Paralleling Power Transformers 4

[BruceC]

Hi,

I need to parallel a Y - Y 4800-208/120 transformer to a 480-208/102 transformer. The 480 transformer is fed by the output of a small induction generator (so it consumes VARs) which is fed by the same 4800-208/120 transformer. Will the 480-208/120 also need to be a Y - Y configuration? Or is this even an issue? In other words, I'm not sure if the 480 transformer appears in parallel, or series with the 4800 transformer that is feeds the induction generator. Can anyone help me with this? Thanks in advance, and this is a GREAT site!

BruceC

 

[Shortstub]

For parallel operation of 3-phase trasformers, their impedance, polarity, and phase-shift must be alike.

Because the 480 Volt unit has a secondary specified as wye, then, too, its primary must be wye. This because a delta primary will introduce a 30-deg phase-shift!

 

[jbartos]

Suggestions/questions to the original posting marked ///\\I need to parallel a Y - Y 4800-208/120 transformer to a 480-208/102 transformer.
///Usually, the same/identical transformers are paralleled. since the first one has primary of 4800V and the second one 480V the transformers cannot be paralleled without some voltage adjustment on the primary side. Also, please, clarify the second transformer secondary 208/102.\\
The 480 transformer is fed by the output of a small induction generator (so it consumes VARs) which is fed by the same 4800-208/120 transformer.
///This can be interpreted as the induction generator is on 480V transformer side and 208/120V secondaries of the transformers are to be connected, e.g. as star-star transformer secondary windings.\\ Will the 480-208/120 also need to be a Y - Y configuration?
///It would be better to connect the transformer secondaries 4800-208Y/120Y to 208Delta/480VStar and generator windings into delta to break the zero sequence current paths. Each wye connection interfacing delta connection should be grounded.\\ Or is this even an issue?
///Yes, it is an issue to break the zero sequence current paths.\\ In other words, I'm not sure if the 480 transformer appears in parallel or series with the 4800 transformer
///The transformers would be connected in series rather than in parallel if the 208/120V sides are connected together.\\ that is feeds the induction generator.

 

[DanDel]

I'm not sure what you mean when you say that "The 480 transformer is fed by the output of a small induction generator (so it consumes VARs) which is fed by the same 4800-208/120 transformer." This sounds like you're saying that the generator is fed by the 4800-208/120 transformer. If you're trying to parallel a generator-driven source with a utility-driven source, you need synchronization equipment. Please clarify your system.

 

[BruceC]

Thank you for your response. I will try to explain in better detail what I am attempting to do.

I have a 150kW Induction generator with a 480 volt, 3-phase output. I would like to connect the power output of this generator to an existing distribution switchgear bus which is fed from a 4800-208/120 volt utility transformer. I know I need to have transformer for the output of the generator to get the same voltage level of 208/120. Syncronizing equipment should not be necessary since this is an induction machine, correct? I also know that a delta-wye connection will cause me problems with the phase shift. I am not concerned with polarity, since that can be corrected with the external wiring from the transformer to the switchhgear and a phase rotation meter.

Again, thank you to all who have and will respond!

BruceC

 

[busbar]

In the US, induction generators have interconnection requirements, although they are typically less complex than those for synchronous generators. If you are planning to generate electricity to reduce your power demand {slowing the watthour meter}, or send back to the utility, {reversing the meter} a contract has to be secured with the serving utility. There are significant safety implications and potential damage to either participant in operating an induction generator.

 

[jbartos]

Suggestion: I agree with busbar to have your intent in agreement with the Utility rules and regulations.

 

[DanDel]

The system you describe has significant switchgear and protection necessities, of which synchronization equipment is only a part. There are also other concerns, as mentioned by busbar and jbartos.
Typically, an induction generator is started by utility power as a motor, and then the prime mover is brought up to increase the rotor speed from below synchronous to above synchronous, which will push power back into the system. Protection for overcurrent(motor start and FLA), and over/underspeed is the minimum. This is no simple project.

 

[BruceC]

Thank you all for the helpful comments!

I already have the necessary protections in place with shunt trip breaker, over/under freq. and voltage etc. Remote emergency stop and run/stop indication. Since the utility provides the field excitation for the generator (excess steam is prime mover, generator is acting as throttling valve) I'm thinking that syncronization isn't an issue. Am I way off base here?

Also, the utility has rate tables that apply to this installation, and have been extremely helpful.

Thank you all again.

BruceC

 

[DanDel]

BruceC
It appears that you have the appropriate protection from your last post.
Your initial question was about transformer configuration. Are you indeed starting the generator as a motor connected to the utility? If this is so, you simply need the correct transformation to connect to the system as a motor,and no synchronization equipment is needed(though a synch-check relay should be used to prevent reclosing the C/B after a trip if the generator is still spinning).

 

[busbar]

Between the utility and the induction machine, the two described transformers would be considered essentially in series.

The inherent 30° delta-wye or wye-delta low-voltage-transformer phase shift should not generally be of concern for generator operation.

Startup synchronization should not likely be required. That may change if operationally the unit is motored up by the prime-mover or stator-induced power. The only obvious resync concerns may be if something on the utility system islands the generator; id est, reclosing relay cycling versus generator time constants. At any rate, your facility is almost assuredly bound by the terms of the utility’s {should have been clearly understood and accounted for in the project-design stage before now} interconnect requirements.