INVENTIO5 N
The present invention relates to the field of power converters, and more particularly to a
method for online monitoring of health of a capacitor by comparing the second
harmonic impedance of the capacitor.
10 BACKGROUND OF THE INVENTION
Electric power converters use electrolytic capacitors for filtering the input voltage or the
output voltage. These converters supply DC voltage to electronic systems of equipment
for which any shutdown may be prejudicial. It has been observed that most failures of
these converters are due to malfunctioning of the electrolytic capacitors. Consequently,
15 several techniques or methods are employed for online health monitoring of capacitors.
A simple online technique to determine the health of capacitor for LC filter, is based on
monitoring the capacitor voltage ripples which increases with the rising electrolytic
series resistance (ESR). (Chen et Al.: IEEE Trans. Ind. Electron., vol. 55, no. 1, pp. 400-
20 406, Jan. 2008.). Harada et Al.: IEEE Trans. Power Electron, vol.8, no. 4, pp. 355 -
361(1993) discloses a similar technique to monitor the capacitor ripple voltage due to
capacitor ripple current. If the voltage ripples exceeds the preset limit, it gives the
indication of capacitor failure. The drawback of the above techniques is that under
transient period, the voltage ripples may grow leading to error in detection. Further, the
25 effect of variation in input voltage, load, duty cycle and temperature on the magnitude
of ripple voltage is not considered. This leads to error in the predicted results from the
technique, thereby limiting its use in actual operating conditions.
Further, techniques were developed. Buiatti et al: IEEE Trans. Instrum. Meas., vol. 59,
30 no. 8, pp. 2134-2143, (Aug. 2010) discloses a predictive maintenance technique for
electrolytic capacitor for boost converter. The technique is non-invasive and uses double
3
estimation of ESR and capacitance for higher accuracy. It can be applied online and
even in real time. It does not require to add any additional hardware in the power stage
nor to even slightly modify it. However, to implement this technique converter
parameters are sampled at high frequency, which requires high speed Analogue to
Digital Converter (ADC), thereby increasing the cost5 .
Abdennadher et al: IEEE Trans. Ind. Appl., vol. 46, no. 4, pp. 1644-1652, (2010)
discloses a technique for online health monitoring of Aluminium Electrolytic Capacitor
(AEC) for uninterruptible power supplies (UPSs). The technique uses kalman filter
10 algorithm for ESR and capacitance estimation incorporating the temperature effect.
However, use of kalman filter makes this technique computationally intensive and may
require additional processor/controller for calculations.
Lahyani et al: IEEE Trans. Power Electron. vol. 13, no. 6, pp. 1199-1207, (1998)
15 discloses a predictive maintenance technique for AEC for switch mode power supply.
The software-added approach acquires the voltage and current ripples across the
capacitor for precise measurement of ESR. The technique also incorporates the effect of
temperature and load variation on the magnitude of capacitor voltage and current
ripples. To incorporate these effect a look up table is created. To create this table,
20 testing of the new converter is required at various load and operating conditions, which
makes the method complex. This method requires testing of each converter at the
manufacturing facility to create the look up table.
Amaral et al: IET Power Electron., vol. 5, no. 3, pp. 315-322, (2012) discloses a simple
25 online fault detection technique to prevent structural failures in aluminium electrolytic
capacitors used in the output filter of step-down DC-DC converters. Input current and
output voltage are used for online estimation of ESR. To incorporate the effect of
temperature on measurement of ESR, mathematical model of ESR as a function of
temperature is also developed using offline technique during initial testing of converter
30 and its components. The measurement system is composed of a digital oscilloscope that
is connected to a microcomputer with Matlab software. The technique eliminates the
4
use of large bandwidth current sensor for capacitor current. However, to calculate ESR
this technique requires knowledge of various converter parameters such as load
resistance, inductance and its resistance, switch resistance etc. Due to error in values of
these parameters, the estimated values using this technique has significant error.
5
Pu et al: IEEE Trans. Ind. Electron., vol. 60, no. 9, pp. 4118-4127,(Sep. 2013) discloses
a scheme for the estimation of the ESR monitoring of DC-link AEC for three-phase
PWM converter. In this, a sub-harmonic external ac current is injected from three-phase
source side due to which ripples are produced across DC-link capacitor. Recursive least
10 square method is used for the estimation of ESR. Since the magnitude of injected subharmonic
external ac current is high which violates the limits mentioned in IEEE 519-
2014 standard for grid feeding solar photovoltaic (PV) system. Therefore, this method
may not be allowed by utilities following the aforementioned standard.
15 Therefore, various online techniques are being developed for the monitoring of health of
the capacitors.
DE102004036211 discloses a method in which an intermediate circuit capacitor is precharged
during starting of the device via a charging resistor. From the measurement of
20 the charging current and a measured voltage curve at the capacitor, delay time of RC
circuit is calculated and compared with pre define delay time. Any deviation in this
difference is the indication of ageing of capacitor. In this method, extra pre charging
circuit is used for health monitoring of AEC. Furthermore, it is not possible to monitor
the health of capacitor during normal operation of the converter.
