Short-circuit currents in three-phase a.c. systems - Part 3: Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth
This part of IEC 60909 specifies procedures for calculation of the prospective short-circuit. currents with an unbalanced short circuit in high-voltage three-phase AC systems operating at nominal frequency 50 Hz or 60 Hz, i.e.
a) currents during two separate simultaneous line-to-earth short circuits in isolated neutral or resonant earthed neutral systems;
b) partial short-circuit currents flowing through earth in case of single line-to-earth short circuit in solidly earthed or low-impedance earthed neutral systems.
The currents calculated by these procedures are used when determining induced voltages or touch or step voltages and rise of earth potential at a station (power station or substation).
This standard does not cover:
a) short-circuit currents deliberately created under controlled conditions as in short-circuit testing stations, or
b) short-circuit currents in the electrical installations on board ships or aeroplanes, or
c) single line-to-earth faults in isolated or resonant earthed systems.
The object of this standard is to establish practical and concise procedures for the calculation of line-to-earth short-circuit currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents through earth from electrical installations, leading to conservative results with sufficient accuracy. For this purpose, the current is determined by considering an equivalent voltage source applied at the short-circuit location with all other sources set to zero. The procedure is suitable for determination by manual methods or digital computation.
This standard is an addition to IEC 60909-0. General definitions, symbols and calculation assumptions refer to that publication. Special items only are defined or specified in this document. This does not exclude the use of special methods, for example the superposition method, adjusted to particular circumstances, if they give at least the same precision.
As stated in IEC 60909-0, short-circuit currents and their parameters may also be determined by system tests.
The calculation of the short-circuit parameters based on the rated data of the electrical equipment and the topological arrangement of the system has the advantage of being possible both for existing systems and for systems at the planning stage.