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Easy, Reliable and Cost-Effective Rapid Shutdown Compliance Utilizing the SunSpec Standard

The requirements of 2017 and 2020 National Electric Code (NEC) section 690.12 – entitled Rapid Shutdown of PV Systems on Buildings – currently mean that module-level power electronic (MLPE) devices must be installed with PV systems installed on buildings, even if they are not appropriate for energy optimization reasons.  As microinverters and standard DC optimizers solutions are closed, proprietary solutions, their use locks installers into depending on a single source for equipment and replacements.

As discussed in the first blog post of this series, the SunSpec Alliance has the goal of creating industry-wide standards to resolve issues like compliance with Rapid Shutdown requirements. Their Rapid Shutdown standard allows for multiple vendors to certify equipment to an open, published standard – allowing installers to choose between interoperable devices and avoid being locked into a closed ecosystem to comply with code.

SMA rapid shutdown certified devices.

SMA has been a global leader in PV technology for almost 40 years.  SMA America will celebrate its 20th anniversary in September 2020 and has long been a technology leader: first with UL certified inverter-integrated AFCI as well as the first to achieve UL1741 SA smart inverter certification. SMA’s success is based on ensuring that it delivers safe and reliable equipment that reduces the time installers must spend on a roof installing or servicing PV equipment.  With Rapid Shutdown now requiring module-level power electronics for rooftop PV systems, SMA faced a daunting challenge to provide a fully code-compliant solution that also minimizes reliability impacts, while ensuring optimal energy yields. The SunSpec rapid shutdown standard provided a perfect complement to SMA’s ShadeFix optimized residential and commercial string inverters to achieve this goal.

The functioning of the SunSpec rapid shutdown process is very straightforward.  Two components are required – a transmitter and some number of receivers.  The transmitter is responsible for imparting the SunSpec rapid shutdown power-line signal onto the DC conductors that go between the array and the inverter.  The receivers are responsible for detecting the signal on those conductors.  While the signal is present, the receivers do nothing. When the signal disappears, the receivers are responsible for lowering the array voltage below the code limits within the code specified time after rapid shutdown is “initiated”. They isolate their modules from the string they are part of, without breaking the string, and provide a low, “standby” voltage on their output conductors if the SunSpec signal is absent. This power-line communication signal is a “keep-alive” signal that does not require any additional wired or wireless communications channel between the array and inverter. It is important to highlight that the SunSpec certified receivers do not perform any power conditioning function or transmit information about the array operation, thereby allowing them to be very simple and robust devices.  This is important, as there may be many hundreds of these devices scattered throughout the harshest environmental conditions of a single large commercial rooftop installation. Time spent servicing failed MLPE devices is time not spent installing. rapid-shutdown-cert_vert-color

SMA America has integrated a transmitter certified to the SunSpec rapid shutdown standard into both our residential Sunny Boy US-41 and commercial CORE1 US-41 inverter lines. This transmitter, when enabled during commissioning, begins to impart onto the DC conductors connected to the array the SunSpec rapid shutdown “keep-alive” signal.

SMA’s approved SunSpec certified rapid shutdown receiver can support most residential and commercial module choices, meaning only this part needs to be sourced when using either SMA’s residential or commercial string inverters. This makes stocking and allocating inventory as easy as possible for installers and equipment distributors. Furthermore, SMA’s string inverters do not require the SunSpec shutdown receiver units to function, so if the PV system does not require rapid shutdown (e.g. a ground mount) then the receivers are simply not installed. Commissioning with SunSpec shutdown is fast since there is no detection step required for these MLPE or a layout map creation step for module-level monitoring.

The SMA inverter-integrated SunSpec transmitters are configured to stop broadcasting the “keep-alive” signal whenever the inverter senses a loss of utility voltage and frequency on its AC terminals. A first responder will shut off utility power to a site as an initial step to fight a fire, so this means the rapid shutdown “initiator” – loss of AC power – is something that would be done even if that responder has no formal training on how to recognize and utilize rapid shutdown equipment on a PV system.

