Phase-to-Ground Clearance Interference with Insulators
Phase-to-Ground Clearance Interference with Insulators
(OP)
Hi all,
when designing an EHV SS, it was noticed that the SA phase-to-ground clearance is interfering with the SF6 Air Bushing insulator (not grounded structure, i understand that there shall be some voltage in between zero and the HV Phase voltage somewhere on the bushing surface as it is working as a potential divider, however, there is no metallic parts along the bushing,.
sketch is showing my concern, clouded part shows the interference!
I don't know whether this clearance is sufficient?! or needs some voltage analysis for the bushing itself,which looks very difficult!!.
Thanks.
when designing an EHV SS, it was noticed that the SA phase-to-ground clearance is interfering with the SF6 Air Bushing insulator (not grounded structure, i understand that there shall be some voltage in between zero and the HV Phase voltage somewhere on the bushing surface as it is working as a potential divider, however, there is no metallic parts along the bushing,.
sketch is showing my concern, clouded part shows the interference!
I don't know whether this clearance is sufficient?! or needs some voltage analysis for the bushing itself,which looks very difficult!!.
Thanks.






RE: Phase-to-Ground Clearance Interference with Insulators
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.
RE: Phase-to-Ground Clearance Interference with Insulators
RE: Phase-to-Ground Clearance Interference with Insulators
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.
RE: Phase-to-Ground Clearance Interference with Insulators
RE: Phase-to-Ground Clearance Interference with Insulators
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.
RE: Phase-to-Ground Clearance Interference with Insulators
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.
RE: Phase-to-Ground Clearance Interference with Insulators
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?
RE: Phase-to-Ground Clearance Interference with Insulators
RE: Phase-to-Ground Clearance Interference with Insulators
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.
RE: Phase-to-Ground Clearance Interference with Insulators
RE: Phase-to-Ground Clearance Interference with Insulators
See page 4 of the ABB technical reference in the link below.
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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
RE: Phase-to-Ground Clearance Interference with Insulators
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.
RE: Phase-to-Ground Clearance Interference with Insulators
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:
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.