A Current Sensor-less Maximum Power Point Tracking Method for PV System Byunggyu Yu, Ahmed G. Abo-Khalil
A Current Sensor-less Maximum Power Point Tracking Method for PV System 1
Byunggyu Yu, 2Ahmed G. Abo-Khalil Kongju National University,
[email protected] 2 Majmaah University on leave from Assiut University,
[email protected] 1, First Author, Corresponding Author
Abstract This paper deals with a conventional single-phase, two-stage energy conversion system which is connected between the PV array and the electrical power system, employing a new, simple and effective MPPT algorithm. The basic operations are based on the scanning of the characteristics of the PV array regularly to obtain the maximum power point condition. At first, the duty-ratio of the boostconverter is set to zero where the PV current is settled to zero. Then the converter duty ratio is adjusted to unity in which the PV current starts to increase while the PV voltage decreases. In this time, the PV voltage is measured and the PV current is calculated and the PV instantaneous power is calculated. The instantaneous calculated power is compared with the previous value until the maximum power point is obtained. The corresponding voltage is saved as a reference for the maximum power point condition in the normal operation. The main advantage of this method is eliminating the current sensor. Meanwhile, this MPPT algorithm reduces the power oscillations around the peak power point which occurs with perturbation and observation algorithms. In addition, the total cost will decrease by removing the current sensor from the PV side. Finally, simulation results confirm the accuracy of the proposed method.
Keywords: Photovoltaic Generation, Power Converter, Maximum Power Point Tracking 1. Introduction In the past few years, Photovoltaic (PV) generation has emerged as a one of the most promising sources of large-scale renewable energy systems. The importance of the PV generation comes from its advantages such as the absence of fuel cost, little maintenance, no pollution, no noise and wear due to absence of moving parts. However, there are other issues with using of PV systems, which are the high installation cost and the low energy conversion efficiency. The voltage-power characteristic of PV array is a non-linear because of the variation that caused by solar irradiation and temperature. Therefore, it is very important for a PV to operate at the maximum power point to reduce the cost of the generated power to the system installation cost. Many maximum power point tracking (MPPT) techniques for PV systems are well established in the literature. The most commonly known are hill-climbing [1], fractional open-circuit voltage control [2], perturb and observe (P&O) [3], and incremental conductance [4], the parasitic capacitance [5], the constant voltage [6]. There are lesser known, but sometimes very appropriate, methods such as maximizing load current or voltage [7], fractional short-circuit current control [8], array reconfiguration [9], linear current control [10], fuzzy control [11], neural network [12], dc link capacitor droop control, pilot cells, current sweep, limit-cycle control, and several others [13]. Generally, there are several methods which are commonly used to determine the maximum power point. With P&O algorithm the operating voltage V is perturbed with every MPPT cycle. As soon as the maximum power point (MPP) is reached, V oscillates around the ideal operating voltage Vmpp. This causes a power loss which depends on the step width of a single perturbation. If the step width is large, the MPPT algorithm will respond quickly to sudden changes in the operating conditions with increasing losses under stable or slowly changing conditions. If the step width is very small the losses under stable or slowly changing conditions will be reduced, but the system will only able to respond very slowly to rapid changes in the temperature or illumination levels. The value of the ideal step width is system- dependent and needs to be determined experimentally. Incremental conductance algorithm has an advantage over P&O method in that it can determine when the MPPT reaches the MPP, while the output power in P&O method oscillates around the MPP. Also, incremental conductance can track rapidly changing irradiance conditions with higher accuracy than P&O method.
International Journal of Advancements in Computing Technology(IJACT) Volume 5, Number 11, July 2013 doi : 10.4156/ijact.vol5.issue11.42
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A Current Sensor-less Maximum Power Point Tracking Method for PV System Byunggyu Yu, Ahmed G. Abo-Khalil
Figure 1. An equivvalent circuit model m of PV aarray.
Figgure 2. PV volltage and currrent characterisstics at a constant temperatuure. T This paper deaals with a connventional sinngle-phase, tw wo-stage energgy conversionn system whicch is connnected betweeen the PV arrray and the electrical pow wer system, employing e a new, simple and effective MPPT algorithm. a Thee basic operatiions are basedd on the scannning of the chaaracteristics off the PV array regularlly to obtain thhe maximum power p point coondition. At fiirst, the duty-rratio of the booostconvverter is set too zero where thhe PV current is settled to zzero. Then the converter dutty ratio is adjuusted to uunity in which the PV currennt starts to inccrease while thhe PV voltagee decreases. Inn this time, thee PV voltage is measurred and the PV V current is callculated and thhe PV instantaaneous power is calculated. The instaantaneous calcculated powerr is compared with the previious value unttil the maximuum power poinnt is obtaained. The corrresponding vooltage is savedd as a referencce for the maxximum power point conditioon in the normal n operattion.
