Paralleled Generators' Loss of Load

They might, but various other things might also happen. The best is that they are equally tuned through the range of possible loads, so that they continue to share equally.

That is understood. But remember the load is suddenly disconnected leaving the gens tied to a common bus. If they were equally sharing the lost load, what happens after the load is gone?

The way the generators share the load (KW) is determined by the throttle settings. If the throttle is raised on one generator with respect to the other, it will assume more than its share of the load. If in this case the load is removed, this generator will tend to "motorize" the other generator. There will be power delivered from the advanced engine, through its generator to the other generator (which is acting like a motor) to the other engine.

Thanks. What if the throttle settings are uniform such that the generators are sharing load equally?

If they are equal under load, then they should both run unloaded when the load is removed. Neither should show any power (KW).

If you are drawing current with both generators in parallel with no load, there likely is a problem with voltage regulation. If the voltages are not set equal, there will be cross current (reactive current) between the two generators.

Also phasing, most importantly, that they might not cancel out one unto another.

If the generators are locked in synchronism they have to be in phase. If not one generator will drive the other, circulating currents will be high. "Motoring" as Rixter pointed out.

The loads were actually disconnected when all evacuating lines were tripped by protection but the hydro gens remained paralleled on the bus. Eventually, the generators tripped on " low governor oil pressure"

It depends on the system....I would think typically one generator would have the lead, any others would shut down on reduced load...this is a condition that is dealt with in the control strategy, which depends on type of load and conditions...

https://www.physicsforums.com/threads/paralleled-generator-protection.324929/

The engine alternators with which I was familiar years ago had governors which had two modes, isochronous and droop, that controlled the load versus speed function. Isochronous mode maintained a constant speed (or frequency) regardless of load. Droop mode allowed the engine to slow down or speed up depending on the load.

The standard procedure was to set one engine to isochronous and set the frequency with the throttle control. Other alternators paralleled with the first had governors set to droop and were adjusted with the throttle to take their share of the load.

Dear Mr. Richard Matisco,

This topic has appeared earlier and you can use the option to search with in CR4

and you will find the following link

https://www.physicsforums.com/threads/paralleled-generator-protection.324929

and you can refer the link below.

https://www.google.co.in/search?q=*Paralleled+Generators%27+Loss+of+Load*&biw=1280&bih=636&start=20&sa=N&psj=1&bav=on.2,or.r_cp.&bvm=bv.114195076,d.c2E&ech=1&psi=wBrAVqLFBI60uAS4iqmQCA.1455430329665.7&ei=6hvAVtWULNiLuwTIhoT4Bw&emsg=NCSR&noj=1

During my college days, our Professor has taught that the generator which has got less Percentage of Voltage Droop Character will continue to work as generator and the other one tend to work as a motor.

DHAYANANDHAN.S

Sudden loss of load = overspeed trip

They will all encourage each other to go faster when the load goes off. No personal experience but I guess without speed or frequency controls they will race away - particularly if diesel driven -it has something to do with needing less fuel at higher speed on no-load (experts here can explain why. I don't know for sure).

There is plenty of evidence to show what can happen to generators that over-speed:

Here's an example:

https://nolstuijt.wordpress.com/2011/11/27/duvha-powerstation-turbine-blowup-sa/

PS: Richard - are you the Chief Engineer of this power station by any chance??

"...They will all encourage each other to go faster when the load goes off...", no they don't since properly set systems don't allow this to happen.

Similarly, the overspeed in this article does not occur in normal operation, it was a result of operator error during testing:

"...They were doing a test of the turbine overspeed protection system, and in short, the protection did not kick in..."

Sorry to disagree with you RAMconsult, but I say they will speed up. We don't know the system is properly set.

My link shows what could happen in overspeed. The reason causing the overspeed is because the controls were bypassed for the test.

The novice level of the OP question makes me think there might not be any controls to start with. Hence the warning.

I am still curious why diesel engines keep on speeding up on no load (if they do that is).

Disagreement is fine, I had exactly the same reaction when my new boss (who happened to be on an IEEE committee on Power System Dynamics) asked me the same question. In my naivete I answered that once the load was removed the turbine-generator would speed up because all the inertia in the rotating mass had to go somewhere. He smiled and said, "...and where does the accelerating torque come from if no steam is entering the turbine?"

It's no different than if you're driving up a steep hill and you shift into neutral without removing your foot from the accelerator, the engine speed pops up beyond redline. On the other hand, if you simultaneously take your foot off the gas when you shift into neutral, the engine speed will see a small rise as the remaining fuel burns off, then immediately slow down once the fuel flow has stopped.

The accident you cited was a triple contingency failure and totally avoidable, a failure of multiple humans, not a mechanical one. In the US the safety systems are routinely checked, rechecked, and checked again, to ensure that something like that doesn't happen.

https://en.wikipedia.org/wiki/Droop_speed_control

Did your new boss figure that the control system could shut off the fuel, water, or steam instantly?

No, we were a bit more sophisticated than that, especially since we were modelling the overall dynamics of the plant. We were fully aware of second and third order effects such valve stroke timing, steam chest clearance volumes, piping sizes/lengths, governor and safety system time constants/delays/inertia, relay settings/operating times/sensitivities, boiler plant/control system dynamics, variances in manufacturers' data sheets, etc., etc.

It was a multidisciplinary effort involving teams from the electrical, mechanical, controls, relaying departments, as well as station operating personnel. After we programmed it in Fortran4, we then verified the models against actual tests at a steam station.

I think you were right to say the generator would speed up; unless you were told beforehand that the steam had been shut off. This seems to have been introduced into the equation afterwards. But I suppose you don't argue with your new boss.

The analogy of driving uphill is OK as an example of a load, but going into neutral is akin to disconnecting the engine - the load is still there.

I think we need two cars connected by a rigid tow-bar to simulate 'parallel' running (and maybe pulling a trailer). And then to go level or downhill to simulate no-load (or losing the trailer) - when without changing the throttle (drivers not paying attention) - they will speed up without knowing.

This is well known to zealous traffic cops who place cameras here to catch drivers before they realise they have inadvertently crept over the speed limit - a nice little earner this !

Whatever the merits of this analogy - appropriate speed/load controls are necessary to run generators in parallel - as many have said here on CR4 - without which, everything else being equal, they will speed up when the load is removed.

No I am not the chief Engineer. Actually I am a protection engineer trying to comprehend the dynamics of the system resulting from the disturbance.

If you're interested in dynamics then you might find "Power System Stability and Control" by P. Kundur interesting, there's even a chapter dedicated to PSS. Heavy reading though, 1,000+ pages!

Thank you

"...No I am not the chief Engineer....."

..good to know....but I never did think you were. Just my odd sense of humour creeping in. Apart from thinking the generators will speed up when the load is removed - for reasons given, I am not the expert here.

It looks as though RAMconsult and others have pointed you in the right direction.

I think RAMConsult has answered it. What you have explained is just what happened: oscillations, then a 'governor oil pressure low' trip. Possibly the oscillations were not 'damped' and the governors struggled with them until the trip. In your view, could a power system stabilizer have made a difference?

Thanks for the feedback! Unfortunately despite its name, a Power System Stabilizer (PSS) is not the panacea that many think it is. Its primary use is to reduce a phenomenom known as SubSynchronous Resonance (SSR) that can be problematic in large geographically diverse power systems. Unless you're experiencing SSR and/or routinely snapping 24" shafts between the turbine and the generator, a PSS is not for you.

Thanks. This is useful information!

If one of the generators is motor driven by the other unit then it will disconnect from the system on a reverse power relay trip.

Yes I agree, GA.