Experiment 3 - Single-phase inverter 1

Transcription

1 ELEC6.0 Objective he Univerity of New South Wale School of Electrical Engineering & elecommunication ELEC6 Experiment : Single-phae C-C Inverter hi experiment introduce you to a ingle-phae bridge inverter which produce 50Hz C upply from a C upply. Modulation cheme for obtaining variable ingle-phae C output voltage will be tudied and teted. Effect of modulation frequency and croover protection delay on the quality of the inverter output waveform will be alo oberved. Bipolar and unipolar witching cheme for the ingle-phae inverter will alo be tudied..0 Introduction Inverter circuit convert C power from a C ource to C of a deired voltage and frequency. hee are widely ued in application uch a tand-by power upplie, induction heating, grid connection of olar P module and o on. he C ource i uually a battery, olar cell, rectified wind generator output or a rectified C output from the fixed C upply from the utility. he input may have characteritic of a voltage ource or a current ource. hi experiment concern a voltage ource inverter in which the input to the inverter i from an ideal C voltage ource.. oltage Source Half-Bridge Inverter half-bridge inverter connect the load between the centre-tap of the C upply, created by uing two input capacitor a hown in figure, and the poitive or the negative terminal of the upply by mean of two witching device. wo tranitor and in figure witch alternately at 50 Hz, with equal ON and OFF time for each tranitor. Note, from the witching pattern of figure (b), that and are witched in a complementary manner with no overlap i.e., and can not be allowed to conduct imultaneouly in order to prevent hort circuit of the C ource terminal. Becaue of finite turn-off time of each witch and it parallel diode, a cro-over (or overlap) protection circuit i eential. C C R Load L. il =/f (a) Figure (b) Experiment - Single-phae inverter F. Rahman/March 009

2 ELEC6 he voltage acro the load terminal i a quare wave of amplitude alternating between + / and - /. When there i inductance in the load the energy return diode and become imperative becaue the current in the inductor at the turn-off of each witching device mut be allowed to continue. For example, at the end of ON duration, the load current continue to flow through and tart to fall under the influence of - / of the lower capacitor and the load. play the ame role when i turned off. In any cae the voltage waveform acro the load remain a quare-waveform. It can be hown that for equal ON and OFF time, the output voltage can be expreed in a Fourier erie a: v o in( n t)...() n n,,5,... = RMS value of fundamental component = o = total RMS value of output voltage = where = f o and f o = Hz Full-Bridge Inverter ingle-phae bridge inverter i hown in figure. he tranitor pair [,] and [,] are turned ON and OFF alternately in a complementary manner at a frequency f o which may be 50 Hz. he feedback diode [,] and [,] alo conduct alternately when witching pair [,] and [,] turn off, repectively. C +. R Load L +. +, v o i L, = /f (a) (b) before, v o n,,5,... in( n t) n Figure...() = 0.9 and o =. Experiment - Single-phae inverter F. Rahman/March 009

3 ELEC6. Croover Protection elay he witching tranitor at the top and bottom of any leg of the inverter mut not conduct imultaneouly to prevent catatrophic hort circuit of the C ource. hu, there mut be a dead-time, d which mut elape before top and bottom tranitor can change tate. he duration of the dead-time i determined by the turn-off time of the witching device ued. ypically thi i of the order of a few microecond. Your experimental circuit include a module which accept L gate driving ignal and produce two witching ignal for an inverter leg with a programmable dead-time d in microecond at the tranition. he timing diagram of figure decribe the operation of thi circuit. + & ON & OFF & ON & OFF d d d Figure. Output voltage control of a ingle phae inverter here are a number of way in which the output voltage of an inverter can be varied. Only the Sinuoidal PWM (SPWM) cheme will be conidered here. Sinuoidal PWM (SPWM) In thi cheme, the reference inuoidal ignal (called modulating waveform) repreenting the deired output waveform of the inverter i compared with a high-frequency triangular or a awtooth carrier waveform a indicated in figure. he comparator output pule become proportional to the level of the inuoid at the centre of the pule. (Hence the term Pule-Width Modulation: PWM). hee output are ued to witch the tranitor pair - and - in figure to to produce the load voltage waveform of figure (b). he reulting inverter output now ha much reduced harmonic, pecially the lower order one (which are more difficult to filter out). Figure alo indicate the inverter output voltage waveform and it harmonic pectrum. Figure 5 how the harmonic profile of a SPWM inverter where p = f c /f o = 5, where f o i the output frequency f c i the carrier frequency. he depth of modulation, m, i the ratio of the amplitude of the reference and the carrier waveform. It can be hown that the RMS value of the output voltage varie proportionately with m, a indicated in Figure 6, for m the linear and overmodulation range. Experiment - Single-phae inverter F. Rahman/March 009

