IEC 60909-3-2003 pdf – 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.
1 Scope 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.
2 Normative reference 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 edition of the referenced document (including any amendments) applies. IEC 60909-0:2001, Short-circuit currents in three-phase a.c. systems – Part 0: Calculation of currents 3 Terms and definitions For the purposes of this standard, the following 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 a three-phase AC system having a resonant earthed or an isolated neutral. 3.2 initial short-circuit currents during two separate simultaneous line-to-earth short circuits ” kEE I r.m.s. value of the initial short-circuit currents flowing with the same magnitude at both locations at the instant of the two separate simultaneous line-to-earth short circuits. 3.3 total earth current I Etot at the short-circuit location r.m.s. value of the earth current at the short-circuit location flowing through the earthing system of a station (power station or substation) or the footing resistance of an overhead line tower far away from a station and through earthed conductors to earth (Figures 4 and 5). Such conductors may be earth wires of over-head lines or sheaths, shielding or armouring of cables. 3.4 earth current I ETn r.m.s. value of the earth current causing the potential to rise above earth at an overhead line tower n in the vicinity of a station 3.5 earth current I EBn r.m.s. value of the earth current causing the potential to rise above earth U EBn of station B, in case of a line-to-earth short circuit at an overhead line tower n in the vicinity of the station B 3.6 partial short-circuit current through earth r·3I (0) part of the total current flowing through earth remote from the short-circuit location and the earthing system of a station, where the distribution of the total current between earthed conduct