Three phase shunt active power filter with unipolar PWM technique by nikeborome

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									                          National Power Electronics Conference (NPEC-10) June 10-13, 2010.


 Three phase shunt active power filter with unipolar PWM technique to
        compensate reactive power and low order harmonics

                   1
                    J.Vikramarajan, 2Rasmi ranjan das and 3Razia sulthana.
           1
            PG Student, School of Electrical Engineering, VIT University, Vellore, India.
   2
    Assistant Professor (senior), School of Electrical Engineering, VIT University, Vellore, India.
       3
        Assistant Professor, School of Electrical Engineering, VIT University, Vellore, India.



Abstract: Active power filter continues to             economic concerns. Non-linear loads result in
attract considerable attention. Because of             voltage and current harmonics in distribution
sensitivity of consumers on power quality and          network.
advancement in power electronics. Active
                                                       Harmonics provide main problems in network
power filter technology is the most efficient
                                                       like power losses and excess heat. Thus,
way to compensate reactive power and
                                                       harmonic limitation seems to be vital.
cancel out low order harmonics generated by
                                                       Nowadays, active power filters (APF) play
nonlinear loads. An active power filter is a
                                                       effective role in distortion recognition and
device that is connected in parallel to and
                                                       elimination. These filters are classified with
cancels the reactive and harmonic currents
                                                       respect to distortion determination strategy,
from the group of nonlinear loads so that the
                                                       inverter    control    techniques,     inverter
resulting total current drawn from the ac main
                                                       topologies and their connection types to the
is sinusoidal. The shunt active power filter
                                                       grid. Shunt filters are connected in parallel
was consider to be the most basic
                                                       with distribution networks. They recognize
configuration for the APF.This project
                                                       current distortions by sampling the line
presents Three phase shunt active power
                                                       current and compensate distorted current
filter with unipolar pulse width modulation
                                                       components to maintain sinusoidal source
technique to make the total input current
                                                       current. In most recommended grids, the
drawn from the three phase AC main as
                                                       reference signal can be extracted by
sinusoidal, by compensating reactive power
                                                       sampling and analyzing the input voltages
and low order harmonics generated by the
                                                       and load currents for harmonic and reactive
three     phase      nonlinear   loads.    The
                                                       power calculations. These control methods
synchronous d-q-o reference frame algorithm
                                                       require rapid and real time calculating blocks
is also used to recognize and extract
                                                       like high-speed digital microprocessors and
harmonic distortions. The validity of proposed
                                                       capable A/Ds, so cost increment, circuit
method and achievement of desired
                                                       complexity and system stability reduction
compensation are shown by simulation
                                                       would be inevitable.
results.
                                                       Current control technique based on unipolar
Keywords: Three phase shunt Active power               PWM      provides     better    stability and
filter, Unipolar PWM technique.                        compensation with loss reduction; efficiency
                                                       increment and dc offset elimination compare
I.INTRODUCTION                                         to bipolar PWM technique. This paper
                                                       presents a hysteresis current control
Increasing demand of electric and electronic           technique based on unipolar Pulse Width
instruments including non-linear loads (e.g.           Modulation (PWM) with time and magnitude
computers, power electronic inverters and              errors control to reduce switching losses and
induction furnace) necessitates investigations         to improve the quality of output current.
about stability maintenance, continuous
energy serving and power quality considering           An active power filter is a device that is
                                                       connected in parallel to and cancels the

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                             National Power Electronics Conference (NPEC-10) June 10-13, 2010.


reactive and harmonic currents from a group            analyze the voltage at the point of installation
of nonlinear loads so that the resulting total         and is controlled as to present infinite
current drawn from the ac main is sinusoidal.          impedance for the fundamental frequencies
A device which converts the pulsating output           and a low impedance for the harmonic
of a rectifier into steady state dc level is           frequencies. In order to perform other
known as Filter. This filters out the                  functions, as reactive power compensation,
undesirable ac component and allows only dc            Flicker/imbalance compensation etc., the
component to reach the load.                           overall system must be equipped with other
                                                       feedback or feed forward control loops.
                                                       Harmonics are sinusoidal voltages or currents
                                                       having frequencies that are integer multiples
                                                       of the frequency at which the supply system
                                                       is designed to operate, that combine with the
                                                       fundamental voltage or current, and produce
                                                       waveform distortion. It is caused by non-
                                                       linear loads. Lower order harmonic is defined
                                                       as the harmonic component whose frequency
                                                       is closest to fundamental one and its
                                                       amplitude is greater than or equal to 3% of
 Fig.1: Block diagram of the system                    the fundamental component.

