whereby a first indicating signal (IP) is formed when the forward voltage exceeds a predetermined first level (U1). In each one of the semiconductor positions there is arranged a recovery protection unit (RP) which, in case of faultless function, initiates a firing pulse when the voltage across the semiconductor exceeds a predetermined second position(URP). The chosen semiconductor position is supplied alone with an activation signal (FPP) and the other semiconductor devices included in the semiconductor valve are brought into a conducting state in dependence on the firing signals (FP) supplied to the respective semiconductor positions. Said activation signal is of a first duration (τ), so chosen that the forward voltage across the semiconductor position during said first duration reaches a level which exceeds the second level. A firing pulse for the chosen semiconductor position and a second indicating signal are formed in dependence on said activation signal. A first function signal (a1) is formed in dependence on said second indicating signal.

Description

1

A method and a device for functional test of a semiconductor valve

TECHNICAL FIELD

The present invention relates to a method and a device for functional testing of an optional semiconductor position included in an electric semiconductor valve, for example a line-commutated valve in a converter for conversion between alternating current and high-voltage direct current, which semiconductor valve comprises a plurality of semiconductor positions with mutually series-connected semiconductor devices, and a device for carrying out the method.

A semiconductor position in this application means a component group comprising a controllable semiconductor device, such as, for example, a thyristor or a gate turn- off (GTO) thyristor, resistors and capacitors, arranged in a manner known per se on the semiconductor device, for voltage protection of the semiconductor device and voltage division with other semiconductor positions included in the valve, a firing channel for receiving and transmitting firing signals for the semiconductor device, an electronic unit for protection and supervision of the thyristor function, an indicating unit for generating an indicating signal when the voltage across the semiconductor device exceeds a predetermined value, and an indicating channel for emitting the indicating signal.

A semiconductor valve, or simply valve, in this application means a set of a plurality of semiconductor positions with mutually series-connected semiconductor devices, which during normal operation, from an electrical point of view, function as a unit. The firing and indicating signals may comprise, in a manner known per se, light guides for non-galvanic signal transmission between different potential levels and then comprise, at their end points, means for conversion between electrical signals and light signals.

The semiconductor device may be electrically fired or directly light-fired. In the former case, circuits, designed in a manner known per se, for conversion of a firing signal, received in the form of a light signal, into an electrical signal, adapted to be supplied to the control gate of the semiconductor device, may be arranged at the indicating unit. In the latter case, the received firing signal may, via the firing channel, be directly supplied to the semiconducor device in the form of a light signal. The indicating signal may be of a short type, by which is meant that a short pulse is emitted when the voltage across the semiconductor device passes the predetermined value in an increasing direction, or of a long type, by which is meant that an indicating signal is emitted as long as the voltage is higher than the predetermined value. Long indicating signals may consist of a continuous signal or be in the form of a pulse train.

BACKGROUND ART

For a general description of the technical background within the technical field in question, reference is made to Ake Ekstrδm: High Power Electronics HVDC and SVC, EKC- Electric Power Research Center, The Royal Institute of Technology, Stockholm 1990, in particular chapter 9.9: Valve Control .

An electric valve, for example included in a converter for conversion between alternating current and high-voltage direct current (HVDC converter) , comprises a normally large number of mutually series-connected semiconductor devices in the form of thyristors. Control equipment for the converter, placed at ground potential, generates a firing order for the valve and control equipment for the valve, also located at ground potential, generates, as a result of a received firing order, a firing signal for each one of the thyristors included in the valve. These firing signals are received by a firing channel, provided for each thyristor, which firing channel transmits the firing signal to an electronic unit arranged at each thyristor. In the case of semiconductor devices capable of being electrically fired, the electronic unit, which is at the potential of the thyristor, comprises circuits for converting a firing signal, received as a light signal, into an electrical firing pulse, which is supplied to the control gate of the thyristor, as well as an indicating unit. In the case where the thyristors are of a directly light-fired type, the electronic unit consists only of an indicating unit which is then connected to the control gate of the respective thyristor.

The indicating signal generated by the indicating unit is transmitted via an indicating channel to ground potential and is used to ensure, in a manner known per se, that the control gate of a thyristor is not supplied with a firing pulse if its off-state voltage in the forward direction has not reached a predetermined value, adapted for a rapid and reliable firing, and for indicating by its occurrence that the respective thyristor is not short-circuited. For the latter purpose the indicating signal is supplied to monitoring equipment for the valve or the converter, whereby the occurrence of an indicating signal entails a confirmation that the respective indicating channel is in operation.

Both the firing and indicating channels are usually designed as optical fibre links and are provided, at their end points, with members for conversion between electrical and optical signals.

With the valve in blocked state, each thyristor takes up part of the voltage across the valve. The voltage division between the individual thyristors is determined by a voltage divider, comprising resistors and capacitors, connected in parallel with the thyristors. The valve is usually dimensioned such that, in the case that one or a few individual thyristors, for example due to an internal short circuit, should lack voltage-blocking capacity, the remaining thyristors during operation under normal voltage conditions are still capable of blocking voltages arising across the valve.

