5-160 G30 Generator Protection System GE Multilin5.6 GROUPED ELEMENTS 5 SETTINGS5The equation above brings an advantage of generating the restraining signal of twice the external ground fault current,while reducing the restraint below the internal ground fault current. The negative-sequence component of the restrainingsignal (IR2) is meant to provide maximum restraint during external phase-to-phase faults and is calculated as follows:(EQ 5.40)The multiplier of 1 is used by the relay for first two cycles following complete de-energization of the winding (all three phasecurrents below 5% of nominal for at least five cycles). The multiplier of 3 is used during normal operation; that is, two cyclesafter the winding has been energized. The lower multiplier is used to ensure better sensitivity when energizing a faultywinding.The positive-sequence component of the restraining signal (IR1) is meant to provide restraint during symmetrical condi-tions, either symmetrical faults or load, and is calculated according to the following algorithm:1 If of phase CT, then2 If , then3 else4 elseUnder load-level currents (below 150% of nominal), the positive-sequence restraint is set to 1/8th of the positive-sequencecurrent (line 4). This is to ensure maximum sensitivity during low-current faults under full load conditions. Under fault-levelcurrents (above 150% of nominal), the positive-sequence restraint is removed if the zero-sequence component is greaterthan the positive-sequence (line 3), or set at the net difference of the two (line 2).The raw restraining signal (Irest) is further post-filtered for better performance during external faults with heavy CT satura-tion and for better switch-off transient control:(EQ 5.41)where k represents a present sample, k – 1 represents the previous sample, and α is a factory constant (α < 1). The equa-tion above introduces a decaying memory to the restraining signal. Should the raw restraining signal (Irest) disappear ordrop significantly, such as when an external fault gets cleared or a CT saturates heavily, the actual restraining signal (Igr(k))will not reduce instantly but will keep decaying decreasing its value by 50% each 15.5 power system cycles.Having the differential and restraining signals developed, the element applies a single slope differential characteristic with aminimum pickup as shown in the logic diagram below.Figure 5–68: RESTRICTED GROUND FAULT SCHEME LOGICIR2 I_2= or IR2 3 I_2×=I_1 2 pu>I_1 I_0> IR1 3 I_1 I_0–( )×=IR1 0=IR1 I_1 8⁄=Igr k( ) max Irest k( ) α Igr k 1–( )×,( )=SETTINGSETTINGSETTINGSETTINGSETTINGSSETTINGFLEXLOGIC OPERANDSACTUAL VALUESRESTD GND FT1FUNCTION:RESTD GND FT1BLOCK:RESTD GND FT1SOURCE:RESTD GND FT1PICKUP:RESTD GND FT1 RESETDELAY:RESTD GND FT1 PICKUPDELAY:RESTD GND FT1SLOPE:RESTD GND FT1 OPRESTD GND FT1 DPORESTD GND FT1 PKPRGF 1 gd MagIRGF 1 gr MagIOff=0Enabled=1Disabled=0AND828002A2.CDRRUNRUNIgd > PICKUPINIGI_0I_1I_2AND> SLOPE *Igd IgrDifferentialandRestrainingCurrentst PKP t RST
