PLL BASED SHUNT ACTIVE HARMONIC FILTER TO COMPENSATE MULTIPLE NON-LINEAR LOADS by iaemedu

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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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 COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

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    PLL BASED SHUNT ACTIVE HARMONIC FILTER TO COMPENSATE
                  MULTIPLE NON-LINEAR LOADS

                                        Smt. Smita Singhai
                                    Govt. Engineering College
                               Sejbahar, Raipur(C.G.) INDIA-492015

                                        Prof. Bharti Dewani
                               Asst. Prof. DIMAT Raipur(C.G., India)



ABSTRACT

        In this paper, three phase shunt active power filter is given for compensating multiple non-
linear loads. The circuit models a standard shunt AHF with IGBT inverter and series inductor on the
AC side and DC capacitor energization. The AHF uses a PLL to generate a reference sinusoidal
source current which is in-phase and has the same RMS gain as the load current. Current control is
implemented through feedback modulation of a dynamic hysteresis band PWM controller. The shunt
line current tracks the reference current within a hysteresis band. By comparing the reference
currents calculated by the controller with the measured values of compensation currents, the
command signals for the inverter semiconductor switches can be produced.

Keywords: Shunt AHF, IGBT inverter, Series Inductor, PLL(phase locked loop), Hysteresis
Switching, Non-linear Load, Harmonics.

1. INTRODUCTION

         Nonlinear loads cause voltage and current waveforms distortion in the ac power network. It
results in harmonic related problems including substantially higher transformer and line losses,
reactive power and resonance problems, over-voltages, over-heating, Electro Magnetic Interference
(EMI) problems, and other undesirable effects. The result is reducing system stability [1]-[3]. Passive
filters alone have been traditionally used to eliminate the harmonics in utilities due to their low cost
and high efficiency. Shunt-connected passive filters, tuned to show low impedances at different
dominant harmonic frequencies, are widely used. Conventionally passive L-C filters were used to
reduce harmonics and capacitors were used to improve the power factor of the ac loads. However,
passive filters have the demerits of fixed compensation, large size, and resonance. Including all these

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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
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drawbacks, these filters also generate fixed quantity of reactive power at fundamental frequency
affecting sometimes the voltage regulation at the PCC. Active filters were developed to mitigate
problems of passive filters. Shunt Active Filters are used for compensating current harmonics. Series
Active Filters are used for compensating voltage harmonics. They are more effective in harmonic
compensation and improved system performance. But using only active filters is a very expensive
solution because it requires comparatively high power converter ratings. Hybrid Active Filter (HAF)
topologies which combine the advantages of both active and passive filters [4]-[6] is more appealing
in terms of cost and performance. They are cost-effective by reducing the KVA rating of the active
filter as much as possible while offering harmonic isolation and voltage regulation [5]. Two kinds of
hybrid active filters have been developed: a hybrid series active filter and a shunt hybrid active filter.
To compensate for both current and voltage system harmonics, a shunt and series active filter
configuration must be used respectively. Integrating this filter serves to eliminate load harmonics
whilst ensuring the supply remains fundamental. For harmonic elimination, active filter can be
classified on the basis of various control technique- open loop system & closed loop system. Open-
loop systems sense the load current and the harmonics it contains. They inject a fixed amount of
power in the form of current (mainly reactive) into the system, which may compensate for most of
the harmonics and/or reactive power available. No reference current is required for this type Closed
loop control systems incorporate a feedback loop providing greater accuracy of current injection for
harmonic compensation as well as reactive power reduction well over open loop system [7]-[10].
There is reference variable to check the performance and accuracy of the filter.[11-18]


2. ACTIVE HARMONIC FILTER

        Proposed methodology uses a combination of a grid current forcing shunt APF with a series
reactor installed at the Point of Common Coupling (PCC) to handle the harmonic and unbalance
problems from mixed loads. [20] The three-phase shunt active power filter is a three-phase current
controlled “voltage-source inverter” (CC-VSI) with a mid-point earthed, split capacitor in the dc bus
and inductors in the ac output (It is essentially three independent single phase inverters with a
common dc bus).




                       Figure 1. Proposed Active Power Filter Configuration



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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

        The circuit models a shunt AHF with IGBT inverter and series inductor on the AC side and
DC capacitor energization. The load consists of two diode rectifiers which are phase-shifted by 30
degrees. The Delta-Y connected rectifier is connected after 10 cycles to change the load from 6-pulse
to 12-pulse.
        The AHF uses a PLL to generate a reference sinusoidal source current which is in-phase and
has the same RMS gain as the load current. The current error between the load current and the
reference current is generated by the IGBT Bridge through hysteresis switching. The AHF aims to
inject this current error at the point of common coupling in order to match the source current as
closely as possible with the reference current.