However, a test of the function of the thyristors included in the valve is necessary so that faulty units may be replaced during planned maintenance work. According to the prior art, the monitoring is carried out as mentioned above by observing the indicating signals transmitted to the test equipment, in which case the absence, indicated short-circuit type faults in the respective thyristor, or faults in the indicating channel, are recorded together with information as to which semiconductor position has been found to lack indicating signal.

If the forward voltage of a thyristor, for example due to disturbances, should exceed a certain level, the thyristor may be fired without a firing signal. To prevent such firings, which may destroy the thyristors, a protective firing is initiated, at a certain forward-voltage level, by an overvoltage protection unit arranged in the electronic unit.

At the end of the conduction interval of a thyristor, the thyristor is blocked by line commutation. After the blocking, the thyrisor has, for a certain limited period of time, a limited ability to take up forward voltage. To protect the thyristors during this recovery period, a so- called recovery protection unit, arranged in the elecronic unit, is activated when the thyristor stops carrying current. If, during the recovery period, the forward voltage of the thyristor should exceed a certain level, which is lower than that voltage level at which protective firing by the overvoltage protection unit is initiated, the recovery protection unit initiates a protective firing of the thyristor.

US 4,377,835 describes such a recovery protection unit. When the thyristor stops carrying current and its forward voltage falls below a certain negative voltage level, a signal is generated which activates the recovery protection unit for a certain period of time, for example 1 ms . If, during this time, the forward voltage exceeds a value which is considered critical to the thyristor, a signal initiating protective firing of the thyristor is generated by the recovery protection unit.

Published international patent application PCT/SE93/00662 suggests a method and a device for functional testing of the overvoltage protection unit of a selected semiconductor position. A semiconductor position is tested by generating a test firing signal, whereupon an indicating signal emitted by the semiconductor position is studied with respect to time. However, this method does not permit a functional test of the recovery protection units at the respective semiconductor positions, and thus not a complete test of the protective functions of the position. SUMMARY OF THE INVENTION

It is an object of the invention to achieve a method and a device for a functional test of the recovery protection unit at an optional semiconductor position during normal operation and without operational disturbances of the converter. This simplifies maintenance and shortens maintenance work significantly.

According to the invention, this is achieved by supplying to the selected semiconductor position alone an activation signal and bringing the other semiconductor positions, included in the semiconductor valve, into a conducting state in dependence on the firing signals supplied to the respective semiconductor position. The activation signal and the firing signals are supplied to the respective semiconductor positions essentially simultaneously. The activation signal is pulse-shaped and of a first duration so chosen that the forward voltage across the semicon- ductor position during the first duration reaches a level at which the recovery protection unit, in case of faultless function, generates an initiating signal. In the case where the recovery protection unit generates an initiating signal, a firing pulse is formed for the selected semi- conductor position in dependence on the initiating signal and the activation signal. An indicating signal is formed in dependence on the initiating signal and the firing pulse and a function signal, indicating that the semiconductor of the selected semiconductor position has been brought into conducting state by the initiating signal, is formed in dependence on the indicating signal and the activation signal.

If for some reason the recovery protection unit should not initiate a firing pulse, the function signal does not occur and the semiconductor is fired when the activation 7

signal ceases. The activation signal and the functional test have such a short duration in relation to a period of the applied alternating voltage that no influence is exerted on the normal function of the valve.

When a recovery protection unit in this way is detected as being faulty, this is recorded in the test device together with the position of its thyristor. During the next stop for routine maintenance work, the electronic unit of the thyristor position may be replaced or repaired.

Advantageous improvements of the invention will become clear from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by description of embodiments with reference to the accompanying drawings, wherein

Figure la shows, in the form of a block diagram, a device for monitoring semiconductor positions in an HVDC converter,

Figure lb schematically shows a 6-pulse valve bridge which is part of an HVDC converter,

Figure lc shows a schematic equivalence circuit for a thyristor with associated RC circuits,

Figure Id shows voltage, current and signal characteristics while blocking a thyristor with a recovery protection unit according to the prior art,

Figure 2 shows in the form of a block diagram an electronic unit with an overvoltage protection 8

unit and a recovery protection unit according to the invention,

Figure 3 shows the composition of a test device for testing electronic units according to the invention,

Figure 4a shows voltage and signal characteristics for a normal firing of a thyristor with an electronic unit according to Figure 2,

Figure 4b shows voltage and signal characteristics for a normal firing of a thyristor with an electronic unit according to a second embodiment of the invention,

Figure 5a shows voltage and signal characteristics during testing of a functioning recovery protection unit according to a first embodiment of the invention,

Figure 5b shows voltage and signal characteristics during testing of a functioning recovery protection unit according to a second embodiment of the invention,

Figure 6a shows voltage and signal characteristics during testing of a faulty recovery protection unit according to a first embodiment of the invention,

Figure 6b shows voltage and signal characteristics during testing of a faulty recovery protection unit according to a second embodiment of the invention . DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description refers both to the method and the device.

Figure la shows part of a valve V of the kind described in the introductory part of the description. The valve comprises N mutually series-connected thyristors Ti , T₂ , ...

