Neutral Current on 3 phase system

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JerryBumgarner

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We are connecting a Trojan UV system. It calls for a 3 phase, 208Y, 4 wire connection. We are providing that power to the system. When the Trojan field rep was starting up the system today, she measured the current in each phase with all 3 UV modules connected. The current was 12 amps in each phase (that was rated current). She then measured the neutral current - 10.8 amps. She will not sign off on the start up unless the neutral current is < 5 amps. I agree that the neutral current should be small. However, I am arguing that the load (UV system) is causing the neutral current. She is arguing that we are providing an unbalanced system to them and that the problem is on our end. Has anyone seen anything similar to this?

Background: This is a new waste water treatment plant and we are the EC. The plant is not running yet so the only load on the transformer is the UV system and the 4 lights that were on in the electric room. Incoming power is 3 phase 480. It is being stepped down to 208 using a 480 Delta/208Y transformer. The transformer supplies a distribution panel and the UV system is fed from a 3 pole, 20 amp breaker. The lamps of the UV system are all connected line-to-neutral (3 lamp modules per system). There are 2 UV systems and both have the same phase and neutral currents. The line-to-line voltage measures 215 VAC (Duke Energy is always high here). Line-to-neutral voltage measures 125 VAC. The neutral of the transformer secondary is bonded to ground. The transformer is grounded to 3 ground rods driven in a triangle.
 
JerryBumgarner said:
We are connecting a Trojan UV system. It calls for a 3 phase, 208Y, 4 wire connection. We are providing that power to the system. When the Trojan field rep was starting up the system today, she measured the current in each phase with all 3 UV modules connected. The current was 12 amps in each phase (that was rated current). She then measured the neutral current - 10.8 amps. She will not sign off on the start up unless the neutral current is < 5 amps. I agree that the neutral current should be small. However, I am arguing that the load (UV system) is causing the neutral current. She is arguing that we are providing an unbalanced system to them and that the problem is on our end. Has anyone seen anything similar to this?

Background: This is a new waste water treatment plant and we are the EC. The plant is not running yet so the only load on the transformer is the UV system and the 4 lights that were on in the electric room. Incoming power is 3 phase 480. It is being stepped down to 208 using a 480 Delta/208Y transformer. The transformer supplies a distribution panel and the UV system is fed from a 3 pole, 20 amp breaker. The lamps of the UV system are all connected line-to-neutral (3 lamp modules per system). There are 2 UV systems and both have the same phase and neutral currents. The line-to-line voltage measures 215 VAC (Duke Energy is always high here). Line-to-neutral voltage measures 125 VAC. The neutral of the transformer secondary is bonded to ground. The transformer is grounded to 3 ground rods driven in a triangle.
Click to expand...

The ballasts and lamps of the UV system present a non-linear load on each leg of the wye. As a result the neutral currents from the three legs cannot cancel and there will be a high neutral current. If put a scope on the neutral current you should see that it consists entirely of harmonics of the supply frequency, plus perhaps a small 60Hz component from residual imbalance of the load currents.
If the three phase input is clean and the supply voltages are balanced, then any neutral current can only be attributed to the load itself.

If the Trojan field rep has not seen this before (and has ever looked!), then possibly the UV system is defective or missing some filtering components designed to limit the non-linearity (distortion power factor).

If you put a 200A balanced resistive or motor load on the system in addition to the UV, the phase currents would be 212A each and the neutral current would still only be 11A. I expect she would be perfectly happy with that.:angel:
 
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Jerry:
As has been mentioned, don't rule out an issue with the equipment. I have had issues twice at my WWTP's due to incorrect wiring with brand new Trojan UV systems.
 
Jerry:
The supply voltage appears clean: 1.732 times phase voltage 125V closely equals the line voltage 215V. Any unbalance may be ascertained from the line voltage values.
 
