Life Prediction of Aluminum Electrolytic Capacitors Used in Two-Level Inverters

Abstract

Aluminum electrolytic capacitors (AL-CAPs) are widely used in two-level inverter applications with having prominent features such as high capacitance – high voltage ratings, energy storage capability and good voltage regulation performance at low-cost. Reports show that AL-CAPs play a critical role on determining a power electronics system’s life performance. With increasing safety concerns brought by standards, it is crucial to deeply analyze AL-CAPs life performance in advance. For this reason, this paper focuses on the life estimation of AL-CAPs used in two-level inverters. In order to do this, a prototype is implemented and a practical and easy to understand method is suggested for selection and life estimation of AL-CAPs based on measurements on prototype. Life improvement methods are also discussed in the scope of the work.

Full text

Abstract - Aluminum electrolytic capacitors (AL-CAPs) are
widely used in two-level inverter applications with having
prominent features such as high capacitance – high voltage
ratings, energy storage capability and good voltage regulation
performance at low-cost. Reports show that Al-CAPs play a
critical role on determining a power electronics system’s life
performance. With increasing safety concerns brought by
standards, it is crucial to deeply analyze Al-CAPs life
performance in advance. For this reason, this paper focuses on
the life estimation of Al-CAPs used in two-level inverters. In
order to do this, a prototype is implemented and a practical and
easy to understand method is suggested for selection and life
estimation of Al-CAPs based on measurements on prototype. Life
improvement methods are also discussed in the scope of the work.
Keywords - Aluminum electrolytic capacitor, dc link capacitor,
life estimation, equivalent series resistance, fast fourier transform.
I. INTRODUCTION
L-CAPS are widely used in a great number of power
electronics applications such as uninterruptable power
supplies, photovoltaic systems, wind turbines, electric
vehicles, led drivers, motor drivers and etc. When
price/performance characteristics are evaluated in such
applications requiring high capacitance and high withstand
voltage ratings, AL-CAPs stand out as the most preferred
capacitor type. On the other hand, according to the research
results shown in Figure 1, AL-CAPs hold the maximum
distribution of long-term possible failures, which is more than
50%, compared to other power electronic devices [1, 2]. It is
widely accepted that the life of a power electronics system is
equal to the life of the used AL-CAPs [3]. For these reasons,
the selection of AL-CAPs and its life estimation play a critical
role on long-life system design.
Figure 1: Distribution of failure for each power component [2]
In this study, primary wear-out mechanisms for AL-CAPs
are investigated in Section 2. Life determinant factors for AL-
CAPs are explained in Section 3. Based on the measurements
on prototype, recommended life prediction method is provided
step by step in Section 4. Results are evaluated and life
increment methods are discussed at the end of the paper.
II. ROOT CAUSES OF WEAR-OUT FAULTS
AL-CAPs used in power electronics systems are expected to
operate under electrical, thermal and mechanical stresses. AL-
CAPs operating under harsh environment conditions start to
wear out over time and they are unable to maintain their
datasheet properties. The main factor that causes AL-CAPs
wearing out mechanism is the evaporation of the electrolyte
liquid in the internal structure shown in Figure 2. With the
decline of the electrolytic liquid over time, the equivalent
series resistance (ESR) of the capacitor increases, the
increased ESR causes capacitor power losses to increase and
hence internal temperature which further accelerates in the
evaporation rate of the electrolytic liquid [4].
Figure 2: Mechanical construction of aluminum electrolytic
capacitor [4]
Figure 3: Equivalent model of aluminum electrolytic capacitor
The change in ESR according to the amount of electrolytic
liquid in the internal structure of AL-CAP is shown in Figure
4. An old rule of thumb says that the reduction in the
electrolytic liquid from 30% to 40% causes the ESR to
increase 2-3 times from its initial value [4].
Life Prediction of Aluminum Electrolytic
Capacitors Used in Two-Level Inverters
V. SUEL1, H. A. ONAY2, M. K. AKINCI3 and T. OZGEN4
1 Akim Metal Research and Development Center, Istanbul/Turkey, vsuel@akimmetal.com.tr
2 Akim Metal Research and Development Center, Istanbul/Turkey, haonay@akimmetal.com.tr
3 Akim Metal Research and Development Center, Istanbul/Turkey, mkakinci@akimmetal.com.tr
4 Akim Metal Research and Development Center, Istanbul/Turkey, tozgen@akimmetal.com.tr
A
International Conference on Engineering Technologies (ICENTE'18)
October 26-28,2018,Konya/TURKEY
_________________________________________________________________________________________________________________
E-ISBN: 978-605-68537-3-9
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Figure 4: ESR dependence on the amount of electrolytic liquid [4]
In general terms, AL-CAPs have three distinct indicators of
the completion of its useful life. Standard [5] suggests that if
any of these three conditions begin to occur, AL-CAPs should
be considered as failed and they should be considered as
unusable:
▪ 20% decrease in capacitance below its initial value
▪ 2-3 times increase in ESR over its initial value
▪ Considerably increase in leakage current
After completing AL-CAPs useful life, the power system
will not be able to provide the expected hold up time. Voltage
fluctuations on AL-CAPs will become so high that its average
value will fall significantly, especially at low line. The
switching elements in the inverter structure (IGBTs,
MOSFETs, etc.) will start to produce more and more heat over
time and total system efficiency will deteriorate significantly.
Eventually, power system will be unusable in a very short of
time [6].
According to the above mentioned critical points, it is
obvious that AL-CAPs directly affect the whole system’s life
performance. For these reasons, selection and life estimation
of AL-CAPs are very important for a reliable and long life
design. In the literature, capacitor life estimation methods are
generally based on capacitance and ESR monitoring, which
are seem to be impractical, rather complex, time-consuming or
quite expensive [1, 7, 8]. In this study, a practical and easy to
understand method is suggested to estimate the life of the
selected AL-CAPs used in two-level inverters. By
implementing the suggested life estimation method, the
information on replacing time of the capacitors are provided
for the users before aforementioned situations occur.
III. LIFE CALCULATION PARAMETERS
The simplest AL-CAP life calculation method is based on
the assumption that life is doubled with every decrease of
temperature by 10°C [6]. Life estimation formula based on this
assumption is given by equation (1):
10/)TT(
0estimated operatedrated
2*LL −
=
(1)
▪ Lestimated: Estimated life
▪ L0: Rated life
▪ Trated: Maximum operating temperature
▪ Toperated: Case temperature under working conditions
Equation (1) is a practical way to get an idea of life,
whereas it gives an erroneous result since it is lack of the
information of current harmonics and operating voltage. In this
study, a life estimation method is presented by considering
current harmonics, temperature and operating voltage effects
to get more reliable and accurate result.
A. Effect of Current Harmonics on AL-CAPs life
In all two-level inverter structures shown in Figure 5, the dc
bus faces with high frequency ripple current due to high
frequency pulse-width modulation (PWM) swtiching of the
inverter side and low frequency ripple current from the source
side [9]. For this reason, dc bus capacitor current frequency
spectrum consists of low frequency (100 Hz and its multiplies)
and high frequency (2*fsw, its multiplies and side-bands)
components. All harmonic contents have an effect on capacitor
power dissipation and its life. Therefore, capacitor life
estimation methods without considering the effects of current
harmonics are not so accurate [10].
Figure 5: General structure of two-level inverters [9]
In order to examine the current harmonics effect on life, the
first step is to obtain all 100 Hz referred values of each
harmonic contents via the datasheet graph shown in Figure 6.
Thereafter, total 100 Hz equivalent rms current value should
be calculated using equation (2). The ripple current factor,
RCF, is then calculated using equation (3).
Figure 6: Frequency factor of permissible ripple current [10]
)I...II(I 2Hz100,n
2Hz100,2
n
1
2Hz100,1Hz100,AC +++

