Replacing SF6 with R134a? 1

[soare0]

Well, all I need is to fill an HV transformer (50kVrms) enclosure with SF6. Simple, ok. Only that all the stuff is needed in a week and the SF6 provider has some problems.

So I must replace SF6 with something else.

R12 freon is a good insulator and it can be used instead SF6 in certain conditions. However, these conditions are not met in this project.


There are also other freons usable for this, but I can have them in 30 days, like SF6.

Well, yesterday It came into my mind that maybe R134a could be a solution. It is ecologically safe (well, this will see in the next decades) chemically inert (a sort off) and most of all, I have it.

The problem is: does anybody knows anything about the breakdown voltage of this gas or other electrical characteristics? I've searched the web but I've found nothing about using R134 in electrical.

Maybe you can help me.

Paul N.

 

[itsmoked]

I don't think using a combustible gas in a transformer would be a wise thing to do...

 

[ScottyUK]

SF6 is pretty unique in its electrical capabilities which is why this oherwise rather unpleasant gas is used for the purpose instead of something more innocuous. You must discuss this with the manufacturer of the equipment because they are the only people who know the geometry of electric field distribution within the equipment and how much margin they built in.

A couple of other things to consider:

Can you get SF6 from, say, your grid operator or a switchgear manufacturer?

If you get this wrong, how long will it take for a new transformer to be built?
If there is catastrophic fault resulting in major injury or fatality, who is liable?


----------------------------------

Your body might be a temple. Mine is an amusement park...

 

[soare0]

Thank you!

First: I didn't knew that R134a is combustible. All the MSDS sheets I've read didn't say this. Oh, only that it can explode when it is mixed with air. Funny indeed!

Second: the transformer is a prototype, build by me. So what to say about the manufacturer opinions...

It is a small one, about 2kVA/50-100kV. It must be delivered very soon and it is a big problem with SF6 delivery.

However, I agree with you and I think I will wait and I'll stick around SF6.

Paul N.

 

[ScottyUK]

You are making an SF₆-insulated transformer? Why?? Sounds like an interesting project! Is any reason why the old fashioned cast resin or oil / silicone fluid designs won't work?

Glad you're playing safe - I must say that I had envisaged one of the huge transmission transformers like those made by Toshiba for example when I posted my caution. A 2kVA SF₆ transformer never even occured to me.

http://www.toshiba.co.jp/f-ene/tands/english/trans/l_gas.htm

----------------------------------

Your body might be a temple. Mine is an amusement park...

 

[CarlPugh]

You may want to consider silicone transformer liquid. Manufactured by Dow Corning and ?
Silicone oil is a good insulator, better than SF6 at atmosphere pressure. Much better heat conductor.
Problems:
Expands, must have some provision for expansion.
Not compatiable with a few materials.
Heavier than SF6.
Expensive, but so is SF6.
Silicon oil is slightly flammable. (Much less than
mineral oil)

 

[soare0]

1. Well, there could be some reasons for somebody building a SF6 transformer, even at 2kVA:
- copper wire. The minimum order for enameled wire - transformer oil resistant grade - was too expensive and, of course, not on stock.
- weight. Well, it is a real pain to have a 1000 kg toy to work with in laboratory (it is for an automated HV testing power supply.
- again,money. 500l SF6 at let say, 2-3 bar it is much cheaper than 500l transformer oil.
- you can put some nasty components (HV voltage dividers) in the same insulation medium like the transformer - in the same enclosure - so if there is a leakage, it is only in one place
- it is chemically inert. With transformer oil you must be a little more... cautious.
- filling the tank with oil is a pain in the... project. The procedure seems to me very complicated (vacuum-heat-vacuum-heat-and so on).

But the main reason was the first, the rest I've discovered on the way. However, modifying a box designed for oil to suport 2 bar SF6, well, believe me this is a nightmare but still possible.

