 Open Access
 Total Downloads : 228
 Authors : M.Priya
 Paper ID : IJERTV2IS100883
 Volume & Issue : Volume 02, Issue 10 (October 2013)
 Published (First Online): 30102013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Mitigation of Harmonics by Evaluation of Control Algorithms for Shunt Active Filter
M.Priya, M.Tech Student Department of
Electrical and Electronics Engineering SR Engineering College Ananthasagar, Warangal, AP, India
Abstract: – Shunt Active Filter produces the reference current that must be supplied by the power filter to compensate harmonic currents demanded by the load. This paper presents different types of SRF methods for real time regeneration of compensating current for harmonic mitigation. The three techniques analyzed are the Synchronous Reference Frame Theory (SRF), SRF theory without synchronizing circuit like phase lock loop (PLL) also called instantaneous current component theory and finally modified SRF theory. The performance of Shunt Active Power Filter in terms of THD (Total Harmonic distortion) of voltage and current is achieved with in the IEEE 519 Standard. The comparison of all methods is based on the theoretical analysis and simulation results obtained with MATLAB/SIMULINK
Index termsSynchronous Reference Frame, instantaneous current component theory, Modified SRF, Active Filter, Harmonics.

INTRODUCTION
The increasing use of power electronic based loads (adjustable speed drives, SMPS, etc) to improve system efficiency and controllability is increasing concern for harmonic distortion levels in end use facilities and on overall power system. The power electronic switching is useful to generate harmonic currents in active power filters that cancel harmonic content from non linear loads. Over the recent years, power quality has been given attention due to the intensively use of power electronic Controlled applications in all groups of engineering, such as controlling or converting AC power to feed electrical loads.
The nonlinear loads have led to the concerns over the allowable amounts of harmonic distortion injected into the supply system. Standards such as IEEE519 have emerged to set and impose limits and recommended practices so that the harmonic distortion levels are kept in check, thereby promoting better practices in the design and operation of power system and electric equipment.
Based on observations from various references, a practical limit of less than 5% of the total harmonic distortion (THD) must be employed by any system designers and/or end users to ensure compliance with the established standards. Many
efforts have been expended to develop active power filters and conditioner that can soften the power quality problems.
One of the cornerstones of the active filter is its control strategy that is implemented in the active filter controller. The performance of an active filter depends mainly on the selected reference generation scheme.
The control strategy for a shunt active power filter generates the reference current, that must be provided by the power filter to compensate reactive power and harmonic currents demanded by the load. This involves a set of currents in the phase domain, which will be tracked generating the switching signals applied to the electronic converter by means of the appropriate closedloop switching control technique such as hysteresis or deadbeat control.
Several methods including instantaneous real and reactive power theory have been proposed for extracting the harmonic content. This paper presents a different modification based on the same principle and compares its performances with sinusoidal source and balanced load condition. The Modified SRF method called, in this paper, Filtered Modified Reference Frame Method (FMRF), because it uses filters and is based on the modified reference frame method.
Fig. 1: Basic principal of shunt current compensation in active

SRF METHODS
Among the several methods presented in the literature, the Synchronous Reference Frame method (SRF) is one of the most common and probably it is widely used method. SRF methods Are described concisely in this. The three methods presented in this section with some results obtained with the above mentioned methods. The nonlinear load considered is a threephase diode bridge rectifier.

Synchronous Reference Theory (SRF)
In the SRF, the load current signals are transformed into the conventional rotating frame dq. Below equation shows the transformation angle as .
The transformation is defined by:
.1
Where voltages or currents are denoted by x.
Fig. 2: Basic Synchronous Reference Frame Configuration
In the synchronous reference frame (SRF) method is a time varying angle that represents the angular position of the reference frame which is rotating at constant speed in synchronism with the three individual phase ac voltages. To implement the SRF method some kind of synchronizing system should be used. Phaselocked loop (PLL) is used to
implementation of this method. In this case the speed of the reference frame is nearly stable, that is, the method behaves as the reference frames moment of inertia is infinite. The dq components fundamental currents are now dc values. The harmonics come into view like ripple. By removing the dc offset we can achieve Harmonic isolation of the dq transformed signal. high pass filters (HPF) were used to accomplish this task. In spite of a high pass filter, a LPF filter is used to gain the reference source current in dq coordinates. Fig 2 illustrates a configuration of the SRF method. There is no need to supply voltage waveform for a SRF based controller. On the other hand voltage information is useful to find the phase position angle. The SRF harmonic detection method can be reasonably summarized as a block diagram as shown in Fig.3.
Fig.3: SRF harmonic detection