T_N with electric firing, connected between two main terminals A^_ , B]_, A₂ , B₂... A_N, B_N, of which only three have been drawn in the figure. An electronic unit TCU, which is thus at high potential, is associated with each thyristor Ti , T₂ , ...T^. In the figure a dash-dotted line marks a dividing line such that parts shown to the right of this line are at a high potential whereas parts shown to the left of this line are at ground potential . A light- pulse generator L is associated with each electronic unit and is connected to the respective electronic unit by a first light guide 1 and a receiving detector D, connected to the respective electronic unit by a second light guide 2, for conversion of the firing signals and indicating signals, which are to be described in greater detail below, from electrical form into light and inversely. Valve control equipment VCU is activated by a firing order VFO for the valve, and the valve control equipment VCU generates a central firing signal CP in dependence on the firing order.

Each thyristor position is associated with a signal selector gate FPG, which receives the central firing signal CP and converts it into an individual firing signal FPi, FP₂, ...FP_N. A firing signal FP is brought from the signal selector gate FPG to the respective light-pulse generator L, and from there via the signal input SD_FP to a light-sensitive diode D_FP (shown in Figure 2) in the 10

respective electronic unit. The firing signal FP is converted in the electronic unit into an electrical signal which, via a conductor 3, is supplied to the control gate of the thyristor in order to bring the thyristor into conducting state. According to the invention, a signal- selector gate FPG may, in addition, receive activation signals FPP which are generated by a test device 7. An activation signal FPP is intended for a chosen thyristor position and is passed via a conductor 8 from the test device 7 to the input of a signal-selector gate FPG, which forwards the activation signal to the respective electronic unic TCU.

Each one of the electronic units also comprises an indi- eating unit of the previously mentioned kind and from these indicating units indicating signals IPi, IP₂, -IP_N' in the form of light signals, are emitted via the light guides 2 to the respective receiving detector D, in which the received light signal is converted into a correspon- ding electrical signal.

Each one of the light-pulse generators L with its associated first light guide 1, circuits arranged in the respective electronic unit TCU for conversion of the firing signal into an electrical signal, and the conductor 3 between the output of the these circuits and the control gate of the thyristor thus form a firing channel. Each one of the indicating units and the associated second light guide 2 and detector D form an indicating channel .

A firing channel, the associated thyristor with resistors and capacitors for voltage protection of the semiconductor devices and the associated indicating channel form a semiconductor position TS .

Figure lb shows a converter in three-phase 6-pulse bridge 11

connection comprising six valves V_]_ , V₂ , ... Vg . In a manner known per se the bridge is connected, as part of a 12-puls bridge connection, between a three-phase ac network, only marked LI, L2 , L3 in the figure, and a dc network, only marked by two conductors 11, 12 in the figure. In a manner described with reference to Figure la, each one of the valves receives a firing signal CP(1), CP(2), ... CP(6), common to the respective valve and generated by valve control equipment belonging to the valve. A firing order VFO is generated in a manner known per se by control equipment for the converter such that the valves are cyclically supplied with firing orders VFO in the sequence V_]_ , V₂ , .. Vg , ι_ .. . Each one of the valves comprises N semiconductor positions, as described with reference to Figure la.

All the thyristors in the valve V are connected in series, and when the thyristors are in non-conducting state, the valve voltage is distributed evenly, with the aid of vol- tage dividers SD, over the blocked thyristors. Figure lc shows parts of these voltage dividers, comprising a series connection of a resistor R₂ and a capacitor C₂ connected in parallel with a thyristor T and a resistor Rη_ , also connected in parallel with the thyristor T^ . In case of voltage changes, the rate of the voltage change across the individual thyristors is limited by the capacitor C₂ in cooperation with the resistors Rj_ and R₂.

Immediately after the current zero crossing in connection with a blocking of the thyristor, this has a limited voltage withstand capability. To protect the thyristor during the recovery time, therefore, a recovery protection unit RP is activated when turning off the thyristor.

To illustrate the function of a recovery protection unit 12

according to the prior art, Figure Id shows voltage and current characteristics for a typical thyristor commutation. Before time t_a the thyristor T is conducting in a first semiconductor valve, for example V_]_ , and the voltage U_τ across the thyristor has a low, positive value.

At time t_a a second semiconductor valve, for example V₃ , is turned on and the commutation of the current to the second semiconductor valve is started. The thyristor current I_τ in the first valve decreases and passes through zero at the time t]-, and then assumes a negative value. At time t_c the thyristor T in the first semiconductor valve starts taking up blocking voltage and the voltage U_τ becomes negative. When the thyristor voltage falls below a value U3 the recovery protection unit is activated for a time XR . During a normal commutation, the voltage U_τ follows the continuous line in the figure between time t_c and time t^ and then it follows the dashed line marked by U'. If, for example, a transient disturbance should occur at time t^, the forward voltage U_τ after time t^ may follow a course according to the continuous line. When the voltage, at time t_e, exceeds a predetermined value UR_P and the recovery protection unit is still activated, a firing of the thyristor is initiated through the recovery protection unit RP . Without a recovery protection unit, the thyristor voltage U_τ would follow the dashed line marked U' ' resulting in a risk of the thyristor being destroyed.

