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9 comments
Member

Join Date: Apr 2012
Posts: 8

Diesel Generator Reverse Power

04/26/2012 8:24 AM

Our diesel generator was in shutdown /idle condition and operator closed the bus breaker. After this a heavy current about 4 times of generator rated current and bus breaker tripped on short circuit protection

What is the reason of so much high current?

What is effect of this on diesel engine and generator?

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Power-User

Join Date: Jan 2007
Location: UK S.Northants
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#1

Re: Diesel generator reverse power

04/26/2012 8:34 AM

You are energizing the field windings from the mains and trying to drive the generator as a motor but the rotor cannot rotate because it is mechanically coupled to the diesel engine. Since the rotor cannot rotate there is no back EMF and the only thing limiting the current flow is the series resistance of the field windings - which is very low.

I hope the windings haven't been damaged, if the protective devices were sized for a stall you might be ok. You will soon know when you try it.

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#4
In reply to #1

Re: Diesel generator reverse power

04/26/2012 4:05 PM

Shouldn't that be stator windings, not field? The field windings are on the rotor.

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Power-User

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#9
In reply to #4

Re: Diesel generator reverse power

05/30/2012 3:36 AM

yes of course it should have been

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#2

Re: Diesel generator reverse power

04/26/2012 8:35 AM

The reason is that a stationary generator presents virtually a short circuit to the incoming connection provided by inadvertently closing the breaker. In the circumstances, the protection device probably protected the generator from catastrophic thermal self-disassembly, collateral equipment damage, building damage, injuries and deaths. Good job it was there and working correctly!

The generator requires local assessment to determine whether the incident has had any damaging effect on it. Repeat the commissioning procedure and testing that was followed immediately post-installation and compare the two sets of results. Do not attempt to use the equipment until this exercise has been completed. If in doubt, contact the original equipment manufacturer for advice, preferably by telephone.

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#3

Re: Diesel Generator Reverse Power

04/26/2012 3:53 PM

I wouldn't expect any damage if it's all been designed correctly. The protection mechanism (bus breaker in this case) must be capable of working before damage is done, as the engine can stop at any time without warning, due to a mechanical problem.

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#5
In reply to #3

Re: Diesel Generator Reverse Power

04/26/2012 5:24 PM

I would expect a lot of damage; it went out on the short circuit protection. Now if the Reverse Power protection had worked that is set to protect the generator.

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#7
In reply to #5

Re: Diesel Generator Reverse Power

04/27/2012 11:02 AM

I was assuming it would have protection, call it Reverse Power protection if you like. If it's not provided, so that the kit can be seriously damaged when an engine stops unexpectedly (which may not happen often but is a definite possibility) I would say that's a gross failure in design.

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#6

Re: Diesel Generator Reverse Power

04/26/2012 11:27 PM

You energized a piece of rotating equipment that was stationary. Fortunately(?) for you the field did (should) not have had any field on it, in effect you energized a large iron core inductor which was not designed for such duty. Depending upon where in the cycle the breaker closed the current could have easily exceeded 7 times normal, unless you had some form of high speed recording device on the bus you will never know, but the 4 times that was observed on a mechanical meter was probably low due to meter inertia.

The effect on the set can only be determined by a thorough internal inspection. The generator may have end-turn distortion or displaced coils, and depending upon the type of excitation system you may have blown diodes, fuses, and welded brushes; after all your fault current created a rotating magnetic field in the stator that in turn induced voltage in the field circuit so everything needs to be checked.

The diesel may have received some severe mechanical shocks as well, so everything in the drive train should be checked for bending and/or distortion.

You are assuming that any damage is limited to the set however this may not be the case. Everything from the set back to any source of power needs to be checked, including transformers, bus bar supports and any other breakers in the fault current path. For example the bus breaker is designed to open under short circuit conditions and continue to operate properly; however the opposite is not true, a breaker does not have to operate properly after closing into a bolted fault. From:

http://www1.cooperbussmann.com/pdf/2b9f092e-4f96-44bc-8d46-49c4c0640c3d.pdf

"...For example, 7.1.11.6.3.1 of UL 489 states "The inability to relatch, reclose, or otherwise reestablish continuity ... shall be considered acceptable for circuit breakers
which are tested under bus bar conditions". In practical terms, this means the circuit breaker doesn't have to work after a fault near the circuit breaker occurs..."

Since we know nothing about the ratings on your equipment we can't tell how much of this is applicable to your particular situation, only trained and experienced personnel on-site have that ability.

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Commentator

Join Date: May 2012
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#8

Re: Diesel Generator Reverse Power

05/13/2012 8:26 AM

the effect on your genset will burn your exciter and your AVR so my suggest to you is need you to replace your generator bigger than the load your are using, or you can install step up transformer that has 380 / 415 ac input and the TR output is 10kv and second TR input is 10KV @ 3ph 415acv that will help you alot but costly more coastly if you buy new and bigger generator, think about it , i done this before when uor generator capacity of 1.5mage watts we are supplying one villege whene suddenly the generator shutdown, it burn the exciter and AVR and i repair it and intall 2 unit of stepdown 415input TR 33kv 415 output .

hope this will help you my friend, at you service (GENTECH 0&M)

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