P12x/EN T/Eb6 - Schneider - error

P12x/EN AP/Eb6Page 46/78Application GuideMiCOM P120/P121/P122/P12321. BROKEN CONDUCTOR DETECTION (P122 & P123 ONLY)Most of the faults that affect a power system occur between one phase and the earth orbetween two phases and the earth. These faults are shunt faults and are caused by lightningdischarges and other overvoltages generating flashovers. They may also arise from birds onoverhead lines or mechanical damage on underground cables, etc.Such faults lead the current to increase appreciably and therefore they can easily bedetected in most applications. Open circuit faults are a different type of faults that canhappen in electrical networks. These faults can be caused by broken conductors, blownfuses or misoperation of a pole of a circuit-breaker.Series faults will not lead to an increase in phase current and therefore they can not beeasily detected by common overcurrent relays. However, this type of faults produce anunbalance that creates negative phase sequence current, which can be detected.The use of negative phase sequence overcurrent is then recommended to detect such faultyconditions. However, on lightly loaded lines, the value of the negative sequence currentcaused by a faulty condition may be very close to, or even inferior, to the full load steadystate unbalance generated by CT errors, load unbalance, etc. As a consequence, a negativesequence protection element would not work for low level of loads.As a solution, MiCOM P122 and P123 have a protection element that measures the ratiobetween the negative and the positive phase sequence current (I2/I1). Using this ratioinstead of the measure of I2 only, the relay will be able to detect a faulty conditionindependently on the level of load on the network, since the ratio is approximately constantwith variations in load current. It is then possible to get a more sensitive setting.NOTE:the Broken conductor function is inhibited if the current value flowingin each one of the three phases is inferior to 10% of the nominalcurrent.Setting GuidelinesOn single point earthed power systems, there is a low zero sequence current flow and theratio I2/I1 that flows is close to100%. On multiple earthed power systems, (assuming that theimpedances in each sequence network are equals), the ratio I2/I1 will be equal to 50%.It is possible to calculate the ratio of I2/I1 corresponding to various system impedances,according to the following equations:E g (Z 2 +Z 0 )I 1F =Z 1 Z 2 +Z 1 Z 0 +Z 2 Z 0I 2F =Where:–E g Z 0Z 1 Z 2 +Z 1 Z 0 +Z 2 Z 0E g = System VoltageZ 0 = Zero sequence impedanceZ 1 = Positive sequence impedanceZ 2 = Negative sequence impedanceTherefore:I 2F Z 0I=1F Z 0 +Z 2As a consequence, for an open circuit in a particular part of the system, I2/I1 can bedetermined from the ratio between the zero sequence and the negative sequenceimpedance. It must be noted however, that this ratio may vary depending on the location ofthe fault. It is therefore desirable to apply a setting that is as sensitive as possible.Practically, the levels of standing negative phase sequence current present on the systemguide the choice of this minimum setting. A system study, or the use of measurement data ofthe relay during commissioning stage are two ways to determine this minimum setting. If thelatter method is chosen, it is important to take measurements during maximum loadconditions, to be sure that all single phase loads are taken into account.