Unsaon Ang Tama nga Pili sa Vacuum Circuit Breakers?

01 Pahayag

Sa mga sistema sa medium-voltage, ang mga circuit breakers mao ang indispensable nga primary nga komponente. Ang mga vacuum circuit breakers adunay dominante sa domestic market. Busa, ang tama nga electrical design dili mapisado gikan sa tama nga pagpili sa vacuum circuit breakers. Sa kahon niini, atong ipangutana kung unsaon ang tama nga pagpili sa vacuum circuit breakers ug ang mga common misconceptions sa ilang pagpili.

02 Interrupting Capacity for Short-Circuit Current Need Not Be Excessively High

Ang short-circuit interrupting capacity sa usa ka circuit breaker dili kinahanglan nga sobra ka high, pero kinahanglan og usa ka margin aron matampok ang future grid capacity expansion nga makadaghan sa short-circuit currents. Pero, sa actual electrical design, ang gipili nga interrupting capacity sa mga circuit breakers kasagaran sobra ka high. 

Pwede mosabot, sa mga end-user transformer substations sa 10kV systems, ang busbar short-circuit current kasagaran nag-10kA, ug sa mas dako nga capacity nga mga sistema, mahimong makaabot sa 16kA. Pero, sa mga electrical design drawings, ang interrupting capacity sa vacuum circuit breakers kasagaran gi-specify ngadto sa 31.5kA, o padayon pa sa 40kA. Kini nga mataas nga interrupting capacity resulta sa wasted investment. Sa kahon niini, ang interrupting capacity sa 20kA o 25kA sulod ra. Karon, pero, ang mga vacuum circuit breakers nga may 31.5kA nga interrupting capacity adunay high demand ug mass-produced, naka-lead sa reduced manufacturing costs ug prices, naka-resulta sa mas widespread nga adoption.

Sa electrical design, ang calculated short-circuit currents kasagaran sa taas nga bahin. Ang rason mao ang system impedance ug contact resistance sa circuit loop kasagaran gibale sa calculation. Sigurado, ang interrupting capacity sa mga circuit breakers kinahanglan basaha batas sa maximum possible short-circuit current. Pero, ang short-circuit protection setting value dili dapat basaha batas sa maximum short-circuit current. 

Kini tungod kay ang arcs kasagaran molabay sa panahon sa short circuits, ug ang arc resistance labi ka taas. Sa design calculations, ang short circuits gitreat isip pure metallic three-phase short circuits, assuming walay arc ug walay contact resistance. Sa actual fault statistics, higit sa 80% sa mga short circuits single-phase, ug ang arcs kasagaran molabay sa panahon sa short-circuit events. Busa, ang actual short-circuit current labi ka baba kaysa ideal nga calculated value. 

Kon ang protection setting value sobra ka taas, makakurta sa protection sensitivity o makapukaw sa failure sa instantaneous protection nga mobuhat. Sa engineering practice, ang problema kasagaran dili na ang circuit breaker failed to interrupt, pero ang protection element failed to activate tungod sa sobra ka taas nga setting values. By the way, ang pure metallic three-phase short circuits dili kasagaran mogamit—molabay lang sila kon ang grounding wires wala gihatagan og remove human sa maintenance sa closing sa breaker. Pero, ang grounding kasagaran gibuhat pinaagi sa grounding switches o grounding trolleys, ug ang interlocking functions gipasabot, makapukaw sa pure metallic short circuits labi ka unlikely.

Sa mga electrical construction drawings, kasagaran makita ang interrupting capacity sa main incoming circuit breaker gi-specify usab ka level taas kon parehas sa feeder circuit breakers. Kini dili kinahanglan. Ang main breaker nag-handle sa busbar short-circuit faults, ug ang feeder breakers nag-handle sa faults sa ilang respective circuits. Pero, near sa load side sa usa ka feeder breaker, tungod sa iyang proximity sa busbar, ang short-circuit current dili labi ka dako gikan sa busbar short-circuit current. Busa, ang interrupting capacities sa main ug feeder breakers sama ra.

03 Electrical and Mechanical Life Requirements Need Not Be Excessively High

Ang electrical life nga gipangutana dinhi dili refer sa number of times ang breaker can open ug close under rated o partial load current sa specified intervals, pero ang number of times it can interrupt short-circuit current without requiring maintenance. Wala may national standard para niining number. Kasagaran, ang manufacturers design para sa 30 such interruptions. Usa ka manufacturer's products mahimo 50. Sa bidding documents para sa user projects, kasagaran makita ang sobra ka taas nga requirements sa number of short-circuit interruptions. Pwede mosabot, usa ka tender document girequire ang 12kV line protection vacuum circuit breaker to interrupt rated short-circuit current 100 times, with a mechanical life of 100,000 operations ug rated current interruption 20,000 times—kini nga requirements unreasonable.

