SL400 Vacuum Contactor Failure to Trip? In-Depth Analysis of Causes and Solutions

In the high-voltage auxiliary power system of power generation enterprises, high-voltage vacuum contactors are used as control electrical appliances for high-voltage motors, transformers, frequency converters, and other electrical equipment. They enable remote control and frequent operation, thus gaining widespread application. If faults of vacuum contactors are not handled promptly, it will directly affect the safe and economical operation of the generating units in power generation enterprises.

Among the vacuum contactors in the high-voltage auxiliary power system of Units 3 and 4 of a thermal power plant, 60 are SL400-type 400A vacuum contactors. From their commissioning in 2015 to the end of 2016, multiple vacuum contactors in the coal handling system experienced failures such as tripping mechanism refusal to trip, tripping coil burnout, and activation of the "control circuit disconnection" alarm signal, resulting in the equipment being unable to shut down. Since one end of the tripping coil is directly connected to the negative electrode, it may also cause direct grounding of the DC negative electrode, leading to protection device failure to operate and posing serious hidden dangers to safe operation. Meanwhile, the need for on-site manual tripping when the vacuum contactor refuses to trip also brings significant safety risks to the operation staff.

1. Working Principle of the Operating Mechanism

The operating mechanism of the SL-400 type vacuum contactor selected by the thermal power plant is a mechanical holding-type mechanism. When the closing coil of the vacuum contactor is energized, the closing moving iron core drives the main shaft mechanism to move under the action of electromagnetic force. The roller on the closing moving iron core comes into contact with the tripping detent, locking the executive component to keep the contactor in the closed state. At the same time, the spring is compressed to store tripping energy, and the tripping detent connecting piece and the tripping electromagnet bending plate are lifted to prepare for tripping.

When the tripping coil receives a pulse power supply, the tripping moving iron core attracts the bending plate to move downward. The bending plate impacts the tripping detent connecting piece, releasing the dead-center position maintained by the closing moving iron core roller and the tripping detent. Under the action of the spring, rapid tripping occurs. The closing moving iron core, driven by the tripping spring, rotates with the main shaft to the position of the limit plate and stops, completing the tripping process.

2. Cause Analysis

2.1 Electrical Aspect

Inspection of the tripping circuit showed that the contact resistance of the secondary plug, auxiliary contacts of the position vacuum contactor, and operation handle contacts was normal. The DC output voltage was approximately 110V, and there was no situation of excessively low voltage on the tripping coil. No phenomena such as poor insulation grounding in the control circuit or loose/worn wires were found.

The tripping control circuit disconnection is an alarm signal triggered by the tripping of the control power vacuum contactor due to long-term energization and burnout of the tripping coil. Therefore, when the SL contactor experiences tripping refusal, electrical causes can basically be ruled out.

2.2 Mechanical Aspect

Inadequate material design of the tripping detent connecting piece: The original materials of the tripping detent, tripping electromagnet bending plate, and connecting piece were carbon steel, which has high magnetism. After multiple energization and tripping operations, the bending plate and connecting piece were gradually magnetized by the magnetic field generated by the coil during the tripping process, resulting in a certain mutual magnetic force and increasing the mechanical resistance of tripping. If tripping failure occurs and frequent operations are performed, the tripping coil will be burned out.

Residual magnetism in the tripping coil after energization: This leads to a decrease in the magnetic flux of the tripping coil, resulting in insufficient tripping torque and unreliable tripping. Frequent tripping operations cause the tripping coil to be energized for a long time, generating heat and eventually burning out.

Mechanical jamming between the tripping detent and the positioning roller: The rotating parts lack lubricating grease. Burrs in the movable parts of the bending plate positioning hole and positioning rod, or deviation of the positioning hole due to wear, cause jamming. After multiple operations of the tripping electromagnet, the tripping friction resistance gradually increases, leading to overload and burnout of the tripping coil.

Frequent startup and shutdown of equipment: Coal handling belt conveyors and coal crushers are equipment that start and shut down frequently. When the tripping refusal fault occurs, these devices have already operated more than 500 times. The tripping coil is often energized and generates heat, which accelerates the insulation aging of the coil to a certain extent.

3. Handling Methods

Material replacement for key components: Replace the material of the tripping detent connecting piece from carbon steel to non-magnetic stainless steel, and replace the fixed screws from galvanized carbon steel to copper screws. This prevents the connecting piece from being magnetized, significantly reduces the mechanical resistance of tripping, and thereby reduces the consumption of tripping energy.

Demagnetization of core components: Demagnetize the tripping electromagnet base plate and bending plate using the tapping method before installation. This further reduces the attractive resistance between these components and the tripping detent connecting piece, increases the tripping force margin, and ensures reliable closing and tripping of the contactor.

Localization transformation of the original coil: Replace the original coil with one with a resistance of approximately 20Ω, increase the number of coil turns to enhance magnetic flux, and maintain the electromagnetic force of the coil operation above a certain value. At the same time, the increased resistance of the tripping circuit reduces the circuit current, lowers the heat generation of the coil during energization, slows down the coil aging rate, and effectively reduces the tripping refusal phenomenon caused by the decrease in tripping coil voltage due to increased contact resistance from burnout and oxidation of auxiliary contacts.

Lubrication and maintenance of mechanical parts: Apply lubricating grease to the tripping detent and positioning roller of the vacuum contactor, as well as the rotating parts of the tripping detent. Polish and trim the burrs and worn parts in the movable parts of the bending plate positioning hole, and perform lubrication and maintenance on the rotating parts of the tripping detent connecting piece. After the minimum tripping action voltage test, the action value is basically controlled between 45V and 55V, keeping the tripping mechanism in good condition and greatly improving the safety and reliability of tripping.

4. Preventive Measures

• Regular maintenance and testing: Conduct minor maintenance once a year and major maintenance once every five years after normal operation, and properly perform mechanism maintenance and preventive tests.

• Strict equipment selection and acceptance: Ensure proper selection of vacuum contactor equipment, and strictly control the quality of commissioning, handover, and acceptance.

• Real-time operation monitoring: Strengthen monitoring during operation to promptly identify and handle problems.

• Optimize maintenance procedures: Further grasp the actual conditions of the equipment, and revise and improve the maintenance process procedures based on fault handling methods and experience.

• Strengthen inspection and management of frequently operated equipment: Enhance the inspection and management intensity of vacuum contactors in frequently operated equipment.

• Focus on mechanical part inspection: Pay attention to inspecting the vacuum contactor's mechanical parts, including checking whether the operating mechanism is well-lubricated, operates flexibly, and has no jamming. Special attention should be paid to checking for jamming between the tripping electromagnet bending plate and the tripping detent connecting piece.

• Utilize unit shutdown periods for maintenance: Make full use of the unit shutdown and standby periods to perform maintenance on the vacuum contactor mechanism and conduct preventive tests such as the closing and tripping coil action voltage test. This helps grasp the deterioration trend and promptly adjust and handle potential issues.

5. Conclusion

The vacuum contactors after handling have been put into operation for nearly one year without any faults such as tripping refusal or coil burnout. The power plant once again inspected the vacuum contactors in the coal handling system that had newly accumulated 500 to 1,000 operations and conducted the minimum tripping action voltage test. The results showed that the DC resistance and insulation of the tripping coils were in good condition, the action voltage value did not increase significantly, and the on-site/remote electric tripping tests were accurate and reliable. This has greatly improved the health level and reliability of the equipment, while reducing maintenance workload and saving maintenance costs.