I. Fault Phenomenon:
A. At 10:00 in Fangcheng anchorage, Guangxi, the emergency generator automatically started and was tested for power supply. The main switchboard connection switch in the centralized control room was pulled down. The emergency generator automatically started within 38 seconds and supplied power to the emergency distribution board. The emergency fire pump and No. 2 steering gear (powered by the emergency distribution board) were started, with a load of 50KW. It ran for 20 minutes. After stopping the emergency fire pump, the emergency generator overspeed alarm sounded and the emergency generator stopped. (The manufacturer of the diesel engine control box at the machine side is Jiangsu Enda)
B. At 13:30 in the afternoon, the emergency generator's automatic start-up and power supply function was tested again. The first test was with a 50KW load. By adjusting the load, the rotational speed of the emergency generator remained stable and no overspeed alarm and shutdown occurred. In the second test, the load was just put on and the machine ran for less than two minutes before the overspeed alarm triggered and it stopped. After resetting the overspeed alarm, in the third test, the machine ran with the load for a short period of time before the overspeed alarm occurred again and it stopped.
C. Every time an overspeed alarm occurs, it is found that the display unit on the control box panel shows an actual rotational speed of around 1795 rpm (rated speed 1800 rpm). The rotational speed is far from reaching the overspeed setting value (2070 rpm). When the overspeed alarm occurs, the red indicator light "OVERSPEED SHUT DOWN" on the outside of the control box lights up, the KA14 relay inside the control box is energized, the main switch of the emergency generator is disconnected, and the emergency generator automatically stops.
II. Analysis and Troubleshooting Process:
a) Firstly, based on the drawings and actual wiring, it is checked that the vessel's emergency generator has two speed measurement probes. The first probe is connected to the governor module, while the second probe is used to measure the speed safety system (SAFE SYSTEM) and the PLC control unit.
b) The second speed measurement signal is routed through the connection panel TERMINAL-2 into two separate paths. One path goes to the security system (SAFE SYSTEM), and the other path goes to the PLC speed measurement input channel. The control box panel indicates that the speed is derived from the PLC output.
c) Based on the above analysis process, the aviation plug of the speed measuring probe and the locking screw of the speed measuring probe were inspected and found to be undamaged. The connection terminals of the speed measuring probe were checked and tightened. However, the overspeed parking failure still occurred during the second and third tests in the afternoon.
d) During the second and third tests, at the moment of overspeed, the display unit on the control box was observed. The actual rotational speed was around 1795 revolutions per minute, and the speed did not reach the overspeed setting value. Additionally, due to the signal from the second speed measurement probe, it was routed through the terminal block TERMINAL-2 into two paths. One path went to the security system (SAFE SYSTEM), and the other path went to the PLC rotational speed measurement input channel. The rotational speed displayed on the control box was derived from the PLC output. Preliminary elimination of the fault of the speed measurement probe.
e) After communicating with the company's onshore supervisor, an inspection was conducted on the PLC program board inside the emergency generator control box. The output voltage of the internal power module of the PLC was measured to be normal at 5.07V. The speed security system (SAFE SYSTEM) was also inspected, and the output voltage of the internal power module was measured to be normal at 5.02V.
f) Due to the activation of the speed override relay KA14, it indicates that the output signal of the speed protection system (SAFE SYSTEM) has been generated. Considering that the actual rotational speed of the emergency generator is not high, it is concluded that the error output from the safety system (SAFE SYSTEM) has caused the speed overrun alarm and the vehicle to stop.
III. Causes of Failure.
It was determined that the abnormal speeding and braking was caused by an incorrect output from the safety system (SAFE SYSTEM) module.
Due to the lack of spare parts for the safety system (SAFE SYSTEM), the company urgently ordered spare parts for the vessel, installed and replaced the safety system spare parts, and repeatedly tested. No phenomenon of excessive speed and braking occurred again.