Renewable Energy Installer February 2014 | Page 23

The ugly truth
Ivan Saha, president and chief technology officer at Vikram Solar, explains what
other module manufacturers are said to deny: The phenomenon of Potential
Induced Degration
What is PID?
Potential Induced Degradation
(PID) is principally observed on
the negative terminal end of
strings connected to floating-
ground PV inverters and can
cause severe degradation in
power output in modern PV
power plants (ground mounted as
well as rooftop). The phenomenon
is caused by migration of mobile
ions from the glass to the p-n
junction of the solar cells under
high negative bias. The current
protocols of module qualification
under IEC 61215/ IEC 61730 do
not include any test to evaluate
this degradation. A special test is
being contemplated under a draft
standard IEC 62804 to address
the issue.
Causes and how to control it
Over the last decade, internal
device construction of crystalline
solar cells has changed for the
sake of efficiency enhancement.
This modification coupled with
the use of thinner encapsulant
(EVA) of low volume resistivity
has contributed to making the
module more susceptible to PID.
At a system level, string voltage
has increased from 600V to 1000V
and beyond over the last decade.
This has also increased the risk
of modules getting affected with
PID. A recent trend in using
‘transformerless’ inverters has
also added to this risk. The main
causes of PID can be attributed at
three levels:
Cell level
It has been observed that higher
emitter sheet resistance (and a
resulting lower emitter thickness)
increase the chances of PID. With
an appropriate combination of
anti-reflection coating (SiNx)
ratio of metal ions in the module
glass have a direct impact on PID.
System level
The use of inverters with a
ground connection reduces the
effect of PID. In floating ground
proprietary to Vikram Solar and
shows a good correspondence
to the Draft Test Protocol from
IEC. Based on this study, a
performance ranking was
applied to various raw material
combinations.
Power degradation (as a percentage) and EL images of modules with the best Cell+EVA
combination as tested by Vikram Solar
thickness and refractive index,
the PID impact on the cell can
be minimised. Apart from that,
a higher base wafer resistivity
of wafer gives a wider depletion
layer at the p-n-junction which is
less prone to PID shunting.
Module Level
The dielectric constant, volume
resistivity and thickness of the
module encapsulant (EVA) are
important parameters that can
impact the effect of PID. Also, the
chemical composition and the
inverters, a negative terminal
grounding kit recommended by
the inverter manufacturer will
ensure that the charge built up
during the daytime is effectively
dissipated during the night.
Evaluation at Vikram Solar
The R&D team at Vikram Solar
carried out tests to understand
the effect of materials, processes
and environment factors on
module PID. This research helped
to set up an internal reliability
protocol. The test protocol is
The best combination
showed a degradation of only
0.08 percent in power output.
PID testing at TÜV Rhienland
according to IEC 62804 (Draft)
standard confirmed that Vikram
Solar modules have a degradation
of 0.2 percent against the pass
criteria of 5 percent. Being the
first and only Indian manufacturer
to achieve this distinction,
Vikram Solar has held seminars
and training sessions to educate
distributors and installers about
the PID phenomenon.
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