Finally, the existence of the SunSpec rapid shutdown open standard means that multiple vendors can provide receiver devices that mirror trends in the PV module market. More devices with a wider range of options and sizes are available to installers than SMA alone could reasonably provide, and module manufacturers can even build the receiver capability within their module’s junction box directly. These devices (and the installer installing them) are not dependent on an SMA inverter to ensure rapid shutdown code compliance, simply another certified transmitter, inverter-integrated or stand-alone.

The existence of the SunSpec rapid shutdown standard has allowed SMA America to improve its ShadeFix optimized string inverters to provide the industry’s fastest-to-install, most reliable and cost-effective solution for any PV system needing to achieve 2017 or 2020 NEC 690.12 compliance.

 

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SMA Sunny Tripower CORE1 Inverters: Repowering Ready

Part II: Maximize and Protect

In the first installment of this series, we focused on MPP voltage. This post will explain how to maximize the number of strings connected to a string inverter and protect the inverter from damage. The strings are limited by the number of terminals and the maximum short-circuit current of each individual maximum power point tracker (MPPT).

CORE1 multiple strings

There are ultimately two advantages of checking the short-circuit current: to maximize the number of strings per inverter and to prevent inverter damage in the event of a string fault with a high DC current. Compared to 600VDC systems, modern PV systems have longer strings with higher MPP voltage, requiring fewer strings in parallel to reach the same capacity. Contemporary inverters can, as a result, handle a comparatively higher voltage with limited current capacities.

Still, a new inverter placed into an existing system must match the current requirements. There are two numbers to check to ensure that’s the case:

  1. Maximum operating input current
  2. Maximum short-circuit current of the inverter/MPPT

First, system age is an important consideration since older modules may not reach their maximum operating input current, even under the best conditions. In most cases, the DC/AC ratio has more influence on clipping than the maximum operating input current. Customers have, however, in some repowering projects, experienced up to a 20% higher module MPP current than the inverter’s maximum operating input current without a loss in production.

It’s also important to focus on the maximum short-circuit current of each inverter MPPT. As mentioned above, there are two reasons to investigate this value. Let’s dive into the specifics of how and why to ensure you’re ready for repowering.

Ready to Repower?

One reason to repower is to protect the MPPT of the inverter from damaging current. These elements include the maximum short circuit current per MPPT, the safety factor and the short-circuit current of the chosen module of two parallel strings.

Let’s look at an example using the SMA CORE1:

  • Module type: Sunmodule Plus SW290 MONO from SolarWorld
  • Standard safety factor: 1.25
  • Maximum short-circuit current: 9.97A

The maximum short-circuit current of the PV design is then calculated by multiplying the short-circuit current of the module (9.97A) by the number of strings (2) and by the safety factor (1.25). Multiplying these values equals 24.93A. The total must be less than the maximum short-circuit current of one MPPT of the CORE1, which is 30A.

Thus, this design works and matches the inverter limits, allowing connection of two strings directly to one MPPT. Three strings would risk permanent damage to the inverter.

The second reason to check the maximum short-circuit current per MPPT is so you can calculate the maximum number of strings that can be connected to each MPPT. In the aforementioned scenario, two strings are the maximum allowed per MPPT. Some installed modules in older designs, however, have a lower short-circuit current. In such cases, the maximum short-circuit current per MPPT must be divided by the safety factor and by the maximum short-circuit current of the modules installed.

When determining the maximum number of strings to connect to one MPPT in parallel, you can round the results down. Let’s look at an example:

  • Module type: Ultra 175-PC from Shell
  • Maximum short-circuit current: 5.43A

And here’s the math: The maximum short-circuit current of the MPPT (30A) divided by the safety factor (1.25) and by the short-circuit current of the module (5.43) results in 4.42 strings. Once rounded down, it shows that a maximum number of four strings can be connected in parallel to one MPPT.

string inverters formula

Why calculate the maximum number of strings, especially when repowering?

The existing strings are typically very short (about 10 to 14 modules), which results in a low string voltage. Combined with a low current, those strings have a low power output. Using the example above with the Shell modules, a string length of 12 modules with two strings per MPPT puts out 25.2kWp of total power under test conditions. The SMA CORE1, 33-US can handle up to 50.0kWp (DC).

