Generator Overcurrent Settings

[nightfox1925]

We have a 500kW, 600V Standby Diesel Generator at site with S.F. = 1.15, FLA = 601A, Max Overcurrent Rating = 1.8 x 601A for 60secs. P.F. = 0.80

The generator is protected by a 800AF molded case circuit breaker with Long time and Instantanous currents settings and associated time delays.

We have set the LTPU to be close to the FLA, hence LTPU = 0.75 (600A). The Instantaneous setting is disabled.

My question is, are we suppose to set the LTPU and time delays such that it will allow the 60 sec temporary overloading and trip instantly on the maximu fault. The present setting seems to trip the 1.8 x FLA at less than 1 second.

 

[rcw retired EE]

Look at what else you are trying to protect, such as the cables or bus downstream. LTPU of 600A is about 125% of the generator FLA which seems OK (ignoring generator decrement for now). But why not set the protection time delay to make full use of the generator capability?

Check the generator decrement curve against your MCB trip curve. During a fault the generator output current will decrease rapidly and could even drop below the FLA. Check that the breaker curve and the decrement curve intersect, otherwise the breaker may never trip on a heavy fault.

 

[waross]

I calculate the FLA as 600 Amps. When you add the service factor that goes up to 690 Amps.
But I haven't seen a service factor on generator specs. It may mean that the prime mover is 15% over powered.???
Is the generator self excited or PMG excited? It makes a big difference to the fault current.
Also, is the generator rated for standby use or prime use? A prime rated generator will generally accept a 10% overload for one hour. A standby rating does not allow overloads.
The 1.15 service factor makes me think that you are using a prime rated set for standby service. There's nothing wrong with that, but it makes a difference to the allowable current.

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

 

[nightfox1925]

Thanks Bill and rcwilson for your prompt response.

I checked the generator specs once more and it is a Caterpillar C15 500ekW, PMG excitation system, 600V, Efficiency at FL=94.5%, Rated PF=80%, Standby Duty, Governor Model=ADEM A4, Generator=Leroy Somer/6134F, Max O/C Rating For 60 secs = 1.8 x Rated FLA, X"d = 0.1151 P.U.

Sorry about the SF=1.15 confusion, it turns out that it was an MR spec and yes there is no S.F. specified by the vendor.

Any recommendations as far as O/C settings are concern?

 

[waross]

For standby duty this set will be at its maximum capability. (The same set rated for prime service would typically be rated at 450 kW or 460 kW. For prime, it may or may not have extra options on the engine.)
I would set the overload protection very close to the full Amp rating of the set.
With a PMG you will have good (for a generator) fault current and the instantaneous trip may be set higher to allow motor starting and transformer energization.

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

 

[nightfox1925]

Hi waross and rcwilson,

I checked the generator parameters as supplied by the genset manufacturer and derived the generator decrement curves by SKM simulation.

The generator breaker and the downstream incomer breaker are both having their long time and short time settings above the decrement curve. This generator does not have any voltage controlled overcurrent protection (51VC).

Are there any other ways to resolve this or this will be acceptable but with conditions?

 

[nightfox1925]

The only option I can think of right now is to set the LTPU and STPU of the breakers to be below (on the left side) of the decrement curve and the instantaneous to intersect the lowest part of the decrement curve.

With this, loadability (in FLA) of the generator is reduced to 400A and hence, loads shall be selected and operated in usch a way that the load current will be below 400A. So much for not having a 51VC.

Regards...

 

[PaultheBigFinn]

In general the generator output circuit breaker does not protect the generator. That is the function of the generator's operating and safety controls.

The function of the output circuit breaker is to protect the feeder the generator is supplying. I would coordinate the generator circuit breaker with the downstream distribution system.

 

[waross]

I don't often see generator breakers trip. The most common cause of a generator breaker trip in the sizes that I am familiar with, (which includes 500kW) is an out of sync breaker closing.
Generators usually manage to burn up without tripping the breaker.

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