Abstract: A protective triggering system protects the thyristor switch element of a pulse generator circuit controlling a pulse operated electrostatic precipitator. The protective triggering is initiated for every pulse in the area around passage of the pulse current from positive to negative, its object being to supplement the spark-over released retriggering system so as to ensure that spark-overs occurring immediately after the current passage across zero do not cause renewed thyristor current in the absence of a triggering signal.

Abstract: A spark-over detector for an electrostatic precipitator has a current sensor (3) which provides a signal (a) which is differentiated twice to produce a signal (c). If the level of the signal (c) is above a predetermined level then a level-detecting circuit (7) passes a signal (d) to an AND-gate (8). A second level detecting circuit (9) receives the signal (a) and if this signal is above a predetermined level passes the signal (e) to a timing circuit (10) which provides a signal (f) from a first preset time after having received the signal (e) until a second preset time after the signal (e) has ceased. Thus, if a spark over starts to occur signals (d) and (f) occur simultaneously and the AND-gate produces an output signal (g) which can be used to trigger the thyristor switch circuit of the pulse generator to conduction to prevent thyristor damage or destruction.

Abstract: A method and apparatus for protecting switching elements, and, in particular, thyristor switching elements, which are used in a pulse generator to supply pulsed electrical voltage to a capacitive load, from damage resulting from a spark-over occurring in that load are described. Specifically, whenever the occurrence of a spark-over is detected, during, for example, the interval of time when a voltage pulse applied across the load is decaying, a trigger pulse is applied by a firing circuit to the thyristors to cause them to assume a substantially conductive state.