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Building out the Smart Grid

Help Wanted: Wind, Solar, and On-Grid Battery Storage [Part TWO of a Series]

The previous article in this series examined how the platform design concept, as used by smart phones, is needed to help bring more engineering expertise to the grid problems of today. He we take a closer look at an application, power conversion, that directly impacts some of the biggest challenges facing the grid—renewables.

BY BRETT BURGER, NATIONAL INSTRUMENTS

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The benefits of a design platform extend beyond the first development cycle.  Looking back at a smart phone, upgrading software is a fairly common and simple task.  About once a year, with the push of a virtual button, the entire operating system is upgraded fixing bugs, adding features, and tweaking the UI.  About once every two years a new hardware upgrade is available and the software stack is migrated from a backup. The system picks up right where it left off but with more storage space, a faster processor, new co-processing units, better camera, speakers and so on.  This demonstrates the value of building on a platform; abstraction of the hardware technology through a layer of software.  From a hardware standpoint, changing processor architectures and other new hardware is not a trivial update, but to the end users that have adopted the platform it becomes as trivial as plugging in the new phone and performing a “sync” operation.  Instant technology refresh.  This is happening with software platforms for the grid as well.  One specific application in need of platform adoption is that of power conversion, or power electronics.

Power conversion happens in inverters which are the hardware nodes that enable DC power sources, like renewables, to connect to an AC grid.  Inverters are one of the problems referenced in the previous article because they add harmonic noise to the grid and, by their digital nature, have no inertia to help absorb grid issues.  In fact, as their penetration grows they reduce the overall grid inertia that exists from spinning base load.  Wind turbines, solar arrays, battery storage systems like Tesla’s PowerWall, all need inverters. 

At the core of the inverter is an “intelligent power module” that consists of a processing control board, analog to digital converters (ADCs) and digital to analog converters (DACs) for connection to sensors and insulated-gate bipolar transistors (IGBTs).  With given set points, the control board drives the IGBTs to digitally create a sine wave of voltage potential (AC) from the constant output of DC potential (Figure 1).  Most inverters today are the product of design teams comprised of experts in digital hardware design, board layout, analog front end design, hardware design language (HDL), signal processing, and of course power and inverter control theory.  All of these engineers work in concert to design inverters.  This design process is lengthy and much of the expertise is used solving problems that are not core to the task at hand, such as glue logic for mating ADCs to an FPGA, writing middleware for application logic to interface with the hardware, and verification of the hardware layout.  Time spent on these tasks is time that isn’t spent on improving inverter control algorithms, responding to customer requests, and differentiating the inverter in a growing market.

Figure 1
3-Phase Inverter Diagram. Upgrading the control system of an inverter leads to efficiency gains and new application use cases, but traditional design makes it difficult to keep up with market trends and silicon level technology.

There is a better way to approach design that will let inverter manufacturers focus more on business core competencies.  Platform based design is the better path for development and will benefit not only inverter manufacturers, but their end customers and utility companies as well.  Looking back to the smart phone analogy, switching to platform development democratizes system design and lets experts focus on their core strength.  iOS and Android enable a wealth of application developers who know little to nothing regarding hardware, operating systems, and middleware development to bring unique products to market that would otherwise live only as a “what if” idea.  The same concept applies to inverter design. 

Traditionally, inverter and control experts would have their software simulation tools and then pass the results “over the wall” to the embedded experts to implement.  This takes time and is subject to a broken toolchain.  With a platform approach, engineers can focus more of their effort on new inverter design techniques and let companies that are experts in control board layout and embedded system platforms innovate on the under the hood components.  The inverter experts interface with the platform API at a level that abstracts away the low level complexities.  This concept eliminates some of the walls that exist with traditional design methods, results in a shorter design cycle and abstracts away much of the hardware dependencies that often cause longer redesigns.

Inverter design is currently pressured from both sides of the industry.  From the market side, applications are becoming more diverse.  Wind and solar arrays are growing in size and demand more efficient inverters to meet the requirements of the generation owners and cleaner outputs to satisfy the requirements of system operators.  On-grid storage systems, mostly batteries, are growing in demand to help control the dynamic generation from wind and solar systems.  Wind, solar, storage…there is not necessarily a one size fits all inverter that covers all of these applications and accounts for the unique requests that always emerge from the end users.  How does time spent laying out the front end analog circuitry or FPGA glue logic help a company compete and win business in this evolving inverter market?  It doesn’t.

The inverter market is just one source of pressure.  The other is the rapid growth of silicon level technology.  Newer, more advanced hardware technology rarely, if ever, is a drop-in physical upgrade to the previous generation.  DIP chips, socketed processors, and quad-flat packages are giving way to the ball grid array (BGA) packages, a significantly more complex component to integrate into a board level design.  Companies that take on the effort of a custom built electronics package, despite their core differentiator being in the inverter and control features, now must spend more time and effort to update their in house knowledge and tools to deal with these complex form factors.  Beyond the physical form factor is the technology trend of heterogeneous processing elements (Figure 2).  New components like the ZYNQ system on a chip (SoC) from Xilinx blend DSP cores, FPGA logic fabric, and microprocessors on a single die and offer a great value in performance flexibility and computations per dollar.  Should an inverter design company spend resources to redesign around the latest processing elements or continue to focus on their market differentiator?  The latest chip available on the open market does not a differentiator make.

Figure 2
Heterogeneous System-On-a-Chip (HSoC). New chip technologies that incorporate multiple processing elements (FPGA, CPU, DSP) on a single die are more difficult to design into a system and require specialized skills and tools. Traditional design manufacturers are having to devote more and more resources just to keep up with existing technology.

In house full design can be attractive from the “control” standpoint, as in the manufacturer controls all of the design, but with this comes added responsibility and burden.  Components like flash memory and non-volatile storage have shorter market cycles and require a large last time buy purchase, a board spin, or at least another round of testing to validate the replacement.  Companies that do choose to stick with traditional design will see their resource allotment to ancillary and overhead in-house services grow and thus may see new competitors enter the market with niche solutions that they could otherwise have covered if they had a more democratized design approach.

Dynapower used a platform design approach for an inverter targeted at advanced carbon battery based on-grid storage solutions.  Power engineers programmed the new product without needing embedded software engineers in the middle for every step of the process.  “The key to this design was the ability for our power engineers to directly program their product without a software engineer in the middle. This new platform and method of development changed our development time from 72 weeks to 24 weeks….These tools take design to the next level. We can have a 90 percent confidence factor in a first design and minimize hardware iterations during the prototype stage.” said Kyle Clark of Dynapower. 

The platform used for this new inverter was the LabVIEW RIO architecture from National Instruments.  Specifically, a single-board RIO control board is mated to an I/O board with circuitry designed to control IGBTs.  The single-board RIO general purpose inverter controller (GPIC) used has a Freescale PowerPC processor and a XILINX Spartan-6 FPGA.  NI recently released an update to the Single-Board RIO that mates to the GPIC.  For future designs, Dynapower can port their existing software IP from the 400 MHz PowerPC processing board to a dual-core ARM9 processing board with ARTIX-7 FPGA fabric.  This means an immediate performance upgrade with no hardware redesign required.  Staying current with technology while focusing on the core problem is the power of a platform. It also represents an enabling technology that is going to help engineers make the next generation grid more accommodating to wind, solar, on-grid storage, and any other technology that requires conversion between DC and AC.

National Instruments
Austin, TX
(512) 683-0100
www.ni.com

Dynapower
South Burlington, VT
(802) 860-7200
www.dynapower.com