88 International Journal for Modern Trends in Science and Technology
MAF-PLL based Shunt Active Power Filter for Current Harmonic Elimination
Pavan Kumar Adivishnu1 | Satyanarayana Vanapalli2
1Associate Professor, Department of EEE, Ramachandra College of Engineering, Vatluru(V), Pedapadu(M), West Godavari(Dt), Andhra Pradesh
2Professor, Department of EEE, Ramachandra College of Engineering, Vatluru(V), Pedapadu(M), West Godavari(Dt), Andhra Pradesh
To Cite this Article
Pavan Kumar Adivishnu and Satyanarayana Vanapalli, “MAF-PLL based Shunt Active Power Filter for Current Harmonic Elimination”, International Journal for Modern Trends in Science and Technology, Vol. 04, Issue 04, April 2018, pp.-88-92.
This paper presents a novel indirect current control technique for current harmonic elimination using a Shunt Active Power Filter (ShAPF). The control technique is developed with the help of Moving Average Filter (MAF) based Phase Locked Loop (PLL). PLLs are used in control circuit of ShAPF for determination of fundamental frequency and phase angle of distorted voltage and/or current signals. The standard PLLs used in control circuit of ShAPF suffers with slow dynamic response. To overcome the drawback a new control scheme based on MAF with PLL is implemented for disturbance extraction from the input signals. The effectiveness of the control scheme is investigated with the help of MATLAB based simulation studies for current harmonic elimination in a power distribution system supplying harmonic sensitive loads.
Keywords: Moving Average Filter, Phase Locked Loop, Shunt Active Power Filter, dynamic response.
Copyright © 2018 International Journal for Modern Trends in Science and Technology All rights reserved.
I. INTRODUCTION
ShAFs provide an effective approach for compensating current harmonics present supply currents in any distribution system. ShAPF were initially introduced by H. Sasaki and T. Machida [1]
for removing current harmonics. When Active Filters are connected across the harmonic load these are called ShAPF. The ShAPF is suitable for suppressing harmonics introduced by nonlinear loads into supply system. These are smaller in size, versatile and provide better damping in comparison with passive filters [2]. The performance of harmonic compensation of any ShAPF is determined by the compensation current tracking method [3, 4]. Several methods have been reported in literature for effective extraction of harmonic components. The approaches such as
using a low pass filter (LPF), pq method, dq method, techniques based on Kalman filtering, Discrete Fourier transform are most commonly in use.
Moran L.A et al [5] used LPF for extraction of reactive and harmonic currents for fixed frequency systems, but the filter introduces a phase shift at the output; this requires a compensator for the phase compensation. The compensator is sensitive to frequency variations; any change in frequency introduces an error in the compensation network, which in turn gets added to the fundamental component of the reference currents.
Akagi. H et. al [6] introduced the pq method, this method gives satisfactory performance under balanced and steady-state conditions, but under supply voltage distortions or variations the reference currents that were produced are ABSTRACT
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International Journal for Modern Trends in Science and Technology
ISSN: 2455-3778 :: Volume: 04, Issue No: 04, April 2018
89 International Journal for Modern Trends in Science and Technology Elimination
inaccurate this leads to inaccurate compensation [7].
PI control is the most commonly used control scheme for achieving good dynamic response and DC signal tracking. For tracking of AC Signal PI Control is not suitable due to its band width limitation and as the current reference of ShAPPF contains series of harmonic components which makes it difficult to maintain stability. To achieve better harmonic compensation the AC signal condition should be achieved with multiple PI current controllers with selective harmonic elimination in multi-reference frames. In this control scheme a series of harmonic current regulators are designed for compensating specific harmonic component in current and all such controllers are connected in parallel. This method requires more computational resources and multiple coordinate transformations and the speed of dynamic response significantly reduced.
This paper mainly discusses extraction of reference currents required to control ShAPF.
Proper Extraction of reference currents provides efficient control. The remaining part of the paper is covers the concepts of MAF-PLL, Harmonic Compensation using ShAPF, Control of ShAPF using MAF-PLL and finally simulation results are presented in section 5 followed by conclusion.
II. MOVING AVERAGE FILTER BASED PHASE LOCK LOOP (MAF-PLL)
A discrete Moving Average Filter based Phase lock loop (MAF-PLL) is used for identification of phase of distorted waves.
The transfer function of Moving Average Filter (MAF) in z – domain is given by
1 1 1 1
z z N z X
z
Y N
(1) Discrete realization of MAF represented by (1) is
shown in Fig.1.
Fig.1: Discrete realization of MAF represented in (1).
As the utility grid supplies AC Voltages it is easy to use fixed length windows, this makes implementation of MAF very easy and achieves high computational efficiency.
The actual implementation of MAF-PLL is shown in Fig.2. The instantaneous three phase signals are sampled and are transformed into direct axis and quadrature axis components using park transformation given in (2), x may be either 3 phase signals of voltage or current.
c b a
q d
x x x
t t
t
t t
t x
x
3 cos 2
3 cos 2
cos
3 sin 2
3 sin 2 sin
(2)
These values xd and xq are given as input to MAF to find out the average value of the signal over a cycle. The output values xd and xq that are available across the output terminals of MAF are transformed into instantaneous values by applying the inverse transformation given in (3) the fundamental component of the input signal is obtained.
