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ISGT14 1569885965
Dept. of Electrical Engineering
University of Malaya
Kuala Lumpur 50603, Malaysia papantec@siswa.um.edu.my
Abstract — A hybrid compensator which is a combination of a three phase four wire shunt active power filter and a parallel passive filter has been presented in this paper. The dominant lower order harmonics as well as reactive power can be compensated by passive elements whereas the active part mitigates remaining distortions and improves the power quality.
Modified phase lock loop based synchronous reference frame control strategy is adopted here for active filtering system. The proposed three phase hybrid line conditioning system can be quite effective for recompensing harmonics, reactive power & neutral current under unbalanced grid conditions. The simulated results obtained by MATLAB/SIMULINK power system block set are examined in detail for the validity of suggested approach. A laboratory prototype has been built on dSPACE1104 platform to verify the feasibility of the suggested
SHAPF controller.
Dept. of Electrical Engineering
University of Malaya
Kuala Lumpur 50603, Malaysia saad@um.edu.my
Index Terms — Active power filter (APF); phase locked loop
(PLL); harmonics; power quality; synchronous reference frame
(SRF)
I.
INTRODUCTION
Owing to the wide application of power electronic converter continuity of power flow become polluted due to the burden of high reactive power, contaminated harmonically and unbalancing load currents. Hence to deliver clean power several methods were proposed by the researchers [1].
as cancelling out nonlinear load harmonic currents. Added with harmonics compensating ability, APFs are also capable for reactive power reduction and load balancing [3]. Due to no resonance problems in the system, APF draws a significant attention over those tuned passive filters. But it is restricted by high cost and low capacity of switching devices.
If we use hybrid configuration it brings down the cost of the active filter significantly and be more practicable in industry applications [4].Because of avoiding complexity and unreliability of series hybrid filtering, a shunt active filter base hybrid filter topology has been considered in this paper for its capability of attenuation and effective operation [5, 6].
Generations of current references using the harmonic extraction method, feedback current controlling technique and the APF inverter circuit behavior in dynamic environment are considered as indispensable role for the APF performance. So, for varying loads, it is essential in active power filters to detect components of harmonics in current with quickly and accurately. Many harmonic revealing approaches were suggested and their dynamic performances were also assessed in papers that can be established in the literature. Among those, control scheme based on SRF is one of the utmost conventional and the most practically applicable method [6]. It performs an excellent job but it requires a PLL circuit for synchronization.
Passive filters whose work as least impedance path to tuned harmonic frequencies Though simple and less expensive but for several drawbacks like fixed compensation, bulky devices and resonance problem of those L-C filters
APF has been developed for complete compensation of distortions [2]. The APFs use power electronics converters whose insert harmonic components to the electrical network
In this paper a new technique constructed on SRF using the adapted PLL algorithm is demonstrated and analyzed its performances for unbalance source condition. Here,
Hysteresis current regulator is used to generate switching signals of APF and for maintaining the dc link capacitor voltage a proportion-integral (PI) controller is introduced.
The authors wish to acknowledge the financial support from university of
Malaya HIR-MOHE project UM.C/HIR/MOHE/ENG/24.
1

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Simulation and technical analysis of those result shows the validation of proposed strategy.
[9]. In this study, the improved PLL developed in fig.2 is put forward for determination of positive sequence components with stability and rapidity.
The structure of this paper is as following: The hybrid active power filter is discussed in section II. Section III describes the control algorithm of APF .Simulation results and their brief analyses are presented in section IV.
Experimental results are presented in section V. Conclusions are drawn finally in section VI.
Figure 2. PLL circuit block diagram
Figure 1. Basic structure pattern of SHAPF
II. HYBRID ACTIVE POWER FILTER TOPOLOGY
The modified PLL circuit use Clarke and park transform for identifying the peak of positive sequence voltage. The three phase unbalance voltages are converted to stationary coordinate system. Through the measured phase angle, the voltages in stationary co-ordination are transformed to DQ.
A PI controller forced D-axis component V d to zero in order to align the mains voltage space vector with Q-axis [10]. The estimated phase angle θ which is attained by integrating the proportion-integral (PI) controller output is in turn used for coordinate transformation process. Here, a second order resonant filter is used for suppress the double fundamental frequency which is created for unbalancing and the gain adaptation block shows convergence of any value of system voltages and creates normalized templates of fundamental
[11]. The rate limiter block works as an attenuator against ripple. The reformed PLL can go satisfactorily as long as the gains of PI are adjusted accordingly under highly inaccurate and disturbed system voltages.
