2018 5th International Conference on Electrical and Electronics Engineering

Comparison of Reference Current Generation Techniques for Shunt Active Power

Filter

Ikramullah Muhammad Ashraf

Department of Electrical Engineering Department of Electrical Engineering
Capital University of Science and Technology Capital University of Science and Technology
Islamabad, Pakistan Islamabad, Pakistan
e-mail: ikramullah_73@yahoo.com e-mail: ashraf@cust.edu.pk

Abdur Rehman
Department of Electrical Engineering
Capital University of Science and Technology
Islamabad, Pakistan
e-mail: rehmanmstel@gmail.com

Abstract—In this paper a comparison is made between three over the past few years. Different methods [5] like K-rated
different control strategies for reference current generation in transformers, separate neutral conductors, zigzag
three-phase three-wire shunt active power filter. Two autotransformer, phase shifting, series line inductors and
approaches are being used for this purpose; one is time domain harmonic filters are employed for mitigation of harmonics.
and the other is frequency domain. The three techniques Active power filters (APF) have been used for harmonics
considered are Instantaneous Reactive Power Theory, suppression and reactive power compensation in order to
Synchronous Reference Frame theory and Discrete Fourier enhance the power quality. Gyugi and Strycula [6] were the
Transform. These methods are deeply analyzed under various first to introduce the concept of Active Power Filters. Due to
non-linear load conditions. The performance of these
significant growth in power electronics, the use of APF has
techniques is evaluated by measuring Total Harmonic
Distortion of the source current before and after compensation.
become the dynamic solution for reduction of harmonics
The results are evaluated and compared using because of having unique advantages as compared to passive
MATLAB/Simulink. filter devices [7]. The Shunt Active Power Filter (SAPF)
connected in parallel with the load is used for mitigation of
Keywords-active power filter; reference current generation; current harmonics. The process involves detection of
harmonics mitigation harmonic component, generation of reference current and
generation of the gate pulses for the power circuit to inject
I. INTRODUCTION the reference current. The power circuit consists of a voltage
With the advancement of technology, the dependency on source inverter with a DC link capacitor. An active power
the electrical energy has been increased greatly. The filter cancels out the current harmonics introduced by the
performance efficiency of different electric apparatus is nonlinear load by injecting reference current in parallel to the
directly affected by power quality. The power-quality load, thus making the waveform of the current supplied
problem is identified by a distortion in the voltage waveform through the source, sinusoidal. The basic structure of Shunt
of the power source from a sine wave, or in the amplitude Active Power Filter is shown in Fig. 1. The source voltage
from an established reference level [1]. Different types of Vs is assumed pure sinusoidal. The load current is
disturbances that affect the power quality are voltage represented by IL. The reference current generation block
fluctuations, voltage sags, voltage unbalance, transients and detects the harmonic current waveform and generates the
harmonics [2]. reference current IR, which is anti-phase to the harmonic
Harmonics is known to be a major issue out of all power current. The switching pulses for voltage source inverter are
quality problems [3]. The increased use of power electronics generated by the current injection controller in order to inject
equipment has resulted in rise of harmonics in the power IR at the point of common coupling (PCC). The injected
system. Harmonics can be defined as currents or voltages current with addition to load current will make the source
which are integer multiples of fundamental frequency [4]. current very near to pure sinusoid. The parameter used to
Nonlinear loads connected with the distribution system result evaluate the performance of APF is Total Harmonic
in harmonic currents. These current harmonics will be Distortion (THD). A THD value close to zero proves a very
responsible for power factor reduction, power system voltage good performance of APF in terms of harmonics mitigation.
fluctuations, decrease in efficiency and interference during Reference current can be extracted using approaches both
communications. The Harmonic currents are increasing in frequency-domain and time-domain. In Discrete Fourier
dramatically in all residential, commercial, and industrial Transform (DFT) which is a frequency domain technique,
installations as the nonlinear loads are increasing rapidly the reference current is extracted from the load current
waveform by Fourier analysis. In the time domain techniques

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like Instantaneous Reactive Power (p-q) theory and wire system is considered, so the zero sequence power and
Synchronous Reference Frame (d-q) theory, coordinate current are ignored.
transformation is used to extract the reference current.
= − (4)

This active and reactive power can be expressed in two

parts i.e. AC and DC as given by equations (5) and (6).

= + (5)

= + (6)

