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The Micro-Inverter. Francine V. Notte NABCEP Certified PV Installer ISPQ Certified PV Instructor. History of the micro-inverter.
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The Micro-Inverter Francine V. Notte NABCEP Certified PV Installer ISPQ Certified PV Instructor
History of the micro-inverter • Micro-inverters are installed on the roof under or next to a module and usually serve one single module at a time. Some serve two modules but the double ones have not been as successful as the micro-inverters attached to a single module. Micro-inverters convert the DC of the modules into AC just as conventionally recognizable inverters do and provide maximum power point tracking. • The AC output goes directly into the homeowners’ load center.
The concept of the micro-inverter has been around for as long as the idea of using solar energy for power in a real and commercially viable way has been. • 1991 saw the beginnings of the first incarnation of the micro-inverter which was rather inefficient and was basically a miniaturized version of a conventional string inverter. It was developed by a US based company, Ascension Technology. In 1994 a version of this was submitted to Sandia Labs for testing.
This design prototype fell under the radar but resurfaced again as a 300W AC module called the SunSine 300W. • The SunSine 300W module didn’t exactly fly off the shelf.
In 1993, Mastervolt introduced the first ever grid-tied micro-inverter, the SunMaster 130S which was a collaborative effort by Shell, Ecofys, and ECN companies. • The OK-4E-100-E micro-inverter was designed by OKE Services in 1995 and distributed to the European market by NKF Kabel. It was later re-branded by Trace (aka Xantrex, aka Schneider Electric) for sale in the US market as the Trace Microsine. • The final version was the OK4All which improved efficiency but still no cigar. The OK series was discontinued in 2003.
All of these early micro-inverters essentially came to nothing. It wasn’t until 2008 that a reliable, and commercially viable micro-inverter was developed and made publically available by Enphase called the M175. This was followed by the M190, Enphase’s most popular and widespread design. Three new models from Enphase have subsequently appeared on the market. • The M210 suitable for the SunPower and Sanyo modules and both 72 and 60 cell modules, and the M215 which supports 60 cell modules only.
The D380 which services two modules with one micro-inverter is being discontinued. • Enphase shipped their 1,000,000th micro-inverter in September of 2011. • The groundwork for this true sine wave micro-inverter began in the late 1980’s by Werner Kleinkauf. So the micro-inverter has been a long-time coming. • The reliability of the micro-inverter is eye-popping for an intricate electronic device constantly subjected to heat and moisture. Emphase claims a mean time between failures of 300 years and has a NEMA 6 rating.
Micro-Inverters vs String Inverters • The main advantage to micro-inverters is their ability to maintain a robust and consistent flow of power even with shade on one or more of the panels. A “string” of modules in a micro-inverter array is in parallel rather than series as with a conventional inverter. • Because the micro-inverters service an individual module, the power performance and the overall health of each module can be tracked and monitored in real time.
Monitoring the array with conventional string inverters consist of checking the aggregate output of each string of modules for performance. If there is a single module in a string that is malfunctioning, the installer would need to go on the roof and find the single module that is effecting the string and ultimately the total output of the array. • With Micro-inverters, a bad module can be detected virtually instantaneously and the best part, identified remotely.
String inverters respond to the least efficient module in a string. For instance if a particular module is slightly more resistive, say 5% more resistive than the rest of the modules in a string the entire string will perform 5% less efficiently. • Variations in modules have no effect on the ultimate output of the array since modules with a micro-inverters are independent contributors to the power output. Different types and different manufacturers’ modules can be used in a “string” of Micro-inverters.
When you build a string for a conventional inverter, all of the modules need to at least have the same electrical characteristics and preferably be made by the same manufacturer. • Which brings us to maximum power point tracking. The way maximum power point tracking works (very basically) is to create a resistance or load which then “defines” the amperage that is flowing from the array or from a single module as in the case of a micro-inverter.
In the case of a string inverter, this resistance is in response to the amperage output of the aggregate of modules in the array. Different modules in the string have different mpp’s which means some modules are not performing as well as they could resulting in a loss of power. • Since each micro-inverter contains a maximum power point tracker, each module performs to its maximum best.