Figure 2 shows, in the form of a block diagram, an embodiment of the electronic unit TCU according to the invention. The electronic unit comprises circuits for achieving a recovery protection unit RP and a possibility of testing this according to the invention. 13

Via a voltage divider R₃ , R₄ , connected between the main terminals A and B on the thyristor T, the forward voltage U_τ across the thyristor T is sensed and a voltage value U_M proportional thereto is supplied to four threshold-value circuits gl, g2 , g3 , g4. A first threshold-value circuit gl generates a signal si when the forward voltage U_τ exceeds a first level Ui , of for example 35 V, in an increasing direction. The signal si is supplied to a first OR-circuit gβ , the output of which is connected to a first dynamic circuit g7. At the output of the first dynamic circuit g7 , a first indicating signal IP is generated when the forward voltatge U_τ exceeds the first level Ui . The first indicating signal IP is supplied to a light-emitting diode D_IP, and from there to the light guide 2.

The indicating signals IP, arriving via the light guides 2 in the form of light, are transformed in the detector D into electrical signals, which via wires 9 are supplied to the testing device 7. At the beginning of each positive half-cycle for the alternating voltage applied across the valve, a first indicating signal IP is thus obtained from each electronic unit TCU for each faultless semiconductor position .

A second OR-circuit gl2 has three signal inputs and, when being supplied to one of an input signal s4, sll or sl5, generates a signal TP, which is amplied in an amplifier gl2 into a firing pulse. The firing pulse is supplied to the control input of the thyristor via the conductor 3 and hence fires the thyristor, that is, brings the thyristor into a conducting state.

As described above, a central firing signal CP is transformed into an individual firing signal FP which is transformed, at the input D_FP of the electronic unit, into 14

electrical form. According to the invention, the firing signal FP is supplied to an inverting input of a second dynamic circuit gl4, which on the negative flank of the firing signal FP generates a signal sl4. A first AND- circuit gl5 is supplied with the signal si and the signal sl4, which AND-circuit, in dependence on these signals, generates a signal sl5. The sigal sl5 is supplied to an input of the second OR-circuit gl2 , which results in firing of the thyristor.

If due to a malfunction, for example absence of a firing signal FP, the thyristor T does not fire, a protective firing is initiated in the electronic unit TCU, according to the prior art, through the so-called overvoltage protection unit. The voltage value U_M is supplied to a fourth threshold-value circuit g4. When the forward voltage U_τ exceeds a fourth level U_PF, the fourth threshold-value circuit g4 generates a signal s4. The signal s4 is supplied to the second OR-circuit gl2 which, as described above, leads to firing of the thyristor T.

A recovery protection unit RP according to the invention comprises a second and a third threshold-value circuit g2 , g3 , a bistable flip-flop g8 , a monostable flip-flop g9 , a third OR-circuit glO, a second AND-circuit gll with a first and a second input, and a third AND-gate g5 with two inputs . Thr third threshold-value circuit g3 generates a signal s3 when the forward voltage U falls below a third level U₃ , of for example -17V. The signal s3 is supplied to the SET-input of the bistable flip-flop g8. At the output of the bistable flip-flop g8 there is generated, in dependence on the signal s3 , a signal s8 which is supplied to the monostable flip-flop g9. In dependence on the signal s3 the monostable flip-flop g9 generates a signal s9 of a duration T_RP of, for example, 1 ms . The signal s9 15

is supplied to an input of the third OR-circuit glO, which, at its output, generates a signal AS, whereby the recovery protection unit RP is activated for the time τ_RP .

The signal AS is supplied to the first input of the second AND-circuit gll .

The second threshold-value circuit g2 in the recovery protection unit RP generates a signal s2 when the forward voltage of the thyristor exceeds a second level U_RP of, for example, IkV, which signal is supplied to the second input of the second AND-circuit gll. This circuit generates a signal sll in dependence on the signals s2 and AS. The signal sll is supplied to an input on the second OR- circuit gl2, and thus initiates a thyristor firing as described above. The signal TP is also supplied to the RESET-input of the bistable flip-flop g8, which is thus reset at each thyristor firing.

According to the invention, a possibility of carrying out a functional test of the recovery protection units on an optional semiconductor position is provided. Therefore, the signal s2 from the second threshold-value circuit is supplied to the third AND-circuit g5. The second input on the third AND-cicuit g5 is supplied with the signal TP, and the third AND-circuit g5 thus generates a signal s5 when the thyristor has been fired through the recovery protection unit RP . The signal s5 is supplied to an input on the first OR-circuit g6 and thus initiates a second indicating pulse IP, as described above.

According to the invention, the firing signals FP, and, at the chosen semiconductor position, an activation signal FPP, are supplied not only to the dynamic circuit gl4 but also to an input of the third OR-circuit glO . The acti- vation signal FPP thus activates the recovery protection 16

unit RP during the duration of this signal. The activation signal FPP is sent by the testing device 7 to that semiconductor position, the electronic unit TCU of which is to be tested. The activation signal FPP is generated simul- taneously with the central firing order CP. As the firing signal FP, the activation signal FPP is pulse-shaped but of a longer duration τ, typically 50 μs at a system frequency of 50 Hz, compared with a typical duration of 1- 2 μs for the firing signal. The thyristors in the non- tested semiconductor positions in the valve V are fired, according to the invention, on the negative flanks by the firing signals FP. The forward voltage U_T across the tested semiconductor position TS rises rapidly, limited in its rate of change by the capacitive part of the voltage divider SD. When the forward voltage U_τ exceeds the second level U_RP the thyristor is fired by the recovery protection unit RP. The duration τ of the activation signal FPP is so chosen that the forward voltage U_T, with knowledge of the expected time rates of change of the voltage at least during a predetermined part of its period after its zero crossing to positive polarity over the chosen semiconductor position, reaches a level which exceeds the second value URP during the duration τ but does not reach the fourth value Up_F, at which a protective firing through overvoltage protection is initiated. If the recovery protection unit should exhibit a malfunction, the thyristor T on the negative flank is fired by the activation signal FPP without being damaged by the test.

