ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

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

Is this used at a facility that uses / tests shipboard equipment? The ungrounded delta is or was used aboard Navy ships to enhance equipment reliability during battle situations. A single phase could become grounded and the equipment will still function until another phase was grounded.

I'm the one who submitted this. It is from one of the hangars at the now defunct El Toro MCAS. It was used for power to the planes as far as I know. At the time it was shut down this xfmr had already been abandoned. It may have been used on earlier planes such as the F4 Phantom, but I am just guessing.

larry

MCAS = Marine Corps Air Station?

If so, it is quite likely it was used to support the equipment used for the little planes the Marines landed on aircraft carriers.

Larry C

Yes, Marine Corp Air Station.

I wonder why it is a 50 Hz transformer.
Joe

Ordering mistake?

Located on a 50 Hz bus used for NATO and Soviet ground gear?

This is in Irvine in Southern California.

I don't think we were ever on a 50hZ system here.

What would happen if it were connected to a system generating 60hZ?

I go along with the idea that this is just some European theater equipment that came home.
Scott probably knows best but my guess is 50hz transformers will work at 60hz although maybe not as efficiently.
If it works I bet GIs use it.

My GUESS is that the transformer will run a little hotter, the voltage regulation will be a little worse, because the internal impeadance will have increased. The output voltage will run a little lower.