GE Multilin L90 Line Current Differential System 5-595 SETTINGS 5.4 SYSTEM SETUP55.4SYSTEM SETUP 5.4.1 AC INPUTSa) CURRENT BANKSPATH: SETTINGS ÖØ SYSTEM SETUP Ö AC INPUTS Ö CURRENT BANK F1(L5)Because energy parameters are accumulated, these values should be recorded and then reset immediatelyprior to changing CT characteristics.Four banks of phase and ground CTs can be set, where the current banks are denoted in the following format (X representsthe module slot position letter):Xa, where X = {F, L} and a = {1, 5}.See the Introduction to AC Sources section at the beginning of this chapter for additional details.These settings are critical for all features that have settings dependent on current measurements. When the relay isordered, the CT module must be specified to include a standard or sensitive ground input. As the phase CTs are connectedin wye (star), the calculated phasor sum of the three phase currents (IA + IB + IC = neutral current = 3Io) is used as theinput for the neutral overcurrent elements. In addition, a zero-sequence (core balance) CT which senses current in all of thecircuit primary conductors, or a CT in a neutral grounding conductor may also be used. For this configuration, the groundCT primary rating must be entered. To detect low level ground fault currents, the sensitive ground input may be used. In thiscase, the sensitive ground CT primary rating must be entered. Refer to chapter 3 for more details on CT connections.Enter the rated CT primary current values. For both 1000:5 and 1000:1 CTs, the entry would be 1000. For correct opera-tion, the CT secondary rating must match the setting (which must also correspond to the specific CT connections used).The following example illustrates how multiple CT inputs (current banks) are summed as one source current. Given If thefollowing current banks:• F1: CT bank with 500:1 ratio.• F5: CT bank with 1000: ratio.• L1: CT bank with 800:1 ratio.The following rule applies:(EQ 5.7)1 pu is the highest primary current. In this case, 1000 is entered and the secondary current from the 500:1 ratio CT will beadjusted to that created by a 1000:1 CT before summation. If a protection element is set up to act on SRC 1 currents, thena pickup level of 1 pu will operate on 1000 A primary.The same rule applies for current sums from CTs with different secondary taps (5 A and 1 A). CURRENT BANK F1PHASE CT F1PRIMARY: 1 ARange: 1 to 65000 A in steps of 1MESSAGE PHASE CT F1SECONDARY: 1 ARange: 1 A, 5 AMESSAGE GROUND CT F1PRIMARY: 1 ARange: 1 to 65000 A in steps of 1MESSAGE GROUND CT F1SECONDARY: 1 ARange: 1 A, 5 ANOTESRC 1 F1 F5 L1+ +=
