1. What is the definition of a String Inverter and what makes it different?

The core of a PV solar array is multiple PV modules which are wired in a series very similar to Christmas lights.  On a residential installation, there are usually two to four “strings” of modules wired in a series.  Those strings are then wired together in parallel to make up a solar array that is then connected to an inverter.

10 years ago, modules were very expensive and typical residential solar arrays were much smaller.  A residential PV installation might only have had a single string of modules to make up the entire array. The term ‘string inverter’ comes from this reference to a single string of modules used with a single inverter. 

Today, the term ’string inverter’ typically refers to an inverter that is between 1kW and 30KW AC in rating and either wall-mounted or mounted to a ground mount array support structure.  Due to these characteristics, string inverters are primarily used in residential installations.

2. Can you provide a brief overview of the current String market and where the PVP HE String Inverter fits into it?

As the price of modules has steeply dropped over the past four years, average residential system sizes in the United States have grown.  There has also been a boom in the number of new PV system installers in the U.S. Lastly, the type of person buying solar for their home or business has changed. 

All of this has had a profound effect on how string inverters are used and what features are important.  The PVPowered (PVP) HE inverter, from Advanced Energy, is designed for the new needs of installers and system owners. With respect to the change in average system sizes we have tailored the line to provide the inverter power level that installers need.  The PVP HE string inverter line is available in 3.8kW, 5.0kW, 6.0KW and 7.0kW sizes.  These models align with the new higher average sizes of residential PV systems that are a result of lower prices for PV.  These are also the sizes that installers want to use as building blocks when creating larger residential system sizes.  We have incorporated features such as an integrated wire-way and have made sure the inverters have uniform physical dimensions which make multiple inverter installation easy, fast and low cost.

We have also focused on making the line of HE string inverters easy to install for installers new to the business and not just those with years of experience.  Many elements of a PV installation that have previously been field installed extras are now included as standard features. These features include: an integrated AC and DC disconnect, integrated fused DC combiner, integrated wire-way, provisions for supplemental AC fusing, and standard integrated ZigBee wireless performance monitoring.

Additionally, to improve safety and prevent inverter damage due to common installation errors, we have incorporated AC and DC overvoltage protection and DC reverse polarity protection. AC and DC over-voltage protection is needed in the case where an installer might make a PV array design error or accidentally hook the inverter up to an AC voltage that the inverter isn’t configured for. DC reverse polarity protection prevents damage to an inverter when an installer makes the simple mistake of mixing up the positive and negative leads of the PV array. These mistakes can happen and often permanently damage inverters so we have worked to prevent damage and make installations smoother.

Many of the new PV installers begin installing residential systems.  As those installers gain experience they often expand to installing commercial systems.  Installers will frequently use the string inverters that they have become familiar with in residential systems on commercial systems. 

To meet that need, the PVP HE inverter design is also well suited for commercial systems.  With power levels at 5kW and 7kW and the ability to be field configured for three phase AC voltages, the PVP HE inverter is an excellent building block for use in commercial systems where installers are more comfortable using string inverters, multiple inverters are preferred for redundancy, multiple MPPT trackers are provided by multiple inverters or it is impractical to install a ground mounted central inverter. 

Finally, owners of PV systems with string inverters increasingly expect that their PV system look and operate similarly to other household appliances that they own.  The PV Powered HE string inverter is the result of eight years’ experience building reliable string inverters for the US market.  We do not want system owners to feel like the inverter is just one more electrical box that needs to be hidden away.  The PV Powered HE string inverter offers an elegant sophisticated design intended to blend into multiple architectural styles. 

Additionally, today’s more sophisticated system owners expect all of their electronics to be connected to a centralized data cloud.  We agree, and have created the first truly connected string inverter with the PV Powered HE inverter line.  Every PV Powered HE string inverter incorporates ZigBee wireless monitoring as a standard feature.  Zigbee Wireless monitoring allows simple and reliable communication between the inverter and an optional in-home Advanced Energy wireless gateway. The gateway can be installed by a home owner or a business IT professional in minutes. Monitoring of the HE string inverter does not require multiple cards and boxes as is common from other inverter manufacturers.  When installed in the home or business, the Advanced Energy gateway communicates with the inverter and MyPVPower.com and allows the system owner and installer to monitor the health and performance of the system absolutely free.  By utilizing ZigBee and not a proprietary communication protocol, the inverter can communicate with thousands of ZigBee enabled devices including home area networks and energy management systems. 

3. What are the major enhancements and new features of the PVP HE String line?

First, the PV Powered HE inverter features a 97 percent CEC efficiency rating across the entire product range.  With inverters from other manufacturers, if an installer selected a lower power model or adjusted the inverter’s AC voltage this might result in a lower CEC efficiency than what is rated for that inverter. This is not the case with the HE string inverter as the CEC efficiency is 97 percent across the entire product range and configurations.   

Also, we understand that efficiency means little if you can’t design an array for the available roof space.  The PV Powered (PVP) HE string inverter line offers a full power MPPT range of 200-500 and a MPPT operating range of 100-500V. Put simply, this means that the PVP HE string inverter offers a large amount of flexibility in how an installer designs an array.  These extra wide ranges also mean that the inverter starts making power earlier in the day and continues to make power later in the day. This flexibility coupled with a 97 percent CEC efficiency means the PV Powered HE string inverter offers industry-leading energy harvest. 

