Kent Sheldon, Power-One

Advancing Inverter Technologies

Kent Sheldon has been on the leading edge of renewable energy technology changes since 1995. He began his career at Kenetech Windpower and Trace Technologies. He then served as the engineering manager at Xantrex, the engineering manager and director of sales at SMA America and the director of sales at Enphase Energy. Kent is now a VP of sales at Power-One, the second-largest global PV inverter manufacturer with more than 800 MW of installed PV inverters to date and a global capacity of 4 GW projected by the end of 2010. Kent received a BS in electrical engineering from California State University, Fullerton.

SP: What drew you out of the large-wind arena and into what was a relatively small PV market?
KS: Purely circumstance. I happened to join Kenetech about a month before its first round of layoffs as it headed into bankruptcy. Over the next year, the power electronics group diversified away from wind into custom hybrid-power projects, mostly for the military. This group spun off to become Trace Technologies, which later became Xantrex through further acquisitions. I was working on lots of odd one-off inverter projects (flywheel, liquid battery, superconductors, fuel-cell). One of these was a 75 kW grid-tied inverter for the first Powerlight project in Hawaii, long before there was a US grid-tied PV market. I didn’t think much of where that inverter development would lead me at the time.

SP: What were your major roles at SMA?
KS: In 2001, the US grid-tied solar market was just getting started, mostly in California. Trace held the vast majority of market share with the Sun Tie inverter, which was failing wholesale in the field. John Berdner had convinced SMA to open an office in California. SMA was relatively small at the time, with a handful of string inverter models. John convinced me to join SMA and introduce the Sunny Boy inverters to the US market. Within a year, SMA held something like 90% market share. My first few years at SMA were spent trying to develop a commercial market for its inverters. SMA believed that string inverters should be used for all PV systems, as in Germany. My experience with central inverters at Trace and Xantrex contradicted this strategy. It took many years before SMA released its first commercial inverter in the US, which was quickly scrubbed due to high price and complexity. It was redesigned into the product family SMA has now. Ironically, I never really sold that many commercial inverters while at SMA, as the current family was released after I joined Enphase.

SP: What drew you to join Enphase Energy, a start-up that was essentially creating a new product class, initially developed for the North American PV market?
KS: Microinverters have always been sort of the Holy Grail of the PV industry. How elegant is the thought of generating utility-grade ac power directly from the PV module? The original NEC Article 690 included language about ac modules long before the technology existed. The thought of a true ac module was exciting, and the microinverter was the first step. Technology had finally advanced to the point where a microinverter was achievable and arguably feasible for the first time, even though most of the power electronics industry was in opposition. Six months prior to leaving SMA, I delivered its company perspective at Solar Power International, saying: “Microinverters cannot compete against string inverters due to low efficiency, high cost and poor reliability.” I had not yet been introduced to Enphase. When I saw what they were up to, my thinking changed.

I had a fun and productive time during my two years at Enphase. I was part of the team that successfully shifted the PV power electronics paradigm. Enphase has a long road ahead in its goal to rule the PV world, but one thing is clear: Microinverters will continue to play an important role in the industry.

SP: What were the biggest obstacles you had to overcome to gain acceptance from the building official and Code community when you helped launch the Enphase product into the market?
KS: Everything in Article 690. It looked different, it smelled different and it didn’t live near the meter. What about all the dc rules? Where’s the dc switch, dc fuses and display on each inverter? We found ourselves answering all of the same tired questions, like what about anti-islanding? The UL 1741 listing was constantly called into question, because there were no specific test provisions for microinverters in the UL standard.

SP: Beyond ongoing questions of reliability due to a short track record in the field, what major concerns did you encounter from the installer community?
KS: Price. Microinverters cost more than string inverters, and installers are focused on up-front cost. Micros can take longer to install, which adds to the cost. Now they have 30 on a rooftop. If one fails, it costs more to replace. The likelihood of repeat failures is higher due to more inverters per system, meaning more truck rolls. The rebate-driven incentive programs don’t reward for energy harvest, so installation cost is critical to installers who see their margins continuously eroded.

Commercial inverters are becoming so inexpensive that micros have little chance of competing anytime soon in the commercial market. The value proposition of higher energy production starts falling apart as the system size grows. You just don’t need module-level MPPT on a large flat array with no shading issues. I think it will be many years before micros are feasible for utility and large commercial projects. Or the micro will need to go through a radical redesign, making it a lot less like a micro.

SP: To what do you attribute the success micros have enjoyed?
KS: I see microinverters performing well in the residential and small commercial markets. They really make sense in places with heavy shading, mixed orientations, small systems and constrained spaces. The large influx of electricians who are entering the PV game can easily understand them. A lot of new blood doesn’t want to worry about shade, layout, orientation, 600 Vdc wiring and all the hard math of string sizing. They understand the ac domain and don’t want to bother with dc.

