Advanced testing key to assessing rotating machine failure risks

The reliability of rotating electrical machines hinges on advanced diagnostic techniques to predict and prevent failures effectively.

Authored By:  Fabian Oettl, Product Manager, OMICRON

Assess the risk of failure in rotating machines

Rotating electrical machines, such as motors and generators, are essential for power generation and industrial applications. Dependable reliability and availability are, therefore, crucial during their operational life.

On-going condition assessments

Premature failure may lead to significant economic losses due to unexpected outages and possible equipment damage. Over time, thermal, electrical, and mechanical stress factors influence motor and generator failure. For an accurate failure risk assessment, accurate, up-to-date condition information about machines and their components is essential to plan their maintenance or replacement effectively.

During routine maintenance outages, the more extended downtime of the machine is used to perform various inspections, off-line tests, and measurements. An important aspect of these inspections is electrical testing, which assesses the quality of the insulation and reveals contact problems and other potential sources of error.

Various electrical tests evaluate the condition.

Various off-line electrical tests can be performed over the complete life cycle of rotating electrical machines to ensure their reliability, prevent premature failures, and extend service life. These diagnostic tests, including measurements of capacitance, dissipation factor/power factor, voltage withstand, partial discharge, and impedance – among others, such as the electromagnetic imperfection test (also known as a stray flux measurement) on stator cores – are performed after a machine has been manufactured, installed on-site and also during periodic maintenance checks to assess its condition state accurately.

The importance of partial discharge testing

 Partial discharges occur in the insulation system of rotating machines, where the local electric field stress exceeds the local electrical strength. It causes a progressive erosion of insulation materials that can lead to their failure.

 Compared with other dielectric tests on rotating machines, the differentiating character of PD measurements allows the identification of single weak points of the insulation system.

  PD in rotating machines (e.g. slot discharges or end winding discharges) causes recognisable patterns. Through pattern analysis, specific root causes can be identified, such as contamination, voids, cracks, ageing, or general deterioration of different insulation components.

How does it work?

Off-line PD measurements are performed when the machine is taken out of service and energised with a high-voltage source. A coupling capacitor is connected to the machine’s terminals, which are connected to the PD measurement device.

Depending on whether the star point is accessible, a single-phase measurement can be done. Otherwise, a three-phase measurement in combination with source separation techniques enables you to identify PD activity in a specific phase.

Several measurements over time enable trending of the insulation condition, which is the most powerful way to recognise a fault in its early stage. Several relevant international standards specify how to make PD measurements on rotating machines, such as IEC 60034-27.

Partial discharge testing on a generator with OMICRON’s MPD 800 system

Recommended diagnostic tools

OMICRON offers a matching electrical testing or monitoring solution for effective condition assessments of rotating electrical machines. Together, these diagnostic solutions provide users with a thorough condition assessment of rotating electrical machines to identify potential problems and assess the risk of failure quickly.

More information about electrical testing on rotating machine is available at: www.omicronenergy.com/machine-testing