FR2382120A1 - METHOD AND DEVICE FOR SURVEILLANCE OF OVERCURRENTS OF A POLYPHASE CURRENT - Google Patents

Abstract

In the overcurrent protection of electrical devices, there is the problem of the overlapping of extreme but still permissible high-load operating states, on the one hand, and low-current short circuits which must be switched off, on the other hand. This mainly applies when low-load operating currents are superimposed on currents from remote short circuits. Instead of the usual fixed limit values for the current comparison of the overcurrent protection, the solution provides limit values (AGG) which are dependent on the minimum value of the phase currents (IR, IS, IT), which are derived from the phase currents via a minimum-value selection circuit of a limit-value generator (GG). The limit value effective in each case is thus dependent in-phase on the current of the phase not affected by a short circuit (apart from the case of a three-phase short circuit which must be covered separately), that is to say on the smallest current. This results in automatic adaptation of the comparison signal representing the limit value in such a manner that with a short circuit which becomes superimposed on a low-load operating state with a low current level, the phase currents are compared with a comparatively low limit value but are compared with a higher limit value in the case of high operating currents. The overcurrent protection can thus be set to be comparatively insensitive so that the range of unnecessary disconnections is restricted without impairing the reliability of the short circuit detection.