SP: Where do you see microinverters fitting into the big picture of the US PV industry?
KS: This is the key question and one of the main reasons I joined Power-One. All forecasts show the US market growing much faster in the utility and commercial sector than in residential. Residential will continue to grow at a good rate, but the magnitude is dwarfed compared to the other sectors. Microinverters run into a wall at the large-commercial–project size. The thought of hundreds or thousands of inverters under the array is difficult to swallow when considering a 20-year system warranty and life expectancy. What will be available for replacement if there are major failures in year 10? “Plug-n-play” doesn’t mean much if the plugs are incompatible in the future.

Utility engineers become very animated when asked how they feel about 50,000 micros on a 10 MW PV farm. They don’t want to think about controlling all these things. How will they interact with each other or respond to utility faults? How will their harmonics compound? Utility engineers can get their heads around a small number of central generators with an on/off switch. They need a major shift in their thinking before they will accept micros in large plants.

SP: What’s your current role at Power-One?
KS: Vice president of sales for renewable energy in North America (it hardly fits on my business card). I started my career working on large inverters and progressed to smaller and smaller ones. Now I’ve come back full circle to MW-size inverters.

SP: Now that you have returned to a central inverter manufacturer, do you think there is significant competition from microinverter and module optimizer manufacturers?
KS: There will certainly be some competition in the smaller system sizes, but I don’t see it happening across the board. A large number of systems just don’t need the advantages microinverters offer. The larger the system gets, the more complicated micro installations become. Communication becomes a real challenge. Module-level data is just way too much information to manage in large commercial or utility plants. Optimizers are interesting, but I could never really see the advantage versus the risk. It’s the worst of both worlds in my mind: all the potential issues of microinverters and central inverters combined in one system.

SP: What differentiates Power-One products, and what are your expectations for increasing its US market share?
KS: I foresee Power-One’s success in Europe being repeated in the US. We will gain significant market share over the next couple of years as we introduce our full line of products and technologies into the US market and increase the PV community’s awareness of the unique features of the Aurora inverters. Our reliability is proving to be the highest in the industry. These inverters have, by far, the widest dc input range of any inverter on the market. All of the Aurora string inverters have two independent dc inputs, meaning you can have different string lengths and orientations, and shading issues are minimized. This is similar to the value proposition of a microinverter, albeit less granular, but at the price of a string inverter. All our central inverters incorporate a modular architecture built of multiple paralleled inverters. System failures can be localized to only a small portion of the inverter or PV array while allowing the rest of the system to continue operating. Repair of a central inverter can be accomplished in a matter of minutes on-site.

SP: Power-One has had a transformerless (TL) inverter available in the US market since the allowance of such inverters in the 2005 NEC cycle. To what do you attribute its general lack of acceptance? What is your projection on the deployment of transformerless inverters in the US market?
KS: Installers are more or less oblivious to the differences between TL and isolated inverters. The installation of both types is nearly identical from a wiring point of view. The 2005 NEC was poorly written and was corrected in 2008. So some of the slow adoption of TL inverters can be blamed on the NEC. More importantly, module companies did not use PV Wire on their modules in the US. NEC 2008 requires that doubleinsulated wires be used between the modules and the TL inverter, because there is no electrical ground connection on the dc side. Most module companies used less expensive single-insulated wire for their US products. The cost saving was trivial; however, this was one way of controlling product allocated to the US market. These modules could not be used with TL inverters commonly used everywhere else in the world. The first year that the majority of modules changed to PV Wire was 2009, and we are seeing a dramatic uptick in TL inverter sales as a result.

I anticipate inverters will transition from isolated to transformerless very quickly. European manufacturers have been waiting for TL inverters to be allowed in the US for many years. Isolated inverters were developed specifically for the US market, to the irritation of European inverter manufacturers. The advantages are simple: The removal of the isolation transformer achieves higher efficiency, leading to higher energy harvest.

SP: What are the benefits of the modular/scalable architecture approaches of the 250- US and 300-US Power-One inverters? And when will these models be added to the CEC’s list of eligible equipment?
KS: CEC listing for Power-One’s Aurora PVI-250 and 300 central inverter models is imminent. The Aurora PVI-250 and 300 are composed of five and six 50 kW inverters wired in parallel. They can be configured a number of ways—master/slave or multi/master or a combination of both. For instance, one 300 kW array may be connected to all six inverters. If one inverter fails, it isolates itself and the rest continue to process up to 250 kW of power from the array. You lose a maximum of 50 kW of power from the array, or a net energy loss of approximately 6%. You can also group the inverters into three sets of 100 kW blocks. Similarly, if you lose one inverter, 250 kW of production is available. If you have a problem in one of the subarrays, you still have 200 kW of production available. Spare inverters can be stored at the site for an immediate response to an inverter issue. The inverters are racked in a cage and can easily be replaced and sent back to the factory for repair rather than sending technicians to diagnose and repair on-site. I see these as important benefits to ensuring maximum system availability.

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