PT Connections 2

[nickoliver]

I have an issue on a co-gen project where the controls on the generator are expecting to see open delta connected PT's (480 to 120V) on each side of the generator breaker for synchronizing to the utility. The controls in the switch gear, where we will be pulling the signal from, have this set up on the generator side but not the utility side. On the utility side the PT's are wired wye and the generator people tell me this will not work due to a phase shift that happens. I need to get new PT's in a hurry but all I can find off the shelf are 277/120 PT's. If I used these and wired the secondary closed delta would they work?? I can't rewire the the existing PT's becouse the switchgear controls need the wye connection.

 

[davidbeach]

Any decent synchronizer or sync check relay can deal with the 30 degree phase shift (any whole number of 30 degree phase shifts, in fact). Given the greatly increased amount of information that wye connected PTs can provide, the use of open delta PTs is simply penny wise and pound foolish.

As for your question, if the synchronizer cannot deal with the 30 degree phase shift, I doubt it could deal with the voltage difference that your 277:120 PTs would produce compared to 480:120 PTs.

 

[weh3]

Star for db.

William

 

[GTstartup]

I agree with DB, most synchronizers have offsets for just this very situation. If you isolate the generator and backfeed both sets of PTs from the same source then the synchroniser will show the phase angle difference. Then you can adjust the offet so the phase angle is equal. This will ensure that you have correctly adjusted the offset

 

[weh3]

But you had better get it right the first time, so you don't wring the alternator off its shaft.

William

 

[ScottyUK]

A phase rotation meter followed by a three-lamp synchronising check direct from the 480V system with synchroniser 'live' and the incoming breaker locked out will all but eliminate the risks of reverse sequence closure or out-of-step closure. Keep people away from the generator during first synch - you are more likely to wreck the engine than the alternator if things go awry - broken or bent conrods and a hole in the block being quite likely consequences.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[nickoliver]

Wow, thanks for the replies. I guess it will work but based on what everyone has said I think I will just get the PT's like the generator manufacturer wants. If we did use the wye connected PT's and something did go wrong I'm sure it would be our fault. A couple hundred bucks for PT's is cheap insurance. What is killing us is the lead time. I guess we will just have to wait.

 

[rcw retired EE]

The open delta and wye PT's will work since you are only looking at phase to phase voltages in the synchronizer. The synchronizer doesn't care how the voltages are developed. Vector A-B will always be in phase with vector A'-B', (A = the open-delta, A' = the wye PT) on either Y-Y or Open Delta PT’s.

The problem is that the bus side of the synch circuit has B phase grounded (open-delta connection) and the generator side grounds the wye point. There is 69 V between B & B’, A-A’, C-C’ even though all of the voltages are in phase.

What to do?

Chase the circuit to see if the phase voltages from the two sets of PT's get tied together anywhere, such as on a common for the synch light circuit or a jumper on the synch scope or a voltage relay looking at phase-to-ground voltage. Then make sure the synchronizer is looking for just phase voltages (A-B or A-C) and not phase to ground voltages. If that's the case, everything will work.

If there is a common ground or a cross connection some where the 69 V difference will blow fuses if you try to synch. Solution is to install two or three 120V:120V VT's to use as isolation transformers and put them on the feed to the synchronizer, preferably on the generator side to get rid of the wye connection. If you have to put them on the bus side, float the secondary and you will avoid the ground loop problems. Wire teh three VT's in wye/grounded-wye if requried.

I know it sounds strange, but I have done this on about 25 large generators (>250 MVA)and several small ones. I have had to use isolating transformers only twice.

To make sure everything works correctly before trying to synch the generator, see if you can lift the generator leads and close the breaker to backfeed power to the generator VT's. This puts the same voltage on both sets of PT’s. Then make sure the synchronizer and all meters show 12:00, in-phase, voltage match, correct rotation, etc.

You can also do this by opening the generator neutral connection, if you can’t get the generator VT’s energized without energizing the generator windings. The windings will be energized at 480 V but will have no current flow.

Good Luck.

 

[davidbeach]

To make sure everything works correctly before trying to synch the generator, see if you can lift the generator leads and close the breaker to backfeed power to the generator VT's. This puts the same voltage on both sets of PT’s. Then make sure the synchronizer and all meters show 12:00, in-phase, voltage match, correct rotation, etc.

That works, but there is often a far better solution. If there is another breaker between the breaker in question and the utility, open (and lock out) that other breaker; then power the gear up from the generator. That way you can see how it all works together. While running in that mode make careful not of the phase rotation on the bus. Make what ever adjustments are necessary in the synchronizer/sync check (I'm not sure which - or maybe both - you are dealing with here) settings that both sides show as being in sync. Then open the generator breaker and close the other breaker. Make sure the bus phase rotation remains the same. At this point you have a system set up to synchronize across the generator breaker.

 

[nickoliver]

Great responses. Thank you.