Generator fault 1

[wbd]

Hello,

What happens on a diesel generator when there is a fault between the generator output terminals and the line side of the generator breaker? Specifically, how is the generator shutdown or does it just run to failure?
I am thinking about a large 480V, 1800kW self contained genset.

Thank you in advance for replies.

 

[waross]

Many sets in that size range are not protected against that type of fault and will run to destruction.
Differential protection may or may not provide protection, depending if the breaker is in the protection zone.
Many sets in that size range do not have differential protection.
If the fault is to ground, ground fault protection may or may not see the fault, depending on the type of protection and the location of the CTs.
Ground fault protection will generally be for protection of loads beyond the breaker.
An exception may be an impedance grounded set with monitoring of the current to ground.
Basic ground fault protection may be incorporated in the generator circuit breaker and will not see the fault.
The last time I was involved in the purchase of a set in that size range, I requested differential protection. My request was ignored by the financial department.


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

 

[robert122]

In large generators, to protect these types of faults differential protection is used. This differential relay when there is fault in between the intended points, will sense and switches off the generator. This is very common type of protection in generators.

 

[catserveng]

My experience is similar to Bill's, differential can be applied but may not protect from all faults. Likely if this is a "packaged" genset it uses the generator control panel to provide electrical protections which are usually pretty basic), along with the start and stop controls. A unit of that size that I get a round most tends to be very "cost sensitive", meaning no one really wants to spend anymore than they absolutely have to most times.

Your application, such as standby, prime or parallel to grid all have a possible impact on how any protection will function and what really is required.

Modern multi-function relays, like an SEL 700G could go a long way in providing very good protections for your generator, but depends on how the generator is installed, grounded and who develops and implements the protection scheme.

If you provided some additional details you may get some better answers, likely your best solution is to hire a local engineer that has experience in generator protection and local codes/regulations.

Here is a link to a paper by Beckwith regarding generator protection,

https://www.beckwithelectric.com/docs/tech-papers/multirelay.pdf

Basler also has some good papers on the subject but their new website sure makes it hard to find

http://www.basler.com

And depending where in the world you are there may be others, but I'm assuming US or somewhere close based on your voltage and rating

Hope that helps, MikeL.

 

[wbd]

This is a standby genset with only a basic control system and a circuit breaker. Just trying to determine the arc flash assuming the flash would propagate to the line side breaker terminals. Therefore it seems that for this type of genset, it would shut itself down based on catastrophic failure.

 

[Parchie]

@wbd,
I've seen a 5 kVA tower light generator burn until nothing flammable is left on the generator side, the drive engine kept running! I guess that's how these small gennys are.

 

[maqhawe]

The genset controller which in some controllers has a built in AVR and ECM, have almost all the standard codes protection, like 50/51, 50N/51N, and 32P, 25 etc protections, in that case that is the one protecting the gensets for any faults before the breaker, it will either shut off the excitation or open the breaker to isolate the genset and then shut it down.
the built in protection is somehow aligned to the breaker protection curve, that's why I have never seen an external filed differential protection on a diesel genset.

 

[waross]

I would not expect to see diff protection on anything less than about 1.5 mw or 2 MW. I wanted differential protection when we were ordering a 1.4 MW set. I was overruled by the bean counters.

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

 

[MatthewDB]

I've seen a 5 kVA tower light generator burn until nothing flammable is left on the generator side, the drive engine kept running! I guess that's how these small gennys are.

Ditto on a 25kVA 480V 3ph generator. The copper melted out and ran onto the floor around the generator. The customer reported that the fire department was afraid to go near because it was throwing balls of flaming copper. Once the copper had finished departing, they got close enough to hit the stop button.