3. PLL (PHASE LOCKED LOOP)

         Three different types of harmonic detection strategies used to determine the current reference
for the active filter. These are-
1. Measuring the load harmonic current to be compensated and using this as a reference command;
2. Measuring source harmonic current and controlling the filter to minimize it; and
3. Measuring harmonic voltage at the active filter point of common coupling (PCC) and controlling
the filter to minimize the voltage distortion.
         Proposed        methodology involves measurement of the load current and subsequent
extraction of its harmonic content. The harmonic components, so extracted, are adjusted for polarity
and used as reference commands for the current controller. For estimation of reference current
various techniques are used- High pass filter method, Low pass filter method, Time domain
approaches- Instantaneous reactive power algorithm, Synchronous detection algorithm, Constant
active power algorithm, Constant power factor algorithm, Fictitious power compensation algorithm,
Synchronous frame based algorithm, Synchronous flux detection algorithm, Frequency domain
approaches- Conventional Fourier and FFT algorithms Sine multiplication technique, Modified
Fourier series techniques.
         Proposed methodology uses PLL         (phase Locked loop) and hysteresis switching for
estimation of reference current.[12]
         Let the load current, input frequency and terminal voltage be the input to the PLL. Three
phase distorted supply voltages are sensed and given to the PLL which generates sine terms. The
sensed supply voltage is multiplied with a suitable value of gain before being given as an input to the
PLL. Here K=1…N, be the gain value assigned for controlling.




                        Figure 2: Proposed reference signal generation (PLL)



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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

        Here IL is the load current VT is the load voltage, ω is the output signal of the adaptive
detecting circuit; and f is the fundamental reference frequency which is in phase with ac source
voltage. As the input sinusoidal reference signal, i.e. the fundamental component of the system
voltage has the same frequency and in phase with the desired fundamental components of load
current and load voltage, the dc component of the output of integrator will tune accordingly until
they are equal in magnitude. The corresponding fundamental real components of the current and
voltage are then extracted from the sampled load current and load voltage. The output signal of the
adaptive detecting current and voltage are just the reactive power and harmonic components of the
nonlinear load voltage and current.

4. HYSTERESIS SWITCHING

      Current control is implemented through     feedback modulation of a dynamic hysteresis band
PWM controller. The shunt line current tracks    the reference current within a hysteresis band. By
comparing the reference currents calculated      by the controller with the measured values of
compensation currents, the command signals        for the inverter semiconductor switches can be
produced.




                              Figure 3: Proposed hysteresis controller

        In proposed methodology, the load current, and the current of active filter be the input to the
shunt firing unit. The gate signal obtained from this unit is the input to the IGBT. Thus obtaining
gate signal by means of hysteresis current controlling technique is performed. The gate signal is
obtained by means of using hysteresis current controlling technique. To detect the current to be
compensated, reference current should be obtained. PLL value is improved by means of RMS value
of load current.

4. SIMULATION RESULTS

        Simulation is carried out on a Matlab /Simulink software.[19] Figure 4 represents the
simulation model. Harmonics generated by non-linear loads is removed by PLL based Shunt Active
Power Filter. Proposed model not only considers the harmonics due to non-linear load but it also
considers the disturbance occurs in supply. In this simulation the input current wave shape is non-
sinusoidal which represents unbalanced supply. Simulation time is 0.25 seconds. Figure 5 (figure 5.1
to 5.6) shows the Simulation Results. Table 1 shows experimental & simulation parameters.


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0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME


                  S.No.                 Parameters                  Value
               Source          Voltage      Vabc            4160*sqrt(2)/sqrt(3)
                               Frequency F                  50Hz
               Load            Two          Resistance      1*10-3 ohms
                               Diode
                               Rectifiers Snubber           1x103ohms
                                            Resistance

                                           Snubber          1x10-6ohms
                                           Capacitance


                      Table 1: SAPF Experimental and Simulation Parameters




                      Figure 4. Simulation Model of Shunt Power Active Filter




Figure 5.1 3 phase source current waveform               Figure 5.2 V-DC BUS voltage waveform


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       Figure 5.3 V-DC BUS LOAD2 voltage                    Figure 5.4 3 phase injected current
                 Waveform                                                Waveform




Figure 5.5 3 phase Reference current waveform             Figure 5.6 3 phase load current of
                  of AHF                                               Waveform

                                   Figure 5. Simulation Results



5. DISCUSSION

       In this Project Shunt active power filter is used to compensate current harmonics by
providing same-but-reverse harmonic compensating current. Hence the shunt active power filter now
operates as a current source providing the harmonic components generated by the load but the phase
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International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 4, July-August (2013), © IAEME

is shifted now by 180o. This phenomenon is for all type of loads considered in a harmonic source.
Moreover, with an efficient control scheme, the active power filter can also compensate the load
power factor. By this, the power distribution system can see the non linear load and the active power
filter as an ideal resistor. To order to present the compensation for harmonic voltage sources, a
simulation model can be presented via facilitating the circuit constants based on a three phase ac
system. For obtaining harmonic free supply at load end, PLL (phase locked loop) & harmonic
switching is used. A PLL is feedback system that fixes relation between output clock phase and input
clock phase. Actually phase of both input signal and output signal are synchronized or locked, hence
name called “Phase Locked Loop”. The hysteresis band current control technique has been proven to
be the most suitable technique for all the applications of current controlled voltage source inverters
in the active power filters. The hysteresis band current control is characterized by its unconditioned
stability, high speed response, and valid accuracy. Simulation results, this system provides unity
power factor operation of non-linear loads with harmonic current sources, harmonic voltage sources,
reactive, and unbalanced components.

5. CONCLUSION

        This project proposes the implementation of a three-phase active power filter together with a
decoupling reactor in series with the load operated to directly control the ac grid current to be
sinusoidal and in phase with the grid voltage. From the simulation results, this system provides unity
power factor operation of non-linear loads with harmonic current sources, harmonic voltage sources,
reactive, and unbalanced components.

6. ACKNOWLEDGEMENT

       The authors owe a great deal of sincere thanks to all of those involved, directly or indirectly,
in the preparation of this research paper.

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