Non intrusive and dynamic method for measuring a distance or the variation thereof through dielectrics

An apparatus for measuring the vibrations of a stator bar of an electric generator while in use, said vibrations corresponding to a distance to be measured over a period, said stator bar acting as a grounded conductive or semi-conductive surface, said distance is an air gap between a stator-side surface of a tightening wedge of said stator bar and a rotor-side surface of said stator bar, the apparatus comprising:

• a capacitive sensor installed on a rotor-side surface of said tightening wedge, said sensor being composed of a first conductive or semi-conductive plate, said first plate being furthest from said stator-side surface of the tightening wedge, and a second conductive or semi-conductive plate which is nearest stator-side surface of a tightening wedge and electrically insulated from the first plate, and both plates being parallel to the rotor-side surface of said stator bar;
• a high frequency signal generator for feeding said first plate of said sensor a high frequency signal while said electric generator is in use;
• a current detector for measuring over time the values of the current generated by said high frequency signal between said second plate and the grounded conductive or semi-conductive surface which faces said stator-side surface of the tightening wedge;
• a treatment unit receiving the measured current, wherein the measured current varies as an inverse function of said distance, thereby enabling the determination of said distance over a period and hence said vibrations.

A method for measuring the vibrations of a stator bar of an electric generator while in use, said vibrations corresponding to a distance to be measured over a period, said stator bar acting as a grounded conductive or semi-conductive surface, said distance is an air gap between a stator-side surface of a tightening wedge of said stator bar and a rotor-side surface of said stator bar, the method comprising:

• using a capacitive sensor installed on a rotor-side surface of said tightening wedge, said sensor being composed of a first conductive or semi-conductive plate, said first plate being furthest from said stator-side surface of the tightening wedged, and a second conductive or semi-conductive plate which is nearest stator-side surface of a tightening wedge and electrically insulated from the first plate, and both plates being parallel to the rotor-side surface of said stator bar;
• feeding said first plate of said sensor a high frequency signal while said electric generator is in use;
• measuring over time the values of the current generated by said high frequency signal between said second plate and the grounded conductive or semi-conductive surface which faces said stator-side surface of the tightening wedge;
• wherein the measured current varies as an inverse function of said distance, thereby enabling the determination of said distance over a period and hence said vibrations.