Reading the article, “Extensive NREL Power Electronics Capabilities,” sent me back to thinking about the years I spent designing and testing high-voltage and high-current electronics. The engineers who did this work were usually more eccentric than nerdish. I think there were a couple of reasons for that: It’s very difficult to analyze the effects of extreme conditions on electronic components and systems — you have to rely a lot on experience and intuition. And most of us also got a kick out of the danger involved — we had to be really careful about how close we came to high voltage and there was always the possibility of something failing with a big bang or flame. One thing that our experience taught us was that extensive testing is absolutely crucial for power electronics — analysis alone will never be enough. High power stresses materials to their limits.
You can pretty much sum up most electronics applications as being either brains or muscles. The body analogy is quite appropriate. Computer circuits have to be carefully designed to protect their delicate brains. However, if muscles are overstressed doing heavy lifting, they can be seriously damaged (as I found out when I tore my rotator cuff). So it is with high-current and high-voltage power electronic devices — overstress them and they will fail. And thousands of amps and thousands of volts can create quite a lot of stress.
Based on all my years of experience and hard knocks with high-power electronics, one paragraph in the NREL article struck me as especially significant: With power electronics “… comprehensive component tests are important, because working backward from an integrated system is more difficult than getting all components on solid ground from the start. Just like how the customized semiconductors stack into components, the tested components stack into a system or device.”
In my day, much of our work was for niche applications, but now, with the race to incorporate renewables into the power grid, power electronics have moved to center stage.
The two most common renewable energy sources are solar and wind.
Most solar power comes from photovoltaic arrays, whose output is DC — but the electric grid is AC. So, solar power has to be converted to AC before it can be connected to the grid. The way that is accomplished is with a circuit called an inverter (not converter — that converts AC to DC). Inverters use power semiconductors that continuously reverse the polarity of the voltage applied to their load, producing a bidirectional waveform to match the standard power line frequencies — 60 Hz in the U.S. and 50 Hz elsewhere.
The other most common renewable is wind power. Most wind generators produce AC power but not at standard power line frequencies. So, they use AC-to-DC solid-state converter, electronics to compensate for source and load variations, and an inverter to switch the power back to AC at standard line frequency.
A microgrid is a localized system of power generation that serves multiple users but does not require connection to the grid. It can either be permanently isolated from the grid (“islanded”) or can be made to connect or disconnect from the grid under various conditions. For example, during periods of high demand, such as a hot summer day, disconnecting the microgrid will reduce the burden on the utility. Or if the microgrid is run by solar power it can connect to the grid at night.
If there is a power failure on the utility grid, the microgrid can automatically disconnect from it and continue to function. A failed microgrid could also be isolated so its effects will not spread beyond its boundaries.
The connection/disconnection is implemented with power semiconductor switching devices.
The article “Bringing More Reliable Electricity to Puerto Rican Microgrids” is a case study of grid-connected microgrids helping a rural town cope with severe power outages.
Testing and Reliability
Power electronics are key for building a more sustainable infrastructure, but they require as close to 100 percent reliability as possible to keep our power supply up and running. And key for reliability is to be able to thoroughly test the ability of components and systems to stand up under the most extreme conditions.