Passive filter for harmonics mitigation in standalone PV system for non linear load

Passive Filter for Harmonics Mitigation

In Standalone PV System for non Linear Load

Mouna TALI, Abdellatif OBBADI*, Abdelkrim ELF AJRI, Youssef ERRAMI Laboratory: Electronics, Instrumentation and Energy Team: Exploitation and Processing of Renewable Energy

Faculty of Science University Chouaib Doukkali Department of Physics Route Ben Maachou, 24000 EI-Jadida, Morocco

natalimou@hotmail.fr obbadi.a@ucd.ac.ma elfajri@hotmail.com errami3@hotmail.com

Abstract-Several studies have been presented regarding the

harmonics mitigation by using different types of filters. Passive

filter is one of them and is employed due to his simplicity,

economical cost and high reliability in power system. This paper

presents an analysis and study of three types of passive filters to

minimize harmonics distortion caused by non linear loads in

standalone PV system. In order to achieve the certain filtering

effect, it is necessary to combine different filter topologies;

generally these topologies can be divided into two categories:

series AC reactor and shunt passive filter such as tuned filters

and high pass filters. This paper presents firstly the basic design

and components of PV system, secondly describes the causes of

harmonics and their effects and presents the means to improve

power quality and to protect the equipment in our power system.

The proposed system is verified by the simulation using

Matlab/Simulink environment.

Keywords- Passive Harmonic Filter; Total Harmonic Distortion

(THD); Photovoltaic cell; PV inverter; LC filter; Non Linear Load.

I. INTRODUCTION

The global energy demand is constantly increasing, and the pollutant nature of fossil energies has increased the interest of the development of renewable energies.

Solar energy is plentiful worldwide, and the best way to produce electricity without pollution[I].We can use solar PV systems for domestic use and store excess electricity in batteries for later use this is a Standalone PV System, or feed into the electricity grid to reduce the electricity bill [2-3]. In standalone PV system power electronic equipment and non linear loads are widely used and resulted serious harmonic problems. Normally, standalone PV system is designed to operate at frequencies of 50Hz. Although certain types of loads produce current and voltage signal with frequencies that are integer multiples of the 50Hz fundamental frequency [4]. These higher frequencies are called electrical pollution that is known as power system harmonics. Harmonics causes obstruction to the normal operation of the equipment or the system. Harmonics are generated by various reasons such as saturation, switching like thyristor/diode rectifiers, cyclo converters can interact adversely in the PV system. It is imperative to analyze, quantify and reduce these harmonics to a level which meets the IEEE 519-1992 standard [5]. A large number of filter topologies and filtering techniques are

proposed in literature including passive, active and hybrid filters [6]. This paper focuses on the analysis and design of harmonic passive filters for a single phase standalone PV system, single tuned filter for low order frequencies and high passive filter for high order frequencies [6-7]. This work is organized as follows section I discusses in brief the modeling of the standalone PV system; section II describes the passive filters design for the harmonics mitigation in the standalone PV system. Figure 1 shows the block diagram of the proposed standalone system.

PV module

Figure 1. Bloc diagram of PV system

The system comprises of a PV module containing a number of PV cells connected in series to obtain a desired DC voltage, a DC/DC boost converter is used to generate a higher DC voltage, a DC/AC inverter is necessary to provide an AC output voltage. For the standalone application and non linear loads, Passive Filter must be used for harmonics mitigations.

II. MODELING OF STANDALONE PV SYSTEM

A. PVCell

Photovoltaic cell generates electricity by converting sunlight, due to the fact that the voltage and current output of a signal PV cell can be too small, PV cell is connected in series or parallel combination to obtain the voltage and current level suitable for practical use. The PV cell can be modeled as a current source Iph in parallel with a diode, shunt resistor Rsh and series Rs resistor as shown in figure 2.

