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