Generator exciter current

[SilverfoxUK]

Hi
I've got an 11kV generator which is delivering just over 2MW.
Over the last 6 weeks the exciter current has started to climb and gone from the expected of 1.5A to 2.5A.
The winding temps have also increased by 15 to 20degC from the expected mid 60degC and is now fluctuating between 60 and 80deg C.
Exciter current does fluctuate though but output is steady.
Output has been steady over this period, no changes to the load etc.
Any thoughts?

 

[edison123]

Causes in order of probability

Failing/failed rotating diodes

Faulty AVR/CT/PT

Turn shorts in exciter field

Turn shorts in exciter armature

Turn shorts in main rotor

Choked cooling circuit for over temp




Muthu

http://www.edison.co.in

 

[SilverfoxUK]

diodes and AVR we are know to be fine. Simply cooling circuit with redundancy which I'm guessing leaves me the expensive stuff.

 

[waross]

Your exciter field is failing.
It may be failing turn to turn.
The AVR is compensating by increasing the exciting current of the exciter.
If the fault is in the armature or load winding it will be causing circulating currents through the shorted turns.
If the fault were in the exciting winding of the exciter the resistance would be almost 1/2 of what it should be.
It is possible but unlikely that almost 1/2 of the winding would short out without going all the way to total failure.
That leads me to suggest that the armature or load winding of the exciter is shorting turn to turn.
The armature will be AC and a turn to turn short will act as a shorted transformer winding.
The increase in current will be disproportionately large compared to the change in resistance.
Turn to turn shorts in AC windings often quickly progress to complete failure.
The failure may be catastrophic, leading to core damage from arcing.
I would immediately shut down and check for damage.
How quickly?
If the exciter has failed by the time you read this, it would not be surprising.
Rather than an exciter fault you may have a fault in the main generator field winding.
While still serious and demanding immediate inspection, the main generator field winding is a DC winding and will not see the rapid avalanche type failure typical of turn to turn shorts on AC windings.

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

 

[waross]

I overlooked the diodes. Nice catch Edison.

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

 

[SilverfoxUK]

this fault was first spotted a few months ago, I wasn't informed, the alarm reset so nothing actioned.
the alarm is coming in a more regularly now but appears to be a little heat related. When I'm talking heat I'm talking 20deg C air temp local to the machine.
the alarm still goes into reset.
I agree the field winding looks likely so a slow and gently expire.

 

[wcaseyharman]

Do you know what the field voltage is?
With field voltage you could calculate field resistance. If the resistance is falling then it is further confirmation that you have shorted field windings.

If you had a flux probe installed that would also help confirm.

Casey

 

[SilverfoxUK]

Hope to be getting some more figures in a few days time.
I've managed to get a much larger snapshot of the winding temperatures and can prove this is cyclic so more relaxed with that.
The machine is coming off bars later this week so I can check values then.