Rotating electrical machines, such as motors and generators, are highly important components in power generation and industrial applications. Dependable reliability and availability are therefore crucial during the operational life of rotating machines. Here, Omicron highlights the importance of assessing risk of failure in rotating machines.
On-going condition assessments
Premature failure may lead to significant economic losses, due to unexpected outages and possible equipment damage. Failure in motors and generators is influenced by thermal, electrical and mechanical stress factors over time. For an accurate failure risk assessment, it is essential to have accurate, up-to-date condition information about machines and their components to effectively plan their maintenance or replacement.
During routine maintenance outages, the longer downtime of the machine is used to perform a variety of 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 condition
A variety of off-line electrical tests can be performed over the complete life cycle of rotating electrical machines to ensure their reliability, prevent premature failures and to 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 accurately assess its condition state.
The importance of partial discharge testing
Partial discharges (PD) 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 single weak points of the insulation system to be clearly identified.
PD in rotating machines (e.g. slot discharges or end winding discharges) causes recognisable patterns. Through the 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 terminals of the machine, which is 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 a trending of the insulation condition, which is the most powerful way to recognise a fault in its early stage. There are several relevant international standards that specify how to make PD measurements on rotating machines, such as IEC 60034-27.
Recommended diagnostic tools
For effective condition assessments of rotating electrical machines, Omicron offers the matching electrical testing or monitoring solution. Together, these diagnostic solutions provide users with a thorough condition assessment of rotating electrical machines to quickly identify potential problems and to assess the risk of failure.
More information about electrical testing on rotating machine is available at: www.omicronenergy.com/machine-testing
This article originally appeared in Electrical Review January/February 2021.