2. Silicone oil. Great ideea,thank you very much, I almost forgot about it. Only that it is much more expensive than SF6 filling. And there are 500l!

I'd better buy the transformer (10000 euro without VAT, a 120kV/10kVA transformer). My overall costs were under 3000 euro (including 40kg SF6,and VAT). And I built it how I liked.

The real power of my transformer it is more than 4kVA at a maximum of 100kV but I only need to use it at 2kVA. However, the costs are not much greater for building let say, a 20kVA transformer. The oil is the most expensive stuff for this but very convenient from some point of view.


Now, I have some questions:


1.The transformer was designed for 100kV maximum voltage (continuous use) in transformer oil.
I think that SF6 at 2.5 bar (absolute) have the same dielectric strength like a good transformer oil.

In this case, if I will limit the maximum voltage at 50kV,it may be ok to use only 1.25 bar absolute pressure for a good insulation. Well, let say 1.5-1.7 bar,for safety.
What do you think?

2.How is the behaviour of the SF6 at Corona discharge? It is a self healing gas, but even so, untill healing...

Thank you for your support.

 

[CarlPugh]

In answer to your questions,
1 If my memory serves correctly, your pressures are correct.
2 Corona in SF6, Have heard different opinions, however the general opinion is that corona in SF6 creates an acid. Continous corona will create a large amount of acid that will destroy nearly everything that it touches.

Every application should be judged on it's own merits, however in the past have researched SF6 and decided not to use it for the following reasons.
1 In the USA there are strict requirements for pressure vessels and over something like 12 or 14 PSI the requirements get tough.
2 SF6 is difficult to work with, you cannot see it. Cannot (easily) tell if container is filled with air or SF6 or the percent of either.
3 SF6 in the USA is getting difficult to obtain.
4 There may be some strict requirements to reclaim the SF6 if a tank is opened to make repairs or change components. You should check the requirements in your country for the requirements.
4 A small leak in an oil tank is not significant. In SF6 it is critical.

Some suggestions. (you are probably already doing these, and no reflection is inteneded on your design)
1 No sharp edges or high electrical stress areas.
2 Use some standards that have been proven in use. Don't for instance say that the breakdown gap for 0.1 inch is 35 kV and then say 0.2 inch is good for 35 kV operating.
3 Use surge shields connected to the start and finish of the high voltage winding.
4 Application affects the design tremdously.
4A If there are no arcs, then it's an easy design.
4B If there are arcs in air, then a more conserative design is required.
4C If there are arcs in a vacuum, you will have problems. Design the transformer like a tank, and expect it to fail.
5 If you are building one transformer, it may be OK to use an unproven insulating material. If you are buildings 500 transformers, you should not consider any material that does not have a proven track record. (using a prototype for a month is not a proven track record)
6 Don't have any floating hardware inside the tank.

Some questions.
If you are building 500 of these, why are you concerned about a minimum order of wire?

How did you specify the wire to use in a mineral oil filled transformer. There are many wire insulations that can be used for mineral oil filled transformers and it doesn't seem possible that none of these are availiable.

Are you building a 50 kV or 100 kV transformer?

SF6 does not cool transformers as well as mineral oil or silicon oil? The transformer KVA rating in SF6 will be much less than in oil.

Sounds like a fun project.
Good Luck
Carl

Safety remark.
Nearly forgot, SF6 is heavier than air. If there is a SF6 leak and some sort of pit or basement is nearby, the pit or basement could fill with SF6.
There will not be any oxygen in the pit or basement and anyone entering could be killed.

 

[itsmoked]

CarlPugh; Intersting comments.

I think the OP stated 500Liters not 500 units. Which probably does relate back to quantity problems.