=
(2)
rated
Hz100,AC
I
I
RCF =
(3)
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B. Temperature Effect on AL-CAPs life
According to the Arrhenius Law, the life of the aluminum
electrolytic capacitor is calculated based on the assumption
that it is doubled with every decrease of temperature by 10°C.
[3]. In order to apply this assumption, the rms value of the
current flowing through the capacitor in any operating
conditions must be smaller than the ripple current rating. The
ripple current ratings are usually given in the datasheets at the
maximum ambient temperature (85°C or 105°C). The
temperature multipliers are needed, as shown in Figure 7, to
find the maximum permissible rms current value for the
working conditions. If the calculated rms current value under
operating conditions does not exceed the ripple current rating,
the capacitor life cycle according to the Arrhenius Law can be
applied to get an idea of life for the selected capacitor [6].
Table 1: Example of Temperature Multipliers [11]
Temperature [°C] 40 60 85
Multiplier 1.89 1.67 1.0
In this study, as a worst case, capacitor core temperature is
recommended as a reference temperature value to make life
calculations more accurate [6].
C. Operating Voltage Effect on AL-CAPs life
Generally, operating voltage has a less effect on AL-CAPs
life cycle performance compared to the current harmonics and
core temperature effects. It is generally considered that
operating voltage does not effect the life for small size radial
type capacitors. On the other hand, operating voltage has an
effect of capacitor life cycle for medium and large size snap in
and screw type capacitors [12]. When investigating the
operating voltage effect on AL-CAPs life, it is necessary to
find the Kv coefficient by equation (4):
n
X
R
vU
U
K







=
(4)
▪ Kv: Voltage factor
▪ UR: Rated voltage
▪ UX: Operating voltage
▪ n: Exponent
25.1
U
U
00.1
X
R









