ABB REF 610 Technical Reference Manual

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ABB REF 610 Technical Reference Manual Manual pdf 21 page image

1MRS 755535 Feeder Protection RelayTechnical Reference Manual - ANSI VersionREF 61021It is possible to block the tripping of an ground-fault element by applying a digitalinput signal to the relay.Note!Elements 51N and 50N can be set to be blocked by the auto-reclose function.4.1.4.4. Thermal protection for cablesThe thermal protection detects long-time overloads during normal operation.Prolonged overloading results in the thermal stress capacity of the cable beingexceeded, which degrades the insulation of the cable, which in turn may cause ashort circuit or an ground fault. The heating up of the cable follows an exponentialcurve, the levelled-out value of which is determined by the squared value of the loadcurrent. The thermal protection may equally well be used to protect dry-typetransformers, capacitor banks, busbars and overhead lines, for instance.The thermal protection element continuously calculates the thermal capacity used asa percentage of the cable’s total thermal capacity. The thermal capacity is calculatedas follows:whereθ1) = thermal capacityI = phase current valueFLA = set full load currentt = time (in minutes)τ = time constant (in minutes)1) θ equals TH LEVEL“When one or several phase currents exceed the set full load current, FLA, element49 will pickup. At the same time, the thermal capacity will start to increase at a ratedepending on the current amplitude and the prior load of the cable.When the thermal capacity, influenced by the thermal history of the cable, exceedsthe set alarm level, 49ALARM the element will generate an alarm signal. Thethermal alarm can be used to avoid unnecessary tripping due to a beginningoverload. The thermal level at various constant currents are presented in the tablebelow:Table 4.1.4.4-1 Thermal level at constant currentsI/FLA Thermal level (%)1.0 90.70.9 73.50.8 580.7 44.4θ I1.05FLA×-------------------------⎝ ⎠⎛ ⎞ 2 1 e t τ⁄––( )× 100%×=