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International Journal of Engineering Trends and Applications (IJETA) – Volume 4 Issue 2, Mar-Apr 2017RESEARCH ARTICLEOPEN ACCESSMagnetic Inrush Current of Transformer ReduceBy Point Wave Switching Method with MATLAB SimulationResultsSachin R. Chavan [1], M. S. Potdar [2]PG Student [1], Assistant Professor [2]Dept. of Electrical Engineering, PES College of EngineeringAurangabad (MS) - IndiaABSTRACTAt the time of transformer energization, a high current will be drawn by the transformer. This current is mentioned transientinrush current and it may rise to ten times the nominal full load current of transformer during operation .This current canproduce mechanical stress to the transformer, causes protection system malfunction and it often affect the power systemquality and may disrupt the operation of sensitive electrical loads such as computers and medical equipment connected tothe system. Energization transients current reduce by use of point on wave switching at the time transformer is initiallyconnected to supply. In this paper the simulations and the experimental results on a three-phase transformer for reductionof inrush currents. An electronic devices three-phase switching controller has been designed and some thyristors are usedfor switching power to the transformers. Reduction of magnetic inrush current and the way to control of energizationtransients currents have become important concerns to the power industry for engineers.Keywords:- (Transformer current, magnetic inrush current)I. INTRODUCTIONInrush currents from transformer and reactor energizationhave always been a concern in power industry. Over thepast several decades, a few methods have been proposed tolimit the inrush currents. Representative examples pointwave switching method and the pre insertion of seriesresistors. Two recent developments in power industry havereignited interests in finding better methods for controllinginrush currents. One of them is the deregulation ofelectricity market. More and more independent powerproducers and co-generators are taking advantage of thesituation. Their proposed generators often need to have thegenerator transformers energized from the system side dueto cost considerations. This results in the problem of largeinrush currents being injected into the supply system.Another development is the increased awareness on powerquality. The power quality consequences of inrush currents.can be quite detrimental. Examples are motor tripping,relay disoperation and so on. There is still a need to findsimpler and low cost schemes to limit the inrush currents.Independent power producers are especially interested insuch techniques. The idea presented in this paper is byobservation and research. In view of the fact that the inrushcurrents are always unbalanced among three phases, aneutral resistor could provide some damping to thecurrents. But this is the basis of the proposed idea. But thisidea is not permanent and convenient The idea is furtherimproved by introducing delayed energization of eachISSN: 2393-9516phase of the transformer by point wave switching method.This improvement has made the proposed scheme almostas effective by point wave switching method theperformance and characteristics of the proposed methodhave been investigated using simulations and experiments.Very encouraging results have been observed. This paperexplains the proposed idea and presents its performancecharacteristics. The results are obtained from simulationand experimental studies.II. REASONS & EFFECTWhen the transformer is charged, a transient current knownas magnetizing inrush current (of magnitude as high as10times of rated current) flows in to the system. This isdue to the nonlinear relationship of flux and magnetizingcurrent as transformer core is in saturation mode. It is notonly the high magnitude of inrush current but itscomposition (rich in DC component and harmonics) andduration are also the cause of concern which severelyaffects the stability of the system. Factors contributing tothe magnitude and duration of inrush current areMagnitude of residual flux in transformer core, Nonlinearmagnetizing characteristic of transformer core, Magnitudeof source voltage at the switching instant and Impedanceand short circuit power of the source.www.ijetajournal.orgPage 1

International Journal of Engineering Trends and Applications (IJETA) – Volume 4 Issue 2, Mar-Apr 2017The key adverse effects includeA. Mechanical and electrical stresses in windings:The amplitude of inrush current can be equal to that ofthe short circuit current and may last longer dependingon system configuration. This can seriously damage thewindings through excessive mechanical stressesSwitching, magnetic inrush current a transformer mayresult in core saturation, power quality decrease where, asmall increase of flux which leads to a large increase incurrent as shown in fig 2.B. Harmonic resonant over voltages:Transformer inrush currents are rich in harmonics and inthe event of resonance, a sustained Harmonic resonantover voltages may exist and if these over voltages lastfor a long period of time, they may eventually damagethe equipment.C. Mall operation of protective relaysD. Voltage dips:Due to high magnitude and asymmetrical nature ofinrush current a voltage dip is observed by the system.The magnitude, duration and unbalance of voltages inthe respective phases are function of system impedance,source, transformer capacities etcFig 1: Voltage, current and flux waveformIII. PROPOSED SCHEMEWhen a transformer is energized, then depending on itsswitching at and high magnetic flux in the transformer core,it may produce a high transient magnetic inrush currentsis produce which up to ten cycle times of its rated no-loadcurrent. The magnetizing inrush currents in powertransformers can cause electromechanical shock ontransformer bushings, core and windings, malfunctioningof protection systems fail and create saturation in the coresevere power quality reduce problem created in the powertransformer. The magnetic inrush currents in transformermay last in about 10 cycles. Energization magnetic currentreduce the performance of a transformer. The design of adevice to control in order to provide voltage delay anglecontrolled switching device to connect the rated voltage totransformer primary circuit at the starting current can bevery effective. The point on wave control is designed toenergize the transformer at the optimal point on wave thevoltage waveform, and its intention is to reducetransformer transient inrush at the time of energization ofstarting current.In a transformer circuit the voltage and the currentwaveform are 90 away apart from each other.Transformer current and flux are normally in phase soangle between current and flux is zero voltage and flux are90 apart as well as shown in fig 1. Without point on waveFig 2: Transient magnetic inrush currentIV. INRUSH CURRENTThe transient component Фt will be decay according to thecircuit time constant (L/R) which is constant, the fluxtransient will through a maximum value of 2Фm So thisphenomenon called as doubling effect. The correspondingexciting current is be very high as the core gets deepsaturation region of magnetization (Bm 2*1.4 2.8T);which may indeed be as high the normal exciting current,(normal exciting current being 0.05 pu) producingelectromagnetic forces 25 times the normal rated current.This is why windings of highly transformer strong braced.Фt (Фm Фr)- Фt cos wtAt wt from the instant of closing the switchequation becomesISSN: 2393-9516www.ijetajournal.orgPage 2