Adding more strings until the chosen DC/AC ratio has been reached makes full use of the inverter capacity—underscoring why this is the second important reason to check on the maximum short-circuit current of the MPPT. Keep in mind, though, when more than two strings operate in parallel, each string requires protection, which is achieved by adding string fuses.

Now that you know how to correctly check both numbers, your repowering is set up for success—ensuring your system will be safe and efficient.

 

 

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SMA Sunny Tripower CORE1 Inverters: Repowering Ready!

Would you like to discover a new level of PV performance? Then this blog post series on SMA Repowering is for you! In this article, our repowering expert Thorsten Hoefer shares insights on which aspects to consider when choosing an inverter for a repowering project and explains which SMA inverters are repowering ready.

Sunny Tripower CORE1 commercial inverter

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New University Study Shows SMA Outperforms the Competition

Whether it’s from passing clouds, roof vents, or chimneys casting shadows, PV professionals must frequently deal with shade. But, what is the most effective way to handle it? A study by the University of Southern Denmark (SDU) now has the answer. 

String level optimization

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Solar Power World Features SMA’s DC Coupled Storage Solution at SPI

Large-scale storage solutions from SMA allow system owners access to new revenue streams.

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A New Era in Power: The Promise of Virtual Power Plants

Distributed energy resources with storage are being aggregated and managed by SMA using ennexOS, the award-winning cross sector energy management platform. 

Distributed renewable energy resources bring a variety of benefits to the electrical grid. However, they also increase complexity due to their intermittent nature. The addition of battery storage can help offset that intermittency while allowing system owners greater self-consumption of their onsite power generation.

But with this distributed network of renewable energy storage systems, how do utilities manage an effective flow of electricity throughout their network while maintaining price and grid stability?

The solution is the Virtual Power Plant enabled by SMA technology.

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Keep the Lights On with the SMA Energy System

If a power outage or other emergencies like tornadoes or hurricanes hit your home, you can keep the refrigerator running and the lights on with backup power from by the SMA Energy System.

In Part I of this blog series, we provided details on how homeowners can maximize energy consumption with the SMA Energy System. In Part II, we elaborate on the options for protected loads and whole home backup power. 

SMA Energy System

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The Secret to Maximizing Your Energy Consumption at Home

With the SMA Energy System, homeowners are able to achieve both whole home backup during power outages and efficient energy management when reducing energy bills requires more than just a PV system.

In Part I of this blog series, we will provide details on how homeowners can maximize energy consumption with the SMA Energy System. In Part II, we will elaborate on the options for protected loads and whole home backup power. 

Long Shot of a Father, Mother and Little Girl Watching TV.

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Energy Reborn: Live the SMA Experience at SPI

Visit our booths – outdoors at the Grand Plaza and indoors at the Smart Energy Microgrid Marketplace – and experience how SMA is leading the evolution of renewable energy with major advancements in solar technology.

SMA @ SPI

 

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Part 2: Commercial PV System Sizing and Design with SMA CORE1 Inverter

PART II

In part one, we introduced the features of SMA’s free sizing and simulation tool and outlined how the CORE1 is designed to provide easier installation, commissioning and monitoring. In part two, we will discuss the benefits of the CORE1 for weather data integration as well as the advantages of using Sunny Design.

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For weather data integration, the CORE1 communications card allows the installation of a sensor module that can integrate temperature, irradiance, and wind speed sensors, reducing the amount of connections, cable runs, and additional external monitoring devices needed.

If we look at the East/West configuration of the array, it is important to account for any possible shading effects. Fortunately, SMA inverters include the Opti Trac Global Peak algorithm, which reduces the effects of shade on the system by allowing the inverters to optimize the Maximum Power Point Tracking of the PV array. Additionally with the CORE1, it is possible to monitor independent strings in order to check for any differences in energy production on each of the channels of the inverter. As it occurs in this case where we have strings of similar size, it is possible to configure two groups (East and West) to compare currents and identify any failures or potential problems on the strings.

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One nice feature of Sunny Design is the possibility of doing a side-by-side comparison of design options. In the image above, we can see the results and overview of both options. Although for alternative 1 there is a higher AC capacity by having two 30kW inverters, the annual energy yield and the performance ratio of both designs is almost the same. The CORE 1 design has a higher DC/AC ratio.

