Hydrogenerator Noise @ 720 Hz 1

[StanBognatz]

I've got a unique situation with a recently re-stacked hydrogenerator. Vitals are:

# poles = 56 ; # stator slots = 360 ; line freq = 60 Hz ; rpm = 128.57

Old design had 4 stator core splits ; New design uses a continuous core stack

Vibration taken directly from the back of the core iron shows a 4.3 g-pk component at 720 Hz, with the next largest response at its first harmonic of 1440 Hz at 0.45 g. The adjacent keybar showed similar but slightly higher radial responses of 4.8 and 0.9 g-pk, respectively. Tangential responses on the keybar were 0.9 and 2.4 g, respectively.

Through each spectrum 120 Hz harmonics are present, but they are comparatively low level, typically 0.05 to 0.09 g.

The 720 Hz vibration creates significant audible noise, with levels that increase directly with load. At FSNL and with excitation on, we have about 75 dbA at 20' outside the generator enclosure. At 35MW load, we have 90+dB, with the predominant 720 Hz component.

Sister units to this problem unit operate at full load with noise levels in the 75 dBA range; no access for vibration measurements at this point. Our client is not accepting the unit for operation based upon the noise being generated. We have had several theories regarding the source of the noise, with possible solutions.

Since I'm an ME, my working knowledge of generator circuitry is limited. I'd like to get some feed back from the EE's out there if they feel there is anything in either the static excitation circuitry or the generation circuitry that would cause this.

It has been suggested that the static excitation frequency would be at 6x-Line, or 360 Hz, and that 720 Hz would also be present. However, the 720 Hz vibration and noise DOMINATE the response spectra.

We are also wondering about the 2x-Line harmonics. Do they suggest stator core looseness, or should they be expected?

I would appreciate any commentary on the issue.

Thanks,
Stan

 

[edison123]

I would suspect loose laminations (not enough axial pressure) or loose keys (not welded or anchored to the stator frame).

You can use the knife test for the laminations (insert a thin edged knife radially between the laminations, if you succeed the laminations are loose)

* If a little knowledge is dangerous, then I am the safest person in the world *

 

[StanBognatz]

Thanks Edison.

Regarding loose laminations, this is brand new re-stack of the core, and I've heard the stator core bolt tensions reviewed in our conference calls, and they did not raise any flags amongst the group. However....no one that I know of did a knife test.

Regarding keys, we had taken data from the "keybars", which we're referring to as the axially mounted bars located around the periphery of the core iron. These to my knowledge are welded to the frame. There has been discussion if clearance exists between the keybars and core back iron, but no measurements that I'm aware of. Do you know if there should be any clearance (by design?) to allow for radial thermal expansion of the core, or should they be 'tight'?

Let's say either or both of the above conditions occur. I would expect 120 Hz harmonics. But do you think that would also produce the dominant 720 Hz component we are seeing?

Also, I just found out that exciter does indeed get its power supply from the generator's output terminals. Could that contribute to the 720 Hz?

Thanks for the input.

 

[edison123]

Stan

Knife test is a simple test to get a quick feel (!!!) of the core tightness.

I do not know how you get the 720 Hz component. Loose laminations could create all sorts of noise, I guess.

We normally do a core shake down every 400 to 500 mm of stack length by doing ring flux test. This aids core settling before tightening. Was this done in your case ?

Other check points

1. Stator roundness (Move one pole around the stator core and check for equal air gaps)

2. Rotor roundness (Move all poles to one fixed point on the stator and check for equal air gaps)

3. Stationary air gap checks in at least 8 x 45 deg (within 5% of average)

4. Rotor AC Pole drop test (to check poles for turn shorts)

I do not think static exciter is creating the problem unless it has a very high ripple content output.






* If a little knowledge is dangerous, then I am the safest person in the world *