Generator Circuit Breakers for smaller machines 1

[sakaran51]

Hi,
According to IEEE Std.C37.013a-2007 for Generators ranging from 10-100MVA, MV/HV Generator Circuit Breakers(GCB)are recommended as against normal distribution circuit breakers used in industrial, commercial and utility systems.The GCBs have special features suitable for
-high X/R ratio of circuit- 50
-high time constant of circuit- 133ms
-% DC component for min. contact parting time- 75% system fed fault/130% Generator fed fault.
-CB withstand capability for delayed current zeros for generator fed asymmetrical fault.
-high Rate of rise of recovery voltage capability
-high out of step switching capability-50%*I
But ,of course, the price of GCB is very high.I am wondering how many clients go for GCB for smaller machines from 10 to 50 MVA.(rated voltage 11kV).
My question is can we use normal 11kV Distribution breakers, as long as calculated circuit symmetrical,asymmetrical breaking currents,DC component and making current values are within limits of circuit breaker?
And there are lot of existing smaller power plant generators are operating with normal distribution circuit breakers, you will agree.
Beyond 50 MVA either we can opt for Unit scheme or GCB scheme.
Your views on this will be very helpful.
Thanks

 

[ScottyUK]

Over here in the IEC world it is unfortunately not uncommon for distribution-class breakers to be employed on generator duty. It is unusual to get a distribution breaker with adequate breaking capability to handle the high X/R ratio because distribution breakers are built down to a cost and the interrupters are made just good (or almost good enough) enough to handle the conditions found on a distribution application.

One 'trick' used is to delay the breaker opening until the DC component has decayed to a level within the distribution breaker's capability. Personally I think this is simply bad engineering, done with the sole aim of allowing a cheaper breaker to be used and placing both the generator and the system at greater risk through delayed fault clearance.

 

[sakaran51]

ScottyUK
Yes I agree by increasing the relay time slightly the
DC Component to be handled by the circuit breaker can be reduced within CB's capacity with its associated risks.But can we select an oversize distribution breaker in terms of short circuit breking current for
smaller generators,say 40kA instead 25kA requirement.

Moreover how to address Transient Recovery Voltage and
Rate of Rise of Recovery Voltage for small machines?Are they very critical?
Thanks.

 

[sakaran51]

ScottyUK
By the way,I am also in IEC world and finding it difficult to arrive at cost effective & technically right solution.This especially true for smaller and cat on the wall rating machines!
Thanks

 

[TugboatEng]

I was of the understanding that the short circuit current of a generator is substantially lower than that of a transformer of a similar rating. You shouldn't need to oversize a distribution breaker for use with a generator?

 

[davidbeach]

The generator has a much higher X/R than a similarly sized transformer. While the current magnitude may be lower, the DC offset is much greater and the current much more difficult to interrupt. The listed breaker rating (20kA or 40kA or whatever) is for currents at or below the test X/R; above that X/R the breaker must be derated. In the worst case, the generator contribution to a fault may be fully offset for the first few cycles, meaning no zero crossings for a few cycles, and with no zero crossings there is no possibility of interruption in a SF6 or vacuum circuit breaker.

 

[ScottyUK]

sakharan51,

Breaker performance isn't really my field of expertise, but in a GCB application you certainly need to be cognisant of RRRV and TRV capability, because generator duty places significantly more severe demands on the interrupter than seen in a typical distribution application. If no one here jumps in with greater knowledge than mine about breaker performance then you might get some help from the technical guys at one of the larger manufacturers such as Eaton, Siemens or ABB, although they will all want to sell you an expensive GCB rather than help you use a distribution-class breaker in a generator application. Eaton published a little booklet which goes into some of the problems found in GCB applications, although you possibly already know most of this:

http://www.eaton.com/ecm/groups/public/@pub/@eaton/@corp/documents/content/pct_1353315.pdf

I think the comparision table on page 4 may be helpful.

TugboatEng,

Faults close in to a generator tend to have a big DC transient component of current which takes a long time to decay, much more so than typically found in a distribution application. The decay time is determined by the X/R ratio at that point in the system. Vacuum breakers generally interrupt on a current zero, so if there's a big DC offset it takes a relatively long time for the first current zero to occur and all that time the interrupter pole is burning away in the arc. Vacuum bottle failure tends to be quite spectacular. One of the reasons why generator beakers are large and expensive compared to distribution breakers with similar thermal ratings is that they don't use the simple vacuum interrupters found in distribution breakers, but use SF[sub]6[/sub] 'puffer' type cross-blast interrupters which can handle the severe DC breaking requirement.

 

[bacon4life]

As long as you are OK with the occasional catastrophic explosion every once in a while you can use distribution class breakers for generators. Since GCB's take up more space, finding physical space to retrofit the right equipment after a failure can be quite challenging.

In addition to the interrupting capability limitations, distribution class breakers may physically beat themselves to death. GCBs often operate multiple times per day whereas a typical distribution breaker only operates a few times per year.

 

[ScottyUK]

The cynics of the world might think that some EPC contractors and/or consultants are ok with risking the occasional burn-down if using cheap gear wins them the job. They normally aren't the ones left picking up the pieces afterwards. :-(

 

[ScottyUK]

Another paper from Eaton explaining the problems faced by breakers in generator service - it's worth a read because it expands somewhat on what I was saying in my earlier post and it incldues a few diagrams which illustrate the problems quite well:

http://www.eaton.com/ecm/groups/public/@pub/@eaton/@corp/documents/content/pct_1603168.pdf

 

[sakaran51]

ScottyUK
The above white paper from Eaton mention in one place, while comparing the parameters
"Field experience has shown that the performance capabilities of some of the standard distribution class vacuum circuit breakers may well be adequate for the protection of smaller machines,rated below 10 MVA."

It looks like unless a thorough fault analysis study is carried out,to determine
-% DC Component
-arcing time- whether it is within the distribution class circuit breaker(generally it is around 15ms for a Dis.CB)to handle delayed current zeros
-TRV parameters with RRRV during fault and out of step condition
-in addition to system and generator fed fault currents
on a case to case basis, especially for smaller generators i.e., from 10- 50 MVA
we can not come to a right conclusion in the selection circuit breakers.

So, as you rightly mentioned, we are at the mercy of GCB manufacturers!

 

[sakaran51]

As a corollary, there is a question arises,why IEEE STD C37.013a-2007 was introduced as an amendment for generator applications of 10 MVA onwards and NOT 50 MVA onwards?

 

[ScottyUK]

At a guess, probably because someone in the industry thought that it isn't acceptable to burn down switchboards when the interrupters fail in a mis-applied distribution breaker. Perhaps also some of the manufacturers who offer distribution breakers at the upper end of the capability range saw a market opportunity if they got their breaker qualified to C37.013a.