In order to get DC part of active and reactive power p and

q signal need to be passed through low pass filter. Low pass
filter will filter out high frequency component and will give
expected signal i.e. Fundamental part.
Figure 1. Basic structure of Shunt Active Power Filter In the next step the reference current in α- β co-ordinates is
calculated by equation (7).
II. REFERENCE CURRENT GENERATION TECHNIQUES
The extraction of reference current is the important part ∗ −
of APF. The exact measurement of the load current ensures ∗ = ∗ (7)
accurate generation of the reference current. The reference
In the final step the reference current in a-b-c co-
current generation should be done properly in order to
ordinates is calculated by using equation (8).
achieve good performance of the filter. Various techniques
are used by the researchers for reference current generation
[8]. ∗ 1 0 1⁄√2 ∗
∗ = −1⁄2 √3⁄2 1⁄√2 ∗ ∗ (8)
A. lnstantaneous Reactive Power (p-q) Theory ∗ ∗
−1⁄2 −√3⁄2 1⁄√2
This theory is very likely used for reference current
generation in active power filters. In this theory variable
transformation is used to transform the co-ordinates from the B. Synchronous Reference Frame Theory
a, b, c, reference frame to the α, β reference frame [9]. In this case, the real currents are transformed into a
In this theory, instantaneous three phase current and synchronous reference frame [11]. The reference frame is
voltages are used as input for doing the transformations [10] synchronized with the ac mains voltage, and is rotating at the
as shown in equation (1) and (2). same frequency. This technique is also known as d-q theory.
The currents are transformed first from a-b-c co-ordinates to
α-β-0 co-ordinates and then to d-q-0 co-ordinates [12]. It is
1⁄√2 1⁄√2 1⁄√2 done using the following equations:
= 1 −1⁄2 1 ⁄2 (1)
0 √3⁄2 −√3⁄2 1 −1⁄2 1 ⁄2
= (9)
0 √3⁄2 −√3⁄2
1⁄√2 1⁄√2 1⁄√2
= 1 −1⁄2 1 ⁄2 (2) i sin(ө) − cos(ө) i ∗
i = (10)
0 √3⁄2 −√3⁄2 cos(ө) sin(ө) i ∗

The source currents and phase neutral voltage are used to The AC component or oscillating value and the DC
derive the instantaneous real and imaginary power component or average value, both are present in the output of
components as given in equation (3). this equation. The AC component contains harmonic
component and is eliminated by using low pass filter so that
0 0 the output of above equation which is the DC component is
harmonic free. This harmonic free signal in 0-d-q rotating
= 0 (3) frame is converted back into a -b-c stationery frame as
0 − shown below.

The zero sequence power only exists in three phase ∗ (ө) − (ө)
systems with a neutral wire. In this paper three phase three ∗ = (ө) (ө)
(11)

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 ∗ 1 0 TABLE I. SOURCE PARAMETERS
∗
∗ = −1⁄2 √3⁄2 ∗ (12) Parameter Value
∗
∗ 1 ⁄2 −√3⁄2 Line to Line rms voltage 380 V
Source frequency 50 Hz
C. Discrete Fourier Transform Source resistance 0.001 Ω
Source inductance 0.001 μH
In this method FFT algorithm is used to determine the Line inductance 1 μH
harmonic components of the load current [13]. The block Line resistance 0.01 Ω
diagram of this method is shown in Fig. 2. The load currents
for three phases are represented by ia, ib and ic and the TABLE II. LOAD PARAMETERS
reference currents by ia*, ib* and ic* respectively.
Simulation THD before Output Loads
Case compensation Resistor RL Capacitor CL
ia ia* I 29.89% 10 Ω --x--
Harmonics II 36.94% 10 Ω 100 μF
ib Fourier Detection 180 degree ib*
III 42.98% 10 Ω 500 μF
Analysis & Phase shift
A. Case I
Addition
ic ic* In this case only resistor is used as output load for
simulation. The load current waveform for the first case is
Figure 2. Block Diagram of DFT Technique shown in Fig. 4 and its corresponding THD is shown in Fig 5.
Reference current is generated by using the three techniques
In the Fourier analysis block frequency components of discussed above and their corresponding waveforms are
the load current are detected [14]. In the second stage shown in Figs. 6-8. All the techniques are evaluated in
fundamental component is separated and the remaining transient state as well as steady state.
frequency components are added to get the harmonics
current waveform. In the final stage harmonics current
waveform is 180 degree phase shifted to get the reference
current.
III. SIMULATION RESULTS
The simulation results for all the three techniques have Figure 4. Load current waveform for Case I.
been obtained by using Matlab/Simulink. For the purpose of
simulation a three phase three wire system with balanced
load has been considered. The proposed APF has been
simulated under three different load conditions. In this
simulation the generated reference current IR is added to the
load current IL to get the Source current IS and the accuracy
of the generated reference current is determined by
calculating THD of IS. A value close to zero indicates
accurately generated reference current as well as the purity of
sinusoidal wave shape. Table I shows the Source parameters
used for simulation and Table II shows the parameters of Figure 5. THD for Case I.
load connected to the source for all the three cases observed.
Fig. 3 shows the simulink model in which a 3-phase non-
linear load is connected to the 3-phase source and the shunt
active power filter is connected in parallel to the load.

Figure 6. Reference current (p-q theory).

Figure 3. Simulink Model. Figure 7. Reference current (d-q theory).

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 Figure 8. Reference current (DFT) Figure 13. Reference current (DFT)

B. Case II C. Case III

In this case a filter capacitor of 100 μF is also used along In the third case value of the filter capacitor is changed to
with the resistor. The capacitor is connected in parallel with 500μF. The load current waveform for the third case is
the resistor. The load current waveform for the second case shown in Fig. 14 and its corresponding THD is shown in Fig.
is shown in Figure 9 and its corresponding THD is shown in 15. The circuit is simulated again by using all the three
Figure 10. The circuit is simulated using all the three techniques. The generated reference currents for all the three
techniques. The reference current waveforms thus generated theories are shown in Figs. 16-18.
by using the three techniques are shown in Figs. 11-13.

Figure 14. Load current waveform for Case III.

Figure 9. Load current waveform for Case II.

Figure 15. THD for Case III

Figure 10. THD for Case II

Figure 16. Reference current (p-q theory)

Figure 11. Reference current (p-q theory)

Figure 17. Reference current (d-q theory)

Figure 12. Reference current (d-q theory)

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