An embodiment of the test device 7 according to the invention is shown in Figure 3. The test device comprises a demultiplexer 712 and a multiplexer 721, with a number of outputs and inputs, respectively, corresponding to the number of semiconductor positions in the valve V, each 17

semiconductor position TS being associated with an output on the demultiplexer and an input on the multiplexer. Each one of the outputs of the demultiplexer is connected, via its own conductor, to the respective signal-selector gate FPG and each one of the inputs of the multiplexer receives, via its own conductor 9, an indicating signal from the respective semiconductor position, as described with reference to Figure 1. The test device further comprises a microprocessor 74, a testing unit 77, a signal bus 75. The signal bus is connected both to the multiplexer 721, the demultiplexer 712, the microprocessor 74 and the testing unit 77.

The test of the thyristor positions may be initiated automatically, directly through the testing unit 77, or from a central computer unit (not shown here) which sends a test order to the testing unit 77. For example, a routine testing of all the semiconductor positions may be carried out after a certain time interval . In the testing unit there is selected at which semiconductor position TS a functional test is to be carried out and address information thereto is sent via the signal bus 75 to the microprocessor 74.

The microprocessor 74 receives from the testing unit 77 address information relating to the chosen semiconductor position in the valve and transmits this information via the signal bus 75, in a manner known per se, to the demultiplexer 712 and the multiplexer 721 and influences them such that the indicating signal IP for the tested semiconductor position TS appears on the output of the multiplexer 721 and such that the activation signal FPP, generated, for example, in the manner described below, and supplied to the input of the multiplexer 712, occurs on the output of the demultiplexer 712 which is connected to the signal-selector gate FPG of the chosen semiconductor 18

position .

While testing the semiconductor position TS , a signal b is generated by the testing unit 77, which signal is supplied to an input of a fourth AND-circuit 763 with two inputs. The second input on the fourth AND-circuit 763 is supplied to the central firing signal CP via a wire 10. At the output of the fourth AND-circuit 763, a signal c is generated in dependence on signals b and CP and is supplied both to an input on the microprocessor 74 and to the input of a monostable flip-flop 764. This flip-flop emits the activation signal FPP of the duration τ when a signal c is supplied to the input thereof. The activation signal FPP is supplied to the input of the demultiplexer 712 and from its output further to the tested semiconductor position as described above.

The output of the multiplexern 721 is connected to an input of a fifth AND-circuit 761, which input is connected to the output of the multiplexer for receiving the second indicating signal IP generated from the chosen semiconductor position. The second input of the fifth AND-circuit 761 is supplied to the activation signal FPP. At the output of the fifth AND-circuit 761, a signal a is generated, when the second indicating signal IP occurs during the duration of the activation signal FPP. The signal ai is supplied to an input of the testing unit 77, in which, in the presence of the signal ai, it is recorded that the recovery protection unit RP of the respective electronic unit is in operation. In the absence of a signal Ά , the respective electronic unit is registered as faulty.

Figures 4-6 show voltage and signal characteristics illustrating the test method. Figures 4a, 5a and 6a relate to a first embodiment of the invention when the electronic 19

unit generates indicating signals of the above-described short type, whereas Figures 4b, 5b, 6b relate to a second embodiment of the invention when the electronic unit generates indicating signals of a so-called long type. Figure 4a shows the time progress during normal, undisturbed operation. At the beginning of the positive voltage half-wave of the applied alternating voltage, the forward voltage UT of the thyristor T exceeds the first level Ui at a first time tι_, and a first indicating signal IP is generated. At a time t₂ the central control unit generates a central firing order CP which is supplied to the signal-selector gates FPG and from there, as individual firing signals, to the semiconductor positions TSi, TS₂ , ... TSN in the semiconductor valve V. At the time t_2a, when the firing signal ceases, the firing pulse TP is formed in the manner described above.

Figure 5a shows the characteristics for a tested semiconductor position TS when the recovery protection unit RP on the semiconductor position TS functions. When the forward voltage U_τ exceeds the level Ui at the time ti, a first indicating signal IP is generated. At the time t₂ a central firing order CP is generated in the central control unit. At the same time, an activation signal FPP is generated in the test device 7. During the duration X of the activation signal FPP, the recovery protection unit is activated and the signal AS generated. At the time t_2a the thyristors in the non-tested semiconductor positions are fired and the forward voltage UT of the tested semi- conductor position rises rapidly. At a time t₃ during the duration of the activation signal FPP, the forward voltage U exceeds the second level UR_P and a signal s2 is generated. In sequence, the signals sll and TP are generated and the thyristor T is fired. The signal s5 is generated, 20

as described with reference to Figure 2, and hence a second indicating signal IP, which leads to generation of a first function signal a± in the test device 7.