Other major enhancements to the PV Powered HE string inverter line include many standard features that an installer previously had to install in the field.  These standard BOS (balance-of-system) features include an AC and DC disconnect, integrated DC combiner with fusing for up to 4 PV strings, provisions for AC fusing and an integrated wire-way. 

We also spent time on the aesthetic design of the inverter, understanding that many system owners want something that blends into their home or building while still appearing to be a high quality appliance.  The industrial design of the PV Powered HE String Inverter meets those goals with a modern design that will blend into multiple architectural styles yet it still elegant and refined.

Finally, we have incorporated ZigBee wireless into each inverter as a standard feature.  Integrating ZigBee wireless into each inverter offers unprecedented standard connectivity of a string inverter. Where monitoring is an optional component in other string inverters, we’ve committed to making inverter direct monitoring standard while making it easy, fast, reliable and accessible to the system owner and to the installer.

4. Explain how the PVP HE String line utilizes the Zigbee monitoring and the benefits to both the installer and end user?

Figuratively speaking, monitoring the performance of string inverters has been stuck in 1995.  To enable performance monitoring in string inverters in the past it required the installation of PC cards in the inverter and usually additional boxes in the home or business.  Installing those PC cards in the inverter can be dangerous and monitoring cards and external boxes require special configuration.  The special configuration usually requires that an installer have special network training.  We recognized that all of these extra hurdles and the associated costs meant that systems were not getting monitored correctly.  Furthermore, the data that came out of these systems was proprietary and couldn’t be shared. 

We believe that system owners and installers should take an active role in understanding how their system is performing and that basic web based monitoring should be free and flexible.  Adhering to this philosophy, we have designed the ZigBee based monitoring solution for the PV Powered HE string inverter to make string inverter performance monitoring fast, easy, simple, reliable and flexible.

The first benefit to ZigBee wireless monitoring in the PV Powered HE string inverter is that it doesn’t require any additional work on the inverter, eliminating that cost and risk.  Secondly, the PV Powered HE string inverter’s integrated ZigBee wireless can communicate with thousands of ZigBee enabled devices such as smart meters, home area networks and energy management systems.  The ZigBee wireless radio’s primary function is to communicate with an Advanced Energy (AE) ZigBee wireless gateway.  Installation of this ZigBee gateway is extremely simple and can be done in minutes.  Once installed, The ZigBee wireless gateway talks to all the inverters at a single installation and sends the information to MyPVPower.com.  Through MyPVPower.com, system owners and installers can monitor their inverters anywhere in the world. 

MyPVPower.com is a service that is totally free for the life of the inverter and is simple, intuitive, and easy to use yet includes powerful features such as an Operations & Maintenance (O&M) portal, daily production and weather information, and data export.  We deliver monitoring value with the right combination of features, functions, and flexibility while lowering the existing hurdles to monitoring.

5. Why did you choose Zigbee over others as the wireless protocol for the monitoring system?

Zigbee is proving to be the standard for smart grid communication.  ZigBee has very low power consumption, is designed for long range communication of small amounts of data, incorporates mesh networking, and has seen wide adoption as the protocol that is used for modern smart grid devices.  Other communication protocols like Bluetooth and Wi-Fi are not well suited for inverter monitoring. They are a shorter range, have higher power consumption, and their setup can be arduous and hit and miss. Anyone who has tried to join someone’s Wi-Fi network without knowing the right Wi-Fi network name or Wi-Fi password can attest to this. Our implementation of ZigBee works and delivers the features and interoperability that system owners and installers want.

6. As a solar installer in an increasingly competitive market, reputation and time are increasingly valuable, what specific features of the PVP HE String Inverters enable Installers to save time and costs?

The PVP HE inverter includes an unprecedented level of integrated BOS, features that take the guess work out of installation and make it safer. Those features include an integrated AC/DC disconnect, DC fused string combiner for up to 4 strings, provisions for supplementary AC fusing, integrated wire-way, optimally placed knockouts to make wire pulls easier, mounting that can be accomplished on a single wall stud, integrated lifting handles to make installation safer, and finally integrated ZigBee monitoring. All these features save hours of installation time and offer both installers and systems owners benefits that they should just expect.

7. What percentage of the cost of a typical solar installation is attributed to the inverter?

On average, 10-13 percent of the cost of a typical solar installation is attributed to the inverter.

8. What is the most asked question by installers when comparing inverter systems?  How about the end user?

The type of question depends on the region in which the installer is located.  In our experience questions related to reliability, inverter performance, performance data monitoring, and customer service are among the most frequent that we receive across the board.

9. When and where will the line be available?

Advanced Energy’s PV Powered HE string inverter line will be available in Q2 2012 through our various distribution partners. Additional information can be found at http://solarenergy.advanced-energy.com/en/STRING.html

10.  Is there anything else you’d like to add?

We are very excited to bring the features that our installers have asked for.  This is a product designed for North American solar installations from the start and is not adapted from a European design.  There are no design compromises and that will be obvious in the experience that installers and system owners will have with their system.

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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