CHAPTER 7 MAXIMUM POWER POINT TRACKING USING HILL CLIMBING ?· 100 CHAPTER 7 MAXIMUM POWER POINT TRACKING…

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    CHAPTER 7

    MAXIMUM POWER POINT TRACKING USING

    HILL CLIMBING ALGORITHM

    7.1 INTRODUCTION

    An efficient Photovoltaic system is implemented in any place with

    minimum modifications. The PV energy conversion system implemented in

    this thesis using neural network is trained for MPP depending upon the place

    of installation. The system implemented using fuzzy logic requires prior

    knowledge about the variation in geographical data. The hill climbing method

    of MPPT implemented by Maheshappa et al (1998), dealt with increasing or

    decreasing the array operating voltage and observing its impact on the array

    output power. This algorithm is independent of place of installation and prior

    study of the geographical data is not required. Any system implemented using

    the hill climbing algorithm is considered to be most efficient system.

    Noguchi et al (2000) proposed a novel maximum-power-point

    tracking (MPPT) method with a simple algorithm by using a short-current

    pulse of the PV array to determine an optimum operating current for the

    maximum output power. Here, the optimum operating current was

    instantaneously determined by taking a product of the short-current pulse

    amplitude and a parameter k because the optimum operating current was

    exactly proportional to the short circuit current

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    Nicola Femia et al (2005) proposed that optimization approach lies

    in customization of the perturb and observe MPPT parameters to the dynamic

    behaviour of the PV system. Kasa et al (2000) presented a perturbation and

    observation method with a capacitor identifier for MPPT. The variation of

    duty ratio was determined by considering its circuit parameters. The actual

    capacitance of an electrolytic capacitor in parallel with the photovoltaic array

    has 50% tolerance of its nominal value. Teulings et al (1993) presented a

    digital hill-climbing control strategy combined with a bidirectional current

    mode power cell that makes to get a regulated bus voltage topology, suitable

    for space applications, by means of two converters. MOSFET-based power

    conditioning unit (PCU) along with a control algorithm to track the maximum

    power point was discussed. Maximum power from each PV array was

    extracted in spite of mismatch in the array characteristics. When the variation

    of duty ratio was determined based on its nominal value, the performance of

    the MPPT was degraded.

    7.2 HILL CLIMBING ALGORITHM

    The hill climbing algorithm locates the maximum power point by

    relating changes in the power to changes in the control variable used to

    control the array. This system includes the perturb and absorb algorithm

    which was proposed by Xiao et al (2004).

    Hill-climbing algorithm involves a perturbation in the duty ratio of

    the power inverter. In the case of a PV array connected to a system,

    perturbing the duty ratio of power inverter perturbs the PV array current and

    consequently perturbs the PV array voltage. Figure 7.1 shows the

    characteristic of PV array curve. In this method, by incrementing the voltage,

    the power increases when operating on the left of the MPP and decreases the

    power when on the right of the MPP. Therefore, if there is an increase in

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    power, the subsequent perturbation is kept at same point to reach the MPP and

    if there is a decrease in power, the perturbation is reversed. This algorithm is

    summarized in Table 7.1. The process is repeated periodically until the MPP

    is reached. The system then oscillates about the MPP. The oscillation is

    minimized by reducing the perturbation step size.

    Figure 7.1 Characteristic PV Array Power Curve

    Table 7.1 Summary of Hill Climbing Algorithm

    Perturbation Change in Power Next Perturbation

    Positive Positive Positive

    Positive Negative Negative

    Negative Positive Negative

    Negative Negative Positive

    V (Volt)

    Max.Power Point (Slope is Zero)

    Slope =P/ v

    P (Watt)

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    7.3 BLOCK DIAGRAM OF THE PROPOSED SYSTEM

    Figure 7.2 shows the entire block diagram of the proposed system.

