230kV Surge Arresters Clearances 2

[rammie47]

Hi,

The general clearances required for 230kV system are: 81” phase to phase and 70” phase to ground

Do these clearances apply for surge arresters as well? For example, we have a transformer vendor who is using these reference distances to mount the arresters away from the bushings on the incoming side. I thought the clearances are not applicable in this case as the bushings themselves are insulated?

Can someone advise the proper application?

Thanks,
RK

 

[VTer]

I would think that at minimum you should maintain the same distance between arresters as you do with bushings. The minimum distance from xmfr bushing L1 to arrester L1 for example is irrelevant from insulation perspective since they are the same potential but arresters L2 and L3 should maintain the minimum clearance distances. Also the same minimum clearances should be kept for all arresters and ground.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[rammie47]

Yes, the distance between the arresters would be same as distance between the bushings. But the distance between the bushing and arrester doesn't need to be so much and I think the vendor is not applying the clearance rule properly?

 

[Power0020]

you would need half the phase to ground between transformer bushing and the surge arrestor live points( on the same phase), and full phase to ground at the earthed points (on the same phase).

this requirement seems weird but this is important to avoid flashing during transients in polluted environments.

 

[cuky2000]

Per IEEE Std C57.12.00-2010, the minimum phase-to-phase clearances for 230 kV power transformers bushings are:
• Live parts : 70 inches (1778 mm).
• Top shed of insulator of bushings: 60 inches (1651 mm).

Phase-to-phase air clearances are driven primarily by the largest clearances required by either BIL or switching Surge Impulse (BSL). Other factors are also considered such as bushing configuration (vertical or Inclined), SA grading ring, altitude (>3.300 ft.),mechanical forces (loading combination of SC, seismic, wind, etc.)

Same phase, external air clearances such as the transformer bushing and surge arrester is not explicitly provided by the standard. This could be estimating by the transformer manufacturer considering the maximum potential differential between adjacent points between the bushing and arrester. The manufacturer should consider the dissimilar shape of bushings and arresters selected the different creeps and uneven pollution during operation that consequently will produce different voltage profiles from top to bottom of the insulators been max. at the top in the metallic energized part and reduced somehow in a parabolic way down to zero at ground potential on the base of the insulator. (See similar discussion on thread238-266323)

Phase-to-ground distance from energized point of bushings or arresters to the metallic base of transformer should be driven by the same rules of buses and post insulators. Similarly the distance from energized points to grade or top of the foundation should meet the local codes (in the USA the IEEE Std C2)