 

[cuky2000]

The refrigerant, Freon R134a [CF3CFH2] (tetrafluoroethane) is not a replacement of the SF6 for dielectric and arc quenching for high voltage applications. However, R134a has excellent dielectric characteristics comparable with SF6 as shown in the information enclose below.
One disadvantage of R134a is that the dissociation products are highly corrosive, so it is important that operating voltages remain well below corona starting voltages. In presence of air, R134a forms highly reactive nitrogen oxides and other corrosive compounds, particularly if there is any water vapor present. Under high pressure, air can also support combustion due to the oxygen content.
Bellow is a comparison of SF6 and R134a including dielectric characteristics collected from difference sources.

 

[ScottyUK]

Carl raises some good points:

As I recall, SF₆ decomposes to form metallic fluorine in the presence of an arc. Fluorine being the most chemically active of the halogens, it reacts readily with water to form HF, which is hydrofluoric acid. Nasty stuff - think 'Aliens' movie and you're on the right lines. The fluorine is ok while the vessel is full of SF₆ but becomes problematic when the vessel is vented and opened up to atmosphere.

The problems with removing moisture from windings will apply to a SF₆ design as much as an oil-filled design as far as I can see. SF₆ switchgear has to be clinically clean and absolutely dry. Usually prolonged purge with dry nitrogen is required for bus duct and switchgear, with vacuum being applied before the SF₆ fill.

SF₆ must be reclaimed and captured in a gas cart or similar. It has major ozone-depletion potential and its release is reportable (in the UK at least). You can't - or shouldn't - just vent it to atmosphere.


----------------------------------

Your body might be a temple. Mine is an amusement park...

 

[soare0]

Well, at startup, I didn't suppose that building a transformer could be a thrilling activity.

ScottyUK: -Yes, indeed, there are environmentall problems with SF6, but if I remember correctly, not with the ozone but with warming potential. However, it is bad anyway. And this is the reason for bringing in discution R134a.
-In a normal transformer it shouldn't be an electric arc present, except in the switchgear, but this one is a laboratory transformer and it hasn't switches in it. So it shouldn't be a problem with decomposition products by means of an arc. I think.

Cuky2000: -The tables submited by you are for freons indeed, but R134a is not there. Or R113 is the same with R134a? I think that the breakdown voltage is somwere there like for R12, but I didn't found any datasheet.
- The dissociation products are highly corrosive for both gases, but SF6 is self healing (I don't know the recovery time, anyway). So if there is no electric arc to decompose SF6, would be the replacement with R134a a problem?
I mention again that atmosferic overvoltage (lightning) shouldn't be a problem (only if the user will keep the window open ), but I'll specify this in MSDS).
- Maintain voltages below corona forming. Well, may God help me to get rid of Corona. An ideea about how to track if there it is a Corona development potential is to keep the transformer at atmosferic pressure and to apply at the primary winding a voltage at such level that the secondary E-fields to be the same like in presurized tank, and to look about Corona. At midnight time...spooky thing!

CarlPugh: - The transformer is 220V/100kV, because it has a primary limiting resistor, in such way that at maximum current the maximum voltage will be 50kV.
This transformer will operate in a test station for dielectric materials, so there will be MANY shortcircuits. I built some HV test stations before but only up to 15kV and the transformers were bought. However, I solved the current limiting problem in the same way.
Of course there are other means too for limiting the currents and the voltages, wich are included in the automation station for this project. Two PLC's and a computer are making the "smart things".
One question: why do you say " If there are arcs in a vacuum, you will have problems."? In the vacuum can't be a pressure rising. Or maybe I didn't understood your statement?
- A pressure sensor and a manometer it are mounted on transformer so it will not work in the case of a leakage. If my container is filled with SF6/R134a above atmospheric pressure, I think that is difficult to have air entering in sistem. But indeed, if the presurre get down to 1 bar, it may be safe to vacuum again the transformer and only then refill it.
- About vacuum. If I make vacuum before filling, the water must get out with air I think. The insulator between windings is Nomex. I think it should not absorb water. Is this so?
The vacuum pump can make 10Pa (150microns Hg column?). Would be enough this vacuum? And how long I must mainatain it?
I have not the facilities for passing dried N2 before filling so I must see if another mean to get out the water vapors is possible. Wouldn't it go along with air?