    In this, the PV array output vary with temperature, insolation, angle of

    incidence and the PV characteristics of the PV cell or array which is used. So

    in order to track the maximum power point for a particular condition, the

    voltage and current is sensed and is scaled to 5V through an operational

    amplifier and is given as an input to the analog channel of the PIC

    microcontroller for making necessary control action. The PIC microcontroller

    tracks the variation of dp/dv which is either positive, negative or zero. If it is

    zero, it doesnt make any change in control signal. Whereas if it is positive, it

    increments the modulation index and if it is negative, it decrements the

    modulation index. The PIC microcontroller sends necessary signal to PWM

    generator which generates gate pulses for triggering the inverter.

    Figure 7.2 Block Diagram of Entire System

    Output AC

    PV Array

    Single Phase

    Inverter

    Transformer

    Control unit for Implementing Hill

    Climbing Algorithm

    PWM Generation and Driving Circuits

    Modulation index

    Voltage and

    Current Sensing

    Gate Pulses

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    Chihchiang Hua et al (1998) proposed to track, the maximum

    power point of the PV panel in real time using a simple algorithm based on

    perturbation and observation (P&O) method which was widely used because

    of its simple feed back structure and fewer parameters. Nobuyuki Kasa et al

    (2005) proposed the digital signal processing kit to control power

    conditioning unit and MPPT including the PV current and pulse width

    modulation calculation. Figure 7.3 shows the flow chart of the implemented

    algorithm by measuring the array voltage and array current information. The

    PV array output is calculated and compared to the previous PV array output

    power. Initially the modulation index (m) value is set and if the final output

    power is equal to the initially measured output power, the control circuit

    maintains the same m value. If it is greater, then m value is increased and

    vice versa.

    Eftichios Koutroulis et al (2001), proposed a simple method in

    which the PV array output power delivered to a load was maximized using

    MPPT control systems, in which the control unit drive the power conditioner

    such that it extracted the maximum power from a PV array. In this method, a

    Buck-type dc/dc converter was used where the duty cycle variation was not

    analysed. To overcome this, PWM technique is implemented to switch on the

    inverter circuit.

    The level of power flow depends on the desired array voltage value

    determined by the MPPT algorithm. There are two possible situations that

    need to be addressed. First, an increase in the array voltage is required, and

    secondly, a decrease is required. The voltage output of the voltage source

    inverter (VSI) is fixed, the power flow is varied by altering the VSI output

    current. If the MPPT algorithm requires a decrease in the array voltage, the

    output current is increased in phase with the grid voltage to a stable

    magnitude determined by modulation index using PWM generator.

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    Figure 7.3 Flow Chart for Calculating Modulation Index Value

    Start

    Measure voltage and current

    Initialize modulation index (m)

    Power (Pin) = voltage * current

    Increases the m

    Measure voltage and current input

    Power (Pfin) = voltage * current

    If Pin =Pfin

    If Pin < Pfin

    If Pin >Pfin

    m = m+1 m = m-1 m = m

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    This causes an increase in the positive power flow towards the load.

    The extra power comes from the array, which causes the array voltage to fall

    to the desired value as suggested by Krein et al (2003). The desired voltage is

    reached, the output current goes down to a level that the power on the array

    and load are equal once again. If an increase in the array voltage is required,

    the opposite effect occurs by which a constant voltage is maintained.

    Algorithm and flow chart:

    The algorithm used for MPPT is discussed below:

    Step 1: Sensing and measuring the voltage and current of PV

    array

    Step 2: Initialize the modulation index to a particular value

    Step 3: The initial power Pin is calculated

    Step 4: Increase the value of m

    Step 5: Sense the PV array voltage and current

    Step 6: Calculate the modified power Pfin

    Step 7: If the change in power is positive, increase m value; if it

    is negative, decrease m value and if there is no change

    in power, m value is retained.

    Step 8: Repeat step 5.

    The above algorithm for MPPT is incorporated in PIC

    microcontroller 18F452 using MPLAB IDE.

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    7.4 SIMULATION MODEL

    Figure 7.4 shows the simulation model of the proposed system. The

    input parameters from the PV array are sensed through hill climbing block.

