Chiller starter Zero Sequence GFI 1

[dithomas]

I have a 540 ton 480VAC 3 PH Y-D closed transistion starter fed from a Y solidly grounded transformer secondary.
The compaint is that the chiller may experience a ground fualt trip on the first start-up after not running for a long period of time. The gfi is reset and the chiller starts and runs. This problem has been happening for years.

When I took a first look at the starter I found that there is a zero sequence (core balance) CT on the line side of the main mold case non automatic breaker. This main is fitted with a shunt trip operated by the GFI relay.

The issue that I want to get your opion on is this:

The installing contractor ran the the phase conductors and the Ground conductor throught the CT. There is no Nuetral conductor.

I am thinking that the ground condcutor is not to be running through the ct and could be causing the problem.


Dan

 

[unclebob]

Exactly, no ground conductor should run through the CT.

 

[dithomas]

Thanks unclebob.

 

[controlsdude]

http://www.bender.org/mobile/knowledgebase/?a=ground-fault-monitoring-grounded-system

http://www.littelfuse.com/~/media/f...elays tn-gf03_ground-fault-misconceptions.pdf

I would do some reading. Google it first to see if it fits your install. Seems to depend of how ground is done at your install. Resistance grounding: low, med. or high resistance grounding.

 

[Parchie]

Could it be that the cable breakout was beyond the point where the CBCT was placed? The quick fix is to run back the ground conductor thru the CBCT before attaching it to the grounding terminal. Just saying if that scenario is the case with your installation.

 

[dithomas]

Parchie
It is possible to remove the ground conductor from the ct window. The cable breakout was above the CT. I have attached a photo.
Dan

 

[KuanYau]

To dithomas (Electrical)
(OP)

1. If your motor is [insulated from any earth except through the insulated PE conductor] which runs through the ZCT, the GFI relay
would NOT function. Under earth-fault condition, [the sum of three-phase currents and the earth-fault current would be equal to zero]. There is NO [spill current]. Therefore the GFI relay would NOT function.

2. Attention: Your motor is [unlikely insulated from earth]. It may be (unintentionally earthed) through the chiller unit, metallic pipes lines, metallic coupling or separate PE conductor mounted on the motor footing. These unintentionally parallel earthing paths would drain some of the earth-fault current when an earth-fault occurred.

3. When an earth-fault occurred, the [sum of three-phase currents is NOT equal to zero]. This spill current would be detected by the GFI relay.

4. [Only the three-phase cables shall go through the ZCT], (excluding the PE conductor).
The PE conductor shall by-pass the ZCT.

5. During motor starting, the [instantaneous sum of three-phase currents may NOT equal to zero], (even without an earth-fault). This spill current may be picked-up by the GFI relay if set at a very low value. Add a time delay relay of 0.5s would usually prevent unwanted tripping of this nature.

 

[Parchie]

Dan,
Yep. Don't let the ground conductors go through the ZCT.

 

[BigJohn1]

"5. During motor starting, the [instantaneous sum of three-phase currents may NOT equal to zero], (even without an earth-fault). This spill current may be picked-up by the GFI relay if set at a very low value. Add a time delay relay of 0.5s would usually prevent unwanted tripping of this nature."

Can you expand on this a little? I agree that running the equipment ground through the CT window can defeat the GFP function, but I'm not familiar with how a normal startup would cause a current imbalance in a healthy 3 phase system.

Where would the differential current be flowing if not in the phase conductors?