SAND2013-0748C
Lifetime Testing of Metallized Thin Film Capacitors for Inverter
Applications
Jack Flicker, Robert Kaplar, and Jennifer Granata
Sandia National Laboratories, Albuquerque, NM 87185, USA
oriented polypropylene film as the dielectric due to low cost,
Abstract — In order to understand the degradation low resistance, and high manufacturing consistency [7].
mechanisms and failure precursors of metallized thin film MTFCs are thought to have longer lifetimes than electrolytic
capacitors (MTFC) used in photovoltaic (PV) inverters, we have capacitors for a number of reasons including better
carried out accelerated testing on MTFCs. By understanding the
degradation mechanisms and precursors of imminent
reverse/over voltage handling, smaller internal heating due to
catastrophic failure, implementation of a prognostics and health lower ESR, and clearing behavior.
management (PHM) plan can be used to optimize PV array Clearing is a mechanism found in MTFCs due to the thin
operations and maintenance (O&M), decreasing cost per watt electrode. In this mechanism, during capacitor operation,
towards the US Department of Energy goals. defects in the dielectric film will short and locally heat at
Index Terms — PV systems, inverter reliability, capacitors. some applied voltage less than the global breakdown voltage
of the device. The nm-scale electrode layer will quickly heat
I. INTRODUCTION via Joule heating and vaporize, isolating the local defect
region through the creation of a pinhole (~5-8 mm2 [8]) in the
In PV inverters, the combination semiconductor switching electrode. The capacitor suffers a small decrease in
and PV array source inductance results in an additional AC capacitance, but lifetime is increased by avoiding catastrophic
component injected onto the nominally DC bus. This AC failure. Eventually it is assumed that the capacitor suffers a
component is known as voltage ripple (Vripple) and exists “soft” failure after capacitor degradation yields a certain
throughout the inverter/module circuit. The PV module is percentage of capacitance degradation (the definition of soft
hypersensitive to Vripple as voltage ripple dramatically reduces failure varies by manufacturer and application).
available output power [1]. Due to soft failure, MTFCs are thought to be inherently
In order to limit this voltage ripple, each inverter requires an safer than electrolytics and offer a clear solution to inverter
energy storage element (i.e. a capacitor) [2]. Many consider reliability [9-12]. However, as more clearing events occur, the
these DC bus capacitors to be the weak link in inverter production of vaporized metal increases pressure inside the
reliability [3] due to constant temperature and power cycling casing. This increase in pressure tends to increase the rate of
and high internal capacitor temperatures . According to degradation [13] as localized failure in one area leaves
SunEdison, over a 27-month period, capacitor issues were neighboring areas ripe for further electrical breakdown (Figure
responsible for 7% of inverter energy loss due to maintenance 1) [8]. The eventual build-up of metallized vapor can cause
and downtime [4]. Therefore, a thorough knowledge of catastrophic failure either through the pressure increasing
capacitor degradation and failure is important to both enough to burst the capacitor casing [14] or the vaporized
increasing capacitor lifetime through improved inverter design metal concentration increasing enough to become conductive
and optimizing O&M to decrease PV array costs. and cause a flashover event [15].
Historically, electrolytic capacitors have been used as the
bus capacitor due to their low cost and high capacitance per
volume. Unfortunately, the use of liquid electrolyte means that
these types of capacitors tend to suffer from high instability
with temperature and time and large equivalent series
resistance (ESR), yielding short lifetimes and the tendency to
catastrophically fail while releasing H2 gas [5].
In recent years, inverter manufacturers, especially those in
utility scale systems, have been moving towards MTFCs.
These capacitors consist of a dielectric plastic film (10-100 Figure 1: Typical capacitance degradation over time of an MTFC.
m), which is metallized on both sides with Al and/or Zn Clearing events result in a steady decrease in capacitance until soft
(~20-100 nm) in pure or alloyed states to form electrodes [6]. failure (A, C=2% in this case). Sometime after soft failure, the
Though a variety of polymer film dielectrics can be used, the degradation rate increases (B) until the conductive vapor inside the
majority of high performance capacitors utilize biaxially MTFC leads to catastrophic failure (C).
