IEC 61865:2001 pdf download

IEC 61865:2001 pdf download.Overhead lines – Calculation of the electrical component of distance between live parts and obstacles – Method of calculation.
3.1.3
temporary overvoltage
oscillatory overvoltage (at power frequency) at a given location, of relatively long duration and which is undamped or weakly damped
(1EV 604-03-12, modified)
NOTE Temporary overvollages suaIIy originat, from switchanQ Operations or taulls (for .xampl., sudd.n load rejection. erngle phase-to-earth faults and’or from nOn-linearities iterro-resonance effects. harmonics).
3.1.4
fifty per cent disruptive discharge voltage
peak value of an impulse test voltage having a 50 % probability of initiating a disruptive discharge each time the dielectric testing is performed
(1EV 604-03-43)
3.1.5
power.frequency with stand voltage
r.m.s. value of sinusoidal power frequency voltage that the equipment can withstand during tests made under specified conditions and for a specified time
(1EV 604-03-40)
3.1.6
lightning overvoltage
transient overvoltage, the shape of which can be regarded for insulation coordination purposes as similar to that of the standard li9htning impulse
[1EV 604-03-30)
3.1.7
front of a voltage impulse
that part of an impulse which occurs prior to the peak
[1EV 604-03-16)
3.1.8
electrical distance D,
reference distance which ensures that the electrical breakdown between any live part of the electrical installation to the body of a member of the public, or any conductive tool or oblect which they could reasonably be expected to be in contact with, is effectively avoided
3.1.9
occupancy at electrical distance T
number of hours, at which the individual or any conductive part to which he/she Is in contact. is taken to be at the limit of the electrical distance
NOTE As a reference. is taken to be I h per year.
3.1.10
annual probability of breakdown of air gap Ra
probability of sparkover
NOTE For a reference occupancy of I h per year at the electrical distance, O, the value a taken tO be to’.
3.1.11
statistical withstand voltage
overvoltage, with the shape of the representative overvoltage, at which the insulation exhibits a 90 % probability of withstand
5.3 Slow-front overvoltages
Slow-front overvoltages are statistically distributed according to a function which can frequently be approximated by a normal (Gaussian) distribution in the vicinity of the statistical overvoltage. 112SF (and UF expressed on a per unit basis) which has only a 2 % probability of being exceeded. In the vicinity of U2,c a normal distribution can be fitted with a coefficient of variation, 2SF- If better information is not available, a suggested value for ‘2SF is 20 %.
The annual frequency of the occurrence of a slow-front overvoltage due to normal switching operations and to delayed auto-reclosure operations after faults, NSF. will depend upon the operation of the line. A typical value is likely to be 1 to 100 overvoltages per year.
In addition to normal switching operations when the line Is energized, faults (for example due to lightning and pollution flashover) will occur on the line, and, in these cases. the breaker opening and reclosing will Interrupt the passage of curreni
In the event that delayed auto-reclosure is used (with a dead time of several seconds) the trapped charge on the line will have decayed to some extent and, if the decay is to zero, the overvoltage on re-energization will be essentially that of the line energization. In the event that high-speed reclosure is used, the overvoltages are potentially higher. The annual number of reclose operations is denoted by SFR The annual frequency of the occurrence of a slow- front overvoltage due to reclosing is related to the fault rate of the line which will depend upon parameters such as line construction, lightning ground-flash density, pollution levels, conductor clashing incidents, etc. A typical annual fault rate is of the order of I to 2 per 100 km of line per year.
5.4 Fast-front overvoltages
The actual probability distribution of fast-front overvoltages occurring on a line is usually not known by utilities or line designers. It Is thus not possible to apply completely the procedure set Out In clause 6. The following provides guidance for assessing the magnitude and frequency distribution of last-front overvoltages.
The major source of fast-front overvoltages is from lightning and the frequency of occurrence of this (either in the form of ground-ftash density or thunderstorm days) is known. Lightning surges can arise due to direct lightning strokes to the line, back flashover of Insulator strings. or induction. Their amplitude decreases and their time to peak increases as the surge travels along the line in each direction away from the stroke. This is because the energy in the surge is dissipated by the production of corona discharges.
Lightning surges that exceed the insulation level of the line will be largely dissipated to ground at towers adjacent to the lightning stroke, so that any residual fast-front surge will then be relatively small. However, lightning surges which are below the insulation level of the line are able to propagate until they are dissipated by the generation of corona discharges. or result in sparkover at a tower because of the statistical variability of the gaps between conductors and the towers’ steelwork, The probability of discharge to earthed parts near the conductor within any span where a direct stroke occurs is not considered here.