3rd International Conference and Workshops on Recent Advances and Innovations in Engineering, 22-25 November 2018
(IEEE Conference Record # 43534)
Harmonic Mitigation in a Transformer Less Grid
Connected Solar PV System Using Modified LCL
Filter
Pankaj Gakhar Dr. Manoj Gupta
Research Scholar, SET, Poornima University Professor, SET, PoornimaUniversity
Jaipur, Rajasthan (India) Jaipur, Rajasthan (India)
gakhar23@gmail.com manojg@poornima.edu.in
Abstract— Since the Penetration of the Solar Photovoltaic makes in not convenient to use due to weight and volume
has been increased rapidly over the decade, the issue of issues. The uncertainties in network impedances make the
maintaining the power quality within the allowable range has LC filter for being not a good choice in spite of having
been a topic of major concern. This paper presents the basic simple structure and easy parameterization. In such scenario,
designing, analysis and implementation of the LCL filter for the LCL filters are getting great attention and preference by
attenuating the grid current harmonics and improving the researchers because of its various positive aspects. It
power quality. The comparative analysis related to stability, provides stability to the system and proves to be cost
complexity, cost and application of LCL filter with L and LC efficient when used with damping techniques[3,4]. Also
filter has been presented and a step by step approach to design
when correctly damped it can also eliminate the resonance
a LCL filter has also been explained in the paper. The
approach has been tested on a 100KW grid connected
problem. It can attenuate higher order harmonics levels at
photovoltaic system and the efficacy of the approach has been lower frequencies to meet harmonic standards.
proved by the reduced THD level up to 2.93%. In this paper a design procedure of a LCL filter for
harmonic mitigation has been proposed. The various types
Keywords- Grid connected PV system, Power Quality, LCL of filter topologies have also been discussed. A step by step
Filter, Frequency response procedure to design a LCL filter has been described and a
design illustration has also been shown with the help of a
I. INTRODUCTION
test system. In the last, the efficacy of the proposed LCL
The motivation given by the Govt. of India, for the filter has been proved.
establishment of sustainable and green energy sources
especially in the field of solar energy had played a vital role II. SYSTEM DESIGN
in increasing the penetration levels of solar PVs into the grid.
These systems for integration of solar PV to the grids are The block diagram of a transfomerless grid connected PV
designed to transfer maximum power but a much higher system is shown in fig1.
attention is required on the various power quality issues
among which harmonics are the major one. While the grid
integration, the power electronic PWM modulated MPPT
converters are used which are responsible for harmonic
pollution in the grid current. The total harmonic distortion in
the grid current is inevitable. So, in order to regulate the
levels of THD in any power system various standards are set
by various international agencies such as IEEE-519[1] and Li Lg
IEEE-1547[2]. Also, as per IEEE-929-2000 standard, Boost Bridge
Solar PV Grid
harmonic distortion of 5% in the current along with Converter Inverter Li Lg
individual bounds of 4% in each harmonic current from 3rd
to 9th and 2% from 11th to 15th along with nearly unity Li Lg
power factor. Cpv Cdc
The switching nature of the inverter is provides big Cf
Cf
challenges in harmonic reduction in the grid currents. In
order to achieve so, various filter designs are available Cf
which are broadly classified as Active Filters and Passive Fig. 1. Proposed LCL filter for a PV connected Grid
Filters. The active filter comprises of active switching
requirements and complicated control algorithms and these The solar photovoltaic panels supply active power to the
are not cost efficient. Whereas passive filters such as L, LC dc-dc converter which is equipped with Perturb and Observe
and LCL are inexpensive. The L type filter is very easy and algorithm for maximum power point tracking. The dc-dc
commonly used in grid connected in inverters but the major converter is used to change the dc voltage levels. The output
drawback of this filter is that while decreasing the ripple of the boost converter is fed to the PWM modulated inverter
current the size of the filter needs to be increased which
• 978-1-5386-4525-3/18/$31.00 ©2018 IEEE
to convert it into ac supply. The output of inverter is fed to designing LCL filter, resonance of filter and current ripple
the grid through the LCL filter. flowing through the ripple are also taken into the account.
The resonance frequency of the filter is calculated by:
III. FILTER TOPOLOGIES
(3)
The various filter topologies used for the harmonic 1
suppression for power quality improvement are as follows: 2 ∗ ∗
The major drawback of this filter is that it is resistless to
the oscillations which intensify near cut-off frequency
which can be eliminated by using damping resistor.
VSI VSI VSI An essential transfer function is:
(4)
Fig. 2. Different types of filter topologies(L, LC and LCL)
Where the voltage of grid is considered to be ideal source
proficient of discarding all remaining frequencies.
A. L Filter Considering 0 i.e. condition for current controlled
It is a first order filter having an attenuation boundary of converter, LCL filter transfer function neglecting the
20db/decade over whole range of frequency. So, a higher damping resistance is given by:
value of inductor is needed for suppressing current
harmonics which leads to the higher voltage drop worsening 1 (5)
the dynamics of system and also high cost[5]. Therefore L
filters are used in the areas wheresufficient attenuation is
available like high switching frequency converters.
