Here is the scenario.
When two shipboard gens (rotating field, brushless exciters) are synchronized and then paralleled, voltage on bus jumps from 460 VAC to 520 VAC. There is also big jump on ampmeters, so they show 90A. Generator breakers don't trip, though.
Both #1 and #2 Governor America Corp. load sharing modules have LEDs for FWD and REV Power. So, FWD and REV LED bars are lit simultaneously on both modules, indicating that something weird is going on.
Both gens work just fine during single generator operation, no issues (I have had all this by phone, by the way).
From my understanding this doesn't look like reverse power flow condition (both gens, and their diesels work just fine, when single-oping). There is no drop in RPMs on both gen sets when paralleled. There are no PF meters on this particular switchboard, so I cannot tell what PFs are.
So,
1) What would I see on on KW-, V-, A-meters when there is a TRUE reverse power condition? I don't believe Volts will jump up? Is there any good online reference on reverse power conditions?
2) What is the potential problem in the situation I described? I forgot to mention, both gens just came from overhaul.
I sympathize with your problem, but since you are only getting this information second hand by phone everything I say will be sheer speculation, with some SWAGS included for good measure. But that has never stopped me before.
So, assuming that everything you have been told is 100% accurate, let the logic train roll:
1. Both generators seem to be able to power the loads and to give a voltage output of 460 VAC on some meter. If it is an RMS meter, this means that they will both deliver the same amount of power to a resistive load or a light bulb. With a motor, you should see very similar performance too. If you are using a peak voltage meter that applies a correction factor to read RMS assuming sine wave output, then they might actually be delivering different power to your loads, but not necessarily noticeable.
2. Since the synchronzing modules are not complaining and the engines are not showing a cyclic load, we will assume that the two gennys are frequency matched and are staying in sync over a long period of time.
3. A voltmeter, ammeter or KVA meter which is not a true phase-sensitive watt-meter will probably not be of any use to you, since they will not tell you which way power is flowing on the average, let alone at different points in the cycle.
Now for the interesting part:
4. What the load sharing modules are telling you, I think, is that during some parts of the AC waveform power is flowing out of each generator while at other times it is flowing into the generator. With enough load on the generators to load them down close to full output (more load than either one could handle alone) the REV lights might go out, but the two generators would still be supplying more or less power to the load during different phases of the cycle.
5. All of the load sharing assumes that both generators are putting out relatively pure sine waves. Whether they really do or not will depend on the correct operation of the brushless field and the rest of the voltage regulating system in each generator. If one of the generators is putting out a distorted wave form (high third harmonic for example), then its regulator could be causing the load to see the correct RMS voltage and current, just not sinusoidal.
6. When you parallel a generator with a pure sine wave output with one that is putting out a distorted waveform, there are several possible side effects:
a. At some parts of the waveform current will try to flow from one generator to the other as well as to the load. Then at other times in the cycle that undesirable flow will reverse.
b. That could cause the REV lights to come on on both generators even though they are both supplying NET power to your load.
c. The result of this back-and-forth flow, interacting with the regulator mechanism of each generator, could cause the RMS voltage at the load to rise, with a corresponding rise in RMS current.
d. That will be bad for your loads, but will not necessarily cause either generator to show immediate distress.
e. But the cyclic (harmonic?) current between the two generators could cause both to overheat even though their engines are not seeing an excessive power load on the average (flywheel effect)
The very first thing I would like to see would be the waveform of each generator output, as shown on a scope or a graphic display voltmeter.
If you do not have that, try just putting an ordinary AC meter between the corresponding output terminals of the two generators to see if there is a voltage being developed across the interconnecting wires. That will give you a clue whether abnormal current is flowing between them.
Or, if you can have them both remain synchronized even though not connected in parallel, just look for a voltage on meter between the two generators, indicating a distorted waveform on one or both.
All of this is betting on the fact that one of the generators came back from servicing with a malfunction which allows it to deliver rated voltage but with a bad waveform.
I welcome other guesses, as well as feedback from you when you get a chance to investigate directly.
BTW, the sign of a reverse power condition is current flowing into the generator while the voltage is positive or flowing out of the generator when the voltage is negative. To see this you have to know both voltage and current at the same moment in time or you have to know the phase relationship between the voltage and current waveforms.
An actual watt meter will be sensitive to the instantaneous power flow, averaged over one cycle, and so will only show the average power, not whether the power flow is reversing direction many times during one cycle. So a wattmeter will not necessarily give you the same information that the FWD and REV lights on the load sharing modules are producing.
To some extent the power flow will also change direction during each cycle if you have a highly inductive or capacitive load, but that should affect both generators equally and hopefully should not cause the REV light to turn on.
