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11. SCOPE OF PRESEKT WORKThe Electrical encrgy available from
solar pancl depends
on th e levelof sun irradiation and conversion chrrradcristicsof
photovoltaiccells. The photocell characteristics are suchthat the
peak power available depends on the irradiationand the cell
configuration. Fig. 1shows th e characteristicsof 8 cells connected
in series and 3 such units connectedparallel (each cell can give
peak output power of 3GW at15V). From the Fig. 1t a n be observed
that the poweravailability is maximum if cell output is utilised at
fiV/cell(120V). This point is known as maximum power point.
Theinverter should utilise this characteristics of solar panelarray
to utilise the maximum power available (Maximumpower point
tracking-MPPT). The output of the inverter iscontrolled to apply a
constant V/F to the motor dependingon the sun irradiation. By
varying the output frequency ofthe inverter the speed of the
induction motor is varied,hence the power consumed by the pump. The
outputvoltage is varied to maintain constant V/F (flux) on
thevoltage applied to the motor.
m600Ax )
EfI
Fig. 1. Power output characteristics of solar cells und er
differentsun irradiation condition (8 Series, 3 Parallel)
A. At sun irradiation on 120mW/cm.q.B. At sun irradiation on
100mWlcm.q.C. At sun irradiation on 80mW/cm.sq.D. At sun
irradiation on bOm'CV/cin.sq.E. At sun irradiation on 4OinW/cm.q.F.
At sun irradiation 011 2OniW/cni.scl.
Space Vector modulation (SVM) principle gives optimumutilisation
of DC bus and less number of switching per cycleif used to gcnerate
the output voltage [1] & [2]. Thismethod ensures less harmonics
and more efficiency in thegeneration of AC variable voltage
variable frequencyoutput.
TheDCvoltage is sensed and the slopeof the power
curveisdetermined to track the power point. In case of a
negativeslope the inverter frequency is increased to bring
theoperating point to the peak of the curve (Fig. 1) and for
apositive slope the inverter frequency is reduced to have
theoperating point at the peak of the curve (Fig.1). Thus acurve
fitting is done in the control circuit to track thecharacteristic
of photovoltaic array and the powerconsumed by the load.
The inverter should withstand the maximum voltage,transients and
also should be suitable for outdoor-installation. The complete
specification of the inverter tobe made for this application is
frozen consideringfollowing: Maximum output current 14A,
nominaloutput 6OV, 50Hq.3 phase to drive the submersible pumpunit.
Protection against over voltage, under voltage, overcurrent, earth
fault, under load, over load, also protectionagainst empty running
of pump and interlock with remoteswitch for detecting the over flow
conditions are included.
111. I M P ~ E M E ~ A T I Q ~Inverter control c
bit microcontrmicrocontroller does all the drivacquisition and
control.
High linear analog opt uplers senses solar cell voltageprecisely
and microcontroller converts into digital with inbuilt A/D
converter. Same isused for undeover voltage protection.
Power Supply for controller is generated with SMPSwhich works on
DC bus with wide input voltage variation(SOV - 320V DC) which also
gives isolated power supplyto gate drive circuits.
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TABLE 1A B C D E
90 66 5.25 346.50 64890 88 4.60 404.80 64892 93 5.45 506.85
66292 98 5.40 529.20 66295 81 5.60 453.60 68495 111 5.05 560.55
68495 121 4.45 538.45 684
AB = Panel output DC voltage - VoltsC = Panel output current -
AmpsD = Panel output power -W attsE = Maximum power available -
Watts
= Irradiation 100% 120 mW/ cm.sq.- mW/cm.sq.
Table 2 shows the panel output obtained under variousirradiation
level with the MPPT inverter and submersibleunit for 864 Watts
solar array configuration.
.,. . IACKNOWLEDGMENT
The authors wish to thank the management of KEC unitTV for the
permission to publish this paper. Fauthors wish to thank
Mr.N.K.BAJAJ, KBL Bhis team for the help rendered in
conductingexperiment.
REPERENCES
TABLE 2A B C D E95 120 5.70 684.0 684.093 117 5.70 666.9 '
70.091 119 5.30 630.7 655.072 112 4.50 504.0 518.4
AB = Panel output DC voltage - Volts= Irradiation 100% 120 mW/
cm.sq. - mW/cm.sq.
DE
= Panel output power -W atts= Maximum pow er availpble -
Watts
[l] J.Holts, "High speed drive system with ultrasonic MOS
FET-PWM-INVERTER and single chip microprocessor control"IEEE trans.
Ind. Applicat., vol IA-23, pp. 1010-1015,1987.
[2] Ganesh C and K. Suresh, "A Low Cost Three Phase IGBTInverter
with Space Vector M odulation" INTRONICS-94.
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