GoldDigger said:
The ballasts and lamps of the UV system present a non-linear load on each leg of the wye. As a result the neutral currents from the three legs cannot cancel and there will be a high neutral current. If put a scope on the neutral current you should see that it consists entirely of harmonics of the supply frequency, plus perhaps a small 60Hz component from residual imbalance of the load currents.
If the three phase input is clean and the supply voltages are balanced, then any neutral current can only be attributed to the load itself.

If the Trojan field rep has not seen this before (and has ever looked!), then possibly the UV system is defective or missing some filtering components designed to limit the non-linearity (distortion power factor).

If you put a 200A balanced resistive or motor load on the system in addition to the UV, the phase currents would be 212A each and the neutral current would still only be 11A. I expect she would be perfectly happy with that.:angel:
Click to expand...

Thank you for that response. I was thinking along those same lines, you just stated much better than I was able to yesterday.
 
JerryBumgarner said:
Thank you for that response. I was thinking along those same lines, you just stated much better than I was able to yesterday.
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Another thing is that she may be thinking <5A is valid for one system and should not exceed that value even though two systems are connected. Two systems will have twice the triplen harmonic current... 2 ? 5A = 10A.

One doubt I have is regarding the way you stated the transformer secondary neutral is bonded to ground... and immediately stated thereafter it is grounded to three ground rods. The three ground rods cannot be the only means of grounding. It must be connected to the 480V system grounding.
 
JerryBumgarner said:
We are connecting a Trojan UV system. It calls for a 3 phase, 208Y, 4 wire connection. We are providing that power to the system. When the Trojan field rep was starting up the system today, she measured the current in each phase with all 3 UV modules connected. The current was 12 amps in each phase (that was rated current). She then measured the neutral current - 10.8 amps. She will not sign off on the start up unless the neutral current is < 5 amps. I agree that the neutral current should be small. However, I am arguing that the load (UV system) is causing the neutral current. She is arguing that we are providing an unbalanced system to them and that the problem is on our end. Has anyone seen anything similar to this?
Click to expand...
I have and I'm with the Gold miner fellow. If the UV ballasts take non-linear current it likely contains third harmonic (180Hz) and upwards. The so-called triple-n harmonics. As GD said, these add rather than cancel in the neutral.
I've come across a few situations where it was so bad it caused the neutral bar to overheat.
 
GoldDigger said:
The ballasts and lamps of the UV system present a non-linear load on each leg of the wye. As a result the neutral currents from the three legs cannot cancel and there will be a high neutral current. If put a scope on the neutral current you should see that it consists entirely of harmonics of the supply frequency, plus perhaps a small 60Hz component from residual imbalance of the load currents.
If the three phase input is clean and the supply voltages are balanced, then any neutral current can only be attributed to the load itself.

If the Trojan field rep has not seen this before (and has ever looked!), then possibly the UV system is defective or missing some filtering components designed to limit the non-linearity (distortion power factor).

If you put a 200A balanced resistive or motor load on the system in addition to the UV, the phase currents would be 212A each and the neutral current would still only be 11A. I expect she would be perfectly happy with that.:angel:
Click to expand...

That's my thought also. One would expect if the loads were identical on all 3 phases one would expect that there would be no neutral current. Measuring a neutral current is commonly a red flag for harmonics. Not that this has anything to do with this issue but K factor transformers have 200% neutrals as well as panels being specified with 200% neutrals. As such this is an indicator the unexpected high neutral currents most likely are due to harmonics.
 
Besoeker said:
I have and I'm with the Gold miner fellow.
Click to expand...

Actually, here in Placerville we do mostly placer "mining". We don't dig mine shafts, we just scoop up the dirt and use sluice boxes or pans to separate out the gold.
At one point in the town's history gold seekers would stake small claims and then dig 6' by 6' holes in the middle of the streets. :)
 
GoldDigger said:
Actually, here in Placerville we do mostly placer "mining". We don't dig mine shafts, we just scoop up the dirt and use sluice boxes or pans to separate out the gold.
At one point in the town's history gold seekers would stake small claims and then dig 6' by 6' holes in the middle of the streets. :)
Click to expand...

I stand corrected young chap!
:D
 
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