IEC TS 61463:2000 pdf download
IEC TS 61463:2000 pdf download.Bushings – Seismic qualification.
As bushings are mounted on apparatus or buildings, the seismic qualification of the bushing must consider the behaviour of the apparatus or building. In the seismic qualification of a bushing all parts should be included, which contribute to the stresses in the critical cross- sections during a seismic event, e.g. the conductor and inner spacer in gas insulated bushings.
Three methods and combinations thereof are described in this technleat speclfication
— qualification by stahc calculation (clause 7);
— qualification by dynamic analysis (clause 8);
— qualification by vibration test (clause 9).
A combination of the methods may be used:
— to qualify a bushing which cannot be qualified by testing alone (e.g. because of size andor complexity of the apparatus);
— to qualify a bushing already tested under different seismic conditions;
— to qualify a bushing similar to a bushing already tested but which includes modifications influencing the dynamic behaviour (e.9. change in the length of insulators or in the mass).
Vibrational data (damping, critical frequencies, stresses of critical elements as a function of Input acceleration) for analysis can be obtained by:
a) a dynamic test on a similar bushing;
b) a dynamic test at reduced test level;
C) determination of natural frequencies and damping by other tests such as free oscillation tests (see annex B) or low level excitation (see clause A.1 of IEC 61166).
The methods result In the value of M which is determined for each part of the bushing on either side of the flange. The stress due to this moment should be combined with the other stresses acting In the bushing and It should be demonstrated that the bushing withstands the combined stress (clause 10).
The different methods of seismic qualification are illustrated in the flow chart given in annex A.
6 Severltles
6.1 At the ground
The ground acceleration depends upon the seismic conditions of the site where the apparatus is to be located. When it is known, it should be prescribed by the relevant specification. Otherwise the severity level should be selected from table 1.
The selected qualification level should be in accordance with expected earthquakes of maximum ground motions for the site location, This level corresponds to S2 earthquakes (see 3.24 of lEG 60068-3-3).
For qualification It should be assumed that.
— the horizontal movements as described in table 1 act in any direction
— the seventies of the vertical accelerations are 50 % of the horizontal:
— both directions may reach their maximum values simultaneously.
The ground motion can be described by natural time histories wtten known, or by artificial time histories, which should comply with the RRS; this is used as input for dynamic analysis or vibation test on the complete apparatus
NOTE Information on the correlahon between seismic qualification levels, seismic zone and seismic scales are given in IEC 60721-2-6 and IEC 60068-3-3.
6.2 At the bushIng flange
The severity at the bushing flange (see figure 4) may be available from the manufacturer of the apparatus and structures (i.e. transformers, gas insulated substations (GIS), building) in terms of RRS or maximum acceleration (8.). Where no information is available, the following simplified formula is used in order to establish an acceleration value at the flange of the bushing.
at= K.
The superelevation (actor K cart be:
— calculated by finite element analysis Including soil interaction or any other careful modelling, or
— derived from results from calculations or tests on comparable apparatus or structures, or
— taken from typical values obtained from experience. So tar very little experience is reported. Unless more background information is available, K should be assumed to be 1,5. See also 8.2.5 of IEC 60068-3-3. table 4.
7 Qualification by static calculation
This method is valid for rigid equipment. It may be extended to flexible equipment. such as a bushing, taking Into consideration the response factor R, as an alternative to the method by analysis. This allows simpler evaluation with Increased conservatism.
Using the static calculation method, the bending moment in the critical cross-section of the part of the bushing under consideration is calculated from an equivalent acceleration of the centre of gravity of that part.
