Rola bellows daxistina dîstanûyên vakûmî

Daxelîna Vacuu Interrupter û Bellows

Bi pêşketina teknolojî û çendînên li ser xafanîkirinê, vacuu circuit breakers ji bo navbera daxelîna elektrikyekî werin herî şopandin. Daxeyên din di demên din de dikarin destpêkên bêtirî yên daxelîna circuit breakers bi bêtirî ve girîng bikin, bi tenêyekî ku hêsan ên li ser dawiyên daxelîna zêdetir û demên karbûnê zêdetir. Di circuit breakers ên mezik-voltî de, vacuu interrupters (VIs) bi taybetmendîyan wan bi rastî hatine şopandin. Ji ber ku vacuu wekî medium ek daxelîna yekemîn taybetmendiyên naverok birazan di vê parçeyê de. Vacuu interrupter wekî komponenta sereke ya circuit breaker a vacuu re hatine şopandin, u bellows li ser rola jî tişt u efektî da war hatine.
Bellows îne metalî ji bo rakirina seal-a ultra-hînî vacuu û tevlîkî digerînî yên electrical contact movîn di chamber interrupter de. Lakin, cihê dan a mechanical lifespan a vacuu interrupter bi tenêyekî ku hêsan ên li ser "vacuu bellows" re hate serbest kirin. Di context a circuit breakers ên din de, daxwazên daxelîna zêdetir bi rêza zêdetir hatine, ku hêsan ên bi loadên dynamic impact-type zêdetir re hate serbest kirin. Loadên wan dikarin oscillations a bellows bi amplitudes zêdetir re trigger bikin, ku hêsan ên bi bêtirî ve mechanical lifespan a bellows bigire. Diwendî lêzêdeyî, li ser daxwazên daxelîna zêdetir di power grids ên din de, simulation a vacuu bellows bi bêtirî ve dizayn a wan bi bêtirî ve optimize bikin, u hêsan ên mechanical lifespan a vacuu interrupters bigire.

Rola Bellows di Vacuu Interrupters de

Bellows, ji stainless-steel sheets rakên, ji bo facilitekirina opening û closing a contacts û hêsan ên li ser maintenance a vacuum environment inside the interrupter re şopandin.
Fatigue resistance a bellows wekî factor a keyî hatine şopandin ku mechanical life a vacuu interrupter determine dikare. Her contact opening û closing operation bellows bi stress re subject dikare, tevahî convolutions a closest to the ends. Bi serbazî mechanical stress a operational movement, bellows post-operation oscillations heye ji bo time ke contact motion cease. Oscillations ên din hêsan ên wear and tear a bellows accelerate dikare, u degradation over time.
Figure 1 specific type a bellows for vacuu interrupters manufactured by Sigma-Netics company show dikare.

Fig 1: Vacuu Interrupter Bellows by Sigma-Netics compan

Mechanical life a vacuu interrupters bi several critical contact motion parameters serbazî influence dikare:

• Steady-state contact stroke or gap: This determines the distance the contacts separate during operation, impacting the electrical insulation and arc-extinguishing capabilities.

• Opening and closing speed: Faster speeds can enhance the switching performance but also impose greater dynamic loads on the components, including the bellows.

• Motion damping at the end of opening and closing stroke: Adequate damping is essential to minimize vibrations and reduce mechanical stress on the bellows and other parts.

• Overshoot and rebound on opening: These phenomena can cause additional wear and tear on the contacts and the bellows, potentially shortening the overall lifespan.

• Mounting resilience: The way the vacuu interrupter is mounted can affect the distribution of forces during operation, influencing the mechanical life of the bellows.

• Contact bouncing on closing: Excessive contact bouncing can lead to arcing and increased stress on the bellows, degrading its performance over time.

Bellows play a dual-role in vacuu interrupters. They enable the movement of the moving contact while maintaining a vacuum-tight seal. Constructed from stainless steel, typically with a thickness of approximately 150 µm, they are engineered to withstand the harsh operating conditions within the interrupter. Three types of bellows have been successfully integrated into vacuu interrupter designs:

• Seamless hydroformed bellows: These are formed without visible seams, potentially offering enhanced integrity and performance.

• Seam-welded hydroformed bellows: Manufactured by welding seams after hydroforming, they balance cost and performance requirements.

• Bellows made from edge-welded, thin stainless-steel washers: Constructed by welding thin washers together, they provide a cost-effective solution for certain applications.

Comprehensive details regarding bellows design and performance can be found in the EJMA Standards.