Figure 6a shows the signal characteristics when the recovery protection unit RP exhibits a malfunction. Up to the time t3 the voltage and signal characteristics are identical with those shown in Figure 5a. At the time t3 , the forward voltage U exceeds the value UR , but because of a malfunction the signal sll fails to appear and the thyristor T is not fired. At the time t₄, when the activation signal FPP ceases, a signal sl5 is generated on the negative flank thereof, which leads to generation of a signal TP and the thyristor T is fired. Because the thyristor is fired after the activation signal has ceased, no first function signal a^ is generated in the test device 7.

From the testing unit 77, the information about a faulty semiconductor position is transmitted, in a manner not shown, to the central computer unit. The information may, for example, be shown or called on a display in the central monitoring unit.

In the second embodiment of the invention, indicating signals IPL of a long type are generated in the electronic unit, that is to say, as long as the forward voltage U_τ of the thyristor T is higher than the first level U_]_, the third indicating signal IPL is generated. The third indicating signal IPL ceases when the thyristor is fired and the forward voltage U_τ becomes lower than the first level U_]_ . The indicating signals IPL are transmitted via the indicating channels to the test device 7. The indicating signals IPL are supplied to the inputs of the multi- plexer 721 and from there to an inverting input on a sixth 21

AND-circuit 762, shown in Figure 3 by dashed lines.

When testing a semiconductor position TS , an activation signal FPP is generated by the monostable flip-flop 764. The activation signal FPP is supplied to a second input of the sixth AND circuit 762. When the third indicating signal IPL ceases during the duration τ_RP of the activation signal, a signal a₂ is generated, which remains until the activation signal FPP has ceased. The signal a₂ is supplied to the input of the testing unit 77. If the thyristor is not fired during the duration τ_RP of the activation signal FPP, the third indicating signal IPL ceases upon firing the thyristor on the negative flank of the activation signal FPP. In the testing unit, the respective semiconductor position TS is recorded as described above.

Figure 4b shows the voltage and signal characteristics during normal operation. Contrary to the first embodiment, the third indicating signal IPL does not cease until the thyristor is fired.

Figure 5b shows the characteristics for a tested semiconductor position TS when the recovery protection unit on the semiconductor position TS functions. When the forward voltage U_T exceeds the level Ui at the time t]_, the third indicating signal IPL is generated. At the time t2 , a central firing order CP is generated in the central control unit and an activation signal FPP is generated in the test device. During the duration τ of the activation signal FPP, the recovery protection unit RP is activated and the signal AS is generated. At the time t _a, the thyristors on the non-tested semiconductor positions are fired and the forward voltage U on the tested semicon- 22

ductor position rises rapidly. At a time t₃ during the duration of the activtion signal FPP, the forward voltage U exceeds the second level UR and a signal s2 is generated. Signals sll and TP are generated in sequence and the thyristor T is fired. The third indicating signal IPL ceases, which leads to generation of a second function signal a₂ which ceases when the activation signal FPP ceases at the time t₄.

Figure 6b shows the signal characteristics when the recovery protection unit RP exhibits a malfunction. Up to the time t3 , the voltage and signal characteristics are identical with those shown in Figure 5b. At the time t₃ , the thyristor voltage UT exceeds the value UR_P, but because of a malfunction, the signal Sn fails to occur and the thyristor T is not fired. At the time t₄ , when the activation signal FPP ceases, a signal sl5 is generated on its negative flank, which leads to generation of a signal TP, firing of the thyristor T and cessation of the third indicating signal IPL. Because the thyristor is fired after cessation of the activation signal FPP, no second function signal a is generated in the test device 7.

The invention makes it possible to carry out an automatic functional test of all the semiconductor positions included in the valve. It is often a question of very large installations with a large number of positions. In each valve the number of thyristors may be up to 250, and in a 12-pulse system the number of units to be tested may thus amount to the order of magnitue of 3000 units. The alternative of carrying out an otherwise necessarily manual measurement control on all the positions is therefore a very comprehensive operation, which may cause additional malfunctions which may be essentially elimi- nated through the invention. Only those positions which 23

have been diagnosed according to the invention thus need to be overhauled manually.

The invention has been exemplified by means of a valve where the actual semiconductor devices consist of thyristors with electric firing. It is conceivable that also directly light-fired thyristors will be used in corresponding applications, and it should therefore be emphasized that the invention may also be applied to functional testing of semiconductor positions comprising these types of semiconductor devices .

The invention may, of course, also be applied to semiconductor valves intended for other purposes than conversion between alternating current and direct current.