25
DE102004035723 discloses a method for determining remaining service life of an
electrolytic capacitor of a frequency converter. Remaining lifetime of an electrolytic
capacitor is calculated with the aid of its calculated core temperature and a
corresponding life. The core temperature is determined using the measured ambient
30 temperature and its calculated power loss. For the life of an aging rate is computed that
is integrated to an actual age, and subtracted from an end of life results in the respective
5
residual life. The power dissipation is calculated based on a measured DC voltage, a
measured motor current, the capacitance of the capacitors and effective internal
resistances. This leads to a large computational effort. Further, accuracy of this method
is limited due to errors in thermal and electrical modeling of the converter.
5
DE102004052977 discloses a diagnostic method for determining the aging state of a
capacitor. In this case, age-dependent parameters of the capacitor like ESR is measured
by means of discharging of capacitors.
10 DE102012105198 discloses a process to calculate the remaining service life of
electrolytic capacitor for an electric drive system by determining the power consumed
by motor, capacitor temperature, and voltage. Further, to calculate capacitor ripple
current using power consumed by motor, a relationship is determined between capacitor
ripple current and power. To establish this relation testing of drive system is required at
15 various load and operating conditions, which make the method complex to realize in
actual product.
US7830269 discloses a method for monitoring degradation of an electrolytic capacitor
in a power converter. In this method capacitor voltage ripple is detected and a pulse
20 width modulated (PWM) signal is generated which have duty cycle corresponding to
magnitude of detected capacitor ripple voltage. If the duty of PWM signal exceeds the
predefined limit, a warning signal is generated to indicate the degradation of the
capacitor. In this method only ripple voltage is used to determine the health of capacitor.
This could lead to error during transient conditions. Further, the magnitude of capacitor
25 ripple voltage also depends on the variation in input voltage, load, duty cycle and
temperature. It is not solely depend on the ESR. Neglecting of this effect leads to error
in the estimated value.
US7719808 discloses a method for predicting faults in a power converter. The
30 efficiency of power converter is calculated based on monitored output voltage and
output current. In this document by using the monitored voltage, current and
6
temperature stresses on the capacitor and by monitoring degradation in efficiency of
power converter, remaining life of the capacitor is estimated. However, the efficiency of
power converter also depends on the variation in load. Therefore, large load variation
can lead to an error in the determination of life of capacitor.
5
US8090548 discloses a method and device for predicting defects of a capacitor. The
method includes determining the ripple voltage, the temperature, and the current of the
capacitor, determining the value of an equivalent series resistance (ESR) and the
capacitance value of the capacitor using a digital filter. In this method, ESR is
10 calculated using switching frequency voltage and current ripples, which require high
sampling rate of ADC, thereby increasing the system cost.
US20140103937 discloses method for estimating the state of health of an electronic
component which is a part of the power distribution system. The said method constructs
15 an impedance matrix between different nodes based on measurement using Spread
Spectrum Time Domain Reflectometry (SSTDR). The degradation of capacitor is
reflected in the constructed matrix. The ESR value of capacitor is obtained by
minimizing the error function. This method requires extra hardware circuitry called
SSTDR test chamber to measure the different impedances, thereby increasing the cost.
20 US20110208452 discloses a non-invasive method and device for determining the
electrical impedance of an electrochemical system for electric power. It teaches the
calculation of impedance of battery by directly measuring the voltage and current across
capacitor and then converting these time domain signals into frequency domain. But, in
case of capacitors used in power electronic converters it is not possible to measure the
25 capacitor current directly by inserting a current sensor in the dc link capacitor. Insertion
of current sensor in dc link capacitor leads to distortion of the capacitor voltage
waveform due to increase of electrolytic series inductance (ESL). Further, additional
current sensor increases the cost of this solution.
30 US8796982 discloses a non-transitory computer readable storage medium has stored
thereon a computer program comprising instructions which, when executed by at least
7
one processor, cause the at least one processor to acquire a DC link current of an
adjustable speed drive (ASD) and transmit the DC link current to a state observer
formulated to represent the ASD. The instructions further cause the at least one
processor to extract at least one component of a DC link capacitor current of the ASD
from the state observer using the DC link current, wherein extracting the at least on5 e
component comprises extracting at least one of a second harmonic component of the
DC link capacitor current and a sixth harmonic component of the DC link capacitor
current. The instructions also cause the at least one processor to compare the amplitude
of the at least one extracted component of the DC link capacitor current to a fault range.
10 If the amplitude of the at least one extracted component of the DC link capacitor current
is within the fault range, the instructions cause the at least one processor to generate an
indication of a phase loss. This method first detect the phase loss condition by
extracting either second and sixth harmonic ripple component of capacitor current and
comparing with pre-defined limits. Further, if phase loss is detected remaining life of
15 capacitor is calculated using a life calculation model. This model requires information
of ripple currents of capacitor and hot spot temperature of capacitor. Therefore,
accuracy of this method is limited due to errors in thermal modeling of the converter
and life calculation model. Further, this method calculate the life of capacitor during
phase loss condition only, which may not be present in all grid conditions.
