Protective Circuit for Thyristor Controlled Systems and Thyristor Converter Embodying Such Protective Circuit

The present invention relates to thyristor protection in general, and more particularly to a circuit for the detection of individual thyristor failure.

In an array of thyristors it is important to detect without delay if any of the thyristors do not operate properly, in order to be able to shut down the entire system before the failure of one thyristor endangers the operation of the others, or puts into jeopardy the overall system operation.

In this regard, it is desirable to be able to sense thyristor failure individually, although it can be sufficient to know that such failure has occurred without the necessity of identifying immediately which thyristor is defective. Such an approach has the advantage of simplicity, reliability and uniformity.

The present invention is particularly applicable to the detection of a thyristor failure due to a short in the device. A thyristor may fail to turn ON and remain OFF because of misfiring under gating control, or because it inherently is no longer capable of being turned ON. This is called thyristor failure in the open mode. A thyristor may also fail because of overvoltage and the occurrence of an arc over the device. The thyristor remains shorted across its main electrodes, independently of any gating or communication condition. In such case the thyristor is said to have failed in the shorted mode. The present invention is with regard to the detection of thyristor failure in the shorted mode. The invention is particularly applicable to a stack of thyristor sharing in common a relatively high voltage. It is important in such instance that failure of one thyristor be detected and that precautionary measures, such as a shut-down, be immediately taken before the entire voltage exceeds the rating of the other thyristors in the stack, in which situation each thyristor could be damaged one after the other.

The present invention resides in providing each thyristor of a stack of thyristors sharing a common voltage in series to one another with individual capacitor means for charging up to a voltage related to the voltage normally applied to each thyristor of the stack, with means for sensing any discharge of such individual capacitor when the associated thyristor is being turned ON, and with means for detecting the absence of any discharge by said sensing means, in regard to any thyristor of said stack when being turned ON, as an indication of thyristor failure.

FIG. 1 shows an array of thyristors in series under a common voltage with the provision for thyristor short failure detection according to the invention;

FIG. 2 illustrates the individual thyristor protection circuit of the invention as employed in the circuit of FIG. 1; and

FIG. 3 illustrates with curves the operation of the circuit of FIG. 2.

Referring to FIG. 1, between positive terminal T.sub.A of a DC source of voltage V.sub.DC applied across opposite DC terminals T.sub.A, T.sub.B and an AC phase line A, are connected in series three thyristors TH.sub.1, TH.sub.2, TH.sub.3 belonging to an array of thyristors within a static-controlled converter converting a DC voltage into an AC voltage, applied to three phase lines A, B, C, for instance. The thyristors are gated for conduction from lines 10. The thyristors are turned OFF by natural commutation, e.g. when the voltage between the main electrodes has the appropriate magnitude and polarity. Thyristor TH.sub.1 is between nodes J.sub.1, J.sub.2, thyristor TH.sub.2 between nodes J.sub.2, J.sub.3 and thyristor TH.sub.3 between nodes J.sub.3 and J.sub.4. Resistors R.sub.1, R'.sub.1, R".sub.1 in parallel with the respective thyristors TH.sub.1, TH.sub.2, TH.sub.3 cause an equal sharing of the voltage supported by the thyristors between line T.sub.A and phase line A.

In accordance with the present invention, a snubber circuit, e.g. a series capacitor-resistor circuit (R.sub.2, C.sub.1), (R'.sub.2, C'.sub.1) or (R".sub.2, C".sub.1), is connected in parallel with a corresponding one of thyristors TH.sub.1, TH.sub.2, TH.sub.3, e.g. with resistors R.sub.1, R'.sub.1, or R".sub.1. When a thyristor is turned OFF, the voltage between the two nodes (J.sub.1, J.sub.2), (J.sub.2, J.sub.3) or (J.sub.3, J.sub.4) is charging the associated snubber circuit. When a thyristor is turned ON, capacitor C.sub.1, C'.sub.1 or C".sub.1 depending upon whether TH.sub.1, TH.sub.2 or TH.sub.3 is the concurrently gated thyristor, discharges itself through resistor R.sub.2, R'.sub.2 or R".sub.2 and the now conductive thyristor. A pulse detector PD, PD', PD" coupled with each snubber circuit, has the capability of detecting the discharge of the associated capacitor when it occurs. By line 1 for capacitor C.sub. 1, line 1' for capacitor C'.sub.1 and line 1" for capacitor C".sub.1, the detected discharge pulse is converted by an A/D converter into a digital signal which, by lines 3, 3', 3", is applied to a NOR LATCH device 4. Should, at the moment of gating, one of the thyristors be in a shorted condition, the snubber capacitor will not have been able to charge and the discharge expected upon firing of the thyristor cannot occur. In the absence of a pulse being detected by the corresponding pulse detector PD, PD' or PD", the digital state of line 3, 3' or 3" will not be the same as on the other lines, and NOR LATCH device 4, in response to such condition, will change its output state to a ONE on line 5, 5' or 5". A failure detector 6 is provided which responds to such occurrence by generating an alarm signal on line 7 at its output either to shut down the installation by line 8, or to display the happening of such thyristor failure.

