The conventional methods of ac to variable voltage and frequency conversion such as ac voltagecontroller and Cycloconverter will gives the more harmonics and low input power factor. Andalso single phase ac to three phase variable ac conversion also very difficult. The above problemscan be overcome by the proposed method in this project, i.e., single phase to three phaseconversion with the two parallel connected rectifiers. With this method of conversion the inputpower factor will be improved. The Proposed Model consists of two parallel single-phase rectifiers,a three-phase inverter, and an induction motor. The proposed topology permits to reduce therectifier switch currents, the harmonic distortion at the input converter side, and presentsimprovements on the fault tolerance characteristics. Even with the increase in the number ofswitches, the total energy loss of the proposed system may be lower than that of a conventionalone. The model of the system is derived, and it is shown that the reduction of circulating currentis an important objective in the system design. A suitable control strategy, including the PulseWidth Modulation technique (PWM), is developed.
Keywords: Ac-dc-ac power converter, Drive system, Parallel converter
INTRODUCTIONA wide variety of commercial and industrialelectrical equipment requires three-phasepower. Electric utilities do not install three-phase power as a matter of course becauseit cost significantly more than single-phase
ISSN 2319 – 2518 www.ijeetc.comVol. 4, No. 3, July 2015
1 Student, Electrical & Electrical Engineering Department, Muffakham Jah College of Engineering and Technology, Banjara Hills,Hyderabad, Telangana State 500032, India.
2 Professor, Electrical & Electronics Engineering Department, Muffakham Jah College of Engineering and Technology, Banjara Hills,Hyderabad, Telangana State 500032, India.
3 Sr. Assistant Professor, Electrical & Electronics Engineering Department, Muffakham Jah College of Engineering and Technology,
Banjara Hills, Hyderabad, Telangana State 500032, India.
installation. Hence we need to conversionfrom single-phase to three-phase. Parallelconverters have been used to improve thepower capability, reliability, efficiency, andredundancy. Usually the operation ofconverters in parallel requires a transformer
Research Paper
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
for isolation. However, weight, size, and costassociated with the transformer may makesuch a solution undesirable. When an isolationtransformer is not used, the reduction ofcirculating currents among different converterstages is an important objective in the systemdesign.
Several solutions have been proposedwhen the objective is to supply a three-phasemotor from single-phase ac mains (Lee andKim, 2007). It is quite common to have only asingle phase power grid in residential,commercial, manufacturing, and mainly in ruralareas, while the adjustable speed drives mayrequest a three-phase power grid. Single-phase to three-phase ac-dc-ac conversionusually employs a full-bridge topology, whichimplies in ten power switches, as shown inFigure 1. This converter is denoted here asconventional topology. Parallel convertershave been used to improve the powercapability, reliability, eff iciency, andredundancy. Parallel converter techniques canbe employed to improve the performance ofactive power filters (Chaer et al., 2009),Uninterruptible Power Supplies (UPS)(Guerrero et al., 2009), fault tolerance of doublyfed induction generators (Flannery andVenkataramanan, 2008), and three-phasedrives (Jacobina et al., 2008). Usually theoperation of converters in parallel requires atransformer for isolation. However, weight, size,and cost associated with the transformer maymake such a solution undesirable (Park et al.,2008). When an isolation transformer is notused, the reduction of circulating currentsamong different converter stages is animportant objective in the system design(Cheng et al., 2008).
In this paper, a single-phase to three-phase drive system composed of twoparallel single-phase rectifiers and a three-phase inverter is proposed, as shown inFigure 2. The proposed system is conceivedto operate where the single-phase utility gridis the unique option available. Compared tothe conventional topology, the proposedsystem permits: to reduce the rectifier switchcurrents; the Total Harmonic Distortion (THD)of the grid current with same switchingfrequency or the switching frequency withsame THD of the grid current; and toincrease the fault tolerance characteristics.In addition, the losses of the proposedsystem may be lower than that of theconventional counterpart. Theaforementioned benefits justify the initialinvestment of the proposed system, due tothe increase of number of switches.
Figure 1: Block Diagram for ConventionalSingle-Phase to Three-Phase Drive
System
Figure 2: Block Diagram for ProposedSingle-Phase to Three-Phase Drive
System
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
METHODS TO CONNECTSINGLE PHASE TO THREEPHASE DRIVE SYSTEMSStatic Phase ConverterStatic Phase Converters operate by chargingand discharging capacitors to temporarilyproduce a 3rd phase of power for only a matterof seconds during startup of electric motors,then it will drop out forcing the motor tocontinue to run on just 1 phase and only part ofits windings. Due to their technology, StaticPhase Converters do not properly power anyclass of 3 phase machinery or equipment. Theywill not in any way power 3 phase welders, 3phase battery chargers, 3 phase lasers, or anytype of machinery with 3 phase circuitry. StaticPhase Converters also will not start deltawound 3 phase motors.
