Inspection and prevention of AC intrusion into DC systems
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This post was last edited by qb655 on 2023-6-6 at 18:30. Our old plant’s system uses fixed-bed reactors, with 4 HN compressors as well as other high and low pressure electrical equipment. The 4 HN machines are 5000KW synchronous machines with a voltage of 6KV. At noon on June 5, Compressor #1HN tripped, indicated as a non-electrical fault, but the cause was not identified from a process perspective; it restarted at 17:01 in the afternoon. At 17:53, the 3 operating synchronous generators tripped simultaneously, with an excitation fault indicated, but all three excitation cabinets were functioning normally. After isolating the non-electrical quantities and excitation trip circuits, Unit 1 tripped at 23:30 at night, and Unit 3 tripped at 23:36; there was no trip information, just as in normal tripping scenarios. Later, isolation was also added to the trip circuit, and it is now operating normally. We suspect that alternating current has infiltrated the DC system, because during the trip, the operators noticed that the red indicator lights indicating operation and the green indicator lights indicating shutdown on the high-voltage switchgear were flashing. After the trip, the light also indicated normal operation. Please discuss whether you have encountered similar faults and how you handled them
To address this situation, we can take the following measures to detect and prevent AC electricity from entering the DC system: 1. Check the grounding of the DC system to ensure it is properly grounded and that the ground wire resistance meets the required standards. 2. Check whether the isolating switches and grounding switches of the DC system are operating properly, and carry out regular maintenance and inspections. 3. Check the filters of the DC system to ensure their effectiveness and prevent high-frequency noise from interfering with the DC system. 4. Add monitoring and protection devices for the DC system, such as those for DC voltage, current, power, temperature, as well as protection against overvoltage, overcurrent, and overload, to detect abnormalities in a timely manner and take action accordingly. 5. Inspect and verify the connection between the DC system and the AC system to ensure it is proper and secure, with no looseness or poor contact. 6. Conduct regular inspections and maintenance, promptly remove dust and other debris to keep the equipment clean and dry, and prevent the effects of moisture and corrosion. 7. Strengthen the training and management of operators to enhance their awareness of equipment safety and operating conditions as well as their sense of responsibility, thereby preventing hazards caused by misoperations and violations of regulations. .
It seems there might be a problem with the DC system, but it doesn’t quite seem likely that all 3 high-voltage motors in operation would trigger their trip circuits at the same time; it’s hard to say for sure. Check the insulation monitoring and event records of the DC system to see if there are any instances of momentary grounding, and whether they correspond to the tripping time of the synchronous machine.
An inspection of the DC panel was carried out, and no signs of grounding or poor insulation were found. The control circuits of the three synchronous generators were also inspected; the control cables on site were removed, and the insulation was found to be in good condition. Ultimately, it was suspected that the control cable was interfered with.
The temporary measure we’re taking now is to isolate the control cables using relays. It is running normally at the moment.
Yesterday, it was found that the indicator lights on several high-voltage motors were flickering again, indicating that the source of interference still exists; it’s just not clear how to identify and resolve it.