New Tools For Diagnostic Measurements On Rotating Electrical Machines

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Michael Krüger
Consulting Engineer – Testing and Diagnosis of Electric Power Equipment
OMICRON electronics GmbH

Co Authors

Fabian Öttl
OMICRON electronics Austria

Wenyu Guo
OMICRON electronics Australia

Florian Predl
OMICRON electronics Australia

Rotating electrical machines are important and expensive assets in electric power systems. Recurring diagnostic measurements on the electrical insulation of stator and rotor are important to ensure the safe operation and prevent unexpected outages and serious failures. Common practise for the insulation diagnosis is the measurement of the Polarisation Index (PI), the measurement of the capacitances and dielectric losses and the measurement of Partial Discharges (PD).

Innovative tools like the Dielectric Response Analysis (DRA), the Frequency Response Analysis (FRA) or the partial discharge measurement with synchronous multi-channel systems enable a more detailed analysis of the insulation quality and the location of faults.

The dielectric response analysis of the insulation with Polarisation / Depolarisation Current (PDC) and Frequency Domain Spectroscopy (FDS) can detect moisture and ageing problems much better than measuring the PI only. With FRA short circuits on claw-pole windings and broken damper bars on rotors of synchronous motors can be found. Simultaneous PD measurements at different points, allow the separation of different PD sources from each other and from interference. Multi-frequency PD measurement systems enable pulse shape analysis. This way PD faults can be seen and localised much easier. Such PD systems can also be used in online monitoring systems with great advantage.

In the paper the new methods of insulation testing and monitoring are introduced and the advantages of these tools are illustrated with practical case studies.

Michael Krüger works with OMICRON Energy in Klaus, Austria since 1999. After working as product manager and head of engineering services, he is now consulting engineer for “Testing and Diagnosis of Electric Power Equipment”.


He studied electrical engineering at the Technical University of Aachen (RWTH) and the University of Kaiserslautern (Germany) and graduated in 1976 (Dipl.-Ing.). In 1990 he received the Dr. degree (PhD) from the Technical University of Vienna.

Michael Krüger has more than 40 years of experience in high voltage engineering and diagnostic measurements on power and instrument transformers, rotating electrical machines, GIS and cables.

He has published more than 50 technical papers about electrical measurements on power and instrument transformers, rotating electrical machines, GIS and cables and holds 15 patents. He is member of VDE for 40 years, Cigre and IEEE and works in several working groups of OEVE, IEC and Cigre.