The symmetrical component method used by elec calc™ for the calculation of short-circuit currents allows a very accurate calculation of these currents according to the characteristics of each motor and this at every point of the installation, and for each of the operating scenarios described in the project.
A reminder of the requirements:
The standard IEC 60909-0 and its French equivalent NF EN 60909-0 “Short-circuit currents in AC three-phase networks” specify:
“Medium and low voltage motors contribute to the initial symmetrical short-circuit current, the peak short-circuits current, symmetrical short-circuit current cut and for asymmetric short-circuits, also to the permanent short-circuit current. The medium-voltage motors must be taken into account when calculating the maximum short-circuit current. Low-voltage motors must be taken into account in plant auxiliaries as well as in industrial installations and the like, for example in the networks of the chemical and iron and steel industries, and in pumping stations (…)
(…) It is permissible to overlook the contribution of asynchronous motors to the short-circuit current in low voltage supply networks if their contribution does not exceed 5% of the calculated initial short-circuit current. without motors (…)
(…) In the calculation of short-circuit currents, it is allowed to overlook these medium and low voltage motors, when, in accordance with the circuit diagram (locking) or the operating process (reversibility), they are not in use at the same time.”
In order to meet these requirements, some guidelines (PD CLC / TR 50480 or FD C 15-500) provide a comprehensive approach that leads to increasing the maximum short-circuit currents of the plant as a function of the ratio of the total power of the motors running simultaneously and the total power of the sources.
This approximate approach is, of course, conservative and fastidious to implement if the installation comprises several operation modes with different sources.
To overcome this difficulty and improve the accuracy of the results, elec calc ™ 2018, thanks to its real-time calculation possibilities, offers the user the choice for each motor whether or not it participates in short circuits.
By Philippe Aupetit – Product Manager for elec calc™2018 software
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