Thyristor protection method and apparatus

Abstract: A method and apparatus for protecting thyristors, particularly those in a power converter, from overvoltages. Voltage detectors determine if the forward voltage of the thyristors has increased above a predetermined level. If such an overvoltage occurs during a predetermined period after the conduction of the thyristor, forced triggering of the thyristor prevents damage to the thyristor. If more than one thyristor is employed, all the thyristors are forced triggered.

Description: The present invention relates generally to a protection method and apparatus for thyristors, particularly for thyristors forming a thyristor valve in a power converter, and more particularly to a protection method for thyristors using forced triggering signals. Thyristors are commonly used in power converters and similar applications. The thyristors may be arranged in a series connection or a series-parallel connection. While these devices have become common, a certain type of failure may occur when an unusual transient pulse occurs during a certain period of operation of the device. Figure 1 of the accompanying drawings shows a prior art thyristor converter that is used for direct current power transmission. Incoming power is converted from alternating current to direct current or vice versa utilising transformer 12 and thyristor converter 10. Reactor 13 helps to smooth the resultant current. An arm of the thyristor converter 10 is shown in Figure 2 and is referred to as a thyristor valve. It includes a number of thyristors 16 l ... 16 l ... 16 N arranged in series. The thyristors are also connected to individual voltage dividing circuits 18 l ... 18 l ... 18 N which serve to make the thyristor voltages uniform. Light guides 14 l ... 14 l ... 14 N transmit the trigger light signals from a pulse generator (not shown) to the respective thyristors. Arrestor 15 serves to suppress thyristor overvoltage. The number of thyristors, N, which are arranged in series is determined by the voltage rating of the convertor. Figure 3 shows graphs of thyristor voltage and current over a period of time when operating as an inverter. Circuit voltage E t , determined by the main circuit, is imposed on the thyristor. At time t₁, this voltage falls to zero and the thyristor becomes conductive, as seen by the graph of the thyristor current. At time t₂, the thyristor is turned off and circuit voltage E t is made negative. Since carriers remain inside the thyristor immediately after conduction, it is not possible to immediately achieve a forwardly-directed withstand voltage (forward recovery), but it is necessary first to impose a set reserve voltage period until the carriers are removed. This reverse voltage period is known as the margin angle γ. If this reverse voltage period is too small, commutation failure may occur in the converter as a whole or there may be an occurrence of partial commutation failure, know, as partial self-triggering. This occurs when series connected thyristors include some elements in which triggering occurs and some elements where it does not occur. When this happens, the entire circuit voltage is then imposed on the elements where triggering does not take place. This results in the element breaking down due to overvoltage or element breakdown due to a self-triggering domino effect which accompanies a rise in voltage above a certain level. In order to compensate for this, other prior art devices have made use of forced triggering protection where triggering signals are forcibly supplied if the margin angle becomes too small due to a decrease in system voltage, voltage distortion or improper control. This protection helps prevent excessive stress on elements and prevents element breakdown. Thus, forced triggering is known to prevent self-triggering of thyristors during times of overvoltage or when the voltage is above a dangerous level. However, this approach has only been used during the margin angle (reverse voltage) period. Specifically, it has not proved satisfactory when overvoltage enters the converter immediately after conduction. At time t₂ shown in Figure 3, E t assumes a reverse voltage to start the margin angle. This period ends at time t₃ when E t becomes zero. The thyristor recovery voltage E tf gradually increases when the residual carriers disappear during the margin angle. This voltage starts to increase from zero at a time t₂₀. This voltage increases and eventually crosses the voltage level V pt which corresponds to the protection level provided by the arrestor 15. It then continues until it reaches the forward withstand voltage V DRM at time t₆. For a high withstand voltage, large current thyristor, the time necessary to achieve forward recovery varies considerably within the range of 1.2 to 1.5 T₂, where T₂ is the on-off time. This forward recovery time is seen in the figure as the time between t₂ and t₆. As can be seen from Figure 3, in normal operation E tf is greater than E t during the forward recover time. Thus, the thyristor forward recovery voltage is always higher than the circuit voltage. However, during this time, if a forwardly directed overvoltage Ov occurs, such as indicated by the dashed line at t₄₀, the circuit voltage may become greater than the forward recovery voltage for a short time. The thyristor may be unable to withstand this voltage and, accordingly, the problem of self-triggering occurs. The thyristor will then breakdown if the voltage is above a certain value (designated as the limit self-triggering voltage V sfi ). V sfi may be approximately one-third to one-half of V DRM . Since the occurence of an overvoltage is random and may occur at any time, it is necessary to protect the thyristor elements during the time period t₂ to t₅. After t₅ there is