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Debugging and verifying microinverters for photovoltaic installations
Testing the microinverters
In the development environment, Enphase
engineers use an SAS to verify power
production and test the voltage compliance
range to ensure they are producing power
at the high- and low-voltage extremes.
They also use it to develop and test MPPT
algorithms.
“The SAS is an integral part of our
engineer’s bench,” said Mark Baldassari,
Enphase director of hardware engineering.
“We have several channels available at
a bench, either using them singly or
paralleling two together to give us higher
current at low voltage.” The parallel
method is necessary because Enphase runs
most tests in the 20-volt range and needs
about 200 watts of power.
In the product verification
environment, Enphase uses the SAS to
make sure its products meet specifications.
Enphase’s engineers use the SAS in
conjunction with an environmental
chamber to ensure products operate
over their full temperature range. The
chamber also includes a vibration/shock
table, which, among other things, lets
them ensure that the microinverter is
maintaining proper compliance with the
photovoltaic array and that the MPPT
algorithm is working correctly.
For long-term reliability tests,
Enphase conducts simulated lifecycle
testing during which they expose the
unit-under-test (UUT) to environmental
conditions it would likely face over several
years. Using the SAS, they simulate the
diurnal variations in illumination while
simultaneously simulating diurnal and
seasonal temperature variation. They
simulate two and a half months per day
and run the tests for two to three weeks.
Static and dynamic testing
Enphase uses the SAS for two types of
tests: static and dynamic. Most of its testing
is done statically, which entails simulating
a particular power point for the solar array
and verifying voltage, current and illumination
of the solar module. For example,
when checking for compliance with CEC
(California Energy Commission) standards,
they test at the minimum, nominal and
maximum rated voltages and at specific
power levels. Each level is one static point.
When the engineers need to
simulate the illumination curve of a
solar module, they use dynamic testing.
The SAS lets them simulate the output
of a solar module over its rated values
for temperature and illumination. A
simulation run starts at low voltage and
low power to simulate cloudy days, then
progresses to high voltage and high power
to simulate sunny conditions.
According to Mark, dynamic
simulation of the illumination curves is
challenging, as is simulating the slow ramp
that occurs at dawn: “Early in the morning,
the solar module will actually produce a
small amount of power just from sky light.
We have to operate in those times too –
below 10 W.”
Enphase microinverters have a special
mode that makes it possible to generate
AC power generation with low input
power – and this is hard to simulate.”
However, Enphase discovered that it can
be done using either an Agilent E4350B
or E4360A, Agilent’s older and newer SAS
units, respectively. “Doing the dynamic
simulation of the illumination is easier
with an E4360As because its response time
is so much faster,” said Mark. “I can load
my user-defined I-V curve tables almost in
real time.”
Measuring MPPT efficiency
Accuracy is extremely important to
Enphase engineers. “An important test
is MPPT efficiency, which measures
the ability of our product to accurately
determine the maximum power production
point of the solar module,” said Mark.
Today, this is an Enphase-internal test;
however, it may soon be mandated as a
regulatory or certification test. Those tests
may require a weighted efficiency rating
that combines “converting efficiency” –
how efficiently the inverter converts DC to
AC – and “MPPT efficiency,” which rates
how accurately the inverter determines the
maximum power point. MPPT efficiency is
the most important: If the unit is operating
at a suboptimum power point, it isn’t
providing the maximum possible power.
To test MPPT efficiency, Enphase
connects an electronic load such as the
Agilent N3306A to the output of an SAS.
They then load the I-V curve table and
switch on the load. A test engineer sweeps
the load through the curve at ±10% or
±15% of MPPT and finds the point of
maximum power production. Next, they
disconnect the load and hook up the
UUT. After the microinverter MPPT
circuit and algorithm find the maximum
power point, the engineer compares the
two results. The MPPT efficiency is the
quotient of those two. Accuracy is very
important, and the E4360A SAS provides
the accurate Enphase needs.
“Using the SAS is the only way we
can develop our MPPT algorithms,” said
Mark. “We get pretty clever running in this
dynamic mode and checking the algorithm
to see if it gets hung up in certain spots.”
Enphase has seen such issues in the field
and has been able to simulate them in
their lab with the SAS.
All of Enphase Energy’s test-system
software was written for GPIB, so they
use that interface for connecting to the
Agilent SAS. “In the future, we may switch
to LAN,” said Mark. “As our other test
equipment evolves, we’ll start doing a
lot more LAN drivers. It’s great that the
E4360A already has that capability.”
Selecting a solar array simulator
Agilent is one of the few companies that
offers a solar array simulator. Enphase considered
designing one on its own – until it
discovered the Agilent product.
“The Agilent SAS could do exactly
what we were looking for,” said Mark. “We
have a lot of solutions, but if we really want
to see the dynamic response of our inverter
and check the MPPT algorithms and
accuracy, then we go to the SAS.”
The fact that the E4350A can be
scaled down to single-inverter size was an
important factor for Enphase.
Win Seipel has been with Agilent/HP
for 40 years in a variety of roles. He has
been a design engineer and has managed
magnetic component design and
manufacturing engineering teams. He
has also been an R&D project manager,
R&D section manager, and R&D
manager. He is currently an applications
engineer focused on aerospace / defense
and specifically solar array simulation
for satellite and terrestrial applications.
Win holds a BSEE degree from Newark
College of Engineering.
www.globalsolartechnology.com
Global Solar Technology – July/August 2009 – 9