Open Wye/Open Delta Voltage Readings 3

[brhodes]

A Lineman approached me today asking my opinion on something. We have an open wye/open delta connected transformer bank and I have a few of questions.

1)Should the primary neutral point float or be grounded and why?
I understood that if the connection is a wye-delta the primary neutral point should float, but never understood completely why.

2)The secondary voltage is 480V with no neutral coming into the service. Inside the building what would the phase to ground voltage be?
To my knowledge it's variable and unpredictable.
The electrician is concerned before connecting his load. He only has 3-phase 480 volt load with the exception of one single-phase dry type transformer (480 to 120).

Here are the voltages the lineman had given me that he read.

~503 volts phase to phase (Our voltage runs a little high), which is okay and works fine for his load.
A-phase to Gnd: 640V
B-Phase to Gnd: 504V
C-Phase to Gnd: 280V

3)Are these numbers common for an ungrounded secondary or floating neutral at the transformer? Would having no reference point at the transformer bank cause these phase to ground voltage readings?

I am going to the site tomorrow. I will verify the primary and secondary connections and see if the primary neutral is floating or not.

With his load being only 3-phase 480 volt and a 480 to 120 volt dry-type transformer, my thoughts would be that he is okay. I would like to have more opinions on it since the phase to ground voltage readings is unusual.

Any help you guys can give would be very much appreciated.

Thanks!

 

[wfowfo]

The primary neutral point needs to be grounded. Otherwise, you are simply connecting it single phase (line to line).
If it isn't grounded on the primary may be why your secondary voltages are so varied. However, even when done correctly, open delta configurations don't have particularly good voltage regulation characteristics compared to a closed bank.

 

[jghrist]

With the secondary ungrounded, the only reference to ground is through cable capacitance, which will be different for each phase, giving different voltages to ground.

As wfowfo said, you need to have the primary neutral grounded, otherwise it isn't an open wye connection, but a phase-to-phase with the two transformers splitting the voltage. The secondary will have 480V across two terminals and 240V across the other two, all in phase or 180° out of phase. Since this isn't what you have, you're neutral is grounded.

The floating of the neutral is for a regular wye-delta bank. This prevents it from being a ground source. The open-wye, open-delta connection will not be a ground source.

 

[waross]

Hi jghrist;
I agree completely with the first part of your post.
However I disagree with your comments regarding neutral grounding on a Wye-delta bank.
"This prevents it from being a ground source."
If the primary neutral is grounded on a bank of distribution transformers, the transformers may be overloaded to the point of burnout by circulating currents caused by unbalanced primary voltages.
In the event of a phase loss the transformer bank will try to replace the missing phase. Its success in replacing the missing phase will depend on the relative size of the transformer bank compared to the load connected to the circuit past the point of phase loss.
Been there, Done that.
I have spent a number of years in a part of the world where this connection is popular. That is, a wye-delta bank with the primary neutral connected/grounded.
Note. It is not the connection to ground but the connection to the system neutral which is the cause of the issues. This is a fine point as the system neutral is well grounded.
Power failures and primary voltage unbalances are frequent in this part of the world.
When a primary phase is lost, transformer bank primary fuses frequently blow . Transformers burn out often. When two primary phases are lost, there is a backfeed of current from both unpowered transformers. These voltages will be determined by the connected load past the point of phase loss, but the maximum sum of the two voltages will be the normal voltage. That's when the freezers and refrigerators start burning up. That is common also.
respectfully.

 

[jghrist]

waross,
Perhaps I was too brief. I didn't mean that being a ground source was an advantage. It is a disadvantage because it can result in the problems that you mention. Floating the neutral will prevent it from being a ground source and causing the problems noted. I was considering grounding the transformer neutral and connecting it to the neutral to be the same thing, my being used to a multi-grounded neutral.

 

[brhodes]

Thanks to everyone that has replied to my questions. It's a big help.
jghrist,
Just to clarify on what you stated in your first reply, are the phase to ground voltages I mentioned earlier common for an ungrounded secondary even if the voltages were recorded at the secondary terminals of the transformers?

Thanks!

 

[jghrist]

I can't say whether such voltages would be common or not, but you should not expect balanced voltages to ground on an ungrounded system. The voltages to ground have no real significance in an ungrounded system.

 

[waross]

Hi brhodes;
This thread deals with voltages to ground in an ungrounded system.
Voltage Unbalance Vector Unknown
thread238-157183
I think that my comments in my last post in particular apply to your situation also. Specifically the last two sentences.
respectfully

 

[cuky2000]

Below is a typical connection diagram for 3-phase with two transformer in open-wye grounded primary, Open_delta grounded with 4-wires in the secondary.

Under normal conditions, the phase-to-ground voltage in A and B are equal in magnitude and phase C to ground should be ~73% larger while phase-to-phase voltages should be twice the voltage A to ground.

Appears that your case have a lost or bad grounding connection.

 

[brhodes]

Thanks to everyone who replied.
cuky2000, the transformer secondary is ungrounded causing the phase to ground unbalanced voltages. Thanks for the diagram.

With that said I will state that the system is running normal. The unbalanced phase to ground voltages is due to the ungrounded secondary. The electrician was worried because there is a 4th wire running into the building and he was getting unbalanced voltage readings. I think the electrician assumed they had a 4-wire system, but the 4th wire is not connected at the transformer. The system is 3-wire so I'm not sure why the 4th wire was pulled in unless they were planning for a change in the future.

Thanks Again!

 

[cuky2000]

Thanks David.

I changed the picture with your comment and added additional info.

Thanks again

 

[davidbeach]

cuky2000, you're welcome. Once I say your change had been made I RF'ed my post for the site management to deleted it to avoid confusion.