Depending on the characteristics and conditions of the installation site, it might make more sense to adopt the alternative one with two Sunny Tripower TL-US inverters. Even though these devices do not have the high integration of the CORE 1, they are lighter, more flexible, and allow installations in high-pitched roofs.

The last step in the design process involves selecting the option that makes the economic sense. Fortunately, Sunny Design offers the possibility of customizing values and settings to perform a profitability analysis, so that costs and payback periods can be visualized. This enables an integrator to make the right decision not only based on technical specifications, but also on financial metrics.

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In summary, if you want to complete successfully a PV project on time, there is no need to cut corners, just make sure that you have the best tools for the job. You can count on SMA, we have the solution for any PV project offering the best support and providing the answers, you need every step of the way.

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Part 1: Commercial PV System Sizing and Design with SMA CORE1 Inverter

PART I

In view of the increasing number of PV commercial installations across the U.S., installers and contractors find themselves under constant pressure to complete more projects in less time. In order to achieve this, they must streamline the installation process while maintaining safety and quality standards.

sunny_design1-1024x707

The first step of the process starts with design. While most of the time the objective is to maximize roof space in order to fit as many modules as possible, it is essential to plan ahead for service and maintenance, leaving enough space between strings of modules in order to allow easy access. Once the system layout has been determined, the equipment for the project must be selected. When choosing the right inverter for the job, one must consider more than just technical specifications. It is also important to keep he installation process in mind, under what conditions the inverters will be working, and all other major requirements of a commercial PV plant, including; monitoring systems, weather stations, string aggregation, shade mitigation, racking structures, and additional BOS.

When considering all these factors it is often difficult to find an inverter that can meet all of the required metrics for the integration. Fortunately, we have the tools and the solutions to help you design a successful project that can be carried out and completed within a tight deadline.

First, SMA’s free sizing and simulation tool, Sunny Design, allows you to size systems correctly by matching SMA inverters with PV curves, and to compare design alternatives with different inverters in order to be able to make the right decision not only based on energy yield but also on economics and architectures.

As an example, we sized a 67-kWp rooftop commercial system with Sunny Design. We designed two alternatives for this project; one with the Sunny Tripower TL-US and one with the Sunny Tripower CORE1 – the latest addition to our commercial solutions portfolio.  For this project, we designed a roof-mounted array with an East/West configuration and a 15o inclination.

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When used in the automatic design mode, Sunny Design will do all the calculations and can offer options based on profitability or energy yield. For the first alternative we have selected a design with two STP 30000 TL-US inverters. The suggested configuration for the array has three strings in parallel connected to each input of the inverter, meaning that we will need to incorporate DC combiner boxes in order bring the six strings into the two MPPT channels of the STP inverters. The results of the simulation, including current and voltage values, can be seen below:

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It is important to note the 1.1 DC/AC ratio of the design. High DC/AC ratios account for module degradation and potential higher energy yields during a calendar year.

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The higher integration of the CORE1 allows you to connect up to 12 strings to the inverter eliminating the need for additional BOS like DC combiner boxes. Thanks to its higher power rating of 50kW it is possible to reduce the number of inverters and the number of connections, improving the overall installation time.

When considering the specification of the PV plant it is clear that the advanced features of the CORE 1 align better with the requirements of the project.

Multiple communication channels allow for easier monitoring and commissioning. Although best practices for monitoring involve using a physical communications channel like Speedwire (SMA’s Ethernet based protocol) because of its reliability and higher speeds, WLAN is the better option when commissioning the inverter on site. It will allow direct communication between the inverter and any smart device that can connect to a wireless network and access the WebUI through a web browser.

In part two of our series, we will outline the benefits of the CORE1 for weather data integration as well as the advantages of using Sunny Design.

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Are you Ready to Achieve Compliance of California Rule 21 Reactive Power Priority Requirement?

Tech Tip Sunny Boy for CA 21

CA Rule 21 Phase1 new requirements will include a Reactive Power Priority setting starting Thursday, July 26.  UL has certified SMA inverters as compliant with this new regulation.

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