20
However, the aforementioned methods or techniques for determining the health of AEC,
either require an additional circuit / sensor or special operating conditions. Further, most
of the known methods uses switching frequency components of voltage and current to
determine the health of Aluminium Electrolytic Capacitors (AEC). This requires high
25 bandwidth sensors and/or high sampling rate of Analog to Digital Converter (ADC).
The methods which utilize low frequency component of voltage and current to
determine the health of capacitor are based upon the special operating condition of
system. Therefore, it is not possible to monitor health of capacitor in real time under
normal operation without using any additional hardware.
30
8
Thus, in view of the drawbacks of the existing methods/techniques and systems, it is
required to develop a method that monitors the health of capacitor in real time under
normal operation of system and does not require special operating condition,
interruptions or components. Also, it utilizes the natural present low frequency (twice
the power frequency) signals to determine the health of capacitor5 .
SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a
basic understanding of some aspects of the invention. This summary is not an extensive
10 overview of the present invention. It is not intended to identify the key/critical elements
of the invention or to delineate the scope of the invention. Its sole purpose is to present
some concept of the invention in a simplified form as a prelude to a more detailed
description of the invention presented later.
15 Accordingly, it is an object of the present invention to overcome the drawbacks of the
prior art.
It is another object of the present invention is to provide a method for monitoring the
health of capacitor by determining the second harmonic impedance of capacitor.
20
It is another object of the present invention is to provide a method for monitoring the
health of capacitor in a single phase grid connected photovoltaic (PV) system.
It is yet another object of the present invention is to provide a method or online
25 monitoring of health of capacitor.
It is still another object of the present invention is to provide a method for monitoring
the health of capacitor that does not require special operating condition, interruptions or
components.
30
9
Accordingly, in one implementation, a method for online monitoring of health of
capacitor in a single phase grid connected PV system is disclosed. The said method
comprising: (i) Sampling of PV voltage, PV current, and inductor current; (ii)
evaluation of capacitor current, ic; (iii) passing the PV voltage, vpv and capacitor
current, ic through the second order generalized integrator, SOGI to extract secon5 d
harmonic ripple component of capacitor voltage and current; (iv) evaluating the second
harmonic impedance of the capacitor; (v) comparing the evaluated impedance of step
(iv) with a predefined impedance value to detect the performance degradation of the
capacitor; and (vi) generating a warning signal after detecting the performance
10 degradation of the capacitor or else resample quantities of step (i).
In one implementation, a method for online monitoring of health of aluminium
electrolytic capacitor in a single phase grid connected PV system is disclosed. The
method comprising: (i) Sampling of PV current and inductor current, ; (ii)
15 evaluation of current in the dc link, idc from the inductor current of (i) and switch states
of inverter; (iii) evaluation of capacitor current, ic; (iv) sampling of PV voltage, ; (v)
passing the PV voltage and capacitor current, ic through the second order generalized
integrator, SOGI to extract second harmonic ripple component of capacitor voltage and
current; (vi) evaluating the second harmonic impedance of the capacitor; (vii)
20 comparing the evaluated impedance of step (vi) with a predefined impedance value to
detect the performance degradation of the capacitor; and (viii) generating a warning
signal after detecting the performance degradation of the capacitor or else wait for PV
voltage to settle to resample quantities of step (i).
25 In one implementation, a method for online monitoring of health of aluminium
electrolytic capacitor in a single phase grid connected PV system with LCL filter is
disclosed. The said method comprising: (i) Sampling of PV current and grid side
inductor current; (ii) evaluation of current in the dc link, idc from the grid side inductor
current of step (i) and switch states of inverter; (iii) evaluation of capacitor current, ic;
30 (iv) sampling of PV voltage; (v) passing the PV voltage and capacitor current, ic
10
through the second order generalized integrator, SOGI to extract second harmonic
ripple component of capacitor voltage and current; (vi) evaluating the second harmonic
impedance of the capacitor; (vii) comparing the evaluated impedance of step (vi) with a
predefined impedance value to detect the performance degradation of the capacitor; and
(viii) generating a warning signal after detecting the performance degradation of th5 e
capacitor or else wait for PV voltage to settle to resample quantities of step (i).
Other aspects, advantages, and salient features of the invention will become apparent to
those skilled in the art from the following detailed description, which, taken in
10 conjunction with the annexed drawings, discloses exemplary embodiments of the
invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above and other aspects, features, and advantages of certain exemplary
15 embodiments of the present invention will be more apparent from the following
description taken in conjunction with the accompanying drawings in which:
Figure 1 illustrates a grid feeding solar photovoltaic (PV) system, in accordance with
the subject matter of the present invention.
20
Figure 2 is a Bode Plot of characteristic impedance of electrolytic capacitor.
Figure 3 is a flow chart of the proposed method, in accordance with the subject matter
of the present invention.
