OHL trip under foggy conditions 3

[RRaghunath]

We are experiencing 132kV OHL trips under foggy conditions. The line is constructed new and passes through hilly terrain.

It happens often in the nights when there is settling of moisture over the insulators and it is reported that flashes across insulator strings along the line can be seen in the darkness before flashover occurs at one of the locations causing the line to trip on earth fault protection. The dust and moisture condensation seems to be responsible for the same.

The line is built with long rod porcelain insulators with 45mm / kV creepage distance.

I would appreciate if you could share your experiences in this regard including suggestions to overcome the problem.

 

[busbar]

Is the line relaying a numerical type that can provide fault-distance reports? That may simplify damage assessment.

 

[RRaghunath]

Busbar,

The line is provided with MiCOM series numerical protective relays, P441 for distance protection and P127 for backup OCEF protection.

How can I utilise the info in these relays and what info to look for.

 

[busbar]

Try locating a relay-applications person through the manufacturer's local office for help with post-fault analysis.

 

[aali2208]

Dear friend,
This usually happens in the line, You can check the fault impedance in P 441, If you have feeded the line parameters then you can also find the distance. What is the total lenght of the line if possible arrange petroling in the night time and closely look the insulators. The Flashing (in case of insulator Problem) tell you which insulator is weak one. Insulator surfaces if cracked can cause this problem too.

 

[jbartos]

Suggestion: The insulator 45mm/kV creepage distance appears to be sufficient, i.e. (45mm/kV) x 132kV = 5940mm which is almost 6 meters. What about a shape of insulators? This is important too.

 

[jbartos]

Suggestion: Visit

http://www.iee.org/OnComms/Branches/UK/England/NorthE/Mersey/Events/docs/MicomProtection.PDF

http://www.levico.com/alstomRelays.htm

http://www.alstom.com

etc. for more info on MiCOM

 

[RRaghunath]

The insulator strings have arcing horns across them. Do you think that the sparkover under moist conditions is taking place across the spark gap and these are better removed (the line is any way protected by Zinc Oxide surge arreters at both ends).

I remember it used to be a practice to provide arcing horns for grid transformer bushings, which has been subsequently discontinued. I am not sure about overhead line transmission lines.

Thanks in anticipation.

 

[YULESWAM]

Though the long rod insulator has acreepage distance of 45 mm/kv Ifeel the protected creepage is not 50% of the total creepage. please try with antifog type insulators which have enough protected creepage distance.Further any chance of chemical fumes outlet in the wind direction?

 

[ghanijan]

KESC of Pakistan facing the same problem as you.After detail consultation and tests of expert is recommended to apply a 5.o MM (3/16 in) film of Slicon Grease on Insolator now the KESC applying the Silicon Gress(SG99) Elecrolube of England on Insolator three time in a year. After applying the SG 99,it minimized the tripping during foggy condition.

 

[GrahamP]

Alternative to Silicon Grease is SILIDRIEL a silicone based paint which is sprayed onto insulator strings. Elecricidad federal de Mexico use it frequently to avoid similar tripping situations. Advantage of paint over grease is that you simply re-spray when neccessary, NO complex time consuming cleaning operation as in the case of grease.

 

[busbar]

An alternative Long-Shot fix may be something like “HVCE-” or “HVCE-WA”-series compnents on page 7 of:

http://energy.tycoelectronics.com/brochures/InsenEPP_0606-02.pdf

 

[cuky2000]

The flashover is more likely to occur during wetting and drying cycles creating what is called as dry-band conducting surface do to the air bound contamination particles.

Long rod polymer insulator and porcelain insulator with semiconductor surface with sufficient creepage distance and BIL level could help to mitigate the frequency of flashover. The final solution could be a compromising between capital cost investment and number of outage to be expected.

Consider investigate the following suggestions:

o Replace the section of insulators with higher BIL rating insulators.
o Semi conducting resistive glaze insulators is an alternative to reduce the flashover R & TV noise.

http://www.ngk.co.jp/DEN/english/product/12/001/main.html

o Coating insulator could be also an alternative

http://www.midsungroup.com/rginsulators.pdf

o Use grading ring on critical insulators. That will help to make the voltage distribution across insulator more uniform and reduce the chance of flashover.
o Explore to use inline surge arrester in section of the line with high rate of failure.
o Check the conductor clearance during critical swing. Line separator could be and alternative in case finding critical spots.
o Revisit the shielding capability of the line and degree of protection of the ground wire.
o Check the tower footage grounding resistance and reinforce with counterpoise as required.

