is reverse current harmful to generators

Not only is it totally possible, but in doing so, the generator engine runs in reverse and will suck the burned fuel exhaust out of the air to convert it back into fuel! Why we have not been doing this for the past century or so I'll never understand...
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So, you do not have a Automatic Transfer Switch (ATS)?

ETA: this is a working system or just in the design phase? If this is a working system, then likely there's already a transfer system in place and maybe reverse power protection relay (a controller with a single function, basically).
 
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I would think the only issue would be if closing the breaker with an out of sync generator, that would cause very big issues and curernts. A diesel generator should be happy to motor along, the fueling to the engine would be near zero. It would seems to be a huge waste of power, but I don't know that it would hurt anything.
 
I would think the only issue would be if closing the breaker with an out of sync generator, that would cause very big issues and curernts.

That's what check sync relays are for.

The OP's system sounds like its PV in parallel with Generation, but that's a guess as there isn't enough info to go on, without knowing the details then its impossible to say really what the effects will be

General rule of thumb is if in parallel then use reverse power protection, but again thats based on several other factors (PV rating, Gen rating etc. etc.)

If its ATS break before make then there is no physical connection between PV and Gen then no reverse power protection required.
 
Hi Kataeb,

Diesel generators do not like running at low output for extended periods of time. Look up 'wet stacking':

Diesel-powered generators are prone to problems associated with operating for
extended periods with little or no load applied. This primarily results from over-sizing of
the generator set to accommodate future loads and from maintenance test running that
is necessary. Conditions such as accumulation of lubricating oil and unburned fuel in the
exhaust stack (wet stacking) and carbon buildup in combustion chambers, on injector
nozzles, piston rings, turbo chargers, exhaust piping and silencers commonly develop.

So if you have so much solar energy that the diesel generator is running at less 65% of full output you may be causing undue wear on the diesel generator. Some diesel plants will have generators of different sizes so they can be run at appropriate outputs, such as one 50 kW generator and one 100 kW generator.

There will naturally be some imbalance in the load on each phase of the generator just due to the different apartments running different loads at different times. The generator will have a specified maximum imbalance that is allowed. Imbalanced load results in negative sequence current in the generator which results in a pulsing torque on the prime mover. Imbalance is mechanically bad for the engine.

Generally I would expect the size of the diesel generator to be much larger than the PV array and the diesel generator is generally sized appropriately for the load and the PV array simply saves some fuel. If this is not the case then I would expect some other means of regulating the output from the PV array such as a battery bank and inverter.

If you tie the PV system in to the diesel system through a three pole breaker and implement protection for the maximum allowable imbalance then you shouldn't have to worry about losing one phase of the PV generation, but as indicated above the diesel engine should be operated far from 0 anyway so reverse power should not be a possibility.
 
Geez - there is some crock in this thread. I have been implementing diesel generator systems since 1992. I have built and automated diesel generator power stations - some large ones - the largest was 15 x 8 megawatt, 11kV generators - a decent power station. Normally the power authorities will not allow reverse power (kW) or reverse var. Added to that they normally require ROCOF and DT protection to disconnect the generator or mains (depends on the requirements). Additionally low load, as mentioned, is not good for diesel generators. Depending on the generator manufacturers requirements at least 40% load has to be on the generator all the time - some manufacturers require 60% load all the time. What happens with low load is the cylinders become glazed and wear rates are horrendous. Oil usage goes through the roof as well. Oil contamination also becomes a huge problem as the seals start leaking oil. Additionally the engine will start to 'slobber' oil running out of the seals around the engine. Then the generator has to be taken to full load for some hours to remove the glazing, or most of it, and stop the 'slobbering'! If low load is consistent the generator is mechanically stuffed! Often we have had to fit an automated load bank to generators that are running under low load to load them up and stop glazing - a huge waste of fuel. Generators do not generally run efficiently either if the load is less than 85/90% - depends on the generator. Stupid idea from the aspect of a guy who has implemented 7-800 generator systems of various kinds at last count - and still going! With a proper generator controller - suggest Woodward - and ATS controllers load can be maintained on the generator to stop it becoming mechanically stuffed - but the PV array would not like getting belted up the a**e either I am sure.
 

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