5-212 N60 NETWORK STABILITY AND SYNCHROPHASOR MEASUREMENT SYSTEM – INSTRUCTION MANUALCONTROL ELEMENTS CHAPTER 5: SETTINGS5Figure 5-106: IEC 255-8 sample operate and reset curvesThe thermal overload protection element estimates accumulated thermal energy E using the following equationscalculated each power cycle. When current is greater than the pickup level, In > k × I B , element starts increasing thethermal energy:Eq. 5-16When current is less than the dropout level, In > 0.97 × k × I B , the element starts decreasing the thermal energy:Eq. 5-17where∆t is the power cycle durationn is the power cycle indext op(In) is the trip time calculated at index n as per the IEC255-8 cold curve or hot curve equationstrst(In) is the reset time calculated at index n as per the reset time equation0.1 1 10 100I / Ipkpt (min)0.010.1110100τ = 30rstτ = 30opT = 10min827724A1.CDRE n E n 1–tt op In --------------+=E n E n 1–tt rst In --------------–=
