This Transformer image was submitted by a Member of ECN, for discussion purposes.
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Pre-Note:The following abbreviations / acronyms may be used in this text:
* ECN: Electrical Contractor Network (this Forum),
* GEC: Grounding Electrode Conductor,
* GES: Grounding Electrode System,
* EGC: Equipment Grounding Conductor,
* SDS: Separately Derived System,
* TVS: Transient Voltage Surges.
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--- Discussion Text regarding attached Transformer Image ---Take notice of the connection setups:
* Primary Side = Wye: Common Star-Point is Grounded to Enclosure + local GES.
Primary Voltage = 4160V
* Secondary Side = Delta (Closed).
Secondary Voltage = 480V.
Member denotes that the Secondary Side (the "SDS") is Ungrounded.
This Transformer is on a Military Installation, which may be reason for the Primary side being a Grounded Wye (i.e.: Mil. Specs.).
I will perform some research, then post any relevant data per the possibility of Mil. Specs.
Other than certain Specification requirements, the use of a Grounded Primary Transformer with a Medium Voltage Circuit feeding it, should help to reduce "Flash-Over" issues to the Secondary Windings, as result of:
* Surges (Lightning-related and TVS from interconnected Equipment),
* Winding Insulation Failure / Breakdown.
A Wye connection would be the most relevant approach, for "Per Instance Primary Side Grounding" type protection of a Low Voltage SDS.
Since the SDS (Secondary Side of the Transformer) is an Ungrounded System, this type of Protection would help reduce & stabilize (to an extent) the Voltage on the Secondary Side, should a Winding Fault occur.
I would imagine the Primary side's Star-Point is bonded to the same Grounding Electrode System created at the Building(s) where this Transformer is used.
*** NOTE ***
Even though the Secondary / SDS is Ungrounded (the System is not physically Grounded), there will be a GES created at the location where this System is used.
The metallic Equipment is bonded to that local GES via the use of EGCs, and similar effective Bonding techniques.
This is to "Drive" the metallic Equipment to the Voltage found at the GES, so as to reduce the shock hazard to Personnel.
There will be
§ "Capacitive Coupling Effects" relative to the SDS's Circuitry, which will place charges on the metallic enclosures of the connected Electrical Equipment.
By Bonding this metallic Equipment to a local GES, the Potential Difference created when the Coupled Charges are placed on those enclosures, becomes stabilized.
I will end here, leaving this topic open for replies and clarifications by others.
Anyone wishing to address this topic may feel free to do so!
Additionally, I would like to get some responses from the Member whom submitted this item for discussion.
Scott
