Generator Circuit breaker and distribution circuit breaker 3

[JAYII]

What are the differences between Generator protection Circuit Breaker and Distribution Circuit breaker( only considering Circuit Breaker it self, W/o relay protection)

 

[dpc]

Probably not much. There are special breakers for very large generators due to high current requirements.

At low voltage, the generator breaker probably must be electrically-operated to allow synchronization.

It would help if we knew what voltage and current rating you were concerned with.

 

[davidbeach]

It may have to do with voltage ratings across open contacts; generator breakers are subject to twice nominal while most other breaker are only subject to nominal.

 

[Eleceng01]

To expand on davidbeach's comment, I would think it might also have to do with the X/R ratio that the breaker was tested at. A generator breaker will have a higher X/R ratio than a distribution breaker due to the X"d of the generator itself (higher and longer lasting asymmetry).

 

[rcw retired EE]

Generator breakers usually have much higher amp ratings than standard switchgear breakers - 7,000A, 10,000 A, 21,000A.

The high X/R ratio at the generator terminals means the fault current may never have a zero crossing for several cycles which is severe duty for a breaker. Breaker designs usually depend on the fault current passing through zero during the interrupting process.

Most generator breakers are also designed to be compatible with an isophase bus system. Each pole/phase is enclosed in a separate metal enclosure.

Generator breakers can be supplied with disconnect switches and grounding switches - devices not normally available in switchgear breakers.

Check the Areva and ABB websites for additional information.

 

[dpc]

This all depends on the size and voltage of the generator. There are thousands of small to medium size medium-voltage generators running with standard drawout metal-clad breakers.

 

[JAYII]

Yes Davidbeach, that was the concern,
The voltage level is 12kV, but the supplier says special breakers have to be used only for more than 10,000kW generators,
Do you have any more information?

 

[rcw retired EE]

JayII - At 12 kV, 10,000 kVA is around 480 amps. With a 25% safety factor, a 600 A rated switchgear breaker could operate as a generator breaker provided its short circuit interrrupting capacity is suitable. They are more economical than dedicated generator breakers.

DPC is correct, there are many, many, small and medium generators operating with distribution style breakers in service as generator breakers. For ANSI equipment, application difficulties start showing up around 3200A (75 mVA at 13.8 kV).

The generator breakers I mentioned before are usulally not considered on units less than 100 MVA.

 

[swgrmfg]

Yes Exactly, confirming the above. Up to 3250A (about 70MVA)we have used off-the-shelf 40, 50, and 63KA circuit breakers, over and over and over. Beyond that (6300 and 8,000A) we apply GCB's such as ABB HGI-3's. The only thing 'special' that the supplier is talking about would be the surge package requirement which is external to the circuit breaker anyway (and sometimes on both sides of the circuit breaker).

 

[Kiribanda]

JAYII,

There are two reasons for specifying Gen CBs instead of normal Distribution CBs in a generator installation.

1)The short cct current may have non-current zeros depending on the X/R ratio.(specially for system fed fault)

2)Depending on the X/R ratio, Gen. circuit breakers should be able to break higher DC component than normal Distrubution CBs which are designed for X/R=17.

2)The CBs should be designed for out of phase switching TRVs which may happen during manual synchronizing.(depends on your generator operation procedures)

By doing a short circuit study you can get the answers for both 1 & 2 above.

You can calculate TRV values knowing all reactances and time constants of the Generators.

IEEE C37.013 was earlier confined to circuit breakers for large generators mainly from 100 MVA and sc currents form 63 to 100 kA.

But now IEEE C37.013A-2007(Ammendment) covers CBs for small generators from 10-100 MVA with sc currents 20-160 kA.

Therefore, you can decide whether to use custom built Gen. CBs or normal Distribution CBs only after doing a complete sc study to find out answerrs for 1,2,3 above.

As an example CUTLER HAMMER has got

a)150 VCP W for Distribution CBs.
b)150 VCP WG for Gen. CBs.

with a considerable price difference I hope.

Hope this helps.

Kiri

 

[waross]

Several generations ago, some manufactures used a special circuit breaker for dual voltage generators.
The breaker was reverse fed, and there were six generator terminals. Three terminals connected to the current sensing unit and three terminals bypassed the current sensing unit. When the generator was used on the high voltage connection, only six terminals were used. On the low voltage connection, the current from one winding went through the current sensing unit and the current from the other winding bypassed the current sensing unit and connected directly to the breakers load break contacts.
Some versions had three extra "dummy" contacts to which the T4, T5, and T6 leads were connected. This allowed the set to be changed from low voltage to high voltage by just changing the position of three shorting straps on the back of the breaker. The ammeters and voltmeters had dual scales and the AVR was permanently connected (power and sense) to one of the high voltage windings.
These "Generator Circuit Breakers" were common on Delco sets made by GM in the 40's and 50's.
I haven't seen one for years. They are sometimes described in old text books as "Generator Circuit Breakers".
If you have been reading an old text book....
respectfully

 

[RRaghunath]

Thanks Kiribanda and Waross.
Kiribanda's reply is comprehensive and Waross's post took me to a different (and interesting) world.
Unless veterans like waross share those things, we may not even know such technologies existed once upon a time.

 

[Hoxton]

I work in the reciprocating (diesel / gas) engine field. Some engine manufacturers specify maximum closing times for generator circuit breakers (70mS) - someone must have had a problem with synchronising in the past!