Power Optimizers • Power optimizers are essentially micro-inverters minus the capacity to convert DC to AC. • They are a DC to DC device which uses maximum power point tracking to optimize the power output of individual modules. It is usually attached to the back of or close to the module just as the micro-inverters are. • It basically monitors the maximum power point of the string inverter and fine tunes the output of the module to match it.
Power optimizers are particularly useful in situations where components are not well matched or the locations of components are far removed from one another. Also, if the orientation or tilt is slightly different among the modules. Power optimizers can smooth out the differences and maximize output. • SolarEdge is the leader in the field of power optimizers. The company shipped 250,000 of their power optimizers called PowerBoxes in 2010 and is expecting to manufacture 500 megawatts of them in 2012.
There are other manufacturers of power optimizers as well as SolarEdge but none are commercially viable. • There are three varieties of technology that drives power optimizers and they are, buck, boost, and a mixture of buck and boost. • Quoting from Photon magazine 2011, Issue #10, a buck power optimizer, “decreases the output voltage of a shaded PV panel and increases the output current to match the current of unshaded modules in the same series”.
Buck devices such as Azuray Technologies and Tigo Energy make, are “most effective in PV systems where shade or mismatch occurs only on a few PV panels. In this case, the buck converter is installed only on those PV panels experiencing shade.” • Boost technology, “increases the module voltage to match the voltage requested by a central inverter optimizing its efficiency”. • The boost technology is used by Eiq Energy, ST Microelectronics, and Sunvision.
SolarEdge and Ampt use the mixed technology of buck and boost which, “can either increase or decrease the output voltage of a PV panel”. • The installer can then either install the SolarEdge PowerBox optimizer on just the shaded modules or on all the modules in a parallel string.
Designing with Micro-Inverters • Micro-inverters have branch circuits analagous to string inverters’ strings. The difference being the modules are in parallel not series so the voltage exiting the micro-inverter remains constant at 240V or 208V depending on the type of system being fed and the amperage adds. • The amperage exiting the micro-inverter is about 1A depending on the type of micro-inverter.
To build a branch circuit of modules the installer needs to check the micro-inverter cut sheet to find out the maximum number of modules in a single branch circuit. In a 240V M215 the maximum is 17 modules. In a 208V M215 the maximum is 21 modules. • The manufacturer recommends a 20A double pole breaker for interconnection regardless of 208V or 240V. • The next question is, “is the module compatible with the micro-inverter?”
On the Emphase website there is a compatibility list about 22 pages long with every conceivable module on it. • Make sure the module and micro-inverter are compatible. The micro-inverter can be damaged if the module is not compatible with it. • As explained earlier in the discussion, Modules come in 60 cell and 72 cell varieties. • The M215 is for 60 cell modules. • The M190 and M210 supports 60 cell as well as 72 cell, Sanyo, and SunPower modules.
The maximum wattage modules that the Enphase micro-inverters will tolerate is 280W. • Just as a design tip, it is best to use the highest wattage module that the micro-inverter can take.
Cost • The cost of a micro-inverter system is approximately the same as a string inverter. The difference is in the time it takes to install. • The micro-inverter installation is about one to one and a half days shorter than the string inverter installation. • Shorter time means more overall profit. • Less time on the roof means less exposure to fall hazards for the crew.
Installing Micro-Inverters • The initial installation of micro-inverters starts when the rails for the modules are installed. Once the rails are completed, the micro-inverters are bolted to the rails in the center of where each module will fall. • Starting with the end of the first rail the first micro-inverter gets bolted to the rail half the distance of one side of the module. This distance depends on whether the module is being mounted in a portrait or a landscape orientation.
The newest way to install the AC cabling for the M215 micro-inverters calls for a single, long AC cable running along the rail to which each micro-inverter is connected. Enphase provides special clips to append the cable to the rail. Bring cable ties just in case. • Previously the M190 connectors were daisy-chained along the rail. One micro-inverter plugging into the next micro-inverter and so on to the junction box. • The new way gives the installer a little more latitude in where the micro-inverter can be placed and still allows for easy connection to the main AC cable.
