Diesel Generator Reverse Power Problem


Thread Starter


Hi there!

Can anyone please tell me how to fix reverse power trip on a diesel generator? I've been working to fix this problem for two weeks but still the problem exists. I had checked all possible parts like CT's, controllers, relays, governor actuator, and even replaced them. There were two things that I had noticed. One, there was no gain during the time the generator synch and the other was the actuator lever did not react during the synchronization causing insufficient fuel supply to the engine. Maybe these two causes the generator to reverse power.

Please help me on this one.
Many thanks!

It's presumed the diesel-generator (generator-set) had been working fine for some time and has just started exhibiting this behaviour.

The amount of power produced by a generator-set (prime mover and generator) is directly proportional the amount of energy flowing into the prime mover. In your case, the prime mover is a diesel engine, so the amount of power being produced by the diesel is a function of the amount of diesel flowing into the engine.

It requires a certain amount of fuel for a diesel-generator just to reach and maintain rated speed to perform synchronization. And, once the generator-set is synchronized to the grid (or with other generators and their prime movers) to increase the amount of power being produced by your diesel-generator it's necessary to increase the amount of fuel flowing into the diesel over and above the amount required just to reach and maintain synchronizing speed. If the fuel is not sufficient to maintain diesel-generator speed at synchronous speed then the grid to which the diesel-generator is synchronized will supply power to the generator--called "motorizing" the generator--to keep the speed at synchronous speed. This is NOT a good condition for diesel generators--because the generator, which has become a motor when this happens (when reverse power is flowing INTO the generator from the grid) is actually spinning the diesel because the amount of fuel flowing into the diesel is not sufficient to keep the gen-set spinning at synchronous speed. Diesels don't like this very much--and that's why there are reverse power relays, to protect the prime movers (the diesel in your case) from damage caused by the generator spinning the prime mover.

If the generator-set doesn't pick up load once synchronized, then there's something wrong with either the fuel rack/control system, or there is some restriction limiting the flow of liquid fuel into the diesel. I've see paper elements of diesel fuel filters swell up and restrict the flow of diesel fuel through them when there is some water in the diesel fuel (which there shouldn't be--but sometimes it happens). Or, the diesel fuel filters are just plugged from dirt. Or the fuel rack has "loosened" and even through the actuator is at full stroke, the fuel rack isn't. Or, the injectors are dirty/plugged.

Reverse power occurs when the energy flow-rate into the generator prime mover isn't sufficient to keep the generator spinning at synchronous (rated) speed. When a generator and it's prime mover are synchronized to a grid, it HAS to spin at the speed relative to the frequency of the grid (synchronous speed). If it doesn't, the grid will supply power to the generator-set to keep it spinning at synchronous speed--and that's reverse power. (The generator actually becomes a motor and drives, spins, the prime mover--which isn't good for some prime movers.)

Solve your fuel flow-rate problem and you will solve your reverse power problem.

Please write back to let us know what you find!
Hi, CSA,

I agree with the fuel problem. One thing puzzling me is that the DG set has enough fuel for synchronizing speed before and during paralleling. Would the problem caused by the reversed action of the actuator?

Yes, it certainly could be caused by reverse action of the governor/actuator. I presumed the unit had been running without problems for some time and just developed this problem.

I have encountered diesel fuel filters that would allow full speed operation--but very little more than that--when swollen with water. This would allow synchronization but not much load, if any. If the engine were struggling just to synchronize once the breaker closed the reverse power relay could operate. I've als seen some diesel generator reverse power relays set to operate at slight positive power (say 100 KW, or 250 KW, depending on the rating of the machine and the size of the grid load and nature of the other generator-sets synchronized to the grid.
Seems easy enough to watch the freq meters on the gen-set to tell if the engine is bogging down-exhaust pyrometers might tell you something also, but they are way too slow.

I agree with the comments about fuel flow problems, but I would expect the load control system to be requesting more rack from the engine.