Loss of Excitation on a Grid-Tied Generator 1

[Umer9]

Hello everyone,

I am trying to get my head around a few things related to loss of excitation on a generator operating in parallel with other generators. We know that when a generator loses excitation while the prime mover keeps it running, it acts like an induction generator but since its field has lost so it draws in VARs from the system but the literature says the amount of VARs drawn depends on initial loading of the generator, how? Also, there is drop in active power loading of the generator, why is it so because primer mover is intact? Could anybody explain it?

Thanks in advance

 

[FacEngrPE]

With loss of excitation there is still residue magnetism.
Now there is the case of
[ul]
[li]Excitation field stays aligned with stator field. No real power transfer.[/li]
[li]Excitation field rotates faster or slower than stator field.[/li]
[/ul]

 

[waross]

The amount of VARs drawn and the Real Power produced depend on the characteristics of the machine as an induction generator, and on the RPM relative to synchronous RPM.
A typical 1420 RPM or a 1460 RPM induction motor will produce close to full rated current at 1860 RPM or 1840 RPM respectively.
Some induction motors are designed with much more slip and must be driven at a much higher speed to achieve full output.

While the VARs drawn will to some extent depend on the loading, vARs will be more dependent on the machine design.
At what speed will the synchronous generator, acting as an induction generator develop full current?
Unknown. (The apparent current will be the vector sum of the active current and the reactive current, acting at 90 degrees, to add some additional complication)

How fast will the unexcited generator run?
That does depend on the loading.
Based on 1800 RPM for comparison, a grid tied set will be running at 5% droop. At full load, the governor will be set at 105% of synchronous speed, or at 1890 RPM.
A base loaded set may be loaded to 80% and so the governor will be set to 1800 + (5% x 80%)= 1872 RPM.
At no load, the set will run at 1872 RPM, but it will be loaded as an induction generator.
How much will it be loaded?
That depends on the unknown characteristics as an induction machine.
Any loading will reduce the speed below 1872 RPM.
So the question of what will be the output of a synchronous generator as an induction generator after it loses excitation depnds on several unknowns, but I would not expect a lot of power to be generated relative to the synchronous capacity of the generator.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

The induction currents will quickly suppress the residual magnetism.
On open circuit, the residual magnetism will generate a potential, but that will be unlikely to be able to support a load.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!