Generator Grounding

[cay1998]

Hello everyone. I'm new to the forum. My background is with generators but I'm now in the transformer business. In an effort to help some of my customers, I'm trying to understand certain situations. I've been given conflicting answers to a couple scenarios and was hoping for some input.

First scenario:

1,000 kW generator 277/480 volt feeding a 1,500 kVA transformer with a 4,160 Y / 2,400 primary and 480 Y / 277 volt secondary.

Since this transformer is being "back fed" and is being used as a step up, I have a grounding question. This is a temporary installation that is feeding a 600 amp 5 kV fused disconnect with the appropriately sized fuses.

How should the transformer's neutral (XO) be managed? XO left floating, XO tied to ground or XO tied through an NGR. Why ?

How should the transformers high voltage neutral (HO) be managed? HO left floating, HO tied to ground or HO tied through an NGR. Why ?

How should the generators neutral connection be managed? Left floating, tied to ground or tied through an NGR. Why ?

Second Scenario:

Same as first except the transformer is a 4,160 D to a 480 Y / 277.

How should the generators's neutral be managed?

How should the transformers secondary neutral be managed?

If any of this is unclear, please let me know.

I'll be looking forward to a response.

Thank you !

 

[cay1998]

Does anyone have any input on this? Thank you.

 

[erickench]

What I don't understand is how you connect 277/480 generator to a 4160 Volt primary. Should'nt the generator be connected to the low side of the transformer?

 

[ohmhead]

Well your step up is fine in my book trans is larger than gen but i guess you load will be the factor .

I would ground neutral for just a simple fact of floating neutral and if your using a leg or not to a grounded conductor even with the step up connection .

Ground it ! at generator or transformer .

To protect a fault condition return path to transformer !

 

[LarryFine]

First scenario:

How should the transformer's neutral (XO) be managed? XO left floating, XO tied to ground or XO tied through an NGR.

The (now) primary's neutral should be left floating, as if it was a Delta load.

Why ?I'm not sure, but I know that's the right answer.

How should the transformers high voltage neutral (HO) be managed? HO left floating, HO tied to ground or HO tied through an NGR.

The (now) secondary's neutral should be bonded, unless you have a good reason not to.

Why ?

For one reason, this will provide you with the lowest line-to-earth voltage.

How should the generators neutral connection be managed? Left floating, tied to ground or tied through an NGR.

Again, grounded would be my first choice.

Why ?

I don't know, I just like the idea. Similar reason as above, I guess.

Second Scenario:

Same as first except the transformer is a 4,160 D to a 480 Y / 277.

How should the generators's neutral be managed?

Same as above.

How should the transformers secondary neutral be managed?

Same as above.

Of course, you won't have a (now) secondary neutral, so you can either float it or bond one phase.

 

[ohmhead]

Well think of it this way .

The phase to ground voltage remains constant during a ground fault .There are very high fault currents which can result in more damage then if a ground point is used .

If the fault is aloud to rotate thur generator phases you have a bad condition more damage is going to happen like generator failure or bigg dollars . kinda a Longer time to trip

Meaning current can now flow into other phases which is a high level during a fault .
Meaning fault clearing can be longer with out a grounded conductor bonded.
Normal faults are to ground its better to go to ground then any other point like other equipment or people and grounding it would limit the trip time safely.

Some point must be grounded you pick it !

 

[cay1998]

Though there are different trains of thought, a transformer is not a smart machine. It does not know a primary from a secondary.

While the ideal situation would be a 480 D to a 4,160 Y / 2,400, I have several customers that have wye to wye and delta wye and would like to use as generator step up.

I have successfully used generator step up units like this in the past with no problem.

What I'm trying to do is to understand what is RIGHT...possibly even...What is CODE?

To address Larry's response. I'm told that the generator should be grounded through an NGR, especially if while paralleling. If stand alone, it should be grounded (through an NGR if it is desired to limit ground fault to a certain level).

I'm told that the low voltage side of the transformer should NOT be grounded, just like you have indicated because it will limit the circulating currents (again, talking about paralleled systems).

I wouldn't see any reason why the 4,160 volt side of the transformer shouldn't be grounded.

Now, when it comes to using a 4,160 D to a 480 Y / 277 to step up a 480 volt generator to 4,160 volt power... I'm told that the generator should be grounded but not the 480 Y / 277 side of the transformer.

I'm still trying to understand this.

Anyone else ?

Thank you all for your input, by the way.

 

[cripple]

Generator Grounding

Here are some things you may want to look over.

When the secondary (WYE) of a DELTA-WYE transformer is energized, instead of the primary (DELTA), the neutral should not be connected to ground or to the enclosure. An unbalanced source might circulate current in the transformer DELTA and cause over-heating. Also the impedance to ground of the transformer might be lower than the system ground impedance and excessive current to ground might over-heat the transformer. Unbalanced conditions could cause a voltage to ground to appear on the enclosure if it was connected to the neutral.

You may discover that the primary protector, having been properly selected and applied per Article 450 of the National Electrical Code, nevertheless, trips or blows when you attempt to energize the reverse operated transformer. This phenomenon can occur because the low impedance winding now serves as the primary and the value of the magnetizing inrush current (Mag-I) is actually much greater than expected. The Mag-I experienced when energizing transformers is similar to the inrush current associated with motor starting.

In conclusion if the transformer was originally designed for primary and secondary as stated in the nameplate, which is validated by the UL mark and if the transformer is reverse-fed or used in an application other than what it was originally intended for, make sure you consult your local codes and UL standards.

 

[jim dungar]

First transformers have low voltage windings and high voltage windings. Primary and secondary have to do with which set of windings feed (supplies the magnetizing current) the transformer.

Depending on winding configurations, such as taps and inductance, it is usually easier if the primary winding is wound further away from the core than the secondary, and due to conductor size it is often easier for the high voltage windings to be on the outside of the low voltage windings. But, these locations are not absolutes.

Now, for the primary wye neutral point. Unless you are a utility you should always allow the "X0" bushing on the primary of a transformer to float, especially if the secondary is a delta. Floating the X0 will allow the voltage across individual windings to 'shift' as necessary to accommodate unbalances on the delta secondary. The X0 bushings on wye-wye transformers can be grounded, but you need to make sure that you are not creating multiple neutral-ground connections.

At the generator, ground the neutral as required by you protection scheme (ignoring the transformer).
At the transformer primary, treat it as if it was a delta connection (let X0 float).
At the transformer secondary, treat it like it the separately derived system it is, ground the neutral per your protection scheme.