Synchronizing Checks

[GTstartup]

We tried this idea to verify synchronizing a 15KV 120 MVA generator across an SF6 Breaker. Note: we also did all the usual checks, backfeed etc.

We constructed six metal plates with insulated backs and taped them to the breaker pole frame right next to the insulators on each pole, for each phase, line side and generator side. We ran six wires from the plates outside the enclosure and using a 4 channel oscilloscope we were able to see the waveform at about 4V P-P induced. We were able to see that, when all phases are in phase, the synchroscope was at 12 and the synchronizer was sending the signal to close (breaker close command was disabled)

My question is - good idea or really, really dangerous and stupid?

PS we already did it with two units, OK I admit it, I've done it a few times before, once in a 230kV yard.

 

[mgtrp]

If I'm understanding your description, it's basically similar to how many vendors provide voltage indicators. I've never heard of anyone doing this to check a generator synchroniser, and I don't think that I would given that there are other more conventional methods of checking.

An alternative method for checking synchronisers:
Check the phase rotation on the bus VT and record for later comparison (see note below if no 3-phase VT).

Close the generator onto a dead bus, and confirm that with the breaker closed, the synchroscope and any sync-check relays etc are showing 12 o'clock/synchronised. Check the phase rotation on the same bus VT as previous and confirm that it is the same as the first check.

Rack the breaker into the test position (or if not possible, disable the close command in some other way) and run through a dummy sync. Check that the synchroniser does sensible things (raises/lowers the speed and voltage appropriately, closes the breaker at the right phase angle and slip frequency, etc). If you have any sync-checks, etc, this is a good time to confirm that they properly block a close signal as well.

Rack the breaker into the connected position and synchronise the unit. Your unit is now online and your synchroniser is commissioned.

Note, if a three-phase VT is not available on the bus, or if the bus cannot be taken out of service, it is also acceptable to remove the stator links and back-energise the generator cables etc from the bus. The generator 3-phase VT is then used for phase rotation comparison.


This method is not affected by VT wiring errors, and does not expose test personnel to anything more dangerous than VT secondary voltage.

 

[GTstartup]

We did both methods, the method I described and also the backfeed. The backfeed method is takes way longer and more labor. So I'm trying to figure out if I should use the induced voltage measuring method instead. The only downside that I can come up with is perhaps safety. This is the part I'd like some feedback for but any opinion about it in general is welcome

 

[waross]

I think that the safety would depend in a large part on the clearances and proximity to live parts of your plates.
I like very much the method of checking actual voltages phase angles across the breaker that will make the final closure. That gives very good safety in the matter of taking possible PT wiring errors out of the picture as they relate to out of sync closures.
As for safety of persons using this method, that would depend on insulation methods and creep and flash-over distances. Could be very good, could be bad. Not enough information as to the physical details.
I will be filing this method away in my memory under possible test methods. Thanks for the tip.

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

 

[LionelHutz]

Do you have a picture? At first read, it doesn't sound like the safest method because an insulation failure in the breaker could expose the operators to lethal voltages.

 

[GTstartup]

I Didn't take a picture but I will next time. Think of 3 sF6 breaker poles on insulators supported by horizontal steel members. The plates can be actually further away than the closest grounded part mounted next to the insulators that support the bus on both sides of the breaker. The plates just have to be insulated. The wave form is well defined, there is no doubts. With somebody watching the sychroscope and the command output of the synchronizer and with a small enough slip it's easy to see. You can also check phase rotation of the system and the generator. I have done it a few times but I think I will set it up properly with high voltage test leads and HV gloves and PPE. The backfeed method is a good method also, but there's always the thought that perhaps something got changed since the backfeed. This method removes all doubt in my opinion.