The transfer function of L filter is:
1 (1)
G s
L s
B. LC Filter
LC filter is a kind of second order filter which comprises
of better dynamics related to damping as compared to L
filter. The LC filter does not provide any gain before cut-off
frequency but after cut-off freq, it gives an attenuation of
12db/octave. It comprises of an easy designing structure.
The Transfer function of LC Filter is
1 (2)
1 L L L
The design of filter is a compromise between high value
of capacitance affecting value of voltage and high value of
inductance required to achieve cut off freq of LC filter.A
large size of capacitor may help in reducing the cost of Fig. 3. Bode Plot of L and LCL Filter
inductor but it may cause high inrush current and reactive
power flow at the fundamental frequency [5]. whereas the transfer function of LCL filter with damping
Therefore, this type of filter is less suitable despite of resistances in series is given by:
having simple design. Also one of the major drawback of
LC filter while using it in grid integration is that its resonant 1 (6)
frequency depends on network impedance[6].
C. LCL Filter
And the transfer function of the LCL filter with damping
The LCL filter has an attenuation of 60db/decade for the resistance in parallel is given by:
frequency beyond resonant frequency. Hence, these can be
used with high switching frequency converters. LCL filter (7)
suits best for the application of PV integration to the grid. It ∗
also has the capabilities of reducing ripple current up to a
decent level. But the filter is required to be designed as per
Both the series as well as parallel damping resistor are
converter parameters very precisely otherwise it may lead to
good in filtering but the series damping resistor has better
resonance and instability in system. Therefore, while
capability of suppressing higher frequencies since the high 1 (9)
frequency harmonic current in series flowing through the
capacitor is greater than parallel.
For maximum power factor variation seen be grid is
considered to be 5%. Therefore, the filter capacitance is to
be considered as:
0.5 (10)
The maximum network current is calculated as:
√2 (11)
3
In an inverter, the maximum ripple current is given by:
2 1 (12)
3
Where m is modulation index of inverter
The maximum peak to peak current is observed to happen
at m=0.5. So,
2 (13)
6
Fig. 4 Bode Plot of LCL filter with series and parallel damping
A ripple content of 10% is considered in rated current.
Figure no. 4 demonstrates the bode plot of LCL filter
with and without damping resistances respectively. The 0.1 (14)
addition of damping resistance mitigated the gain spike,
furbishing the overall response and rolling of -180 degrees Hence,
for high frequency in place of -270 degrees. It can be seen in (15)
the bode plot that the closed loop bandwidth must be in the 6
range of 1000 Hz where the phase shift is around -90
degrees.
The LCL filter is expected to reduce the ripple up to 20%,
IV. LCL FILTER DESIGN PROCEDURE resulting in 2% ripple in the network current [8][9].
While designing the LCL filter the size of the inductor
plays a vital role. The higher size of inductor reduces the
current ripples but it also reduced the tracking speed of the 1 (16)
current.It also leads to the higher size and weight along with
cost. So, under such circumstances certain constraints are
defined to get the optimal value of inductor which are as
follows [7]: WhereKais the attenuation factor.
The grid side inductor value for a LCL filter is calculated
1. The voltage drop across the inductor should be less than as:
the 5% of the rated network voltage.
2. The peak value of the harmonic current should be (17)
within 10%-20% of the inverter current.
3. The inrush current should be as small as possible. Where is the limiting factor for to be less than .
A series resistor is added with the capacitor in order to
The parameters required for filter designing are: attenuate ripples at switching frequency for eliminating
• Line to Line R.M.S Voltage, VL-L resonance. This resistor should be of a value one third of
• Phase Voltage, VPH filter capacitor impedance at resonant frequency.
• Active Power delivered by inverter, P 1 (18)
• DC Link Voltage, VDC 3
• Grid Frequency, Fg Here, 2
• Inverter's Switching Frequency, Fsw
• Resonance frequency, Fres
The base impedance and the base capacitance is
calculated as
(8)
V. LCL FILTER DESIGN ILLUSTRATION& SIMULATION VI. CONCLUSION
ANALYSIS: This paper presents a step by step procedure to design a
TABLE I. SYSTEM PARAMETERS
LCL filter for a grid connected inverter system. It helps in
coupling with utility grid as well as it reduces the current
Parameters Value ripples. The efficacy of the proposed filter has been tested in
Grid Frequency, Fg 50Hz
results as the harmonics in the grid current has been reduced
PWM Frequency, Fsw 20KHz
DC Link Voltage, VDC 1450 to 2.93%. The harmonic levels are under the allowable
Nominal Power, P 100KW range as per standards. This filter is also applicable to the
Line to Line Voltage, VL-L 415V small as well as large inverter systems used in various
Grid Side inductor, 1.6mH applications such as solar photovoltaic, fuel cell etc.
Inverter Side Inductor, 2.7mH
Capacitor, 207 F REFERENCES
Damping Resistor, RD 0.73Ohms
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Fig. 5. Harmonic levels in grid current