A.
Circuit topology
Fig. 1 presents the basic structure of SHAPF. The proposed hybrid filter structure comprises of shunt passive filter and shunt active filter. A 3-leg inverter with split capacitor works as APF. It is designed to be associated in parallel with the single phase and three phase loads that’s considered as a non-linear and unbalance load for a 3-phase
4-wire distribution system [7] while its complex control circuitry and massive dc link capacitors are necessary for perfect operation. The inner point of every branch is attached to the power network through an inductor which is used to filter the ripples of inverter current. The considered LC passive filter at 5th harmonic tuned frequency is connected in shunt to the power line before the load. It provides low impedance trap to harmonic to which the filter is tuned and correspondingly aids in reduction in active filter power rating.
III. CONTROL ALGORITHM
A.PLL Operation
In order to safe and consistent operation of active power filter under unbalance and distorted grid voltage situation phase and frequency extraction of positive sequence fundamental component of voltage should obtain quickly and accurately[8]. Because of dynamic behavior SRF-PLL performance is dissatisfactory under non ideal voltage mains
For non-ideal mains voltage
Vsabc =
⎡
⎢
V
⎣
⎢
⎢
V
V a b c
⎥
⎥
⎦
⎤
⎥
=
⎢
⎢
⎡
⎢
⎢ ⎣
V
V
V a b c
+
+
+
⎥
⎥
⎤
⎥
⎥ ⎦
+
⎢
⎢
⎡ V
⎣
⎢
⎢
V
V a b c
−
−
_
⎥
⎥
⎤
⎥
⎥ ⎦
+
⎡
⎢
V
⎣
⎢
⎢
V
V a 0 b 0 c 0
⎥
⎥
⎦
⎤
⎥ (1)
= V +
⎡
⎢
⎢
⎣
Sin
Sin
Sin
(
(
θ
θ
θ
−
+
2 π
2 π
/
/
3 )
3 )
⎥
⎦
⎤
⎥
+ V −
⎡
⎢
⎢
⎣
Sin
Sin
Sin
(
(
θ
θ
θ
−
+
2 π
2 π
/
/
3 )
⎤
⎥
3
⎥
⎦
+
⎡
⎢
Sin
Vo
⎢
⎢
Sin (
⎣ Sin (
θ
θ
θ
−
+
2 π
2 π
/
/
3 )
3 )
⎥
⎦
⎤
⎥
(2)
Using transform, the voltage vectors are:
αβ
=
⎡
⎢
⎣
α
β
⎤
⎥
⎦
=
[ ]
abc
(3)
Where,
[ T
αβ
] = 2
3
⎡
⎢
⎢
⎢
1
⎣
0
− 1
2
3
2
− 1
2
3
2
⎥
⎥
⎦
⎤
⎥
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So
V
αβ
=
⎡
⎢
V
V
α
β
⎤
⎥ =
⎡
⎢
−
V
V
+ Sin
+ Cos
θ
θ
+
+
+
+
V
V
+ Sin θ
− Cos
−
θ −
⎤
⎥
(4)
Performing d-q transform
V dq
=
⎡
⎣
V
V d q
⎤
⎥ =
[ ]
V
αβ
= ⎡
⎢
−
Cos
Sin
θ
θ
^
^
Sin
Cos
θ
θ
^
^
⎤
⎥
⎡
⎢
−
V
V
+
+
Sin
Cos
θ
θ
+
+
+
+
V
V
+
−
Sin
Cos
θ − ⎤
⎥
θ −
= ⎡
⎢
−
V
V
+
+
Sin
Cos
( θ
( θ
+
+
−
+
θ
θ
^
^
)
)
+
+
V
V
− Sin
− Cos
( θ
( θ
−
−
+ θ
+ θ
^ )
^ )
⎤
⎥
(5)
The estimated phase angle= θ
θ
^ ; assuming
θ
θ ^ ≈ successfully tracks the phase at ^ = + = θ −
So,
⎡
⎢
V
V d q
⎤
⎥ ≈
⎡
⎢
− V
V
−
−
+
Sin
V
( 2 θ
+ Cos
^
(
)
2 θ ^ )
⎤
⎥
ω t . PLL
(6)
Here 2 θ ^ is the double frequency to be eliminated. It is the basic concept of the modified PLL structure. It can provide the positive sequence component by cancellation of 2 ω oscillations
The system under observation is three phase-four wire wherever active component i d
and oscillating part i q
are reflected. After the load currents i d
and i q
are found, they are allowable to pass over a low pass filter to separate ac and dc part where the active and reactive fundamental current components (i dDC, i qDC) are obtained. The both currents
