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.
Mastervolt Sunmaster 130S
Micro-Inverter
• All of these early micro-inverters essentially
came to nothing. It wasn’t until 2008 that a
reliable, and commercially viable microinverter 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 microinverter in September of 2011.
• The groundwork for this true sine wave microinverter 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 eyepopping 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 microinverter.
• 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 microinverters 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.
SolarEdge Power Optimizers
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 microinverter.
• 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 microinverters 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.
The Old Method
M190 Connection Setup (Older Type)
Tyco Module Connectors
AC Cable Connectors
Multi-Contact (MC)Connectors
New Type of AC Cable Connection
(120V/208V)
Cap which fits over micro-inverter
connection point during shipping
Cable End Showing Wire
(120V/208V)
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 microinverter 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 microinverter 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 microinverter 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 microinverters 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.
Enlighten and The 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.
Enphase Internet Display
• 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.
Thank you and have a
great and sunny day.