By Engr. Kanthar Balanathan DipEE (UK), GradCert (RelEng-Monash), DipBus&Adm (Finance-Massey), C.Eng., MIEE, Former Director of Power Engineering Solutions Pty Ltd, Consulting Electrical EngineersLeading Engineer of the World 2006, UK Authority Award

1. General         

The author, engineer (A) joined a thermal power station as a Senior Engineer in Sept 1998. The station had a combination of thermal, gas turbines, and combined cycle plants. After meeting the staff, the engineer went on a walk around the station to acquaint himself of the plant. The author has extensive experience on thermal generators, Gas Turbine generators and large HV motors and Transformers. As he was ging towards Unit one he observed a consultant who had a PhD doing a temperature rise test.

• Plant

The PhD said that he is testing the Unit for rotor short turns by doing the temperature rise test.

• Issues

We SriLankan engineers although we did not have sophisticated test gear in SL in the 70s, we had our own way of testing for vibration detection unless it requires advanced testing, then the owners will get down foreign experts with sophisticated equipment. The author decided his Sri Lankan method and found out the Unit had vibration. He checked the Bentley Nevada computer screen, and the vibration was above normal.

With some doubts the engineer checked the Pedestal on the excitation side of the generator. He was not satisfied because he thought that there was vibration on the pedestal and decided that it could be a soft Footing” issue. After analysing he decided to check the Unit file. The unit file had some interesting information. The unit had been installed with Flux Probe (FP). The flux probe is similar to the RSO test where the probe will detect a rotor short of turns. The Unit had the FB wired out to a rf TB.

The engineer then decided to download a series of samples at various loads the following days after the Consultant had finished his work. The flux probe was invented by Generatortech, Inc in Ohio. 

The engineer analysed the samples and found that there was no indication of rotor short of turn. The engineer called Ohio and discussed with Mr. David Albright (DA) who was the inventor of the FB. Those who are interested in knowing more please refer to site given below. After discussion the Director asked me to send samples at various loads and he will analyse and send a reply. The Technical Manager was a mechanical engineer with an additional mechanical engineer supporting him. The engineer attempted to explain the Station Manager, Tech Manager. None of them would agree and finally spoke to the Operations Manager who had a Physics degree (Special) and he agreed. Finally, all agreed to carry out a vibration test on the pedestal and based on the results came to conclusion that the pedestal has a soft footing”. BY this time the engineer (A) had decided what the real issue was. The previous year (Latter part of the year) the unit had undergone a rotor refit of rewedging, and the unit had new BN gear fitted. The engineer (A) concluded that one or two wedge may be tight fitted which may have caused minor thermal bowing and the soft footing has caused the vibration. The engineer (A) checked with EPRI reports and found out that in 1964 a generator in California had similar incident of vibration. The engineer was already given access to EPRI by the Station.

The company invited an engineer from Bentley Nevada who came and did the same temperature rise test. After all, the engineer invited the BN engineer for a meeting at midnight. The discussion was on pros and cons and the engineer (A) explained with refence to EPRI. Both agreed and the SM was conveyed. The Eng. (A) was asked to submit a report to the GM.

The rotor was sent to the workshops for refitting with the same rotor bars but with new insulation. The engineer wanted to be at site when the remove the bars. One bar was found to have foreign body which caused the tightness.

At site, they found the pedestal base concrete foundation breaking like biscuits.

The unit was repaired and reinstalled and was online with no vibration. The engineer (A) has photocopies of all related matters and was happy that he was correct. The engineer was allowed to practice as an Electrical and mechanical engineer by the professional body in the UK.  The engineer had published a book also on power systems. Also, the credibility and competency of the SriLankan engineers in Power Systems.

Contact: http://www.generatortech.com/B-Page2-Theory-Overview.html

Cause of pedestal vibration

Foundation degradation ・ Bearing pedestal stiffness ・ Natural frequency excitation.

https://turbolab.tamu.edu/wp-content/uploads/2018/08/Case_Study_11.pdf

Thank you,