Triphen Harmonic Neutral Loads

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David P

New member
We are doing a project that requires 1700 Compact Flourescent Bulbs
We are using 2-200 Amp 3 Phase 120/208 Lighting Contactor Panels.
The A,B, & C Phases are Balanced @ 50 Amps per leg but the Neutral is @ 129 Amps. I have never seen this.
Question is, is this normal on Compact Flourescent Bulbs and if it is, is it going to cause any problems in there transformers or any of there other equipment, and if this is true what should we do to correct it.
 

renosteinke

Senior Member
Location
NE Arkansas
First, get a real meter ... a "true RMS" meter ... and check your readings again. Then check your wiring, and make sure the circuits really are fed from different phases. It looks to me like you're using one phase twice.

Then, take time to read the little sticker in the fixture that warns you of this issue. You just might have to get away from your shared neutral, and run a separate neutral for each 'leg.'

If there's enough of an issue, you might have to -carrying it to an extreme - completely redesign the distribution system and replace the transformers.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Given the type of load is it possible that triplen harmonics are present. They become additive in the neutral. You would need a true RMS meter to accurately test the current as Reno stated. Your numbers seem off for only harmonic current. Typically it would not exceed 1.73 times the phase current which you have greatly exceeded with your measurements.
 

iwire

Moderator
Staff member
Location
Massachusetts
The A,B, & C Phases are Balanced @ 50 Amps per leg but the Neutral is @ 129 Amps.

Just a shot in the dark .... is there any chance it was 12.9 amps and someone did not notice the decimal?

The only reason I ask is I have seen people have trouble with auto ranging meters. The range can change between measurements and if you are not paying attention this can be a problem.
 

dbuckley

Senior Member
My understanding is that there is no limit of 1.73x.

I've seen (on the Fluke RMS clamp) nearly twice the balanced phase current.

Thus I'm tempted to believe the results a first blush, even though they are out further than I've seen, and thus would stongly suggest getting the right test equipment in to do a power quality analysis to determine if this is instrumentation or real.

Does the switchgear hum or buzz? If its buzzing rather than the deep 60Hz hum then that is evidence of triplen or higher harmonics.

As long as this load is a small part of the total load on the transformer, then all will be well. if its a significant part of the transformer then I would expect heating.

Worth checking the other things as noted above, just in case this is all a red herring :)
 

Smart $

Esteemed Member
Location
Ohio
... Typically it would not exceed 1.73 times the phase current which you have greatly exceeded with your measurements.

My understanding is that there is no limit of 1.73x.

...
The theoretical limit is 100% triplen harmonic current on each of the phase conductors, while the neutral current value would be the phase currents arithmetically summed. So I'd say 3x is the theoretical limit.
 

iowa58

New member
i had a similar issue, i needed to add some temporary lighting on a ckt with some HID wall packs so without thinking much about it i put up a couple keyless holder and compact flourescent lamps. i noticed this morning that the ckt that i put those on was off, i checked that panel and sure enough that ckt was tripped well at that time of the morning they have it scheduled to test the generator under load i thought maybe the ampereage from the hot restart time was too much for it to handle, i reset the breaker and put my amp meter on it and it was drawing right at 95 amps, mind you this was on 10awg 30a ckt. I went back later in the day after they had cooled and drew the contactor in and sure enough it tripped again, i was pretty sure before that it wasnt just the hot restart that was drawing 95 amps but i had to make sure. i went and unhooked the 2 compact flourescent bulbs and tested it again and it was drawing the correct amount, only around 12 amps. both times i was reading the phase conductor (single phase application). When i reset the breaker while the generator was running under load i could definitely hear and tell it was dragging the generator down with that addition 95amp load. can someone explain to a relative novice what triplen harmonics is and why 2 simple little compact flourescent bulbs can add an additional 80 amp load on a ckt? i understand at this point that it is triplen harmonics but i would like to know why so if someone ever asks me i can tell them something more than "triplen harmonics" any help would be great..
 

__dan

Senior Member
put my amp meter on it and it was drawing right at 95 amps, mind you this was on 10awg 30a ckt. I went back later in the day after they had cooled and drew the contactor in and sure enough it tripped again, i was pretty sure before that it wasnt just the hot restart that was drawing 95 amps but i had to make sure. i went and unhooked the 2 compact flourescent bulbs and tested it again and it was drawing the correct amount, only around 12 amps.

can someone explain to a relative novice what triplen harmonics is and why 2 simple little compact flourescent bulbs can add an additional 80 amp load on a ckt? i understand at this point that it is triplen harmonics but i would like to know why so if someone ever asks me i can tell them something more than "triplen harmonics" any help would be great..

Harmonics are sinewaves whose frequency is a multiple of the fundamental frequency at 60 Hz. The odd order harmonics add up instead of cancelling in the neutral on multiphase shared neutral circuits, ie, 60 x 3 = 180 Hz, 60 x 5 = 300 Hz ..

The problem sounds more like a shorted circuit with current flow limited by the length of the wire. I would suggest bench testing the CFL's. They're not going to draw 80 amps on the bench unless they're shorted / defective.
 

richxtlc

Senior Member
Location
Tampa Florida
The theoretical limit is 100% triplen harmonic current on each of the phase conductors, while the neutral current value would be the phase currents arithmetically summed. So I'd say 3x is the theoretical limit.

If he is using a true RMS meter, it looks like about 86% harmonics. This is a totally new circuit, not an add-on, so it is possible to have 129 amps in the neutral.
The other problem is that the transformer supplying the circuit may over heat due to the higher frequency being trapped in the delta. In addition to having one neutral per phase, you may have to change the transformer to one that is K-rated for the required kVA.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Iowa, welcome to the forum! :)

... i reset the breaker and put my amp meter on it and it was drawing right at 95 amps, mind you this was on 10awg 30a ckt.

i went and unhooked the 2 compact flourescent bulbs and tested it again and it was drawing the correct amount, only around 12 amps. both times i was reading the phase conductor (single phase application).

can someone explain to a relative novice what triplen harmonics is and why 2 simple little compact flourescent bulbs can add an additional 80 amp load on a ckt?
Harmonics do not add on unshared conductors, so that isn't the problem. In no particular order:

When you connect and disconnect the CFL's, do you disconnect wires or just unscrew the bulbs? (Bad socket?)

Have you tried only connecting one CFL at a time? (Tip: When troubleshooting, make only one change at a time.)

Did the CFL's light up at all? My bet is no. Thy another temporary light, like an incandescent bulb in a rubber socket.

The problem sounds more like a shorted circuit with current flow limited by the length of the wire.
My exact thought as I was reading the OP.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor

richxtlc

Senior Member
Location
Tampa Florida
My comments concerning the size of the wire were related to the current in the neutral. What was the circuit designed to carry? If there were a short circuit as suggested and that the current was limited by the length of the conductors, it would have to be one very long circuit.
 

richxtlc

Senior Member
Location
Tampa Florida
We are doing a project that requires 1700 Compact Flourescent Bulbs
We are using 2-200 Amp 3 Phase 120/208 Lighting Contactor Panels.
The A,B, & C Phases are Balanced @ 50 Amps per leg but the Neutral is @ 129 Amps. I have never seen this.
Question is, is this normal on Compact Flourescent Bulbs and if it is, is it going to cause any problems in there transformers or any of there other equipment, and if this is true what should we do to correct it.
What was the original circuit designed to handle? What is the rating on the OCPD?
What was the voltage across each p-g? If you were using a True RMS meter, it should tell you what the dominant frequency was when you took the reading, was it 180Hz or something else?
 
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