1 Non Contact Electrical Characterization of PV Films Review of conductivity/dielectric/insulation resistance testing • Direct Current (DC) Insulation Resistance Standard Test (IEC 62788-1-2, 2014 developed for PV insulators) • Alternating Current (AC) Insulation Resistance Test (ASTM D149 modified for printed circuit boards dielectrics) • Brief intro to polarization and conduction processes in dielectrics • real and imaginary AC conductivity • correlation between DC and AC conductivity Non-contact AC conductivity measurement using a 7 GHz resonant cavity • AC conductivity of PET – effect of exposure to environmental conditions • Estimation of the corresponding insulation resistance change Jan Obrzut, NIST, MML, Div 642 ([email protected]) Contributors: Xiaohong Gu, NIST, EL 3 rd Atlas/NIST Workshop on Photovoltaic Materials Durability December 8 and 9, 2015
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Non Contact Electrical Characterization of PV Films...Non-contact AC conductivity measurement using a 7 GHz resonant cavity • AC conductivity of PET – effect of exposure to environmental
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Non Contact Electrical Characterization of PV Films
Review of conductivity/dielectric/insulation resistance testing • Direct Current (DC) Insulation Resistance Standard Test
(IEC 62788-1-2, 2014 developed for PV insulators) • Alternating Current (AC) Insulation Resistance Test
(ASTM D149 modified for printed circuit boards dielectrics) • Brief intro to polarization and conduction processes in dielectrics • real and imaginary AC conductivity • correlation between DC and AC conductivity
Non-contact AC conductivity measurement using a 7 GHz resonant cavity
• AC conductivity of PET – effect of exposure to environmental conditions • Estimation of the corresponding insulation resistance change
Requirements for the design qualification and approval of terrestrial photovoltaic modules suitable for long-term operation in general open air climates
Insulation resistance passing criteria a) the degradation of maximum output power does not exceed the prescribed limit after
each test nor 8 % after each test sequence; b) no sample has exhibited any open circuit during the tests; c) there is no visual evidence of a major defect, as defined in Clause 7; d) the insulation test requirements are met after the tests;
R >= 400 MΩ if area < 0.1 m2, Dielectric withstanding voltage 1000 V for 1min R *area >= 40 MΩ m2 at 500 V if area > 0.1 m2
e) the wet leakage current test requirements are met at the beginning and the end of each sequence and after the damp heat test;
f) specific requirements of the individual tests are met.
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HV DC Insulation resistance test ( IEC 62788-1-2, 2014)
S
sCRt
i Rve
RVti Si +≈ − /0)(
i(t) V0
Ri
RS CS
is RR >>
Rs Cs
Ri V0
itest =V0/Rs ≈ 2pA, Rs = 103 V/(2 x 10-12 A) = 5 x 1014 Ω ρ = Rs As / ts = 2.2 x 1017 Ωcm
Rs-field (0.1 m2) = 2.2 x 1017 Ωcm *0.046 cm /103 cm2 ≈ 1013Ω ρmin (Rs-min-field = 4 x 108 Ω, ) ≈ 8.6 x 1012 Ωcm
(V0/Rs )max-test ≈ 5 x 10-8 A, will get there in about 1.3 105 cycles, if linear conditions stay
0
200
400
600
800
1000
1200
-2
-1
0
1
2
3
4
0 1 2 3 4 5
Appl
ied
Volta
ge (V
)
Curr
ent (
pA)
Time (h)
Example Data
Current (pA) Voltage (V)
SRV0
As =20.5 cm2
ts = 0.046 cm
Step voltage stimulus – current decay response in time domain
transient specimen current
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Pros: • Simple instrumentation, easy visualization • Widely accepted • Large data base of performance (failure rate) in commercial applications
(oldest standards , R. Bartnikas, 1982 )
HV Direct Current Insulation Resistance Test
Cons: • Long testing (electrification) time to read the steady state leakage current is-∝ =
V0/Rs • Arbitrary acceptance criteria, often based on historic performance (R> 400 MΩ) • All signals are transient until the steady state conditions are reached, then vs = V0 • Test results are difficult to link with the physical mechanism of aging and reliability
projection (the current decay and kinetics of charging are consequence of several processes acting simultaneously: dipolar polarization, charge transport, space charge )
Step voltage stimulus – current decay response in time domain
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Alternating Current (AC) Insulation Resistance Test
)()i( 0 ϕω += tRVt
isin
0 5 10 15 20
-600-400-200
0200400600
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
V 0 (V)
t (ms)
V0 VRi
V Ri (V
)
)1(i −= ∗
∗∗
RiS R
VVZ
o5892 .−≈= τ∆πϕ f
Sinusoidal voltage stimulus – alternating current response at single frequency
in time domain )()v( 0 tVt ωsin=
Rs
∗SZ -complex impedance
sC
Ri ∗RiV
∗V
In frequency domain ))((0 ϕω +=∗ tjVV exp
-89.3
-89.2
-89.1
-89.0
0 500 1000 1500 2000 250044.5M
44.6M
44.7M
44.8M |ZS|
|ZS|
(Ω)
Voltage (V)
Phas
e φ (d
egree
s)
φ
ϕ= -90° real (ideal) capacitance 0> ϕ >-90° complex cap with loss (ZS*) ϕ = 0 real impedance , ZS = Rs
Under UV, Temp stress conditions Humidity dramatically accelerate loss of insulation resistance. Mechanical elongation creates additional conducting paths.
Ambient sunlight
50 days
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Tensile test results
0.0 %
0.48 %
0.65 %
1.07 %
1.78 %
17.60 %
(a) (e)
(f) (b)
(c)
(d)
Crack density increases with RH
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• Direct Current (DC) High Voltage Insulation Resistance Test: step voltage – current decay in time domain easy to visualize, commonly used
• Alternating Current (AC) Insulation Resistance Test phasor transform in frequency domain fast, physical mechanism of charge transport at high frequencies eliminates ambiguity with ionic current (redox process)
• Demonstrated non-contact resonant cavity test method for conductivity of PET samples. The method operates at 7 GHz and is sensitive to aging effects caused by the UV, T, RH ambient and accelerated stress. No direct correlation with the life test.
• sensitive, fast, non-invasive, small specimens (New IEC std developed at NIST).
0 ; )(0 == ′′∞ ετ iiiVR cR ,/
)( )()0()( ωωωω ε CRAC iiii ++== ′′
ACDC RR >
Summary
Thank you!
( ))(|111
S|sin
Zϕ−≈
∗SR
3rdAtlas/NIST Workshop on Photovoltaic Materials Durability December 8 and 9, 2015