4 ELEC6 e c v tri tri max v tri max - tri max Comparator output & ON & ON v o + d v o - d 0 t on f o n = f +/- f f +/-f o,f o f +/-f o,f o,f o (b) Figure Figure 5.78 omax d.0.0 Figure 6. Experiment - Single-phae inverter F. Rahman/March 009 m

5 ELEC6.5 Bipolar S Unipolar Switching Scheme Figure illutrate the bipolar (complementary) witching cheme in which both diagonal tranitor pair are turned ON or OFF together and the potential acro the load i either + or - (for a full-bridge inverter). However intead of turning off both diagonal tranitor at the end of an ON pule, only one in a leg may be witched off, thu allowing the load current to circulate through one feedback diode and the other tranitor of the ame leg which remain ON, without the load current having to ink through the C ource. hi lead to reduced harmonic in the output voltage and lower witching loe. hi i one form of unipolar witching, o called becaue the output voltage witche between the upply voltage and zero during each half cycle, not between + and - a in the bipolar witching cheme. See Figure 7 for waveform in a uni-polar witched inverter. Figure 7 f = 50 Hz, f o = 50 Hz, m = 0.8. Equipment IGB/MOSFE inverter leg with energy return diode -phae diode rectifier module LC filter compriing of one mh/5 inductor and capacitor bank with four 00C/600 MF capacitor connected in two group of two in parallel and then in erie giving a centre tap. three-phae load reitor bank mh/5 inductor for load inductance three-phae PWM module three-channel cro-over protection module iolated current tranducer; / iolated voltage tranducer; /50 four-channel ocillocope C voltmeter and ammeter module C voltmeter and ammeter module Experiment - Single-phae inverter 5 F. Rahman/March 009

6 ELEC6. Experiment In thi ection, a ingle-phae full-bridge inverter uing four power MOSFE witche with feedback diode a hown in figure 6 will be tudied. Filter Idc PHSE 5 50Hz I o CRO dc C Iac.... I R ac LO o CRO L o CRO I R Sig Gen X-OER PROECION ELY + o CRO d = 0 uec Figure 8. he experimental et-up For the quare-wave inverter, the ignal generator i et for quare-wave L output and it frequency will be et at 50 Hz. Connect the SG output to the cro-over protection module. For the SPWM inverter, the modulating waveform for the ingle-phae inverter i upplied by the ame ignal generator which mut be et for ine-wave output. he SG output i connected to the pule-width-modulator followed by the cro-over protection module, a indicated in Figure 9. he amplitude and frequency of the modulating waveform can be adjuted via the ignal generator. he witching frequency of the pule-width modulator (f ) and the dead time d can be adjuted through the PWM and cro-over protection module.. Identify the C voltage upply to the inverter which i obtained from a three-phae diode bridge rectifier followed by an LC filter, a indicated in figure 8. You hould ue the autotranformer control and it iolator witch to apply, adjut or remove the C upply from the inverter. he filter capacitor of the rectifier provide for a low impedance C voltage ource. he C voltage rating of thi electrolytic capacitor i however 00, o that the rectifier output C voltage mut not exceed thi.. he R-L load conit of the one phae of the load bank in erie with a load inductor. he full-bridge inverter circuit i already connected a hown in figure 8. o not alter the power circuit, epecially when the C upply to the inverter i ON. Iolated tranducer are ued to oberve the load, tranitor and the energy return diode current and the load voltage waveform. ll of thee tranducer output ignal and the tranitor witching ignal may be connected to the CRO. he CRO hould alo be ued for reading the RMS output value. Experiment - Single-phae inverter 6 F. Rahman/March 009

7 ELEC6 Square-wave Inverter. Set the SG for quare-wave and 50 Hz output. Connect the ignal generator L output to the croover protection circuit to produce the witching ignal for,, and a indicated in figure 6. Set the croover dead-time d to 0µec. Oberve thee waveform on the CRO.. Switch the auto-tranformer ON and increae the C voltage to the inverter lowly to 50. o not exceed 00. djut the load reitor until the load current i about rm. You will not alter the load reitance during the ret of the experiment..5 Record the C input voltage and current from the C meter, and the C RMS output voltage and current value from the CRO..6 Print the C input current and load voltage and current waveform from the CRO clearly indicating their value to cale. lo record one MOSFE and it energy return diode current waveform. Note the effect of different dead-time ( d ) on the output current waveform..7 Record the fundamental and higher order harmonic in the load voltage and current from the CRO. hee data are in db. Sinuoidally modulated PWM (SPWM) inverter - Bipolar Switching urn the C upply to the inverter to zero. Set the SG for ine-wave ouput with and et the output frequency to 50 Hz. Set the amplitude of inewave output to 0. Connect the SG output to the PWM module. he ret of the connection of the inverter remain a for the fullbridge inverter of fig. 6. Set the PWM modulator witching frequency f c to khz. he maximum allowable dead-time for each witching frequency i indicated in able. able. Maximum allowable croover dead-time d S PWM witching frequencie PWM Switching Frequency, in khz Maximum elay, d, in ec Record the waveform of the tranitor witching ignal on the CRO. Note that amplitude of 0 of modulating ignal from the SG will produce full modulation..9 Switch the C upply to the inverter ON and increae it lowly to about 00. Increae the modulating waveform amplitude lowly and record the RMS load voltage v the amplitude of the modulating ignal, by varying the modulating ignal amplitude in the range of 0 0. he actual amplitude of the SG output divided by 0 i the depth of modulation m. Experiment - Single-phae inverter 7 F. Rahman/March 009