Sobra ka taas nga number of short-circuit interruptions dili kinahanglan. Ang short-circuit fault mao ang major electrical incident. Kada occurrence kinahanglan tratuhon isip serious accident requiring root cause analysis ug corrective actions to prevent recurrence. Busa, sa effective service life sa usa ka circuit breaker, it will only interrupt short-circuit faults pipila ka beses. Ang mas taas nga system voltage, mas dako ang damage caused by short circuits, pero mas baba ang probability of occurrence. Busa, ang medium-voltage circuit breaker capable of interrupting 30 short-circuit faults sulod ra. Type testing for short-circuit interruption costly. Para sa 12kV vacuum circuit breaker, kada short-circuit interruption test kasagaran nag-cost about 10,000 RMB. Conducting excessive tests makakurta sa high costs ug unnecessary.

Does a higher number of successful interruptions mean better interrupting capability? Kini usa ka common misconception. Ang key sa vacuum circuit breaker short-circuit interruption testing nasa first ten operations. Kon ang breaker successfully interrupts the specified current in the first ten tests, its subsequent performance generally reliable. Statistical data from type tests show that the probability of failure highest during the first ten interruptions ug gradually decreases as the number of interruptions increases. After 30 interruptions, the likelihood of failure in subsequent tests nearly zero. Busa, being able to interrupt 30 times does not mean it cannot interrupt 50—it simply means further testing unnecessary.

Regarding the mechanical life of vacuum circuit breakers, wala need for excessively high requirements. M1 class originally not less than 2,000 operations, ug M2 class only 10,000. Now, manufacturers competing in mechanical life—one claims 25,000, another claims 100,000. In bidding processes, participants compare mechanical life values, which meaningless for distribution-use vacuum circuit breakers. But, in specific applications like frequent switching of motors, arc furnaces, or automatic capacitor compensation circuits, vacuum contactors more suitable (SF6 circuit breakers commonly used for switching medium-voltage capacitor banks). Contactors have mechanical and electrical lives exceeding one million operations (their electrical life measured by rated current interruption, not short-circuit current). No need to compete on mechanical life in circuit breakers.

04 Excessive Requirements for Other Electrical Parameters

The short-time withstand current of a circuit breaker refers to its ability to withstand the thermal stress of short-circuit current during a fault. This is not the same as temperature rise. Temperature rise testing involves passing rated or specified current through the breaker for a long time and ensuring that temperature rise at various points does not exceed specified limits. The short-time withstand current of a circuit breaker is generally tested for 3 seconds.

Within this time, the heat generated by the short-circuit current must not damage the breaker. A 3-second thermal withstand capability is sufficient. The reason is that after a short circuit occurs, time-graded protection may involve intentional delay to ensure selectivity. For time-based protection, a 0.5-second delay between adjacent breakers ensures selectivity. If breakers differ by two levels, the trip delay is 1 second; if three levels, 1.5 seconds. A 3-second withstand capability is already sufficient. However, some users or designers insist on a 5-second thermal withstand capability, which is truly unnecessary.

During the closing process of a circuit breaker, the moving and fixed contacts may bounce. If the bounce time is too long or the three-phase closing asynchronism is large, breakdown and restrike may occur between the contacts. Restrike causes a charge-discharge process in the circuit, increasing the steepness and amplitude of overvoltage. This overvoltage is known as contact restrike overvoltage.

Its hazard may even exceed the current chopping overvoltage of vacuum circuit breakers, threatening the turn-to-turn insulation of transformers and motors. Therefore, the contact bounce time and three-phase asynchronism should not exceed 2ms. Current circuit breaker parameters are manufactured to meet this requirement. However, some users demand values less than 2ms, even requiring no more than 1ms, which exceeds current technical capabilities.

05 Negative Issues Caused by Excessively High Starting Current of Vacuum Interrupters

The starting rated current for medium-voltage vacuum interrupters is 630A. Currently, some manufacturers no longer produce 630A versions, and the minimum starting current has increased to 1250A. This is related to vacuum interrupter manufacturing. However, it brings a series of negative consequences. Because the starting current of vacuum interrupters is too high, vacuum circuit breakers assembled with these interrupters must match the interrupter’s current rating. 

As a result, all associated components—such as pole columns, plug-in contacts on pole columns, and fixed contacts in switchgear—must also match the interrupter’s current rating. This leads to severe waste of non-ferrous metal materials in most cases. For example, a 12kV vacuum circuit breaker may supply only a 1000kVA transformer, whose 10kV side rated current is merely 57.7A. However, since the vacuum interrupter starts at 1250A, the circuit breaker must be rated at 1250A. Consequently, all accessories of the breaker must have a rated current of at least 1250A, and the fixed contacts in the switchgear must also be rated at no less than 1250A, resulting in significant waste of non-ferrous metals.

Worse, users or designers insist that the current-carrying capacity of the main conductors in the switchgear must match that of the circuit breaker—i.e., the conductor’s current-carrying capacity is designed for 1250A. In reality, a capacity of 60A is sufficient, and as long as the minimum cross-section of the circuit conductor passes dynamic and thermal stability checks, there is considerable room for saving materials.