In this work, we conduct accelerated testing on MTFCs to voltage for 30 minutes. After the 30-minute holding period,
demonstrate increased degradation rates over time and the capacitor was discharged through the resistor bank in
eventual catastrophic failure. The electrical performance over under two minutes. Once the capacitor is fully discharged,
time of stressed capacitors in analyzed in order to determine five averaged measurements of the capacitance and ESR are
precursors to MTFC failure. By understanding the precursors taken using the LCR meter. The capacitor is then charged and
of failure, it is possible to develop a suitable PHM scheme to the process is repeated until failure.
decrease O&M costs for a PV array and therefore decreasing
the cost per watt of PV energy. III. RESULTS AND DISCUSSION
II. EXPERIMENTAL PROCEDURE After 901 hours of testing at 900V and 85oC (1,513 hours
total), the capacitor demonstrated catastrophic failure (Figure
To safely test the high voltage, large capacitance (1 kV, 3). As a result catastrophic failure, the capacitor packaging
1mF) typically used in PV inverters, a test setup was was breached and the encapsulant can be seen. Although it
constructed as shown in Figure 2. The capacitor under test is cannot be determined using this setup if catastrophic failure
enclosed in a two-box design. Once the outer plexiglass box was due to pressure build-up or flashover, the expansion of
is breached, an autoshunt circuit discharges the capacitor via a encapsulant indicates internal heating of the capacitor, which
brass bar through a 5k (100W) resistor before a user can would indicate failure due to flashover.
contact the capacitor inside the inner plexiglass box.
The capacitor under test is stressed by temperature and/or
voltage through the use of a Kiethley 7001 high voltage switch
matrix card. Temperature control is provided by an Omega
CN740 temperature controller with an SRT051-40 tape heater
and an SA2C-J J-type thermocouple. DC voltage stress is Figure 3: (left) 800V, 600F MTFC before accelerated test. (right)
provided by a Kiethley 2410 power supply. Electrical After accelerated testing, MTFC packaging has burst in a
measurements of capacitance and ESR are taken via an catastrophic failure and encapsulant has expanded out of the can.
Agilent 4263B LCR meter.
To safely measure the electrical characteristics with an LCR The results of electrical measurements during testing of
meter, the capacitor must be completely discharged through a capacitance and ESR over time are shown in Figure 4 and
bank of resistors. In order to minimize discharge time while Figure 5, respectively. The capacitance data looks similar to
still limiting current through the switch matrix (10VA max), the degradation that is expected from a MTFC. There is a
resistance steps of 100k, 50k, 4.5k, and 20m were used long period of slow, steady degradation following by a period
by the combination of multiple resistors in parallel. of accelerating degradation. Shortly after the capacitor passes
the degradation failure point (defined by the manufacturer as
98% of original capacitance), the capacitor catastrophically
fails.
Figure 2: Circuit schematic of capacitor accelerated test setup. The
user is protected with a dual Plexiglas box system with automatic
Figure 4: MTFC capacitance over time during an accelerated
shunt and both electrical and mechanical interlocks. The capacitor
lifetime test (900V, 80oC). The grey box indicates faulty reading due
can be accelerated by voltage and temperature.
to a loose connection. Following degradation failure (dashed line)
the capacitor catastrophically failed.
In order to test the electrical degradation of a MTFC under
voltage and temperature stress, a 800V, 600mF polypropylene In addition to the characteristic degradation of the MTFC
capacitor with a listed lifetime of 200,000 hours at rated capacitance, the ESR also demonstrates degradation. For the
voltage and 60oC (5,000 at rated voltage and 85oC) was tested much of the testing period, the ESR stays below 10m. As
at voltages of 850 and 900 V and temperatures of 25 oC, 50oC, the period of accelerating capacitance degradation occurs
and 80oC. (approximately 1,200-1,400 hours), the ESR increases
The capacitor under test was charged to the holding voltage
dramatically to around 38m.
with a maximum current flow of 10mA and held at that
This increase in ESR is closely tied to the accelerating
capacitance degradation due to a positive feedback REFERENCES
mechanism. A larger ESR yields more internal heating during
charge/discharge or in response to DC bus ripple. The internal [1] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, "A Review of
heating yields more degradation, which increases the ESR as Single-Phase Grid-Connected Inverters for Photovoltaic
the process repeats itself, eventually leading to failure. In Modules," IEEE Transactions on Industry Applications,
addition to being closely tied to the degradation of the vol. 41 (5), pp. 1292-1306, 2005.
[2] C. Siedle and V. D. Ingenieure, Comparative Investigations
capacitor, ESR is an easier quantity to measure in situ than of Charge Equalizers to Improve the Long-Term
capacitance. Therefore, with appropriate sensor data, a large Performance of Multi-cell Battery Banks vol. 245: VDI
ESR increase could be used in a PHM scheme to signal Verlag, 1998.
imminent catastrophic failure of an MTFC used in the field. [3] Y. Xue, K. C. Divya, G. Griepentrog, M. Liviu, S. Suresh,
and M. Manjrekar, "Towards Next Generation Photovoltaic
Inverters," 2011 IEEE Energy Conversion Congress and
Exposition (ECCE), 2011.
