March 1, 2010

Restraining Torsional Vibration

By Alexander G. Beckford and Benjamin A. Mancuso

Consumer’s Energy Campbell Station Success Story

Consumer’s Energy J.H. Campbell Generating Complex recently completed an aftermarket upgrade to the excitation system on the 800-MW Unit 3 that unexpectedly affected the power train dynamics. Following the alteration, testing confirmed there was a generator torsional mode very near the critical 2x grid frequency (Figure 2). In fact, the unit passed through 120 Hz during operation (from low load to full load). The risk of equipment damage and failure was too high to continue operation in this condition, so various alternatives were explored.

2.    Pre-tuning results. Testing of the generator end of the power train confirmed that there was a torsional mode at 2x the operating frequency (120 Hz or 3,600 rpm) under all operating conditions (see red arrows). Continued operation under these conditions would soon cause mechanical damage of the power train. Source: GE Energy

The usual method of coupling inertial rings could not provide sufficient margin from the stimulus frequency to meet standard separation margins so this option was discarded.

The Campbell staff contacted GE to explore other technical options. GE engineers used this opportunity to develop, test, and install the first full-scale torsional dynamic tuner in the spring of 2007 (Figure 3). With minimal alterations and down time (<300 pounds of hardware installed at the free end of the shaft), the torsional dynamic tuner achieved over 150% of recommended frequency margin under all plant operating conditions, including start-up (Figure 4).

3. Post-tuning results. After using the first full-scale torsional dynamic tuner, GE engineers were able to determine the amount of weight that should be added to the generator-exciter end of the power train shaft to achieve over 150% of the recommended frequency margin. Source: GE Energy


4.    Tuner hardware. About 300 pounds of weight was added to the generator shaft end to achieve the required torsional frequency margin. Courtesy: GE Energy

— Contributed by Alexander G. Beckford (alexander.beckford@ge.com), a principal engineer, and Benjamin A. Mancuso (benjamin.mancuso@ge.com), an engineer, with the Generator Services Engineering division of GE Energy.