Instantaneous Current Component (idiq) Theory:
Fig.1 shows the schematic block diagram of the shunt active filter with controller. The block diagram consists of variable sensing system, Reference Estimation System, pulse width modulation signal generator and system controller.
The variable sensing block senses the system variables like supply current, load current and compensating current, DC link voltage or current. Pulse width modulation signal generator and system controller generate switching signals for converter switches based on the error produced by reference signal and actual system variables.
this constitutes the average current components. The first harmonic current, negative sequence currents & the entire higher order current harmonics, , are transformed to nondc quantities and undergo a frequency shift, and so, comprise the components of oscillatory currents. These assumptions are applicable under sinusoidal mains and balanced voltage conditions. Eliminating the average current components by HPFs the currents that should be compensated are and.
Fig.4: AF control system based on the instantaneous active and reactive current component Id – Iq method.
In this method the currents Ici are obtained from the instantaneous active and reactive current components Id and Iq of the nonlinear load. In the same way, the mains voltages vi and the polluted currents Ii in as in the previous method by 2 and 3. However, the load current components are derived from a synchronous reference frame based on the Park transformation, where the below equations represents the instantaneous voltage vector angle 4
. 2
..3
..4
With transformation the direct voltage component is udq=u=
and the quadrature voltage component is always null, Uq=0 , so due to geometric relations 4 becomes
.5
Instantaneus active and reactive load currents Id and Iq can also be decomposed into oscillatory and average terms Id=I*d+Id , and Iq=I*q+Iq . The first harmonic current of positive sequence is transformed to dc quantities, i dq1h i.e.,
..6

Modified (idiq) Theory
The method suggested in this section is based on the modified (idiq) method (FMRF). The principle is the same. However there are two differences in the determination of the instantaneous position of the rotating reference frame. In spite of using the voltages to calculate the transformation angle, low pass filters (LPF) are used to reduce harmonics of the network signals, and consequently use on the control process approximate sinusoidal waveforms, fig.6.
Fig. 5: Principal of modified (idiq) method
The second modification consists in separating the coefficient and to use a filtered coefficient. This new modification is important because the system will presents better results to inverse sequence components. These concepts are presented in fig. 5 using block diagrams. The modified synchronous reference frame method has excellent results in balanced sinusoidal and unbalanced ac mains.
In all cases studied in this paper, the load is a three phase diode bridge with an inductive circuit on its dc side. The LPF, LPF (cutoff frequency fc) and LPF (cutoff frequency fd) have different purposes. LPF, LPF are set to filter the ac mains and to avoid the influence of voltage harmonics presented on the network point of common coupling. The LPF is set to avoid the oscillation of the parameter that is due to the inverse sequence component. The low pass filter used for LPF, LPF, and LPF, the type of LPF are of 4th order Butterworth and 1st order
chebyshev type with appropriate cutoff frequencies. In this method the cutoff frequency of the filter was set at 8 Hz and the cutoff frequency of the alpha and beta filters were also set to 60 Hz in Butterworth filter and passing band frequency of 20 Hz is used in chebyshev type of LPF.
III RESULTS AND ANALYSIS
In order to evaluate the performance of all the methods simulation studies are carried out. In FMRF method it is observed that the supply current is close to sinusoidal and it remains in phase with the supply voltage, therefore, unity power factor is maintained at the output of supply system. From the figures 6 to 17 are results corresponding the three SRF Theories and Comparison is shown in the Table 1
Fig 6: Performance of SRF theory: (1) Load current
(2) Source current (3) Compensating current
4000
3000
2000
1000
0
1000
12
10
8
Mag
Mag
6
4
2
0
Fig. 8: Performance of SRF theory: (1) Load current
(2) Source current (3) compensating current
Selected signal: 10 cycles
0 0.05 0.1 0.15 0.2
Time (s)
Fundamental (50Hz) = 469.6 , THD= 3.84%
0 20 40 60 80 100
Harmonic order
4000
3000
2000
1000
0
1000
15
Mag
Mag
10
5
0
Selected signal: 10 cycles
0 0.05 0.1 0.15 0.2
Time (s)
Fundamental (50Hz) = 469.6 , THD= 1.01%
0 5 10 15 20
Harmonic order
Fig. 9: SRF Theory, Chebyshev type filter: (1) Source current for 10 cycles (2) FFT analysis
Fig. 7: SRF Theory, Butterworth type filter: (1) Source current for 10 cycles (2) FFT analysis
Fig. 10: Performance of idiq theory: (1) Load current
(2) Source current (3) Compensating current
150
100
50
0
50
0.15
Mag
Mag
0.1
0.05
0
Selected signal: 10 cycles. FFT window (in red): 7 cycles
0 0.05 0.1 0.15 0.2
Time (s)
Fundamental (50Hz) = 10.06 , THD= 2.02%
0 200 400 600 800 1000
Frequency (Hz)
Fig. 11: idiq Theory Butterworth type filter: (1) Source current for 5 cycles (2) FFT analysis
Fig. 14: Performance of Modified idiq theory: (1) Load current
(2) Source current (3) compensating current
Selected signal: 10 cycles
4000
3000
2000
1000
0
1000
15
Mag
Mag
10
0 0.05 0.1 0.15 0.2
Time (s)
Fundamental (50Hz) = 469.6 , THD= 1.01%
150
100
50
0
Fig. 12: Performance of idiq theory: (1) Load current

