In power systems, circuit breakers not only assume the basic responsibility of breaking fault current, but also directly affect the operational safety and service life of transformers. Especially in medium and high voltage applications, Vacuum Circuit Breaker (VCB) and SF6 Circuit Breaker (SF6 Circuit Breaker) are two mainstream technology routes. How to combine the characteristics of the two to develop a reasonable protection and maintenance strategy is the key to extend the life of the transformer.
The central role of circuit breakers in transformer protection and life expectancy
Impact of fault opening and breaking on the insulation life of transformers
The insulation life of a transformer directly determines its service life, and short-circuit, overload and other fault currents are the main factors that accelerate the aging of insulation. When an abnormality occurs in the system, the circuit breaker must reliably open the fault current within a very short time. If the opening delay or incomplete, high current will make the winding temperature rise rapidly, resulting in brittleness of insulation materials, performance degradation; Frequent arc impact will also trigger partial discharge, destroying the stability of the insulation structure, significantly shortening the life of the transformer.
With fast breaking capacity, low probability of re-ignition of high-quality circuit breaker, can effectively reduce the instantaneous overcurrent and arc impact on the transformer, reduce the thermal stress and electrical stress accumulation, significantly delay the insulation degradation, for the long-term reliable operation of the transformer to provide protection.
Key requirements for transformer protection
Transformer protection requirements for circuit breakers are strict and comprehensive, mainly including four aspects: first, overcurrent protection, timely removal of overload, to avoid long-term overheating damage; second, short-circuit protection, to achieve millisecond fault isolation, to prevent the expansion of the accident; third, selective protection, accurate positioning of the fault area, to avoid non-essential blackouts; fourth, high reliability and repeatability of the opening ability to ensure that the performance of the multiple actions after the stability. Therefore, the circuit breaker should not only have the ability to open and close, but also be fast, accurate and stable.
Comparison of Maintenance Characteristics of Vacuum Circuit Breakers and SF6 Circuit Breakers
Maintenance characteristics of vacuum circuit breaker
Vacuum circuit breaker takes vacuum as the arc extinguishing medium, which has the advantages of simple structure, no gas pollution, good environmental protection and so on. Maintenance focus mainly includes: regular testing of contact wear and contact resistance, to avoid long-term opening and closing caused by ablation affecting the conductive properties; check the spring energy storage and operating mechanism to ensure good lubrication and flexible action; monitoring the vacuum interrupter chamber sealing, to ensure that the vacuum degree is stable, to maintain the ability to extinguish the arc. Overall, vacuum circuit breaker maintenance cycle is long, low cost, especially suitable for medium voltage system frequent operation scenario.
cSF6 circuit breaker relies on sulfur hexafluoride gas to realize arc extinguishing and insulation, and has excellent performance in high-voltage and large-capacity systems. Its maintenance requirements are higher: continuous monitoring of gas density is required, as insufficient pressure will seriously reduce the insulation level; regular inspection of the sealing structure is required to prevent gas leakage; gas purity and decomposition products are analyzed to avoid impurities affecting the performance of the equipment. Although SF6 circuit breakers are widely used in 110kV and above systems, the maintenance process is complicated, and there are environmental control requirements, so operation and maintenance costs are relatively high.
Optimize circuit breaker selection and maintenance strategy to extend transformer life span
Circuit breaker selection principle
Circuit breaker selection directly affects the protection effect and transformer life, and needs to be comprehensively determined in combination with system conditions: firstly, to ensure that the breaking capacity matches with the system short-circuit capacity; secondly, to select according to the voltage level, vacuum circuit breaker is preferred for 10-35kV medium-voltage distribution system, and SF6 circuit breaker is preferred for 110kV and above high-voltage/ultra-high-voltage system; and at the same time, to pay attention to the arc-extinguishing performance, rekindling At the same time, pay attention to the arc extinguishing performance, re-ignition probability, voltage recovery characteristics, as well as site temperature and humidity, pollution level and other environmental factors to ensure equipment suitability.
Preventive maintenance strategy
Scientific maintenance can effectively reduce the risk of transformer failure. Real-time tracking of the number of openings, contact loss, current waveforms and other data through condition monitoring; the use of predictive maintenance and data analysis to identify potential hazards in advance; regular mechanical characteristics, insulation performance and other tests; optimize the protection setting to ensure that the circuit breakers and relay protection devices work together. These measures can reduce problems such as false activation, refusal to activate, delayed opening, etc., and reduce the damage to the transformer from the source.
Conclusion
Vacuum circuit breakers and SF6 circuit breakers have their own technical advantages: vacuum circuit breakers are easy to maintain, environmentally friendly, and suitable for frequent operation; SF6 circuit breakers have outstanding performance in high-voltage, large-capacity scenarios.
The differences in arc extinguishing methods and maintenance characteristics of the two directly affect the operating status and life of the transformer. For power system design and operation and maintenance personnel, according to the voltage level, operation scenarios, reasonable selection, combined with condition monitoring, predictive maintenance and other fine management, not only can enhance the reliability of circuit breakers, but also effectively reduce the transformer stress damage, is to extend the life of the equipment, improve the safety and stability of the power grid level of operation of the key.
