IEC 60909-3:2009 pdf download – Short-circuit currents in three-phase AC systems – Part 3: Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents flowing through earth
1 Scope and object
This part of lEC 60909 specifies procedures for calculation of the prospective short-circuitcurrents with an unbalanced short circuit in high-voltage three-phase a.c. systems operatingat 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 ortouch or step voltages and rise of earth potential at a station (power station or substation) andthe towers of overhead lines.
Procedures are given for the calculation of reduction factors of overhead lines with one or twoearth wires.
The standard does not cover:
a) short-circuit currents deliberately created under controlled conditions as in short circuittesting stations, or
b) short-circuit currents in the electrical installations on board ships or aeroplanes,orc) single line-to-earth fault currents in isolated or resonant earthed systems.
The object of this standard is to establish practical and concise procedures for the calculationof line-to-earth short-circuit currents during two separate simultaneous line-to-earth shortcircuits and partial short-circuit currents through earth,earth wires of overhead lines andsheaths or shields of cables leading to conservative results with sufficient accuracy.For thispurpose,the short-circuit currents are determined by considering an equivalent voltagesource at the short-circuit location with all other voltage sources set to zero.Resistances ofearth grids in stations or footing resistances of overhead line towers are neglected，whencalculating the short-circuit currents at the short-circuit location.
This standard is an addition to IEC 60909-0.General definitions，symbols and calculationassumptions refer to that publication.Special items only are defined or specified in thisstandard.
The calculation of the short-circuit currents 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.The procedure issuitable for determination by manual methods or digital computation. This does not excludethe use of special methods,for example the super-position method，adjusted to particularcircumstances, 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.
2Normative references
The following referenced documents are indispensable for the application of this document.For dated references, only the edition cited applies.For undated references, the latest editionof the referenced document (including any amendments) applies.
IEC 60909-0:2001，Short-circuit currents in three-phase a.c. systems – Part 0: Calculation ofcurrents
IEC/TR 60909-2;2008，Short-circuit currents in three-phase a.c. systems – Part 2: Data ofelectrical equipment for short-circuit current calculations
Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
two separate simultaneous line-to earth short circuits
line-to-earth short circuits at different locations at the same time on different conductors of athree-phase a.c. network having a resonant earthed or an isolated neutral
3.2
initial short-circuit currents during two separate simultaneous line-to-earthshort circuits IKEE
r.m.s value of the initial short-circuit currents flowing at both short-circuit locations with thesame magnitude
3.3
partial short-circuit current through earth IE8
r.m.s. value of the current flowing through earth in a fictive line in the equivalent earthpenetration depth é
NOTE In case of overhead lines remote from the short-circuit location and the earthing system of a station, wherethe distribution of the current between earthed conductors and earth is nearly constant, the current through earthdepends on the reduction factor of the overhead line(Figures 4 and 5). In case of cables with metallic sheaths orshields,earthed at both ends in the stations A and B, current through earth between the stations A and B(Figures9a) and 10a)), respectively between the short-circuit location and the stations A or B(Figures 9b) and 10b).
3.4
total current to earth IETtot at the short-circuit location on the tower T of an overheadline
r. m. s. value of the current flowing to earth through the footing resistance of an overhead linetower far away from a station connected with the driving point impedances of the overheadline at both sides,see Figure 5
