Crane install

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Razzap

Razzap

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Recently my company built an addition, and extended the overhead crane rail approximately 300 feet. The original crane rail was about 400 feet, for a total of 700 feet. The original building is fed from a 480 V, 3 phase Y -Y transformer. The new addition is fed from a 480 V, 3 phase Y-Delta transformer. Both primaries are fed from the same POCO. The two crane rails were not electrically connected together, but when the crane traveled across the gap, the forward shoes were in contact with the new system, while the back shoes were in contact with the original system. Sparks flew and the main breaker tripped out at the original building. The electrical contractors checked phasing between the original and new crane rail and it was determined that phasing was OK. My question is, what went wrong? I am thinking that it might have something to do with transformer polarity, but I cannot seem to find much info on the Web. The electrical contractor is now saying that they will have to replace the new Y-Delta tranny with a Y-Y. From what I have read, paralleling 3-phase transformers does not work because of polarity issues. When the crane bridged the gap between original and new, it effectively put the transformers in parallel. Also, it seems that phase B original and phase B new have about 144 V between them, measured at the crane rail. I am not really involved in this dilema, but just an electrician, building machine tools and watching everyone scratching their heads over this problem. Any ideas on how to solve this?
 
That was my suggestion, too. I was told voltage drop was an issue, but of course there are ways around that. I forgot to mention that this 700 feet of crane rail is only one of six identical crane installs. Plus each pair of rail currently supports four cranes, with more to come. Every foot of floor space is accessable by crane. This issue is holding up production in the new addition, because no one can move material the entire length of the building.
 
Razzap said:
Recently my company built an addition, and extended the overhead crane rail approximately 300 feet. The original crane rail was about 400 feet, for a total of 700 feet. The original building is fed from a 480 V, 3 phase Y -Y transformer. The new addition is fed from a 480 V, 3 phase Y-Delta transformer. Both primaries are fed from the same POCO. The two crane rails were not electrically connected together, but when the crane traveled across the gap, the forward shoes were in contact with the new system, while the back shoes were in contact with the original system. Sparks flew and the main breaker tripped out at the original building. The electrical contractors checked phasing between the original and new crane rail and it was determined that phasing was OK. My question is, what went wrong? I am thinking that it might have something to do with transformer polarity, but I cannot seem to find much info on the Web. The electrical contractor is now saying that they will have to replace the new Y-Delta tranny with a Y-Y. From what I have read, paralleling 3-phase transformers does not work because of polarity issues. When the crane bridged the gap between original and new, it effectively put the transformers in parallel. Also, it seems that phase B original and phase B new have about 144 V between them, measured at the crane rail. I am not really involved in this dilema, but just an electrician, building machine tools and watching everyone scratching their heads over this problem. Any ideas on how to solve this?
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You need to check the phase offset of Y-Y versus Y-Delta
 
fmtjfw said:
You need to check the phase offset of Y-Y versus Y-Delta
Click to expand...
Exactly, even though the phase rotation is the same, the two systems are 30 degrees out of phase.
For two 480 volt systems that are 30 degrees out of phase the math says you will have 143.39 volts between the two "B" phases.
 
The set-up you describe simply has to be illegal. I cannot see how you can have two sources supply the same conductors- even for an instant. How would you LOTO? What use would overcurrent protection be?
 
renosteinke said:
I cannot see how you can have two sources supply the same conductors- even for an instant.
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For the records, yes you can parallel transformers, usually without much problem, especially when they are fed from a common source. This occurs regularly in Main-tie-Main double ended substations or other equipment with closed transition switching. Co-generation is another common example of multiple sources feeding single sets of conductors.

However, paralleled sources usually have the ability to provide a tremendous amount of fault current. The crane power equipment is probably not rated for this situation, so at a minimum it may not be in compliance with 110.9 and 110.10.
 
renosteinke said:
The set-up you describe simply has to be illegal. I cannot see how you can have two sources supply the same conductors- even for an instant. How would you LOTO? What use would overcurrent protection be?
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There are two separate segments of trolley rails. This is demonstrated by the fact that the OCPD trips when the crane spans the gap between the two segments of rails. This causes the 140V difference in the two sources to provide enough current to trip. When the trolley is not spanning the gap, there is no current between the two sets.

The use of multiple trolley sources on multiple segments is common. The trolley pickups are designed to span the gap, or enough residual rotation is in the mechanical parts to carry across the gap. Usually some sparking occurs from voltage drop differences but not enough to appear to be a fault.
 