978-1-4799-7336-1/14/$31.00 ©2014 IEEE

Rs I

Figure 2. Model of a PV cell

By applying Kirchhoff's law in the equivalent circuit of solar cell, the current generated I can be obtained as:

(1)

Where Iph is the light generating current which depends on the solar irradiance, Id is the current flowing through the diode which depends on the solar cell temperature; Ish is the current flowing in the equivalent shunt resistance of the solar cell [2].

Vd = V + RsI

Ish = (V + IRs)/Rsh qVd Id = Is[ eXP(nKT) -

1

n : ideality factor of p-n junction of cell. Vd: voltage across the diode. q: electronic charge: 1,602 x 10-19 C. K: Boltzmann's constant: 1,38x1O-23• T: solar temperature in Kelvin scale. Is: rated short circuit current of solar cell.

B. DC/DC Boost Converter:

(2)

(3)

(4)

A Boost converter is proposed and preferred of DC/DC converter in standalone PV system because he can step up small DC voltage produced by PV panel to a higher level suitable for the DCI AC inverter [2].

C. Full-Bridge Inverter:

Single phase Full-Bridge inverter is used to convert the DC output voltage of the DC/DC Boost Converter into AC voltage required for an AC loads, in the standalone PV system. Pulse Width Modulation (PWM) is used to create proper gating signals for switches. The gate signals are pulses obtained by comparing a reference sinusoidal signal Vref with a triangular signal Vc [2-3-4].

In general, there is much harmonic component in output inverter voltage, thus by choosing a high value of the carrier frequency facilitates filtering of current and reduces the harmonic output voltage but power switches have a limited time of conduction then it's necessary to find a compromise, hence a LC filter is designed and used to filter the high frequency harmonic in output inverter voltage.

The most commonly used indices to quantify voltage and current distortions are voltage and current THD that can be calculated as follows.

THDx =

"hmax X2 .:Jh=2 h

X� (5)

Where x can be the voltage or the current.

D. LC filter:

To reduce harmonics contained in output inverter voltage and to create a clean output sinusoidal voltage the LC Low pass filter is used. It is placed between the inverter and the load in a standalone PV system. The LC filter chosen is a second order which eliminates all high order harmonics; the filter inductance value L is calculated such that the voltage drop across the inductor is less than 3% of the inverter output voltage Vo [2].

2rr. f. L. ILmax < 0,03. Vo (6)

Where hmax is the maximum RMS (Root Mean Square) Load current, f is output voltage frequency 50Hz and Vo is the RMS value of inverter output voltage, the filter capacitance value C is then calculated from the resonance relation as given in:

1 C =

(2rrfo)2L

Where fo is cut off frequency.

III. HARMONIC PASSIVE FILTER

A. Non linear loads:

(7)

Non-linear loads consisting of components such as rectifiers, lighting electronic ballasts, fluorescent lights generate and inject current and voltage Harmonics in the power system. The main problems are additional power losses in the electrical equipment, errors in measurement. Therefore mitigation is required to maintain Power Quality and improve energy efficiency and reduce the potential for device failure by using Harmonic Passive filters [8].

B. Series-connected AC Reactor :

Series AC Reactor is constituted of an inductor connected in series with the non linear loads, this type of configurations is considered as a low pass filter. Figure 3 illustrates the basic configuration of series-connected reactor in the power system. It has the ability to pass low frequency harmonics and provides high impedance to high frequency harmonic currents to limit their proliferation into the power system. The merit of this filter is in its low cost, small size and provides no system resonance condition. The value of the inductor is set to a voltage drop of between 3% and 5% of the nominal voltage of the network [9].

Li

Voltage source

Non linear load

Figure 3. Series AC Reactor

C. Shunt Passive Filters:

Shunt Passive filters always been considered as a good solution to solve harmonic current problems [10], shunt

passive filters can be classified into three basic categories as follows: 1. Band pass filters (of single or double tuned). 2. High pass filters (of first, second, third-order or C-type). 3. Composite filters. Shunt passives filters as shown in figure 4.

