Abstract- with the arrival ofsemiconductor and power electronic devices and their easier controllability hascaused wide use of nonlinear loads. But the use of power electronic devices isresponsible for harmonic and reactive power disturbances. These harmonics createsthe disturbance in normal operation, excessive heating in the equipments etc.so it is necessary to eliminate these harmonics problems. So importanceis being given to the development of Active Power Filters to solve theseproblems to improve power quality among which shunt active power filter is usedto eliminate voltage and load current harmonics and for reactive powercompensation. The shunt active power filters have been developed based onSynchronous Reference Frame Algorithm Method.
Synchronous Reference Frame (SRF)Algorithm is used to extract the harmonics components. Hysteresis band currentcontrol (HBCC) technique is used for the generation of firing pulses to theinverter. This system is simulated using MATLAB and results are observed. Keywords– Harmonics, Hysteresiscurrent control, Shunt Active Power Filter, Synchronous ReferenceFrame Algorithm I INTRODUCTIONNow days,power system uses large number of power electronic devices to control the powersystem equipments.
However power electronic based equipments which includesadjustable speed motor drives, electronic power supplies, electronic ballastsare responsible for the rise in power quality related issues.1. Thesenonlinear loads appear to be important sources of harmonic distortion in apower distribution system. These harmonics reduces the quality of power, lowefficiency, low power factor. Hence to overcome these problems of harmonicspassive filters have been used.
But due to some disadvantages, namely it willintroduce system resonances that can move a harmonic problem from one frequencyto another, it is difficult to design the filters to avoid leading power factoroperation for some load conditions. To overcome these disadvantages, activepower filter have been developed.2 The Active Power Filter (APF) based onpower electronics technology is a viable solution for power conditioning tosuppress the harmonics in the power system. With recent developments in powerelectronic switches, the Active Power Filters (APFs) have been applied tomitigate the problems created by non-linear loads. One of the most commonlyused active filters is the Shunt Active Filter (SAF) which is used to eliminatethe unwanted harmonics and compensate reactive power consumed by non-linearloads 3. The Shunt Active Power Filter is connected in parallel with the line through a couplinginductor. Its main power circuitconsists of a three phase three-legcurrent controlledvoltage source inverter with a DC link capacitor.
An active power filter operatesby generating a compensating current with 180 degree phase opposition and injects it back to the line soas to cancel out the current harmonics introduced by the nonlinear load. This will thussuppress the harmonic content present in the line and make the current waveform sinusoidal. So the process comprises of detectingthe harmonic component present in the line current,generating the reference current, producing the switching pulses for the power circuit,generating a compensating currentand injecting it back to the line4-7. Figure.1 Three phaseshunt active power filter II.
SHUNTACTIVE POWER FILTERShunt active power filtersare widely used in power system to compensate reactive power and currentharmonics. It can also play the role of static VAR generator in the powersystem for improving and stabilizing the voltage profile. Shunt active filtercompensate current harmonic by injecting complementary current that of producedby nonlinear load. shunt active filter acts as a current source by introducingthe harmonic components created by the load.
Consequently, the current harmoniccomponent present in the load current got cancelled and the source currentremains sinusoidal. By the use of proper control scheme, APF can also improvesystem power factor. However the performance of SAPF largely depend on thecontrol strategy which is responsible for generating complementary harmoniccurrent to cancel out the current harmonics present in the load current. Thereare several control strategies like, Instantaneous power theory based onsymmetrical 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 signalsapplied to current control algorithm.
III SYNCHRONOUS REFERENCE FRAME ALGORITHM Number of controlstrategies being used for the determination of reference currents in shuntactive 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 thethree-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 explainsthree-phase SRF-theory, used for harmonic component extraction. Figure.2 Reference Frame Transformation Figure.
3 Block diagram of SRF basedalgorithm In thismethod, the source currents (ia, ib, ic) arefirst 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 whichallows the evaluation of specific harmonic component of the input signals and alow pass filtering stage LPF. Now, the two phase current quantities i?and i? of stationary ??-axes are transformed into two-phasesynchronous (or rotating) frame (d-q-axes) using equation (2), where Cos? andSin? represents the synchronous unit vectors which can be generated using phase-lockedloop 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 DCpart and the AC part represents the harmonic component. This harmonic componentcan be easily extracted using a high pass filter (HPF), as implemented in Fig2.
The d-axis current is a combination of active fundamental current (iddc) and the load harmonic current (ih). The fundamental component ofcurrent rotates in synchronism with the rotating frame and thus can beconsidered as dc. By filtering id, the current is obtained, whichrepresents the fundamental component of the load current in the synchronous frame.Thus, the AC component idh can be obtained by subtracting iddc part from the total d-axis current (id), which leaves behind theharmonic component present in the load current.
In the rotating frame theq-axis current (iq) represents the sum of the fundamental reactiveload currents and part of the load harmonic currents. So the q-axis current canbe totally used to calculate the reference compensation currents.Now inversetransformation is performed to transform the currents from two phasesynchronous frame d-q into two-phase stationary frame ?-? as per equation (3). (3) Finally the current from two phase stationary frame ??0 istransformed back into three-phase stationary frame abc as per equation (4) andthe compensation reference currents ica*, icb* and icc*are obtained. (4) Where, (5) IV HYSTERISIS BANDCURRENT CONTROL The hysteresis band current control (HBCC)technique is used for pulse generation in current controlled VSIs.