Rotary Phase ConverterA rotary phase converter, abbreviated RPC,is an electrical machine that produces three-phase electric power from single-phaseelectric power. This allows three phase loadsto run using generator or utility-supplied single-phase electric power. A rotary phase convertermay be built as a motor-generator set. Thesehave the advantage that in isolating thegenerated three-phase power from the singlephase supply and balancing the three-phaseoutput. However, due to weight, cost, andefficiency concerns, most RPCs are not builtthis way. Rotary Phase Converters ProvideReliable, Balanced, and Efficient Three PhasePower Quick and Effective Three PhaseElectricity.
All converters can be mainly categorized intotwo groups: one is cascade type and anotheris unified type. In cascade type, the PWM
converter for power factor correction and thePWM inverter for speed control are connectedin series with large DC-Link capacitor and twostatic power converters are operated andcontrolled in separate. In this type, specificnumber of switches, to compose the converterand inverter, are required. Therefore, therequired number of switches cannot bereduced significantly. On the other hand, in theunified type, conventional concepts of PWMconverter and inverter are merged togetherand same converter handles the functions ofPWM converter (power factor correction) andPWM inverter(motor control) at the same time.As an added advantage, the input inductor,this is commonly used in the PWM.
COMPONENTS IN THEPROPOSED CIRCUITEffect of Source InductanceIn the Figure 1, lg’, la’, lb, lb’ are input sideinductors these inductors are called sourceinductors. The input side bridge rectifier is afully controlled rectifier. The presence of sourceinductance introduces an additional mode ofoperation of when firing angle is less thancertain value. When there is an inductor inseries with each input line, it is necessary tofind out its effect. The effects are
1. The reduction in output voltage
2. The duration of commutation overlap.
3. The relationship between the firing angleand the commutation overlap.
DC Link CapacitorA simple analytical expression for the currentstress on the DC-link capacitor caused by theload-side inverter of a voltage DC-link-converter system is derived. The DC-link
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
capacitor current RMS value is determinedfrom the modulation depth and by theamplitude and the phase angle of the inverteroutput current assuming a sinusoidal inverteroutput current and a constant DC-link voltage.
Despite neglecting the output-current ripple,the results of the analytical calculation are within8% of measurements made from digitalsimulation and an experimental system, evenif the output-current ripple is relatively high asin the case of low-frequency IGBT invertersystems. The simple analytical expressionprovides signif icant advantages oversimulation methods for designing the DC-linkcapacitor of PWM converter systems.
IGBT FundamentalsThe Insulated Gate Bipolar Transistor (IGBT)is a minority carrier device with high inputimpedance and large bipolar current-carryingcapability. Many designers view IGBT as adevice with MOS input characteristics andbipolar output characteristic that is a voltage-controlled bipolar device. To make use of theadvantages of both Power MOSFET and BJT,the IGBT has been introduced. It’s a functionalintegration of Power MOSFET and BJTdevices in monolithic form. It combines thebest attributes of both to achieve optimaldevice characteristics. The IGBT is suitablefor many applications in power electronics,especially in Pulse Width Modulated (PWM)servo and three-phase drives requiring highdynamic range. Control and low noise. It alsocan be used in Uninterruptible Power Supplies(UPS), Switched-Mode Power Supplies(SMPS), and other power circuits requiringhigh switch repetition rates. IGBT improvesdynamic performance and efficiency andreduced the level of audible noise. It is equally
suitable in resonant-mode converter circuits.Optimized IGBT is available for both lowconduction loss and low switching loss.
MODELLING AND CASESTUDYMathematical Analysis of the ModelThe system is composed of grid, inputinductors ( , , , and ), rectifiers (A andB), capacitor bank at the dc link, inverter, andinduction machine. Rectifiers A and B areconstituted of switches , , , and ,and , , , and , respectively. TheTheinverter is constituted of switches , , ,
, , and . The conduction state of theswitches is represented by variable to ,where = 1 indicates a closed switch while
= 0 an open one.
From Figure 2. The following equations canbe derived:
...(1)
...(2)
...(3)
...(4)
...(5)where and symbols like r and lrepresents resistances and inductances of theinput inductor , , , and .
