Nitrogen Blanket VS Conservator Tank in a Power Transformer 9

[subash148]

Actually i am reviewing a single phase, 90 MVA, 13.8 kv/132 kV ODWF power transformer specification. Is it possible to have a nitrogent blanket as well as Coservator tank with buchholz relay in the same transformer? As far as i know purpose of both is to keep Oil pressure constant inside the transformer. Or do i have to select between one of them?

Please clarify on this.

SG

 

[waross]

A nitrogen blanket may seriously inhibit the operation of a Buchholz relay.
You may, however, place the nitrogen blanket above the oil in the conservator tank.
The purpose of either a nitrogen blanket or a conservator is to exclude moisture and air from the transformer oil.
The Bucholz relay detects gases generated as a result of an internal fault in the transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Usually nitrogen blanket ( trade name:inertair) is used with out conservator where nitrogen is kept at a positive pressure with arrangement for automatic fill or bleed provision. In such arrangement, sudden pressure relay is used in place of Buchholz relay.There are certain disadvantages for such system esp for EHV Transformers due to bubble formation in oil with change insolubility of N2 in oil.

Constant pressure over oil is obtained with conservator + free breathing silicagel breather or conservator + air bag( Atomoseal).In these cases Buchholz relay is invariably provided.

 

[squeeky]

Dear Subash148, I presume you are wanting to keep moisture out. I will make a few observations. Excuse me if I relate information you already know. The normal silica gel “breather” has some limitations and I am starting some trials on a few modifications. The problem is not only water but the rust in the vapour space. You will not stop all moisture. The moisture enters the conservator. The oil heats up from either internal or external temperatures. The moisture enters the vapour phase and then when the conservator cools, it forms a liquid layer on the surface of the vapour space of the conservator. This then rusts. The rust drops off and falls to the bottom of the conservator where the excess water sits. It now breaks down to small particles. If the conservator is not drained regularly to check for water, it enters the main tank. Water and ferrous in your main tank is not good, need I say more.
We always fit oversized breathers because of the people problem in the maintenance where they either don’t do it because they don’t go or will do it next time. The next problem is tunnelling. The breather input at the bottom centre for the moist air and the output at the top centre to enter the conservator. Unless you are very observant, the colour on the outside may mask the true path of the air and colour inside. Some companies put a spiral in the gel reservoir. I have inserted a section of plastic pipe into the top hole with a blank plug at the bottom and sufficient holes in the top to the air to get through. The area matches the output hole area plus 10% . This forces the air flow to the outer edges where you can see any colour changes.
Then I place a VPCI or VCI ( Vapour phase corrosion inhibitor) in the pipe and this product protects any metal surfaces in the vapour space. The word vapour can be replaced with volatile if you go onto the internet and run a search. I am presenting a paper to the international Sugar Convention in South Africa on Wednesday. They work under the title ISSCT. You may be able to pick it up from there after the conference has closed or I can send it to you. No rocket science, just facts.
One other thing. If your silica gel goes white at the top, you already have pin holing in the conservator from the inside. The moisture is at such a high level it is affecting the silica gel at the top.

 

[FPelec]

Dear Subash148,
according to my experience, nitrogen blanket without conservator is not a common solution for a 90 MVA power transformer, at least in the European industry. We generally use conservator + maintenance-free (regenerative) dehydrating breather for transformers with rated power higher than 10 MVA.



Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[krisys]

For the large transformer (140/155MVA, 220/33kV) which we had in my previous organization, the transformer had a top membrane to protect the oil from the atmospheric air. Additionally, it had a nitrogen blanketing between the oil and the membrane to provide an additional cushion.

I do not recall it clearly. But both Nitrogen blanketing and conservator is being given for some large transformers.

 

[prc]

squeeky,it will be nice if you can attach your paper on breather at this site.

 

[subash148]

Thank you all for your informative feedback.

SG

 

[dpc]

In the US, it used to be extremely rare to see a conservator-type transformer. Inert gas systems are commonly used up through the largest transformer sizes. I have been involved with units up to 800 MVA and I'm sure there are larger ones. I am not aware of a single US manufacturer of large power transformers (yes, we still have some) who uses a conservator design as a standard. With inert gas systems, a sudden pressure relay is used in lieu of a Buchholz relay.

 

[subash148]

Thanks DPC, very valuable information.

SG

 

[DTR2011]

Sealed transformers with N2 blanket also typically have alarms on the N2 system that can be utilized for Main Tank pressure and N2 tank pressure - typically wired into a SCADA system.

The RPR rise relay in modern units is mounted close to ground level, vs. Bucholtz relay, up high. If there are requirements for testing, with the side mounted RPR, this can be done with ease with a simple test set (from Mfg) or a DIY made kit.

In the USA Sudden Pressure / Rapid Pressure Rise relay may be part of required routine testing. Having this device on the ground minimizes the need for a unit outage and also minimizes clearance issues getting up near the HV section of the transformer.