(i dAC, i qAC) ac parts are related with the responsibility for active and non-active harmonic components. The filters used in the circuit are the 2 nd order butter worth type and their cut off frequency is identical to one half of the fundamental frequency. With consideration of the reactive current,
Passive filter provides its DC value while VSI delivers an
AC voltage to sink the harmonics [12]. The filtered active and non-active currents from eqn.2 are used for generation of the accurate references to the modulator
⎣
⎢
⎡ i dAC i qAC
⎦
⎥
⎤
=
⎣
⎢
⎡ i i d q
⎦
⎥
⎤
−
⎣
⎢
⎡ i dDC i qDC
⎦
⎥
⎤
(8)
The three phase load currents are measured using halleffect current sensor and converted into d-q-0 by means of a rotational frame synchronous with the system voltage positive sequence in eqn.1.
As well as providing harmonic currents, the dc voltage of
PWM VSI should be maintained for accurate operation. The voltage of capacitor is controlled by regulating the reactive current as shown in fig 3.Then the abc frame reference currents are
⎡
⎢ i
CA
⎢
⎣ i i
CB
CC
⎥
⎦
⎤
⎥
=
⎡
⎢
⎢
⎢
⎢
⎢
⎣ sin( sin( sin(
ω
ω s s t t
ω
− s t )
2 π
+
2
3
π
3
)
) cos( cos( cos(
ω
ω s s t t
ω
− s t
2
)
π
+
2
3
π
3
)
)
⎤
⎥
⎥
⎥
⎥
⎥
⎦
⎡
⎢ i i d q
*
*
⎤
⎥
(9)
⎡
⎢ i
⎢
⎣ i i q d
0
⎥
⎦
⎤
⎥
=
2
3
⎡
⎢
⎢ sin(
⎢
⎣
⎢
⎢ cos(
1
ω
ω
2 s s t ) t ) sin( ω s t cos( ω s t
1
−
−
2 π
2
3
π
3
)
)
2 sin( ω s t + cos( ω s t +
1
2
2 π
2
3
π
3
)
) ⎥
⎥
⎥
⎦
⎤
⎥
⎥
⎢
⎣ i i
⎡
⎢ i
A
B
C
⎥
⎦
⎤
⎥
(7)
s t is considered here as the transformation-angle of positive sequence source voltage and it is delivered by the proposed phase lock loop.
The dc voltage can be build up in APF and regulate across dc capacitors by itself . The capacitance is designated such that the voltage ripple is less than 1%. Selection of dc value confirms that the current time derivatives of converter supply must be required for the compensation of selected harmonics
[13]. Using this concept the capacitor voltage is chosen from:
C dc
=
3 ..
ω
π
.
V
I dc f
( p − p ) max
(10)
In order to control inverter current actively the nominal dc bus voltage V dc should be larger than or identical to line-line peak voltage [13]. The value of voltage is preferred for a specific non-linear load which is function of rated load power and the compensated maximum distortion. According to the capacity of the system it can be selected as
S
<
dc
≤ 2
S
.
Figure 3. SRF based control block diagram
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4 variation of load current harmonics and switching frequency of the devices.
IV. SIMULATION RESULTS
Figure 4. DC voltage optimization with PI approach
A proportional–integral controller is used to regulate the voltage of DC link and added to active part of the fundamental load current. The obtained capacitor voltage is matched with a set reference value. The loss component is then handled concluded a PI controller which contributes to the zero steady error in tracking the reference current signal and it is shown in fig 4. The supply current peak value is calculated using the output component of PI controller.
