Strang phenomena on 380 kV In-Out Arrangement OHTL

[Inrushman]

Hi all,

It has been reported to our department that a new substation has supply by in-out arrangement from an existing double circuit 380kV OHTL, experiences frequent FLASHOVER on the two suspension towers adjacent to the tapping point, the flashovers occured almost at same time daily it is around 6 a.m., where the weather is very humid, i have investigated various causes assuming that the suspension insulators ( Vee Type ) on the existing towers had been slightly tilted during stringing of the cut conductors to the new terminal tower, field investigation by binoculars proofs perfect alignment...the Trip log showed both differential and distance relay trip on all phases on one side, and on center phase - (Blue) on the other phase, one two different days.

I think that some trapped charge is accumulating between the two lines ( continuous and cut one ), and unbalance due to the diffraction angle of the exiting towers ( about 3 degrees ), caused this (potential) over-voltage on certain adjacent towers,...

Any ideas?

 

[davidbeach]

First question that has to be answered is why 6am? Are there capacitors at that station that get switched on every morning at 6? If so, you probably have a condition where your insulation is just marginal. The ringing due to capacitor switching may add just enough voltage to cause the flash over.

 

[Inrushman]

no capacitors on this SS, or on 380 kV network completely, at 6 a.m., the humidity is very high,

 

[alehman]

How close is "almost the same time each day"?

Properly designed insulation shouldn't flash over, even with fog, so the first suspect as a recurring voltage event. Do you have a voltage recorder?

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[Inrushman]

Alan,

This is correct, TL must survive fogs, this is correct for the existing line " erected about 20 years ago ", but,when the modification took place, the flashover occured on the two adjacent towers only, this flashover took place for about 4 days ,at 6 a.m. everyday, the flashover was possible to be seen by naked eye from the SS operator.

We are going to check the voltage log on both the new SS and the remote ends ( sending & receiving ), however, the network in this region is comparatively stable where no load rejection is probable to occur.

 

[RRaghunath]

Could it be that at 6:00AM the loads drop in the system giving rise to over voltages (at the receiving end of long EHV OHLs) - Ferranti effect.
What are the lengths and are there any Shunt reactors at the substation!!

 

[jghrist]

Someone should inspect the insulators for damage. Maybe there are cracks in the porcelain that causes flashovers when moisture is present. If the insulators are polymer type, moisture may be getting in the interface between the polymer and the fiberglass rod in the middle.

 

[cuky2000]

Check if there is migrating birds in the route of the TL.

Check also the "dry band". Mechanism. This could be more significant than the humidity for F0.

 

[Eleceng01]

I don't want to change the OP but this reminded me of a story that a colleague told me. They were experiencing a fault at the same time every day (3:40 pm)that they couldn't find the cause of. So they finally sent a field crew out to the line and they observed a child get off of the school bus, grap a pipe from a nearby ditch and throw it skyward into the line - causing there "mystery" fault.

Doesn't seem to be the cause here, but I had to share.