M190 Connection Setup (Older Type) Tyco Module Connectors AC Cable Connectors
New Type of AC Cable Connection (120V/208V) Cap which fits over micro-inverter connection point during shipping
This is the Enphase 215 Micro-inverter. Notice the plug configuration where it fits into the large AC cable. Notice also that it takes a single bolt to bolt it to the rail rather than two which the older models required.
Normally there are leads with connectors from the micro-inverter where the modules attach to it. • The picture doesn’t show them presumably because of the two types of connectors in general use, the MC connectors and the Tyco connectors, and I’m guessing Enphase did not want to endorse one over the other. • The installer needs to know what type of connector will be on the end of the module leads before ordering the micro-inverter so the two connectors will be compatible.
Let me stop here for a moment and say, that I am not trying to endorse or sell Enphase micro-inverters or any particular product. At the moment Enphase dominates the market and is typical of the way micro-inverters install and operate. • There is the CyboInverter from Cybosoft and the Enecsys micro-inverter SMI-D360W-72-UL. The Cybosoft inverter has a maximum capacity of four modules per unit. • Direct Grid Technologies has a UL1741 listing for their DGM460 micro-inverter.
At this point in the installation, each micro-inverter is ready to be plugged into the main AC cable. One end of the cable is capped off and the other is inserted into a junction box on the roof to be connected to the wires in conduit going down to the AC disconnect if the installer choses to install one and then on to the main load center. However, the 20A breaker in the main load center is perfectly fine to use as the AC disconnect. • One more step needs to be completed before the installation and electrical connection of the modules.
Each micro-inverter comes from the manufacturer with a serial number imprinted on two stickers. One stays with the micro-inverter on the roof and the other is placed on a layout diagram of the array once the rails and micro-inverters have been installed. • The diagram indicates where each micro-inverter is located on the roof. This is used to upload information about the array to Enphase via its internet program called Enlighten.
Once the information reaches Enlighten it is used to track the performance or indicate a problem with each individual module and its specific location on the owner’s roof or ground mount. • It is the installer’s responsibility to do the uploading to Enlighten.
After all the duplicate micro-inverter stickers go on the diagram the modules can then go on the rails and be electrically connected to each micro-inverter. • Electrical connection of any of the components can be done in any sequence such as the modules to the micro-inverter or the micro-inverter to the AC cable or the AC cable to the wires in the junction box on the roof. • Since the micro-inverters are UL1741 listed, no current flows until five minutes after the micro-inverters are connected to the active utility grid.
That completes the installation. The installer is now ready to upload the array information and track the array performance through the Enphase gateway device called, Envoy.
There are two components to the data collection for the Enphase micro-inverters; Enlighten which is the internet program on the Enphase website and the Envoy which is the datalogger. The Envoy collects the data from the array and conveys it to Enlighten and/or displays it directly. • It’s not necessary for data collection with the Envoy to have an internet connection.
If the customer does have the internet in their home they will be able to go to www.enphase.com and bring up their data. If there is no internet available at the array site the customer will not be able to use the full range of data collection offered by Enphase. • Initially, the installer uses the company log-in to upload the customer’s array information to the Emphase website. Enphase then provides the customer with a user name and a password to access their data. The customer receives an email with the Enlighten user name and password.
If the customer does not have the internet, the limited data from the array can be read directly off of the Envoy gateway display. • The Envoy as well as access to the internet data is the least expensive of all residential data acquisition systems at around $300. • Installation of the Envoy is extremely easy. All it requires is to be plugged into an ordinary wall outlet. The array data is transmitted via the residential house wiring to the Envoy. • Hooking up to the internet requires an ethernet cable from the Envoy to the home’s wireless or wired router.
Once the customer is on the internet they can view their installation as real time performance or accumulated performance per month, per day or the lifetime of the system. • Also, data can be accessed for each module just by clicking on the module on the diagram.
