Synchronising generators 2

[maqhawe]

Hi,

I need some advise please, please see the attached SLD

I have three standby gensets running in parallel, 400V, from the 400V bus bar to the two transformers, I am going to keep the breakers at a normally close position. I don't expect any problem in terms of voltages as my transformers are identical.
my master controller will only control the utility incomer, and do a soft mains return, that is when there was load shedding or something.

my only worry is when I want to do a test with load, whereby the Utility will be live, which means the 400V bus bar will be live, my gensets will be synchronizing to the live bus bar,
I am worried about the phase shift that will be imposed by the start-delta transformer's, can the 30 Deg phase shift affect the synchronizing of each gensets?

 

[waross]

I don't think that I said "Acceptable".
I can't give you warranty if the suppliers deny warranty.
You should be talking to the transformer manufacturers.
The transformer manufacturers are the ones who know if their transformers are safe to back feed and the transformer manufacturers are the ones to give or deny warranty.
With a wye:delta four wire connection you may have circulating current issues in the event of unequal voltages or phase angles.
With a wye:delta three wire connection (floating neutral) you may have overvoltage switching transients.
With a NGR you may have the worst of both worlds.
or
The potential problems may not develop. The close connection to the generators reduces the likely-hood of circulating currents.
There is still the warranty issue.

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

 

[mher1]

Hello waross
Thanks for the tips.
Unequal Voltages and Phase Angles are unlikely because all transformers will be fed from the same bus and at the same time they have identical parameters.
Anyway, I do appreciate your comments.

 

[rockman7892]

Waross

I had a general question above the following comment you made in the thread above:

"It will bring up the voltage on the two transformers as if they are Generator Step-up Units, without any energization surge"

When having the transformer connected to the generator as it comes up to speed does reduce any transformer inrush current since the generator voltage is ramping up as it comes up to speed? I guess since the generator starts at a lower voltage as it ramps up the transformer inrush is significantly reduced? This sounds like the preferred method as opposed to bringing the generator up to speed and then trying to energize transformer in which case there will be a large inrush current associated with energizing the transformer? Is this typically how GSU's are energized?

 

[mher1]

Yes Waross you are correct the inrush current will be reduced as the gen ramps up voltage.
But since I have 3 transformers to energize I am thinking of time delayed magnetization..

 

[mher1]

Sorry Rockman, I should have addressed my reply to you...

 

[maqhawe]

Rockman,

on that statement of inrush current being reduced, it will depend on the operating philosophy of your standby substation,
if its automatic, that means all the paralleling breakers of the gens must be kept open, they will only be closed once the unit is up to speed and ate least at 90% rated voltage, depending on which genset controller you will be using.

if everything is manual then, you can keep one breaker closed and start that unit as it ramps up it energizes the transformers, once its up to speed, the other units can be started and synched to the live bus.

if you worried about an inrush current, you can program your system to close the transformer feeding breakers once you have all two gensets running at full speed.

Conventional wisdom on how large of a transformer
a single generator set can energize varies between 5
and 20 times the kVA rating of the generator set. The
limiting factor typically is that the peak inrush current
will be high enough to trip a circuit breaker.