Generator Breaker

[shlim]

Hi there, this is my first posting since I subscribed to Eng-Tips.

I have attended a generator failure in a diesel driven power plant. According to the plant, there had been multiple tries to sync this particular Gen to the grid. After replacing a few electronics component, the Gen still failed to sync. Lastly, they decided to swap gen breaker from another unit. The operator claimed that during the process, he had mistakenly switch to close the breaker's contacts, closing to generator to the grid, instead of switching from 'local' to 'remote' (in order for the control room to take control).

My question is, it possible that the operator can closed the breaker 'locally' from the control switch at the front of the VCB cubicle? My understanding is there should be a sort of protection relay to prevent the breaker from closing if the Gen had not meet synchronization criteria.

Thank you.

 

[davidbeach]

It depends. There are no universal rules or design requirements; without examining the detailed drawings (if they're even accurate) for that particular installation it is impossible to know.

 

[racookpe1978]

It could have happened. There are "local" overrides (actually, "local actions that are not prevented by remote interlocks to prevent disaster") that may have permitted the breaker to shut. And, if things were exactly right, it could have closed successfully and not exploded in his face.

 

[waross]

My question is, it possible that the operator can closed the breaker 'locally' from the control switch at the front of the VCB cubicle?
The answer seems to be-- YES
Why did the generator fail so sync? This may have to be answered before the replacement will produce.
One possibility is that the generator had already failed and someone has reset the trip history. Rather than an accident waiting to happen this may have been an accident waiting to be discovered.
Another reason for a failure to sync may be the frequency setting.
A generator coming online is often set a fraction of a cycle fast. For example, a generator set at grid frequency plus 0.5Hz (60.5Hz or 50.5 Hz) will pass through syncronization every two seconds. This is quite easy to sync and the generator will pick up enough load to avoid reverse power trips.
If an operator "tweaks" the speed setting to exactly grid frequency, the unit may sit "out of sync" indefinitely. This may happen with a new, smart operator who is striving to be perfect in the execution of his duties.
I spent a few years as system engineer for a very small utility. We had manual control and very few interlocks. The breakers could be closed at any time. Fortunately the windings were more robust than the keyway keys in the drive couplings. We had lots of breaker trips on sync attempts and once or twice a year, sheared drive keys.


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

 

[dpc]

It's certainly possible. Use of permissive contacts to prevent out of synch closing is by no means universal. I've worked on 600 MW units with no such interlocks - only a synch scope.

 

[waross]

Seeing the title of this thread took me back quite a few years. A "Generator Breaker" used to be the name of a special three pole, nine stud breaker specifically for dual voltage generators. On the high voltage setting, all the current went through the trip unit.
On the low voltage setting, the current from one set of windings went through the trip unit and the current from the second set of windings bypassed the trip unit.
I haven't seen an old style "Generator Breaker" for about 25 years and that one was very old, probably from the 1940's.


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

 

[ScottyUK]

Interesting Bill -


When I hear 'generator breaker' I envisage a very large severe-service breaker capable of dealing with the arduous switching conditions encountered in generator duty: high operating currents, very high fault currents, and high X/R with long time to first current zero. It just shows that you can't take words at face value!

Have you got a link to one of the oddities? I've had a quick look on Google images and drawn a blank. There are plenty of examples of my type of generator breaker.

 

[shlim]

Appreciate all your comments and opinions. It looks like I will have a lot more questions for the plant operator.

Thank you.

 

[odlanor]

There is no protection relay to prevent the breaker from closing, because you have to close breaker to detect
the synchronization criteria not satisfied for good synchronization.
We have a dedicated very fast protection for wrong synchronization occurred after breaker closed.
See logic diagram attached.

 

[odlanor]

ERRATA
There is no protection relay specific to prevent the breaker from closing, because you have to close breaker to detect
the synchronization criteria not satisfied for good synchronization.
We have a dedicated very fast and selective protection for wrong synchronization occurred after breaker closed.
See logic diagram attached.

 

[shlim]

Odlanor, do you mean that after we have close the breaker, only then we would able to know if the generator have meet the sync criteria, which can be observed on the synchroscope?

 

[davidbeach]

No, synch or not can be fully determined with the breaker open.