Performance of 3 legs- 4 wire Hybrid active power filter for filtering current distortions, compensation of neutral current, load current balancing and reactive power mitigation have been examined under unbalance source-unbalance load condition. The goal of this case study is to demonstrate the validity and evaluation of proposed approach, if the source is greatly unbalanced. Using power system blockset in matlabsimulink for three phases four wire power network with shunt active and shunt passive filter the presented simulation results are obtained. The results are specified before compensation, compensation using only passive filter and after the operation of hybrid filter in the following simulation studies. Three and single phase diode rectifier nonlinear loads are coupled in power system for producing unbalance, reactive current and harmonics in load current as well as neutral current. Single phase diode rectifier with RL load in
DC side is connected in ‘c’ phase for evaluation of the dynamic performance. The widespread simulation outcomes are discussed below.
For generation of commanded compensating current switching strategy is developed widely. A hysteresis comparator is used here for quick response and appropriate no.sinusoidal current tracking capability. Moreover it is easy for implementation than other switching methods. The active filter switching patterns are decided by the current controller
[13]. The desired current, I la
(t), and the injected inverter current
I la
*(t) are compared with one another. The logic of switching is as followed:
If I l a
< (I l a
*-hb) upper switch S1 is OFF & lower switch S2 is
ON in leg of ‘a’ of active power filter.
If I l a
> (I l a
*-hb) upper switchS1 is ON & lower switch S2 is
OFF in leg of ‘a’ of active power filter.
Correspondingly in the legs’b’ & ‘c’ the switches are initiated. Here ‘hb’ is hysteresis band and determine the a
When 3 phase source voltages are not balanced properly, actual values of phase voltages are not identical and negative sequence fundamental voltage component will be present in system voltage.
The three phase’s unbalanced voltages are considered as:
V a
= 100 sin ω t,V b
= 80 sin( ω t-120) andV c
= 50 sin( ω t+120)
Simulation results with passive filter and hybrid active filter for eliminated harmonic current and load current balancing under this condition are shown in fig.6 b c d e f g h i
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Figure 6. Simulation results for SHAPF operational performance a) Source voltage b) load current c)THD before compensation d)passive filtering load current e)THD after passive filtering f)Load current after hybrid filtering g) THD after hybrid filtering h)injecting current i) DC link voltage j)instantaneous reactive power k) instantaneous active power l)Source neutral current
Before filtering the non-linear load current THD level is
21.20%. With passive filter alone the three phase obtained currents are not pure sinusoidal. As mains voltage negative sequence part is eliminated with hybrid filter, balanced and sinusoidal load currents are achieved after compensation.
TABLE I. SIMULATION RESULTS FOR UNBALANCE SOURCE
Load current
THD (%)
A-phase
B-phase
C-phase
Neutral
RMS(A)
A-phase
B-phase
C-phase
Before filter Passive filter HAPF
Compensation of reactive current, neutral current elimination and dc link voltage is maintained properly with the proposed method. Comprehensive compensation of load currents and their harmonics level are shown in table1.under unbalance source & unbalance load condition.
V. EXPERIMENTAL RESULTS
The performance of the power inverter is first verified under balanced grid condition. Before turning on the inverter source voltage is sinusoidal but load current is highly
9.87 6.17 1.14 filter the value of load current THD is reduced with 11.2%.
But when APF is connected in shunt with line and passive a b c d e f g h
Figure 7..Experimental results for unbalance source a) Supply voltage (25v/div) b) Load current before compensation (0.8A/div) c) %THD before compensation d) Load current after passive filtering e) %THD after passive filter f) DC link voltage and load current after compensation g) %THD after
SHAPF compensation h) injected filter current i) PLL output (sinwt, coswt vector)
5 i

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TABLE II. SIMULATION PARAMETERS
Parameters
Source impedance
Load impedance
Passive Filter
Shunt APF
Values
R=0.01ohm,L=2mH
3-phase diode rectifier R=80 Ω ,L=60mH
1-phase diode rectifier R=0.1
Ω ,L=1 mH
5 th harmonic tuned, R=0.2
Ω ,L=5mh,C=40 μ F
R=0.001ohm,L=2mH,TwoDClink capacitor=2100 μ F, Vdc=220 V,HB=±2A
100V(RMS),50 HZ Source voltage
Parameters of the SHAPF system used in this study are listed in table II. In addition a comparison between proposed topology and [3], [9] is shown in table III in terms of control method, THD% and balancing the load current according different conditions. The harmonic extraction algorithms are built on same concept with slight modification and DC link voltage control is maintained through considering the active or reactive part of the filtered ac current. Regarding their performance it is evaluated and the proposed technique formed better compensation.
