Abstract- be important sources of harmonic distortion in a

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.


– Harmonics, Hysteresis
current control, Shunt Active Power Filter, Synchronous Reference
Frame Algorithm

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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



                      Figure.1 Three phase
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.







              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


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





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).



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).




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.













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.




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



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%.


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6Mohammad Monfared, “A New
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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

   Bhivpuri Road , Karjat.







   Ms. Pranita P. Chavan received his B.E degree in Electrical
Engg. from Mumbai University, in2002.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.