5-232 L30 Line Current Differential System GE Multilin5.7 CONTROL ELEMENTS 5 SETTINGS5To ensure element accuracy for high overcurrent conditions, the maximum value of I/(k x I B) is limited to 8, even when real-istically it is exceeding this value.The reset time of the thermal overload protection element is also time delayed using following formula:(EQ 5.25)In the above equation,• τrst = thermal protection trip time constant.• Tmin is a minimum reset time settingFigure 5–109: IEC 255-8 SAMPLE OPERATE AND RESET CURVESThe thermal overload protection element estimates accumulated thermal energy E using the following equations calculatedeach power cycle. When current is greater than the pickup level, In > k × IB, element starts increasing the thermal energy:(EQ 5.26)When current is less than the dropout level, I n > 0.97 × k × I B, the element starts decreasing the thermal energy:t rstτrstkI B( )2I2 kI B( )2–----------------------------- ln× T min+=0.1 1 10 100I / Ipkpt (min)0.010.1110100τ = 30rstτ = 30opT = 10min827724A1.CDRE n E n 1–tΔt op In( )---------------+=
