NON-LINEAR PWM CONTROLLED SINGLE-PHASE BOOST MODE GRID-CONNECTED PHOTOVOLTAIC INVERTER WITH LIMITED STORAGE INDUCTANCE CURRENT
Abstract
A non-linear PWM controlled single-phase boostmode photovoltaic grid-connected inverter with limited storageinductance current is proposed in this paper. The circuit topology,control strategy, steady state principle characteristic and designcriterion for the key circuit parameters of this kind of inverter areinvestigated in depth, and important conclusions are obtained.The inverter’s regenerating energy duty ratio 1-D which decreaseswith the decline of the grid-connected voltage is real-time adaptedby sampling and feeding back the inverting bridge modulationcurrent, and the average value of the modulation current in eachswitching cycle tracks the reference sinusoidal signal to get highquality grid-connected current. The active control of the energystorage inductance current and the balance of the voltage step-upratio are realized by adding a by-pass switch connected in parallelwith the energy storage inductance and using two kinds ofswitching pattern namely boost pattern and freewheeling pattern.The theoretical analysis and experimental results of the 1kVA110VDC/220V50Hz photovoltaic grid-connected inverterprototype show that it has the advantages such as single-stageboost conversion, high conversion efficiency, high quality ofgrid-connected current waveform, low value of energy storageinductance, simple control etc.
EXISTING SYSTEM:
Three-phase boost mode grid-connected inverter adoptingtwo-loop control strategy with motor speed outer loop and dclink current inner loop has obtained better performance. TheOne-cycle control method derived by the Buck dc-dc converterhas also been applied for three-phase boost-type grid-connectedinverter . However, the traditional single-phase boost modeSPWM inverter does not meet the basic principle of boostconverter when the output voltage is lower than the inputvoltage. Namely whether the energy storage inductance L ismagnetizing or regenerating energy, its current is alwaysincreasing, which lead to inverter cannot get sinusoidalgrid-connected current. Therefore, the inverter whoseregenerating duty ratio 1-D is increasing with the decreasing ofthe grid-connected voltage un has inherent defects such as largeenergy storage inductor and its current, serious outputwaveform distortion, and low conversion efficiency. For all this,some new solutions on circuit topology and control strategy areproposed .A current-fed Z-source inverter has been proposed which cansuccessfully buck and boost voltage in reference , but thisinverter has more complex circuit topology due to overmuchenergy storage element amount. A one-cycle controlledsingle-phase Z-source inverter with unsymmetricalcharacteristic is proposed in reference. This inverter hassimple circuit topology, input and output sharing the sameground, but it can only buck voltage, the conversion efficiencyand THD of the output voltage are not given. A differentialinverter that two bi-directional boost dc-dc converters areconnected in parallel in the input side and connected in seriesreversely in the output side is proposed in reference. The two bi-directional boost dc-dc converters operatesimultaneously and respectively output low-frequencypulsating sinusoidal voltage with the same dc component and180º phase difference.
PROPOSED SYSTEM:
An inverterwhose power switch is connected in series between the positiveend of the input source and energy storage inductor, and afreewheeling diode is connected in parallel between thenegative end of the input source and energy storage inductor inthe traditional circuit is proposed in reference. The serialconnected power switch and the inverting bridge powerswitches are controlled by the hysteresis control strategy of theoutput voltage and energy storage inductor current, the inverteris operating in boost Mode or Buck Mode, and the dualsinusoidal half-wave energy storage inductor current andoutput sinusoidal voltage with high frequency (HF) ripplewhich are synchronized with the output voltage are obtained.This scheme reduces the energy storage inductor to 0.3mH, butthe output filter capacitance is still as high as 20µF, theconversion efficiency is only 78% and THD of the outputvoltage is as high as 3.87%. A single-phase boost mode inverterwith HF link whose output and input end connects in parallelwith a flyback AC-DC energy regenerating circuit is proposedin reference. The flyback AC-DC energy regeneratingcircuit operates only when the output voltage is falling and theabsolute value of the output voltage is smaller or equal to theinput voltage multiplied by turn ratio of the transformer. Theexperimental waveforms are not given and this scheme is at thecost of increasing the complexity of the circuit. An activenonlinear modulation control strategy is proposed in reference. Based on the derived nonlinear modulation compensationfunction restraining harmonic waves of the grid-connectedcurrent, the proposed control strategy respectively extracts thedc and ac component of the energy storage inductance currentand compares with the grid-connected current reference to getthe nonlinear PWM modulation signal which can inhibit thirdharmonic wave of the grid-connected current. The proposedcontrol strategy can effectively suppress the third harmonicwave of the grid-connected current, but there still are defectssuch as large energy storage inductance (21mH) and THD ofthe output current waveform up to 4.42%. A control strategybased on the inverter’s output current involving feedforwardand feedback terms is proposed in reference, whichimproves the quality of output waveforms, but the energystorage inductance L is still as high as 10 mH, the conversionefficiency and THD of the output voltage are not given. Aparallel-resonator is connected in series between the inputsource and energy storage inductor of the traditional inverter,and a output voltage and current feedback control strategy withproportional resonant is adopted in reference. Theparallel-resonator can filter out the second and fourthlow-frequency harmonic component in energy storage inductorcurrent and reduce the energy storage inductor and the inputlow-frequency ripple to some extent. The control strategyeffectively improves the quality of output waveform, but theresonant inductors in the parallel-resonator are high up to 5 mHand 10 mH and the energy storage inductor value of the inverteris 5 mH. Only light load waveform is provided and theconversion efficiency is not provided.To overcome inherent defects of the traditional single-phaseboost mode inverter, a nonlinear PWM controlled single-phaseboost mode PV grid-connected inverter with limited energystorage inductance current is proposed and deeply researched inthis paper, and important conclusions are obtained.
