Paper type: Essay Pages: 9 (2104 words)
IMPACT OF HARMONICS ON POWER SYSTEM AND SUITABILITY OF IT’S MITIGATION TECHNIQUESShubhada V. Ghanwat.Ankita R. Patil.Harshada S. Tapale.Department of Electrical Engineering,P.E.S.’s Modern College of Engineering, Pune.Savitribai Phule Pune University.ABSTRACT-This paper presents the study of harmonics present in the system and harmonic filters primarily used in electrical system. The main objective is to analyze and reduce harmonics in the system to get desired output and system response for particular operation. This paper describes the hardware design of active filter along with comparative study with simulation results of active filter in MATLAB.
I.INTRODUCTION: The presence of harmonics in a power system can give rise to a variety of problems including equipment overheating, reduced power factors, deteriorating performance of electrical equipment, the incorrect operation of protective relays, interference with communication devices, and in some cases, circuit resonance to cause electric apparatus dielectric failure and other types of severe damage. Even worse, harmonic currents generated in one area can penetrate into the power grid and propagate into other areas, resulting in voltage and current distortions for the entire system.
This phenomenon has become a major concern for power quality due to the ever-increasing usage of electronic devices and equipment in power systems.The harmonic is defined in many literatures as a component of a periodic wave having a frequency that is an integral multiple of the fundamental power line frequency. The meaning of the harmonic can be easily explained using the following example. Let f’D. K. JoshiAssit. Prof. Department of Electrical Engineering,P.E.S.’s Modern College of Engineering, Pune.Savitribai Phule Pune University.represents a fundamental frequency, the second harmonic has frequency 2f Hz, third harmonic has frequency 3fHz ,and so on. The 2nd ,4th,6th,etc.,are called even harmonics while the 3rd,5th,7th,etc.,are called odd harmonics. There are two types of harmonics:1. Voltage Harmonics2. Current Harmonics The harmonic distortion occurs when non-linear loads, such as rectifier, inverter, adapters, etc., are feeded from power systems and changes sinusoidal wave at a fundamental frequency to different non-sinusoidal waves as shown in Fig.1Fig.1 Effect of Non-linear load on electrical power system.II. HARMONICS AND ITS EFFECTS:Table 1. Harmonics and its effects: Sr.NoEffects of Harmonics1. General Heating,Thermal or voltage stress, aging of electrical insulations2. Motors Flux distribution in air gap,cogging or crawling3. Generators High stressed mechanical forces , temperature rise4. Transforme-rsIncreases audible noise, Increases losses like Cu losses, stray flux losses, Iron losses, causes losses5. Power cables Voltage stress and corona6. Electronic Equipment Malfunctioning 7. Metering Erroneous operation8. Switchgear And Relaying Increases heating and losses, slower operation ,changes in operating characteristicsFig.2 Effect of HarmonicsNon-linear loads:The effects discussed above are because of the non-linear load. These are the loads in which the current wave form does not resemble the applied voltage waveform due to a number of reasons, for e.g. the use of electronic switches that conduct load current only during a fraction of power frequency period. Therefore,e can conceive non-linear loads as those which Ohm’s law cannot describe the relation between V and I.Those loads are listed below:Power Electronics:Variable frequency drivesPower converters CycloconvertersCranesElevatorsUPSBattery ChargersInvertersARC Devices:Fluorescent LightingArc furnacesWelding machinesIII.Mitigation techniques:Passive Filters: Passive filters are very much helpful for mitigation of harmonic component & used traditionally. There is a continuous development has been reported in this technique for the better use of filter & convert the filter more useful to achieve the optimum approach to utilization with reduced rating & cost. The use of passive filter in the mitigation of harmonic in 3 phase system use the utilizing reactor & capacitor is the most significant development in the field of harmonic distortion mitigation.Active Filter:To reduce the harmonics conventionally passive L”C filters were used and also capacitors were employed to improve the power factor of the ac loads. But the passive filters have several drawbacks like fixed compensation, large size and resonance problem. To mitigate the harmonics problem, many research work development are developed on the active powerFilter (APF)IV.Case Study:Set up :We connected the power quality analyzer Fluke 430-II across various non-linear loads like makino A98, A99, CNC machine, vfd, etc.Modeling