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Table of Contents

Case Study

Resolving Overvoltage Issue in CAT 3512C Emergency Generator on Jack up drilling rig

Table of Contents

Background

Industry
Oil & Gas
Solution
Generator
Troubleshooting
Make & Model
CAT 3512C
Engine
Asset
Jack-up
Drilling Rig

Objective

To diagnose and resolve an overvoltage issue in the Emergency Generator that prevented the Circuit Breaker from closing during a blackout and to ensure operational readiness of Main Generator No.1 with a successful load trial.

Challenge / Problem Context

  • Emergency Generator started but its Circuit Breaker failed to close.
  • Customer observed an overvoltage issue during blackout conditions.
  • Emergency Generator could not be started for testing due to ongoing rig operations.
  • Loose PT (Potential Transformer) wire connection found in AVR input circuit.
  • AVR had been programmed using Main Generator No.1 settings, which caused compatibility issues.
  • Intermittent common alarms reported in Main Generator No.5.
  • Winding and insulation resistance needed to be verified to rule out internal faults.

What Did We Do?

  • Conducted a detailed safety and technical briefing onboard with the Chief Electrical Engineer and ET team.
  • Performed insulation resistance and winding resistance tests on the Main Stator, Main Rotor, Exciter Rotor and Stator, PMG Stator, and Diode Assembly—all results were satisfactory.
  • Identified a loose PT wire connection in the AVR input circuit and corrected it.
  • Bypassed the AVR and used external 12V DC excitation to test the alternator voltage at idle and rated RPM—results were stable and confirmed alternator health.
  • Reconnected the AVR and tested voltage output at both idle and rated RPM, confirming normal voltage of 600V AC.
  • Reviewed schematic drawings of the Circuit Breaker control circuit of Main Generator No.1 and conducted a successful load trial.
  • Investigated and resolved intermittent alarms in Main Generator No.5 by inspecting and reinstalling the harness for the lubricating oil and fuel pressure sensors.
  • Verified and documented the results of all diagnostic tests, including winding and insulation resistance measurements, and completed the report for customer validation.

Results That We Achieved

  • Resolved overvoltage issue by identifying and correcting a loose PT wire in the AVR circuit.
  • Validated alternator functionality with successful testing at idle and rated RPM, confirming 600V AC at full speed.
  • Confirmed AVR stability after reinstallation and eliminated risk of future overvoltage during operation.
  • Ensured operational readiness of Main Generator No.1 by successfully conducting a load trial with satisfactory performance.

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