Emax Low voltage air circuit-breakers - ABB Download Center

Switching and protection of capacitorsOperating conditions of circuit-breakersduring continuous service for capacitorbanksAccording to the IEC 60831-1 and 60931-1 Standards, capacitorsmust be able to operate in service conditions with arated rms current of up to 1.3 times the rated current Icn ofthe capacitor. This prescription is due to the possible presenceof harmonics in the mains voltage.It should also be kept in mind that a tolerance of +15% isadmissible for the capacitance value corresponding to itsrated power, so that the circuit-breakers for switching capacitorbanks must be selected to permanently carry a maximumcurrent equal to:ln = 1.3 x 1.15 x lnc = 1.5 x lnc.Current for connecting capacitor banksConnection of a capacitor bank can be compared to a closingoperation under short-circuit conditions, where the transientmaking capacity Ip takes on high peak values, above all whencapacitor banks are connected in parallel with others that arealready powered. The value of Ip needs to be calculated foreach individual situation because it depends on the individualcircuit conditions and can in certain cases even have a peakvalue equal to 100-200 x Icn for a duration of 1-2 ms.This fact must be taken into account when selecting the circuitbreaker,which must have a suitable making capacity, and whensetting the overcurrent release, which must not cause unwantedtrips when the bank is connected.Selecting the circuit-breakerUsing the information on the rating plate of the three-phasecapacitor bankQn = rated power in kvarUn = rated voltage in Vthe rated current of the capacitor bank is determined as follows:6Inc =Qn x 10 3, in A.3 x UnThe following conditions must be verified for the circuit-breaker:Rated current ln > 1.5 lncOverload protection setting l1 = 1.5 x lncShort-circuit protection setting l3 = OFFBreaking capacity lcu > Ik, at the point of installation.ABB SACE 6/41