and control of the single-phase THSeAFAverage and small-signal modeling Based on the average equivalent circuit of an inverter ,the small-signal model of the proposed configuration can be obtained as in fig.4 . Hereafter, d is the duty cycle of the upper switch during a switching period, whereas v and i denote the average values in a switching period of the voltage and current of the same leg. The mean converter output voltage and current are expressed by following relations:Fig.4. Small-signal model of transformerless HSeAF in series between the grid and the loadvO=(2d-1)VDCWhere the (2d-1) equals to m, theniDC=mifCalculating the theThevenin equivalent circuit of the harmonic current source leads to the following assumption:vh(j‰) =-jihCHPF.‰hIf the harmonic frequency is high enough, it is possible to assume that there will be no voltage harmonics across the load. The state-space small signal ac model could be derived by a linearized perturbation of the averaged model as follows_x=Ax+BuHence we obtain,ddtvCfvCHPFisifil = 001Cf001CHPF-1Ls-1Lf0-1Ls01LL-rcLs-rcLf0 1Cf00-1CHPF-rcLs-rcLf000-RLLL—vCfvCHPFisifil+0000001LS0001Lf01LS0-1Ll—vSVDCvhMoreover, the output vector isy = Cx+DuBy means of (10) and (12), the state-space representation of the model is obtained as shown in Fig. 4.The transfer function of the compensating voltage versus the load voltage, TV _CL(s), and the source current, TCI (s), are developed in the Appendix. Meanwhile, to control the active43228850507Fig. 5. Control system scheme of the active part.part independently, the derived transfer function should be autonomous from the grid configuration. The transfer function TVm presents the relation between the output voltages of the converter versus the duty cycle of the first leg converter’s upper switchTVs=VcompVO= rCCfs + 1LfCf s2 + rCCf s + 1 13TVms=Vcompm= VDC · TV (s). 14The further detailed derivation of steady-state transfer functions is described in Section V.A dc auxiliary source should be employed to maintain an adequate supply on the load terminals. During the sag or swell conditions, it should absorb or inject power to keep the voltage magnitude at the load terminals within a specified margin. However,if the compensation of sags and swells is less imperative, a capacitor could be deployed. Consequently, the dc-link voltage across the capacitor should be regulated as demonstrated in Fig. 5.B. Voltage and Current Harmonic DetectionThe outer-loop controller is used where a capacitor replaces the dc auxiliary source. This control strategy is well explained in the previous section. The inner-loop control strategy is based on an indirect control principle. A fast Fourier transformation was used to extract the magnitude of the fundamental and its phase degree from current harmonics. The control gain G representing the impedance of the source for current harmonicshas a sufficient level to clean the grid from current harmonics fed through the nonlinear load.The second proportional integrator (PI) controller used in the outer loop was to enhance the effectiveness of the controllerwhen regulating the dc bus. Thus, a more accurate and faster transient response was achieved without compromising the compensation behaviour of the system. According to the theory, the gain G should be kept in a suitable level, preventing the harmonics from flowing into the grid , . As previouslydiscussed, for a more precise compensation of current harmonics, the voltage harmonics should also be considered. The compensating voltage for current harmonic compensation is obtained fromFig. 6. Block diagram of THSeAF and PI controller.vcomp_i(t) = (€’GiS + vL)€’[| €’ GiS1 + vL1| · sin(‰St€’)].Hereby, as voltage distortion at the load terminals is not desired, the voltage sag and swell should also be investigated in the inner loop. The closed-loop equation (16) allows to indirectly maintain the voltage magnitude at the load side equalto V €— L as a predefined value, within acceptable marginsvcomp_v = vL €’ V€—L sin(‰St). (16)The entire control scheme for the THSeAF presented in Fig. 5 was used and implemented in MATLAB/Simulink for real-time simulations and the calculation of the compensating voltage. The real-time toolbox of dSPACE was used for compilation and execution on the dsp-1103 control board. The source and load voltages, together with the source current, are considered as system input signals. According to Srianthumrong et al. , an indirect control increases the stability of the system.The source current harmonics are obtained by extracting thefundamental component from the source currentv €—com_ref = vcomp_v €’ vcomp_i + vDC_ref (17)42741856350where the vDC_ref is the voltage required to maintain the dc bus voltage constantvDC_ref (t) = VO_DC · sin(‰St). (18)A