Power company transformer

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sparks1

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I've noticed that on most single phase pole mounted transformers, that are used for 120/240 volts secondary, have one primary H1 line feeding into the top of the transformer.
Can someone explain, why is this?
How is the primary H2 connection made?
It appears from a distance looking up, that the H2 connection come off the side of the transformer and is grounded to the secondary drop and ground rod at the pole.

What is the device that is wired in series with the primary fuse called. I looks like a lighting arrestor?
 
Re: Power company transformer

The transformer you describe is common, and is suitable for single phase use in solidly grounded wye systems, where the H2 can be grounded. The H2 is actually attached to the case, and the exterior case ground carries current back to the system neutral. It is not suitable for installation in a three phase bank.

As far as the thing in series with the cutout, it probably isn't an arrester. They are connected in parallel. It might be a current limiting fuse, or could you be looking at the H1 bushing?
Jim T
 
Re: Power company transformer

It is not suitable for installation in a three phase bank.

Actually Jim, we will connect them into an open wye - open delta and in a wye - wye configuration. Of course, we use the single bushing transformers for 1? all the time. We use two bushing transformers and connect them into a wye with a floating neutral - delta configuration.

What is the device that is wired in series with the primary fuse called. It looks like a lighting arrestor?

Lady, I am a loss also. I wouldn't think it is a current limiting fuse for a transformer but we do use current limiting fuses on capacitor banks. I would have to see a picture to be able to answer your question.

A typical installation today consists of a stinger (wire) from the primary phase to the cutout, the cutout, the stinger from the bottom of the cutout to the top of the tank mounted lightning arrester and on to the single primary bushing. :D
 
Re: Power company transformer

Charlie
You are correct, it is appropriate for 3 phase banks. I found a reference to ANSI C57.12.00 which describes that particular type as suitable for wye connection in a solidly grounded system.
Thanks
Jim T
 
Re: Power company transformer

Jim T,
It might be a current limiting fuse.
Decription of it, It looks like a roman candle. Maybe it is a current limiting fuse?
It's not the H1 bushing.
 
Re: Power company transformer

Charlie,
Do you have a diagram of how the primary wye H2 lead is connected to the transformer case to show how the exterior case ground carries current back to the system neutral. I'm assuming that its a wye,wye and that the current goes back from the center of the primary wye to the center of the secondary wye. Where doe it go form there?

I can't stay up late.

[ February 16, 2005, 09:31 PM: Message edited by: sparks1 ]
 
Re: Power company transformer

Sorry, I don't have a diagram of a wye - wye configuration. However, connect the primary into the wye configuration and connect the tanks to the system neutral and pole ground.

On the secondary side, connect the coils into a wye configuration (120 volt coils since the coils must be paralleled) and connect the center point to the system neutral and the pole ground.

The common point of both the primary and secondary are tied together and grounded. On the primary side, the current flows into each individual bushing, goes to the neutral point and back out through the other two transformers. This is reflected on the secondary side of the bank. The current doesn't pass from the primary to the secondary side through the neutral connections, it is through the interactions of the magnetic flux with the coils. :D
 
Re: Power company transformer

Charlie,
When you say the common point of the wye and the primary and secondary are tied together and grounded, do you mean a wire connects from the primary common point to the secondary common point and then to the pole ground?
I realize that the transformer windings are magnetically coupled. What I'm wondering about is the neutral current path on a 120/240 volts Delta mid point secondary.
What is the returning neutral current path from the center tab. We know it passes through the other windings on the negative half cycle. This neutral current, therefore must be induced back into the primary from the secondary and from there back to the power source (power station) on the negative half cycle ?
Where does is go from there?

Does any of this neutral current returning back on the negative half cycle at the center tap
go to ground at this point under normal conditions?
My guess is, it would take the least path back through the transformer...

The device I mentioned earlier in the post, that comes after the primary fuse looks like a roman candle. I don't have a picture. What is your guess ?
 
Re: Power company transformer

The roman candle thing probably is a current limiting fuse. They are often used where there is a likelihood of pole mounted transformer failure. A utility that I worked for about 15 years ago had a pole mounted transformer fail violently, spreading hot oil all over a parking lot. They had to paint a bunch of cars as a result. After that, we came up with an application guideline to install CL fuses in all pole mounted transformers, existing and new, when subjected to a certain high side fault current level.
Jim T
 
Re: Power company transformer

Jim, that is interesting. We do not use CL fuses on anything but capacitor banks.

Does any of this neutral current returning back on the negative half cycle at the center tap go to ground at this point under normal conditions?

No, the connection to ground does not enter into the workings of the transformer at all. The ground point is to stabilize the voltage to ground, provide a path for lightning strikes, etc. It is no different than a car battery that is insulated from earth and you connect the positive post of the battery to earth. Nothing happens and no current flows. :D
 
Re: Power company transformer

Charlie
As I was leaving the utility industry back in the late 80's, "Probability Of Rupture" was an item to specify in cap banks. We looked for a less than 10% probability. As you know, the capacitor tank is very small and an arc inside will quickly generate sufficient gas to rupture them.
The utility I worked for experienced a problem in a couple of transformers where the design of the gas release system was not adequate to keep the tank from failing. I was not involved in the investigations, but knew about them.
Our system was a 13.8/23.9 system, and within a few miles of the substation the fault currents were high enough to cause problems. I don't remember the exact fault current thresholds that caused problems. To this day, they install CL fuses on all pole mounted transformers within that radius of the substation.
Jim T
 
Re: Power company transformer

I wonder if our system is different (as far as transformer faults go) since we generally don't use transformers smaller than 25 kVA on a 13.2 kV/7.62 kV system? :D
 
Re: Power company transformer

Charlie
I've worked in numerous 15 kv systems, and don't remember a problem in 35 years. In fact the only time I have been around that kind of tank failure was this one area.
These failed back in the early '80s. As far as KVA, the larger the tank, the more likely the fault produced gasses will be contained. Look at padmounts. They have a much larger tank, and often have CL fuses anyway.
I'm sure that manufacturers can provide info on advances in tank rupture technology.
I would be interested in hearing if others use or see CL fuses in polemount transformers.
Jim T
 
Re: Power company transformer

charlie,
Can you confirm this for me, that the neutral current passes thru from the secondary to the primary on the negative half cycle and goes back to the power station?
 
Re: Power company transformer

I need help from someone to explain this but the primary current will not mix with the secondary current even though the have a single point connection. The primary current will return to the substation through the MGN and the earth. The secondary current will return to the transformers through the secondary neutral. :D
 
Re: Power company transformer

It was once explained to me that a transformer is nothing more than a generator. Isolated as if it was not connected to any other source of power. The primary is the fuel that it runs on, as it is loaded the more fuel it uses to produce more output. The fuel volume is the primary current.
 
Re: Power company transformer

Charlie, help is here! The primary and secondary currents do not mix because current must flow in a closed loop. There is no such loop connecting primary and secondary, only a common ground. For that matter, there will be no secondary neutral current at all if the load is balanced. Bet you knew that!
 
Re: Power company transformer

Hurk, you can model a transformer as an AC source in certain problems, but its operation is quite different from that of an alternator.
 
Re: Power company transformer

Rattus,
Take for example, a transformer with a 120/240 volts delta connected secondary with the center tapped neutral. This carries the unbalanced load to the center tap point and then through the winding of the closed loop as it continues to flow around the closed loop the current is magnetically induced back into the primary and back to the poco in either direction. Is this the correct path!
Thanks.
Phil
 
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