P12x/EN T/Eb6 - Schneider - error

P12x/EN AP/Eb6Page 66/78Application GuideMiCOM P120/P121/P122/P123Figure 19 hereafter shows a CB failure condition. After the t BF timer elapses, the relaydoesn’t detect the opening of the CB pole. Therefore, a CB FAIL signal is given.CB failed« CB FAIL » signal« CB pole closed » internal signalTrip order20 samplest BF timer20 samples 20 samples 20 samples 20 samplesI< thresholdI< threshold20 samples20 samples 20 samples 20 samples 20 samplesP0035ENaFIGURE 19: CB NOT YET OPEN BEFORE tBF EXPIREDFigure 20 hereafter shows an other case of normal CB operation. Once the fault is cleared,the phase current signal takes time to decrease due to the de-magnetisation of the phaseCT. It is a typical case for TPY class CTs which are built with air gap in their magnetic core.Before the drop off of the t BF timer, the relay has detected an opening of the CB pole, thusno CB failure signal is given as it is required. A basic Breaker Failure element based on ansimple undercurrent element would detect a false CB failure condition as the current signalvalue is outside the I< bandzone at the t BF timer drop off.NOTE:Both « CB pole closed » and « CB pole opened » internal signalsmentioned in the above diagrams are derived from the Circuit BreakerFailure function algorithm. They are not affected by the status of therelay opto-inputs wired to the 52a and 52b CB auxiliary contacts.« CB pole closed» internal signalTrip ordert BF timer20 samples 20 samples 20 samples 20 samples« CB pole opened »detectionI< thresholdI< threshold20 samples20 samples 20 samples 20 samples 20 samplesP0036ENaFIGURE 20: DE-ENERGIZATION OF THE CT PHASE