# Connebting two generators in parallel



## Mohammed Ahmed (Jan 25, 2021)

What I know synchronization is required when connecting two generators in parallel, one of the requirement before connecting two generators is to make sure they have the same voltage rating. My question, is it required to match phase voltage or line voltage together,. I see one question in NCEES# 523 two generators have different connection ( one in delta while the other generator in wye but they have the same line voltage 2.4 KV)?


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## RedRaider2020 (Jan 25, 2021)

You can't parallel a delta and a wye transformer. The 30 degree phase shift will cause the magic smoke to come out. It's best if both are exactly the same. You can parallel a delta wye to another delta wye though as long as they are the same. They can have different MVA and impedance but then you have to ensure that you don't overload the smaller of the two. So both line to line and phase to phase voltage must match and they must be connected with the same phase rotation.


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## Zach Stone P.E. (Jan 26, 2021)

Mohammed Ahmed said:


> What I know synchronization is required when connecting two generators in parallel, one of the requirement before connecting two generators is to make sure they have the same voltage rating. My question, is it required to match phase voltage or line voltage together,. I see one question in NCEES# 523 two generators have different connection ( one in delta while the other generator in wye but they have the same line voltage 2.4 KV)?


Since both of the generators in the problem are three-phase, the voltage given for each generator is the three-phase line voltage rating, and not the individual phase voltage rating which will be different for the wye connected generator (smaller by a factor of the square root of three, and lagging by 30 degrees).

The line voltages have to be synched. The power grid does not care nor can it tell if the generators connecting to it are delta connected or wye connected. All it can "see" is the line voltage being applied to the grid terminals.



RedRaider2020 said:


> You can't parallel a delta and a wye transformer. The 30 degree phase shift will cause the magic smoke to come out. It's best if both are exactly the same. You can parallel a delta wye to another delta wye though as long as they are the same. They can have different MVA and impedance but then you have to ensure that you don't overload the smaller of the two. So both line to line and phase to phase voltage must match and they must be connected with the same phase rotation.



While it's true that it is generally a *very bad* idea to parallel transformers with different connections for the reasons you described (such as one that is delta wye and one that is wye-delta), paralleling generators with different three-phase connection types should be okay as long as they are synced correctly.

For example, let's say a delta connected and wye connected generator are syncing to the grid, like the NCEES problem in this post, and that the grid for reference is balanced and positive with an angle of zero degrees for the A-line voltage.

For the *delta* generator:

the phase rotation of the generator must match the positive sequence rotation of the grid (ABC).
the phase voltage *magnitudes* being supplied by the *delta* generator must match the line voltage magnitudes of the grid.
the speed of the generator rotor is slightly sped up or slowed down until the A-phase voltage of the generator is at zero degrees, matching the A-line voltage of the grid, then the speed is set back to 60 Hz.
the generator is now synched and the generator break can close, connecting the generator to the grid.
For the *wye* generator:

the phase rotation of the generator must match the positive sequence rotation of the grid (ABC).
the phase voltage *magnitudes* being supplied by the *wye* generator must be smaller by a factor of the square root of three so that the resulting line voltage magnitudes of the generator match the line voltage magnitudes of the grid.
the speed of the generator rotor is slightly sped up or slowed down until the A-phase voltage of the generator is at minus 30 degrees (-30º) so that the A-line voltage being supplied by the generator is at zero degrees (0º), matching the A-line voltage of the grid, then the speed is set back to 60 Hz.
the generator is now synched and the generator break can close, connecting the generator to the grid.
Unlike transformers that will always phase shift accordingly depending on their connection type, generators as a power source can be sped up or slowed down during the synching process to match their phase shift with respect to the grid.

For what it's worth, just about all of the utility and co-gen turbo-generators (sync gens) I've worked around in the past from the small (1.5 MW) to medium (55 MW) range were all wye connected and fed to a delta wye generator step-up (GSU) transformer with the exception of just one very old generator that was delta connected.

In my experience, they are usually wye connected to allow a neutral for both grounding and protection/monitoring purposes.


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## akyip (Jan 26, 2021)

Zach Stone P.E. said:


> Since both of the generators in the problem are three-phase, the voltage given for each generator is the three-phase line voltage rating, and not the individual phase voltage rating which will be different for the wye connected generator (smaller by a factor of the square root of three, and lagging by 30 degrees).
> 
> The line voltages have to be synched. The power grid does not care nor can it tell if the generators connecting to it are delta connected or wye connected. All it can "see" is the line voltage being applied to the grid terminals.
> 
> ...


This is pretty informative. Never thought that specifically about synching 3-phase generators with different phase windings/connections. Thanks for the FYI!


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## RedRaider2020 (Jan 26, 2021)

Zach Stone P.E. said:


> Since both of the generators in the problem are three-phase, the voltage given for each generator is the three-phase line voltage rating, and not the individual phase voltage rating which will be different for the wye connected generator (smaller by a factor of the square root of three, and lagging by 30 degrees).
> 
> The line voltages have to be synched. The power grid does not care nor can it tell if the generators connecting to it are delta connected or wye connected. All it can "see" is the line voltage being applied to the grid terminals.
> 
> ...


Dang, sorry about that. I was totally thinking about transformers instead of generators.


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## Mohammed Ahmed (Jan 27, 2021)

thanks all for sharing very useful information..


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## Zach Stone P.E. (Jan 27, 2021)

RedRaider2020 said:


> Dang, sorry about that. I was totally thinking about transformers instead of generators.


No problem! I'm not the original poster, just wanted to share what I've seen in the industry.


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