Rr Lr

(a) Tuned filter (b) High pass filter

Figure 4. shunt passive filters

D. Single Tunedjilter design:

The single tuned filter consisting of inductor L[, capacitor Cr and small damping resistor Rf are connected in parallel with non linear loads to provide low-impedance paths for specific harmonic frequencies, thus resulting in absorbing the dominant harmonic currents flowing out of the load. Furthermore it also compensates reactive power at system operating frequency. The impedance versus frequency of this filter is shown:

1 + RfCfS + LfCfS2 Zf(S) =

CfS

Where S = j2rr[

(8)

Generally the filter capacitor is sized for a known reactive power compensation Qc required to improve power factor, Cr can be expressed as:

Qc ( 1 ) Cf =

2rrf1 U2 1 -

n2 (9)

Where U is the supply voltage, n is the harmonic order and fJ is a fundamental frequency.

At the harmonic frequency fn = n. f1 the filter reactor provides a series resonance.

(10)

The inductive value of the filter can be obtained from equation (l1) as:

1 Lf =

2 (2rrfn) Cf (11)

The value of the low-impedance Rr for each single-tuned filter is affected by the quality factor of the filter Q.

(12)

The quality factor Q determines the sharpness of tuning. Usually, a value of Q ranges between 20 and 100. High Q­value filter gives the best reduction in harmonic distortion. The interaction of the filter with the source reactance Ls always creates a parallel resonance condition addition to the series resonance frequency of the filter [11].

1 f - -:::---:-;====::::::::

p -

2rr( .JLf + Ls)Cf

E. High pass jilterdesign:

(13)

Higher order filter is a single-tuned filter where the Lh and Rh elements are connected in parallel instead of series. This connection results in a wide-band filter having impedance at high frequencies limited by the resistance Rh. Its total impedance is given by:

(14)

The values of capacitor Cit and reactor inductor Lh can be calculate with formula (9) and (11).

The value of resistance is calculated for a specific quality factor [12] as given by the equation (15):

Value of Rh should be Low to have Less Power Loss.

F. Analysis and description of system:

The previous descriptions imply that the passive harmonic filters can be characterized by their impedance variation with frequency. The harmonic currents and voltage of a system with a non linear load and a harmonic filter can be analyzed approximately by using the model shown in figure 5.

Passive filter

(b) IS�[ 1

Zs Zr t vs Non Ih

Linear Load

Figure 5. Hannonic circuit model of a system with a nonlinear load and a harmonic filter.

Where the non linear load is modeled as a current source of harmonic Ih, passive filters are modeled as impedance elements. The harmonic currents to system source and harmonic filter, and harmonic voltage in the system can be found as:

Zs = Zsource + ZLi (16)

(17)

Zs If =

Z +

Z . Ih

f s

Zf· Zs Vs =

Z +

Z . Ih

f s

Zfs· Zfh Zf =

Zfs +

Zfh

Is 1 H(S) = -=

Ih 1 + Zs Yf

1 1 1 Yf =-=-+-Zf Zfh Zfs

(18)

(19)

(20)

(21)

(22)

Where Zr is the equivalent impedance between tuned filter impedance Zfs and high order filter impedance Zth. Equation (17) and (19) show that the hannonic current Is and voltage Vs can be reduce by the harmonic filter. In the composite passive filter, combination of two lower order filters 3rd and 5tl1is designed to suppress lower harmonic frequencies, and one second order high pass filter is used for eliminating the high order frequencies.

2. = If is the equivalent admittance of the composite passive Zt filter is shown in figure 5:

1

Zf

80 ____________ � _________ _

60 _______ _

40 _

20

Frequency ( Hz)

Figure 6. Impedance frequency plot of passive filters combination

Figure 6 shows the impedance modulus plots for 3rd, 5th harmonics and after 7th harmonics high pass filter provides low impedance to attenuate high frequency harmonics.

