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Nickname: Bill Schweber     Articles(90)    Visits(107825)    Comments(4)    Votes(21)    RSS
Bill Schweber writes about analogue design trends and their impact on the industry, and gives us his expert insight on the role that analogue plays in our largely digital world.
Bill is the editor-in-chief of Planet Analog and a registered professional engineer.
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Posted: 06:04:28 PM, 29/04/2016

Megavolt tests expose extreme engineering challenges


With all the attention given to low-power design and products, we tend to forget that there's another world out there with power levels that are tens of orders of magnitude greater—and working with them is a radically different world in every respect. A recent article in IEEE Spectrum, "Inside the Lab That Pushes Supergrid Circuit Breakers to the Limit," was a dramatic reminder of the engineering challenges of anything having to do with the electric grid's high-tension power lines in the megavolt/kiloamp regime. Next time you hear engineers moan that their design has to “sip electrons,” perhaps you should suggest that they read this article and they’ll hopefully soon stop.

Forget everything you think you know about "electricity" when you are at these levels. The article focuses on testing of circuit breakers for these power lines, and it's truly another world. There's no need for me to reiterate the article; you can read it yourself. The author’s detailed description of what a circuit breaker must do and deal with at these levels is astonishing: we're in the world of plasmas, gas quenching, switching of kA in milliseconds, mechanical contact issues that you won't be able to anticipate, and more. Every aspect of the test setup had to be custom built, as there are no off-the-shelf fixtures for this kind of work. The test facility even had to build a special generating and storage system to supply the power for the tests. Every decision and action requires careful, deliberate thinking and risk assessment.

I'm fascinated by engineers who are not only at the extremes of design along one or more performance or operational parameters, but must also devise and build ways to test their designs. Sometimes, as in the case of the power-grid circuit breakers, there is a set of operational features in their favor: the tests are reasonably close to final conditions, they can be repeated as needed under carefully controlled conditions, and changes can be made and then the devices are retested.

However, not all tests involving systems at high power levels (whether electrical, chemical, or mechanical) have this characteristic. Often, the test process is so complex and difficult to set up or execute that any the test/modify/retest cycle is too expensive or time consuming. The implications of this point were clearly explained in the excellent book "Apollo: The Race to the Moon,” where an “interlude chapter” steps back and provides a big-picture overview of the differences between aircraft test and big-rocket test. They paraphrase Joe Shea, Apollo Program Manager, as saying it didn't matter if they tested the Saturn V rocket six times instead of four, or eight instead of six … statistically, the extra successes (if they were successes) would be meaningless – all they would do is use up pieces of precious, costly hardware (time and dollars) that could have been used for real missions.

There are other cases when a project can’t be fully tested. The recently published book "The Right Kind of Crazy" about the Mars Curiosity Rover mission discussed the implications of the obvious: that many aspects of the design of space-exploration systems must be simulated, assessed, and analyzed to an extreme, because there is no way to replicate some of the real operating conditions. For the Mars mission, one such topic was the parachute which slowed the Lander down so the "sky crane" could hover and lower the Rover to the Martian surface. You can replicate the extreme cold and vacuum of space, but how do you test deploying that parachute at hundreds of miles/hour in the Martian atmosphere and then using retrorockets to stabilize the platform in a low-g environment? The answer is that you can’t.

What’s your experience with higher voltages and currents? What‘s the highest power level for which you have had to design for? What was your biggest surprise or memorable example of culture shock?
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