Source current (3) Compensating current
Selected signal: 10 cycles. FFT window (in red): 6 cycles
5
0
0 5 10 15 20
Harmonic order
Fig.15: Modified idiq Theory Butterworth type filter:

Source current for 10 cycles (2) FFT analysis
0.1
0.08
Mag
Mag
0.06
0.04
0.02
0
0 0.05 0.1 0.15 0.2
Time (s)
Fundamental (50Hz) = 10.29 , THD= 2.00%
0 200 400 600 800 1000
Frequency (Hz)
Fig. 13: idiq Theory Chebyshev type filter: (1) Source current for 6 cycles (2) FFT analysis
Fig. 16: Performance of Modified idiq theory: (1) Load current

Source current (3) compensating current

4000
3000
2000
1000
0
1000
12
10
8
Mag
Mag
6
4
2
0
Selected signal: 10 cycles
0 0.05 0.1 0.15 0.2
Time (s)
Fundamental (50Hz) = 453.9 , THD= 2.95%
0 5 10 15 20
Harmonic order
Load Perturbati on Response 
30 ms 
40 ms 
20 ms 
25 ms 
10 ms 
10 ms 
Requirem ent Of Ripple Filter 
no 
yes 
yes 
no 
yes 
no 
Load Perturbati on Response 
30 ms 
40 ms 
20 ms 
25 ms 
10 ms 
10 ms 
Requirem ent Of Ripple Filter 
no 
yes 
yes 
no 
yes 
no 
BW = Butterworth, CH = Chebyshev (Type of Filter Used)
IV. CONCLUSION
This paper presents the mitigation of harmonics by evaluation of control algorithm. Results are similar with gained
source THD under IEEE 519, but under various filter type the
Fig. 17: Modified idiq Theory Chebyshev type filter: (1) Source current for 10 cycles (2) FFT analysis
In real filtering, a Butterworth type filter is normally chosen, but chebyshev filter is also equally compatible for preparing experimental prototype. This particular filter type was chosen, in order to obtain magnitude and phase characteristics as close as possible to an ideal filter since its magnitude response is maximally flat in the pass band and is monotonic in both pass band and stop bands. To minimize the influence of the HPFs phase responses, an alternative HPF (AHPF) can also be used by mean of a lowpass filter (LPF) of the same order and cutoff frequency, simply by the difference between the input signal and the filtered one, which is equivalent in performance.
TABLE 1
Comparison of the Different SRF Methods
chebyshev type filter is having superior performance compare to Butterworth filter for all methods. The Synchronous Reference Frame method is one of the most common and performing more accurate for detection of harmonics in active filters. An enhanced Synchronous Reference Frame Method for the organize of active power filters was presented. It is called FMRF and is based on the same principle as the SRF method. So finally it concludes that the isolation of harmonics does not depend on the position of the RRF (rotating reference frame) & its dependson the speed of the system. So, the reference frame can be detected by introducing some delay with ac power filters. Compared with other methods, this latest method presents some benefits due to its ease and its rudeness to perturbations on the ac network.
paramete rs 
SRF Theory 
IdIq Theory 
Modified SRF Theory 

Filter type 
BW 
CH 
BW 
CH 
BW 
CH 
Source Current THD(%) 
1.01 
3.84 
2.02 
2.00 
1.01 
2.95 
5th Harmonic 
2.46 
1.6 
1.7 
1.89 
2.51 
2.44 
7th Harmonic 
1.52 
1.4 
1.15 
1.23 
1.57 
1.52 
9th Harmonic 
0.03 
0.02 
0.28 
0.30 
0.01 
0.02 