Electromagnetic simulation software played a key role in the design of underground switchgear that saves space and improves safety. In designing a new version of its underground distribution switchgear, S&C Electric Co. (Chicago, Illinois, U.S.) set out to design a unit two-thirds the size of its previous model.

The basic design concept is to use a pressurized gas rather than a vacuum as the arc interruption technology. Normally, a long and expensive series of tests would have been required to configure the magnetic field to provide a sound interruption. S&C engineers saved time and money by simulating their original design, visualizing the magnetic fields and making the changes needed to get the fields exactly right.

The design uses a rotating arc interrupter that consists of a moving contact, stationary contact and a magnetic coil. The interrupter uses a swinging blade assembly as the moving contacts, which make or break the electrical circuit. When the blade separates from the stationary contact during opening, an arc is drawn between the two contacts. As the blade continues to open, the arc is transferred from the stationary contacts to the magnetic coil. The current in the coil creates an axial magnetic field that forces the arc to accelerate around the rim of the coil.

To further aid in the interruption, the magnetic field is optimized to be out of phase with the current and keeps the arc moving even as the current approaches zero. The arc is extinguished at the current zero and will not reignite if the arc has been sufficiently cooled by the rotation.

It would have taken about a month to build each prototype and run the tests needed to see how well it worked. Today, software makes it possible to simulate a magnetic field and aid in the process of estimating the arc-field interactions. Simulation was attractive because it would allow the company to know whether a design was effective before it was built, and because it would allow engineers to try out different designs in their attempt to achieve effective containment while minimizing costs.

The initial design was in the ballpark, but the timing and magnitude of the fields wasn't quite right in the area where the contacts are separated. Within a few iterations, engineers created an alternate design that provided a geometry that offered a good chance of interrupting the current efficiently. The company then built a prototype, tested the interrupter and discovered it worked as expected.
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