power transformers

[alexc]

I am experiencing the following situation: a 480/208-120V (delta-wye) transformer was used to step up voltage for 480V equipement. As such, 208-120 became primary, neutral x0 being connected as well to the neutral of the installation. Other single phase loads (and possible three phase) were connected to the 120/208 as well. After approx. one year, the neutral wire was found completely burned.

What is the difference if x0 is not connected to the system's neutral? In other words, would the wye connection generate such an unbalanced current to effectively burn the neutral?

 

[jghrist]

It's possible that the wye-delta transformer was feeding a ground fault on the 120/208 volt system and that the protection on the primary side (120/208) did not clear the fault before the neutral failed. A wye-delta transformer is a ground fault source.

 

[alexc]

Thanks jghrist.
Let me reformulate, my information is a little confusing.

The electrical service in the building is 120/208V. Because they had some equipment that requires 480V (three wires), and it happened that they had available a 480/208-120V transformer (300 KVA, SquareD, Dry type), they decided to connect it to step up the voltage. So they connected the 120/208V (wye) as primary to their electrical service.

In these circumstances, what would be the difference between connecting or not connecting the neutral x0 of this transformer to the installation?

 

[jghrist]

Without the neutral connected, the phase-neutral voltages will not be stable on the 120/208 primary (floating neutral). One primary winding could have nearly zero volts and the others could have nearly 208 volts. This means no stable phase-phase voltage on the 480 side because the phase-phase secondary voltage is at a set ratio to the corresponding phase-neutral primary voltage.

My previous comments on the transformer being a ground fault source are still valid. The transformer will feed a ground fault on the 120/208 system.

 

[busbar]

jghrist has explained the situation well.

(T)he transformer will supply part of the unbalanced phase-to-neutral loads and line-to-ground fault currents. The transformer bank is said to be a ground source. Such connections on Y-grounded… primaries are a hindrance to providing reliable and sensitive ground-fault protection at the source…. Furthermore, such transformer banks are subject to serious overloads under certain open-conductor fault conditions. The YG-D connection is a ground source on the primary side. §2.6 IEEE C57.105-1978

 

[jwerthman]

I agree that with the grounded neutral on the 480-120/208 transformer, it was probably carrying a significant neutral unbalance continuously or a very high neutral current during a fault. However, I respectfully disagree that the neutral can not be disconnected. If the existing 120/208 system neutral is properly grounded at its source, then the 120/208-480 volt transformer neutral can be operated ungrounded with no problem. The neutral on the system will be as stable as it was before the step-up transformer was connected. The delta 480 volt winding will keep the neutral of the step-up transformer stable, grounded or not. This type of connection is quite common on utility distribution systems.

 

[jghrist]

With the 120/208 neutral properly grounded at its source, the neutral on the system will be stable. If the transformer neutral is not connected to the system neutral, however, this does nothing to make the transformer neutral stable.

A delta primary winding will keep an ungrounded secondary wye winding stable because the source keeps the delta winding voltages stable. With the delta winding as the secondary, there is no source voltage to keep the winding voltages stable. Unbalanced load will cause unbalanced voltages both on the delta secondary and on the ungrounded wye primary.

I don't recall ever seeing an ungrounded wye-delta connected transformer on a utility distribution system. That isn't to say that they don't exist, but the most common connection on North American utility systems is grd wye-grd wye. Most common on industrial systems and on utility systems in the rest of the world is delta-grd wye.

 

[jbartos]

Suggestion: The previous posting needs to be amended by a harmonic content situation. Even, if the 120/208-480V transformer has an ungrounded neutral with the 120/208V source properly grounded, there may be a harmonic content flowing in the neutral that may thermally deteriorate the neutral, if the neutral is not adequately overrated.

 

[DanDel]

I agree with jwerthman about the disconnected neutral. The transformer neutral may not have a stable voltage to ground, but the system neutral(if properly grounded at the source) will.
The secondary(480V delta) winding will have no real reference to ground, but this would have no effect on why the neutral wire burned, which was the original question.

I'm assuming that when alexc said "After approx. one year, the neutral wire was found completely burned." that this meant that the cable had overloaded and caused the insulation to melt, as well as the cable to burn, along its entire length, and the cable was no longer connected because it burned through at one point. Please correct me if my assumption is wrong.
I believe that leaving the neutral disconnected will not cause a problem.

 

[advidana]

Something is wrong and you need help. Where was the neutral grounded, if the transformer neutral was grounded at the at the primary. Then the neutral could have burn out because it was getting a return pass from the rest of the 120/208 volt system which is grounded at the service or from the source of the 120/208 volt system. Try to find out why the neutral burn out in the first place. Have a Profession Electrical Engineer and a testing company come in to assist you if need be. This installation could cause more damage and or a fire.

advidana
7-23-03

 

[jwerthman]

I still contend that the 120/208 neutral on the transformer can operate ungrounded with no problem. I have seen this connection many times in utility distribution systems in the midwest, most commonly for 12470-240 volt three phase service with a center tap on one 240-volt leg for 120/240 volt. The HV neutrals on these banks are usually floated to prevent them from being ground sources.

Reference ANSI C57.105, Table 1.

 

[jghrist]

True, ANSI C57.105 does not say anything bad about an ungrounded wye-delta connection. I'll have to admit I'm wrong. Also I'm wrong about not having seen the connection before.

I don't understand how the phase to (transformer) neutral voltages would remain stable, though, if there is no connection of the transformer neutral to the source neutral and there is no voltage source on the delta side.

 

[stevenal]

Assuming balanced source voltages, balanced three phase loading, and equal phase impedances on the transformer; line to transformer neutral voltage will be stable. Add some non-ideal conditions, though, and letting the neutral float to its natural neutral position will tend to even things up better than tieing it down. Add a resistive connection for example, and two windings will hog the load and cause current to flow on the neutral; and cause a voltage drop from N to N'.

A single phase example: We rented a single phase generator to backfeed a transformer in order to heat up a single phase line following some bad weather. The crew went ahead and connected the generator and transformer neutrals together. The result was a large current imbalance and high neutral current. Engineering told them to disconnect the neutral, and all was fine.