power transformers
power transformers
(OP)
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?
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?






RE: power transformers
RE: power transformers
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?
RE: power transformers
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.
RE: power transformers
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
RE: power transformers
RE: power transformers
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.
RE: power transformers
RE: power transformers
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.
RE: power transformers
advidana
7-23-03
RE: power transformers
Reference ANSI C57.105, Table 1.
RE: power transformers
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.
RE: power transformers
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.