CONCLUSION
1) A nonlinear PWM control strategy based on invertingbridge modulation current is proposed. The size of 1-D istimely adjusted by detecting and feeding back modulationcurrent im, and the quality of output waveform is improved.2) A circuit topology of the single-phase boost modegrid-connected inverter with additional bypass switch of theenergy storage inductor and two types of switching pattern withlimitation current of the energy storage inductor are proposed.The active control of the energy storage inductor current isrealized by the freewheeling state of energy storage inductorreplacing the magnetizing state. The problems such as excessenergy of the energy storage inductor and too large step-upratio of the inverter can be effectively solved and theconversion efficiency is also improved.3) There are six kinds of equivalent circuits and eightoperating intervals in one LF output period, there are four kindsof operating situations and eight operating intervals in one HFswitching period, and the relationship between step-up ratioand duty ratio of the equivalent switch is derived.4) Design criteria of the key parameters such as the energystorage inductor, limitation current of energy storage inductor,input and output filter are derived.5) The experimental results of the designed and developed1kW 110VDC/220V50HzAC inverter prototype show that theenergy storage inductance is significantly reduced, conversionefficiency is increased, and the quality of output waveform isimproved.
REFERENCES
[1]M.K. Nguyen, T.V. Le, S.J. Park, and Y.C. Lim, “A class of quasi-switchedboost inverters”,IEEE Trans. on Ind. Electron., vol.62, no.3, pp.1526-1536, Jul. 2015.
[2] S.S. Nag and S. Mishra, “Current-fed switched inverter”, IEEE Trans. onInd. Electron., vol.61, no.9, pp. 4680-4690, Nov. 2014.
[3] B. Singh, C. Jain, and S. Goel, “ILST control algorithm of single-stage dualpurpose grid connected solar PV system”,IEEE Trans. on PowerElectron., vol.29, no.10, pp. 5347-5357, Dec. 2014.
[4]Y. Tang, X. , and Y. He, “Activebuck-boost inverter”, IEEE Trans. onInd. Electron., vol.61, no.9, pp. 4691-4697, Dec. 2014.
[5] J.R. Espinoza, and G. Joos, “A current-source-inverter-fed induction motordrive system with reduced losses”,IEEE Transactions on Ind.Applications, vol. 34, no. 4, pp. 796-805, Aug. 1998.
[6]Y. Chen, K. Smedley, “Three-phaseboost-type grid-connected inverter”,IEEE Trans. on Power Electron., vol. 23, no. 5, pp. 2301-2309, Sept.2008.
[7] Q. Lei, D. Cao, and F.Z. Peng, “Novel loss and harmonic minimized vectormodulation for a current-fed quasi-z-source inverter in HEV motor driveapplication”,IEEE Trans. on Power Electron.,vol. 29, no. 3, pp.1344-1357, May 2014.
[8]Y. Tang, S.J. Xie, C.H. Zhang, “Single-phase Z-Source inverter”,IEEETrans. on Power Electron., vol. 26, no. 12, pp. 3869-3873, Feb. 2011.
[9] R.O. Caceres and I. Barbi, “Aboost DC-AC converter: analysis, design, andexperimentation”,IEEE Trans. on Power Electron., vol.14, no. 1, pp.134-141, Jan. 1999.
[10] Y. Fang and X. Ma, “A novel PV microinverter with coupled inductorsand double-boost topology”,IEEE Trans. on Power Electron., vol. 25,no.12, pp. 3139-3147, Oct. 2010