wye wye transformer problems

[chuck crossan]

what are the problems from a 4160 3phase to 480 3phase wye wye transfromer and the HO XO are tied together internal and grounded external? It is a 1500kva and supplying power to pumps and a 480 to 120/208 3phase power transformer that is also powering pH and flow instruments. The coustomer is having some strange control problems that appear to come from the power source.
can any one avise

 

[templdl]

Re: wye wye transformer problems

the HO XO are tied together internal and grounded external

?!!
WOW, If that's so you have a big problem.
It is of my opinion should only feed the pri H1-H2-H3 with L1-L2-L3 conductors leving the H0 unconnected and isolated. You don't need the neutral. You also must bring an EGC and bond it to the enclosure.
Then, the secondary you treat as a separately derived system where the X0 is bonded to the enclosure.

 

[beanland]

Re: wye wye transformer problems

Y-Y connection with both star-points grounded and bonded should pose no problems, as long as both are well bonded and well grounded. This is a common utility transformer connection. It is the same as three single-phase 2400V transformers. IF the utility is supplying the grounded 4160V with an ungrounded delta source, it should also work UNLESS there is a phase-ground fault on the distribution system.

 

[templdl]

Re: wye wye transformer problems

beanland,
Good point.
I made the "assumption" that the transformer was being feed from a grounded wye where the X0 of a wye connected sourse was grounded and then the H0 of the primary of the transformer was also grounded providing a ground path between the H0 of the sourse and the transformer.

 

[templdl]

Re: wye wye transformer problems

beanland,
Question. I got to thinking a bit more about this. When feeding the wye primary from a wye secondary would the X0 if that wye secondary also be grounded? If so it would mean that the X0 of the upstream transformer and the H0 if the transformer being feed would both be grounded and permitting current flow through the ground.
Dave

 

[don_resqcapt19]

Re: wye wye transformer problems

Dave,
On utility distribution systems with a grounded primary conductor, that conductor is bonded to the secondary grounded conductor.
Don

 

[templdl]

Re: wye wye transformer problems

Thanks Don.
Does that mean that is one were to has (3) Y-Y transformers feeding one to the next that essentially all of their primary and secondary grounded conductors would be linked together?
Would the H0 be bonded to the X0 if the first transformer, the H0 and X0 bonded in the second and the H0 and X0 bonded in the third and each grounded would there be a posibility of allowing ground current to flow between the X0 of the first transformer to the H0 of the second and the X0 of the second to the H0 of the third unless a 4th conductor (neutral?) linking them was included.
I just would like to understand the configuration correctly and assure that ground current isn't being turned loose which may not even be an issue.
Dave

 

[jtester]

Re: wye wye transformer problems

If the secondary is grounded wye, and the primary is wye, the primary must be grounded as well.

In utility systems, the neutral wire is the same as the ground wire, and the neutral is common to both primary and secondary. Therefor all of the primary neutral bushings would be tied together, and then tied to the secondary neutrals, which are tied to the transformer tank ground and the system neutral wire running with the circuit(s) on the pole.

Utilities aren't concerned about separating neutrals and grounds the way the NEC is.

Jim T

 

[templdl]

Re: wye wye transformer problems

Thanks Jim,
I did find it interesting that

Utilities aren't concerned about separating neutrals and grounds the way the NEC is.

That kind of makes you wonder about how concerned they are about stray ground current doen't it?
Dave

 

[jtester]

Re: wye wye transformer problems

quote

That kind of makes you wonder about how concerned they are about stray ground current doen't it?

The utility neutral system presents much less impedance than the earth, so probably more than 80% of return currents flow in the wires, but certainly utilities recognize that there are currents flowing in the ground at all times. It is usually only a problem at dairies, swimming pools, etc., but not always. I worked for utilities for over 18 years, and had employees chasing stray voltages on a regular basis.

There have been many posts here about stray currents and what utilities will and won't do to mitigate them.

Jim T