ABB AFC094AE02 Control circuit board industrial module

Description

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The circuit-breakers for the protection and switching of capacitor banks in LV
shall:
1. withstand the transient currents which occur when connecting and
disconnecting the banks. In particular, the instantaneous magnetic and
electronic releases shall not trip due to these peak currents;
2. withstand the periodic or permanent overcurrents due to the voltage
harmonics and to the tolerance (+15%) of the rated value of capacity;
3. perform a high number of no-load and on-load operations, also with high
frequency;
4. be coordinated with any external device (contactors).
Furthermore, the making and breaking capacity of the circuit-breaker must be
adequate to the short- circuit current values of the installation.
Standards IEC 60831-1 and 60931-1 state that:
– the capacitors shall normally function with an effective current value up to
130% of their rated current Ir (due to the possible presence of voltage
harmonics in the network);
– a tolerance of +15% on the value of the capacity is allowed.
The maximum current which can be absorbed by the capacitor bank Icmax is:
Therefore:
– the rated current of the circuit-breaker shall be greater than 1.5⋅Ir;
– the overload protection setting shall be equal to 1.5⋅Ir.
The connection of a capacitor bank, similar to a closing operation under shortcircuit conditions, associated with transient currents with high frequency (1÷15
kHz), of short duration (1÷3 ms), with high peak (25÷200 Ir).
Therefore:
– the circuit-breaker shall have an adequate making capacity;
– the setting of the instantaneous short-circuit protection must not cause
unwanted trips.