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DISTRIBUTION FOCUS
How Electromagnetic Simulation Helps To Design Underground Switchgear
Underground distribution systems (UDS) can save valuable space in both residential and commercial areas. In residential neighbourhoods, UDS can eliminate unsightly green boxes that often spoil a view. In large commercial or manufacturing buildings where switchgear takes up space, these areas could otherwise be rented or used to increase productive capacity.
Electromagnetic simulation software played a key role in the design of this unique underground switchgear - saving room and improving safety. In designing a new version of its underground distribution switchgear, S&C Electric Company set out to design a unit only two-thirds the size of its previous model. The basic design concept uses 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 magnet 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 tank in the new unit was reduced from 33 to 24 inches high and manufacturability was improved by eliminating the need for vacuum technology. A decision was made to use sulfur hexaflouride as the environment for their arc interruption mechanism because of its unparalleled dielectric capabilities and the fact that it would eliminate the expense of building a vacuum chamber.
The UDS is designed to switch circuits while minimizing damage to equipment in the event of a fault, by automatically interrupting power.
A UNIQUE DESIGN CONCEPT
The new design uses a rotating arc interrupter, also known as an arc spinner, which consists of a moving contact, a 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 initially separates from the stationary contact during opening, an arc is drawn between the two. 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, which in turn, forces the arc to accelerate around the rim of the coil. This spinning along the rim cools the arc.
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 re-ignite if the arc has been sufficiently cooled by the rotation.
"Configuring the interrupting elements was a very challenging task," said Mike Ennis, Engineering Manager for S&C Electric. "When you strike the arc, it generates a magnetic field whose interaction with the arc is critical to the performance of the device. We have to deliver the right magnetic field to the right place to get a sound interruption. The danger if you get the magnetics wrong is that you can't extinguish the arc and current continues to flow. This could damage equipment and make it more difficult to restore power. In the past, we would have had to run an extensive series of laboratory tests to get the field right. 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 also because it would allow engineers to quickly try out different designs in their attempt to achieve effective containment while minimizing costs.
GETTING THE FIELDS RIGHT
Engineers began by entering the geometry for the initial concept design into OPERA 2D software from Vector Fields in Illinois, because its graphical user interface reduces the time needed to model complicated interrupter design, offers outstanding technical depth and breadth, and has a very robust solver that converges to a solution in even the most complex geometries.
They then entered materials properties for the components involved in the analysis and defined the currents and conductivities involved. They used transient analysis to view the formation of the magnetic field in the critical tens of milliseconds before the current is interrupted. The analysis took about 30 minutes to converge on a Dell 410 workstation with two Pentium 3 processors.
The initial design was in the ballpark but the timing and magnitude of the fields weren't quite right in the area where the contacts are separated. One major advantage of electromagnetic simulation is that the designer was able to view the direction and magnitude of electromagnetic fields throughout his area of interest. Knowing where the fields needed to be changed helped them determine what changes in geometry and materials were needed to improve the design.
Physical testing, on the other hand, would have provided measurements on the performance of the design but very little information on why it did or did not work. When the team decided on a change, they updated the model and re-ran the analysis to see what effect it had on the magnetic field.
Each design iteration provided an understanding of the sensitivity of the magnetic field to changes in geometry and materials. Within a few iterations, each of which took only a couple of hours, they had created an alternate design that, according to the simulation software, provided a geometry which offered a good chance of interrupting the current efficiently.
The company then built a prototype, tested the interrupter and discovered that it worked as well as had been hoped for.
INCREASED ADDED VALUE
"The interrupter is the heart of the new product so, once we had that all set, we could focus on the other aspects of the design with the assurance that they were less likely to have to change," said Ennis. The load interrupter switches and fault interrupters are completely sealed in a rugged stainless-steel tank and insulated with SF6 gas. The cable enters and leaves the tank via 600 ampere bushings or 200-ampere bushing wells.
The next generation Vista tank is only 24 inches tall, nearly a foot shorter than the previous generation design. And while the new gear is designed to match the footprint of the previous-generation Vista, the tank itself is not quite as deep, providing extra space for cable terminations.
Open, closed and ground positions are immediately visible directly below the large viewing windows. In addition, the simplified fault-interrupter design uses a three-position switch with arc-spinning contacts for all interrupting duties, providing an extremely straightforward operating sequence.
"This project clearly demonstrates the advantages of electromagnetic simulation in the design of distribution switchgear," said Ennis. "Engineers can determine the values of electromagnetic fields at every point in the problem domain and can quickly determine what changes need to be made in order to extinguish the arc within a few tens of milliseconds."
He added, "The ability of analysis to provide values over the entire problem domain gives engineers a better understanding of their design and often helps them to improve its performance. In this case, the result was that we were able to produce a new generation of our product that meets tough performance specifications while requiring about 30 per cent less space and, at the same time, we have been able to replace a purchased item with one that is designed and manufactured entirely by S&C." ET
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