25
Figure 4 is a Simulink waveform of inverter output voltage, grid voltage and inductor
current.
Figure 5 is an actual and reference PV voltage during transient.
30
11
Figure 6 is an illustration of difference in constructed capacitor current from inductor
current to actual capacitor current (a) including all harmonics (b) filtered second
harmonic component.
Figure 7 is a Waveform of dc-link voltage (a) including all harmonics (b) filtere5 d
second harmonic component.
Figure 8 is a Waveform of inverter output voltage (50 V/div), grid voltage (50V/div)
and inductor current (5 A/div), Time: 5 ms/div.
10
Figure 9 is a Reference (10 V/div) and actual voltage (10 V/div) during maximum
power point tracking.
Figure 10 is a waveform of dc-link voltage ripple (500 mV/div) and current ripple
15 (50mA/div) for Capacitor C1: C=2.263mF, ESR=40.3mΩ.
Figure 11 is a Waveform of dc-link voltage ripple (1 V/div) and current ripple
(100mA/div) for Capacitor C2: C=1.852mF, ESR=58mΩ
20 Figure 12 illustrates a grid feeding solar photovoltaic PV system with LCL filter.
Persons skilled in the art will appreciate that elements in the figures are illustrated for
simplicity and clarity and may have not been drawn to scale. For example, the
dimensions of some of the elements in the figure may be exaggerated relative to other
25 elements to help to improve understanding of various exemplary embodiments of the
present disclosure. Throughout the drawings, it should be noted that like reference
numbers are used to depict the same or similar elements, features, and structures.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
30 The following description with reference to the accompanying drawings is provided to
assist in a comprehensive understanding of exemplary embodiments of the invention. It
12
includes various specific details to assist in that understanding but these are to be
regarded as merely exemplary.
Accordingly, those of ordinary skill in the art will recognize that various changes and
modifications of the embodiments described herein can be made without departing fro5 m
the scope of the invention. In addition, descriptions of well-known functions and
constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the
10 bibliographical meanings, but, are merely used by the inventor to enable a clear and
consistent understanding of the invention. Accordingly, it should be apparent to those
skilled in the art that the following description of exemplary embodiments of the present
invention are provided for illustration purpose only and not for the purpose of limiting
the invention as defined by the appended claims and their equivalents.
15
It is to be understood that the singular forms “a,” “an,” and “the” include plural
referents unless the context clearly dictates otherwise.
By the term “substantially” it is meant that the recited characteristic, parameter, or value
20 need not be achieved exactly, but that deviations or variations, including for example,
tolerances, measurement error, measurement accuracy limitations and other factors
known to those of skill in the art, may occur in amounts that do not preclude the effect
the characteristic was intended to provide.
25 Features that are described and/or illustrated with respect to one embodiment may be
used in the same way or in a similar way in one or more other embodiments and/or in
combination with or instead of the features of the other embodiments.
It should be emphasized that the term “comprises/comprising” when used in this
30 specification is taken to specify the presence of stated features, integers, steps or
13
components but does not preclude the presence or addition of one or more other
features, integers, steps, components or groups thereof.
The present invention relates to a simple, cost efficient method to monitor the life of
capacitor by determining the second harmonic impedance of the capacitor for a singl5 e
phase grid connected PV system. The said method measures inductor current and PV
current to evaluate the current in the dc link capacitor. It utilizes low frequency
component (twice the power frequency) measurements to determine the health of
capacitor. The key advantage of the present method is that no additional circuits or
10 sensors are required. To avoid error in transient the execution period of this method lies
between settling time of outer voltage loop of inverter controller and step time of
Maximum Power Point Tracking (MPPT) algorithm.
Further, all the existing methods considered electrolytic capacitors to be unusable when
15 ESR value reaches 2.8 times of the initial ESR value and capacitance decrease in excess
of 20% of the initial value, however in the method of the present invention a new limit
based upon second harmonic impedance of capacitor is predefined to compare and
analyze the remaining life of capacitor.
20 Using this method it is possible to monitor the health of capacitor in real time under
normal operation of system. This method does not require any extra hardware
circuitry/sensor or special operating condition. The present method is implemented by
using existing sensors, digital processor/controller used for maximum power point
tracking (MPPT) controller and closed loop control of the inverter. In the present
25 method the number of current sensors are optimized by eliminating the large bandwidth
current sensor to measure capacitor current, which reduces the system cost. Monitoring
of second harmonic impedance requires lower sampling rate of analog to digital
converter (ADC), which makes the said method easier to implement.
30 The present method may be implemented in the existing digital processor/controller
used for MPPT, voltage and current control of the inverter.
14
In a single phase grid connected PV system as shown in Figure 1, grid power oscillates
with double the fundamental frequency. It causes the second harmonic ripple in dc-link
voltage. Besides second harmonic, switching ripples are also present in the dc-link
voltage. The second harmonic ripple lie in the capacitive region of impedanc5 e
magnitude plot of capacitor as shown in Figure 2. Based on this information, the present
method extracts the second harmonic ripple component present in the capacitor voltage
and current. The root mean square (RMS) of second harmonic ripple voltage and current
is evaluated and ratio of the two provides the impedance of the capacitor. The present
10 method steps are explained via flow chart as shown in Figure 3.