The controlmethod offers good stability, gives a very fast response, provides goodaccuracy and has got a simple operation. The HBCC technique employed in anactive power filter for the control of line current is shown in Figure 4. Itconsists of a hysteresis band surrounding the generated error current. Thecurrent error is obtained by subtracting the actual filter current from thereference current. The reference current used here is obtained by the SRFmethod as discussed earlier which is represented as Iabc*.
The actual filtercurrent is represented as If abc. The error signal is then fed tothe relay with the desired hysteresis band to obtain the switching pulses forthe inverter. Figure.4 Hysteresis Band Current Controller The operation of APF depends on the sequence of pulse generatedby the controller.
Figure 5 shows the simulation diagram of the hysteresiscurrent controller. A band is set above and below the generated error signal.Whenever this signal crosses the upper band, the output voltage changes so asto decrease the input current and whenever the signal crosses the lower band,the output voltage changes to increase the input current. Accordingly switchingsignals are generated. Figure.5 Simulation diagram of hysteresiscurrent control The switching signals thus generated are fed to the powercircuit which comprises of a three phase three leg VSI with a DC link capacitoracross it.
Based on these switching signals the inverter generates compensatingcurrent in phase opposition to the line current. The compensating current isinjected back into the power line at the PCC and thus suppressing the currentharmonics present in the line. The overall simulation block diagram is shown inFigure 6. Figure.
6 Overall simulation diagram. V. SIMULATION RESULTS AND DISCUSSIONAfter simulation of three phase transmission line having nonlinear load with SRF based shunt active filter the harmoniccurrent is compensated within a permissible limits of IEEE standard. In thisthe 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 tonon linear load. These harmonics distort the source current as shown infigure.7. Also if the THD is cheked, then Total Harmonic Distortion (THD) spectrumin the system without filter is shown in Figure.
7, which indicate a THD of 15.59% These compensatingcurrent is produced by the filter when we are injecting this compensatingcurrent we get the source current with minimum harmonics. The source currentafter the injection of compensating current is shown in Figure 8. The THD withactive power filter included is observed to be 3.77% which is within the allowable harmoniclimit. 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 VI. CONCLUSIONSThe SAPF explainedin this paper compensate the line current harmonics generated due to thenonlinear loads in the system. HBCC technique used for the switching pulsegeneration was found to be effective and its validity is proved based onsimulation results. Thus SRF based SAPF has been proved to be effective to keepthe harmonic content in power lines within the permissible limit of IEEEstandards i.e. THD is 3.77%.
F.McGranaghan, Santoso and H.W.
Beaty, “Electrical Power Systems Quality”,second edition McGraw-Hill, 2002, USA 2JoaoAfonso,Mauricio Aredes,Edson Watanabe, Julio martins “Shunt active filter forpower quality improvement.” International conference UIE 2000- Electricity fora sustainable Urban Development , Lisboa, potugal, 1-4 Novembro 2000 pp683-691.3DeepathiJoseph, “P-Q Theory for Shunt Active Filter using Ramp Comparator” IEEE transactionon International conference on Power, Energy and Control.
2013.4Preeti Yadav, Swati Maurya, “Single phase shunt active power filter forharmonic filtering” International Journal of Emerging Technologyand Advanced Engineering, Volume 4, Issue 4, April 2014.5AlbertoPigazo, “ARecursive Park Transformation to Improve the Performance of SynchronousReference Frame Controllers in Shunt Active Power Filters” IEEE Transactions On PowerElectronics, Vol.
24, No. 9, September 2009.6Mohammad Monfared, “A NewSynchronous Reference Frame-Based Method for Single-Phase Shunt Active PowerFilters” Journal of Power Electronics,Vol. 13, No.
4, July 2013.7Diyun WU, “Design and Performance of a Shunt ActivePower Filter for Three phase Four-wire System” 2009 3rd InternationalConference on Power Electronics Systems and Applications.8Leszek S.
Czarnecki,”Instantaneous Reactive power p-q theory and Power properties of 3-phasesystem”, IEEE Transactions on PowerDelivery, Vol. 21, No. 1, pp.362-367, Jan.
2006. ACKNOWLEGMENT Ms. Dipeeka P. Sawant received his B.E degree in Electrical Engg. from Pune University,in 2012. Now she is doingM.
E. in Electrical Power System from Yadavrao Tasgaonkar Institute of Engg. AndTechnology Bhivpuri Road , Karjat. Ms. Pranita P.
Chavan received his B.E degree in ElectricalEngg. from Mumbai University, in2002.And M.Edegree from Pune University in Electrical Power System in 2004 . Now she is working as Assistant Professor In Yadavrao Tasgaonkar Institute of Engg. AndTechnology Bhivpuri Road , Karjat.
She has Total Experience spans of over 11 years.