Done that. A 40 MW hydroelectric generator with a delay in the breaker. Pressed the ON button when synchronoscope was in right position. Took breaker almost a second to close, so phasing was way wrong when it closed.
What happened was that we lost the 110 kV line - not just the generator - and that the phones started ringing a lot. No damage to the generator or switches or anything else.
This happened when I was at school and was doing summer work at a power station. It was a week-end and all the other guys had left the premises when someone called and asked me to connect G2 to the grid. But he didn't tell me about the delay in the breaker.
The generator will be pulled into synch by the grid. The transient torque on the generator may break the shaft, cause winding movement, breaker failure, etc. Not good.
A newly qualified guy tried to prevent a power interuption by paralleling a tripped turbine generator with one at synchronous speed. You can explain how it would be possible to get an appearance of synchronization from the scope under these circumstances, but he wasn't and the tie breaker exploded.
Yes, dpc. They sent a guy from the main office to inspect that. We had done a quick inspection without finding any cracks - and he didn't find any either.
I think I have mentioned in other threads the little power house with a mix of 900KW and 350 KW diesel machines. The scope was connected wrong and when it showed synchronism the machines were 30 deg. out. They ran like that for several years. Occasionally they would shear a key in a coupling.
I started to ask questions when I saw that every time they brought a machine online, the scope would jump to the one o-clock position. It was something like all the generators having the PTs line to line and the main board having the scope PT connected line to neutral. I am still amazed at how little damage was done.
respectfully
My experience: Windings can get torn loose and fail a week later. It is also a good way to knock all of the dust off the building rafters.
A large steam turbine was accidently put back on line just after its high voltage ring bus breakers were opened. The turbine's GSU transformer HV disconnect switch shaft broke and gave the operator false indication that the turbine was isolated. Standard procedure was to close up the ring bus for security once the offline unit was isolated. When the first breaker closed out of synch, the spinning turbine tore loose from the foundation and rolled through the control room. Flying parts killed a truck driver on the public road next to the plant. Operator survived. (Story told to me by two electrical designers for that utility. Can't vouch for its veracity.)
(Note that the typical "Inadevertant Energization Protection" scheme would not catch this failure since the turbine was still at speed and voltage.)
You always want to close the breaker in synch, or darn close to it. Especially if the unit is big.
well what we have here at the power station where i work is internal combustion engines driving the generator. what u think could happen to the engines if generator not synced. as what u were saying with the turbine, rcwilson?
You could shear the shaft between the engine and the generator or worse. Worse would be rolling the whole set off its foundation or putting a 90[°] bend in the shaft between the engine and the generator.
I have known of several instances that caused a lot of damage, expense and down time. Similar to some of the tales above. Remember that the ac generator (alternator) IEC (and ANSI) specs only require the machine to be designed to sustain a three phase fault on the machine terminals - torques resulting from faulty synchronising are MUCH higher than this. This causes the damage as listed above to keyways, couplings, gearboxes, windings and people. Advice: get it right, make sure procedures and relay sequences do not allow you to get it wrong AND put "check synchronising" relays (separate from the main synchroniser) in the generator breaker "close" circuit - especially if the breaker is remote from the control panel - like it is on most high voltage sets. This will guard against people carrying out a panel wiring modification and shorting out the "close" contacts on the main synchroniser. This can close the breaker as soon as the start button is pressed! A check synch. relay will guard against this - if it is set up right!
If the system is designed correctly you can't do it. Read up on the application of device no. 25. Lots of application notes out there you can google up.
