Abstract- of power electronic devices to control the

Abstract- with the arrival of semiconductor and power electronic devices and
their easier controllability has caused wide use of nonlinear loads. But the
use of power electronic devices is responsible for harmonic and reactive power
disturbances. These harmonics creates the disturbance in normal operation,
excessive heating in the equipments etc. so it is necessary to eliminate these
harmonics problems. So importance
is being given to the development of Active Power Filters to solve these
problems to improve power quality among which shunt active power filter is used
to eliminate voltage and load current harmonics and for reactive power
compensation. The shunt active power filters have been developed based on
Synchronous Reference Frame Algorithm Method. Synchronous Reference Frame (SRF)
Algorithm is used to extract the harmonics components. Hysteresis band current
control (HBCC) technique is used for the generation of firing pulses to the
inverter. This system is simulated using MATLAB and results are observed.

 

Keywords – Shunt
Active Power Filter, harmonics, Synchronous Reference Frame Algorithm,
Hysteresis Current Control

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I    INTRODUCTION

Now days,
power system uses large number of power electronic devices to control the power
system equipments. However power electronic based equipments which includes
adjustable speed motor drives, electronic power supplies, electronic ballasts
are responsible for the rise in power quality related issues.1. These
nonlinear loads appear to be important sources of harmonic distortion in a
power distribution system. These harmonics reduces the quality of power, low
efficiency, low power factor. Hence to overcome these problems of harmonics
passive filters have been used. But due to some disadvantages, namely it will
introduce system resonances that can move a harmonic problem from one frequency
to another, it is difficult to design the filters to avoid leading power factor
operation for some load conditions. To overcome these disadvantages, active
power filter have been developed.2 The Active Power Filter (APF) based on
power electronics technology is a viable solution for power conditioning to
suppress the harmonics in the power system. With recent developments in power
electronic switches, the Active Power Filters (APFs) have been applied to
mitigate the problems created by non-linear loads. One of the most commonly
used active filters is the Shunt Active Filter (SAF) which is used to eliminate
the unwanted harmonics and compensate reactive power consumed by non-linear
loads 3.

The Shunt Active Power Filter is connected in parallel with the line through a coupling
inductor. Its main power circuit
consists of a three phase three-leg
current controlled
voltage source inverter with a DC link capacitor. An active power filter operates
by generating a compensating current with 180 degree phase opposition and injects it back to the line so
as to cancel out the current harmonics introduced by the nonlinear load. This will thus
suppress the harmonic content present in the line and make the current waveform sinusoidal. So the process comprises of detecting
the harmonic component present in the line current,
generating the reference current, producing the switching pulses for the power circuit,
generating a compensating current
and injecting it back to the line
4-7.

 

                  

                      Figure.1 Three phase
shunt active power filter

 

 II. SHUNT
ACTIVE POWER FILTER

Shunt active power filters are widely used in power system
to compensate reactive power and current harmonics. It can also play the role
of static VAR generator in the power system for improving and stabilizing the
voltage profile. Shunt active filter compensate current harmonic by injecting
complementary current that of produced by nonlinear load. shunt active filter
acts as a current source by introducing the harmonic components created by the
load. Consequently, the current harmonic component present in the load current
got cancelled and the source current remains sinusoidal. By the use of proper
control scheme, APF can also improve system power factor. However the
performance of SAPF largely depend on the control strategy which is responsible
for generating complementary harmonic current to cancel out the current
harmonics present in the load current. There are several control strategies
like, Instantaneous power theory based on symmetrical components, Generalized
Instantaneous reactive power theory, Synchronous reference frame theory(SRF),
Synchronous detection method(SDM), etc. In this paper, SRF theory is used to
generate the reference signals applied to current control algorithm.

 

 

1 Synchronous Reference Frame Algorithm

              Number of control
strategies being used for the determination of reference currents in shunt
active power filters namely

              Instantaneous
Reactive Power Theory (p-q theory), sliding mode control strategy, Unity Power
Factor method, One

                Cycle
Control, Fast Fourier Technique etc. Here, SRF theory is used to evaluate the
three-phase reference 3currents(ica*,

                icb*,
icc*) by the active power used filters by targeting the source (ica,
icb, icc) current Fig.2 shows the block diagram which    

              explains
three-phase SRF-theory, used for harmonic component extraction.