Check the enclose reference for additional info in this topic.

http://www.platts.com/engineering/issues/ElectricalWorld/0105/0105ew_trcomposit.shtml

http://tdworld.com/ar/power_insulator_selection_complex/

http://www.ewh.ieee.org/soc/pes/lpdl/archive/2_Stan_Grzybowski_paper.pdf

 

[RRaghunath]

Thanks to every one for the responses. These have been interesting.

Busbar,

especially the one on Creepage extenders (HVCE & HVCE-WA) by Raychem. I have read the brochure, it appears to me as a workable yet low cost solution. Ofcourse, I am yet to make a comparison between Silicone paint vs the Creepage extenders.

Before getting in to costs, I would like to know how useful is the technique and know about the installations where the creepage extenders have been used. I am contacting the manufacturers any way. However, I would appreciate if you could tell me your experiences.

Similarly, any observations on the arcing horns across the insulator strings being more of a nuisense value, specifically in foggy environment.

 

[Rocketir]

I'm sorry I can't comment on the arching horns. But I would persue the relay reports to see if you are getting a fault in the same area each time. This would pinpoint a problem area rather than just suspect the whole line.
After reading your problem, it brought to mind a problem I had with frequent momentaries in the morning time. The problem was finally found. Someone was playing target practice with the insulators and conductor. Hope you find you problem and a solution. I know how frustrating this can be.
Jes

 

[sparky1976]

Hai,

I experincing the same problem withmy 150kV dist line, its only 20 km, but near coastal line,(aprox 1km).
We choose to use polimer onsulaotr with more leakage distance but the same (almost the same)phisical length with existing ceramic fog insulator, to minimaze stringing and retension work.
The existing design fall to light polutant, and the new design fall to heavy polutant.
as mention above the choice are :
1. change to polimer. Its the most suitable for my site.
2. change with RG Glaze insulator.(will increase lost since one set insulator will need aprox 1mA to evenly heat the surface)
3. Grease (our linesmen has lots of bad experince with greasing and mostly "degreasing" after 2 to 5 years aplication).
4.Add more fog bell to the insulator string, this will require retensioning, and ground clearance.
5. hot washing/energized washing, you can use corn cob/nutshell or demin water. do it freaquenlty, (this option will do better if you install one of the insulator monitoring system.
6. as mention before creepage extender.

from all the choices only greasing and hot washing that could easily done without shutting down the line, the other option is not easily done hot. Fortunately we have double circuit so I can shut one line down during the work changing to polimer.

goodluck
3.

 

[xabproject]

Dear RRN,
see if the following is of any use.
It is pertinent to mention that there are number of rock salt (Kala Namak) factories in the area near Panki. These factories generate fumes of hydro sulphuric gas which get deposited on insulators. This deposited layer start conducting during foggy weather resulting in flashovers and trippings of the lines. Therefore washing of the insulators before onset of winter is essential. In the past also, flash overs had taken place near Panki area causing tripping of lines. To prevent this, UPPCL had changed conventional insulators with antifog insulators on both 400kV Panki - Muradnagar and 400kV Obra-Panki lines during March,99 and November,99. These changes were carried out in the areas near to rock salt factories. During site visits it was gathered that UPPCL had not carried out washing of line insulators this year. Large number of trippings on a single night, however, indicate the need for washing of insulators in area around Panki and these lines are also needed to be regularly maintained.
Best regards

 

[Fundaspray]

RRN

These problems are specific to some areas and therefore, the creepage distance aopted for any installation should take care of the local conditions. Post commissioning sloution are difficult & expansive. This may require some kind of simulation studies. Some utilities have used 55mm/kV creepage distance based on their experience. It is not the creepage alone that decides the performance but also the protected length & the profile factor and these have clearly been defned in the IEC 815.

 

[vrb]

Check with Tennessee Valley Authority. I believe they experienced the same problem on a line running parallel to the Applachian Mountains in Eastern Tennessee.