Claims

24CLAIMS

1. A method for functional testing of an optional semiconductor position (TS╬╣_, TS₂ , ... TS_N) included in an electric semiconductor valve (V) , for example a semiconductor valve in a converter for conversion between alternating current and high-voltage direct current, which semiconductor valve comprises a plurality of semiconductor positions with mutually series-connected semiconductor devices (Ti, T₂ , ... T^) , wherein each of the semiconductor devices may be brought into a conducting state by being supplied with a firing pulse and each one has two main terminals (A╬╖_, B^ , A , B₂ , ... A_N, B_N) between which a semiconductor voltage (U_╧ä╬╣, U_T2 / ΓÇóΓÇóΓÇó U_TN) , referred to as forward voltage, occurs, which voltage periodically assumes both a positive and a negative polarity, a first indicating signal (IP) is formed when the forward voltage exceeds a predetermined first level (U_]_) with a positive polarity, and in each one of the semiconductor positions there is arranged a recovery protection unit (RP) which, in case of faultless function, generates a initiating signal (s2) when the forward voltage exceeds a predetermined second level (U_RP) with a positive polarity, characterized in that the chosen semiconductor position alone is supplied with an activation signal (FPP) and the other semiconductor devices included in the semiconductor valve are brought into a conducting state in dependence on the firing signals (FP) supplied to the respective semiconductor positions, whereby said activation signal and said firing signals are supplied to the respective semiconductor positions essentially simultaneously and whereby said activation signal is pulse-shaped and of a first duration (╧ä) , so chosen, with knowledge of the expected time rate of 25

change of the forward voltage during at least a predetermined part of its period after its zero crossing to a positive polarity across the chosen semiconductor position, that the forward voltage across the semiconductor position during said first duration reaches a level which exceeds the second level, whereby, in the case where the recovery protection unit generates an initiating signal, a firing pulse for the chosen semiconductor position is formed in dependence on the initiating signal and said activation signal, a second indicating signal is formed in dependence on the initiating signal and the firing pulse, and a first function signal (a^) is formed in dependence on said second indicating signal and said activation signal, indicating that the semiconductor of the chosen semiconductor position has been caused into a conducting state in dependence on the initiating signal.

2. A method for functional testing of an optional semi- conductor position (TS^, TS , ... TS_N) included in an electric semiconductor valve (V) , for example a semiconductor valve in a converter for conversion between alternating current and high-voltage direct current, which semiconductor valve comprises a plurality of semiconductor positions with mutually series-connected semiconductor devices (T]_, T₂ , ... TJVJ) , wherein each of the semiconductor devices may be brought into a conducting state by being supplied with a firing pulse and each one has two main terminals (Aj_, B^ , A₂ , B , ... A_N, B_N) between which a semiconductor voltage (U i U_T2 ΓÇóΓÇóΓÇó UTN) ' referred to as forward voltage, occurs, which voltage periodically assumes both a positive and a negative polarity, a third indicating signal (IPL) is formed when the forward voltage exceeds a predetermined first level (U_]_) with a positive polarity, and in each one of the semiconductor positions 26

there is arranged a recovery protection unit (RP) which, in case of faultless function, generates a initiating signal (s2) when the forward voltage exceeds a predetermined second level (U_RP) with a positive polarity, characterized in that the chosen semiconductor position alone is supplied with an activation signal (FPP) and the other semiconductor devices included in the semiconductor valve are brought into a conducting state in dependence on the firing signals (FP) supplied to the respective semiconductor positions, whereby said activation signal and said firing signals are supplied to the respective semiconductor positions essentially simultaneously and whereby said activation signal is pulse-shaped and of a first duration

(╧ä) , so chosen, with knowledge of the expected time rate of change of the forward voltage during at least a predetermined part of its period after its zero crossing to a positive polarity across the chosen semiconductor posi- tion, that the forward voltage across the semiconductor position during said first duration reaches a level which exceeds the second level, whereby, in the case where the recovery protection unit generates an initiating signal, a firing pulse for the chosen semiconductor position is formed in dependence on the initiating signal and said activation signal, the semiconductor device is brought into a conducting state in dependence on said firing pulse, said third indicating signal ceases in dependence on the semiconductor device being in a conducting state, and a function signal (a₂) is formed in dependence on said activation signal and the cessation of said third indicating signal, indicating that the semiconductor of the chosen semiconductor position has been caused into a conducting state in dependence on the initiating signal. 27

3. A method according to any of claims 1-2, in which a protective firing of the semiconductor device is initiated when the forward voltage exceeds a fourth level (U_PF) which is higher than the second level (U_RP) , characterized in that the duration (╧ä) of the activation signal is so chosen that the forward voltage does not exceed the fourth level during the functional test.

4. A method according to any of claims 1-2, in which the chosen semiconductor position has a signal input (D_FP) and is supplied with a firing signal on the signal input, which firing signal is pulse-shaped and of a second duration smaller than said first duration, characterized in that firing pulses for the chosen semiconductor posi- tion are formed in dependence on signals supplied to the signal input ceasing and on the activation signal being supplied also to said signal input.

5. A method according to any of claims 1 and 3, when claim 3 depends on claim 1, wherein said first and second indicating signal (IP) are of a short type, characterized in that the function signal (a_) is formed in dependence on the activation signal and the second indicating signal occurring simultaneously.

6. A method according to any of claims 2 and 3, when claim 3 depends on claim 2, wherein said third indicating signal (IPL) is of a long type, characterized in that the function signal (a ) is formed in dependence on the activation signal occurring in the absence of the first indicating signal.

