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Generator grounding & protection with a wye-wye transformer

Generator grounding & protection with a wye-wye transformer

Generator grounding & protection with a wye-wye transformer

Given a generator 1.6MW 480Y/277V 60Hz and a GSU transformer with primary and secondary windings wye connected, neutral internally tied and solidly grounded.
What is the best method of grounding and minimum protection (relaying) necessary for the generator?

RE: Generator grounding & protection with a wye-wye transformer

On my question above, I'm referring to grounding the generator neutral. Is a floating neutral possible? Why and why not? If the neutral will be grounded, what are the options? Is reactance grounding better than resistance grounding? And, what if utility company requires to limit the zero sequence current to say 25Amps?

RE: Generator grounding & protection with a wye-wye transformer

Since the generator is connected to the utility via a YG-YG transformer, the neutral will need to be grounded via a high impedance or ungrounded. High impedance grounding is normally accomplished with a resistor.

A better arrangement may be to use a transformer connection that isolates the grounds (e.g. delta-wye) so you can solidly ground the generator.

RE: Generator grounding & protection with a wye-wye transformer

Any GSU installation should first be considered with high impedance grounding of the generator and a delta winding on the generator side of the GSU.  Often the high side winding of the transformer will be a grounded wye, but that can vary.  There would need to be significant reasons to use a different configuration, as any other configuration will provide less protection of the generator.

RE: Generator grounding & protection with a wye-wye transformer

A question for alehman and davidbeach.
As I understand the original post, the transformer primary is solidly grounded.
Will this transformer neutral ground not defeat any high impedance ground on the generator?
Are there any problems with removing the transformer primary ground and then grounding the generator neutral with a high impedance arrangement?
The transformer may then be considered to have a floating neutral, but it will be connected by the neutral cable to the generator neutral which would be high impedance grounded.
The high impedance would in fact be grounding not just the generator but the generator-transformer combination.
Are there any problems with this arrangment.
Please don't take this post as advice, I'm asking, not telling.
Thanks fellas.

RE: Generator grounding & protection with a wye-wye transformer

Another question. Isn't there what is known as 'swinging neutral' with Y/Y trafos and somewhere (to one of neutrals ?) a zigzag trafo is needed ?

"Most people stop working when they find a job"

RE: Generator grounding & protection with a wye-wye transformer

The YG-YG transformer acts as a 'pass-through' for the zero sequence network from the primary to the secondary (with the addition of the transformer impedance). If the utility network is solidly grounded, the generator will see a nearly solid ground. If the utility network is ungrounded (unlikely), there will be no zero sequence current path for the generator and it will behave as ungrounded.

Low voltage generators are most commonly solidly grounded (at least in my part of the world). Impedance grounding could be used as well. Again I think it would be desirable to isolate the grounds with a different transformer connection so you have control over it and can arrange relaying as you want.

See Blackburn Chapter 4.

RE: Generator grounding & protection with a wye-wye transformer

In theory, you could break the bond between the neutrals of the primary and secondary of the transformer and leave the generator side ungrounded.  Then it would be possible to ground the generator as desired, but with no neutral loads to be served the generator would be well served by high resistance grounding, even though it is a LV generator.

The reason I said "in theory" is that if the transformer came from the manufacturer with the neutrals bonded, it is likely only insulted for phase-to-neutral voltage rather than for phase-to-phase voltage and running with the bond open would be asking for more trouble.

Best thing to do is replace the transformer with a proper GSU rather than trying to patch things together.  The only saving grace here is that it is an LV generator, which has pretty light voltage stresses on its insulation system, and was built with the expectation of minimal protection, mostly just overcurrent.

RE: Generator grounding & protection with a wye-wye transformer

david beach
Thank you for the explanation. It has answered my question.

RE: Generator grounding & protection with a wye-wye transformer

o    Generator: 1.6MW, 480Y/277V, 60Hz.
o    GSU Connection: YG-YG (solidly grounded).
o    [i]Assumption: Industrial application interconnected with a power utility in US.