The present invention provides a method for online monitoring of health of aluminium
electrolytic capacitor in a single phase grid connected PV system, the method
comprising:
15 i. Sampling of PV current and inductor current,
ii. evaluation of current in the dc link, idc from the inductor current of step
(i) and switch states of the inverter shown in Figure 1.;
iii. evaluation of capacitor current, ic;
iv. sampling of PV voltage, ;
v. passing the PV voltage and capacitor current, ic 20 through the second order
generalized integrator, SOGI to extract second harmonic ripple
component of capacitor voltage and current;
vi. evaluating the second harmonic impedance of the capacitor;
vii. comparing the evaluated impedance of step (vi) with a predefined
25 impedance value to detect the performance degradation of the capacitor;
and
viii. Generating a warning signal after detecting the performance degradation
of the capacitor or else wait to resample quantities of step (i) after
settling of PV voltage.
30
15
The present invention provides an online method comprising of sampling of the PV
current and inductor current to estimate the capacitor current. The PV voltage and
capacitor current is passed through the second order generalized integrator (SOGI) in
order to extract their second harmonic component. The Phase locked loop (PLL) is used
in the closed loop control of converter to give information of second harmonic gri5 d
frequency. This information is used by SOGI for maintaining the bandwidth across
twice the grid frequency. The output of SOGI is the second harmonic ripple component
of capacitor voltage and current. The ratio of the root mean square (RMS) of these two
gives the second harmonic impedance of capacitor. If this estimated impedance crosses
10 the predefined limit, then this gives the indication of capacitor failure. Else, the
sampling of dc-link quantities is done again. Some delay is provided in fresh sampling
of quantities. This is due to change in PV reference voltage due to MPPT. The actual
PV voltage takes some time to reach steady state due to plant and controller dynamics.
This transient period of PV operating point is excluded in order to prevent error in
15 sampling. All the samples of dc-link quantities are taken when system is operating at
steady state.
In the present invention the evaluation of capacitor current from inductor current and
PV current is done as below:
20 DC-link current ( ) is evaluated from inductor current ( ) by using switching states of
converter as given in eqn. (1). For unipolar pulse width modulation (PWM), switching
states are given in Table I, where 1 and 0 represents the ON and OFF state of insulatedgate
bipolar transistor (IGBTs) respectively.
(1)
25 PV current is being sensed for MPPT controller. This is also used for the evaluation of
capacitor current as given by,
(2)
Table I: SWITCHING STATES OF PWM CONVERTER
condition
1 1
16
1 0
0 1
0 0
The Extraction of second harmonic ripple current of capacitor is required for evaluating
capacitor impedance. The evaluation of capacitor current from PV and inductor current
eliminates the use of extra current sensor. Also due to low order harmonic used for
impedance calculation, the required sampling rate of is low. This make5 s
the present method easy for implementation and is cost effective.
In an implementation, the method for monitoring the health of capacitor in a single
phase grid connected PV system as shown in figure 1, is simulated in
10 MATLAB/Simulink. The specifications of the system and converter are given in Table
II and III, respectively. PV voltage, PV current and inductor current are sensed for
closed loop control and MPPT algorithm (perturb and observe). Unipolar PWM
technique is used to reduce the filter size. The PLL tracks the grid phase and frequency
(via sensing grid voltage) for unity power factor (UPF) operation. The PV voltage, PV
15 current and inductor current are sampled at switching frequency for the implementation
of present method.
Table II: SPECIFICATION OF SYSTEM
Parameter Value
Maximum Power ( ) 2kW
Voltage at 365 V
Current at 5.48 A
Short Circuit Current 6.3 A
Open Circuit Voltage 390V
Grid Frequency 50Hz
Grid Voltage 230 V
17
Table III: CONVERTER COMPONENT
Parameter Value
Inductor (L) 3mH
DC Link Capacitor (C) 1.62mF
ESR of Capacitor (ESR) 0.095Ω
Switching Frequency 10kHz
Referring now to Figure 4, which shows the Simulink waveform of inverter output
voltage, grid voltage and inductor current. It shows the grid voltage and inductor current
for unity power factor (UPF) operation. Referring now to Figure 5, showing the actua5 l
and reference PV voltage during transient. Change in reference of PV voltage due to
MPPT causes transient in actual PV voltage is shown in Figure 5. Therefore sampling of
dc link quantities and inductor current is done after sampling of PV voltage. Due to
sampling of inductor current at switching frequency, constructed capacitor current
10 which is distorted from the actual capacitor current (if directly sensed) as shown in Fig.
6(a). However, when it is passed through the SOGI, the performance of both the
constructed and actual capacitor current is similar as shown in Fig. 6(b). The RMS of
the extracted second harmonic component from the constructed is 1.61A, with an
error of less than 1%.
