Generator Circuit Breakers

[Arevaengineer]

What are the advantage and disadvantage of having a Generator Circuit Breaker between the Generator and Transformer. In some installations I noticed a GCB is not used and the generator is directly connected to the transformer.
Thanks in advance for your valuable feedback

 

[rasevskii]

It depends on how the station service is obtained amongst other things. Imagine a geographically isolated hydro plant with one or two units connected only onto a 110kV line. To get station service the generator step up transformers have to remain energized all the time, supplying a SST at say 13.8 KV., therefore the GCB is necessary to disconnect the generator.

In other cases where the station service is obtained elsewhere from a dedicated transformer from the transmission voltage, or another source as a local grid or house generator, the GCB may be omitted. The unit would then be synchronized on the HV side of the generator transformer.

regards, rasevskii

 

[GTstartup]

Also depends on rating and cost. On larger Units the current rating is too high or cost prohibitive to be located on the low side of the transformer.

 

[waross]

Ah memories.---
Time was when a "Generator Circuit Breaker" was a specific device. They were used on dual voltage sets in North America.
A generator circuit breaker for three phase service was a nine terminal device. The first set of three terminals connected to the fixed contacts, the second set of terminals connected to the internal connection between the movable contacts and the trip mechanism. The third set of contacts connected to the free end of the trip unit. The terminals were often posts that extended out the back of the breaker. The selection jumpers for high or low voltage were often mounted on the terminal posts. The current was monitored through only one winding. This allowed the same breaker to be reconnected for either voltage. I haven't seen one in years The last one I saw was probably made in the 40's.
Now if you want to use a circuit breaker to switch the output of a generator......

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

 

[ScottyUK]

Switching a GSU transformer is usually a big and unwelcome shock on the system, being hard on switchgear and causing transients which can take a while to calm down. Energising the transformer gently as the generator builds excitation is much more graceful and generally less stressful on everything involved.

GSU transformer maintenance is an operational nightmare if all the station auxiliaries including workshops & admin buildings are are fed from it. Trying to run an outage from mobile generator power while you're working on the GSU transformer is fraught with problems.

The main saving is cost - you can save a very expensive transmission voltage switchgear bay and an expensive transmission system transformer in return for introducing a breaker on the generator bus and using a cheaper distribution-class transformer fed from the generator bus for house load.

If you are going to do it cheap and use the GSU transformer to supply house load then make sure the station has a fully rated alternate supply off the local distribution system for the times when your GSU transformer is unavailable.


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

 

[kh2]

We may start by saying that the major objective of operating a power plant is to achieve the highest possible plant availability at the lowest possible cost.

The use of a generator circuit breaker (GCB) could achieve this goal because of the following advantages:
1) Installation of the GCB directly in the generator connections provides a clear and logical plant arrangement.
2) GCB improves the protection of the generator and its step up transformer such as the differential protection zones can be arranged to achieve maximum selectivity.
3) In case there is no GCB, if an internal fault arcs occurred in the step up transformer, the current of the fault arc is delivered by the system from the high voltage side and by the generator. The current fed from the system is interrupted by the high voltage side circuit breaker, but the generator continues to deliver a significant portion of the fault-arc current until the generator is de-excited, which could take seconds. But if there were a GCB , the generator fed short circuit current is interrupted within cycle and the serious internal or external damage to the transformer can be prevented.
4) The presence of the generator breaker increases the general availability of the auxiliary equipment in the powerhouse. Also provides rapid and selective clearance of all types of faults and avoids expensive damages caused by delay clearance and consequently long down times for repair.

On the other hand the advantages gained by installing a breaker only on the high voltage side of the transformer are:
1) Savings for not to pay for two circuit breakers.
2) To have more available space in the plant for other uses.
3) There will be less trouble shooting in case of trips.
4) There will be less maintenance because of less equipment.

There was another important historical reason for not installing GCB on the large generators built 50-60 years ago. The reason was the limit on the maximum ratings of the available breakers at the time. For example if you had a generator 90 MVA at 13.8 kV in 1955, you need a GCB 4000 amps. At that time the maximum rating was 3000 amps, a Magna Blast breaker, and there were no vacuum breakers or SF6 breakers in the market that are priced low enough to justify the cost.

Hope this helps.

KH2

 

[slavag]

Hi.
We install GCB only in special cases.
I more like block generator-transformer and aux load many years supplied from generator bus duct w/o GCB. For start, have a RAT and SAT systems.
Best Regards.
Slava

 

[ScottyUK]

kh2,


Can I comment on your list?

1) A dedicated generator / GSU block is equally 'logical', perhaps more so because the GSU is a dedicated purpose transformer and isn't involved in other functions like supplying house power.
2) The differential zones can still be designed to provide a highly discrimatory scheme with well-defined overlaps between zones.
3) Agree, although whether the reduction in fault energy would be enough to save the transformer is a different question.
4) Depends on the station auxiliary design. Most generators require maintenance, as do GSU transformers. With a GSU providing house power the GSU can't be maintained without shutting down the site. This is especially a problem on smaller single machine, single-shaft CCGT stations where there is only one GSU and there's no alternate supply from the transmission system or utility. If you have multiple GSUs, each with an auxiliary transformer, then it's an arguable point.


1) If you have a separate station auxiliary transformer then you save a GCB, but have to install an extra breaker in the transmission substation.
2) Agree.
3) Hahahahahaha! You just go trouble-shooting in the dark instead, because there's no house power. Seriously, troubleshooting is just as bad with either system.
4) There'll be less maintenance because you won't be able to release it for maintenance. Then one day it will fail through lack of maintenance and you'll be waiting about two years (last time I looked) for a new GSU transformer.


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

 

[kh2]

ScottyUk,

Thanks for your comments. I think that the generator circuit breaker is preferable in power plants with large units. There is no argument that there are cases the GCB is not cost effective. Agree that GCB is not a requirement.

When I responded to the question, I had in mind the power plants with two units or more each 50 MVA and above. At these plants the GCB increases the security and power plant availability. And, these plants have a black start capabilities such as an emergency generators to supply the station service bus.

The generator circuit breaker in these plants is essentially used to interrupt the short circuit current that can be produced by the generator. This is a security element. Its function is to protect the transformer and the network in case of a fault current in the generator. Also, the voltage decay after the unit shutdown on the high voltage transmission line between the power plant and the switchyard, which is one to two miles long, could take few seconds to decay. This is a safety issue.

We may agree that the generator circuit breaker provides a clear and logical plant arrangement, which makes it easier and safer to oparate and maintain.

KH2