1, Fault phenomenon
For several years, it has been inferred that there have been intermittent start-up faults on the bow side of the ship, mainly manifested as a longer duration of current during the second stage start-up, the disconnection of the side push main contactor (air circuit breaker) Q73 with a rated current of 1000A, and the motor stopping.
Observing the process of side pushing start, when the ship starts normally shortly after leaving the dock, the maximum current in the three-phase digital ammeter of the generator main switch during the first stage start process instantly rises to 1600 to 1800A (the side pushing ammeter is about 1600A), then slowly decreases, and after a few seconds, it drops to around 1000A, and then quickly drops to around 270A, holding for two to three seconds. The second stage start begins, and the current instantly rises to 1000 to 1100A, and then quickly drops to around 270A, indicating successful side pushing start.
Looking at the reactive power compensation device again, when the power factor decreases, the reactive power compensation device first inputs 2 compensation capacitors according to the design program, and then increases 2 compensation capacitors every 6 seconds until the designed power factor is reached. If the power factor is too high, the compensation capacitors are gradually withdrawn.
The process of failure of side push start is not significantly different between first level start and normal start, except that during second level start, the side push ammeter briefly rises to 1000A and then remains basically unchanged, while the digital ammeter of the generator main switch briefly rises to 800-900A and slowly decreases, then drops to 300A or even 160A after seven or eight seconds, and then the current suddenly drops to zero, and the side push start fails. At this time, Q73 has been disconnected.
During the entire start-up process, the generator voltage and frequency were normal, and there were no alarms in the system. The restart operation can be carried out immediately, but the chance of successful restart is only one third or four.
Upon inspection of the main contact of the Q73 side push contactor, it was found that there is often arcing and erosion. After grinding the main contact, the side push can be successfully started again, but after several normal starts, it may fail to start again.
After several months of dock inspection, the occurrence of startup faults became more frequent, and the observed startup process showed similar behavior, except for an increase in current.
2, Fault analysis
From the above perspective, even in the event of a startup failure, if the generator, reactive power compensation device, and autotransformer work normally and the first level startup is basically similar, then the problem lies in the second level startup.
The contactor Q73, which has failed to start once or several times, will disconnect under high current conditions, and its main contact will inevitably experience arcing and erosion. The contact area is too small and the contact area of the three phases is not consistent, causing significant contact resistance and serious uneven phase current, sometimes even approaching a phase loss operation state. At this time, although the generator voltage regulation function and reactive power compensation device are normal, they are not enough to meet the requirements of motor acceleration. In addition, the design of the generator and reactive power compensation device is far away from the side push motor, and the motor continues to have high current while the generator current becomes smaller and smaller. Although the mechanism is complex and difficult to investigate, it can also be understood as a pathological manifestation.
After grinding the main contact, the contact area of the three-phase contact increases and approaches, and it is natural to start the lateral push again successfully.
However, how did the originally intact main contactor contacts first experience arc erosion? Similarly, last year we also purchased a new Q73 main contact, which broke its perfect golden body in just a few days after a startup failure.
That is to say, the first arcing erosion of the main contactor contact is not a problem with the contact itself, but rather caused by the disconnection of the main contactor Q73 during high current. Only by understanding the reason for the disconnection of the main contactor Q73 during high current can the problem be thoroughly solved by targeted measures.
The side push control system of this wheel is equipped with a PLC. Due to professional expertise and the ability to restore the normal operation of the side push by grinding the main contact, many people have not been able to thoroughly investigate the cause of the startup failure for a long time.
After careful study of the circuit and in-depth inspection and analysis, we have ruled out the possibility of Q73 protection disconnection caused by overheating of the side push motor, autotransformer, Q73 overload protection, etc., as there was no alarm and it could be restarted immediately without resetting. At the same time, we found that although there was a side push current signal transmitted to the PLC, there was no switch or contact in the motor control circuit for PLC output disconnection, so we can also rule out the possibility of PLC command disconnection.
Considering that each malfunction only occurs during the startup process, the possibility of poor contact between the control circuit switches and contacts has been ruled out.
The remaining possibility is that the auxiliary contactor K81 will disconnect by itself or Q73 will trip by itself. The power supply of the wheel motor control system is the side push 380V power supply, which outputs 220V through transformer T73. Due to the fact that there is only one ammeter on the side push starting control box, we detected a significant drop in voltage during the starting process using a multimeter.
After testing with a voltage regulator, it was found that the undervoltage coil of the main contactor Q73 will trip when the voltage drops to 185V, while the trip voltage of K81 is much lower.
During the process of starting the side push, the continuous high current causes a decrease in the current at the generator end, resulting in a drop in the input voltage of the side push control box power supply. As a result, the output voltage of transformer T73 is much lower than 220V, which can easily cause Q73 to trip on its own.
Now we can conclude that:
The disconnection of the main contactor Q73 when the side push start fails is due to undervoltage!
3, Troubleshooting
After the ship dock inspection is put into operation, marine organisms and other pollutants begin to grow or accumulate at the side thrust hub, blade, channel, and fence, and the water flow resistance begins to increase. The starting current and operating current of the side thrust main motor will also increase accordingly.
When the starting current increases to a certain extent, it will cause the main contactor Q73 to trip automatically.
The improvement corresponding to the automatic tripping of the main contactor Q73 due to undervoltage during startup is relatively simple. The 220V power supply on board is used instead of the motor control system power supply obtained from the side push main power supply, ensuring the stability of the motor control system power supply and not being affected by the starting current.
经过如此改进后,实践证明即使海生物等污染物增长,目前来说一级启动最大电流增加到 2600A,二级启动电流最大增加到 1500A的情况下还能稳定成功启动,启动完成后零位电流增大到 400A.
另外,为保证侧推的正常,适时清洁侧推及其通道是十分必要的.
After such improvements, it has been proven in practice that even with the increase of pollutants such as marine organisms, the maximum current for the first stage startup can still be stably and successfully started when the maximum current for the second stage startup is increased to 1500A. After the startup is completed, the zero position current increases to 400A.
In addition, it is necessary to clean the side push and its channels in a timely manner to ensure the normal operation of the side push
4, Conclusion
For electrical equipment with overcurrent tripping, the contacts of the contactor/air switch should be checked as early as possible to ensure good technical condition.
When the equipment malfunctions, what we need to do is not just reactive repairs or temporary solutions, but also to dig through the phenomenon and grasp the essence, and find a complete solution to the problem.
Only through continuous learning and research can we accumulate experience, learn lessons, and continuously improve our work skills and management level.