II.SHUNT ACTIVE POWER FILTER

A shunt active filter is designed to be
connected in parallel with the load. It detects
the harmonic current of load and injects into
the system a compensating current, identical
with the load harmonic current but in opposite
phase. Therefore, the net current drawn from
the distribution network at the point of
coupling of filter and the load will be a
sinusoidal current of only fundamental                 Fig.3: Block diagram of a simple power
frequency. Fig 2 shows the principle of                system with APF OFF
operation and the connection scheme of a
shunt active filter.




                                                          
                                                       Fig.4: Block diagram of a simple power
                                                       system with APF ON

                                                       The above diagrams describe the function of
Fig.2: shunt active filter
                                                       Active Power Filter in a system. When the
Harmonic current and/or negative sequence              APF is OFF, the input current is distorted.
current compensation and dc link voltage               When the APF is ON, the input current is
regulation between both active filters. The            sinusoidal and the distortions are reduced,
purpose of the shunt active filter is mainly to        APF will inject the harmonic currents 180
compensate current harmonics generated                 degree out of phase in to the Point of
from the distributed lines. Thus, it has to            Common Coupling (PCC) as shown in Fig.1.

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                           National Power Electronics Conference (NPEC-10) June 10-13, 2010.


III.DISTORTION DETECTION BY                              considered     in   this     paper.   Finally,
REFERENCE FRAME D-Q-O                                    compensated currents are determined by
                                                         adverse park application on d and q axis to
Several methods are used to recognize and                be injected to the network after tracing and
extract harmonic distortions which are                   reconstruction as shown in fig.1.
classified as frequency, time and time-
frequency      approaches.      Fast      Fourier
Transformer (FFT)[3] and adaptive neural
network     [2]     in    frequency      domain,
synchronous reference frame theory d-q-o
(SRF) [4] and instantaneous active and
reactive power theory (pqr)[5] in time domain
and the other methods such as small wave
technique and one-cycle control or separation
with suitable digital or analogue filters[6] have
                                                         iL = ig + iapf                            (6)
wide applications.
 In this approach, reference frame algorithm             Conduction and switching losses of diodes
is used due to simplicity in calculation and             and IGBTs in inverters increase voltage ripple
implementation. Having measured three-                   in DC-link which affects the performance of
phase currents in a-b-c orientation,                     the filter. These effects are controlled by a
transformed to d-q-o by park equation:                   feedback loop where PI regulator compares
                                                         the DC-link voltage with a reference voltage
                                                         to extract d component of current.




Reference frame rotates synchronous with
fundamental currents. Therefore, time variant
currents with fundamental frequencies would
be constant after transformation. However,
harmonics with different speeds remain time              Fig.5: Synchronous d-q-o reference frame
variant in this frame. Thus, currents would be           based compensation algorithm.
separate simultaneously to DC and AC parts.
                                                         IV.HYSTERESIS BAND CURRENT
i d = id + i%d                            (3 )           CONTROL APPROACH

i q = iq + i%q                             (4 )          Reference currents are generated by DC to
                                                         AC converters using a current control
                                                         technique such as a hysteresis control. The
                                                         hysteresis band is used to control load
AC part of d axis and whole current in q axis            currents and determine switching signals for
are used for harmonics elimination and VAR               inverters gates. Suitable stability, fast
compensation. Zero current is produced due               response, high accuracy, simple operation,
to a three-phase voltage imbalance or                    inherent current peak limitation and load
waveform distortions which have not been                 parameters variation independency make the

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                            National Power Electronics Conference (NPEC-10) June 10-13, 2010.


