Maximum Power Point Tracker with Ripple Correlation Control as Charger Controller
Felix Yustian Setiono1, Leonardus Heru Pratomo1
1 Departement of Electrical Engineering – Soegijapranata Catholic University Jl. Pawiyatan Luhur IV/1 Semarang, 50234, Indonesia
[email protected], [email protected]
Abstract - Charger controller system is a combination between MPPT, that using ripple correlation control topology; and battery charger, using buck dc chopper with proportional integrator control topology; that made constant dc voltage used for battery charging. Experimental result of the system has efficiency about 91%.
Keywords – photovoltaic, maximum power point tracker, ripple correlation control, charger controller
I.
I
NTRODUCTIONThe usage of photovoltaic has been increased nowadays, even for commercial or residential. In order to achieve optimum power output of photovoltaic, is using system that could maximize photovoltaic in its optimum work point. This system is called maximum power point tracker (MPPT).
Nowadays, there are many methods of MPPT in three decades. In this paper, will used concept of ripple correlation control (RCC) for MPPT usage.
These concept ago used for dynamic optimization for motor controlling, used ripple that appear in every converter that using switching topology, to get information about operating area control. This topology is fast convergence to load or source shedding, and has efficiency about 99% (Trisham et all., 2006)
Usually MPPT using as charging controller for battery charger, where MPPT combined with battery charger as charger controller sytem. This battery charger used dc chopper in buck type with proportional integrator controller topology, where the output will made regulated dc voltage that used for battery charging.
This paper will be more focussed on maximum power point tracker that used ripple correlation control
topology. The others controls topologies will be not deeply explained.
II.
R
IPPLEC
ORRELATIONC
ONTROL The working of photovoltaic is to change sun rays into electrical current using physical principle of semiconductor principle in cells (pn junctions).Electrical model of photovoltaic shown on figure 1. If sun intensity increased, so do the current that produced by the cells will be increased too.
Fig. 1 Electrical model of photovoltaic
Photovoltaics using maximum power point curve where the maximum power point is the maximum performance of cells output. This maximum power point depends on intensity of sun that received by cells, weather conditions and climates, and load factors. (Pallab et all., 1996)
Fig. 2 Photovoltaics performance curve
If photovoltaics connected directly into electrical load, so the cells voltages will be decreased directly (drop voltage), but the cells currents depends on the intensity of sun rays that received, so the photovoltaics will never reach the maximum power point, with any load factors.
Fig.3 Operating point of cells in direct load connected (in I-V curve)
Charger controller system is combination of MPPT and battery charger. MPPT system that used in this paper is using ripple correlation control (RCC) topology. This topology using dc choppers boost type, where described below.
Fig. 4 MPPT topology
Fig. 5 Photovoltaics working curve
As shown on figure 5, the inductor currents iL and photovoltaics power output P, where working condition is above iL or below iL*. For this condition, we declared that i = iL and C = 0. When iL is under iL*, currents ripple appear along the curve to an in-phase power ripple; and made the product of time derivative of iL (diL/dt) and P (dp/dt) is positive. When iL is above iL*, the current
ripple and power ripple are out of phase, so the product of diL/dt and dp/dt are negative. (Trisham et all., 2006) This equations could be declared as:
diL dt
dp
dt >0⇒IL<I¿L diL
dt dp
dt <0⇒IL>I¿L
(1) The equation above is one form of ripple correlation control (RCC) law.
If iL increased when equation (1) value above zero, and then decreased, so iL must be approach to iL*. Other way to made it by doing integration of equation (1), as:
d=k
∫
dpdt didtLdt(2) Where d is duty cycle of switch S and k value as constant and positive. Inductor current increase and decreased following duty cycle d, so d must be able to follow inductor current iL.
Equation (2) using derivative from signal that could be directly measured. So the optimum point declared when dp/diL = 0; so the control law become as:
d = k ∫ di dp
L
dt
(3)
The possible operating equation since the integration value as zero is when iL achieve iL*. Intregrand of equation (3) isn’t really avaiable on circuits operation.
This difficult to achieve signal ratio for noise in equation (3) even convergence will be very slow.
Scaling the integrand of equation (3) on positive side will change on convergence, but equation still could be in the convergence. We could be declared this as alternative rule of control, where using scaling of integrand (diL/dt)2, will be value as positive when iL
changes.
d=k ∫ di dp
L
( di dtL)
2dt =k ∫ dp dt di
L
dt dt
(4)
Equation (4) similar to equation (3) but made from different point of view. Integration of equation (4) will controlling dp/dt = 0. Equation (1) until (4) could be declared if voltage parameter changed with current; even negativr value of k still be in used, where current and voltage will be contrast value in an operation with photovoltaics.
Condition on above will occured if control system asymtotically convergence with optimal condition for P is unimodal and current derivative value only zero for limiting time period. This condition will occur on photovoltaics and will continue on normal switching condition and converter on continued conduction condition. (Trisham et all., 2006)
Fig. 6 RCC controller topology
For battery charger control, it used proportional integrator controller topology, where series connected with MPPT. This topology used for increasing combined signal that appear from reference signal that has been declared with true signal that appear from system., then this signal will have an integrating process dan combined in proportional controller. The controlled triangle signal will combined with the proportional output signal to made a PWM control signal to control the switch.
Fig. 7 Proportional integrator controller topology
III.
I
MPLEMENTATIONImplementation of system proposed is using a boost dc chopper as MPPT with RCC topology that series connected with buck dc chopper as battery charger with proportional integrator topology.
