Analysis of three-phase networks in disturbed operating conditions

Calculating three-phase networks

The dimensioning of an installation and the equipment to be used, the settings for the protection devices, and the analysis of electrical phenomena often require calculation of the currents and voltages in electrical networks.

The purpose of this “Cahier Technique” is to set out a simple method of calculating all these parameters in three-phase networks subject to disturbance using the symmetrical components method, and to provide specific application examples.

Asymmetrical and symetrical operations

In normal, balanced, symmetrical operation, the study of three-phase networks can be reduced to the study of an equivalent single-phase network with voltages equal to the phase to neutral voltages of the network, currents equal to those of the network and impedances equal to those of the network, known as cyclic impedances.

If the asymmetry is significant, simplification is no longer possible because the relations in the various conductors cannot be determined by means of a cyclic impedance for each element of the network. The general method based on Ohm’s and Kirchhoff’s laws is possible, but it is complex and laborious.

The symmetrical components method described in this document simplifies the calculations and provides a much easier solution by reducing it to the superposition of three independent single-phase networks.

After a brief review of vector concepts, this method is explained by reference to basic applications on various types of short-circuit, followed by worked examples of actual cases.

AUTHOR: Schneider Electric expert | Benoît de METZ-NOBLAT