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Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
Hi all,
I recently discovered by some searching that distilled water has an extremely dielectric constant of around 80 i.e: 80 kV/mm. This made me wonder due to it's constant being about 40 times higher than mineral oil, why don't they use it in transformers? I would think it has a much better thermal conductivity than oil.
At the Haywards substation, they use a closed water cooling system for the HVDC thyristors and I saw their distilling plant for the water that circulates around the system.
How could someone create a batch of extremely pure distilled water and what equipment does it involve?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You don't use it in transformers, as you don't have any use there for high dilectric constant. You need high dielectric strength.
However, why isn't it used in capacitors? Dielectric contstant is at a premium there.
unfortunately a) residual conductivty when pure is many orders of magnitude worse than most dielctrics b) conductivity when impure is many orders of magnitude worse than that c) water is really really easy to make impure (with its high er, it rips ionic compounds apart)
It *is* used by Sandia for energy storage for their Z pinch experiment as the dielctric in 1ohm transmission lines which get charged up to 1MV, where it's worth the hassle of trying to use it for that extraordinray dilectric constant. It's continuously recirculated through an ion exchange filter to try to keep it pure, it's cooled to reduce residual conductivity, and the line charged and discharged within a second
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yep, dielectric constant and dielectric strength are two different things. Water's dielectric constant of 80 means that if you took a capacitor with air dielectric (or strictly speaking, vacuum) and filled it with water, its capacitance would increase by a factor of 80.
Highly purified water has a reasonable dielectric strength, but only for very short pulses like the ones used in the Z-pinch.
Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
OH ok, thanks for the info guys. Makes sense - distilled is very easy to impurify and once it's impurified, the dielectric strength does lower drastically so I can see why you would need a distilling plant to constantly keep the water clean.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Actually the dielectric strength remains pretty high. That's the electric field at which you get a sudden breakdown. Has nothing to do with the increasing conductivity as ionic impurities build up in the water.
The unavoidable conductivity of water, even in ultrapure form (multi megohm-cm), limits its use to capacitors or low-Z transmission lines that do not need to hold a charge for as long as a whole millisecond.
With more conductive water, the capacitors might self-discharge in microseconds. Not the same as breaking down from overvoltage.
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Dielectric constant of distilled water and uses