15 Due to fluctuating grid power, the dominant second harmonic ripple in PV voltage is
shown in Fig. 7(a). By passing the PV voltage through SOGI, the extracted second
harmonic component of PV voltage is shown in Fig. 7(b).
Degradation of electrolytic capacitor:
20 Due to loss in electrolyte, the ESR of electrolytic capacitor rises exponentially and
capacitance value falls linearly with time.
(3)
where, = initial ESR value, k = constant, depends on size and construction of
capacitor, t= operating time in hours, T = operating temperature of capacitor in C, E =
25 activation energy per Boltzmann’s constant.
18
and,
(4)
where, = capacitance value (mF), λ= coefficient depending on design of capacitor, A =
parameter depends on type of capacitor.
The second harmonic impedance (Zc2) of capacitor is given by5 ,
(5)
This impedance increases due to degradation of capacitor. Thus, results in larger second
harmonic voltage ripple across capacitor. Due to ageing, using (3) and (4) the variation
in capacitor parameters (ESR and C) is implemented in Simulink model. The RMS of
10 second harmonic voltage and current ripples of capacitor are evaluated for . The
results are given in Table IV.
TABLE IV: VARIATION OF SECOND HARMONIC IMPEDANCE WITH
DEGRADATION OF CAPACITOR
Time
(hours)
ESR
(Ω)
Capacitance
(mF)
100 Hz component
(in Ω)
10KHzcomponent
0 0.095 1.62 2.05 0.98 0.11
10000 0.125 1.49 2.23 1.07 0.15
20000 0.183 1.36 2.44 1.18 0.22
30000 0.343 1.23 2.73 1.34 0.39
35000 0.607 1.16 3.00 1.50 0.67
37000 0.879 1.14 3.24 1.65 1.22
39000 1.579 1.11 3.96 2.13 1.50
40600 4.398 1.09 6.30 4.64 2.71
41000 7.942 1.01 7.44 8.07 3.20
15 Due to degradation of capacitor, the second harmonic impedance of capacitor increases.
This results in rise in second harmonic ripples in capacitor voltage. The dominant
switching harmonic ripple in capacitor voltage and current are also given in Table IV.
The observation of Table IV reveals that second harmonic ripple is the dominant
19
harmonic in comparison to switching harmonics. The rise in second harmonic voltage
ripple is significant due to the degradation of capacitor. Therefore, it is more viable to
monitor the second harmonic impedance for health detection of capacitor. Curve fitting
is done on the data of provided by simulation results due to following reasons:
5
I. For the health prediction of electrolytic capacitor.
II. For determining the limit on beyond which ripples in capacitor voltage may
exceed the safe limit of converter.
10 The shape of the fitted curve is exponential and the equation is given by,
where, inverse of ‘a’ is the time constant (τ) of second harmonic impedance curve.
Operation of electrolytic capacitor with duration beyond five times τ the will cause
abrupt rise in . This will lead to sudden rise in ripples in capacitor voltage. Thus, the
15 replacement of capacitor is suggested at t=5τ.
In an implementation, the method of present invention monitors the health of aluminium
electrolytic capacitor in a single phase grid connected PV system with LCL filter.
Figure 12 illustrates a grid feeding solar photovoltaic PV system with LCL filter. The
20 said method comprising: (i) Sampling of PV current and grid side inductor current; (ii)
evaluation of current in the dc link, idc from the grid side inductor current of step (i) and
switch states of inverter; (iii) evaluation of capacitor current, ic; (iv) sampling of PV
voltage; (v) passing the PV voltage and capacitor current, ic through the second order
generalized integrator, SOGI to extract second harmonic ripple component of capacitor
25 voltage and current; (vi) evaluating the second harmonic impedance of the capacitor;
(vii) comparing the evaluated impedance of step (vi) with a predefined impedance value
to detect the performance degradation of the capacitor; and (viii) generating a warning
signal after detecting the performance degradation of the capacitor or else wait for PV
voltage to settle to resample quantities of step (i).
30
The invention is now illustrated by way of non-limiting examples
20
EXAMPLES
Example 1:
A prototype of single phase grid connected PV system is built in order to implement the
present method. The built prototype of single phase inverter feeds 120W of power a5 t
unity power factor. Specifications of PV module and inverter components are given in
Tables V and VI, respectively. The experimentations are performed with two different
values of dc-link capacitors as tabulated in Table VI.
