Thermal Sensitivity and Turbo Generator

Dear Friends

On September 2008, I just finished rewound of GE 237 MVA / 15 kV / 3000 RPM Generator.

Vibration after rewound rotor is normal, and site balancing correction was performed to reduce the vibration, normal standard after rotor has been finished rewound its must be overspeed to made all copper bar rotor setteled, unfortunately its can't to caaried out due to electrical demand, our city need much more electrical power.

Generator has been test by using "thermal sensitivity" method with folowed GE published paper.

Test method:

Step 1 : is to isolate the effects of megawatt loading from VAR loading on the field. Vibration changing as a function of megawatt loading is not a thermal sensitivity mechanism. Megawatt loading may result in rotor vibration excursions as a result of bearing alignment shifts.

Step 2: The first part of the test is to apply constant field current to the field and then to vary the megawatt loading on the generator from 15–60%

The second part of the test is to apply a constant megawatt load to the generator (approximately 60–80%) and then raise field current to maximum rated field current. Each test point should be held until steady state is reached. If the unit is unable to reach maximum field current attainable without a vibration excursion, the series should be repeated but be limited to the maximum field current attainable without exceeding acceptable vibration limits. Again, detailed test data should be taken. A significant change in vibration or phase angle with an increase in field current at constant megawatt load would indicate that the field is thermally sensitive.

This test should then be reversed; that is,decrease field current from its maximum value back down to the starting point. Again monitor all test data. If the vibration and phase angle return to their initial values, then the type of thermal sensitivity can be considered reversible and, in many cases, can be overcome with a compromised balance that moves the thermal vector through zero so that vibration limits are not violated. However, if the vibration levels do not return to the original values and remain high, then this field vibration is considered to be irreversible and corrective actions may involve modification to the field.

If the total vibration of the field stays within acceptable limits, the field is not considered "thermally sensitive." Vibration performance is frequently plotted on a polar chart, because vibration is characterized by amplitude and phase angle. If the vibration vector stays within the 2 or 3 mil circle, or whatever is chosen as an acceptable vibration level, the vibration is not considered to be a problem. This is true even if the phase angle changes and the vibration moves around the interior of this circle.

The change in vibration and phase angle within the polar plot from the starting operating point to the end operating point is called the thermal vector.

Test Result

Bearing 5X		Bearing 5Y
Ampl	Angle	Ampl	Angle
30.2	336	18.3	25
60	66	28	228

Thermal vector:

Bearing No. 5X : Vibration thermal vector = 66 micron

Bearing No. 5Y : Vibration thermal vector = 26 micron

Bearing 6X		Bearing 6Y
Ampl	Angle	Ampl	Angle
82.2	70	47.3	196
150	79	80	209

Bearing No. 6X : Vibration thermal vector = 62 micron

Bearing No. 6Y : Vibration thermal vector = 49 micron

Based on those test result and refer to the GE standard for thermal vector, the result is still in the safe range or not concider to thermal sensitivity problems.

Questions:

1. what is the vibration cause ?, the highest vib amplitude is 150 micron at bearing no. 6X, alarm setting by customer for this machine is 110, refer to GE manual for this machine is 180 and trip 210 micron.

During test (monitoring by Bently NEvada Instrument), there is not misalignment found.

2. could you advise me what is the problem cause ?

3. its may cause by copper bars rotor has not settled yet in the rotor slot, due to overspeed is not carried out yet ?

Photos documentation.

Rotor finish clean-up & ready to winding re fitting

Rotor winding being in re fitting process

Fault Typical1. cooling duct of turn insulation is not in line with the cooling duct of the copper bar, causing bar to bar short

Fault Typical 2 : inter turn insulation at endwinding has been move out, causing bar to bar short.

Fault Typical3 : Turn insulation breakdown close to cooling duct,

location in the slot portion

Typical fault 4: Hot spot on the turn insulation

Fault Typical 5; Inter turn insulation was loose (unbonded) to copper bar

Low speed balancing when rotor finish rewinding

RSG Inspection after Rotor complete rewound,

RSG was inspected when rotor stand-still (RPM = 0) and

when rotor running without excitation at >0 RPM up to 3000 RPM, no fault indication on the winding during these test.

Following are repair activities taht has been carried-out:

1. NDT on rotor body, steel wedges, bearing shaft, bearings, coupling, etc

2. Degaussing on LP, HP Turbine incl. bearings, casing, etc and rotor generator body, shaft, journal, etc

3. Rotor rewinding using with the old copper bars (all of them has been clean-up)

4. Replace all rotor insulation and upgrade material from class B to class F

5. Low speed balancing by IRD machine

6. Insitu balancing and vibration analyzer by Bently Nevada Instrument

If you have an idea please reply this discussion topic.

Thank you and best regards

Siswanto