COMMENT: YG-YG connection may not the most desirable configuration because during ground fault event, the current is divided in some undetermined ratio between the ground legs on utility and customer side of the transformer. This imprecision makes it difficult to determine the setting on the ground fault relay and to coordinate it with other relay in the network.  On the other hand, harmonic may be a concern since small and mid size generators are prone to generate high level of third harmonics while the YG-YG transformers do not suppress the third harmonic as the D-YG does. The consequences are additional cost for filters schemes or nuisance tripping since the OC relay may see the high level of harmonic as a ground fault.

Referring to grounding the generator neutral:

- What is the best method of grounding and minimum protection (relaying) necessary for the generator?
    The protection of the generator will be greatly imposed by the transformer winding configuration (YG-YG) and the utility requirement for IPP interconnection at the common point of delivery . Since appears the transformer connection is alredy set, the other compliance should be with your local utility. NOTE: Request information for the interconnection requirement for small power generation and consult the IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power System.

a-    Is a floating neutral (ungrounded) possible (at the generator)?
Yes, it is possible to have ungrounded neutral. Although this limits the phase-to-ground fault current, generators are seldom ungrounded because of the risk to produce high transient overvoltages during fault and difficulties associated with fast fault location.

b-    If the neutral will be grounded, what are the options?
The methods most commonly used for generator grounding are high-impedance, low-resistance, reactance and grounding transformer. Check the ANSI/IEEE STD C37.101 & 102 “Guide for generator ground protection”. NOTE: Solidly grounded generator neutral is not recommended in mid and large size unit since this practice can result in high mechanical stresses and excessive fault damage.

o    Is reactance grounding better than resistance grounding?
The industrial practice in the US prefers resistance grounded over reactance grounded because reduce significantly the fault current without risk of overvoltage.  Reactance grounded, on the other hand, have typical limiting fault current values of 25% to 100% of the generator three-phase fault and much higher overvoltage than resistance grounded.

o    What if utility company requires limiting the zero sequence current to say 25 Amps?
High resistance grounded could meet this requirement. The most economical choice appears to be connecting directly the resistor to the generator grounding.

RE: Generator grounding & protection with a wye-wye transformer

Thanks for all the replies.
Originally, the GSU is intended to be D-YG. First, this was changed to YU-YG. Now, the utility company supplying the transformer is saying if the neutral bonding will be removed the transformer will lost the factory UL rating. That's why a YG-YG comes up.
The question is, how important is this UL rating?
Also, if I may ask for an advice, is it better on the IPP side to proceed with YU-YG or YG-YG?

RE: Generator grounding & protection with a wye-wye transformer

If the Y-Y transformer has a factory installed bond between the two neutrals, the transformer is insulated for the phase-neutral voltage, not for the phase-phase voltage.  Do not disconnect the bond, the UL listing is the least of your worries.  Often with these transformers the H0 terminal is not brought out, the only way to get to it is via the bond, and if H0 is not grounded you will have an insulation failure, sooner or later.

The good news is that LV generators are almost always solidly grounded, but don't usually have a ground source on the terminal end of the generator in addition to the neutral end of the generator.


1. Replace this transformer with a proper GSU in D-YG configuration.

2. Run solidly grounded and live with the high ground fault currents.

3. On thorough analysis of the electrical system, it may be possible to use the transformer at hand and leave the neutral end of the generator ungrounded.  There can be voltage transients during the period between applying the field and closing the breaker tying the generator to the utility.  Do not do this if the generator can supply load  without also being connected to the utility.

The first option is far and away the best option.

RE: Generator grounding & protection with a wye-wye transformer

I concur with David. Many utilities are going to YG-YG transformers in lieu of D-YG because they are less expensive. I dislike them because they prevent the facility from controlling it's own grounding.

A single 1.6M 480V generator should be able to withstand it's own ground fault current if that is a concern.