2. P PV array ch haracteristiics S Solar cells aree devices thatt convert phootons into elecctrical potentiial in a PN juunction, of whhich equiivalent circuitt is shown in Figure F 1. Duee to the compllex physical phenomena insside the solar cell, mannufacturers ussually presentt a family off operating ccurves (V-I) as a shown in Figure 2. Thhese charracteristics aree obtained byy measuring tthe array volt--ampere for a different illuumination vallues. From m these characcteristics, the ooptimum voltaage or current, correspondinng to the maxiimum power ppoint, can be determineed. It is clearlly seen in Figgure 2 that thhe current inccreases as the irradiance leevels incrrease. The maximum m pow wer point incrreases with a steep positive slope prooportional to the illum mination. T main paraameters whichh influence thee illumination levels on a suurface at a fixeed tilt on earthh are The the ddaily and seassonal solar patth, the presencce of clouds, mist, m smog annd dust betweeen the surface and the sunlight, s and tthe shade of thhe object posittioned such that the illuminaation level is rreduced, etc. The equation of tthe PV outputt current is exppressed as a fuunction of the array voltage
I I sc - I o e
q (V IRs ) KTk
- 1} - (V IIRs )/Rsh
((1)
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A Current Sensor-less Maximum Power Point Tracking Method for PV System Byunggyu Yu, Ahmed G. Abo-Khalil
whhere V and I reepresent the PV V output voltaage and currennt, respectivelly; Rs and Rsh are the series and shunnt resistance oof the cell in F Figure 1; q is tthe electronic charge; Isc is tthe light-generrated current; Io is the reverse r Saturaation current; K is the Boltzzman constant,, and Tk is the temperature in Kelvin. Equuation (1) cann be written in another form as [3]
I I sc {1 K1 [e K 2V 1]} - (V IIRs )/Rsh m
((2)
whhere the coefficcient K1, K2 annd m are definned as K1 0.01175, K 2 K 4 /(Voc ) m ,
K 4 ln(( K1 1) / K1 ) , K 3 ln[( I sc (1 K1 ) I mpp ) / K1I sc ],,
m ln( K 3 / K 4 ) / ln(Vmpp / Voc ) Imppp is the currennt at maximum m output pow wer, Vmpp is thee voltage at m maximum pow wer, Isc is the sshort circuuit current andd Voc is the opeen circuit volttage of the arraay. Eqquation (2) is only applicaable at one paarticular oper ating conditi on of illuminnation G and cell tem mperature. T The parametter variationss can be calcuulated by me asuring the variation of the shorrt-circuit curreent and the open-circuit volttage in these cconditions usinng the parameeters at the norrmal illum mination and cell temperaature. Equatioon (2) is used for the V-I V characteriistics for varrious illum mination and ffixed temperatture (25[°C]) iin Figure 2.
3. S System conttrol and esttimation F Figure 3 show ws the circuit cconfiguration oof the proposeed PV power conditioning c ccircuit (PCS). The propposed PCS is composed of a dc–dc boost converter to step-up the PV P voltage to the operating DC linkk voltage and a dc–ac inverrter to invert tthe dc link vooltage to ac vvoltage with uunity power faactor corrrection. Withoout using the dc current seensor for the PV current, MPPT controol is achievedd by estim mating the PV V current estim mator.
Figure 3. PV P power circcuit system connsisted with bboost converterr and full briddge inverter
Figuree 4. Typical sttructure of DC C/DC boost connverter
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A Current Sensor-less Maximum Power Point Tracking Method for PV System Byunggyu Yu, Ahmed G. Abo-Khalil
Vi
Vi
Vo
Vo
Normal duration
Short-Circuit duration
Open-Circuit duration
Short-Circuit duration
Pmax
Open-Circuit duration
(a) (b) Figure 5. Equivalent circuits of the DC/DC boost converter during switching on and off
Normal duration
Time Toc
Tsc
Top
Toc
Tsc
Top
Figure 6. MPPT control sequence of the proposed method
3.1. Boost converter and MPPT control algorithm The main advantages of the boost converter in Figure 4 are higher efficiency and reduced component count and it converts the unregulated voltage into desired regulated voltage by varying the duty cycle at high switching frequency lowering the size and cost of energy storage components. When a boost converter operates in continuous mode, the current through the inductor (IL) never falls to zero. The output voltage can be calculated as follows, in the case of an ideal converter (i.e. using components with an ideal behavior) operating in steady conditions. During the On-state, the switch S is closed, which makes the input voltage appear across the inductor, which causes a change in current (IL) flowing through the inductor during a time period (t) as shown in Figure 5(a). During the Off-state, the switch S is open, so the inductor current flows through the load as shown in Figure 5(b). Applying Kirchhoff’s rules around the loops and rearranging terms yields an intuitive result: Vo 1 (3) Vi 1 D From the above expression it can be seen that the output voltage is always higher than the input voltage (as the duty cycle goes from 0 to 1), and that it increases with D, theoretically to infinity as D approaches 1. If we considered the operation of the boost converter only during the on-off states we can estimate the converter current without a need to a current sensor. If D is set to zero for time longer than the switching frequency -which is the open circuit condition- the output voltage in this case is equal to the input voltage and the inductor current is zero. On the other hand if the duty ratio is set to 1 – which is the short circuit condition- the output voltage will be zero and the inductor current can be calculated as:
Vi iL R L
diL dt
where R is the inductor internal resistance. By solving the differential equation in (4), the instantaneous current can be expressed as: R t V iL i [1 e L ] R
(4)
(5)
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A Current Sensor-less Maximum Power Point Tracking Method for PV System Byunggyu Yu, Ahmed G. Abo-Khalil
D=0 i, Vmax =0 D=D-dD D D=1 i= equationn p(t)=v(t)*i((t)
If p(t+dt)