10 Table V: SPECIFICATION OF PV SYSTEM
Parameter Value
Voltage at 80 V
Current at 1.7 A
Short Circuit Current 2.2 A
Open Circuit Voltage 100 V
Table VI: INVERTER PARAMETERS
Parameter Value
Inductor (L), CRGO core 3mH
DC Link Capacitor (C), AEC
Capacitor C1: C=2.263mF, ESR=0.0403Ω
Capacitor C2: C=1.852mF, ESR=0.058Ω
Switching Frequency 10kHz
Grid frequency (f) 50Hz
Grid Voltage 45V
FSBB20CH60C (IGBT module) is used as inverter, which includes integrated short
15 circuit protection and inbuilt gate drivers. Agilent Technologies made E4360A is used
to emulate solar PV source. DSP TMS320F2808 is used for the closed loop control and
to implement the proposed technique. PWM signals for IGBT’s are generated using
interrupt-based timer modules of the controller. OPAMP-based differential amplifier
21
circuit is used to attenuate feedback signals in order to make it compatible with
controller’s ADC. Inductor and PV currents are measured using LEM-LA-25P current
transducers. For the operation of OP-AMPS, transducers, IGBT driver circuit and
controller, a regulated ±15V supply is generated from a linear power supply circuit. The
inverter output voltage, inductor current and grid voltage are shown in Figure 85 .
Inductor current is of the same phase as that of grid voltage, and power factor is found
to be 0.95. Total harmonic distortion (THD) of the grid voltage and inductor current are
in the range of 1.7%- 1.8% and 2.5%-2.8% for two different dc capacitors, respectively.
The perturb and observe MPPT algorithm is implemented to extract maximum PV
10 power. The waveform of reference PV voltage and actual PV voltage is shown in Figure
9. Since the power fluctuate at twice the grid frequency (50Hz), the dc-link voltage has
the dominant ripple of twice the fundamental frequency (100Hz). Furthermore, the dclink
ripple magnitude depends on the value of dc-link capacitor.
15 For Capacitor C1 and Capacitor C2, dc-link voltage ripple and PV current ripple are
shown in Figure 10 and 11, respectively. From these waveforms it is clear that as dclink
capacitance decreases the magnitude of ripple voltage increases. For capacitor C1
and C2, dc link voltage ripple is 2.94 V and 3.36 V, respectively.
20 Example 2: For two values of capacitance, capacitor impedance at 100Hz ( ) is
evaluated as discussed. is obtained and tabulated in Table VII along with the
measured impedance at 100Hz using LCR meter. It is observed that reduction in
capacitance value and increase in ESR value, increases.
25 Table VII: Results
Capacitor C1:
C=2.263mF, ESR=0.0403Ω
Capacitor C2:
C=1.852mF, ESR=0.058Ω
(using LCR meter) 0.70438 Ω 0.8623 Ω
(Experimentally) 0.7567 Ω 1.011 Ω
22
Some of the important features of the present invention, considered to be noteworthy are
mentioned below:
• The evaluation of low frequency component of capacitor current using inductor
current.
• Using low frequency measurements to determine the health of capacitor5 .
• Using low sampling rate to determine the health of capacitor.
• No additional circuits or sensors are required.
• The evaluation of capacitor current from PV and inductor current eliminates the
use of extra current sensor.
10
The methodology and techniques described with respect to the aforesaid embodiments
can be performed using a machine or other computing device within which a set of
instructions, when executed, may cause the machine to perform any one or more of the
methodologies discussed above. In some embodiments, the machine operates as a
15 standalone device. In some embodiments, the machine may be connected (e.g., using a
network) to other machines. In a networked deployment, the machine may operate in
the capacity of a server or a client user machine in a server-client user network
environment, or as a peer machine in a peer-to-peer (or distributed) network
environment. The machine may comprise a smartphone, server computer, a client user
20 computer, a personal computer (PC), a tablet PC, a laptop computer, a desktop
computer, a control system, a network router, switch or bridge, or any machine capable
of executing a set of instructions (sequential or otherwise) that specify actions to be
taken by that machine. Further, while a single machine is illustrated, the term "machine"
shall also be taken to include any collection of machines that individually or jointly
25 execute a set (or multiple sets) of instructions to perform any one or more of the
methodologies discussed herein.
• The machine may include a processor (e.g., a central processing unit or
CPU, a graphics processing unit or GPU, or both), a main memory and a static
memory, which communicate with each other via a bus. The machine may
30 further include a video display unit (e.g., a liquid crystal display or LCD), a flat
panel, a solid state display, or a cathode ray tube or CRT). The machine may
23
include an input device (e.g., a keyboard) or touch-sensitive screen, a cursor
control device (e.g., a mouse), a disk drive unit, a signal generation device (e.g.,
a speaker or remote control) and a network interface device.
• The disk drive unit may include a machine- readable medium on which is
stored one or more sets of instructions (e.g., software) embodying any one o5 r
more of the methodologies or functions described herein, including those
methods illustrated above. The instructions may also reside, completely or at
least partially, within the main memory, the static memory, and/ or within the
processor during execution thereof by the machine. The main memory and the
10 processor also may constitute machine- readable media.
• Dedicated hardware implementations including, but not limited to,
application specific integrated circuits, programmable logic arrays and other
hardware devices can likewise be constructed to implement the methods
described herein. Applications that may include the apparatus and systems of
15 various embodiments broadly include a variety of electronic and computer
systems. Some embodiments implement functions in two or more specific
interconnected hardware modules or devices with related control and data
signals communicated between and through the modules, or as portions of an
application-specific integrated circuit. Thus, the example system is applicable to
20 software, firmware, and hardware implementations.
