Phase-to-Ground Clearance Interference with Insulators 2

Inrushman

I am not clear on what is your concern here.

If the concern is the safety working zone then you shall comply to applicable safety standards plus good utility practice.

If your concern is the insulation/corona issues related to the electric field strength, you could hire a research group (like university) to do a field analysis, e.g using software tool like FemLab. There always be a potential gradient along nonmetallic parts and it is nonlinear depends on the permittivity, its geometry and the surrounding objects.

You have to move the LA such that clearance circle is not fringing on bushing.

The voltage gradient over the surface of an insulator depends on variable conditions such as moisture and contamination, so is considered indeterminate. NESC calls for 8.5' above grade for indeterminate voltages.

If the circle represents your live part to ground clearance, I think you are okay since it does not intersect a grounded metal part. You cannot have a live to indeterminate voltage clearance without every insulator being a problem. You could just as easily cloud your insulator stack, which is entirely within your clearance circle.

I agree with stevenal. You are in better shape here because any surge voltage will be limited by the surge arrester.

The risk of flashover is acceptable for adjacent insulators connected in the same phase with less than the usual phase to ground clearance.

A simplified practical approach to determine the minimum acceptable clearance may be between half to 2/3rd the phase-to-ground clearance between the grading ring of the surge arrester and the top termination (pothead). An insulation coordination study could provide additional details.

_{The above recommendation is based in the fact that two identical clean post insulators installed side by side and connected in the same phase, the voltage difference between two adjacent points is null. Therefore, the clearance theoretically could be zero. From the practical point of view, the difference in geometric (i.e. grading ring, height, etc) and other random factors such as pollution that may produce uneven surface voltage distribution during transient conditions, the clearance larger than half phase-to-ground clearance could work OK.}

Cuky,you are mentioning that risk of flashover is admissible between insulators of same phase.I believe the
SF-6 to air insulator must be a graded condenser bushing RIP type whose field distribution can be affected by the LA field which may not be a graded one.

All,
Thanks for you helpful reply,
Cuky2000, may I know the reference for the selection of 1/2 or 2/3 of phase-to-ground clearance as a check between insulators shanks / heads?

Parallel insulators are pretty common at less than 1/2 the phase to ground clearance.

Is the SF6 bushing connected to the arrester? If so, the clearance from the arrester terminal to the middle of the SF6 bushing is electrically the same or greater (has at least the same breakdown voltage) as the distance from the bushing terminal to the middle of the bushing. Actually, because the clearance from the arrester to the bushing is all air, the breakdown voltage will be higher than the path from the bushing terminal along the bushing.

This is within the Ø-grd clearance circle. I don't see a problem. You can move the arrester further from the bushing, but you will reduce the protective margin because of increased arrester lead length. You will not increase the distance from an energized part to the middle of the bushing because some part of the jumper from the bushing to the arrester will still be as close as your arrester is now.

So prc has me thinking. The difference between the (non-conducting) arrester and the bushing is that while both have a top to bottom voltage gradient component, the bushing also has a component that goes from inside to outside. So I'm guessing that prc is not so much concerned with flashover as with bushing gradient distortion from the arrester presence that could stress the bushing insulation. Am I close prc? Transformer manufacturers must deal with this. Transformer arresters are commonly mounted on standoff brackets putting them some distance away from the bushing, even though closer would improve arrester performance as jghrist mentioned.

Inrusman,

See page 4 of the ABB technical reference in the link below.


Beware that the calculated phase-to-ground distance on the USA standard practice is usualy larger than the calculated by the ABB recommendation. Therefore, the 1/2 to 2/3 of the phase-to-ground clearance may be conservative.

See also a representative voltage profile of typical ceramic and polymer insulator

Stevenal,you are right.I dont want a bushing failure and consequent disaster of a transformer fire.In bushings top 10-30 % will be at nearly 100 % voltage and then condenser starts.There voltage is graded for uniform axial and radial voltage distribution.As per the sketch given, 100 % voltage of LA ring comes in line with the 60 or 70 % voltage of bushing creating a voltage difference in the air clerance gap.This can affect the LA operation too.

ABB recommendation also says half-to-ground clearance at top part- inclined bushing top to LA top and ground clearance at bottom.At top as mentioned earlier voltage will be same and reduced clearance is allowed. In the present case bushing is vertical and when you provide ground clearance at bottom,automatically top also will be having the same clearance.

I thought in these matters,a liitle conservative approach is always beneficial.

PRC,

Insulation coordination and electrical clearances in particular is one of the most controversial and misunderstood subject in our industry. I believe that there is not realistic a design that claim is free of risk of failure. At best, only a mitigation of risk to an acceptable level can be achieved. The fundamental performance or reliability criterion is based on the consequence of failure and on the expected life of the equipment. For example, it is common define a performance reliability criterion with an accepted MTBF bout 50–200 years for AIS and about 200–800 years for GIS.

Below are two quotes from two IEEE Std that I hope help enlighten this issue:

IEEE Std 1427 said:

The performance criterion upon which the insulation strength or withstand voltages and clearances are selected is based on an [highlight]acceptable probability of insulation failure[/highlight] and is determined by the consequence of failure, required level of reliability, expected life of equipment, economics, and operational requirements. The criterion is usually expressed in terms of an [highlight]acceptable failure rate[/highlight] (number of failures per year, years between failures, risk of failure, etc.) of the insulation configuration.

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IEEE Std 1313.2 said:

… The procedure differs for self- and non-self-restoring insulation. For self-restoring insulation (disconnecting switches and bus support insulators), a statistical BSL exists and the insulation’s strength characteristics may be defined by CFO and [σ]_f. In contrast, for non-self-restoring insulation (the internal insulation of the transformer or bushing), the BSL is a single valued function. The probability of failure is assumed to be 100%, when the voltage exceeds the BSL…………… …….[highlight]The required BSL is the arrester switching impulse discharge voltage multiplied by a protective ratio. The minimum protective ratio is 1.15%;[/highlight] however, larger margins have been used.

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Enclosed is graph with phase-to ground clearance per various standards.
_{The clearance values per IEEE Std 1427 are based on a 605 kV/m critical flashover overvoltage gradient (CFO) a value which has been found to represent the typical geometry for an air-insulated substation. Other traditional clearances are based on CFO gradient that varies from about 540 kV/m to 750 kV/m. The clearance suggested per IEEE Std 1427 is 2.5 to 3.75 times lower than conventional clearances values. The reduce clearances should not substitute the one required by safety standards such as the NESC in the USA or maintenance practice. The IEEE Std 1427 should be taken as acceptable options to be used when economics, space limitations, or other considerations justify the resulting benefits.}