It just hit me on what your problem is, if you have a Wye that is probably center grounded while the other is a delta, probably corner grounded, when the crane crosses over, the shoes are creating a dead short between the grounded phase on the delta side and the ungrounded phase on the wye side. If both were either wye or both delta, grounded the same, you would not have this problem
 
hillbilly1 said:
It just hit me on what your problem is, if you have a Wye that is probably center grounded while the other is a delta, probably corner grounded, when the crane crosses over, the shoes are creating a dead short between the grounded phase on the delta side and the ungrounded phase on the wye side. If both were either wye or both delta, grounded the same, you would not have this problem
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This does not appear to be a grounding issue. The issue is the 30? phase shift between the wye supplied system and the delta supplied system. Two 480 volt systems that are 30? out of phase will have about 144 volts between B phase of one system and B phase of the other system. That is the voltage that OP stated he measured in post #1.
 
don_resqcapt19 said:
This does not appear to be a grounding issue. The issue is the 30? phase shift between the wye supplied system and the delta supplied system. Two 480 volt systems that are 30? out of phase will have about 144 volts between B phase of one system and B phase of the other system. That is the voltage that OP stated he measured in post #1.
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I think that is a problem also, but if the grounding is done as I stated, it would be a short circuit that would trip the Y-Y service like the op stated when the rails were connected by the shoes. Maybe the op will be back with more info on that.
 
hillbilly1 said:
I think that is a problem also, but if the grounding is done as I stated, it would be a short circuit that would trip the Y-Y service like the op stated when the rails were connected by the shoes. Maybe the op will be back with more info on that.
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If that were the case you would have more than 144 volts across the open bus. It looks like the lowest voltage across the open bus with the type of grounding that you described would be 277 volts.
 
don_resqcapt19 said:
If that were the case you would have more than 144 volts across the open bus. It looks like the lowest voltage across the open bus with the type of grounding that you described would be 277 volts.
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True, if it is was properly bonded, but it is also odd that only "B" phase has the 144 volts difference, there should some phase shifting on the other phases as well. ........unless it is a resistance grounded system. Without seeing all voltages to ground and phase to phase, it's just a guessing game!
 
Last edited:
Originally Posted by renosteinke
The set-up you describe simply has to be illegal. I cannot see how you can have two sources supply the same conductors- even for an instant. How would you LOTO? What use would overcurrent protection be?


The crane rails in the original building are fed from the same service, but two different distribution panels. There is a gap in the rails at midpoint of the original building. One disconnect is at one end of the building, the other disconnect is at the other end. In order to LOTO, both disconnects must be turned off. If only one disco is off, and the crane is in the "live" section, it can still be run back to the "dead" section and span the gap. Since both sets of shoes are hot, the leading shoes now would energize the "dead" section, making it hot again. I have pointed that out to others that work on the cranes and were unaware.
With the new addition, now there are three discos per crane rail to deal with.
I will try and get back with more info on how the new Y-Delta is grounded. The original building Y-Y is grounded at the center, 480V / 277 V.
 
hillbilly1 said:
True, if it is was properly bonded, but it is also odd that only "B" phase has the 144 volts difference, there should some phase shifting on the other phases as well. ........unless it is a resistance grounded system. Without seeing all voltages to ground and phase to phase, it's just a guessing game!
Click to expand...
No matter what the grounding is, the 30? phase shift will cause a problem. The grounding, as you described, would make the issue worse. Even if the systems are not properly bonded, if one system is a grounded wye and the other is a corner grounded delta, you would see at least 277 volts between the conductors of the different systems.

I agree that there would be voltage differences on all of the phases as a result of the 30? phase shift, but there would also be voltage differences on all phases from the grounding that you described. I just assumed that they only measured the voltage across one phase and not that only B phase has a voltage difference.
 
don_resqcapt19 said:
No matter what the grounding is, the 30? phase shift will cause a problem. The grounding, as you described, would make the issue worse. Even if the systems are not properly bonded, if one system is a grounded wye and the other is a corner grounded delta, you would see at least 277 volts between the conductors of the different systems.

I agree that there would be voltage differences on all of the phases as a result of the 30? phase shift, but there would also be voltage differences on all phases from the grounding that you described. I just assumed that they only measured the voltage across one phase and not that only B phase has a voltage difference.
Click to expand...

I was agreeing with you on problems being caused by the phase shift, but I think the grounding is causing the biggest problem, both need addressed. The second service being a delta was probably due to the POCO trying to save money with smaller transformers.
 
Razzap said:
Originally Posted by renosteinke
The set-up you describe simply has to be illegal. I cannot see how you can have two sources supply the same conductors- even for an instant. How would you LOTO? What use would overcurrent protection be?


The crane rails in the original building are fed from the same service, but two different distribution panels. There is a gap in the rails at midpoint of the original building. One disconnect is at one end of the building, the other disconnect is at the other end. In order to LOTO, both disconnects must be turned off. If only one disco is off, and the crane is in the "live" section, it can still be run back to the "dead" section and span the gap. Since both sets of shoes are hot, the leading shoes now would energize the "dead" section, making it hot again. I have pointed that out to others that work on the cranes and were unaware.
With the new addition, now there are three discos per crane rail to deal with.
I will try and get back with more info on how the new Y-Delta is grounded. The original building Y-Y is grounded at the center, 480V / 277 V.
Click to expand...

There is usually a 10' isolation section at the end of each main section of crane rails. Turn off a section, and the two adjacent isolation sections turn off too. That way the shoes can span the gap and energize the isolation section, but it can't energize the main run by backfeeding it from an adjacent run.

But all the sections still need to be supplied from a common voltage source.
 
After a lot of discussion, the POCO is going to replace the Y-Delta transformer with a Y-Y unit. I'll keep everyone posted on the results.
 
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