Men, am I in trouble?

Thank you again!

 

[ScottyUK]

soare0,

Have you considered obtaining a transmission grid voltage transformer on the surplus market? These are typically oil filled, but there are SF₆ types available too although these are ofhen designed to be integrated into switchgear. That way you have a product where someone else has done all the R&D and debugged the design for you. And it will be safe. A VT typically has a burden rating quoted which is a small fraction of thermal rating. Perhaps scottf is following this thread and will comment further?


----------------------------------

Your body might be a temple. Mine is an amusement park...

 

[CarlPugh]

soareO

Arcs in vacuum are nasty because vacuum is such a good insulaton. There will be a loose particle or sharp point or whatever and a burst of current. The vacuum will open the circuit when the current is high and there will be a high voltage transients. This may repeat many times.

Three possible methods to determine when the insulation under vacuum is dry.
1 Put a bucket under the vacuum pump outlet. When the vacuum pump stops discharging water, the insulation is dry.
2 Measure vacuum, when vacuum improves, the insulation is dry.
3 This works with paper insulation in an oven, with nomex in a vacuum? Measure the power factor of the major insulation capacitance. When the power factor increases significantly, the insulation is dry.
Problems with above.
If the tank leaks, you will never get a dry indication. Tank should be pressurized overnight at 5 PSI. If there is any drop in pressure, there is a leak.
Vacuum drying is a slow process. Days or weeks depending on the transformer design. Heat on transformer will speed up drying process.
Too good a vacuum is bad. With a good vacuum, some plastics and other materials will evaporate. Single stage rotary (vane type) vacuum pumps are frequently used.

 

[itsmoked]

Another way to check for leaks would be:
1) Vacuum
2) Add traceable refrigerant
3) Use a refrigerant detector

This would let you find The leak as compared to; its got a leak..somewhere.

 

[cuky2000]

Q1:…The tables submitted by you are for freons indeed, but R134a is not there. Or R113 is the same with R134a? I think that the breakdown voltage is somewhere there like for R12, but I did not found any datasheet. The breakdown voltage of R134a according to the ASTM standard D-2477, 1984 ranging from 6.2 kV to 6.6kV at standard ambient pressure and temperature. For additional reference check table 1 in the enclose link

http://www.solvay-fluor.com/docroot/fluor/static_files/attachments/i_11.pdf

Q2:….So if there is no electric arc to decompose SF6, would be the replacement with R134a a problem? Under normal operation without arching, most likely R134a will work OK.
[blue]^{NOTE: Beware that arc may be develop by winding failure or by an external surge overvoltage source.} [/blue]

Q3:...atmospheric overvoltage (lightning) should not be a problem (only if the user will keep the window open ), but I will specify this in MSDS). Overvoltage may be produced by traveling wave if lightning strike nears the power distribution system. Keeping the window closed or open may not be relevant. Proper surge arrester may be considered to mitigate any dangerous overvoltage.

Q4: An idea about how to track if there it is a Corona development potential ……
a)During design and construction phase, select transformer winding wire size with sufficiently large. Avoid sharp edges and small radius to minimize high concentration of charge and discharge OV.
b)During operation, corona may be detected by monitoring partial discharge development.

I hope this could help.

[blue]^{PS: In the future, consider a small size (10 kVA) Station Service Voltage Transformer (SSVT) similar to the one shown below.}[/blue]

http://paradoxecorporation.com/Productos/Kuhlman/StationServiceVoltageTransformer.pdf

 

[Laplacian]

You can't rely on vacuum alone to deplete moisture in an enclosure; at least not in humid environments. Simply applying a vaccuum will cause some of the moisture to form ice where it will never be drawn out with the air. Best to use a gas to displace the moisture (nitrogen), then pull a vacuum as someone else mentioned.