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Causes of Reverse Power in Generators
What are the causes or reverse power in generators?

What are the causes or reverse power in generators?

In my case sometime reverse power alarm came as I am using caterpillar c18 generators. One of my generator some time give this alarm. Kindly explain what are the causes and their solutions.

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- Motors convert amperes into torque.

- Motors get their amperes from generators.

- Generators convert torque into amperes.

- Generators get their torque from the prime mover coupled to them (in your case, a reciprocating engine).

- Prime movers produce torque from the energy flowing into them (diesel fuel; natural gas; steam; etc.).

Synchronous generators--and their prime movers--synchronized together all run at the same speed (remember you have to start the prime mover and get the generator up to rated speed in order to be able to synchronize to the other generators and their prime movers--and once the generator breaker is closed the prime mover and generator can only run at a speed that is proportional to the frequency of the grid).

Most people think that when they increase the load of a generator and its prime mover that the speed of the two will increase--but it doesn't! Once synchronized to a grid with other generators (even a small grid!), the speeds of all the generators are fixed by the frequency of the grid. And, the torque being supplied by the prime movers to each of the generators they are coupled to must be sufficient to keep the generators spinning at rated speed at a minimum. If the torque being provided is more than required to maintain rated speed, then the generator will sense the extra torque and convert it to amperes. If the torque being provided is exactly equal to that required to maintain rated speed there will be ZERO amperes produced by the generator. If the torque being provided by the prime mover to the generator is less than that required to maintain rated speed then the other generators and their prime movers will provide amperes to the generator to keep it (and the prime mover) spinning at rated speed--because they all have to spin at the same speed. (You can't have one generator running at 49.7 Hz, and another one at 52.4 Hz, and another one running at 50.1 Hz, and have 50.0 Hz coming out of the receptacle on the wall. It doesn't work that way; there's no smoothing thing or averaging thing for frequency.)

When the torque being supplied to the generator is not sufficient to keep it running at rated speed the other generators and their prime movers will supply amperes to the generator to keep it spinning at the same speed. This is called "reverse power"--since the amperes will be flowing into the generator which isn't getting sufficient torque from its prime mover instead of flowing out of the generator.

In this case, the generator actually becomes a motor and drives the prime mover to keep them both spinning at synchronous speed (the same speed as all the other generators synchronized together as a function of the grid frequency). This is also called "motorizing" the generator. There is really very little difference, mechanincally, between motors and generators--just the directions of torque and amperes.

It's NOT good to use the generator (as a motor) to drive the reciprocating engine (or just about any other prime mover). So, there are protective relays to monitor the direction of current flow and open the generator breaker to protect the prime mover (again, the electrical machine coupled to the prime mover doesn't really care if it's a motor or a generator (it's designed to act as a generator), but the prime mover doesn't like being driven by a motor (or a generator which has become a motor)).

Since the torque produced by the prime mover is a function of the energy flowing into the prime mover, if the torque falls below that required to keep the generator spinning at its rated speed that means the prime mover control system (the governor) isn't doing it's job properly. Or, there is something which is restricting the flow of energy into the prime mover (in the case of a reciprocating engine, dirty fuel filters; low fuel supply pressure; a non-working fuel pump; etc.).

So, when a generator--and its prime mover--are tripped by reverse power that means the prime mover is not providing or producing sufficient torque to keep its generator spinning at its rated speed and the other generators synchronized to the grid will provide amperes to the generator, causing it to act like a motor, and spin the prime mover.

You need to understand why the prime mover isn't producing sufficient torque to keep the generator spinning at its rated speed.

There's one more possibility--but it gets very complicated. There are two basic governor modes: Droop and Isochronous (speed control modes). IN GENERAL, most governors are always in Droop speed control mode when multiple generators are synchronized together on a grid. If one governor is operated in Isochronous speed control mode when it should not be, then that could cause a problem.

OR, if there is a over-all control system for a group of generators (sometimes called a PMS, or Power Management System) that attempts to control the loads of multiple generators synchronized together there can be issues if the PMS is not properly configured OR operators think they need to over-ride the PMS in some instances....

That's about it. Without understanding a LOT more about the configuration of the generators and their prime movers at your site it's really not possible to say much more. BUT, again: The most common cause of reverse power tripping is insufficient energy flowing into the prime mover--regardless of it's governor mode, or whether there is a PMS or not. Insufficient energy flowing into the prime mover equals reverse power (amperes flowing INTO the generator instead of out). Amperes flowing into a generator causes the generator to become a motor which is keeping the generator--and its prime mover--spinning at a particular speed. And, most prime movers (especially reciprocating engines and steam turbines) DO NOT like to be spun by their generators (which have become motors). It's as simple as that.

Hope this helps!