→
5n =
00.2
U
U
25.1
X
R









→
3n =









X
R
U
U
00.2
→
1n =
IV. APPLICATION EXAMPLE
In this section, for an easy understanding of the
recommended AL-CAP life estimation method, step by step
calculation is examined based on the input parameters for a
two-level inverter application in Table 1.
Table 2: Two-level inverter design inputs
Selected DC bus capacitor 1500 µF 400 V
Supply Voltage 220 V-rms, 50 Hz
AL-CAP Operating Voltage 320 V-dc
Inverter switching frequency 10 kHz
Control Method Space Vector PWM
(SVPWM)
Cooling Natural Cooling
Selected DC bus capacitor is 1500 µF, 400 V aluminum
electrolytic capacitor with screw terminals of part number
B43455 from EPCOS [13]. For the life calculations,
measurement setup in Figure 8 is established. The schematic
view of the measurement setup is also shown in Figure 9. The
used equipments are listed in Table 2.
Figure 7: Test setup
Figure 8: Schematic view of test setup
▪ CH1: Line voltage, Uline
▪ CH2: DC bus capacitor voltage, Udc
▪ CH3: Line current, iline
▪ CH4: DC bus capacitor current, icap
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Table 3: Equipment list
Input source AC Grid; 220 VAC, 50 Hz
Oscilloscope Yokogawa / DLM2034 / 350 MHz
Oscilloscope
Voltage Probe Yokogawa / 701927 / 150 MHz High
Voltage Differential Voltage Probe
Current Probe Yokogawa / 701928 / 50 MHz Current
Probe
In order to measure dc bus capacitor current accurately, the
current probe should be demagnetized before measurement. In
addition, the current path length should be as short as possible.
The recommended connection for dc bus capacitor current
measurement is shown in Figure 10.
Figure 9: Measurement connection of dc bus capacitor current
An oscilloscope view of the measurement is shown in
Figure 11. Harmonic spectrum of the dc bus capacitor current
is also shown in Figure 12. According to the Figure 12, current
harmonics frequency spectrum is well coincided with theory,
which consists of low frequency harmonics (100 Hz and its
multiplies) coming from the input side and high frequency
harmonics (20 kHz and its multiplies) coming from the
inverter side [9].
Figure 10: Osciloscope view of measurement setup
Figure 11: Harmonic spectrum of dc bus capacitor current
Equivalent 100 Hz values for each harmonic current are
calculated by using frequency factors in Figure 13. Total rms
current value referred to 100 Hz is calculated as 2.90 Arms via
equation (2). Thereafter, ripple current factor is obtained as
0.54 via equation (3). Finally, the voltage factor is calculated
as 3.05 according to Table 3 values and equation (4).
Figure 12: Frequency factor of permissible ripple current
Table 4: Measured and calculated values for current harmonics
Harmonic
Frequency
[Hz]
RMS
Current
Value
[mA-rms]
100 Hz
Referred
Coefficients
100 Hz Referred
RMS Current
Values
[mA-rms]
100 1750 1,00 1750
200 1600 1,15 1391
300 1390 1,25 1112
400 1135 1,30 873
500 840 1,33 632
600 590 1,38 428
700 360 1,40 257
800 175 1,42 123
20.000 1200 1,45 828
40.000 250 1,45 172
60.000 230 1,45 159
80.000 150 1,45 95
100.000 80 1,45 55
Low frequency
harmonics
High frequency
harmonics
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EPCOS provides useful life curves in capacitor datasheets
[10]. By entering calculated ripple current factor value to the
vertical axis of Figure 14, the nominal life can be obtained for
various core temperatures. As a result, the life of the selected
capacitor is obtained by multiplying voltage factor and
nominal life value which is given in Table 4.
Figure 13: Useful life curve of selected AL-CAP [13]
Table 5: Life calculation results
Operating
Core
Temperatures
[°C]
Life
Prediction
[k-hours]
70 114
80 60
90 24
V. EVALUATION OF THE RESULTS
In this study, instead of the complex and costly life
estimation methods of dc bus capacitor in the literature, a
practical and easy to implement method using the information
of capacitor current harmonics, core temperature and operating
voltage is presented. For this purpose, the life estimation of a
selected AL-CAP used in a two-level inverter application is
performed. According to the results, the most dominant factors
for determining the life are observed as current harmonics and
core temperature. In the future studies, input line filter effects
to reduce current harmonics and forced cooling method (fan,
heatsink etc.) to reduce core temperature on AL-CAP life will
be investigated to improve the life performance of AL-CAPs.
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[10] “Aluminum Electrolytic Capacitors”, EPCOS Data Book, 2017.
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[13] EPCOS, B43455 Series Aluminum Electrolytic Capacitor Datasheet.
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