The circulating current can be defined fromand or and , i.e.,
...(6)
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
Introducing i0 and adding (3) and (4),relations (1)-(4) become:
...(7)
...(8)
...(9)
where,
...(10)
...(11)
...(12)Relations (7)–(9) and (5) constitute the
front-end rectifier dynamic model. Therefore,va (rectifier A), vb (rectifier B), and vo
(rectifiers A and B) are used to regulatecurrents ia, ib, and io, respectively. Referencecurrents and are chosen equal toand the reference circulating current ischosen equal to 0. In order to both facilitatethe control and share equally current, voltage,and power between the rectifiers, the fourinductors should be equal, i .e.,
and . Inthis case, the model (7)-(9) can be simplifiedto the model given by
...(13)
...(14)
...(15)Additionally, the equations for , , and
can be written as
...(16)
...(17)
...(18)In this case (four identical inductors), the
circulating current can be reduced to zeroimposing
...(19)When , ) the system
model (7)-(9) is reduced to
...(20)
...(21)Then the model of the proposed system
becomes similar to that of a system composedof two independent rectifiers.
PWM StrategyThe inverter can be commanded by using anadequate Pulse Width Modulation (PWM)strategy for three-phase Voltage SourceInverter (VSI), so that it will not be discussedhere. In this section, the PWM strategy for therectifier will be presented. The rectifier polevoltages va10, va20, vb10, and vb20 depend on theconduction states of the power switches, i.e.,
...(22)
where vc is the total dc-link voltage.Considering that v*
a, v*b, and v*
o denote thereference voltages determined by the currentcontrollers (see Section IV),we found
...(23)
...(24)
...(25)
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
The gating signals are directly calculatedfrom the reference pole voltages v*
a10, v*a20,
v*b10and v*
b20. However, (23)-(25) are notsufficient to determine the four pole voltagesuniquely from v*
a, v*b and v*
o. Introducing anauxiliary variable v*
x = v*a20, that equation plus
the three Equations (23)-(25) constitute a fourindependent equations system with fourvariables ( , , , ). Solvingthis system of equations, we obtain
...(26)
...(27)
...(28)
...(29)From these equations, it can be seen that,
besides v*a, v*
b and v*o, the pole voltages
depend on also of v*x. The limit values of the
variable v*x can be calculated by taking into
account the maximum v*c/2 and minimum “v*
c/2 value of the pole voltages
...(30)
...(31)where v*
c is the reference dc-link voltages,v*
max = max and v*min = min with = {v*
a, 0,v*
a/2 + v*b/2 – v*
o/2, v*a/2 – v*
b/2 – v*o/2}.
Introducing a parameter (0 < < 1), thevariable v*
x can be written as
...(32)When = 0, = 0.5, and = 1 the
auxiliary variable has the following values= , = v*
xave = ( + )/2 , and= , respectively. When =
= a converter leg operates with zeroswitching frequency. Once v*
x is chosen, polevoltages v*
a10, v*a20, v*
b10, and v*b20 are defined
from (26) to (29). The gating signals areobtained by comparing pole voltages with one(vt1), two (vt1 and vt2) or more high-frequencytriangular carrier signals. In the case of double-carrier approach, the phase shift of the twotriangular carrier signals (vt1 and vt2) is 180°[see Figures 5c and 5d]. The parameter ìchanges the place of the voltage pulsesrelated to va and vb . When v*
x = v*xmin ( = 0) or
v*x = v*
xmax ( = 1) are selected, the pulses areplaced in the begin or in the end of the halfperiod (Ts) of the triangular carrier signal [seeFigures 5a and 5c]. On the other hand, whenv*
x = v*xave the pulses are centered in the half
period of the carrier signal [see Figures 5band 5d]. The change of the position of thevoltage pulses leads also to the change in thedistribution of the zero instantaneous voltages(i.e., va = 0 and vb = 0). With = 0 or = 1 thezero instantaneous voltages are placed at thebeginning or at the end of the switching period,respectively, while with = 0.5, they aredistributed equally at the beginning and at theend of the half period. This is similar to thedistribution of the zero-voltage vector in thethree-phase inverter. Consequently, influences the harmonic distortion of thevoltages generated by the rectifier, as it willbe shown in Section V.