no problem since there is satisfactory recovery to the forward withstand voltage. During the time period t₂ to t₃ (the margin angle) it is possible to utilise conventional force triggering protection, as described above. However, during the time period t₃ to t₅, such prior art devices cannot provide protection. Accordingly, during this vulnerable period, the thyristors may breakdown due to overvoltage. US - A1 - 4 377 835 discloses applying a forced triggering signal to a thyristor when a voltage of 1Kv is detected whilst the thyristor has reduced ability to take up off-state voltage without self-ignition. According to a first aspect of the invention, there is provided a method of protecting a thyristor connected in an electrical circuit, comprising the steps of detecting the forward voltage of the thyristor; determining a set time period following the conduction period of the thyristor; determining whether the forward voltage equals a predetermined level; and generating a forced triggering signal and applying it to the thyristor to prevent damage to the thyristor if the forward voltage equals said predetermined level during said set time period, characterised in that said predetermined level is zero. According to a second aspect of the invention, an apparatus for carrying out the above method comprises a thyristor connected in an electrical circuit; forward voltage detecting means arranged to determine the forward voltage of the thyristor; a period setting means for determining a set time period following the conduction period of said thyristor; comparison means for determining when the forward voltage equals a predetermined level; and means for generating a forced triggering signal and applying it to the thyristor to prevent damage to the thyristor if the forward voltage equals the predetermined level during said set time period, characterised in that said predetermined level is zero. In order that the invention may be more readily understood, it will now be described, by way of example only with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a prior art thyristor converter; Figure 2 is a block diagram of a thyristor value in Figure 1; Figure 3 id a graph describing the voltage and current of the thyristor shown in Figure 2; Figure 4 is a block diagram of apparatus including a first embodiment of the present invention; Figures 5a and 5b are time charts of various signals which occur in the apparatus of Figure 4 during normal conditions and overvoltage conditions, respectively; Figure 6 is a block diagram of a second embodiment of the present invention; Figure 7 is a block diagram of the forward recovery protection circuit shown in Figure 6; Figure 8 is a block diagram of the period setting circuit shown in Figure 7; Figure 9 is a time chart of signals which occur in Figures 7 and 8; Figure 10 is a block diagram of an alternative form of the period setting circuit shown in Figure 7; Figure 11 is a time chart of signals shown in Figure 10; Figure 12 is a block diagram of a third embodiment of the present invention; and Figure 13 is a block diagram of a fourth embodiment of the present invention. Referring to Figure 4, thyristors 16 l ... 16 l ... 16 N are arranged in series between reactors 13 and in parallel to arrestor 15. Light guides 14 l ... 14 l ... 14 N transmit light signals to trigger the thyristors. Voltage dividing circuits 18 l ... 18 l ...18 N and forward voltage detection circuits 20 l ... 20 l ... 20 N are provided for each thyristor. The forward voltage detection circuits determine the forward voltage for each thyristor and transmit a light signal indicative of this voltage through light guides 14₁₀ ...14₁₀ ...14 NO . These light signals are then converted back to voltage signals C l ...C l ...C N in circuit 23a. Thus, each of these signals indicates the forward voltage for each corresponding thyristor. At the same time, an overvoltage for the thyristor valve as a whole (total voltage of the elements) is detected by overvoltage detection resistor 21 and overvoltage detection circuit 22. This voltage is also converted to a light signal and transmitted to a converter circuit 23b to reform an electrical signal, d. The overvoltage detection level in circuit 22 is set at slightly lower than N times the self-triggering voltage limit, V sf2 (shown in Figure 3). Thus, overvoltage signal d indicates that the circuit as a whole has reached a dangerous overvoltage condition. A forward voltage signal processing circuit 24 detects the forward voltage signal for each thyristor and determines whether each signal has reached zero due to self-triggering. When the zero level is reached, output signal C₁₀..C₁₀..C N0 are produced. The signals are combined in an OR gate 25A to produce output C o . Similarly, this signal and signal d are combined in OR gate 25B to produce output signal f. Thus, this signal provides an indication that at least one of the thyristors or the circuit as a whole has an unusual forward voltage signal. A protection period setting circuit 26 determines the protection period for the device. That is, it determines the period t₃ - t₅ during which time the thyristors are vulnerable to an overvoltage failure. Output signal e is indicative of this time period. The period setting circuit receives input signal a which starts at time t₁ and ends at time t₂. This signal is indicative of the conduction period and is usually 120°. This signal is received from a converter control unit (not shown). Signal b, which is indicative of the reverse voltage period (margin angle) t₂ - t₃ is also received by the period setting circuit. This signal allows the period setting circuit to determine the beginning of the protection period signal e. This signal is received by AND gate 27 which also receives signal f. When both