TABLE III.COMPARISON BETWEEN RECOMMENDED TOPOLOGY
AND [3], [9]
Topology description
Property
Control Method
PLL
RMS current
Unbalance source
THD%
Unbalance source
Hybrid filter(shunt active +shunt passive) proposed
Modified
SRF,Vdc=220 v
Pre-filter+
SRF PLL
unbalance load
Phase a-7.02A
Phase a-1.98
Unbalance load
Shunt active
(Zaveri et al.,
2012)
Idq control
Balance load
Phase a-6.2A
Phase a-4.18
Balance load
UPQC,(Kesler
&Ozdemir,
2011)
SRF,
Vdc=700v
Balance load
Phase a-14.3A
Phase a-3.3
Balance load
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
VI. CONCLUSION
A new SRF control technique based on advanced PLL used in SHAPF has been successfully alleviated the load current harmonics along with reactive power compensation, reduction of excessive neutral current and the load current balancing under non ideal supply and unbalance load condition. It can also be seen that regulation of DC voltage is stable, free from overshoot and no steady state error. The computer simulation and experimental verification proves its effectiveness and this will be really useful for the distribution system. As compared to simulation results, the experimental
THD is slightly more due to accuracy limit of sensors and sampling time limit of DSP in dSPACE.
[13]
S. Rahmani, A. Hamadi, K. Al-Haddad, and A. Alolah, "A DSPbased implementation of an instantaneous current control for a three-phase shunt hybrid power filter," Mathematics and
Computers in Simulation, 2012.
J. Tian, Q. Chen, and B. Xie, "Series hybrid active power filter based on controllable harmonic impedance," IET Power
Electronics, vol. 5, pp. 142-148, 2012.
N. Zaveri and A. Chudasama, "Control strategies for harmonic mitigation and power factor correction using shunt active filter under various source voltage conditions," International Journal of Electrical Power & Energy Systems, vol. 42, pp. 661-671,
2012.
C.-S. Lam, M.-C. Wong and Y.-D. Han, "Hysteresis current control of hybrid active power filters," IET Power Electronics, vol. 5, pp. 1175-1187, 2012.
T. Messikh, S. Mekhilef, and N. A. Rahim, “ Adaptive Notch
Filter for Harmonic Current Mitigation”, International Journal of
Electrical and Electronics Engineering, Vol. 1 No. 4 2008, pp.
240-246.
Y. Suresh, A. Panda, and M. Suresh, "Real-time implementation of adaptive fuzzy hysteresis-band current control technique for shunt active power filter," IET Power Electronics, vol. 5, pp.
1188-1195, 2012.
P. Salmeron and S. P. Litrán, "A control strategy for hybrid power filter to compensate four-wires three-phase systems,"
IEEE Transactions on Power Electronics, , vol. 25, pp. 1923-
1931, 2010.
R. Belaidi, A. Haddouche, and H. Guendouz, "Fuzzy Logic
Controller Based Three-Phase Shunt Active Power Filter for
Compensating Harmonics and Reactive Power under Unbalanced
Mains Voltages," Energy Procedia, vol. 18, pp. 560-570, 2012.
M. Kesler and E. Ozdemir, "Synchronous-Reference-Frame-
Based Control Method for UPQC under Unbalanced and
Distorted Load Conditions," IEEE Transactions on Industrial
Electronics, vol. 58, pp. 3967-3975, 2011.
R. K. Sinha and P. Sensarma, "A pre-filter based PLL for threephase grid connected applications," Electric Power Systems
Research, vol. 81, pp. 129-137, 2011.
V. F. Corasaniti, M. B. Barbieri, P. L. Arnera, and M. I. Valla,
"Hybrid power filter to enhance power quality in a mediumvoltage distribution network," IEEE Transactions on Industrial
Electronics, vol. 56, pp. 2885-2893, 2009.
Mekhilef, S.; Abdul Kadir, M. N.; Salam, Z., “ Digital Control of
Three Phase Three-Stage Hybrid Multilevel Inverter”, IEEE
Transactions on Industrial Informatics, Volume 9, Issue: 2, pp.
719 - 727 (2013)
S Mekhilef, M Tarek,’’ Single-phase Hybrid Active Power Filter withAdaptive Notch Filter for Harmonic Current Estimation’’,
IETE Journal of Research , vol 57, pp.20-28, 2011
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