[4] A. Kaushik and A. Golnas, "Proceedings of SPIE,"
Reliability of Photovoltaic Cells, Modules, Components,
and Systems IV, 2011.
[5] J. Ho, T. R. Jow, and S. Boggs, "Historical Introduction to
Capacitor Technology," Ieee Electrical Insulation
Magazine, vol. 26 (1), pp. 20-25, 2010.
[6] L. Hua, L. Fuchang, Z. Heqing, D. Ling, H. Yongxia, and
K. Zhonghua, "Study on Metallized Film Capacitor and Its
Voltage Maintaining Performance," IEEE Transactions on
Magnetics, vol. 45 (1), pp. 327-330, 2009.
[7] N. Henze and J. Liu, "Reliability Considerations of Low-
Figure 5: MTFC ESR over time during accelerated lifetime testing. Power Grid-Tied Inverter for Photovoltaic Application,"
The grey box indicates faulty reading due to a loose connection. 24th European Photovoltaic Solar Energy Conference and
Prior to catastrophic failure the capacitor demonstrated an ESR Exhibition, Hamburg, Germany, 2009.
increase. [8] C. W. Reed and S. W. Cichanowskil, "The Fundamentals
of Aging in HV Polymer-film Capacitors," Ieee
IV. CONCLUSION Transactions on Dielectrics and Electrical Insulation, vol.
1 (5), pp. 904-922, 1994.
Capacitors are important to the operation of PV systems [9] L. Gu, X. Ruan, M. Xu, and K. Yao, "Means of
because they control voltage ripple on the DC bus, altering the Eliminating Electrolytic Capacitor in AC/DC Power
operation point of the PV system. Since these capacitors are a Supplies for LED Lightings," IEEE Transactions on Power
significant issue in terms of reliability, there has been a trend Electronics, vol. 24 (5), pp. 1399-1408, Jun 2009.
[10] J.-S. Lai, H. Kouns, and J. Bond, "A low-inductance DC
towards replacing electrolytic capacitors with metallized thin
bus capacitor for high power traction motor drive
film. inverters," Industry Applications Conference, 2002. 37th
Sandia National Labs has built a test setup to perform IAS Annual Meeting. Conference Record of the, 2002.
accelerated lifetime testing of large MTFCs. This testing has [11] R. Mirzahosseini and F. Tahami, "A lifetime improved
resulted in capacitors that demonstrate both capacitance and single phase grid connected photovoltaic inverter," Power
ESR degradation in response to temperature and voltage Electronics and Drive Systems Technology (PEDSTC),
stress. Additionally, MTFCs have been shown to 2012 3rd, pp. 234-238, 2012.
catastrophically fail though continued use past the soft failure [12] J. Worden and M. Zuercher-Martinson. (2009, May) How
point. Future work will consist of testing a larger statistical Inverters Work. SolarPro. 68-85. Available:
sampling of MTFCs in order to develop knowledge of http://xiazai.inktronics.com.cn/Uploadfile/DownFile/
precursors to failure that could be used in a PHM scheme to 10-08-13/100813131907_hjkg.pdf
[13] W. Sarjeant, J. Zirnheld, and F. MacDougall, "Capacitors,"
optimize O&M, thereby decreasing the operating costs of PV
IEEE Transactions on Plasma Science, vol. 26 (5), pp.
arrays. 1368-1392, 1998.
[14] W. Sarjeant, F. MacDougall, D. Larson, and I. Kohlberg,
ACKNOWLEDGEMENTS "Energy storage capacitors: Aging, and diagnostic
approaches for life validation," Ieee Transactions on
This work was funded by the DOE Office of Energy Magnetics, 1997.
Efficiency and Renewable Energy. Sandia National [15] A. Schneuwly, P. Groning, and L. Schlapbach,
Laboratories is a multi-program laboratory managed and "Uncoupling Behaviour of Current Gates in Self-healing
operated by Sandia Corporation, a wholly owned subsidiary of Capacitors," Materials Science and Engineering B-Solid
State Materials for Advanced Technology, vol. 55 (3), pp.
Lockheed Martin Corporation, for the U.S. Department of
210-220, 1998.
Energy's National Nuclear Security Administration under
contract DE-AC04-94AL85000.