Fig.2: Block Diagram for extraction of phase angle and fundamental using MAF-PLL.
q d
c b a
x x
t t
t t
t t
x x x
3 sin 2
3 cos 2
3 sin 2
3 cos 2
sin cos
(3)
When the source voltages and currents are ideal and balanced, the dq components of the source vector appears as DC values.
III. HARMONIC CURRENT COMPENSATION USING SHAPF
Fig.3: Schematic of ShAPF used for simulation.
90 International Journal for Modern Trends in Science and Technology Fig.3 shows the schematic representation of
ShAPF. The ShAPF is controlled to inject / draw the harmonic current into / from the utility, so that the harmonics on the AC side of the utility will get cancelled and it makes the source current in phase with the source voltage.
3.1 Currents required for compensation
From Fig.3 the instantaneous currents are written
as is
t iL
t ic t (4) Source voltage is given by
t V
tvs msin (5)
When any non-linear load is connected to the utility, the source current get distorted, this distorted current contains harmonic components in addition to fundamental component of current.
n
n n
L t I n t
i
sin
1
(6)
n
n
n n t
I t
I L t
i
sin sin
2 1
1 (7)
Instantaneous power of the load is given by
t v
t i tpL s *L* (8)
n
n n m
m m
L
t n I t V
t t I V t
I V t p
sin sin
sin cos sin cos
sin
2
1 1
1 2
1
(9)
The above equation i.e (9) represents the instantaneous real power, reactive power of fundamental component and harmonic power consumed by load.
From (6) it is clear that the ShAPF need to meet the demands of reactive power and harmonic power required by load. If this is achieved by ShAPF then the source current is in phase with the utility voltage and pure sinusoidal one. To meet this filter must supply the required compensation current whose value is given by
t i
t i tic L s (10)
For accurate and instantaneous compensation of reactive and harmonic power it is necessary to estimate the fundamental component of load current i.e is(t) is taken as reference current.
3.2 Reference Current Estimation
Ideal compensation requires the utility current to be pure sinusoidal and is in phase with the source voltage irrespective of load current.
The desired currents after compensation are given by
3 sin 2
3 sin 2
sin
*
*
*
t I
t I
t I
i i i
sp sp
sp
sc sb sa
(11)
IV. CONTROL OF SHAPF USING MAF-PLL To verify the correctness of the MAF – PLL described in section 2, the developed PLL is used for disturbance extraction of a harmonic polluted source currents. The system shown in Fig. 4 is considered for analysis. A Three phase supply is used to feed a non-linear load comprising of a diode rectifier with a resistive load at its DC side. This non-linear load introduces a distortion into supply currents with Total Harmonic Distortion equal to 15.18%. A ShAPF is connected across the load terminals for absorption of the disturbance caused in the supply currents. The ShAPF injects / absorbs distorted currents in phase opposition / in phase to those of the supply distortion to make the supply current free from distortions or harmonics.
The ShAPF consists of a Voltage Source Converter. The gate pulses for the Voltage Source Converter are generated by a hysteresis type pulse generator. These reference currents are generated using MAF – PLL.
V. SIMULATION RESULTS
The system is simulated for balanced supply voltages. The ShAPF is added to the system at t = 0.1 Sec. The load current, source current and the filter currents are measured before and after addition of ShAPF to the system.
Fig.4: Load Current in Amps for Phases A, B and C before and after addition of ShAPF.
91 International Journal for Modern Trends in Science and Technology Elimination
Fig.5: Source Current in Amps for Phases A, B and C before and after addition of ShAPF.
Fig.4 shows the three phase currents of load before and after ShAPF addition. Fig.5 shows the source current before and after addition of ShAPF, Fig.6 shows the reference current estimated by MAF – PLL and Fig.7 shows currents injected by ShAPF. In Fig.8 Total Harmonic Distortion of Supply current before addition of ShAPF are presented. Fig.9 represents Total Harmonic Distortion of Supply current of Phase A before addition of ShAPF. Total Harmonic Distortion of reference currents using MAF – PLL applied to ShAPF is presented in Fig.10.
VI. CONCLUSION
In this paper a new indirect current control method based on MAF-PLL applied to ShAPF is presented. The Control Structure of MAF-PLL is implemented with the help of MATLAB - SIMULINK. The implemented scheme is applied to a ShAPF for for current harmonic elimination. The model is simulated for the different operating conditions of utility voltage for sag / swell and harmonics. The simulation thus obtained reveals that this method can effectively eliminate current harmonics from distorted utility currents and maintain THD in utility currents within the limits.
Fig.7: Reference Current in Amps for Phases A, B and C applied to ShAPF.
Fig.8: Current injected by ShAPF in Amps into Phases A, B and C
Fig.9: Total Harmonic Distortion in Phase A of Source current before addititon of ShAPF.
Fig.10: Total Harmonic Distortion in Phase A of Source current after addition of ShAPF.
Fig.10: Total Harmonic Distortion in Phase A of reference current estimated by MAF-PLL
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