Synchronous Generator - Voltage colapse with load 1

[m.maran]

Hello,

I've receaved an aparently new generator for maintance due to voltage control system failure.

But I've noticed that the problem was not the voltage control system.

The generator works fine without load. It reach the nominal voltage with normal field current and voltage.

But when any charge is conected to it the voltagem drops abruptly from 380 V to less thes 50 V.

It is a small synchronousm salient pole, two pole generator, 3-phase 8.5 kVA. I eliminated all the voltage control system and did some test directily applying and reading voltages and currents. I've use an independent manually controled voltage source.

1) I've already tested the stator without the rotor, aplying AC voltage on the terminals and current was equilibrated. So I do not belive that it I have problem with stator conections.

2) I´ve already tested the stator and rotor ohmic resistance and it is fine. Reforcing that conection in fine.

3) Open load test is ok, excitation is under expected.

4) Short circuit circuit, is not able to performe. The current doesn't rise with field voltage rise.

Does anybody has some idea about what is happening to this machine?

Thanks,

Best regards,

Maycon A. Maran

 

[waross]

Check the rating of the AVR.
A generator may require twice the voltage/current on the field under load as compared to no load.
If, for instance you had a 480 Volt AVR connected to 240 Volts, it is possible to develop full voltage at no load but experience voltage collapse when a load is added.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Hope you are keeping the excitation system in Auto. If it is in 'Manual' the AVR function will not be effective when there is a sudden inrush power drawn by charge.
Another point is - For such small generators, the phase circuits will have CTs that feed the excitation system (and thus boost its power) whenever there is sudden change in the generator output. Just check whether the CTs are in circuit (and not just left shorted in the terminal block).

Rompicherla Raghunath

 

[m.maran]

Dear Waross

First, thank you.

I'm testing without AVR. I'm ajusting manually the field current. With load, even if I rise the field current there is no answer from the stator terminal voltage,

I'm searching for some reference to the level of field current, there isn't this information. Only stator data.

Best regards.

M. Maran

 

[m.maran]

Dear Mr. Raghunath

Thank you for your help.

I have completly isolated the AVR system (the fuse open the circuit because the current field keep high for a long time and the stator voltage does not rise). I'm with a voltage controled source directly feeding current into the machine bushes terminals. So CT's and AVR are totally out of the tests.

I've tried several steps of load, for 0,5 kW to 5 kW of resistive load.

 

[crshears]

Hello m.maran,

For future reference, your post more appropriately belongs in the Electric Motors and generators Engineering forum . . . but since it's already here, don't bother double-posting it or trying to move it there; quite a few of us monitor both fora, and will have seen it anyway.

Just saying...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[m.maran]

Dear Crshears

Sorry, new user always do some mess. I'll be more careful next time. Thanks.

Best Regards.

M. Maran

 

[waross]

As an example of manual control: I applied 12 volts to a generator field and got close to rated voltage.
Then I connected a load. It now took about 24 Volts applied to the field to develop close to rated voltage.
As to CTs;
Two possibilities.
1: There was an old excitation system that I havn't seen for years. The last new one I saw was in the late 70's.
This excited the field with a constant voltage transformer. The output of a CT was dropped across an adjustable resistor and the developed voltage was rectified and summed with the rectified output of the constant voltage transformer.
Many sets had no voltage regulator as this system would give about 5% regulation.
This system had no electronics and as rugged as an old hammer.
An AVR could be added to improve the regulation to about 2% or 3%.
If the AVR failed the set could still be used with slightly poorer regulation.
But, about 15 years ago I had an application for a set going to a remote area and thought that the old constant voltage plus CT system may be a good idea.
I contacted the F.G. Wilson company who at the time were the third largest set manufacturer, world wide.
I was informed that that type of excitation system had not been offered for quite some time.
2: Larger sets at times used a CT boost to avoid or lessen voltage collapse under short circuit conditions.
In the event of a fault, the CT boost would hold the voltage up to develop enough current to ensure dependable operation of the protection devices.
While there are still applications for this system, as PMG excitation became more common, this system became less common.

I would be very surprised to see a CT boost system on a set this small.

Be aware that under manual control it may take a 200% or more increase in field current to hold the voltage up under load.


Bill
--------------------
"Why not the best?"
Jimmy Carter