One end of the bellows is securely fixed by brazing it to the end plate of the vacuu interrupter, while the other end is brazed to the moving terminal and moves in tandem with it as the contacts open and close. In a vacuu interrupter, the bellows are subjected to impulsive motion during contact operations. The opening speed of the moving contact can rapidly increase from 0 m/s to as high as 2 m/s in less than 100 µs. At the end of the contact stroke, whether opening or closing, the moving end of the bellows comes to an abrupt stop

The frequency of these open-close operations varies depending on the duty cycle. In some cases, they can occur numerous times, while in others, they are rare. The motion imparted to the bellows is far from uniform, and it is common for the bellows to oscillate multiple times during a single opening or closing operation. For those interested in analyzing this bellows motion, a general analytical approach has been developed to determine the dynamic stresses experienced by the bellows under impulsive motion.

Most vacuu interrupter manufacturers source their bellows from well-established bellows manufacturers and collaborate with them to achieve the desired bellows lifespan. This is typically accomplished by incorporating the bellows into a practical vacuu interrupter and conducting mechanical life tests on a statistically significant number of vacuu interrupter samples. A specified mechanical life can then be assigned to the vacuu interrupter with that bellows using Weibull analysis. Usually, the mechanical life limit of a vacuu interrupter is determined by the number of operations the bellows can endure before fatigue failure occurs.

When mechanically testing a vacuu interrupter, it is crucial to subject the bellows to the same operating parameters it will encounter in a switching device. These parameters include the total travel (operating gap plus over-travel), maximum opening speed, maximum closing speed, and the effects of acceleration and deceleration. Testing the bellows within the vacuu interrupter ensures that it undergoes all the manufacturing steps that the finished device will experience. For instance, it should be exposed to all the heating and cooling cycles required for vacuu interrupter manufacturing. These processes will inevitably anneal the metal of the bellows, altering its granular microstructure and, consequently, its performance characteristics.

The mechanical life of a specific bellows depends not only on the above-mentioned operating parameters but also on its own physical attributes. These include the type of stainless steel used, its length, diameter, thickness, the number of convolutions, and its ability to dampen motion once the contact stops moving. It is feasible to design bellows that can reliably perform the normal 30,000 operations required for most vacuu circuit breakers and vacuu reclosers, and even exceed 10^6 operations for vacuu contactors. However, despite vacuu interrupter manufacturers' efforts to design their products to meet the specified mechanical life of various switching devices, most vacuu interrupters do not reach their stated mechanical life when deployed in the field.For more insights into the failure reasons of Vacuu Interrupters (VIs), please refer to the relevant article.

The vacuu interrupter designer must take precautions to prevent the user from twisting the bellows when installing the vacuu interrupter into a mechanism. A twisted bellows can have its mechanical life severely reduced, potentially to less than 1% of its designed lifespan. The torque that can be applied to the thin-walled bellows in a vacuu interrupter before permanent twisting is relatively low, approximately 8.5–11.5 Nm. To avoid bellows twisting, the designer should insert an anti-twisting bushing into it. This bushing can be locked in place by attaching it to the end plate of the interrupter. The inner surface of the bushing is shaped or features a keyway to prevent any rotation of the moving copper terminal attached to the bellows (as shown in Figure 2). The bushing material can be metal or a plastic such as Nylatron. When using plastic materials like Nylatron and Valox, caution is necessary. These materials can only be used in applications where the maximum permissible temperature they will experience is limited. For example, for Nylatron, the temperature at which its tensional strength is reduced to 50% after 100,000 hours is approximately 125°C (it can withstand higher temperatures for short periods without deforming due to its glass fiber content), and for Valox DR48, it is around 140°C. There are also more expensive, higher-temperature plastics available, such as “Ultem 2310 R.”

Fig 2: Examples of Anti-twist Bushings for Bellow Protection

The material used for these anti-twist bushings has a maximum permissible temperature of approximately 180°C. It can withstand short-term exposure (around 1 hour) to temperatures exceeding this limit without significant deformation.

For vacuu interrupters operating at higher circuit-breaker voltages, a longer contact stroke is necessary. For instance, at 72.5 kV, a stroke of roughly 40 mm is required. To accommodate this extended stroke, the bellows must be proportionally lengthened. However, very long bellows do not open and close in a uniform manner. Instead, they tend to squirm during movement. As a result, the inner convolutions of the bellows may rub against the copper (Cu) terminal. This friction can substantially reduce the bellows' lifespan.

To address this issue, specialized bellows with internal pads have been developed. These pads slide along the Cu terminals, minimizing wear and tear. An example of such a bellows design is illustrated in Figure 3.