phase-locked loop was used to obtain the reference angular frequency (‰s). Accordingly, the extracted current harmonic contains a fundamental component synchronized with the source voltage in order to correct the PF. This current representsthe reactive power of the load. The gain G representing the resistance for harmonics converts current into a relative voltage. The generated reference voltage vcomp_i required to clean thesource current from harmonics is described in (15).According to the presented detection algorithm, the compensated reference voltage v€—Comp_ref is calculated. Thereafter, thereference signal is compared with the measured output voltage and applied to a PI controller to generate the corresponding gate signals as in Fig. 6.C. Stability Analysis for Voltage and Current HarmonicsThe stability of the configuration is mainly affected by the introduced delay of a digital controller. This section studies the impact of the delay first on the inclusive compensated system according to works cited in the literature. Thereafter, its effects on the active compensator is separated from the grid. Using purely inductive source impedance (see Fig. 4) and Kirchhoff’s law for harmonic frequency components, (19) is derived. The delay time of the digital controller, large gain G, and theFig. 7. Control diagram of the system with delay.Fig. 8. Closed-loop control diagram of the active filter with a constantdelay time “high stiffness of the system seriously affect the stability of the closed-loop controlled system.Ish(s)=Vsh-VComp- VLhLss The compensating voltage including the delay time generated by the THSeAF in the Laplace domain [see (1)] isvComp = G · Ish · e €’”s €’ VLh. (20)Considering (19) and (20), the control diagram of the system with delay is obtained as in Fig. 7.For the sake of simplicity, the overall delay of the system is assumed to be a constant value ” . Therefore, the open-loop transfer function is obtained G(s) =GLss e”s.From the Nyquist stability criterion, the stable operation of the system must satisfy the following condition:G(s) <Ls2″ e”s.A system with a typical source inductance Ls of 250 јH and a delay of 40 јs is considered stable according to (22) when the gain G is smaller than 10©. Experimental results confirm the stability of the system presented in this paper. Moreover, theinfluence of the delay on the control algorithm should also be investigated. According to the transfer functions (13) and (14), the control of the active part is affected by the delay introduceby the digital controller. Thus, assuming an ideal switching characteristic for the IGBTs, the closed-loop system for the active part controller is shown in Fig. 8.The open-loop transfer function in Fig. 8 turns to (23), where the ” is the delay time initiated by the digital controllerF(s) =PIG · TVm · e”s=rCCfVDCs + VDC.(Kps + Ki)e”ss · (LfCf s2 + rCCf s + 1)A PI controller with system parameters described in Table I demonstrates a smooth operation in the stable region. By means of MATLAB, the behavior of the system’s transfer function F(s) is traced in Fig. 9. The root locus and the Bode diagram ofthe compensated open-loop system demonstrate a gainmarginSIMULATION RESULTS:-V. CONCLUSIONHarmonics generated from the non-linear load were analyzed using the hardware of power filter along with its MATLAB simulation.The Active power filter gives better results as compared to Power filter and this is observed with the help of waveforms obtained from MATLAB simulation models.Thus we can say that these harmonic mitigation techniques can be applied to various fields to mitigate or reduseharmonicc content present in the electrical system, so as to improve power quality and power factor.VI. ACKNOWLEDGEMENT:Authors wish to thank the authorities of Progressive Education Society’s Modern College of Engineering, Pune , Maharashtra, India for their necessary help.VII.References:IEEE Std 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical PowerSystems, New York, NY: IEEE. Harmonics& Power system by Francisco. C. De. La Rosa, DistributionControl system, Inc. Hazelwood, Missouri, U. S.A. Joseph, S. and J.R. Subjak, 1990. Harmonics_Causes, Effects, Measurments and Analysis: An Update IEEE Transaction on Industry Application, Vol. 26. No 6.S.Janpong,K-L Areerak,K-N Areerak.A literature survey of neutral network applications of shunt active filters.Barbow,Hayes & Horowitz,Rizzoni,Passive FilterECE60L Lecture Notes, Spring 2004,Vcomp = G.Ish €’ VLh. (1)vO = (2d- 1)VDC 6x™ = Ax + Bu. (9)iDC = mif .y = Cx + Du 11vcomp_i(t) = (€’GiS + vL)€’[| €’ GiS1 + vL1| · sin(‰St€’)].15vcomp_v = vL €’ V—L sin(‰St).16v€—com_ref = vcomp_v €’ vcomp_i + vDC_ref (17)vDC_ref (t) = VO_DC · sin(‰St). (18)
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