International Journal of Engineering Trends and Applications (IJETA) – Volume 4 Issue 2, Mar-Apr 2017Фt ( Фm Фr)- ФmcosUsuallyωL1 r1,consequently)[ωt ]In subsequent half periods Фt gradually decease till itreduces and the core flux requires the steady-state value.Because of the low time constant of the transformer circuit,distortion effects will be generated of the transient may lastseveral seconds. The transformer switching transient plusesis referred to as the inrush current. The initial core flux isnot be zero as assumed above but some residual flux valueФr. Because of retentivity, as shown in fig the transientwill now be even more severe, resultan Фt Фm Фr andthe core flux will now go through the high value of (2Фm Фr) as shown in fig 3.is connected to the optocoupler. The input of theoptocoupler is the output of the peak detector circuit. Thedevices are used in each phase for controlling the magneticinrush currentThe peak detector detect the first peak valueand send the these signal to the optocoupler . The TRIACfires to control the phase angle in between 0 to 90 degree.After closing the cycle the relay is operate to trip theCircuitFig 3: Magnetic inrush currentV. CIRCUIT DESCRIPTIONAn IC TCA785 as a zero crossing detector was used tocontrol the phase shift of the thyristor at the angle between0 degree to 90 degree. The angle of thyristor is reduce thenmagnetic inrush current is reduce. Tha angle of thyristor isdepending on magnetic inrush current. The phase anglecontrol, microcontroller, optcoupler, phase detectorelectronics devices are used to control the magnetic inrushcurrent. The switched dinister diode thyristor devices arecapable for switching current pulse. The principal ofswitched dinister diode thyristor operation are consideringpowerful control from blocking conducting state. Duringworking of transformer high magnetic inrush current israpidly am simultaneously filled by an electronic holeplasma. Fig 4 connection of magnetic inrush current withload Fig show the main circuit diagram, the primary of thetransformer is connected to the point on wave switchingcircuit. In this circuit TRIAC is used to switching the cycleduring the inrush current is drawn. The TRIAC firstterminal is connected to supply side and second terminal isconnected to the load side. The gate terminal of the TRIACISSN: 2393-9516Fig 4: Circuit descriptionThe magnetic inrush current is reduce 2 to 3 times which isbefore the 6 to 7 times full load current This occur due tomagnetic circuit current lags by an angle.VI. SIMULATION RESULTSMatlab simulink has been used to simulate inrush currentof a three-phase, 450 kVA, 500kV/230kV, Grounded Y/Dtransformer, and the initial transformer fluxes have beenconsidered. Magnetizing inrush current at these conditionswas very high (up to 1800 Amps) as it is shown in Figure.To simulate reduction of current inrush, supply voltage forphase A was initially connected at zero degree waveformfor this simulation. For 50 Hz supply we can write; Timeduration for 50 cycles 1,000 msec. Time duration for onewww.ijetajournal.orgPage 3

International Journal of Engineering Trends and Applications (IJETA) – Volume 4 Issue 2, Mar-Apr 2017cycle 20 msec. Time duration for cycle passing from 0 to 90 5 msec. For circuit breakers closing time insequence of ABC, firstly assume that we supply phase A atzero degree waveform as mentioned. Phase A will have itsmaximum voltage at 90 in 5 msec time. Next, phase B isconnected 120 apart from phase A, therefore: 120 90 210 , and the time duration for the cycle passing from 0 to 210 11.666 msec. Next for phase C: 240 90 330 , and time duration is 18.333 msec for cycle to passfrom 0 to 330 . Following closing times of 5 msec,11.666 msec, and 18.333 msec for circuit breakers ofphase ABC, the reduction of inrush current was remarkableFigure 6: Current ABC at zero degree voltage appliedFig 5: Reduce the magnetic inrush currentFigure 7: Reduction of inrush currents at sequential closing of CBsISSN: 2393-9516www.ijetajournal.orgPage 4