signals are present, a signal g is given to gate pulse generator 28 which then outputs light signals through the light guides 14 l ..14 l ..14 N to forcibly trigger the thyristors. This then protects the thyristors during the vulnerable time t₃ - t₅ but only if an overvoltage situation is sensed either in an individual thyristor or in the circuit as a whole. Gate pulse generator 28 also produces other signals. The generator receives forward voltage signals C l ..C l ..C N from the circuit 23A. Signal b is produced by the end of signal a and the beginning of the C signals. The end point of signal e is determined in the period setting circuit 26 by setting the maximum value expected for the length of this pulse. Figure 5a shows the normal operation of the device of Figure 4 without an overvoltage while Figure 5b shows the operation of the same device when an overvoltage occurs. The top line in each figure is similar to the timing chart in Figure 3 except that two different lines are provided for E tf . This is to indicate a range of possible voltages which occur normally in slightly differing thyristors. In Figure 5b, the overvoltage signal is indicated by a dotted line. In both figures, signals a and b indicate the respective time periods t₁ - t₂ and t₂ - t₃. Under normal conditions, the various C signals detect a zero voltage at time t₃ and stay at the "1" voltage level since no overvoltage is determined. On the other hand, in the overvoltage situation, the C signals start at the "1" level at time t₃ and change to the "0" level at times t 3B - t 3D . There are slight differences in the sensed times since there are slight differences in the characteristics of the various thyristors. However, at their individual times self-triggering occurs. The first self-triggering signal C₁ causes signal C₀ to occur. This then also causes the generation of signal g from AND gate 27 since it occurs during the time period t₃ - t₅, as determined by signal e. Signal g then causes forced triggering signals to be supplied to all the thyristors. In the example shown, signal d is not produced since self-triggering takes place before the overvoltage reaches the overvoltage detection level V OVT for the thyristor valve as a whole. However, if an overvoltage occurs where the voltage is comparatively low and the change in voltage over time is small and occurs during the time period t₄ - t₅, it may be difficult for self-triggering to take place. As a result, the overvoltage is detected when it becomes greater than the detection level and an overvoltage signal d and forced triggering signal g are produced to cause forced triggering signals to be supplied to the thyristors. Although the protection period has been set at t₃ - t₅, it is also possible to instead use the protection period as t₂ - t₅ and omit the conventional protection for forced triggering when the margin angle is small. This can be done by setting the period setting circuit 26 to produce signal e at the beginning of received signal b rather than the end thereof. One form of converter control protection action is to cause gate shifts. If there are concerns that this could cause unwanted action by self-triggering detection signals C₀ and overvoltage detection signals d, it is possible to temporarily look these two signals by the gate shift signals. The above embodiment utilizes a single signal processing arrangement which receives signals from detectors associated with each of the thyristors. It is also possible to instead utilize a detection device which detects only voltage from one representative thyristor. The second embodiment of the invention is shown in Figure 6. Voltage detector 20 detects the voltage or rate of change of the voltage of thyristor 16. A voltage divider circuit 18 is connected to the thyristor in a manner similar to Figure 4. Likewise, the reactor 13 is connected to one terminal of the thyristor. The voltage detector 20 is a circuit made of resistors or a combination of a resistor and a capacitor. The detected voltage is received by forward voltage recovery protection circuit 32 which serves to determine the protection period and to detect overvoltages. A forced trigger signal may then be applied to the thyristor. A light triggering signal is carried by light guide 14 and converted to a voltage signal a. This signal is amplified to produce signal b which is applied to the thyristor and which determines the turn on period of the thyristor. Signal a is also applied to the overvoltage recovery protection circuit to determine the proper time protection. Figure 7 shows a more detailed block diagram of the forward voltage recovery protection circuit 32. A protection period setting circuit 34 receives input signal a and voltage detection circuit c from the voltage detector 20 to determine the protection period which lasts for a set time from zero voltage t₃. An overvoltage detection circuit 36 receives output c from the voltage detector 20 and produces an overvoltage signal e if it exceeds a set level (V OL shown in Figure 9). If AND gate 38 receives signal e at the same time as signal d representing the desired protection period, a gate trigger signal command is produced and amplified in amplifier 40 to produce forced trigger signal f which is applied to the thyristor gate as shown in Figure 6. Figure 8 shows one embodiment of the protection period setting circuit 34. Input signal a is applied to an inverter 100 whose output g is received by AND gate 103. Input signal c is received by diode 101 which is connected to voltage detector 102. The output of the voltage detector, signal h, is also applied to AND gate 103. The output of AND gate 103, signal i, is applied to one shot multivibrator 104 which produces output signals d. Figure 9 shows a time chart of the various