5. On-site Turning of Generator Slip Rings
On-site Treatment of Generator Slip Rings
During operation of the N7.5-2.35 condensing steam turbine generator set, sparking occurred at the generator slip rings. Specialized tooling was used on-site to grind the slip rings.
Causes:
1) Substandard carbon brush quality; impurities within the brushes caused gradual wear on the slip rings during prolonged operation. This resulted in uneven arc surfaces on the slip rings, leading to poor contact and electrical sparking between the brushes and slip rings.
2) Failure to properly lapping the contact surfaces of replacement carbon brushes or improper installation caused brush chattering during operation, damaging the slip rings.
3) Failure to promptly clean accumulated carbon dust and debris from the slip ring area during unit shutdowns can contaminate the slip ring surface, causing poor contact.
Hazards of Severe Slip Ring Sparking:
1) Slip ring overheating: Sparking causes localized temperature spikes, potentially burning out the slip ring or triggering ring fire (arc propagation between positive and negative brushes), further damaging the brush assembly and windings.
2) Shaft current damage: Shaft currents generated by ring fires cause pitting on shaft journals and melting of Babbitt bearings.
3) Emergency shutdown: Sparking increases contact resistance, leading to uneven current distribution and forcing unit tripping.
Remediation Procedure:
After unit shutdown,Clean the area around the slip rings.
Remove the generator end cover.
Inspect and assess slip ring wear.
Install and secure the specialized grinding fixture.
Adjust the grinding depth based on inspection findings.
While the unit is in handwheel rotation mode, perform slip ring grinding.
Polish the slip ring surface post-grinding.
Clean the slip rings again after processing.
Sparking Phenomenon in Generator Slip Rings
On-site Turning of Slip Rings
Post-grinding operational performance
Surface finish of the slip ring after grinding
Surface finish of the slip ring after grinding
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