 

[ScottyUK]

Agreed - any check-sync relay can do that. That's the whole purpose of them.

I'm not sure what Odlanor would achieve by trying to measure sync conditions across a closed breaker because the two systems will inherently be in sync at that system node, however briefly.

 

[waross]

Hi Scotty
First, size; mostly on small sets from about 30 KVA to 200 KVA, at 240/480 Volts (or 220/440 Volts, these were OLD)
From Westinghouse Maintenance Hints, large 1974 edition.
"Dual Voltage Circuit Breakers
The dual voltage circuit breaker is a method of using one circuit breaker to protect dual voltage equipment with different current ratings for each voltage rating. This breaker has two sets of load terminals, with the second set referred to as "center tap studs". They are not field mountable, and are always rear connected. The line and the load connections must always be made to the rear connecting studs.
By far the leading application has been in portable engine generator sets where the voltage may be changed frequently."

For gen-set applications they were reverse fed so that there was one set of load terminals and two sets of line or feed terminals.

Imagine a 200 Amp moulded case circuit breaker, old style with an electromechanical trip unit. The breaker has rear studs from both the line and the load terminals. There is another set of studs extending from the connection between the breaker contacts and the trip unit.
Call the studs connected to the trip unit Studs 1, call the Studs connected to the connection between the contacts and the trip unit Studs 2 and call the Studs connected to the contacts Studs 3.
On an alternator, the 1-4, 2-5, 3-6 windings would be connected between Studs X and Studs 1. (Studs X were not part of the breaker)
The load would be connected to Studs # 3.
For low voltage use, Studs X would be shorted together by links, forming a wye connection.
The 7-10, 8-11 and 9-12 windings would have the 10, 11 and 12 leads permanently connected together to form a wye. The 7, 8. and 9 leads would be connected to Studs 2.
For high voltage use, the links would be removed from Studs X and the leads 7, 8, and 9 would be moved to Studs X.
The voltage could be changed without changing the breaker.
The trip unit monitored the current through windings 1-4, 2-5, and 3-6, on both high and low voltage settings.
I hope this is understandable.



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

 

[dpc]

There is no protection relay specific to prevent the breaker from closing, because you have to close breaker to detect
the synchronization criteria not satisfied for good synchronization

Sorry, this is incorrect. As noted by others, that is the specific function of synch-check relays.

 

[ScottyUK]

Hi Bill,

I had to draw it out but think I've figured it out from your explanation. I've never seen anything like it before. Thanks.

 

[odlanor]

guys,
you’re right! I wanted to summarize the introduction but I ended up writing nonsense! Forget my comment. I guess there is a thread about this logic diagram; I posted to show Shlim another protection relay.
I just wanted to introduce a protection against wrong, poor, bad synchronization in generators, as seen at attached logic diagram.
The basic idea is to put in an instantaneous overcurrent relay in service to detect the current in the input circuit of the machine only at interval generator is standstill ready to be energized or during synchronization.

 

[slavag]

We are always prevent close of GCB from any local push button.
Close command of GCB is from synchronizing system only. In any case, I would like add synchrochek relay in the SWG, near to breaker

Odlanor: dead machine protection is always added

 

[odlanor]

-Dedicated protection against wrong synchronization
Despite all the prevention measures used in synchronization circuit: synchronism check, synchroscopy, automatic synchronizer, dead-line bar alive, etc, the synchronization information wrong that occurred in Brazilian plants reached the level of discomfort. It worked out, the decision to investigate the possibility of implementing a specific protection against wrong sync generator.

Slavag,
I believe there are more wrong synchronization than energization of dead machine.
This diagram caters to both cases.

 

[DTR2011]

Of course it is possible for someone to close the breaker via a breaker control switch. This is likely a poorly thought out control scheme for the generator breaker.

There are ways of allowing a VCB to be closed (assume for testing), which incorporate a breaker "cell" switch. This switch determines if the breaker is in the disconnect or test position. It would be in series with the close circuit, to to only allow a local close, if the breaker is out of the cubicle or in test position.

Some GCB control schemes do not have a local trip/close switch for the very reason the OP cited. To prevent operator error.