 

 

           

       

 

              

                          

      

 

 

 

 

Figure.2 Reference Frame Transformation

 

 

 

 

 

 

 

 

 

 

                                                                              

 

 

 

Figure.3 Block diagram of SRF based
algorithm

 

In this
method, the source currents (ia, ib, ic) are
first detected and transformed into two-phase stationary frame (??-0)    

from the three-phase stationary frame (a-b-c), as per equation
(1).

 

 

 

 

 

                                   
    (1)

 

Here two directand inverse parks transformation is used which
allows the evaluation of specific harmonic component of

the input signals and a low pass filtering stage LPF. Now,
the two phase current quantities i? and i? of stationary
??-axes

are transformed into two-phase synchronous (or rotating)
frame (d-q-axes) using equation (2), where Cos? and Sin?

represents the synchronous unit vectors which can be
generated using phase-locked loop system (PLL).

 

                                         (2)

 

The d-q currents thus obtained comprises of AC and DC parts.
The fundamental component of current is represented by

the fixed DC part and the AC part represents the harmonic
component. This harmonic component can be easily extracted

using a high pass filter (HPF), as implemented in Fig 2. The
d-axis current is a combination of active fundamental

current (id dc) and the load harmonic current (ih).
The fundamental component of current rotates in synchronism with the

rotating frame and thus can be considered as dc. By filtering
id, the current is obtained, which represents the fundamental

component of the load current in the synchronous frame. Thus,
the AC component idh can be obtained by subtracting id dc

part from the total d-axis current (id), which
leaves behind the harmonic component present in the load current. In the

rotating frame the q-axis current (iq) represents
the sum of the fundamental reactive load currents and part of the load

harmonic currents. So the q-axis current can be totally used
to calculate the reference compensation currents.

Now inverse transformation is performed to transform the
currents from two phase synchronous frame d-q into two-phase stationary frame
?-? as per equation (3).

 

                                              (3)

 

Finally the current from two phase stationary frame ??0 is
transformed back into three-phase stationary frame abc as per equation (4) and
the compensation reference currents ica*, icb* and icc*
are obtained.

 

                                                  
(4)

 

 

Where,

 

                                                   (5)

 

 

2.      
Hysterisis Band Current Control

 

The hysteresis band current control (HBCC)
technique is used for pulse generation in current controlled VSIs. The control
method offers good stability, gives a very fast response, provides good
accuracy and has got a simple operation. The HBCC technique employed in an
active power filter for the control of line current is shown in Figure 4. It
consists of a hysteresis band surrounding the generated error current. The
current error is obtained by subtracting the actual filter current from the
reference current. The reference current used here is obtained by the SRF
method as discussed earlier which is represented as Iabc*. The actual filter
current is represented as If abc. The error signal is then fed to
the relay with the desired hysteresis band to obtain the switching pulses for
the inverter.

 

 

 

 

Figure.4 Hysteresis Band Current Controller

 

The operation of APF depends on the sequence of pulse generated
by the controller. Figure 5 shows the simulation diagram of the hysteresis
current controller. A band is set above and below the generated error signal.
Whenever this signal crosses the upper band, the output voltage changes so as
to decrease the input current and whenever the signal crosses the lower band,
the output voltage changes to increase the input current. Accordingly switching
signals are generated.

 

 

 

Figure.5  Simulation diagram of hysteresis
current control

 

The switching signals thus generated are fed to the power
circuit which comprises of a three phase three leg VSI with a DC link capacitor
across it. Based on these switching signals the inverter generates compensating
current in phase opposition to the line current. The compensating current is
injected back into the power line at the PCC and thus suppressing the current
harmonics present in the line. The overall simulation block diagram is shown in
Figure 6.