CONTROL STRATEGYFigure 3 presents the control block diagramof the system in Figure 2, highlighting thecontrol of the rectifier. The rectifier circuit ofthe proposed system has the same objectivesof that in Figure 1, i.e., to control the dc-linkvoltage and to guarantee the grid power factor
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
close to one. Additionally, the circulating currentio in the rectifier of the proposed system needsto be controlled. In this way, the dc-link voltagevc is adjusted to its reference value v*
c usingthe controller Rc, which is a standard PI typecontroller. This controller provides theamplitude of the reference grid current I*
g. Tocontrol power factor and harmonics in the gridside, the instantaneous reference current i*gmust be synchronized with voltage eg, as givenin the Voltage-Oriented Control (VOC) forthree-phase system. This is obtained viablocks Ge-ig, based on a PLL scheme. Thereference currents i*a and i*b are obtained bymaking i*a = i*b = i*g/2, which means that eachrectifier receives half of the grid current. Thecontrol of the rectifier currents is implementedusing the controllers indicated by blocks Ra
and Rb. These controllers can be implementedusing linear or nonlinear techniques. In thispaper, the current control law is the same asthat used in the two sequences synchronouscontroller described in Jacobina et al. (2001).These current controllers define the inputreference voltages v*
a and v*b. The homopolar
current is measured (io) and compared to itsreference (i*o = 0). The error is the input of PIcontroller Ro, that determines the voltage v*
o.
The calculation of voltage v*x is given from (30)
to (32) as a function of ì, selected as shown inthe Section V. The motor there-phase voltagesare supplied from the inverter (VSI). Block VSI-Ctr indicates the inverter and its control. Thecontrol system is composed of the PWMcommand and a torque/flux control strategy(e.g., field-oriented control or volts/hertzcontrol).
SIMULATION MODELINGAND RESULTSIn this chapter the work carried out onmathematical model of the system usingMATLAB/SIMULINK software. The results ofsimulation have been discussed.
Figure 3: Control Block Diagram
Figure 4: Matlab Model UsingConventional Control
Figure 5: Output Waveforms
(a)
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
Figure 5 (Cont.)
(b)
(c)
(d)
(e)
Figure 5 (Cont.)
(f)
(g)
(h)
Note: a) Waveform of both Grid voltage and current, (b) DCvoltage, (c) Waveforms of Rectifier Pulses, (d) waveform forindividual Line voltages without filter, (e) Waveform ofIndividual Line voltages with Filter, (f) Waveform for statorCurrent, (g) Waveform for ElectroMagnetic torque and (h)rotor speed.
Figure 6: Matlab Model Using BothRectifier Combination
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
Figure 7: Waveforms
(a)
(b)
(c)
(d)
(e)
Figure 7 (Cont.)
(f)
(g)
Note: (a)Waveforms of both Grid current and voltage, (b) DCvoltage (c) Waveforms of Individual Line Voltages withoutfilter, (d) Waveform of Stator Current, (e) Waveforms ofIndividual Line Voltages with Filter, (f) Waveform for RotorSpeed and (g) Electromagnetic Torque.
CONCLUSIONA single-phase to three-phase drive systemcomposed of two parallel single-phaserectifiers, a three-phase inverter and aninduction motor was proposed. The systemcombines two parallel rectifiers without the useof transformers. The system model and thecontrol strategy, including the PWM technique,have been developed. The completecomparison between the proposed andstandard configurations has been carried outin this paper. Compared to the conventionaltopology, the proposed system permits toreduce the rectifier switch currents, the THDof the grid current with same switchingfrequency or the switching frequency withsame THD of the grid current. The initialinvestment of the proposed system (due to highnumber of semiconductor devices) cannot be
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Int. J. Elec&Electr.Eng&Telecoms. 2015 Shaik Barakathulla Basha et al., 2015
considered a drawback, especiallyconsidering the scenario where the citedadvantages justify such initial investment.
FUTURE SCOPEIt is quite common to have only a single phasepower grid in residential, commercial,manufacturing, and mainly in rural areas. At thesolar power plant and industries it isconvenient convert single phase supply to threephase supply. This system is very useful foragricultural to run three phase motor.
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2. Cheng P-T, Hou C-C and Li J-S (2008),“Design of an Auxiliary Converter for theDiode Rectifier and the Analysis of theCirculating Current”, IEEE Trans. PowerElectron., Vol. 23, No. 4, pp. 1658-1667.
3. Flannery P and Venkataramanan G(2008), “A Fault Tolerant Doubly FedInduction Generator Wind Turbine Usinga Parallel Grid Side Rectifier and SeriesGrid Side Converter”, IEEE Trans. PowerElectron., Vol. 23, No. 3, pp. 1126-1135.
4. Guerrero J, Vasquez J, Matas J, CastillaM and de Vicuna L (2009), “Control
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7. Lee D-C and Kim Y-S (2007), “Control ofSingle-Phase-to-Three-Phase AC/DC/AC PWM Converters for Induction MotorDrives”, IEEE Trans. Ind. Electron.,Vol. 54, No. 2, pp. 797-804.
8. Ojo O and Kshirsagar P M (2004),“Concise Modulation Strategies for Four-Leg Voltage Source Inverters”, IEEETrans. Power Electron., Vol. 19, No. 1,pp. 46-53.
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