An experimental protoype of an on-line monitoring system for electrical machines based on impedance spectroscopy was designed, realized and tested in a high voltage laboratory. To be able to take measurements in parallel with the normal operation of the machine, excitation and sensing must be done on high potential levels, and in presence of important grid currents. High voltage coupling is done capacitively, for insulation voltages up to 20kV on the prototype. This allows showing the technology not only on breadboard level but with voltages equivalent to the operation of a power generator. The current measurement is realized with a high frequency current transformer based on a Rogowski coil.
The measurements are acquired via an Ethernet link and processed on a PC. The implemented FRA (Frequency Response Analysis) has a similar function to a commercially available off-line measurement, but works on-line. In particular, mechanical deformations of windings and core can be detected.
Several aspects of this work are original and go beyond of what was published in this field previously:
- Rogowski coil for free-air installation over a high voltage conductor or bus-bar
- Current comparator circuit using a double Rogowski-coil
- Test signal injection with capacitif/inductif coupler
- On-line frequency response measurement
This project allowed to make an important step forward between a first demonstration of feasibility with low-voltage bred-board components, on the one hand, and a real pilot installation, on the other hand.
The high-voltage laboratory demonstration is an indispensable prerequisite and reference for pilot tests in a power plant.
Further developments that are initiated starting form this project, inculde :
- Extending the result processing to automatic anomaly detection
- Defining requirements of a potential commercial product