Generator electric failure due to grounding of the Alternator Rotor windings as a result of vibration on a more than 20 years old Japanese built Reefer vessel – the case include installation of a Container power pack on deck to restore the full el power required for the cargo
On a 22 years old Reefer vessel the CE reported uneven distribution of the currents during parallel operation of the three generators. It was observed as well a significant drop of the voltage (app. 20 to 40 V) of Generator #3 when a powerful consumer was started – for example Main air compressor. A new equivalent type AVR was ordered and installed. The distribution of the currents improved slightly, but the voltage drop remained.
After two month period the Alternator during single operation tripped and caused black out. It was restarted and noted that the voltage reaches only 250V and cannot be increased to reach 440V.
After disconnecting the diodes of the rotating rectifier (exciter) the resistance of the Alternator was measured and found that the Rotor was grounded and needed to be rewound.
The vessel was about to load cargo and all three generator were required to cool the cargo. It was decided to install a deck power generator in a container in order to substitute the missing el. Power. The plan and the installation drawing were submitted to Class BV and after approval the Power pack with suitable power and parameters was installed on deck. The additional generator was connected to the Main switchboard instead of the failed generator and synchronization was executed from Main Switch board with the provided Remote control of the power pack –controlling the frequency, voltage and emergency stop. In this synchronization solution the el. Safeties of the ship remain and the additional Powerpack will be disconnected automatically by the Main Switchboard in case necessary. The electric cables of the power pack were chosen big enough to carry the el current. They were run through sealed/cemented opening from the accommodation till the backside of the ECR Main Switch Board – around 35 m. The generator was provided with its own fuel diesel tank and was supposed to be refilled periodically by the crew.
The failed Alternator was disconnected and dismantled by the Crew. It was taken out of the ER by means of a shore crane. The Alternator was landed for rewinding.
After disassembling the Alternator it was found that the free end bearing journal was damaged and need to be reconditioned (rebuilt). A class approved procedure was chosen for the reconditioning of the rear journal bearing and the bearings were ordered new for replacement. The class approved repair procedure of the repair company was: Take material sample, compile welding procedure, pre-machine journal to 1.2mm in diameter, weld out and carry out post welding heat treatment, Weld journal with two runs of automated high technology welding.
The rewinding was executed by a Class certified local company. The rotor was requested to be varnished and baked 2 times. All diodes on the diode assembly were replaced with new. The rotor was dynamically balanced with printed report. Once the windings were done the following tests were performed with Baker Surge Tester D12R.: MEGGER TEST, HI POT TEST, RESISTANCE TEST, SURGE TEST & POLARITY TEST.
Alternator was assembled and placed in test bed & aligned with drive motor. Alternator was run up to full speed and bearing temperatures checked, rotor float checked and recorded for 30 min. The excitation circuit was connected and voltage output checked across three phases. Small load was applied where the volts and amps to be checked and recorded. A complete test report of the repairs was given on completion. All the above tests were witnessed in the presence of approved surveyor and Company representative.
The generator was transported as airfreight to the next convenient port and was shifted inside ER with a shore crane. It was positioned on place by the crew and aligned by a specialized company service provider: alternator was connected to flywheel, foundation bolts, shims and fixing pins mounted, crankshaft deflection was done and alignment as per cylinder 6 crankweb, electrical connections were restored, and cable connections shifted back from Deckset to DG#3 during blackout mode. Test run without load was performed, then parallel test till 200 kW and single run 300 kW – everything found OK.
After Investigations it was concluded that the reason of the alternator was vibration. This was proven by the pitting on the backside of the bearing casing of the free end, as well as by Vibration analysis report done by specialized company. This was the reason also for the damaged free end bearing journal.