Kataeb
Member
is reverse current really harmful to generators? i did some research on the subject, but it is not clear if this is a real danger to have ac current flowing back to a diesel generator. appreciate your ideas. thank you
is reverse current harmful to generators
- Thread starter Kataeb
- Start date
Mickey
Lifetime Supporting Member
Reverse current or reverse power means that instead of using the generator as a power source you are putting power into the generator and are using it as a motor. Is it dangerous? Well, you can always try it out, but please wait until I've arrived at a safe distance.
James Mcquade
Member
applying reverse current (power) to a generator means that the generator is now a motor.
so how can a generator run as a motor at the same time? it can't !
I heard a story from a professor about a shipyard.
9 ships with their generators tied together and a 10th was added incorrectly.
when they threw the switch, the generator became a motor and it tore the mounting frame out and went through 3 bulkhead walls and the hull of the ship.
james
so how can a generator run as a motor at the same time? it can't !
I heard a story from a professor about a shipyard.
9 ships with their generators tied together and a 10th was added incorrectly.
when they threw the switch, the generator became a motor and it tore the mounting frame out and went through 3 bulkhead walls and the hull of the ship.
james
tragically1969
Member
Yes it is but under normal standby use as a single set it’s not going to happen, however it is important if you are in sync with the mains supply or other generators as the biggest engine will always win and if you get the sync wrong you will end up reverse powering your generator set, a relay with reverse power protection would normally be used in this situation to trip the generator supply breaker off
Last edited:
V0N_hydro
Member
I can sync a pelton turbine and then close off the water and have the synchronous generator act as a motor and rotate the turbine and while the generator is motoring it makes no difference it could do it for ever and be fine.
I'm told many large hydro electric units will operate in this way instead of stopping and starting them they are put on 'sync condense' where they just motor with the excitation on.
For a diesel prime mover it probably depends on the engine. what happens when you let off the throttle in a vehicle and have the inertia of the vehicle drive the engine? Nothing bad, but it is designed for it. If the diesel engine for the generator is still putting fuel in the engine when it is being motored that is probably not good.
Consult your manufacturer.
I'm told many large hydro electric units will operate in this way instead of stopping and starting them they are put on 'sync condense' where they just motor with the excitation on.
For a diesel prime mover it probably depends on the engine. what happens when you let off the throttle in a vehicle and have the inertia of the vehicle drive the engine? Nothing bad, but it is designed for it. If the diesel engine for the generator is still putting fuel in the engine when it is being motored that is probably not good.
Consult your manufacturer.
The current always circulates from the side with the higher voltage to the lower.
The generators designed to be connect in parallel with the power grid are already designed so their alternator generates a voltage slightly higher than the nominal one of the network.
Therefore unless the network has an unexpected voltage rise it is very rare that the current is reversed
The generators designed to be connect in parallel with the power grid are already designed so their alternator generates a voltage slightly higher than the nominal one of the network.
Therefore unless the network has an unexpected voltage rise it is very rare that the current is reversed
+1
I've worked on gas turbines operated this way, speed up, phase to grid shut turbine fuel off and let generator act as a motor. It's done this way to phase compensate the grid if you're operating one.
I've also worked on a uncommon type of power plant where a huge boiler is running a gas turbine, compressor gives air to boiler and boiler flue gas drives turbine and generator gives electric power to grid. During start up the generator is run like a motor giving power to the compressor to pressurize boiler.
But I don't know why one would run a diesel generator that way?
Mickey
Lifetime Supporting Member
I have seen this with a natural gas prime mover. It was running like this for hours at the normal RPM (900 RPM). No damage at all to the generator or the prime mover.
As mentioned above the breaker fail closed. Reverse power alarm also failed. Just lucky I guess.
As mentioned above the breaker fail closed. Reverse power alarm also failed. Just lucky I guess.
tragically1969
Member
Electrical and mechanical damage.
The generator supplier or system designer will normally dictate what protection hey need on the sets/system, ultimately its about asset protection, why risk damaging a $1M generator set for the sake of a $500 relay ?
tragically1969
Member
Under what situation would that happen ?
we have a solar pv supply, connected to a diesel generator grid, supplying a building. each apartment is supplied by one phase plus neutral.
if at the load side, one of the phases got switched off, then will have the solar pv feeding current on one phase, back to the generator.
thank you
if at the load side, one of the phases got switched off, then will have the solar pv feeding current on one phase, back to the generator.
thank you
tragically1969
Member
I don't understand what you mean without seeing a diagram.
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