Effects of Loss of Excitation in a Synchronous Generator

Thanks for your reply..

What I meant by the over-speeding thing was that let's suppose that Excitation is lost, that means magnetic connection b/w rotor & stator is lost & the governor was set to maintain synchronous speed/freq. at a certain load, & now since rotor is free the speed will increase & it should trip on Overspeed?

As you mentioned Alternator motoring/reverse power will not occur on loss of excitation. Can there be any other cause (specifically electrical) of reverse power in a generator other than loss of Prime mover power?

Regards,

MA

Corona , good to learn . . .

Vesna Vulovic is crew member from my former country which survived a fall from 10.000 meters .

So in your experience Alternator motoring/reverse power never happened because of Loss of Excitation or some fault in Excitation circuit?

When the excitation is lost and the generator remains connected to the grid and prime mover continues to supply power, the generator starts acting as "Induction generator". It no more remains a "synchronous generator". It draws excitation current from the grid and runs at some "slip". The control room will notice rise in speed of this particular set (other sets running at synchronous speed). The slip value depends on various parameters of the generator (various reactances etc.) and the amount of load delivered by this set. The stator current would exceed its rated value (assuming that it is supplying its rated load at that moment) as it would be the vectorial sum of load current and the excitation current drawn from the grid. As the generator is not running at synchronous speed, the rotor direct axis and quadrature axis would be cutting stator field axis. As the two axes reactances are different, the currents and the reactance drop would fluctuate at slip frequency. This in turn will make fluctuation in the terminal voltage of the generator. The final value of this fluctuation will depend on the grid strength and its parameters. What percentage of grip capacity is the generator under question will decide the amount of fluctuation in grid voltage.

Ultimately which parameter causes the tripping of the set will be decided by generator and the grid parameters. But if allowed to run as induction generator, then generator will be damaged due to excessive stator current. The power station accessories (various motors etc.) may trip due to fluctuation in grid voltage.

Dear MAfzal,

The replies posted here by CR4 Members are very excellent.

During my college days our Professor taught - the LOSS OF EXCITATION will result in 2 things.

1. The Voltage developed will fall and beyond a limit the generator will trip.

2. If generator is NOT tripping, but NO VOLTAGE at the terminal will make the Generator to draw power from the Grid, and try to run at Synchronous Speed and run as SYNCHRONOUS MOTOR, and Power transmitted from Prime Mover to the Generator will be zero. The current drawn for running as Synchronous Motor will have a very Low Power Factor.

Now I request the authors of the reply RAMConsult, CoronaCameraMan, pcchatur who posted the replies - to comment on the above explanation.

Thanks,

DHAYANANDHAN.S

Dear Dhayanandhan,

I have following comments on ur points

1.The Voltage developed will fall and beyond a limit the generator will trip.

That is true for a stand alone generator, excitation is lost, voltages drop the generator trips. Don't think its that simple for the scenario of Gens. running in parallel connected to the grid.

2. On your second point I conclude that your saying that generator shall now go in Reverse Power mode as a motor. My view & RAM's view as I understand is that generator will draw only Reactive Power from the grid & alternator motoring doesn't occur on reactive power? Kindly further share your views on that

Regards,

MA

Dear Mr MAfzal,

Sorry for the delay in replying.

When the power generation is lost on account of loss of excitation or prime mover trips, the Generator will draw a small power from the grid or other generator to keep the equipment running to suit the frequency in order to keep the machine IN-STEP and it will not PULL-OUT of step.

DHAYANANDHN.S

Then what keeps the rotor instep with the grid? A loss of excitation trip means that there is no longer a magnetic field on the rotor to couple the mechanical energy from the prime mover/rotor shaft to the stator and on to the grid.

This is equivalent to climbing a hill in your manual transmission car and pushing on the clutch or putting your automatic transmission into neutral, or a loss of hydraulic fluid in your automatic transmission/torque converter, there will be no coupling of power from the engine to the transmission/drive wheels.

Your car will slow down, and if you have not removed your foot from the throttle, your engine will speed up. The resulting mismatch in speeds is equivalent to the pullout condition, and will cause great damage if you do not match the clutch plate speed to the engine speed before you re-engage.

If the loss of excitation occurs at or near zero load, then you may not notice it until you try to load the machine, but sooner or later your control operator will notice an unexpected increase in VARs flowing into your machine as the grid tries to provide the magnetic flux in the air gap that would normally be provided by the field winding.

In any case it is an abnormal operating condition that must be avoided to prevent major damage to the generator, that's what the Loss of Excitation, Out of Step, and Reverse Power relays and others are there for.

No excitation?