I've seen utilities retrofit Sudden Pressure / Rapid Pressure Rise relays with stainless tubing and valves (if the device is located in the HV area) so they can be tested from the ground. There is a Doble presentation on it. The testing can then be done in 15-30 minutes without lifting equipment, transformer outages, etc.

I seen the results of conservator type transformers that have not been assembled correctly, have bladders rupture, etc. An N2 filled unit (at least here), is a bit more straightforward for construction & maintenance folks to deal with.

 

[prc]

dpc,To my understanding in US, Inertair air oil preservation system is rarely used nowadays and limited to only medium sized transformers with less than 220 KV rating.This is not because of large transformer manufacturers have closed down,but due to some inherent problems with Inertair system.In fact many old systems are being retrofitted with constant oil pressure systems.

Inertair system- with out conservator, but with a gas space above oil level in tank for filling with N2 gas - was first introduced by Westinghouse during 1930's . N2 was produced by passing air over certain chemicals which absorbed O2 and N2 pressure inside tank varied between -5 psi to +10 psi as the oil expanded or contracted with load or ambient temperature. N2 will dissolve in oil by volume 8-10 % ( 20-90C) and with pressure by Henry's law 8-16 % ( 0-1barg). This will supersaturate oil and during sudden cooling of oil may release fine nitrogen bubbles inside insulation structure creating corona ( oil BDV drops more than 40 %) leading to breakdown. Engineers understood this by 1950's ( 1957 R B Kauffman etc, The effect of transformer oil preservation methods on the dielectric strength of oil, AIEE Vol 76 part3- 1960 A T Cahdwick etc Oil Preservation Systems-factors affecting ionisation in large transformers AIEE Vol 79 , April 1960) In 1983 Doble conference, there was a presentation by Westinghouse Muncie &Georgia Power company engineers -E M Petrie etc Reliability Improvemnets on operating gas space Power transformers - where they explained how they were retrofitting 500 kV transformers by COPS system due to some failures from bubble formation.

In today's inertairsystems the N2 pressure is limited to 0.2 psig to 6-7 psig with out avoiding negative pressure.There are also systems where a constant flow of N2 is made over the oil level without creating overpressure. N2 generators by membrane technology are also available.The more popular system today all over the world is avoiding air contact with oil in conservator by using Atmoseal( by GE in 1950's-also called bladder, air cell, rubberbag etc)

 

[dpc]

Inertair air oil preservation system is rarely used nowadays and limited to only medium sized transformers with less than 220 KV rating.

If you are talking about primary voltage ratings of 220 kV (Not BIL), then this constitutes the vast majority of transformers built in the US. So even if your statement is true, I wouldn't say that the use of inert air systems in the US is "rare". I am involved mainly in distribution substations with operating primary voltages up to 230 kV. I could count the number of conservator units I've seen in over 40 years on one hand. If you are talking about EHV transmission transformer, you may be correct - I really don't know.

My personal opinion is that if one system was clearly superior to the other, it would have won everyone over by now.

 

[prc]

dpc,I meant 220 kV rated voltage.Iknow that that in US inertair system is still used for MV transformers and self supplied such a system to US customer some 10years back. Anyway Iam faraway from US market and my understanding is only through literature and my contacts in US and Canada whom I contacted before my above posting.

 

[prc]

squeeky,using a corrosion inhibitor is something new to me.What is it? A liquid that will vaporise? Will it not pollute the oil ? Transformer engineers take care of rust going inside main tank by giving a 2 inch projection inside conservator tank, for the Buchholz pipe from main tank.By this way rust will remain only at the bottom of the conservator tank. I know an Indian customer who insists for stainless steel conservator for his distribution transformers. With thin plate used for such units,rusting can cause pitting and finally a hole through which rain can enter inside. So in your case also, a bettter solution will be to go for AISI 304L steel pipe for the breather.
It seems you are considering the "rain" inside the conservator is due to the moisture in the breathed in air.But my understanding is different. The air inside the conservator will be reasonably dry due to silicagel breather.But the oil will have water dissolved in it. But in a large transformer(say 100MVA) after a couple of years in service will have 100 litres of water. This will be distributed as 99 litres in paper and balance one litre in oil.This is because of the vast difference in solubility of water in oil and paper. Paper-80,000 ppm, oil (new) 40 ppm(20C) 300 ppm (70C ),Aged oil - 150 ppm (20C) 800 ppm (70C) (Ref CIGRE Brochure 349 of June,2008)
So when oil with dissolved water meet a dry air interface, the relative saturation of water in both media will try to be same. The only way is for water in the oil to separate out as free water and collect at bottom.Then more water will be released from paper to oil and the chain goes on, creating a real rain inside conservator with free breather.

 

[squeeky]

Dear People, I'm back home in South Africa from yet another trip. The paper has limited pictures and the presentation went down very well. Many pictures. The presentation is 6.5MB. If you require that, please let me know. The paper is 3.5MB. I hope it goes through.

It is for the sugar industry but there were people from all over the world. It did generate a lot of interest.