ECN Forum
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! smile

Additionally, I would like to get some responses from the Member whom submitted this item for discussion.

Scott


Description: Isolated Transformer: Wye Primary w/ Grounded Star-Point, Delta Secondary (Ungrounded) Submitted by ECN Member
Attached picture wye_delta_L_LV_MCAS01.jpg
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.
Quote
What would happen if it were connected to a system generating 60hZ?


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.
I found this on the net

http://ed-thelen.org/1401Project/TricksFor50-60cycles.html
Common "Iron" Core Power Transformers the kind we have.

Executive Summary: Raising the applied frequency from 50 Hertz to 60 Hertz is just fine.

I am placing quotes around "Iron" because actually the metal is sheet steel with more than the usual amount of silicon to increase its resistivity. The sheets are insulated from each other with oxide to reduce losses and heating due to "circulating currents". For transformers designed for 50 Hertz, raising the applied frequency 20 percent to 60 Hertz is no big deal. :-)) Actually the "iron" losses will decrease a little due to reduced "iron" saturation and hysteresis loss. :-)) Losses due to "circulating currents" will increase a little.

Going the other way, from 60 Hertz design to 50 Hertz usage, is a *BIG DEAL*. The applied input voltage should be reduced to 5/6 name plate rating or bad things can/will happen. They need about 20 % more "iron" to work at 50 Hertz.

Reference - most any transformer book and also
http://www.sola-hevi-duty.com/products/powerconditioning/pdfs/opchars.pdf
50Hz equipment on 60Hz supply will usually operate with very few problems.

It is 60Hz equipment on 50Hz supplies that has problems of overheating primarily due to issues with the magnetic circuit.
When you will get in trouble is with synchronous motors.
Originally Posted by JBD
50Hz equipment on 60Hz supply will usually operate with very few problems.

It is 60Hz equipment on 50Hz supplies that has problems of overheating primarily due to issues with the magnetic circuit.
Operating 60Hz transformers at 50Hz will cause the core to saturate, at which point the impedance drops to nil and the coil is essentially a short circuit. The transformer must thus be voltage derated by 20%. I had the misfortune of witnessing this first hand when some 60Hz transformers were mistakenly installed at a 50Hz site- the engineer that designed it swore up and down it would instantly explode into flames, but thankfully his calculations were just conservative enough to run. We recorded twice as much kVA going in as coming out, and even at light load with covers off and big fans blowing, the transformers were close to the max rated temperature.

Operating a 50Hz transformer at 60Hz will often cause excessive core hystersis and core heating due to the larger core and especially due to the wider core laminations, which provide much lower impedance to eddy currrents. I have not witnessed it, but another engineer I work with witnessed an oil transformer explode due to heat generated form this.

Dual-rated 50/60Hz transformers use thin 60Hz core laminations built up to a core size thick enough to support 50Hz at the rated voltage without saturation. They're generally less efficient than dedicated 50Hz or 60Hz, but are often cheaper as a single design and assembly line can provide equipmenet that works anywhere in the world.
This is the kind of equipment found in the Pacific theatre, especially Japan -- there's lots of 50 Hz there right outside the base.

Back in the day, rotary phase-converters for 400 Hz aircraft power were common as dust. They used additional poles within a motor-generator scheme to bump the frequency. It'd take the whole output of this XFMR to drive the load of a big aircraft while it's on the tarmac.

Now VFDs have made them out-dated.

Interesting to note the various tap options on the primary.
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