hysteresis current control as one of the best           A.Unipolar control
current control methods of voltage source
inverters. In this approach the current error,          There are four switching states in this
(difference between the reference and                   approach. In unipolar control, when Vg>0
inverter currents) is controlled in hypothetical        (reference current in ascendant slope
control band surrounding reference current.             (dic/dt>0), S3 is on in the reference current
                                                        half cycle, S1 and S2 turn on and off
e = i*c - iapf ( actual )                  (7)
                                                        periodically in switching cycles. Thus V0 and
When the load current exceeds the upper                 zero voltages produce between P and N so
band, the comparator output activated so the            error signal slope is negative in hysteresis
output voltage is changed in such a way to              band and periodically decreases and
decrease the load current and keep it                   increase. Originated current variations in
between the bands and deactivated at lower              reference current half cycle is as follows:
limit. Switching frequency varies with respect          dic 1
to distance between upper and lower band.                  = ( vg + v0 )First half cycle of switching              (9)
The other parameters like inverter-network              dt  L
inductance and DC link voltage affect                   dic 1
                                                           = ( vg ) Second half cycle of switching                (10)
significantly on the switching frequency.               dt  L
Adaptive band definition considered as
                                                        As the output voltages have three levels, +V0,
proper solution in this problem to fix the
                                                        -V0 and zero, the two switching states (similar
switching frequency [7]. The highest
                                                        to bipolar) cannot control the load current
switching frequency is as follows [8]:
                                                        sufficiently. In this case more bands are
                 vdc                                    required to achieve different switching states
f sw (max) =                                (8)
               9 HB.L                                   corresponding to different output voltages. As
Where, HB and L are hysteresis band and                 shown in Fig.5, the reference current has a
load inductance, respectively.                          positive dic/dt from 0- 90° and the load
                                                        current can follow the reference current
V.UNIPOLAR PULSE WIDTH                                  based on two voltage levels, +V0 and zero
MODULATION CONTROL                                      volts. But when the reference current has a
                                                        negative dic/dt, (90- 270°), the output voltage
Fig.6 shows a full bridge Three-phase                   of inverter has to be changed in such a case
inverter schematic that is connected in                 to generate negative dic/dt for the load
parallel to a non-linear load. The inverter can         current, thus more band are required to
be controlled in unipolar PWM method.                   change the voltage level from +V0&0 to
                                                        –V0&0. Current variation in the second half
                                                        cycle of the reference current is:
                                                        dic 1
                                                           = (- vg - v0 )         First half cycle of switching   (11)
                                                        dt  L

                                                        dic 1
                                                           = (- vg      )   Second half cycle of switching        (12)
                                                        dt  L
                                                        VPN has unit polarity in each half cycle of the
                                                        reference current, so this approach called
                                                        unipolar control.




Fig.6: Typical shunt APF connected in
                                                                  (           )
                                                        = - vg - - vg - v0 = v0                                   (13)

parallel with nonlinear load.



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                            National Power Electronics Conference (NPEC-10) June 10-13, 2010.


It is shown that the current control based on             zero to –Vo at t5. In this case, when the slop
unipolar PWM has a low switching losses or                of reference current is close to the slop of
better performance compare to the bipolar                 inverter output current, then the time error
PWM due to less number of switching                       control improves the quality of the APF and
transient per switching cycle.                            pushes the inverter current into the main
                                                          area.




Fig.7: Current and voltage waves with
hysteresis band control (for APF under
Unipolar control mode)

VI.UNIPOLAR BASED ON TIME AND
MAGNITUDE ERRORS CONTROL

In hysteresis current control based on
unipolar PWM, there are two upper bands
and lower bands in order to change the slop
of inverter output current based on there level
voltages, +Vo, 0 and -Vo. The idea is to keep
the current within the main area but the                  Fig.8: (a) noisy load current with upper
second upper and lower bands are to change                and lower bands (b) unipolar current
the voltage level in order to increase or                 control based on time and magnitude
decrease the dic/dt of inverter output current.           error control.
As shown in Fig.8.(a) Al cannot be very small             For magnitude error control, two switches in
as the noisy signal changes the switching                 inverter legs are controlled with a main band
time due to instantaneous comparison                      and two other ones by the second band in a
between the load and the reference currents               unipolar PWM. Control signals originated by
and increases the switching losses; and it                the unipolar command circuit based on the
cannot be big as the total harmonic distortion            hysteresis current control are shown in Fig.9.
may be increased.
 In APF, load current has several different
slops within one cycle and to have a fast
current tracking, the control algorithm in
unipolar current control has been defined
based on magnitude and time errors control
as shown in Fig.8.(b). In this case, the
second upper or lower band values can be
big enough in order to remove the noise
issue of the inverter output current but the
second decision to change the level is based
on time error. For example, when the load                 Fig.9: Proposed unipolar control circuit
current exceeds the first upper band at t4, the
                                                          In the first positive half cycle (Vg>0), S3 is
output voltage of inverter is change from +Vo
                                                          permanently      on.     Inductance    voltage
to 0. The controller waits for At, if the inverter
                                                          calculated as follows:
output current does not cross the second
upper band within this period, then the
                                                          vL (ON )= vg         0 < t < dTs
controller changes the output voltage from

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                                National Power Electronics Conference (NPEC-10) June 10-13, 2010.


