Naked Science Forum
Non Life Sciences => Technology => Topic started by: paul cotter on 31/07/2023 09:57:38
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About 15years ago I was running a 1000kva diesel powered alternator at 60% load, in parallel with the grid, when the excitation failed. Not much happened other than the current shot up but not to dangerous levels and the pf went into a lead. The conventional wisdom is that such a scenario can lead to pole slip( ie desynchronisation ) with potentially severe consequences for the rotating diodes or worse to the coupler and/or the crankshaft. This was a brushless four pole salient pole 1500rpm lv machine. My question is this: with the massive damper fitted to all similar alternators that I have seen would the alternator not default to asynchronous operation wherein the damper would be analogous to a squirrel cage rotor, in the case of excitation failure?
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Synchronous alternators are designed to operate in synchronism with the grid, meaning their rotational speed and phase must match that of the grid. If the alternator loses synchronization with the grid, it can lead to pole slip, where the magnetic poles of the rotor shift out of alignment with the stator poles. This can cause mechanical stress and damage to the alternator's structure.
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Yes, I know all that, my question relates to the possibility of asynchronous operation with the damper an analogue of a squirrel cage rotor.
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Ah, but your correspondent has used AI to produce useless garbage, and must therefore be respected as the forerunner of all good things.
Sorry I can't help - the elec eng book is covered with dust and my father, who knew all this stuff (he designed power stations in the 1950s and 60s) took most of his encylopaedic knowledge to the grave.
True anecdote about heavy electrical engineering. I proudly announced that I had just built a 30 kW precision x-ray generator using the most accurate technology available to stabilise the 300 kV DC supply to +/- 10V. My dad said "Thirty kilowatts? That's a ladies' drink!"
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Sorry I can't help - the elec eng book is covered with dust and my father
Could you move him to one side?
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About 15years ago I was running a 1000kva diesel powered alternator at 60% load, in parallel with the grid, when the excitation failed. Not much happened other than the current shot up but not to dangerous levels and the pf went into a lead. The conventional wisdom is that such a scenario can lead to pole slip( ie desynchronisation ) with potentially severe consequences for the rotating diodes or worse to the coupler and/or the crankshaft. This was a brushless four pole salient pole 1500rpm lv machine. My question is this: with the massive damper fitted to all similar alternators that I have seen would the alternator not default to asynchronous operation wherein the damper would be analogous to a squirrel cage rotor, in the case of excitation failure?
I faced similar situation in 2007. The natural gas processing plant I was working at had 3 gas engine generators which were commissioned in around 2001. Two run synchronously and one as backup. Each unit has 750 kVA capacity. One night, one of the running generator failed, and the backup took over automatically. It was found that the rotating diode was faulty. Without it, there would be no magnetic field induced by rotor coil to the stator coils, and no electric power would be generated.
From the perspective of the engine, losing power load looks like a running locomotive detached from the train. There would be momentary speed up until the governor regulate it back to set speed. The damper or flywheel reduce the acceleration or deceleration rate of engine rotation. The exact effect may vary according the position of faulty component and type of failure, as shown in the article below.
https://hal.science/hal-01063655/file/IEEE_TEC_2014_BACHA.pdf
Considering the second figure in the article above, if the faulty generator unit was running in parallel with other generators, then it will be seen like an inductive load from the perspective of those other generators. But it will not act like a normal asynchronous inductive motor, due to the rectifying diodes. They will block half phase of the current produced by (rotating) excitation coil, induced by stator's main coils which are still connected to other generators.
Moreover, in the case of asynchronous induction motors, the rotor essentially consists of short-circuited coils. It's different from the rotor used in brushless generator shown above.
https://en.wikipedia.org/wiki/Excitation_(magnetic)#Brushless_excitation
https://www.tutorialspoint.com/synchronous-generator-construction-and-working-principle
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Thanks Hamdani for that detailed response. I wish to make a couple of points. What I was dealing with was an excitation failure, ie the avr, and not the diode assembly. Diode failure can produce weird effects, one that comes to mind was a meccalte 500kva which was producing a high output voltage( no load ) with no avr connected and about 90volts ac appearing on the exciter stator winding!!--replacing the diodes sorted the problem which I reckon was due to remnant flux causing a build up of current through a faulty diode. When excitation fails there is no change in watts, only in vars and as you state correctly the faulty alternator will become inductively reactive, ie supplying at a leading power factor with no change to engine loading. As you state correctly an asynchronous rotor consist of shorted heavy conductors but that is exactly the same form as the damper that is an integral part of every salient pole synchronous rotor that I have ever seen.
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Modern generators are usually equipped with some protective electronics and built in diagnostics tools. They address common failures and prevent them from causing further damages by shutting down the unit and disconnect it from the loads.
There's mechanical-electrical analogy for RLC circuit. Some of us are unaware that there are two types of analogies, which may create confusion.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=86432.0;attach=34059)
In the case of rotating equipment like generator and motor, the mass should be replaced by rotational inertia.
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In a well designed plant, loss of excitation protection together with many other protection systems would be implemented. I was dealing with a basic system. That analogy chart looks a bit dodgy, to me, maybe just a poor choice of symbols.
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From my experience, the massive damper in alternators like the one you described does indeed act like a squirrel cage rotor in asynchronous operation. This is especially true in cases of excitation failure. The damper's primary role is to suppress rotor oscillations and help maintain synchronicity, but in the event of a loss of excitation, it can provide a kind of "fallback" mode of operation. It's not ideal, and efficiency takes a hit, but it's definitely better than a complete system failure or damage to the alternator.