Electricmo
Senior Member
- Location
- Missouri
- Occupation
- Lineman
We are talking wye/wye transformer banks here. Not open wye/ delta banks. Thanks for sharing your knowledge of the subject though.
Wye Grounded Primary and Wye Grounded Secondary Transformer
- Thread starter bobby ocampo
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mivey
Senior Member
Yes. It does not matter how X0 and H0 are connected.
To make it a non-SDS you have to also have the other X and H terminals connected or on the same winding side of the transformer.
In other words, if the high side and low side can both path electrons through the same winding then it is not a SDS. An SDS keeps primary and secondary electrons separated by electro-magnetic fields between the windings.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
IMO, the simple act of 'connecting' a neutral point to ground is not sufficient to define an SDS versus a Non-SDS.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Another wrench in the works is, I don't see how one could connect a primary neutral to ground unless it is under the MGN provision in 250.184(C), so if it's LV, I can't see a code compliant way to land the primary neutral even if you wanted to.
But back to the basic question, IMO if I connect XO and HO "directly" and not just thru the GEC's, it's not an SDS. That is what the definition says.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
What prevents the common GES from interconnecting the neutrals?
It seems that it's electrically impossible to have a truly separately-derived system with any grounded secondary.
- Occupation
- Licensed Electrician
Didn't I say that already?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Let's be clear, There are a several different things being discussed:
1. Whether and when such and such meets the NEC definition of SDS.
2. Whether in practice such and such is truly isolated, electrical any different, etc
It seems to me the OP is talking about #1
kwired
Electron manager
- Location
- NE Nebraska
Separate windings for separately derived sytem means separate circuits for primary current and secondary currents, other than whatever point is somehow connected to ground. Arrangement of grounded conductors for NEC applications normally means that a grounded conductor should only carry primary or secondary current and not both in normal operation. Is ok if it would happen to carry fault current in an abnormal situation.
What is not a separately derived system is an autotransformer. Those have a common conductor that carries both input and output current simultaneously during normal operation, and the "coils" are in series in some way rather and are not limited to just magnetic coupling through the core being the only interaction between them
What is not a separately derived system is an autotransformer. Those have a common conductor that carries both input and output current simultaneously during normal operation, and the "coils" are in series in some way rather and are not limited to just magnetic coupling through the core being the only interaction between them
bobby ocampo
Senior Member
Then what defines a Separately Derived System?
bobby ocampo
Senior Member
If both HO and XO are bonded are they still a separately derived system?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
According to the Code, if they are bonded to each other it is not an SDS. If they are both bonded to a GEC, even the same one, it can still be an SDS.
kwired
Electron manager
- Location
- NE Nebraska
Not sure I completely agree.
By their definition the grounded conductor is not a common conductor that carries normal operating current of either primary or secondary coil(s). Grounded conductors of any two separate systems is still going to have electrical continuity between them as there is only one "ground".
You do get stray currents on utility supplied MGN systems because of the MGN concept, but once you get beyond service equipment, all separately derived systems have separate grounded conductors and are only supposed to connect to grounded conductors of other systems via the grounding electrode conductor and associated bonding jumpers.
By their definition the grounded conductor is not a common conductor that carries normal operating current of either primary or secondary coil(s). Grounded conductors of any two separate systems is still going to have electrical continuity between them as there is only one "ground".
You do get stray currents on utility supplied MGN systems because of the MGN concept, but once you get beyond service equipment, all separately derived systems have separate grounded conductors and are only supposed to connect to grounded conductors of other systems via the grounding electrode conductor and associated bonding jumpers.
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Kwired, not sure where you are getting that from. I am not seeing anything in there talking about "normal operating current" let alone grounded conductors.
kwired
Electron manager
- Location
- NE Nebraska
Maybe a little reading between the lines of what "other than those established by grounding and bonding connections" means.
To me it basically means that a secondary conductor should not carry current of the primary system, whether grounded or not and the only electrical connection between primary and secondary should be via grounding/bonding conductors and that they should be arranged that they won't carry normal operating current.
Now throw in separately derived vs non separately derived generator system - the SD arrangement we switch the grounded conductor in the transfer switch because if we don't we can end up with stray currents on parts that were not intended to carry current, wiht NSD arrangement we still have two sources but we arrange things so that there is still only one neutral to ground bond point and that keeps non desired current from following non current carrying components - in normal operation. If there is a fault condition we still want current to flow, and hopefully enough it will operate an overcurrent device rather quickly.
WKB PE
Member
- Location
- Central Illinois
- Occupation
- Professional Engineer
I am currently designing a system for crypto currency miners. They are stepping 480/277 volt Y down to 415/240 Y via autotransformers. They need the 240 for the computers. The wiring diagram for the autotransformers show that the HO and XO are the same terminal in the transformer. Therefore this is not a separately derived system and no bonding of neutral to ground and no grounding electrode is required in my opinion. I would say the same for any other non separately derived system except over 600 volt Y to below 600 volt Y transformer. The last design that I provided for a 13.8KV Y to 480/277 Y pad mount had the HO and XO already bonded together when the liquid filled pad mounted transformer arrived on site. I confirmed with a utility company engineer friend of mine that this was acceptable and bonded neutral to ground and provided grounding electrode system same as any other service. I guess my opinion (and I may be wrong) is below 600 volt if the XO and HO are bonded or the same terminal you do not have a separately derived system and therefore do not need any neutral to ground bonding jumper or grounding electrode connection. Below 600 volt If the HO and XO are not bonded and not the same terminal in a Y:Y transformer I believe that I would provide a neutral to ground bonding jumper and grounding electrode system on the secondary side only. I do not think you should bond the primary neutral to ground as this would be a code violation.
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