8 ELEC6 Sig Gen P U L S E W I H M O U L O R X - O E R P R O E C I O N E L Y d + Figure 9..0 With the dead-time et at 0 ec, and for the ame load, increae the modulator witching frequency f in tep from khz to 0kHz. Record the RMS value of the fundamental and a few higher order harmonic component of the load current waveform from the CRO uing it FF facility for each f in db. Print the output current waveform for f = and 0 khz. o not keep the inverter running with too high a witching frequency (above 0 khz) for a long time.. With the witching frequency et at 0 khz, adjut the dead-time from μec to μec (i.e., upto the maximum allowable a indicated in able ), oberve the effect of deadtime variation in the load current waveform. Record the RMS value of the fundamental and a few higher order harmonic component in the load current waveform from the CRO for a few f and for a few m, in db. Print the output current waveform for f = and 0 khz. o not keep the inverter running with too high a witching frequency (above 5 khz) for a long time.. djut the variac to reduce the C upply to the rectifier to zero. Unipolar Switched SPWM Inverter. In one unipolar SPWM cheme, a ine and it inverted waveform are compared with two ynchronied carrier waveform and the two comparator output are ued to drive the inverter witche - a indicated in figure 0. Connect the two modulating ignal to two pule width modulator a hown in figure 8. Reconnect the four output from the cro-over protection circuit to the MOSFE a indicated in figure 0. Oberve the modulating and the PWM witching waveform on the CRO. Sig Gen SP BOR INERFCE C C B P W M M O U L E X - O E R P R O E C IO N M O U L E # # # # + B + B Figure 0. Experiment - Single-phae inverter 8 F. Rahman/March 009

9 ELEC6. djut the amplitude of the SG output to 0 (i.e., m = ). Increae the C link volatge to 00 lowly. Set the dead time d = 0 μec. For witching frequencie of, ay khz and 5kHz oberve the output voltage and current waveform and record fundamental and the dominant harmonic in the output current waveform in db, from the CRO. Print the output current waveform for f = and 5 khz. djut the variac to reduce the C upply to the rectifier to zero. 5. Report For the quare-wave inverter: 5. Compare the meaured RMS voltage of the ingle-phae quare-wave inverter with the reult from equation. From the CRO reading of harmonic in db, the rm value of each harmonic, including the fundamental, can be found from the converion factor given below: I in db 0 log I n 0 nrm Plot the harmonic pectrum of the load voltage and current in term of their RMS value (not in db). Comment on the relative value of thee harmonic compared to the fundamental. 5. On the print-out of the output volage, MOSFE and energy return diode current and the C current waveform, clearly label thee waveform indicating their value, duration and polaritie. Explain all thee waveform in relation to the load current waveform, uing thee label. For the SPWM inverter: 5. What wa the effect of increaing the witching frequency with a fixed dead-time of 0 ec on the harmonic pectum of the load current? Elaborate thi by plotting the RMS value of variou harmonic current including the fundamental on a graph paper for a few witching frequencie. 5. What wa the effect of increaing the dead time at a fixed witching frequency on the harmonic pectum fo the load current. Elaborate thi by plotting the RMS value of variou harmonic current including the fundamental on a graph paper for a few dead time (in ec) and f = 0kHz. 5.5 Comment on the rating of the feedback diode for inductive load. hee diode provide a path for reactive energy of the load inductance to return to ource. Will thee diode alo allow the trapped energie in the inductance aociated with wiring from the C ource to the inverter to return to the C ource? If not, what additional protective meaure would you ugget? 5.6 Plot the variation of the RMS output voltage with depth of modulation m. 5.7 Plot the harmonic pectrum of the load current for the unipolar and bipolar witching for f = khz and d = 0 ec. What i the lowet order harmonic frequency in the load current? Explain the reaon for the diffence between the two lowet order harmonic frequencie for the two witching cheme. Experiment - Single-phae inverter 9 F. Rahman/March 009