3 phase generators/transformers operating in parallel 2

[powereng91]

Is it possible to parallel connect a generator that is fed into a delta-wye step up transformer to a number of other generators which are fed into delta-delta step up transformers. Basically I want to connect these generator fed transformers all to a common bus. They are all operating at the same frequency and the star point of the delta-wye transformer is tied to earth through an NEX. To my understanding the line voltage on the wye side of the delta-wye transformer will be leading the current by 30 degrees. For the delta-delta transformers the current will lag the voltage by 30 degrees. So the angle between the voltage and current will be the same for all transformers. My question is whether it would be possible to sync these generators. I have limited experience working with 3 phase generators so any help would be appreciated. Thanks.

 

[dpc]

Yes, it is possible if the generator has a dedicated transformer. The generators would be synchronized across their own generator breakers and the phase shift between the other generators due to the transformer phase shift differences would not be an issue as long as there were no other connections between the generators.

If you are in the US with ANSI transformer, then for the delta-wye transformer, the low side voltage will lag the high side voltage by 30 degrees, unless the transformer is non-standard. For the delta-delta transformers, there will be no phase shift. The phase shift relates to the voltage only. The phase angle between the voltage and current is determined by the power factor of the loads.

 

[powereng91]

Thanks for your reply. So if the generators are synced on the primary LV side of the transformers then on the other side of the transformers the delta-wye line voltages will lead the delta-delta transformer line voltages by 30 degrees. Are you saying this will not be an issue? or should they be synced somehow on the secondary HV side of the transformers so that the phase shift differences are not an issue?

 

[waross]

The synchronizing gear should be in the same place as the breakers. Generally there is no breaker between the generator and the GSU transformer. If there is a breaker it is for protection and is always closed unless a fault causes it to open.
I assume that you are closing between the GSU and the bus. That is where the sync gear must be.
Old school: The incoming generator will be running a little above frequency and that will cause the synchro-scope needle to rotate slowly clockwise. When the synchro-scope needle is at 12 o-clock, indicating synchronism, the breaker is closed and the set picks up a little load. The throttle or prime mover control is then advanced to increase the loading.
Everything is in phase at the bus. There may be phase differences between the bus and the various generators introduced by wye;delta connections and transformer vector groups but everything is in phase at the bus.
A generator has difficulty energizing a transformer of the same capacity as the generator at full voltage due to the transformer inrush. GSUs are always direct connected to the generator and the voltage builds in both the generator and the transformer together, avoiding inrush issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[powereng91]

@Waross There are additional breakers between the generator and the transformer for protection. The generator and the transformer are of the same capacity. So to prevent transformer inrush should I put the synchronising gear across the CB's between the transformer and the bus and close here when they are in sync?

 

[waross]

Yes.
The breakers between the generators and the GSU transformers should be closed BEFORE the generator is started.
The synchroscope PTs should be across the breaker that is used to go on-line. If one PT is at the generator and one Pt is at the bus and there is a wye;delta transformer in between, you will have issues with the phase angles. Generally, after going on-line the synchroscope hand stays pointing at 12:00 O-clock. If you are able to successfully close in on an out of phase condition then after closing the 'scope will not be pointing at 12:00 O-clock but will be some degrees away from 12:00 O-clock. eg. If there is a 30 degree phase angle the scope will be 30 degrees away from 12:00 O-clock.
Yes it is possible to close in with a 30 degree phase angle error. The breaker may trip 2 or 3 or 5 times before it stays in. About once a year you may shear a coupling.
This was a real plant that I corrected after it had run for several years with the wrong PT connections.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[squeeky]

Glad to see we pass on errors so others may learn from others mistakes. It is easier that way. We always close the breaker on the MV bus side. We also sense the voltages at that level. We also add a syncheck relay as a back up. It is a simple, inexpensive, basic device just as a back up.

Transformer vector groups are always matching. Tap charger are set for the best regulation.

 

[powereng91]

Just wondering if anyone knows whether there will be issues with harmonics under this arrangement if a non-linear load is connected. I understand that the ydn1 transformer will nullify 3rd harmonics preventing it to be reflected to the delta side. For the delta-delta transformers I think a path is created for circulating currents of the 3rd harmonic. Will this cause issues? Any thought?

 

[powereng91]

@waross @dpc

 

[dpc]

Third harmonic currents behave like zero sequence current in many respects. They will circulate in the delta winding, but cannot flow out of it. But not all non-linear loads will create third harmonics.