MAXIMUM POWER POINT TRACKER

N.PANDIARAJAN

V-I characteristics

The current from the PV-panel depends directly on insolation. This means that the V-I characteristics of a panel would change whenever there is a change in the insolation.

The i – v and p – v characteristics of a solar cell at 28 C and 1000W/m2

The i – v characteristics of a solar Module at various insolation levels in a day

100 mW/cm2

80 mW/cm2

60 mW/cm2

40 mW/cm2

InsolationIncrease

1

2

3

0 5 10 15 20 25

Curre

nt [A

]

Voltage [V]

The i – v characteristics at various temperature levels

TemperatureIncrease

1

2

3

0 5 10 15 20 25

Curre

nt [A

]

Voltage [V]

o75 C

o50 C

o25 C0o C

What is load characteristics?

For a PV system, the load may be the battery to be charged, a DC water pump, a DC motor or an Inverter connected to the grid.

Whatever may the load, the load is characterized by its rated voltage and rated current.

This is load V-I characteristics.

LOAD RESISTANCE

In other words the load resistance is given by

Rated voltage of the load/ Rated current of the load

It is constant does not vary.

Maximum power transfer theorem

The maximum power is transferred to the load when,

the source resistance = load resistance

Diesel Generator Characteristics

In a conventional power source, say diesel generator the power output is increased by increasing the diesel feed to generator.

The diesel generator is constant voltage source and increase in power output results in increased current output.

This increased power output is given effect by adding the additional loads in parallel to existing load.

If the load is not ready to be put on, then the diesel input is reduced and thus load matching is a continuous process.

source resistance of PV panel

The internal resistance of solar cells is a function of both the sunlight intensity and the current drawn.

When the load resistance is constant, the internal resistance of solar cells has a much wider range of 1–10k, it varies continuously.

Maximum Power Point Tracker

To match the load resistance with internal resistance of PV systems the

buck-boost converter is connected between the solar cells and the load circuitry.

This buck-boost converter is called the maximum power point tracker.

V-I characteristics of PV panel The current from the PV-panel

depends directly on insolation. This means that the V-I

characteristics of a panel would change whenever there is a change in the insolation.

Let us take up V-I characteristics of PV panel and it can be drawn as follows.

V-I characteristics of

PV panel

Let Ro1 be the given load resistance. At instant 1 , ISC1, VOC1 and Pmp1 are the short

circuit current, open circuit voltage and maximum power point of the module.

1/Ro1 is the slope of the load line that cuts the V-I characteristics of the module corresponding to Pmp1.

Hence, maximum power is transferred to the load at instant 1

At instant 2, (reduced insolation) ISC2, VOC2 and Pmp2 are the short

circuit current, open circuit voltage and maximum power point of the same panel at reduced isolation.

Now 1/Ro2 is the slope of the load line that would cross the characteristic curve corresponding to Pmp2.

But Ro1 is the actual load and that cannot be changed.

Hence we need to translate the load Ro1 to Ro2 such that maximum power can be transferred to it.

This is achieved by changing the value of duty cycle Dmp which controls the power conditioner circuit.

The power conditioner circuit could be any one type of dc-dc converters.

Circuit Diagram of MPPT

Circuit Description

In circuit, the actual load R3 is connected to the output of a buck-boost converter.

The input resistance of the converter is same as the output resistance of the PV cell/module.

the capacitor C1 is provided at the output of the PV module.

Circuit Description

L1 and C2 are the part of the buck-boost converter.

A PNP transistor TIP127 is an active switch S1 and diode IN4007 is passive switch S2 in the converter.

R1 and R2 are two resistors for biasing the transistor.

Microprocessor TL494

Switching mode pulse width modulation control circuit TL494 is used for controlling the duty cycle.

The duty cycle Dmp can be set by varying the values of C3 and R5 connected to pin numbers 5 and 6 of TL494.

Relationship

The relationship between the output resistance of the PV cell/module and the actual load is given by

Rout(module=R(load)*(1-Dmp)2(sq)/Dmp(sq)

for maximum power transfer using buck-boost converter.

Load Matching

Since R(Load) cannot be changed, the right hand side quantity can be matched to Rout (module) by changing the value of Dmp in the above circuit diagram.

Feedback network and control technique

For adjusting Dmp, we need a feedback network and control technique based on the voltage and current of the PV module, which would change whenever there is a change in the insolation.

MPPT

Therefore, maximum power point tracking system(MPPT) tracks the maximum power point of the

PV cell/module and adjusts Dmp from the feedback network based on the voltage and current of the PV module to match the load.

MPPT control techniques Several MPPT control techniques are available in the literature namely, Hill-Climbing/Perturb and Observe, Incremental Conductance, Fractional Open-Circuit Voltage, Fractional Short-Circuit Current, Fuzzy Logic Control, Neural Network, Ripple

Correlation Control, Current Sweep, DC Link Capacitor Droop Control, Load Current or Load Voltage Maximization, dP/dV or dP/dI Feedback Control, Linear Current Control, and state-based MPPT