Y Y Transformer Grounding
- Thread starter nkbhinge
- Start date
- Location
- New Jersey
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- Journeyman Electrician (retired)
Even with a Wye primary you do not connect a neutral to it and you do not ground the primary H0. (fixed error)
Last edited:
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
Sorry yes primary H0. I'll make the correction thank you.
So basically, if I have a service disconnect switch before the transformer, I have a MBJ at the service disconnect switch. From this service disconnect switch, I bring 3 Lines, and EGC? No neutral? And at the transformer primary, do not connect neutral to ground? But do it at transformer secondary with SBJ? Does this seem to make sense? If yes, why then it is called Yg-Yg? Sorry for too many questions.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
Are you talking about the "WYE-WYE Always Tie" thing?
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
I believe that only applies when the transformer has a set of delta windings on it (secondary or tertiary). As that would enforce the condition that the sum of the primary L-N voltages is 0, by drawing extra current on the primary if necessary. Which is a problem if the primary system at the transformer is not perfectly voltage balanced.
As to the Yg-Yg question, the primary wye system's neutral would be earthed only at its source, not at this transformer. The secondary wye could be installed as an SDS, in which case you connect the secondary's neutral to a GEC to earth it, and install an SBJ on the secondary side. Or it could be grounded directly by connecting the primary neutral to the secondary neutral, making it not an SDS. The latter has a longer fault clearing path for ground faults (relying on the MBJ/SBJ on the primary wye system). Either way the primary and secondary neutrals get interconnected, either through the GES for the SDS, or directly.
Cheers, Wayne
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
I'm pretty sure even ungrounded, without a neutral you need to tie the primary and secondary H0-X0 to keep the voltages stable.
That y-y always tie, goes back a long time in my schooling, so I may be remembering it wrong. I remember "Alley Cat Bad Dog" though and my resistor color codes, so school wasn't a total waste
D. Castor
Senior Member
- Location
- Port Angeles, Wash
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- Electrical Engineer
I don't think it is advisable to leave the primary Y totally ungrounded if the secondary Y is grounded unless there is a tertiary delta winding in the transformer to supply current for secondary ground faults.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
Here is what I remember about Y-Y connections. If H0 and X0 aren't tied and you have a low impedance fault to P-P or P-N/G your voltage will spike on the phase or phases that are not involved in the fault.
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
You might be correct. Can you explain this further as it relates to the OP?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
In case 2, you would not both install an SBJ and directly connect the primary neutral to the secondary neutral. That puts primary neutral current on the grounding system, as it puts the SBJ in parallel with the MBJ. So you do one or the other, to get an SDS or not.
Also, in the case 2 diagram, what you have labeled an SSBJ is just the EGC, same as case 1.
I'm not 100% clear on whether and how case 1 would work, but suspect it would give you unstable secondary L-N voltages when you have L-N loads.
Cheers, Wayne
If its not an SDS, then a new system is not 'created' so would the ECG and Neutral of entire fault clearing path of the 208 'side' not need to be sized to minimum size per 250.122 for the 208 side? this would in effect be a TN-S - TN-S wye.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Presumably your first writing of 208 was meant to be 480, and I am responding as such.
That is an interesting point, as the secondary depends on the MBJ from the primary for ground fault clearing. And while this is certainly true from a technical perspective, I wonder if the NEC is sufficiently clear about this anywhere.
In the diagram shown this would not be a big change, as the transformer is supplied directly from the service disconnect with the MBJ.
Cheers, Wayne