• The present disclosure contemplates a machine readable medium
containing instructions, or that which receives and executes instructions from a
propagated signal so that a device connected to a network environment can send
or receive voice, video or data, and to communicate over the network using the
25 instructions. The instructions may further be transmitted or received over a
network via the network interface device.
• While the machine- readable medium can be a single medium, the term
"machine- readable medium" should be taken to include a single medium or
multiple media (e.g., a centralized or distributed database, and/or associated
30 caches and servers) that store one or more sets of instructions. The term
"machine- readable medium" shall also be taken to include any medium that is
24
capable of storing, encoding or carrying a set of instructions for execution by the
machine and that cause the machine to perform any one or more of the
methodologies of the present disclosure.
• The term "machine- readable medium" shall accordingly be taken to
include, but not be limited to: tangible media; solid-state memories such as 5 a
memory card or other package that houses one or more read- only (non- volatile)
memories, random access memories, or other re- writable (volatile) memories;
magneto- optical or optical medium such as a disk or tape; non- transitory
mediums or other self- contained information archive or set of archives is
10 considered a distribution medium equivalent to a tangible storage medium.
Accordingly, the disclosure is considered to include any one or more of a
machine- readable medium or a distribution medium, as listed herein and
including art- recognized equivalents and successor media, in which the
software implementations herein are stored.
15
25
WE CLAIM:
1. A method for online monitoring of health of capacitor in a single phase grid
connected PV system, the method comprising5 :
i. Sampling of PV current and inductor current, ;
ii. evaluation of capacitor current, ic;
iii. passing the PV voltage , and capacitor current, ic through the second
order generalized integrator, SOGI to extract second harmonic ripple
10 component of capacitor voltage and current;
iv. evaluating the second harmonic impedance of the capacitor;
v. comparing the evaluated impedance of step (iv) with a predefined
impedance value to detect the performance degradation of the capacitor;
and
15 vi. generating a warning signal after detecting the performance degradation
of the capacitor or else resample quantities of step (i).
2. The method as claimed in claim 1 wherein the capacitor is an aluminium
electrolytic capacitor.
20 3. The method as claimed in claim 1 wherein the sampling is done at switching
frequency.
4. The method as claimed in claim 1 wherein evaluation of capacitor current is
from PV current and inductor current.
5. The method as claimed in claim 1 wherein second order generalized integrator,
25 SOGI is adapted to use second harmonic grid frequency information from Phase
locked loop, PLL to maintain bandwidth across twice the grid frequency.
6. The method as claimed in claim 1 comprising unipolar PWM technique to
reduce the filter size of second harmonic component.
7. The method as claimed in claim 1 comprising providing the generated warning
30 signal to a system hosting the capacitor.
26
8. The method as claimed in claim 1 comprising a delay time between settling time
of outer voltage loop of inverter controller and step time of Maximum Power
Point Tracking (MPPT) algorithm to avoid error in transient of the execution
period of the method.
9. The method as claimed in claim 1 comprising replacing the capacitor in respons5 e
to the generated warning signal.
10. A method for online monitoring of health of aluminium electrolytic capacitor in
a single phase grid connected PV system, the method comprising:
i. Sampling of PV current and inductor current, ;
10 ii. evaluation of current in the dc link, idc from the inductor current of step
(i) and switch states of inverter;
iii. evaluation of low frequency capacitor current, ic;
iv. sampling of PV voltage, dc-link quantities;
v. passing the PV voltage and capacitor current, ic through the second order
15 generalized integrator, SOGI to extract second harmonic ripple
component of capacitor voltage and current;
vi. evaluating the second harmonic impedance of the capacitor;
vii. comparing the evaluated impedance of step (vi) with a predefined
impedance value to detect the performance degradation of the capacitor;
20 and
viii. generating a warning signal after detecting the performance degradation
of the capacitor or else wait for PV voltage to settle to resample
quantities of step (i).
25 11. The method as claimed in claim 7 wherein the sampling is done at switching
frequency.
12. The method as claimed in claim 7 wherein evaluation of capacitor current is by
PV current and inductor current.
27
13. A method for online monitoring of health of aluminium electrolytic capacitor in
a single phase grid connected PV system with LCL filter, the method
comprising:
i. Sampling of PV current and grid side inductor current;
ii. evaluation of current in the dc link, idc from the grid side inductor curren5 t
of step (i) and switch states of inverter;
iii. evaluation of capacitor current, ic;
iv. sampling of PV voltage;
v. passing the PV voltage and capacitor current, ic through the second order
10 generalized integrator, SOGI to extract second harmonic ripple
component of capacitor voltage and current;
vi. evaluating the second harmonic impedance of the capacitor;
vii. comparing the evaluated impedance of step (vi) with a predefined
impedance value to detect the performance degradation of the capacitor;
15 and
viii. generating a warning signal after detecting the performance degradation
of the capacitor or else wait for PV voltage to settle to resample
quantities of step (i).