Referring to FIG. 2, the pulse detector PD is shown to consist, illustratively, in a pulse transformer PT.sub.1. An additional resistor R.sub.3 is connected in parallel with resistor R.sub.2 of the snubber. The snubber network R.sub.2, R.sub.3 and C.sub.1 is calculated in order to limit the rate of rise of the voltage applied to thyristor TH.sub.1. In normal operation capacitor C.sub.1 charges up in accordance with such applied voltage. When thyristor TH.sub.1 is gated, capacitor C.sub.1 is discharged over the thyristor and across the primary winding 11 of the pulse transformer PT.sub.1. At the secondary side 12, the transformer exhibits a voltage signal V.sub.S of approximately 2 microseconds, as shown in FIG. 3 for illustration by curve (c) in presence of the relaxation curve (b) representing the discharge of capacitor C.sub.1 with a time constant ##EQU1##

If the voltage V.sub.S does not appear, this indicates that thyristor TH.sub.1 has failed in the short mode.

FIG. 2 also shows the A/D converter used after pulse detector PD to derive a digital signal characterizing the occurrence of a voltage signal such as V.sub.S. The two ends of the secondary winding 12 of pulse transformer PT.sub.1 extend over parallel lines 13, 14, line 14 being connected to ground. Across lines 13 and 14 is a resistor R.sub.5. The non-grounded line 13 goes into a diode D.sub.1 which has its cathode connected to the anode of a steering diode D.sub.2 interposed between the positive voltage of the logic circuit and the output line 3. Summing junction S therebetween goes to ground via a resistor R.sub.6. R.sub.5 has a value of about 1.5 K.OMEGA. and R.sub.6 amounts to 22 K.OMEGA.. The operation of the A/D converter is as follows: when V.sub.S appears on line 13, the voltage on the line reaches a level for which diode D.sub.1 is forward biased and becomes conductive. The voltage across resistor R.sub.6 raises itself above ground to the point when diode D.sub.2 also becomes forward biased, thereby applying the voltage of line 15, namely a logic ONE, to line 3.

Referring again to FIG. 1, when the logic state of lines 3, 3', 3" is a ONE for all lines, NOR LATCH device will establish at its output on lines 5, 5', 5" logic ONES. Should, however, the absence of a voltage V.sub.S on any of the pulse detectors PD manifest itself, at least one of line 3, 3', 3" will be a ZERO, rather than a ONE, and the output state of line 5, 5', or 5" will be a ZERO, rather than a ONE.

Referring to FIG. 3, the overall operation of the circuits of FIGS. 1 and 2 is better understood from a consideration of curves (a) through (g). Up to time t.sub.1 thyristor TH.sub.1 supports one-third of the voltage applied to the chain of three thyristors V.sub.J1,J4. Accordingly, capacitor C.sub.1 is charged to a voltage V.sub.C1 (curve (b)). At time t.sub.1, when the thyristors are gated by a firing pulse (curve (e)) and thyristor TH.sub.1 is turned ON, thyristor TH.sub.1 no longer supports V.sub.J1,J4 as shown by curve (a), whereas capacitor C.sub.1 discharges over thyristor TH.sub.1 with a time constant of relaxation determined by ##EQU2## as shown by curve (b). As a result, pulse transformer PT.sub.1 exhibits at its secondary, e.g. on line 13 (FIG. 2) a voltage signal V.sub.S (see curve (c)). Consequently A/D converter 2 changes state at its output (line 3) as evidenced by the logic state of curve (d). Later in the cycle, the circuits are reset, namely the NOR LATCH device 4 by line 21 and the failure detector 6 by line 22, as evidenced at time t.sub.i on curve (f).

At time t.sub.2, when a second firing pulse (curve (e)) has been applied to the thyristors, thyristor TH.sub.1 is assumed to be shorted as evidenced by the absence of a pulse V.sub.S (curve (c)) since there has been no build-up to the voltage 1/3V.sub.J1,J4, nor any correlative charging to V.sub.C1 (curve (b)) on the snubber capacitor C.sub.1. Now the NOR LATCH device will change state as evidenced by curve (g). The NOR device is reset at time t'.sub.2 by the reset pulse of line 21, as evidenced by curves (f).

Two other conditions, other than a shorted thyristor might happen which will prevent a signal V.sub.S on line 1 from being translated by A/D converter 2 into the normal logic ONE on line 3, 3' or 3". One such condition occurs when the thyristor has failed in the open mode. In this case the thyristor fails to be turned ON. Therefore, the snubber circuit is not being discharged upon gating. However, such failure is only temporary and in all likelihood upon the next half-cycle, gating will cause the thyristor to be turned ON normally. Therefore, despite such apparent absence of snubber discharge at one gating instant, this failed-open condition should not be taken for a shorted mode failure. In order to prevent a false alarm on line 7, the failure detector is conditioned so that on such occurrence the failure will be ignored. One way of doing this is to require a repeated occurrence of a critical logic state on line 7 before lines 8 and 9 are being activated.

Another false shorted failure condition occurs when the thyristor is gated at very low gating angles. In such case, capacitor C.sub.1 is unable to reach a potential high enough to provide the amount of charge required to produce upon being discharged a signal V.sub.S of sufficient magnitude. Diode D.sub.1 would not be forward biased and no change of state will appear at summing point S, e.g. on line 3. This situation is avoided by causing the failure detector 6 to be disabled automatically by line 20 whenever the thyristor gating circuit has been adjusted for low gating angles.