Energy harvesting in PV systems: micro-inverter vs. micro-converter
Both µConverters and µInverters have the
potential to achieve dramatic manufactur-
ing cost reductions in the power conver-
sion circuitry due to the ability to produce
very large numbers of identical units in
fully automated manufacturing lines. It
has been suggested that at sufficiently low
power, or fine-grained levels, this technol-
ogy could be compatible with integrated
circuit or nanotechnology manufacturing
techniques, with resulting price break-
throughs. While somewhat improbable,
it could at some intermediate level be
possible, and certainly the propagation of
safety features such as ground-fault detec-
Figure 5. SolsticeTM DC-DC converter.
tion and isolation into the array would be
extremely valuable. A notional µConverter
As newer technologies permit reliable, of the DC-DC converters. This reduces array based on the Solstice™ architecture
cost-effective operation at the module the required wire gauge, saving installed is shown in Figure 7, where the individual
level this will inevitably be introduced. cost, and reduces the inherent voltage drop outputs are connected in parallel at a
Meanwhile, a trend in the panel industry between the array and inverter. voltage near the maximum permitted for
to develop higher voltage and higher Solstice allows the physical array that type of system. The output stage of
power panels, makes the Solstice™ style layout and electrical architecture to stay the µConverter power circuit has rectifier
system increasingly attractive. Solstice TM the same, but moves the MPPT and the diodes which function as blocking diodes,
combines the best features of central and control of the DC voltage out of the guarding against back feed from the
distributed power conditioning. It is a central inverter and into the array. This line. The ability to extract the maximum
distributed energy management and power allows the individual solar panel strings to amount of power from each PV module,
conditioning system that individually operate at their best power point without the ability to use different PV modules,
controls the energy production at the solar being affected by any other part of the even mixing different PV technologies,
array string level while feeding the entire array. Instead of forcing each solar panel and the ability to instrument each module
array output through a single, central in a large array to have its operating points for power, temperature and other charac-
inverter. compromised by paralleling losses due teristics to achieve a fine grained view of
String level energy management to mismatch, soiling, or partial shading, array performance, are all very powerful
is accomplished by moving the MPPT these impacts are significantly reduced impacts when considering the total cost of
function to the array sub-combiner box or eliminated. By separating the array a system.
where individual array strings are normally into string level increments of power, the
paralleled for connection to the central magnitude of almost all system ground System level comparison
inverter. Once the string level MPPT is faults is reduced to the level of string In this section, comparison will be done
achieved, DC-DC converters then connect currents. The ability to sense, identify and on system level: µInverters vs µConverters
the paralleled strings at a constant output mitigate ground faults as they occur within with central inverters. Critical points will
voltage level to Solstice’s highly optimized strings or sub-strings, rather than across the be investigated such as reliability, efficien-
inverter, allowing the inverter to constantly entire PV array, improves system safety and cy, control system simplicity and system
run in its “sweet” spot as it is not required performance. cost.
to constantly track variable input voltages.
Wiring losses are also reduced due to the promise and reality of Manufactur- Calculated reliability (MTBF) vs lifetime
higher, regulated DC voltage at the output ing Scales Any power electronics used, either at the
Figure 6. (left) Conventional system with combiner box; (right) String level conversion—Solstice™.
10 – Global Solar Technology – October/September 2009
www.globalsolartechnology.com
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