7. A device for functional testing of an optional semiconductor position (TS_]_, TS₂ , ... TS_N) included in an 28

electric semiconductor valve (V) , for example a semiconductor valve in a converter for conversion between alternating current and high-voltage direct current, which semiconductor valve comprises a plurality of semiconductor positions with mutually series-connected semiconductor devices (Ti, T₂ , ... TN) , wherein each of the semiconductor devices may be brought into a conducting state by being supplied with a firing pulse (TP) and each one has two main terminals (A_1; B^ , A , B₂ , ... A_N, B_N) between which, when the semiconductor valve is in operation, a semiconductor voltage (UT₁₍ UT , ... UT ) , referred to as forward voltage, occurs, which voltage periodically assumes both a positive and a negative polarity, which device further comprises a first level-sensing member which generates a first indicating signal (IP), when the forward voltage exceeds a predetermined first level (U_]_) with a positive polarity, which device further comprises a recovery protection unit (RP) with a second level-sensing member (g2) which, in case of faultless function, generates a initiating signal (s2) when the forward voltage exceeds a predetermined second level (U_RP) with a positive polarity, which device is characterized in that it comprises means (763, 764, 712) which supply to the chosen semiconductor position alone an activation signal (FPP) and which supply to the other semiconductor positions, included in the semiconductor valve, firing signals (FP) for bringing these into a conducting state, whereby said activation signal and said firing signals are supplied to the respective semi- conductor positions essentially simultaneously and whereby said activation signal is pulse-shaped and of a first duration (x) , so chosen, with knowledge of the expected time rate of change of the forward voltage at least during a predetermined part of its period after its zero crossing to positive polarity across the chosen semiconductor 29

position, that the forward voltage across the semiconductor position during said first duration reaches a level which exceeds the second level, and that the device further comprises, means (glO, gll, gl2) which, in the case where the recovery protection unit generates an initiating signal, generate a firing pulse for the chosen semiconductor position in dependence on the initiating signal and said activation signal, means (g5, g6 , g7) which form a second indicating signal (IP) in dependence on the firing pulse thus formed, and means (721, 761) which form a function signal (a╬╣_) in dependence on said second indicating signal and said activation signal, indicating that the semiconductor of the chosen semiconductor position is brought into a conducting state in dependence on the initiating signal.

8. A device for functional testing of an optional semi- conductor position (TS^_, TS₂ , - TSjj) included in an electric semiconductor valve (V) , for example a semiconductor valve in a converter for conversion between alternating current and high-voltage direct current, which semiconductor valve comprises a plurality of semiconductor positions with mutually series-connected semiconductor devices (Ti, T2 , ... TN) , wherein each of the semiconductor devices may be brought into a conducting state by being supplied with a firing pulse and each one has two main terminals (A_]_, B]_, A2 , B2 , ... A_N, B_N) between which, when the semiconductor valve is in operation, a semiconductor voltage (UT_]_, UT2 , ... UT_N) , referred to as forward voltage, occurs, which voltage periodically assumes both a positive and a negative polarity, which device further comprises a fifth level-sensing member which generates a third indicating signal (IPL) when the forward voltage 30

exceeds a predetermined first level (U^) with a positive polarity, a recovery protection unit (RP) with a second level-sensing member (g2) which, in case of faultless function, generates a initiating signal (s2) when the forward voltage exceeds a predetermined second level (U_RP) with a positive polarity, which device is characterized in that it comprises means (763, 764, 712) which supply to the chosen semiconductor position alone an activation signal (FPP) and which supply to the other semiconductor positions, included in the semiconductor valve, firing signals (FP) for bringing these into a conducting state, whereby said activation signal and said firing signals are supplied to the respective semiconductor positions essentially simultaneously and whereby said activation signal is pulse-shaped and of a first duration (╧ä) , so chosen, with knowledge of the expected time rate of change of the forward voltage at least during a predetermined part of its period after its zero crossing to positive polarity across the chosen semiconductor position, that the forward voltage across the semiconductor position during said first duration reaches a level which exceeds the second level, and that the device further comprises, means (glO, gll, gl2) which, in the case where the recovery protection unit generates an initiating signal, generate a firing pulse for the chosen semiconductor position in dependence on the initiating signal and said activation signal, and means (721, 761) which form a function signal (a2) in dependence on said third indicating signal and said activation signal, indicating that the semiconductor of the chosen semiconductor position is brought into a conducting state in dependence on the initiating signal.

9. A device according to any of claims 7-8, wherein the 31

chosen semiconductor position has a signal input (D_FP) for receiving firing signals when the semiconductor valve is in operation, which firing signals are pulse-shaped and of a second duration, smaller than said first duration, characterized in that firing pulses for the chosen semiconductor position are formed in dependence on pulse- shaped signals supplied to the signal input ceasing and on the signal input being arranged for receiving the activation signal.

10. A device according to any of claims 7 and 9, when claim 9 depends on claim 7, wherein the indicating signal (IP) is of a short type, characterized in that said means for forming the function signal comprise an AND circuit (761) with an input for receiving the activation signal and with an input for receiving the first indicating signal .

11. A device according to any of claims 8 and 9, when claim 9 depends on claim 8, wherein the indicating signal (IPL) is of a long type, characterized in that said means for forming the function signal comprise an AND circuit (762) with an input for receiving the activation signal and with a negating input for receiving the first indi- eating signal.