Flashover on a newly installed 230/13.8 KV transformer cable box
Flashover on a newly installed 230/13.8 KV transformer cable box
(OP)
Recently we have experience a flashover on a newly installed 100/133 MVA, 230/13.8 KV transformer cable box. The flashover happened two days after the transformer was energized in one high voltage (230 KV) oil filled cable box. An internal high energy arcing occurred in the transformer cable box between the box cover and top shield holding points.
My question: what is the optimum radius around the conductors (insulation distance between current carrying conductor and box wall) that will ensure cable box safety? Is 350mm is enough?
Also, what is the adequate paper insulation thickness around the conductors? Is 3mm is enough?
Is there any pressure relief device that can relief the generated pressure by high energy arcing flashovers?
Thanks
My question: what is the optimum radius around the conductors (insulation distance between current carrying conductor and box wall) that will ensure cable box safety? Is 350mm is enough?
Also, what is the adequate paper insulation thickness around the conductors? Is 3mm is enough?
Is there any pressure relief device that can relief the generated pressure by high energy arcing flashovers?
Thanks






RE: Flashover on a newly installed 230/13.8 KV transformer cable box
First of all you may consult the article from this link.
"Oil-filled cable pothead with venting means and tool therefor"
http://www.patentgenius.com/patent/4273951.html
I'll try to calculate in a simple way the insulation solicitation.
The voltage to earth will be 230/sqrt(3) =133 kV. The test voltage may be 212 KV [shock wave]. The insulation is paper –air in series [or paper-oil]
For paper 3 mm is good. For air 350 may be fair, but better will be 500[as for air maximum permissible electric field intensity will be 32 KV/cm in normal conditions, but for the warm condition 20 KV/cm will be possible.]
For oil 150 KV/cm and for paper 100KV/cm will be maximum permissible.
For oil 100 mm will be enough [if oil will be instead of air]
For electric field intensity approximate formula is:
E [air] =U*epsrpaper/(epsrpaper*thickair+epsrair*thickpaper) [KV/cm]
epsrair = relative permittivity of air =1
epsrpaper=relative permittivity of paper=2.3
epsroil=relative permittivity of oil=2.5
thickpaper =paper thickness =0.3 cm
U=212 KV
There are many types of safety relief valve. I think that this may be useful:
http://www
Never the less you have to consult the manufacturer prior to purchase the valve.
Best Regards
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
Eair=epsrpaper/R2*U/(epsrpaper*ln(R3/R2)+epsrair*ln(R2/R1))
For a minimum radius of copper of 35 mm the maximum air Electric Field Intensity will be 24 KV/cm and this it seems to me tolerable. See attachment
Regards
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
Is it more practical to add additional pressure relief device or move the top one to be closer to cable termination. I think this will be more effective to prevent cable box flashovers. In case of two devices, can both devices be of the same size and relief capacity knowing that cable box expansion factor is very small compared to transfomer main tank that makes handling the pressure generated by arc is very difficult(could reach 20 bars).
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
As Scotty said,a pressure relief device will only save blasting of the cable box that too if you are lucky the fault occurred very near to the PRD.
The patent could not be understood as the figures are not seen with it.
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
The paper insulation of the transformer winding is indeed more solicited and the stress may reach the 80 KV/cm or more as prc said. But still I have to believe that this pot head is outside located and for this configuration my appreciation is correct.
Best Regards
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
Thanks for the sketch - I think I see where this is going now.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
TechARA, diff (87T) was operated?
Best Regards.
Slava
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
A PRD may be the best you can do. A major arcing fault evolves gas a such as colossal rate that it is unlikely that any practical RV will deal with the volume, especially as you will have mixed flow of liquid and gas. The basic answer is that under major arcing fault conditions your transformer is going to suffer damage. The PRD will, given time, at least prevent the tank from becoming a highly pressurised vessel which would pose a serious risk to structures and personnel near it. I have never personally seen a PRD fitted to a cable box.
To give you an idea of just how energy is let loose during a fault, this is the wreckage of a 48MVA transformer on my site which suffered a massive internal failure:
ht
h
The large PRD operated correctly, it was simply overwhelmed by the rate of gas evolution.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
Usually PRD contacts will be actuated after the gas opertaed relay contacts are operated.You cannot be sure of PRD operation if the arc point is slightly away from the PRD location.Purpose of sudden pressure relay is same as Buccholtz relay.SPR is used in nitrogen sealed transformers as Buccholtz relay is not feasible in that case.
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
Thanks a lot for the your great explanation.
Best Regards.
Slava
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
RE: Flashover on a newly installed 230/13.8 KV transformer cable box
I read now about some intresting idea: used arc protection into trafo tank, based on the fiber optic.
On this moment are few problems.
withstand of connector: +80 deg C and vibration.
As protective relay used relay like to ABB's REA.
Regards.
Slava