Persistent Sparking in Red Phase Electrode Chamber of LRS During Startup of a Newly Installed 3.4 MW, 6.3 KV Wound Rotor Induction Motor Driving a Vertical Mill

Hello,

I am seeking expert advice on a concerning issue with persistent sparking (just started) within the red phase electrode chamber of the Liquid Resistance Starter (LRS) for a new 3.4 MW, 6.3 kV wound rotor induction motor. This motor, which drives a vertical mill, has been in operation for only 6 months and is always started unloaded.

Issue Description: As soon as the motor receives the startup command (MV breaker closes) and the electrodes begin to move downward, sparking occurs, significantly illuminating the compartment up to 70% and producing a sound reminiscent of 5 to 6 rounds of pistol fire in burst mode. This sparking lasts for about 3 to 4 seconds before ceasing. Each electrode is housed in a separate compartment, yet the electrolyte is shared, maintaining a level higher than the compartment height within the tank.

Diagnostics Conducted: Motor Side:

• Insulation Resistance: Both stator and rotor have shown excellent insulation resistance.
• Rotor Inspection: No signs of internal sparking or damage.
• Slip Ring Check: No sparking observed during startup or in running state.
• Cable Inspection: Insulation and continuity tests confirm all cables from the slip ring to the LRS are in good condition.

LRS Side:

• Tank Inspection: The tank was emptied and inspected with no visible abnormalities.
• Functional Testing: All LRS functionalities tested satisfactory.
• Connection Integrity: All power and control terminations are secure.
• Short Circuit Contactor: Continuity and insulation resistance with the ground during open and close conditions are satisfactory.
• Electrolyte Replacement: The electrolyte has been replaced with a fresh, dust-free batch that meets specified resistance values.

It's important to note that no major or relevant maintenance activities have been performed recently that could relate to this sparking issue. This adds to the challenge of diagnosing the problem, as there have been no changes in operational or maintenance routines that might otherwise explain the onset of the issue.

Questions:

• Could this sparking be related to the electrode movement configuration, electrical imbalances in the red phase, misalignment of the electrodes, or issues with the properties of the electrolyte?
• Are there specific diagnostics or lesser-known tests that could pinpoint the root cause of this sparking?
• Could environmental factors or other unconsidered elements contribute to this issue?

Any insights or suggestions for further diagnostics or corrective actions that could safely and efficiently resolve this issue would be greatly appreciated.

Thank you!