PD Monitoring Identifies The Cause of Dust Contamination
two months after machine cleaning, drying, and repainting a critical 10kV, 6.7MVA motor, a visual inspection uncovered substantial contamination with white powder-like dust. It's very likely that this dust has a reasonable surface conductivity, which poses a risk for strong surface discharges that can eventually lead to flashover. Additionally, the core material that was not covered with paint showed relatively strong corrosion. This indicated that the machine had seen very strong discharge activities, with the produced ozone acting as a corroding agent. Unfortunately, this process is a selfsupporting cycle – dust contamination leads to surface discharge, which causes stronger corrosion, which causes surface discharge, and so on. However, this behaviour is unusual only two months after overhaul and operation.
PARTIAL DISCHARGE MEASUREMENTS
Due to the criticality of the motor for supplying power to chemical process equipment, an extended outage for shop testing and additional overhaul would have caused large economic losses and posed additional risks. The customer wished to maintain the motor's operation for seven months without a significant outage. Therefore, online PD measurements were conducted to investigate.
The ICMsystem from high-voltage measurement specialist, Power Diagnostix Systems, was used for partial discharge measurements after installing coupling capacitors and conducting PD calibration.
Phase-resolved partial discharge (PRPD) patterns obtained with the ICMsystem, as shown in Figure 1, indicated severely thermally aged insulation and strong surface discharges, which were very likely correlated with the dust contamination. Additionally, sparking or slot discharges were indicated by a pattern predominantly found with phase V, which increases with the machine's loading. Typically, loose wedges and subsequent deterioration of the ground wall insulation and slot corona protection due to bar vibration, are the cause of this type of slot discharges.
Generally, the discharge activity with this machine was relatively strong, with the strongest surface and slot discharges measured at values of up to 20nC and 25nC, respectively. Motors of similar age and size typically show activities below 5nC when in good condition.
CONCLUSION
PD measurements confirmed that the observed dust contamination was not benign. Despite the recent refurbishment, PRPD analysis confirmed strong surface discharge activity on top of severe thermal ageing. Simple cleaning and repainting would not be sufficient to allow this machine to continue reliable operation.
RECOMMENDATIONS
The typical recommendation based on the result of this testing would be immediate rewinding or replacement. However, the customer accepted the risk with further operation since the machine needed to remain uninterrupted for approximately seven months. Therefore, Power Diagnostix recommended installing a partial discharge monitoring device (ICMmonitor portable), which would use the already-installed coupling capacitors in the termination, which remained from the service job. Partial discharge activity could then be continuously monitored for the next seven months, and increased activity could be exported via a 4-20mA signal, via alarm contacts, or via a Monitoring Web Server (MWS), which PD experts from Power Diagnostix could then access for remote support and further analysis. Of course, it was recommended that rewinding or replacement was carried out as soon as operationally feasible or if increased PD activity was observed.
Power Diagnostix also recommended investigating the source of the dust within the cooling system. Once found, the corroded material could be suitably coated or painted to avoid further contamination and a subsequent increase in surface discharges.
