We have are supplying power to our Oxygen plant through 1.5 KM long cables. Cables are terminated to a 25 MVA Ynyn Transformer (33 kV / 6.6 kV) in open yard. LAs are provided just before the Transformers. Name plate details of LAs are:

Metal Oxide, Rated voltage - 36 kV, Discharge current - 20 kA, Pressure relief class - 40 kA, MCOV - 30 kV, Long duration discharge class - 4, Type METOVAR.

We are facing series of LA failures particularly just after the rain. Source end Transformer is Ynyn0d11. Can you help me in identifying the probable reasons of failure?

Sounds like manufacture problem, either bad design or poor quality control. After rain water get inside of the LA housing increase the leakage current,metal oxide heated up and eventually thermal run away.

Are the LAs mounted correctly? Usually there is a small vent at the top or bottom to let gases out. Is it possible that rainwater gets inside this way? It may be that the LAs are mounted horizontally but they should be vertical, or horizontal but with the vents on the topside.

New installation or old?

rasevskii

@ PWRTRAN: How can we check / test the LAs for manufacturing / design problem. Can you suggest some change in specification / inclusion of tests while ordering? we are monitoring the 3rd harmonic leakage current periodically and it is in the order.

@ Rasevskii: LAs are vertically mounted with explosion vent on upper side.

RAgrwal,
Per your post,33 kV (max=36 kV) network seems to be effectively earthed.
If your 33 kV network is effectively earthed then how the Rated Voltage (Ur) of the LA was selected
as 36 kV instaed of 28 kV? For me 36 kV rated voltage is too high on an effectively earthed network which might overstress
the LA during a surge. Any thoughts?

RAgrwal - try silicone rubber housed LA's, they don't need vents, so better sealing.

Yes, our system is effectively grounded. I don't have any idea, how rated voltage was selected. Is there any guidelines for its selection in case of LAs?

RAgrawal,
1) If your 33 kV network is eff earthed then it is having an Earth Fault Factor less than 1.38.That means your arresters should have MINIMUM of 28 kV rated voltage.Since your arresters are 36 kV rated and also the discharge current is 20 kA, the two protection margins offered to the transformer for chopped wave & BIL seems to be very narrow (may be less than standard 20%).Therefore,that issue has to be studied seperately to confirm whether the transformer is offered 20% margin.
2)Your LA has Pressure Relief class=40 kA seems to be adequate for a an earth fault at the location to re-seal.
3)After considering all, it looks like something unusual happens after a rain.As two of our members pointed out are they mounted correctly?If yes,then it looks like a manufacturing
(quality) problem.

Hope this helps.

Can you help me in identifying the probable reasons of failure?

Something to look for possible root caused of the arrester failure is a scenario of sustained temporarily overvoltage (TOV) such as ferroresonance that may caused the arrester exceed its rated energy absorption capability forcing the metal oxide blocks (ZnO) in thermal runaway failure mode.

Although it is recognized that a lower distribution voltage a YnYn transformer are less prone to ferroresonance still there are fairly level of risk in increased susceptibility to ferroresonance of grounded wye-wye at 33 kV with high capacitance provided by the 1.5 km UG line that may interacting with the series transformer magnetizing impedance.

Ferroresonance can more notorious under light loading conditions due to single phase opening such as reclosing on the primary line (if single pole operation) at the HV source transformer. Other possible cause of single phase issues is a faulty connector or splice opening, or an overhead line feeding the cable opening that should be investigated. Those even are likly to happen often during raining season.

Thanks Cuky, is there a way by which i can check the presence of ferroresonance? There is no visible sign (hunting etc.) of ferroresonance in our system.