P12x/EN T/Eb6 - Schneider - error

P12x/EN AP/Eb6Page 30/78Application GuideMiCOM P120/P121/P122/P12311. THERMAL OVERLOAD PROTECTION (P122 & P123 ONLY)Thermal overload protection can be applied to prevent damages to the equipment of theelectrical plant when operating at temperatures that are above the values designed formaximum withstand. A prolonged overloading causes excessive heating, which may result inpremature deterioration of the insulation, or in extreme cases, insulation failure.MiCOM P122 & P123 relays incorporate a current based thermal replica, using load currentto reproduce the heating and cooling of the equipment to be protected. The element thermaloverload protection can be set with both alarm and trip stages.The heating within any plant equipment, such as cables or transformers, is of resistive type(I²R x t). Thus, the quantity of heat generated is directly proportional to the current squared(I²). The thermal time characteristic used in the relay is based on current squared, integratedover time.MiCOM P122 & P123 relays automatically use the highest phase current as input informationfor the thermal model.Protection equipment is designed to operate continuously at a temperature corresponding toits full load rating, where heat generated is balanced with heat dissipated by radiation etc.Over-temperature conditions therefore occur when currents in excess of rating are allowed toflow for a certain period of time. It can be shown that temperatures during heating followexponential time constants and a similar exponential decrease of temperature occurs duringcooling.In order to apply this protection element, the thermal time constant (Te) of the plantequipment to be protected is therefore required.The following sections will show that different plant equipment possesses different thermalcharacteristics, due to the nature of their construction.11.1 Time Constant CharacteristicThis characteristic is used to protect cables, dry type transformers (e.g. type AN), andcapacitor banks.The thermal time characteristic is given by:Where:e⎛ −t⎞⎜ ⎟⎝ τ ⎠=22( I − ( k × I ) )2( I − I )FLC2pt = Time to trip, following application of the overload current, Iτ = Heating and cooling time constant of the protected plant equipmentI = Largest phase currentI FLC = Full load current rating (relay setting ‘Thermal Trip’)k = 1.05 constant, allows continuous operation up to < 1.05 I FLCI P = Steady state pre-loading current before application of the overloadThe time to trip varies depending on the load current carried before application of theoverload, i.e. whether the overload was applied from “hot” or “cold”.Curves of the thermal overload time characteristic are presented in the chapterP12x/EN TD/C55 of the Technical Guide.