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Topic Title: Retrofilling distribution transformers with Midel 7131
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Created On: 28 January 2009 02:01 PM
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 28 January 2009 02:01 PM
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dconstance

Posts: 3
Joined: 30 April 2002

Hello everyone!

Having searched previous forum topics I've not found any references to the use of Midel 7131 as a distribution transformer insulating oil, so hopefully someone out there can help me out.

I am being pushed by our insurance company (FM Global)to retrofill a number of distribution transformers on our private 11kV network with Midel 7131 on the grounds of reducing the fire risk to our operating plants. I am not keen to do this because a) the transformers are all outdoors in bunded compounds b)all transformers are well maintained c) it looks like it will cost £4000 more to fill a 1000kVA transformer with Midel than mineral oil.

In addition I am picking up mixed views on Midel from a few sources that have retrofilled transformers.

I would be interested in hearing of any experiences anyone may have had in retrofilling small (500kVA to 2000kVA) distribution transformers with Midel - good or bad!

Thanks in advance

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DC
 28 January 2009 03:19 PM
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OMS

Posts: 19837
Joined: 23 March 2004

In my experience, it is almost impossible to totally flush an existing mineral filled TX unit before refilling with MIDEL so you end up with a mix of insulating/cooling fluid on board.

As you say, cost will be some 30% more for MIDEL.

Aside from the environmental issues, if the TX are external then increasing the fire resistance of the fluid achieves very little benefit for a lot of pain.

I can understand replacing existing units with hermetically sealed midel units as part of a time expired swap out process but to simply dump and replace seems rather pointless

Regards

OMS

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Failure is always an option
 28 January 2009 04:17 PM
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IanLloyd

Posts: 65
Joined: 17 April 2002

Never retrofilled an ONAN with Midel.

I agree with all OMS has said and also, thinking aloud, would the temperature/cooling characteristics of the transformer alter if it's been designed for oil but is retrofilled with Midel? Is thermal capacity etc. of Midel identical to the oil or would there have to be some consideration given to possibly de-rating the transformer (or, maybe even uprating!?!?)? Just curious to know I guess, in case I ever find myself in that situation.

Edited: 28 January 2009 at 04:18 PM by IanLloyd
 28 January 2009 04:43 PM
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dconstance

Posts: 3
Joined: 30 April 2002

Thanks to both replies posted so far.

OMS - Your point on contamination of Midel with mineral oil is interesting. Depending on the level of contamination this could impact on the potential flash point, and also the future disposal route. I have been advised by contractors to order an additional drum of Midel purely for flushing, which pushes up the costs again.

Thermal capacity according to the manufacturer of Midel is improved compared to mineral oil. Flashpoint is raised from around 140C for mineral oil to over 300C for Midel. You could run your Midel filled transformer at a higher temperature i.e. overload situation. However, this would then have a potential knock on impact to the life of the other transformer components. Best to consult the transformer manufacturer before trying this.

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DC
 28 January 2009 06:03 PM
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OMS

Posts: 19837
Joined: 23 March 2004

OMS - Your point on contamination of Midel with mineral oil is interesting. Depending on the level of contamination this could impact on the potential flash point, and also the future disposal route. I have been advised by contractors to order an additional drum of Midel purely for flushing, which pushes up the costs again.


To be fair, flushing with one spare drum of MIDEL isn't going to totally flush the windings etc - they are (by definition) completely saturated with mineral oil. I once had to ask the question of lindley thompson regarding a mis specification on a transformer - this was a 1MVA unit that had been ordered with mineral but should have had MIDEL on board - in thier considered opinion, it was cheaper and less risky to buy a new transformer (with the appropriate credit for the mineral unit) as the time and effort taken to flush was excessive if you wanted any kind of assurance.

Personally, I would go back to your insurance company and innocently ask the quastion as what stance they would take in the event of a claim where you had (on thier instruction) invalidated all of the declared characteristics of the transformer and an incident/accident occured

Regards

OMS

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Failure is always an option
 28 January 2009 09:03 PM
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dlane

Posts: 690
Joined: 28 September 2007

I would agree with the responses so far. The mineral oil will be impregnated within the winding insulation and won't come out by merely flushing with Midel.

You will also have moisture within the insulation and the impregnated oil. I do not know what the saturation level of Midel is but if it is less than mineral oil, as the transformer is heated up and cooled through load cycling you may get free water being created within the transformer. You definately don't want that.

I would also ask how old are your transformers? Nominal life would probably be around 30 years. If they are getting old and due for replacement then there is no financial gain from putting in Midel to buying a new transformer.
I take it that there are no problems with PCB contamination within the oil to think about as well?

Kind regards

Donald Lane
 28 January 2009 10:57 PM
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dconstance

Posts: 3
Joined: 30 April 2002

Good points!

I will be discussing this is more detail with our insurer shortly, hence my enquiry to arm me with more information. I did ask some months ago what premium reduction we could expect if we adopted their recommendation. So far no actual figure has been forthcoming, just the usual statements regarding reducing risk to our business.

In answer to Donald's questions, the transformers in this case are less than 10 years old and PCBs are not an issue. I would not expect to spend any significant money on these units for many years yet. As you say, 30 years life is not unusual so it is not a case of changing to Midel on replacing the transformer.

I have obtained some contacts from our insurer as examples of where their other customers have converted to Midel. I'll post an update on this when I have some feedback.

Regards
DC


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DC
 04 October 2012 02:48 PM
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tspoors

Posts: 1
Joined: 23 September 2010

One particular advantage of retro-filling with Midel for me is that I can dispense with an expensive to maintain fire supression system. You need to add up the total lifetime cost of retro-filling and not just the capital cost.
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