IEC 60909-3:2003 pdf free download – Short-circuit currents in three-phasea.c. systems
1 Scope
This part of lEC 60909 specifies procedures for calculation of the prospective short-circuitcurrents with an unbalanced short circuit in high-voltage three-phase AC systems operating atnominal frequency 50 Hz or 6O 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 fiowing through earth in case of singlie 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 ortouch 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, orc) single line-to-earth faults in isolated or resonant earthed systems.
The object of this standard is to establish practicai and concise procedures for the catcuiationof line-to-carth short-circuit currents during two scparatc simultancous linc-to-carth shortcircuits and partial short-circuit currents through earth from electrical installations，leading toconservative results with sufficient accuracy. For this purpose, the current is determinedby considering an equivalent voltage source applied at the short-circuit location with allother sources set to zero. The procedure is suitable for determination by manual methodsor digital computation.
This standard is an addition to lEC 60909-0.General definitions，symbols and calculationassumptions refer to that publication. Special items only are defined or specified in thisdocument. This does not exclude the use of special methods, for example the superpositionmethod, adjusted to particular circumstances. if they give at least the same precision.
As stated in lEC 60909-0， short-circuit currents and their parameters may also be determinedby system tests.
The calculation of the short-circuit parameters based on the rated data of the electricalequipment and the topological arrangement of the system has the advantage of beingpossible both for existing systems and for systems at the planning stage.
3.1 two separate simultaneous line-to-earth short circuits
ine-to-earth short circuits at different iocations at the same time on different conductors ofa three-phase AC system having a resonani eartted or an isoiated neutrai.
3.2 initial short-circuit currents during two separate simultaneousline-to-earth short circuits MEE
r.m.s. value of the initial short-circuit currents flowing with the same magnitude at bothlocations at the instant of the two separate simultaneous line-to-earth short circuits.
3.3 total earth current l=tot at the short-circuit location
r.m.s. value of the earth current at the short-circuit location flowing through the earthingsystom of a station (power station or substation) or the footing resistance of an overhoad linetower far away from a station and through earthed conductors to earth (Figures 4 and 5).Such conductors may be earth wires of over-head iines or sheaths, shieiding or armouring ofcables.
3.4 earth current ‘Ern
r.m.s. value of the earth current causing the potential to rise above earth at an overhead linetower n in the vicinity of a station
3.5 earth current IEBn
r.m.s. value of the earth current causing the potential to rise above earth UEpn of station B, incase of a line-to-earth short circuit at an overhead line tower n in the vicinity of the station B