HV Winding Insulation

[mbrooke]

How are high voltage windings insulated in power transformers? I take its not enamel? Is it the paper they are wrapped in?

 

[LarryFine]

The volts/turn ratio is the same as the secondary, so there's no issue for adjacent windings. I believe they use insulation between winding layers, and they keep the winding ends away from each other inside the enclosure.

 

[mbrooke]

Alright, looks like its covered with enamel. What about 15-69kv and 115-765kv?

 

[LarryFine]

Alright, looks like its covered with enamel. What about 15-69kv and 115-765kv?

Do I have "Google" written on my forehead?!

j/k. I really don't know the details, but I'm sure you could search for them.

I imagine the former range is done like the video. I imagine the volts/turn ratio is still the main determinant.

 

[electrofelon]

How are high voltage windings insulated in power transformers? I take its not enamel? Is it the paper they are wrapped in?

I have wondered similar things. Two adjacent wires of course wont have full system potential between them. What is the typical or maximum potential difference between two adjacent wires? Wouldnt "above and below" have much more difference than "left and right"?

 

[sameguy]

Types of Transformer Windings: Cylindrical, Helical, Crossover & Disc Winding | Electrical4U

Core and Windings of Three Phase Core Type Transformer There are different types of windings used for different kinds of applications and arrangements. Windings are the conductors wrapped in various forms like helical, disc, cylindrical, crossover which generates MMF that is carried by the core...

www.electrical4u.com

Try this

 

[rnatalie]

Some of our larger step-down transformers just use air to insulate the adjacent turns of the windings.

 

[mbrooke]

Do I have "Google" written on my forehead?!

j/k. I really don't know the details, but I'm sure you could search for them.

I imagine the former range is done like the video. I imagine the volts/turn ratio is still the main determinant.

Nope, but you're smart enough to have the answers!

 

[mbrooke]

Types of Transformer Windings: Cylindrical, Helical, Crossover & Disc Winding | Electrical4U

Core and Windings of Three Phase Core Type Transformer There are different types of windings used for different kinds of applications and arrangements. Windings are the conductors wrapped in various forms like helical, disc, cylindrical, crossover which generates MMF that is carried by the core...

www.electrical4u.com

Try this

So they just use paper?

 

[paulengr]

So they just use paper?

Almost, sometimes.

If we have say 100 turns with 100 V in a form wound transformer (not random wound) the voltage turn-to-turn is 100 V / 100 turns = 1 V. There is usually thin almost transparent enamel on the wiring.

If we need more it gets separated by wood spacers and/or paper. The whole thing is soaked in varnish or epoxy, tending more to epoxy these days. On larger transformers we use VPI...put a vacuum on it until bubbles stop, then pressurize to push varnish/epoxy in, cycle several times until there are no voids left. The varnish/epoxy+wood/paper composite is part of the insulation. The rest is the air/oil which both insulated and cools the coils, and is why we need all those little wooden spacers in the first place.

 

[mbrooke]

What about the side resting against the metal core? Do the wooden spacer reduce efficiency? Those gaps look huge, where as smaller transformers literally have the wingdings on top of one another touching the windings between them.

 

[JoeStillman]

Here's a look at the innards of a 69kV : 13.2kV 7.5 MVA substation

 

[mbrooke]

What under that paper(?)? Bare or enamel wire?

 

[JoeStillman]

I think it's enamel and that isn't paper. Its some kind of polymer.

 

[paulengr]

I think it's enamel and that isn't paper. Its some kind of polymer.

We still call it paper. It is upgraded paper but still cellulose product. The original paper had a maximum temperature of 55 C rise. Now it’s 65 C rise.

 

[mbrooke]

What is it made of specifically?

 

[paulengr]

What is it made of specifically?

It is just paper that has been “chemically modified”.

http://www.electro-wind.com/10-mil-0-25-mm-thick-grade-k-thermally-upgraded-kraft-press-paper-flexible-laminate-105-c-brown-48-wide-roll.html

Cellulose Paper - Weidmann Electrical Technology AG

Cellulose insulation papers designed for a wide variety of applications, combining experience and the latest equipment to provide best in class solutions.

www.weidmann-electrical.com

Of course DuPont has more expensive stuff.

Transformer Insulation Paper | Nomex® 910 vs Kraft Insulation

Combining an electrical-grade cellulose and high-temperature aramid polymer, transformer insulation paper made with Nomex^® 910 outperforms kraft paper and TUK.

protectiontechnologies.dupont.com

The thing is most of the insulation value is coming from the varnish or epoxy. So we really need something porous to soak up the liquid insulation and provide enough of a structure to hold it.

One of the oil tests for transformers is DP (degree of polymerization). It measures the molecular weight of cellulose in the transformer oil (saw dust). New transformers start out around 800+. When it drops to 100, the transformer is structurally unsound. They can also sample the insulation directly for this test. The acids that oil develops when it breaks down attacks the paper. The water that the paper breaks down into attacks the oil. So maintaining moisture as low as practically possible prevents this chemical reaction from occurring which makes oil filled transformers last. In air cooled transformers the epoxy/varnish is the defense against moisture and no oil acids are present so although they have a reputation for shorter life, I’ve dealt with one from 1978 (1000 kva 7200:480) that has seen hard life on a piece of mobile mining equipment showing no signs of failing any time soon. It’s younger than me but still plenty of years left on it.

So paper is the product of choice along with outright wood wedges for good reason and there is no reason to change.

 

[mbrooke]

What about the wire itself. Is it just bare copper?

 

[paulengr]

What about the wire itself. Is it just bare copper?

Yes sometimes. The ampacity is greatly increased by increasing surface area so larger transformers often switch fo a rectangular profile instead of circular.

Also aluminum is more economical so the vast majority of transformers are aluminum except special uses.

 

[mbrooke]

So with all the spacing, is the transformer larger or less efficient?