MPPT that used has input of current and voltage from photovoltaics, and calculated using current transducer LEM HX10P and voltage transducer as resistor divider.
Then this transducers outputs will be multiplied using some multipliers AD633. Outputs from multipliers will be mixed in integrator controllers that used for signal reinforcement, then connected with comparators that used for compare signal from integrator and a controlled triangle signal to made a control signal for switch.
Battery charger sytem itself will have operating condition as described. Voltage intensity for battery charging will be controlled by controller that used for stop flow of charging current if the battery has full capacity. Controller that be used is proportional integrator controller where this controller has input from mixed signal from battery voltage and reference voltage.
Output of controller will be compared with a controlled triangle signal in a comparator to achieve PWM control signal for switch.
Fig. 8 Battery charger control topology
So the combined system will become as shown in figure 9 below.
Fig. 9 Combined system topology
Result of system is regulated dc voltage that be used for battery charging process. MPPT that used will be forced system to achieve the maximum condition of photovoltaics, and battery charger itself will made a regulated constant dc voltage in continuous.
Fig. 10 System implementation
IV.
E
XPERIMENTALExperiment on system is using two photovoltaics brand BP Solar that series connected with each parameters as:
Maximum power output (Pmax) : 55 watt
Maximum voltage (Vmp) : 16,5 volt
Maximum current (Imp) : 3,33 ampere
Open-circuit voltage (Voc) : 20,6 volt
Short-circuit current (Isc) : 3,69 ampere The implemented system will be connected to various types of loads, in order to find out the drop voltages of photovoltaics when connected with various loads variations. And also to find out the efficiencies of the photovoltaics on various types of loads. The loads itself used of some 20 watts resistors.
Parameters that calculated are input voltages and input currents from photovoltaics that will be multiplied into input power of photovoltaics in watts. And also output voltages and output currents of implemented system that also multiplied into output power in watts. Efficiency of implemented system will be calculated as comparison of input power and output power in percentages.
Input voltages from photovoltaics will be declared as Vpv, input currents from photovoltaics as Ipv, power generated by photovoltaics as Ppv. Output voltages of implemented system will be declared as Vout, output currents of sytem as Iout, and power output of system as Pout. Percentages symbol (%) will be declared as efficiency of impelemented system.
Experiment time for implemented system did on sunny condition on around 10 am to 2 pm (10.00-14.00 in Indonesian time) at day. And also at cloudy condition on around 10 am to 2 pm (10.00-14.00 in Indonesian time) at day also.
This result will be shown in table 1 and 2 below;
voltages will be volts, currents will be in amperes, powers will be in watts, and efficiency will in percentages.
The first experiment did on cloudy condition at 10 am to 2 pm at day shown in table 1 below.
TABLE 1
EXPERIMENT ON CLOUDY CONDITION Load
s Vpv Ipv Ppv Vout Iout Pout % 15 Ω 31,7 2,6 81,
8 7,6 9,8 74,
5 91,1
20 Ω 31,7 2,6 81,
8 7,4 9,7 71,
8 87,8
35 Ω 31,7 2,6 81,
8 7,8 9,7 75,
7 92,5
39 Ω 31,7 2,6 81,
8 7,8 9,7 75,
7 92,5
60 Ω 31,7 2,6 81,
8 7,9 9,7 76,
6 93,7
The second experiment did on sunny condition at 10 am to 2 pm at day shown in table 2 below.
TABLE 2
EXPERIMENT ON SUNNY CONDITION Loads Vpv Ipv Ppv Vout Iout Pout %
15 Ω 36 3 108 10 9,8 98 91
20 Ω 36 3 108 10 9,8 98 91
35 Ω 36 3 108 10 9,8 98 91
39 Ω 36 3 108 10 9,8 98 91
60 Ω 36 3 108 9,75 9,8 95,6 88,5
From the tables above, it could be declared that the implemented system will be forced the output voltages and output currents to achieve the maximum power conditions of photovoltaics. The variations of output powers that generated by photovoltaics is because of the variations of intensities of sun rays that happened at the time of experiments.
V.
S
UMMARYThe implemented system has average efficiency about 91.01%, with a series combination of MPPT as boost dc chopper with ripple correlation control topology and battery charger as buck dc chopper with proportional integrator control topology, that been used for charger controller for battery charging process. Efficiency rates itself depends on intensities of sun rays that received by the photovoltaics, weathers and climates conditions, and loads shedding factors.
A
CKNOWLEDGEMENTThe authors would like to thank for Philip T. Krein and Jonathan W. Kimball for their early contribution involving this work. And thank also for Slamet Riyadi for the discussions and for Agung Nugroho for implementation of system proposed.
R
EFERENCES[1] Trishan Esram, Jonathan W. Kimball, Philip T.
Krein, Patrick L. Chapman, and Pallab Midya, Dynamic Maximum Power Point Tracking of
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5, pp.1282-1291, Sept. 2006.
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[3] Jonathan W. Kimball and Philip T. Krein, Digital Ripple Correlation Control for Photovoltaic Applications. IEEE Power Elec. Conf., pp. 1690- 1694, 2007.
[4] Roger A. Messenger and Jerry Ventre, Photovoltaic Systems Engineering, 2nd ed.. New York, USA: CRC Press, 2004.
[5] Muhammad H. Rashid, Power Electronics : Circuits, Devices, and Applications, 2nd ed. New Jersey, USA: Prentice Hall, 1993
[6] Slamet Riyadi, Koneksi Photovoltaic ke Sistem melalui VSI berbasis Kendali Arus untuk Pembagian Beban. Proceedings of CITEE 2009.
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