Ruhunin bellows a cikin vacuum interrupters

Bayanin Vacuum Interrupters da Bellows

Wannan lokaci, wajen ilimi da kuma zafiya na yaki na duniya ya haifar da vacuum circuit breakers suka zama muhimmanci a cikin fanni na electrical engineering.
A tarihin power grids na gaba zai iya tabbatar da abubuwa masu kyau ga switching performance na circuit breakers, musamman ga gidaje masu kyau da wasu inganci. A cikin medium - voltage circuit breakers, vacuum interrupters (VIs) sun samun shahara sosai. Wannan shine saboda amfani da vacuum a matsayin interrupting medium ya ba da muhimmanci da ba a nan. Vacuum interrupter ya zama mafi girma a cikin vacuum circuit breaker, kuma bellows suna da muhimmanci da kayan aiki a cikin vacuum interrupters.

Bellows mai sarrafa suna da takarda don in ba da ultra - high vacuum seal, amma haka suna iya sauyar da movement na moving electrical contact a cikin interrupter chamber. Amma, mechanical lifespan na vacuum interrupter yana da tsarin kasa mai sarrafa ta haka bellows. A cikin circuit breakers na gaba, wannan bayanin gidaje masu kyau zai iya haifar da dynamic impact - type loads masu kyau. Wanannan loads zai iya haifar da bellows oscillations da keke masu kyau, kuma haka za su rage mechanical lifespan na bellows. Kuma saboda hakan, wajen simulation na vacuum bellows ya zama babban abu don in yi amfani da shi wajen yin tashizawa da design, kuma haka zai iya rage mechanical lifespan na vacuum interrupters.

Rolin Bellows a Cikin Vacuum Interrupters

Bellows, masu ingantaccen stainless - steel sheets, suna da takarda don in ba da opening da closing na contacts, amma haka suna iya ci gaba da maintenance na vacuum environment a cikin interrupter.
Fatigue resistance na bellows yana da muhimmanci wajen ya shiga mechanical life na vacuum interrupter. Duk opening da closing operation yana rage stress na bellows, musamman convolutions da ke daidai zuwa ends. Ba haka ne, stress na operational movement, bellows suna da post - operation oscillations idan contact motion ceases. Wanannan oscillations sun rage wear and tear na bellows, kuma haka za su rage degradation na bellows a lokacin da rike.
Figure 1 ta bayyana bellows na vacuum interrupters mai sarrafa da Sigma - Netics company.

Fig 1: Vacuum Interrupter Bellows by Sigma - Netics compan

Mechanical life na vacuum interrupters yana da muhimmanci saboda abubuwan critical contact motion parameters:

• Steady - state contact stroke or gap: Wannan yana nuna distance na contacts separate during operation, kuma yana rage electrical insulation da arc - extinguishing capabilities.

• Opening and closing speed: Gidaje masu kyau zai iya rage switching performance, amma haka zai iya rage dynamic loads a cikin components, including the bellows.

• Motion damping at the end of opening and closing stroke: Adequate damping yana da muhimmanci don in rage vibrations da kuma rage mechanical stress a cikin bellows da wasu parts.

• Overshoot and rebound on opening: Wanannan phenomena zai iya rage additional wear and tear a cikin contacts da bellows, kuma zai iya rage overall lifespan.

• Mounting resilience: Yadda ake mount vacuum interrupter zai iya rage distribution of forces during operation, kuma yana rage mechanical life na bellows.

• Contact bouncing on closing: Excessive contact bouncing zai iya haifar da arcing da kuma rage stress a cikin bellows, kuma zai iya rage performance na bellows a lokacin da rike.

Bellows suna da dual - role a cikin vacuum interrupters. Suna iya sauyar da movement na moving contact, amma haka suna iya ci gaba da vacuum - tight seal. Mai sarrafa ne stainless steel, typically with a thickness of approximately 150 µm, suna da takarda don in ba da harsh operating conditions a cikin interrupter. Three types of bellows suna da amfani a cikin vacuum interrupter designs:

• Seamless hydroformed bellows: Wadannan suna da takarda don in ba da enhanced integrity da performance.

• Seam - welded hydroformed bellows: Mai sarrafa ne welding seams after hydroforming, suna da takarda don in ba da balance cost and performance requirements.

• Bellows made from edge - welded, thin stainless - steel washers: Mai sarrafa ne welding thin washers together, suna da takarda don in ba da cost - effective solution for certain applications.

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

Yawan bellows an fixe ita da brazing ita zuwa end plate na vacuum interrupter, amma yawan muka an brazing ita zuwa moving terminal da yake sauya da ita a lokacin da contacts open and close. A cikin vacuum interrupter, bellows suna da impulsive motion during contact operations. Opening speed na moving contact zai iya rage kadan da 0 m/s to as high as 2 m/s in less than 100 µs. A karshe na contact stroke, whether opening or closing, moving end na bellows zai iya stop abruptly.

Frequency na open - close operations yana rage depending on the duty cycle. A wasu abubuwa, suna da wanda suke faru many times, amma a wasu, suke da wata. Motion imparted na bellows ba uniform bane, kuma bellows suna da takarda don in ba da 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 vacuum 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 vacuum interrupter and conducting mechanical life tests on a statistically significant number of vacuum interrupter samples. A specified mechanical life can then be assigned to the vacuum interrupter with that bellows using Weibull analysis. Usually, the mechanical life limit of a vacuum interrupter is determined by the number of operations the bellows can endure before fatigue failure occurs.

When mechanically testing a vacuum 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 vacuum 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 vacuum 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 vacuum circuit breakers and vacuum reclosers, and even exceed 10^6 operations for vacuum contactors. However, despite vacuum interrupter manufacturers' efforts to design their products to meet the specified mechanical life of various switching devices, most vacuum interrupters do not reach their stated mechanical life when deployed in the field.For more insights into the failure reasons of Vacuum Interrupters (VIs), please refer to the relevant article.

The vacuum interrupter designer must take precautions to prevent the user from twisting the bellows when installing the vacuum 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 vacuum 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 vacuum 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.