I got my hands on an old 3kVA step-down control transformer that I can adapt into a beauty of a 1:1 isolation transformer. But I'm wondering how worried I shouold be about insulation breakdown on my floating secondary?

Is there any way to stabilize the secondary voltage relative to ground? I was thinking that something as simple as grounding the transformer case might provide a secondary voltage reference via capacitive coupling. Yes/No?

I don't have any information on the BIL, all I know is the maximum primary voltage is 504V.

Anyone have any answers? Thanks.

-John

I have seen some capacitive coupling at transmission voltages on an ungrounded potential transformer secondary and it wasn't pretty.
However, I have no idea if it would ba a problem at your voltage.
How do you plan to energize the primary? I'm thinking if you ground one side of the primary you won't have a problem.

But I'm just guessing.....

John - I'm kinda confused about something. Maybe I'm misreading here, but how are you gonna make a 1:1 isolation out of a step-down transformer?

Nevertheless - just because it's an isolation transformer does not preclude you from grounding it (the secondary, that is). And grounding the case won't (or shouldn't) have any affect on the secondary, separately derived, electrical system. I'd like to believe there's no connection there.

My suggestion, if you really really want it to be floating and ungrounded, is to look into high resistance grounding. Caution here - although it will sort of establish a reference for the system, it would probably not help in clearing a fault. I'm sure other members here can give more info on this.

Back in my Navy days on a submarine (SSN), all the electrical systems were ungrounded. Equipment grounds were continuous everywhere, but the systems themselves were ungrounded. They consisted on 120V and 450V AC, 120V 400Hz AC, and around 300VDC - all completely ungrounded.

Can you provide a little more info regarding your transformer and your application?

Radar

So I take it that you want to clamp your floating secondary from going too far from ground potential without losing your isolation??? You might try placing 2 equal value resistors of appropriate wattage in series across the secondary. Tie the midpoint to ground through 2, series inverse zener diodes or a bi-directional transorb of the clamping value that you want. The diodes should never conduct under normal circumstances. Is that what you are looking to do?
Joe

[This message has been edited by JoeTestingEngr (edited 02-12-2006).]

Are you planning on winding a new sscondary on the transformer?

What is it going to feed?

If used as designed, this transformer has four windings, two primary and two secondary. All of them are pottend and ungrounded, but depending on how the taps are connected, any or non of them can be grounded.

The two secondary windings are identical; they have a 1:1 ration. So, if I can use one of those as my primary and leave the other floating and use it as my secondary, it makes me a nice isolation transformer.

The reason I'd like to have a floating secondary is so I can use this transformer to power equipment on my test-bed and it will help reduce the risk of shock by removing the potential-to-ground.

My concern with leaving the secondary totally ungrounded is that the insulation on the secondary windings may be stressed to the point of breakdown. I think using a very high-resistance ground might do the trick.

JoeTestingEngr, I was simply thinking of grounding one side of the secondary through a 120k resistor, which would limit any current to 1mA. With the setup you describe would the secondary be basically ungrounded all the time, unless potential-to-ground exceeded the breakdown voltage, and then the secondary would develope a high resistance ground , right?

WFO, what happened with the ungrounded PTs that was so bad?

Thanks for the replies so far.

-John

John, I think we'll probably all agree that this won't be a 3KVA transformer in the mode that you are planning to use it. The thing that doesn't thrill me about your 120 Kohm soft ground concept, is the possibility that it becomes a hard ground without your knowing it. That is why I like center tapped or derived center tapped sensing or clamping.

Consider other possibilities like those used in "Hot" and "Grounded Structure" relaying. It sounds like your unused windings would be at much higher voltages because of the way you are cheating on this application. But what if you use one of the windings to produce a hi-z, low voltage, floating supply. That supply can be imposed between the center tap reference and ground and sensed to make sure it's there. If a 20 some volt supply drops to near zero, you detect it as a grounded system. If the system gets elevated, it can be sensed as "hot". If one leg or another gets grounded, you can detect the half-voltage peaks at the center tap. It all really depends on how fancy you want your test bed to get.
Joe
Oh, and to your specific question, right.

[This message has been edited by JoeTestingEngr (edited 02-12-2006).]

John, if this transformer is a typical 120/240v to 12/24v unit, you will have no problem. Energizing one primary winding at 120v will produce 120v in the other primary winding, and 12v in each secondary winding.

The secondaries can be left floating, as their output voltages will not vary (much) with the primary-to-primary current transfer. You can use either or both secondary windings if you have an application for them.

