Imbalanced phase load

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gobble

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Ok well I know what the problem is, I was tagged on a service meter for the meter being overloaded 750 amps. This is a airfield electrical system. The system is a 400 amp 480/277V feeding a vault containing 4 constant current regulators. I checked each phase load with the ccr's turned up to step 5 which is the highest load on these units. When I went to the ats to check the currents I was suprised by the amount of vibration coming from the conductors. The loads were as follows phase A 421A phase b 390A and phase C 100A. Obviously the phasing needs balanced and I'm sure it will help with the situation of overloading. My questions are what would cause the conductors to vibrate, how does induced magnetic fields work? Secondly if there was a power outage and generator was online what damages could occur to the windings given such an unbalanced phase C? Thank you for any responses


Joe
 
Welcome to the forum.

I don't know what you mean by the meter was overloaded 750A.

I've only know conductors to vibrate noticeably when overloaded to the point just before they trip, or should trip a breaker or blow a fuse.


Generators can handle imbalance if they are big enough for the connected load.
 
ActionDave said:
Welcome to the forum.

I don't know what you mean by the meter was overloaded 750A.

I've only know conductors to vibrate noticeably when overloaded to the point just before they trip, or should trip a breaker or blow a fuse.


Generators can handle imbalance if they are big enough for the connected load.
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I was really confused as well, the main should have tripped being that it's only a 400A main. The tag said nothing more then the utility companies name and that there was an overloaded meter.
 
Way back in Jr High science class they used iron filings to demonstrate a magnetic field. Run a Conductor vertically through a horizontal piece of paper. Gently adding a few filings to the area around the conductor and applying DC current would cause the filings to arrange themselves around the wire. The magnetic field. Changing the current to AC would cause the field to vibrate. Something will move and in your case it is the wires.

Same reason a transformer hums or vibrates. It’s why wires rattle in the raceway when large loads are energized.
 
ptonsparky said:
Way back in Jr High science class they used iron filings to demonstrate a magnetic field. Run a Conductor vertically through a horizontal piece of paper. Gently adding a few filings to the area around the conductor and applying DC current would cause the filings to arrange themselves around the wire. The magnetic field. Changing the current to AC would cause the field to vibrate. Something will move and in your case it is the wires.

Same reason a transformer hums or vibrates. It’s why wires rattle in the raceway when large loads are energized.
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Thanks for the reply, so basically it's a constant changing magnetic field, in this instance the change would be at 60hz, correct?
 
171201-1016 EST

120 Hz, every half cycle.

Make a hairpin loop of wire and apply a current thru this and it will try to open to a circle. Like magnetic fields push each other apart, and unlike attract each other.

Take two wires perpendicular to a plain (sheet of paper). One wire has current flowing into the paper, and the other coming out. Plot the magnetic field around each.

See https://www.quora.com/Why-do-two-wi...h-current-flowing-in-opposite-direction-repel
A little more than half way down see pictures by Tariq Merchant.

.
 
gobble said:
Ok well I know what the problem is, I was tagged on a service meter for the meter being overloaded 750 amps. This is a airfield electrical system. The system is a 400 amp 480/277V feeding a vault containing 4 constant current regulators. I checked each phase load with the ccr's turned up to step 5 which is the highest load on these units. When I went to the ats to check the currents I was suprised by the amount of vibration coming from the conductors. The loads were as follows phase A 421A phase b 390A and phase C 100A. Obviously the phasing needs balanced and I'm sure it will help with the situation of overloading. My questions are what would cause the conductors to vibrate, how does induced magnetic fields work? Secondly if there was a power outage and generator was online what damages could occur to the windings given such an unbalanced phase C? Thank you for any responses


Joe
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Is your service a CT supplied meter? It would say CL 20 on the nameplate. To change load ratings, the POCO would have to change CT's and verify transformer and conductor sizing. If you have 421A on any phase, that may be overloading the meter, depending on the TRF (thermal rating factor) and ratio of the CT's. They now make 400A three phase Wye meters, but they don't seem to be popular. I'd be surprised if that's what you have. A 400A meter is designed to run at 320A (80%) per phase continuously. Meter amps are per phase, not additive. In many jurisdictions, anything three phase requires CT's for greater than 200A.
 
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meternerd said:
Is your service a CT supplied meter? It would say CL 20 on the nameplate. To change load ratings, the POCO would have to change CT's and verify transformer and conductor sizing. If you have 421A on any phase, that may be overloading the meter, depending on the TRF (thermal rating factor) and ratio of the CT's. They now make 400A three phase Wye meters, but they don't seem to be popular. I'd be surprised if that's what you have. A 400A meter is designed to run at 320A (80%) per phase continuously. Meter amps are per phase, not additive. In many jurisdictions, anything three phase requires CT's for greater than 200A.
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I haven't been in the meter panel, it was loto by the utility company and didn't feel the need yet to go into the panel. I will be balancing the load when I can get around to it. I don't know if your familiar with ccr's but there airfield lighting power supplies. They will draw current as needed to supply 6.6A on the primary side of the lighting circuit. They will push up to 5.5kv just to assure the runway lights actually illuminate. Given the lenghts of the crcuits I would think we have a short in the field and that's the reason the current was so high. But this is a 4 wire 3 phase so I assume there are ct's in the meter. Once I balance the load and test the ccr's I will further diagnose the field wiring. Hopefully it's an easy find because field shorts in an airfield generally are a pita to pin point. There are over 4000 lights on the field controlled by 4 regulators fun times haha oh yes and every light or sign has its own series isolation transformer
 
gobble said:
I was really confused as well, the main should have tripped being that it's only a 400A main. The tag said nothing more then the utility companies name and that there was an overloaded meter.
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Your highest current was 421 amps - may trip the main if it runs at that level for long enough. Ambient conditions may speed up or retard how long it takes to respond as well. Your main is (probably) a thermal-magnetic device, it takes some time for a slight overload to cause a trip, but a sudden increase of current like what happens in short circuits and ground faults causes rapid tripping on high magnetic fields produced in such events - see below.

gobble said:
Thanks for the reply, so basically it's a constant changing magnetic field, in this instance the change would be at 60hz, correct?
Click to expand...
Put current through a conductor and a magnetic field is produced around it. Put multiple conductors with such fields close to one another and they attract/repel one another according to magnetic polarity - apply an AC current and those magnetic fields are constantly changing at the same rate as the frequency of applied voltage. It is these fields your clamp on ammeter is reading when you use such a meter. Higher the current level the stronger the fields are and the more likely the conductors or other objects under magnetic influence are to move and cause noises.
 
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