The bode magnitude of transfer function of combination ( filters H(s) shown by equation (21) is plotted to assess the overall filter performance as shown in figure 7.

r-_+_-+-+++++H----+--+-+-+�++--�_+_++++++_-+_ .' -�- �- t - :- t t t , " '''' , . , '''' , " "" , , " '''' " . . , '''' " ' " "" ' " . , '''' ___ , __ J __ ,_ oJ_ '- � .JoJ ______ J ___ J __ L _ . _ L .J.'- L L ______ '- ___ L __ , __ '- .J.1 J oJ.J ______ J ___ J __ • _ .1_'_ L •• , , " '''' ' " . , '''' " ' " "" " " '''' , , " '''' " . . , '''' " ' " "" " " '''' , , " '''' " . . , '''' " ' " "" " ' " '''' , , " '''' " . . , '''' " ' " "" " , . '''' , , , ','" , , ' " '''' , " , ' "" ' " " ,'" , , " '''' ' " . , '''' " ' " "" ' " " '" --_. __ ... __ ._ ... _ ......... _----_ ... _-_. __ ._ ._ .... _ .... _----_ .. _-_ .. __ .-_ ..... _ ........ _----_ ._-_ . __ ._ .. _.- ... , , " '''' ' " " '''' " ' " "" ' " ' " " , , " '''' ' " " '''' " ' " "" ' " ' " "

-Oo '---��! �!�! -,-,! !�!o..L ! :_-'-!�! �!�!�!-'--'! !-"-'!:_�! �!�! �! !w!-,-,! !L: _�!�! �!�!�! w!!= i:�: ·10(J 10' 102

(Q (r adls)

Figure 7. Bode magnitude diagram of the transfer function for the passive filter.

The transfer function can be evaluated at low and high frequencies. For low frequencies, it has a OdB gain from OHz to the parallel resonant frequency in passive filter (Or. Hence, the harmonics filtering is divided between the two filters: the low order harmonics are compensated using the tuned filters, while the high-order harmonics are filtered by the high pass filter.

IV. SIMULA nON RESULTS:

The standalone PV system with passive filter connecting to a full wave bridge rectifier with RL Load is proposed and simulated using Matlab/Simulink in this paper as shown in figure 8, the simulation is carried out as series AC reactor alone, and combination of series reactor and two single tuned filters to the 3 rd and 5th harmonics and a high pass filter to compensate higher order harmonics.

ltl)] ,

---------------------------------- ._---------------- ------------------_ .

Voltage source Passive Filters Non Linear Load

Figure 8. Passive filter in standalone PV system with non linear load.

The harmonic spectra and THD for source voltage and source current are analyzed with no filter, with a series AC reactor and with composite filters.

Figure 9 illustrates the voltage waveform were sinusoidal after LC filter connecting.

Inverter ouput voltage with LC filter

:��V\ ..........•••••• I\ •..........••••• I\ ••..........•••• V\ •••..........•• I\ ••••.......... j 20: �···· ·

·

·

·

·

·

···M ····· ·

·

·

·

···M · ····· ·

·

·

·

·

·

·M · ·

·

····· ·

·

·

·

· ,M ········ ·

·

·

·M ·400"------'- ----''----'-- --'------'- ----''----'-- --'-----'-----' o � � � � � � � � � m

Time (,)

Figure 9_ Output voltage of the inverter with LC passive filter.

Figure 10 (a) and (b) show the source current and source voltage wavefonns before filtering.

Source current waveform Is

('1 J1JIY!JJJJJ o � � � � � � � � � m

Time(s)

SourcevoltagewaveformVs (blrSZ\zE\J1l\J o � � � � � � � � � m

Time (,)

Figure 10. source current and source voltage before filtering.

Figure 11 (a) and (b) show harmonic spectrum of source current and source voltage before compensation.

Figure II. (a) Current Harmonic Spectrum without passive filter.

Figure II. (b) Voltage Harmonic Spectrum without Passive filter.

Figure 12 (a) and (b) show the source current Is and sourc, voltage Vs wave forms after the fIrst solution: series inductor installation, as a result is much closer to sinusoidal one.