signals found in Figures 6, 7 and 8. The top line is similar to the time charts in Figures 3 and 5. As can be seen in the figure, signal g is merely the inverse of signal a. Signal h is produced by the voltage detector 102 when signal c reaches the zero point at t₃. The conjunction of these two signals in AND gate 103 causes one shot 104 to produce a signal d of width Δt. Figure 10 shows another embodiment of the circuit shown in Figure 8. However, in this circuit input c is also applied to diode 105 and a second voltage detector 106. A voltage detector determines when the voltage of c is negative, that is, t₂ - t₃. This produces signal j which is combined with signal d from one shot 104 in OR gate 107 to produce a new output signal d′. Figure 11 shows signals j and d′ from Figure 12. As can be seen, signal j merely adds the margin angle to the existing Δt in the OR gate to produce the output d′ which includes the time t₂ - t₅. As mentioned above in regard to Figures 4 and 5, the protection signal may be extended to include the margin angle desired. Figure 12 shows another embodiment of the present invention. In this embodiment, the thyristors utilized are electrical and light trigger thyristors which have both electrical and light trigger inputs. The same forward recovery protection found in Figure 6 may be possible by replacing the single branch type light guide 14 of Figure 6 with a partially branched light guide 14₁ (2-branch type). Figure 13 shows another embodiment of the present invention used with light trigger thyristors. The light guide 14 of Figure 6 is replaced by a 2 x 2 branched ligth guide 14₂ in order to have the same forward recovery protection as in Figure 6. A gate trigger command signal, a, is partially branched at point A and leads to photoreceptor element 42 of the FR protection circuit 32. The output of the protection circuit 32 is sent through light-emitting element 44 and reenters the light guide at point B so that signals b1 (which is the same as A) and b2 both enter the thyristor. The embodiments described above make it possible to ensure a forced triggering protection against overvoltage during the period immediately after the thyristor turn off in a system where each thyristor is provided with a voltage detector and a FR protection circuit. Since protection is effected for each individual thyristor in the series, protection is provided at high speed and it is possible to protect the device even against very fast lightning surge overvoltage. Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the scope of the invention as set forth in the accompanying claims.

Claims: A method of protecting a thyristor (16₁ to 16 N ) connected in an electrical circuit, comprising the steps of detecting the forward voltage of the thyristor; determining a set time period following the conduction period of the thyristor; determining whether the forward voltage equals a predetermined level; and generating a forced triggering signal and applying it to the thyristor to prevent damage to the thyristor if the forward voltage equals said predetermined level during said set time period, characterised in that said predetermined level is zero. A method according to claim 1, characterised in that said set period starts when a reverse voltage across the thyristor becomes zero. A method according to claim 1 or 2, characterised in that said thyristor (16₁ to 16 N ) is one of a plurality of similar thyristors connected together in the electrical circuit, and triggered signals are generated and applied to all the thyristors if the forward voltage imposed on any of the thyristors equals zero during said set time period. Apparatus for carrying out the method of claims 1, 2 or 3, comprising a thyristor (16₁ to 16 N ) connected in an electrical circuit; forward voltage detecting means (20₁ to 20 N ) arranged to determine the forward voltage of the thyristor; a period setting means (26) for determining a set time period following the conduction period of said thyristor (16₁ to 16 N ); comparison means (24) for determining when the forward voltage equals a predetermined level; and means (28, 27, 25B, 25A) for generating a forced triggering signal and applying it to the thyristor (16₁ to 16 N ) to prevent damage to the thyristor if the forward voltage equals the predetermined level during said set time period, characterised in that said predetermined level is zero. Apparatus according to claim 4, characterised in that said set period starts when a reverse voltage across the thyristor becomes zero. Apparatus as claimed in claim 4, characterised in the said set period includes a margin angle. Apparatus as claimed in claim 4, 5, or 6, characterised in that said thyristor (16₁ to 16 N ) is connected electrically in series with a plurality of similar thyristors, each having forward voltage detecting means, and means for generating a forced triggering signal and applying it to the thyristors if the forward voltage of any of the thyristors equals zero during said set time period. Apparatus as claimed in claim 7, characterised in the provision of an overvoltage protection circuit (22, 2, 3B) for detecting when the voltage across said series connected thyristors equals a predetermined over voltage level and for causing the forced triggering signal to be generated when the voltage equals the predetermined level. Apparatus as claimed in any of the claims 4 to 8, characterised in that the or each forward voltage detecting means (20₁ to 20 N ) is connected to the comparison means by means including a light guide (14₁₀ to 14 no ).

Inventors: ITO KATSURO C O INTELLECTUAL P, TAKAHASHI TADASHI C O INTELLEC, ITO, KATSURO, C/O INTELLECTUAL PROPERTY DIV., TAKAHASHI, TADASHI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H02M1
IPC codes: H02M1, H03K17