Ground Fault and Total Harmonic Distortion

[spikes2020]

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?

 

[jim dungar]

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.

 

[spikes2020]

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.

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.

 

[jim dungar]

These readings are from the CTs in the breaker its self and it only has 3 CTs, one on each phase.

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.

 

[spikes2020]

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.

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.


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.