(a)

(b)

Figure 12. source current is and source voltage Vs with series inductor.

Figure 13 (a) and (b) show harmonic spectrum source current and supply voltage after series inductor installation.

Figure 13. (a) Current Harmonic Spectrum with series inductor.

Fundamental (50Hz) = 248.8 , THD= 6.63%

Frequency (Hz)

Figure 13. (b) Voltage Harmonic Spectrums with series inductor.

After series inductor installation THD in current was reduced to 8,31% from 17% whereas voltage hannonic distortion is reduced to 6,63% from 14,1%, it is clear that is an acceptable mitigation, through it is not coming with the limits specifIed by the std IEEE 519, its advantage is also that it does not cause the problem of resonance.

Figure 14 (a) and (b) show the source current and source voltage wavefonns after combination of shunt passive fIlter and series inductor.

(.) 1?k/\J\I\/\J 0.01 002 0.03 004 0.05 006 0.07 008 0.09 0.1

Time(s}

Source voltage waveform Vs

�)�� o � � � � � � � � � ru Time (s)

Figure 14. Source current and source Voltage with composite filter.

Figure 15 (a) and (b) show harmonic spectrum source current and supply voltage after Passive Filter combination.

The source current THD is drastically improved by use of Combination of passive fIlters to 1,09% and also the supply voltage is reduced to 2,03%.

Figure 15. (a) Source current spectra after filtering.

Figure 15. (b) Source Voltage spectra after filtering.

TABLE 1. TOTAL HARMONIC DISTORTION FOR CURRENT AND VOLTAGE.

Total harmonic Distortion THD

Harmoni Current source [s Voltage source Vs c order Without With With Without With With filter series passive filter series Passive illductor filter inductor filter

3,d 11.99% 7.00% 0.05% 10.70% 5.2% 0.12% 5" 6.74% 3.73% 0.02% 6.76% 2.92% 0.09% 7" 3.55% 2.03% 0.8% 4.35% 1.71% 0.79%

THD 17.00% 8.31% 1.09% 14.10% 6.63% 2.03%

Table I shows performance of THD of source current and source voltage of the system before compensation and with combination of passive filters. After connecting a line reactor we can see that source current and source voltage are improved as compared to the previous case, through it is not coming with the limits specified by the std IEEE 519, after connecting shunt passive filter, it is observed that the distortion of the mains current and voltage decreased to a level as mentioned in the std IEEE 519, THD of the voltage and current are lying below 2,03% and 1,09% respectively where as the limits specified by IEEE 519.

The R L, C parameters of the PFs for the simulation are given in table II and III.

TABLE n. V ALVES OF DESIGNED PASSIVE FILTERS

Filters C(F) L(H) R(Q)

Series reactor Li= 15e-3

Tuned filter C3m-60 e-6 L3,d -18.7 e-3 R3m -0.05

Tuned filter C5th-333.38 e-6 L5th -12.ISe-3 R;th -O.OS

High pass filter CHP-50 e-6 LHP -4.2 e-3 RHP -20

TABLE lll. PARAMETERS USED FOR SIMULATION

LC filter Lc = 2.2SmH; Cc = 4.7 JlF

PV source VDC=400v Vs=220V,f=50Hz

Load RL=IOf2; LL=25mH

v. CONCLUSION:

This paper has presented a harmonic mitigation study in the standalone system, using three types of passive filters namely, series reactor and shunt passive filters: single tuned and high pass filters in eliminating harmonics. Line reactor offers the advantage of his simplicity and low cost it provides no system resonance condition and its can achieve a significant reduction in harmonics but the total harmonic distortion cannot be below to 5% that is why use of shunt passive filters was necessary to improve system quality.

The proposed solution allows a better performance compensation of the source current and source voltage at a high level THDI = 1,09% THDV= 2,03% in the simulation. Our results meet the IEEE 519 recommended harmonic standard.

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