Do keep in mind that your current capacity at 120v is not limitless. At 3Kva, with the primaries in parallel, I get 25 amps, meaning 12.5a per winding, and 250 amps with the secondaries in parallel.

Unless I'm wrong, you should be able to get that 120v at 12.5a from your 1:1 use. That's quite a beast! I'd love to have it for its low-voltage uses. Let me know if you ever want to get rid of it.

On second, thought, forget it. It won't work. Let me do you a favor and take it off your hands. I'll even cover shipping costs, because I'm such a nice guy!

Quote:
"WFO, what happened with the ungrounded PTs that was so bad?"

We were going to do a Doble power factor test on an old oil filled potential transformer that was 69 kv primary (yep...phase to phase) and 120 volt secondary.

The primary fuses were GE's that were so old and decrepit that someone had actually wrapped them with #12 AWG to preserve continuity through them (so much for coordination). Since they were in such bad shape, we decided to make sure there was absolutely no load on the secondary. We opened the junction box and found that each leg (the ground and the hot) had a fuse in them. Not knowing any better, we pulled both fuses.

Instantly there was a buzzing and popping in the junction box. We went to get a voltmeter (one of those old Tripletts with a re-settable breaker) and it tripped the thing BEFORE WE EVEN TOUCHED THEM TO THE TERMINALS!

So we put on the high voltage gloves, tried it again, and pegged the meter on the 10 Kv scale.

What had happened is by pulling both fuses, we had removed all reference to ground on the secondary. Being a phase to phase primary, there was no ground reference there, either.
So the thing acted like a capacitor, with the secondary being the other plate. The popping we heard was the voltage building up in the junction box until it reached a point where it flashed over.

If we're talking about a small-scale supply for bench-testing equipment, then I'd just leave the secondary floating. I use a 1:1 xfmr like that when working on vintage AC/DC live chassis radio and TV equipment.

I don't know what you'll be running on the testbed, but in some cases equipment you connect might apply an indirect ground to the transformer anyway. For example, I can isolate a live-chassis TV from the supply not just to reduce the shock risk but also to allow the direct connection of a grounded oscilloscope or signal generator which would not otherwise be possible.

I think I'm gonna go for the "soft-ground" concept that JoeTestingEngr suggested, with a little indicator so I know if/when the secondary grounds-out.

Thanks for the helpful replies, gentlemen. I appreciate it.


-John

You're welcome John. Please forgive me for lusting after that transformer of yours. You might be able to use an unused primary to rig up a "press to test" function for your ground detection/clamping scheme. If you use a very small fuse and a current limiting resistor, you can test the function, and not just the lamp. I know that it might seem like overkill but since we're in a theory forum, might as well brainstorm.
Joe

John, One thing to consider is the fact that many indicators require more current than you are planning on sensing. For instance, you originally mentioned limiting your ground current to 1mA. An LED is normally biased at about 10 times that. Most lamps will be very dim if you're lucky at 1mA. So I was thinking that you might just try putting a neon bulb at your center point reference. I seem to recall that neons conduct at about 60 volts. Neon lamp assemblies contain current limiting resistors and flow currents more on the scale you are working with. A "T" network of 100ks across the top with a <100k (TBD) on the bottom going to ground through a neon might hold the winning ticket
Joe

[This message has been edited by JoeTestingEngr (edited 02-16-2006).]

And don't forget to do an insulation test on the windings.

primary to earth.
secondary to incoming earth.
primary to secondary windings.

In case of a 110 Volts transformer use 250 Vdc.
In case of a 240 Volts transformer use 500 Vdc.

A Megger reading at least gives you an idea of the overall condition of the insulation materials used in the TF.

Should be 50 M.Ohms or better for a 240 Volts TF.
Chhers Ray

I don't know what are you planning to do, but why do you want a floating secondary?

Regards,

Joe.-

I don't know what are you planning to do, but why do you want a floating secondary?

Regards,

Joe.-

Joe,
Things can get very exciting while troubleshooting TVs or other appliances with a hot chassis. The scope probe ground lead is at ground potential and the chassis ground isn't. Sparks will fly if you try to connect the two. The best bet is to float the appliance. I didn't have an isolation transformer at home. My scope had an input transformer and three prong plug. I would use the 2 prong adapter to deliberately float my scope, since I couldn't float my TVs. I realized that I had a shock potential if I touched earth and the scope at the same time. It's much safer to keep your scope grounded and float the appliance.

That said, I always liked the saying, "Whatever floats your scope."
Joe

[This message has been edited by JoeTestingEngr (edited 02-22-2006).]