Ground Fault and Total Harmonic Distortion

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spikes2020

spikes2020

Member
Location
Nashville, TN
Occupation
Electrical Engineer
We have a weird issue, 2 breakers, a feeder in a sub and the main in a panel for some equipment. The feeder breaker is set to 300A ground fault at 0.2 second trip time, while the main in the panel is set to 1200A and 0.1 second. The main panel breaker is tripping first on GF. This is random trips while in operation and there appears not to be any ground faults on the system.

Under further investigation i see that this panel has 30-50% THD on each phase and it is reporting close to 500% TDD. Looking at the harmonics its 3, 5, 7th are crazy high and i assume these are from AC-DC rectifiers, most likely this is from VFDs or power supplies.

This is a 1000A 480V breaker, that most likely feeds several motors (single and 3 phase), plc's, and lots of other equipment. I think they are only pulling maybe 100-150A.

What would be recommended ways to solve this problem? Cap-inductor filters? Isolation transformer? Replace VFDs? Neutral-grounding resistors?

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Have you confirmed the proper installation of the neutral CT on these breakers. I am amazed at the number of installation errors with these CTs, typical problems are surrounding all neutral conductors to not being wired with proper polarity.
 
jim dungar said:
Have you confirmed the proper installation of the neutral CT on these breakers. I am amazed at the number of installation errors with these CTs, typical problems are surrounding all neutral conductors to not being wired with proper polarity.
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The sub breaker is where i am taking these readings it was just installed to feed this new equipment. I looked at the sub main breaker meter that was installed a long time ago and it has similar readings. These readings are from the CTs in the breaker its self and it only has 3 CTs, one on each phase. This equipment gets tested every 3 years so i am 99.9% confident that it is accurate. The panel breaker dose not have a way to get readings only the sub feeder and the sub main.
 
spikes2020 said:
These readings are from the CTs in the breaker its self and it only has 3 CTs, one on each phase.
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If the source is a grounded wye, your breakers should have a CT for each set of conductors or they need to be programmed as 3W devices. When programmed as 3 wire they might not show In current, so I would confirm their setup. Testing agencies typically test per the breaker settings, they rarely question if those settings are proper.

I have seen many more breaker install and setting errors than I have problems with load harmonics.
 
jim dungar said:
If the source is a grounded wye, your breakers should have a CT for each set of conductors or they need to be programmed as 3W devices. When programmed as 3 wire they might not show In current, so I would confirm their setup. Testing agencies typically test per the breaker settings, they rarely question if those settings are proper.

I have seen many more breaker install and setting errors than I have problems with load harmonics.
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Ill look at that, but the sub feeder breaker that i am getting these readings isn't tripping, its the panel breaker at the equipment tripping on 1200A ground fault. The panel breaker is only 3 phases and is a insulated molded breaker with only basic settings, see below for settings.
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Also i looked at another substation that is feeding more of this new equipment and getting similar readings. It has a newer meter. So i think these readings are accurate.
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The only time I've seen a large breaker tripping randomly on ground fault, one of the feeder conductors had a gouge in the insulation. Replaced the conductor and never had another problem.

For your situation, not sure why the 300A breaker has more delay than the main - usually the main has more delay to allow the branch breaker to trip first. Also, do all the other feeder breakers have ground fault sensors?

But those harmonics do sound really high. In particular 5,7 and 11. According to this Siemens paper, three phase bridge rectifiers have harmonics at those particular frequencies. Not sure if this paper would be any help, but I think i would be trying to narrow down where those harmonics are coming from.

 
steve66 said:
The only time I've seen a large breaker tripping randomly on ground fault, one of the feeder conductors had a gouge in the insulation. Replaced the conductor and never had another problem.

For your situation, not sure why the 300A breaker has more delay than the main - usually the main has more delay to allow the branch breaker to trip first. Also, do all the other feeder breakers have ground fault sensors?

But those harmonics do sound really high. In particular 5,7 and 11. According to this Siemens paper, three phase bridge rectifiers have harmonics at those particular frequencies. Not sure if this paper would be any help, but I think i would be trying to narrow down where those harmonics are coming from.

Click to expand...

Thanks! Yeah all AC-DC machines (rectifiers) have bad 5 and 7th harmonics, so i am assuming the cause is a VFDs or power supplies or both. But for this size load what solutions do i have? I would like to give a solution tomorrow in our meeting and all i have is a big Capacitor-Reactor filter. I was wondering if there was newer tech out there to solve this issue. I think this equipment was built using cheap Chinese VFDs that are causing this problem and replacing those is a solution but i don't think we are going to go for that one.

I think they jacked up the current to 1200A to stop it from tripping so much. There isn't a ground fault on the system or we would know.... 1200A makes a bit of a spark. I'll get the coordination back in line once we solve the harmonic problem.
 
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