Generator Syncronisation

[ruggedscot]

A query for the group....

Looking at a situation just now and trying to work out what would happen. Take an example you have two rotary UPS units - each UPS is connected to the same source so they will be syncronised to the incomming utility supply.

If I measured between like for like phases on the system when running normally Id expect to have zero volts between those phases. Outputs being galvanically isolated from inputs.

If I lose the incomming supply the system will freewheel - ie the common source is now defunct and the system operates without reference to each other this will mean that outputs on each set will differ. The diesels kicking in and running each generator this meaning that it would be possible to see the phse difference go from 0 to 180, or am I seeing it wrong here ? Would as the system ran syncronised before stay pretty much syncronised and there would be no real difference between the like for like phases on a three phase panel board.

Rugged

 

[Skogsgurra]

Say again. Feel thick tonite...

Gunnar Englund

http://www.gke.org

 

[ruggedscot]

Okay here goes, sorry for the long explinations that are murky clear !

Right one supply two UPS units two outputs each output three phase.

utility supply is on and okay UPS units are fine output healthy - measure between phase A on set 1 and phase A on set 2 zero volts as expected....

Okay now utility goes off, UPS goes to diesel and units run on diesel - if the generators which were running on utility syncronised through utility switch to diesel should the output remain in step or will there be a significant phase shift between like for like ?

Rugged

 

[itsmoked]

I'm still lost.. Are you saying these two R-UPSs are running and their outputs are synchronized by virtue of both being sync'd to the utility normally?

Now the utility is removed.

Yes! If there in no interconnection between the two sets there would be no reason for them to remain in sync.

Is there any interconnection?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ruggedscot]

I wasnt quite sure thought that if the frequency lock was tight then when the utility dropped it would remain in sync but the more I think about it the more that I see that this wouldnt be so - too much variance if they were not speaking to each other....

Thanks for the input and contribution on this one.

Rugged

 

[itsmoked]

Yeah. If you have non rotary UPi they are synthesizing the line freq and can talk over a data line to stay synced but as you have rotary and they probably aren't talking there would not be any inherent sync unless their outputs are tied together and since they are UPS's not generators you probably don't have them connected at the outputs.

You also have the question of how are they brought back to sync when the utility returns..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ruggedscot]

No question of bringing them back - dual supplies to load (n+1) so UPS when on line and run from utility they remain pretty well synced together by way of supply and like for like no volts - Im sure that if utility fails and I check like for like phases then there will be voltage variance here....

Rugged

 

[itsmoked]

Yes, yes, but when the utility goes they rapidly become unsynched to the utility. When the utility comes back what mechanism is used to re-sync them to the utility?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

If they are the type that are just motors running engines running a generator then when the utility returns the motors just reclaim the mechanical load and the output frequencies remain what they are and the same lack of nonsych remains. For that matter there would not be any syc between them normally. I don't think this is the style you have or you wouldn't be getting your zero voltage(sync'd) readings.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Rugged,

If these are the Anton Piller / Eurodiesel type of UPS there is no reason why they will remain in synch unless their outputs are tied in a redundant scheme of some form.

Keith / Gunnar,

They are based on a special design of synchronous machine so the output is inherently locked to the mains input. Once they are on diesel power the engine can shift speed to re-synch to the mains. Eurodiesel's website gives some idea of how these machines work.

http://www.euro-diesel.com/total_technical.html

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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Ah thanks Scotty I was asking for curiosity and because ruggedscot should know the answer if these are in his facility. (kind of ask testing)


Wow, that is one impressive design. Very elegant. Best I've seen.

My only question is the normal mode statement that sez it filters out all the noise/spikes/etc. I do not see why running a motor (synchronous or otherwise) off a noisy line somehow filters things farther down the line. Unless the non-mentioned gray blob between D1 and the KS is the filter, (which really means the "system" filters the spikes not the stato-alternator per se.

Thanks again.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

Ruggedscot:

The situation you mention is very common and there are solutions. We specify dual bus UPS systems all the time.

If you intend to keep the output of two UPS sources in sync with each other, even if they are on different sources or running on battery, what you need is "Load Bus Sync" (LBS) controls. Each manufacturer has their version of it.

The UPSs (Rotary or static) are normally designed to sync with its Bypass (static or automatic bypass). This for obvious reasons, that when the UPS inverter fails, the load can be immediately bypassed to the bypass source without problems.

The LBS controls becomes active when the outputs of the two (or more) UPS systems goes out of phase (normally set at 6 –7 degrees) and will select one of the UPS s master (selectable) and will force the rest of the UPSs to sync with the master. LBS controls are essentially designed to “fool” the ‘slave” UPS to think that their bypass source is the output of the master. This of course comes with a caveat; during the period LBS is active, the slave UPSs will be inhibited to go their own bypass source as its output will not be in sync the bypass source. Normally the LBS control includes a switch that can pre-select any of the UPS as Master. For more you need to talk to the ‘right’ people (read engineers) of the manufacturer.

Also once the LBS becomes active, it takes several seconds to drag the two (or more) outputs in to sync.

 

[ScottyUK]

Keith,

I'm pretty sure the grey blob is some form of reactor. I'll have a look in my notes once I get to work and see if I can confirm that.

These are excellent machines - they're very popular with the big financial institutions. Hospitals can't afford them. There is something in that situation.

Rugged,

Is this the type of rotary you have, or is it the Merlin / Cat type with a rotary energy store and a power electronic output converter?


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Sometimes I only open my mouth to swap feet...

 

[ruggedscot]

I never really got time to persue but did today -

The answer is that they do not remain in sync as soon as the utility drops - Id thought this but we had a discussion and some ideas and views were mooted and it made me take a step back and think about it a little too deeply.

The UPS set up is pretty stright forward and its a medium to large set up.

looking back through some checks and past history I found that during a utility outage the sets do lose sync and there will be a difference between like for like phases. When on utility this does correct and the two outputs become sync'd but indirectly - what happens is that the units electronically work to maintain the input and output of each set in close tolerance this is to ensure that if they have to go to bypass there is a limited level of impact on the output as both are pretty much together the load transfers over easily. Now as they are both fed from the same utility supply it follows through that they will be roughly in sync. The internal construction of the unit is basically a motor generator the utility powers the motor which spins the generator and the critical load is fed from this generator output.

So there you go. two seperate outputs roughly in phase and supplying the load. Hence a single phase dual fed piece of kit taking two supplies in only really sees no difference between the two inputs.

When the supply fails and it goes over to diesel then you get freewheeling if there is no sync between the sets and you can get a phase to phase voltage across the two supply lives - due to double supplies sharing the same neutral earth tie.

Rugged