Energy harvesting in PV systems: micro-inverter vs. micro-converter
module or string level, for the fine-grained
energy harvesting, whether converter or
inverter based, are presumably all well
designed, use high quality components and
are well manufactured. As such they are
likely to have very high MTBFs. Indeed,
leading µInverter manufacturers claim to
have MTBFs of several hundreds of years
and the µConverters even have higher
MTBFs, since they have fewer components.
However those numbers are for 25ºC
ambient temperature and not the typical
temperature extremes encountered with
outdoor installations near to the PV array.
MTBF is a measure of failure rate during
the useful life period of a product’s life,
with reference to the bathtub curve of
reliability shown in Figure 8, the MTBF
is essentially a measure of how close the
bottom of the bathtub curve is to the floor
Figure 7. Solstice topology.
(x-axis). A 100-year MTBF implies that
1% of the population of product in the
component.
field will fail per year during the useful life
As we said
period. If this were the only consideration
before, central three
then 20-25 year lifetimes would readily be
phase inverter with
achieved. However, the lifetime of a well
µConverters does
designed product, even with an extremely
not have this issue
high MTBF, is invariably dictated by any
since it requires very
inherent, unavoidable weaknesses or domi-
small amount of bus
nant wearout mechanisms that exist in the
capacitance. Film
design or are created in the fabrication
capacitors used for
process.
this application have
In balanced three phase inverters
very high reliability
Figure 8. Typical lifetime bathtub curve.
there are no line frequency energy storage
and no wearout
requirements, so voltage source topologies
mechanisam.
are typically used with a DC-BUS that uses
When it comes to reliability, once
high reliability metalized film capacitors
a product has been made, the system
for the DC-BUS capacitance. Single phase
operating temperature is almost everything.
inverters, as shown in Figure 9, by contrast,
For any well-built design, the easiest path
have a large oscillatory power component,
to increased reliability is lower temperature
 
of:
through either improved thermal design
Figure 9. Single phase inverter.
Power
ALTERNATING
=V
RMS
.I
RMS
.{-cos(2ωt)} (1)
or decreased loss. Most non-mechanical
(or non fatigue) failure mechanisms are
Thermal shock is the final temperature/
where VRMS.IRMS is the output power.
modeled by the Arrhenius equation:
reliability consideration which can typically
It is possible to utilize inductive energy
be controlled in the case of a grid con-
storage, so current sourced inverters,
λ(t)=A*exp(-Ea/R*T) (2)
nected inverter.
or to use active filters to supply this ac
Examining the power electronics
component, and some modern µInverters
where Ea is the Activation Energy of the
solutions of the bulk DC-AC, distributed
use these techniques, however most
failure process, k is Boltzmann’s constant
DC-AC, and distributed DC-DC combined
persevere with capacitive storage using the
(8.617 x 10-5 in eV/K). The constant A is
with bulk DC-AC solutions, with respect
DC-BUS to supply the entire, double line-
a scaling factor. This relationship yields
to temperature, the key consideration is
frequency, momentary power requirement.
the 10˚C, ½ life relationship for electro-
ambient temperature and incident solar
To provide the pulsating energy in a single-
lytics. This relationship also says that the
energy. Shaded environments, sunshields
phase inverter from film capacitors without
physical failure mechanism is based on a
on housings, ability to heat in extremely
an excessively large inverter could also be
diffusion mechanism. The other dominant
cold conditions are all key to minimizing
addressed by control techniques that are
failure mechanisms in power electronics
the operating temperature and the
tolerant of large DC-BUS voltage ripple.
are due to fatigue, the driver for which is
temperature extremes. In general the
Failing these various alternative approaches
thermal cycling (ΔT) which causes thermal
environment on the back of a module is
it is standard to use the traditional voltage-
expansion and contraction, and also the
likely to be the most challenging in a solar
sourced approach. To be cost effective the
mismatches between the coefficients of
PV field.
capacitors consist principally of electrolytic
thermal expansion of bonded materials
capacitors, which are prone to accelerated
resulting in cyclical interface stresses.

wearout due to the moisture within the
12 – Global Solar Technology – October/September 2009
www.globalsolartechnology.com
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