RE: Generator grounding & protection with a wye-wye transformer


A D-YG transformer is no longer an option at this point. In your 2nd option, both generator and transformer are solidly grounded and the neutral tied together? How about the concern on harmonics? How to live with the high fault current?
On the 3rd option, the set-up is generator-ckt. breaker-transformer. The transformer will always be energized and backfeeding for the generator auxiliary loads before synchronization is made in the ckt. breaker for tying to the utility. Is this a set-up that must be avoided?

Thank you.

Thanks also cuky and alehman for the additional info.

RE: Generator grounding & protection with a wye-wye transformer

Hi Alehman,

Regarding the use of YG-YG in this project, I am not sure if economics is the driving point since the IPP developer and not the utility pay often it.

I am also uncertain if the 1.6M 480V generator should be able to withstand its own ground fault current without grounding impedance. The reason for say that is that mid size generator zero sequence is very low causing the phase-to-ground fault is the most severe requiring limit by adding external zero sequence impedance in the neutral to ground point.
1) In addition to cost, utility move to YG-YG in the 60’s after the D-Y was the norm because was perceived less prone to ferroresonance. The exception of that are the grounded-wye primary three-phase transformers having amorphous metal cores and three-phase transformers having five-leg stacked cores. Transformer is less susceptible to ferroresonance as larger is their rated capacity and lowers the voltage. See the enclose link from ANSI/IEEE Std C-37: http://cuky2000.250free.com/Ferroresonance_Pwr_Systems.jpg

2) "According to ANSI C50.13-1977, the maximum stresses that a generator is normally designed to withstand is that associated with the currents of a three-phase fault at the machine terminals. Because of the relatively low zero-sequence impedance inherent in most synchronous generators, a solid phase-to ground fault at the machine terminals will produce winding currents that are higher than those for a three.-phase fault. Therefore, to comply with this standard, generators must be grounded in such a manner to limit the maximum phase-to less than, the three-phase fault current."

RE: Generator grounding & protection with a wye-wye transformer

Loads are tapped between breaker and transformer?  That should be fine as long as there is no way for the generator to feed the loads without the utility.  If you do that, you are then looking the system providing the grounding of the neutral, and no reason that shouldn't work.  You won't have ground fault currents as high as you would with the neutral grounded and can use the YG-YG transformer.  Again, this is all dependent on a system study showing that everything works properly under all normal and abnormal operating conditions.

You live with the high fault current by using switchgear with high enough fault ratings.  Watch your SLG fault value, in the solidly grounded case it can be significantly higher than the three phase fault value.

RE: Generator grounding & protection with a wye-wye transformer

In none of the comments have I seen the issue of unstable neutral.  Be aware that if the neutral of the generator is not connected to the neutral of the transformer, there will be no defined phase voltage to ground/neutral.  You can solidly ground the transformer but if the generator neutral is left ungrounded the combination of the generator and transformer have no way to stabilize the neutral.

RE: Generator grounding & protection with a wye-wye transformer

I just want to be sure that I understood everyone correctly. So, in summary:

1. The transformer (YG-YG) will have be solidly grounded. (As in, just wire/connect the X0 terminal to ground.)

2. The generator neutral should be wired (connected) to the transformer neutral.

Here is what I'm not sure. If a grounding resistor is needed, where should it be connected? In series with 1 above and maintain one point of grounding? Or aside from 1 above, connect this grounding resistor to the generator neutral and have two points of grounding?

RE: Generator grounding & protection with a wye-wye transformer

Grounding resistor would be in parallel with the solid ground at the transformer.  Any resistance parallel with zero resistance still equals zero resistance.  The only possible benefit of the grounding resistor would be to provide a ground reference for the generator while the breaker is open.

RE: Generator grounding & protection with a wye-wye transformer

The enclose information is a summary from the Special Report of the IEEE Relay Committee regarding the application of generator grounding resistor  interacting with a generator step-up transformer (GSU) in YG-YG winding configuration serving local load and interconnected with an utility substation.

I hoping this information help enlighten this discussion in benefit of the project and the member of this forum.

Good luck all.


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