s1 : OFF , S 2 : ON                          (14)         Fig.10,Fig.11,Fig.12 respectively.They are as
                                                          follows;
VL (OFF ) = Vg - v0       dTs < t < Ts
s1 : ON , s2 : OFF                           (15)

For constant-frequency operation and quasi-
steady-state analysis, the average inductor
voltage is approximately balanced during
each switching cycle, that is:
vL (ON ).dTs + vL (OFF ).(1- d )= 0           (16)
Substituting (18) and (19) in (20):
v g .d + (v g - v0 )( - d )= 0 When vg > 0
                     1                        (17)

v g .d + (v g + v0 )( - d )= 0 When vg < 0
                     1                        (18)

Combining       above     equations  provide
relationship between duty ratios of switches
and input ac voltage as well as dc bus
voltage of APF inverter, that is
vge = v0 ( - d )
          1                             (19)
vge = vg
vg > 0
                                             (20)
vge = - vg
vg < 0

VII.SIMULATION RESULTS

Simulations are performed using Matlab and
Simulink platform for a non-linear load (a full
bridge diode rectifier with RL-load) and a
three-phase voltage source inverter produce
and cancel the distorted currents as an APF
with the specifications shown in table 1.
TABLE I




According to simulation results.the load
current, inverter injected current and
compensated supply current are shown in

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                          National Power Electronics Conference (NPEC-10) June 10-13, 2010.


VIII.CONCLUSION                                        [7] K. Murat, O. Engin, “An adaptive
                                                       hysteresis band current controller for shunt
As we all know that nowadays the active                active power filter,” Elsevier Electric Power
power filter technology is the most efficient          Systems Research 73(2005) pp. 113-119.
way to compensate reactive power and low
                                                       [8] M. Rukonuzzaman, and M. Nakaoka,
order harmonics, which has been generated
                                                       “Single-phase shunt active power filter with
due to nonlinear loads of the systems. So
                                                       harmonic detection,” IEEE proc. Electr.
finally synchronous reference frame theory
                                                       Power Appl. Vol. 149, n0.5, September 2002.
for distortion detection and three phase shunt
active power filter with unipolar pulse with           [9] Moleykutty George, 2004. Modeling and
modulation technique to compensate reactive            simulation of a current controlled three-phase
power and low order harmonics which has                shunt active power filter using MATLAB/PSB:
been generated by the three phase nonlinear            AIUB Journal of Science and Engineering, 3:
loads of the system has been successfully              11-18.
implemented. So that the total current drawn
                                                       [10] Janko Nastran, Rafael Cajhen, Matija
from the three phase ac main is sinusoidal.
                                                       Seliger and Peter Jereb, 1994. Active Power
Thus problems like power losses and Excess
                                                       Filter for Nonlinear AC Loads: IEEE Trans.
heat caused by the harmonics in network has
                                                       Power Electronics, 9: 92-96.
been controlled and solved.
                                                       [11] E. Destobbeleer and L. Protin, 1996.
IX. REFERENCES                                         On the Detection of Load Active Currents for
                                                       Active Filter Control: IEEE Trans. Power
[1] Ambrish Chandra, Bhim Singh, B.N.
                                                       Electronics,11: 768-775.
Singh and Kamal Al-Haddad, 2000. An
Improved Control Algorithm of Shunt Active
Filter for Voltage Regulation, Harmonic
Elimination, Power-factor Correction, and
Balancing of Nonlinear loads:IEEE Trans.
Power Electronics, 15: 495-507.
[2]    A. Ametani, “Harmonic reduction in
thyristor Converters by harmonic current
injection,” IEEE Trans8.Power, Appar.Syst.
1976, 95, pp.441-449.
[3] M. Rukonuzzaman and M. Nakaoka, “An
advanced active power filter with adaptive
neural network based harmonic detection
scheme,” IEEE power Electronics Specialist
cascade, PESC, Vancouver Canada, 2001,
pp. 1602-1607.
[4]    M. C. Benhabib and S. Saadate, “New
Control approach for four wire active power
filter based on the use of synchronous
reference frame,” Elsevier Electric power
systems Research 73, 2005, 353-362.
[5] H. Akagi, Y. Kanazawa, and A. Nahae,
“Instantaneous reactive power compensators
comprising switching devices without energy
storage components,” IEEE Trans. Ind. Appl.
1984, 20, pp. 625-630.
[6]   T. Kwan, and K. Martin, “Adaptive
detection and enhancement of multiple
sinusoids using a cascade of IIR filters,” IEEE
Trans. Circuits Syst. 1989, 36, pp. 936-947.

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