 

 

 

 

Figure.6 Overall simulation diagram.

 

 

III. SIMULATION RESULTS
AND DISCUSSION

 

After simulation of three phase transmission line having non
linear load with SRF based shunt active filter the harmonic
current is compensated within a permissible limits of IEEE standard. In this
the source current waveform without filter in a-phase is shown in Figure 7.
when filter is not connected in the system the harmonics are produces due to
non linear load. These harmonics distort the source current as shown in
figure.7. Also if the THD is cheked, then Total Harmonic Distortion (THD)
spectrum in the system without filter is shown in Figure.7, which indicate a
THD of 15.59% These
compensating current is produced by the filter when we are injecting this
compensating current we get the source current with minimum harmonics. The
source current after the injection of compensating current is shown in Figure
8. The THD with active power filter included is observed to be 3.77% which is within the
allowable harmonic limit. Figure.8 shows the THD spectrum with active power filter
in the circuit.

 

 

 

 

Figure.7  Source current and THD spectrum without SAF

 

 

 

Figure.8 Source current and
THD Spectrum with SAF

 

 

 

 

 

IV.   CONCLUSIONS

The SAPF explained in this paper compensate the line current harmonics
generated due to the nonlinear loads in the system. HBCC technique used for the
switching pulse generation was found to be effective and its validity is proved
based on simulation results. Thus SRF based SAPF has been proved to be
effective to keep the harmonic content in power lines within the permissible
limit of IEEE standards i.e. THD is 3.77%.

 

 

 

References

Dugan.C.Roger,
M.F.McGranaghan, Santoso and H.W.Beaty, “Electrical Power Systems Quality”,
second edition McGraw-Hill, 2002, USA

1          
Joao Afonso,Mauricio Aredes,Edson Watanabe, Julio
martins “Shunt active filter for power quality improvement.” International
conference UIE 2000- Electricity for a sustainable Urban Development , Lisboa,
potugal, 1-4 Novembro 2000 pp 683-691.

2          
Deepathi Joseph, “P-Q Theory for Shunt Active Filter
using Ramp Comparator” IEEE transaction on International conference on Power,
Energy and Control. 2013.

3          
 Preeti Yadav,
Swati Maurya, “Single phase shunt active power filter for harmonic filtering” International
Journal of Emerging Technology and Advanced Engineering, Volume 4, Issue 4,
April 2014.

4          
Alberto Pigazo, “A Recursive Park Transformation to
Improve the Performance of Synchronous Reference Frame Controllers in Shunt
Active Power Filters” IEEE Transactions On Power Electronics, Vol. 24, No. 9,
September 2009.

5          
Mohammad Monfared, “A New Synchronous Reference
Frame-Based Method for Single-Phase Shunt Active Power Filters” Journal of Power Electronics, Vol. 13,
No. 4, July 2013.

6          
Diyun WU, “Design
and Performance of a Shunt Active Power Filter for Three phase Four-wire
System” 2009 3rd International Conference on Power Electronics Systems and
Applications.

7          
Leszek S. Czarnecki, “Instantaneous Reactive power p-q
theory and Power properties of 3-phase system”, IEEE Transactions on Power Delivery, Vol. 21, No. 1, pp.362-367,
Jan. 2006.

 

 

 

 

                                                                   

VI.  ACKNOWLEGMENT

 

 

 

 
Ms. Dipeeka P. Sawant received his B.E degree in Electrical Engg. from Pune
University, in 2012. Now   

  
she is doing
M.E. in Electrical Power System from Yadavrao Tasgaonkar Institute of Engg. And
Technology

   Bhivpuri Road , Karjat.

 

 

 

 

 

 

 
   Ms. Pranita P. Chavan received his B.E degree in Electrical
Engg. from Mumbai University, in 2002. And                              

    
M.E degree
from Pune University in Electrical Power System in 2004 . Now  she is working as Assistant

     Professor In  Yadavrao Tasgaonkar Institute of Engg. And
Technology Bhivpuri Road , Karjat. She has

     Total Experience spans of over 11 years.