CHAPTER 5: SETTINGS CONTROL ELEMENTSN60 NETWORK STABILITY AND SYNCHROPHASOR MEASUREMENT SYSTEM – INSTRUCTION MANUAL 5-2115The thermal overload protection element corresponds to the IEC 255-8 standard and is used to detect thermal overloadconditions in protected power system elements. Choosing an appropriate time constant element can be used to protectdifferent elements of the power system. The cold curve characteristic is applied when the previous averaged load currentover the last 5 cycles is less than 10% of the base current. If this current is greater or equal than 10% than the basecurrent, then the hot curve characteristic is applied.The IEC255-8 cold curve is defined as follows:Eq. 5-13The IEC255-8 hot curve is defined as follows:Eq. 5-14wheret op = time to operateτop = thermal protection trip time constantI = measured overload RMS currentIp = measured load RMS current before overload occursk = IEC 255-8 k-factor applied to IB , defining maximum permissible current above nominal currentIB = protected element base (nominal) currentTo ensure element accuracy for high overcurrent conditions, the maximum value of I/(k x IB ) is limited to 8, even whenrealistically it is exceeding this value.The reset time of the thermal overload protection element is also time delayed using following formula:Eq. 5-15whereτrst = thermal protection trip time constantTmin = a minimum reset time setting THERM PROT 1 RESET:OffRange: FlexLogic operand THERM PROT 1 BLOCK:OffRange: FlexLogic operand THERMAL PROTECTION 1TARGET: Self-resetRange: Self-reset, Latched, Disabled THERMAL PROTECTION 1EVENTS: DisabledRange: Disabled, Enabledt op opI 2I 2 kI B 2–----------------------- ln=t op opI 2 I p2–I 2 kI B 2–----------------------- ln=t rst rstkI B 2I 2 kI B 2–------------------------- ln T min+=
