Medium voltage switchgear application guide

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Medium voltage switchgear application guide 2 - MOTOR(cont’d) ■ BREAK FAULT CURRENTS fuse time-current curve t 125 150 200 250 t d I I d I dm date 11/94 - B•3•2 - revised 05/2004 When capacity a fused must contactor be compatible is used with as the a control need arising device, from the co-ordination contactor breaking the fuse and the different protective mechanisms. with The short-circuit fuse breaking current capacity possible. must be higher than or equal to the maximum When breaking a circuit capacity breaker must or be a higher contactor than alone the maximum is used as short-circuit a control device, current its possible. The Fuse specific protection motor which type they must of stress protect that and the fuses the network must withstand on which comes it is located. from the ■stress ■rated current due to (Ir) the motor ■starting current time Id(Id= 6 to 7 Ir for direct starting) tddepends on the td ■the number of consecutive motor (values startings. between 1 and 30 seconds) ■stress ■rated voltage due to the (< 11 network ■prospective short-circuit kV). Only current. fuses with Ur ≤12 kV are used ■choice ■ideal starting of fuse current characteristics The ideal starting current calculation Idmis equal (Idm) 2.4 Idif the ratio Id/ to: - 2 Idif the ratio Id/ Ir given by the manufacturer is a design value This Idmvalue guarantees Ir given two by consecutive the manufacturer start-ups is or a measured six start-ups value. intervalsper hour. at regular ■to record determine the Idmand the fuse starting rating shown in figure 1 time value on the fuse time-current curve as intersection - choose the value immediately higher than the Idmand tdright angle On When figure the 1, fused the fuse contactor rating is to installed be taken in is the 250 cubicle, A. by 20%. Idmmust be increased The Circuit motors breaker are protected protection ■thermal image against overloads, by the following protection devices: ■independent ■earth overcurrent time for overcurrent insulation for faults, short-circuits, inverse ■against component slow start-ups maximum and rotor-locking. for unbalance, The undervoltage motors must relay. be Ingeneral, protected against a single voltage relay is drops installed by on a time-delay carries out load shedding to all the associated devices. the busbar and page 8
Medium voltage switchgear application guide 2 - MOTOR(cont’d) ■ WITHSTAND THE FAULT SWITCHING CURRENT (Ik) The equal contactor to the peak or circuit short-circuit breaker current making value. capacity It is equal must be to 2.5 higher Ikaccording than to the IEC. ■ The BREAK motors are CURRENTS sensitive DURING to overvoltage STARTING since, WITHOUT for technological EXCESSIVE reasons, OVERVOLTAGE are not as well insulated. The switchgear operation creates an overvoltage they insulation for each start-up. which cause Repeated it to wear overvoltage down prematurely, creates weak if not points destroy in the it. turn Devices and do not using need the motor SF6 technique protection do devices. not generate dangerous overvoltage Rotating devices are arc likely techniques to provoke should higher be kept currents if they than work the well. rotating Puffer arc technique therefore more dangerous overvoltage (see“selling points” folder, and overvoltage file). date 11/94 - B•3•2 - revised 05/2004 page 9
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Medium voltage switchgear application guide

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