    According to the variations in the PV array parameters, the corresponding

    modulation index (m) value is obtained. Salas et al (2006) implemented a new

    algorithm for MPPT. The algorithm was programmed in a PIC

    microcontroller and according to the panel input parameters, the duty cycle is

    varied in order to track the maximum power output. Based on this technique

    in this proposed work, the m value is varied using hill climbing algorithm and

    the corresponding PWM pulses are produced to trigger the inverter.

    Figure 7.4 Simulation Model of the PV System Using Hill Climbing

    Algorithm

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    Figure 7.5 shows the simulation block of hill climbing algorithm.

    The input parameters current and voltage is sensed from the PV panel. Any

    positive change in power indicates increase in m value; and any negative

    change in power indicates decrease in m value and no change in power

    indicates to retain the m value.

    Figure 7.5 Hill Climbing Simulation Block

    Based on the variation in m value obtained using hill climbing

    algorithm, the corresponding gate pulses are produced in the PWM circuit.

    These pulses are used to trigger the MOSFET used in the inverter circuit. The

    output voltage generated by the inverter is given by the equation (7.1).

    *2dc

    acVV m (7.1)

    where m is modulation index, the ratio of amplitude of sine wave to

    triangular

    Vdc is the dc supply given to inverter.

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    7.5 SIMULATION RESULTS

    The bridge inverter circuit requires four gate pulses in order to

    trigger the MOSFET. The first two pulses for one arm of the inverter bridge

    are generated by comparing the triangular and the zero phase shifted sine

    wave such that each pulse is a complement of the other. In the similar way

    pulses for the second arm is generated by comparing the triangular wave and

    180 degree phase shifted sine wave as suggested by Krein et al (2004).

    The variation in the modulation index given to the PWM generation

    generates gate pulses to trigger the inverter and produces a constant output.

    Figure 7.6 shows the output waveform of PWM generator. In this, the gate

    pulses G1, G2, G3 and G4 are given to the corresponding MOSFETS T1, T2,

    T3 and T4 respectively.

    Figure 7.6 Gate Pulse output

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    The variation in the modulation index given to the PWM generation

    generates gate pulses to trigger the inverter and produces a constant output.

    The gate pulses G1, G2, G3 and G4 are given to the corresponding

    MOSFETS T1, T2, T3 and T4 respectively. Figure 7.7 represents the inverter

    output current and voltage waveforms.

    Figure 7.7 Inverter Output Current and Voltage Waveform

    7.6 HARDWARE IMPLEMENTATION

    The hardware is implemented using hill climbing algorithm for

    tracking the maximum power from the solar panel. Figure 7.8 shows the solar

    panel used for this work. The output of the panel namely voltage and current

    is sensed and given to the PIC microcontroller for determining the variation in

    power. The hill climbing algorithm for traction of maximum power point

    depending on variation in solar intensity is implemented using this

    microcontroller.

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    Figure 7.8 Solar Panel (1812=216 cells)

    The first requirement in designing the hardware is solar panel and

    its specifications. The specification of the panel used is represented in the

    Table 7.2. It indicates the rating of open circuit voltage, short circuit current,

    peak power delivered by the panel, etc.

    Table 7.2 Solar Panel Specification (1812 = 216 cells)

    S.No. Parameter Value

    1 Open circuit voltage (Voc) 62.8 V dc

    2 Peak voltage (Vp) 50.7 V dc

    3 Short circuit current (Isc) 6.4 A dc

    4 Peak current (Ip) 5.8 A dc

    5 Peak power (Pp) 295 W

    Based on the specifications of the PV panel rating listed in

    Table 7.2 the control circuit parameters are designed.

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    7.6.1 Control Circuit

    Figure 7.9 Control Circuit of the PV System

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    7.6.1.1 Sensing Panel Parameters

    The control circuit is shown in Figure 7.9. The panel parameters

    like voltage and current are sensed and transferred as an input to the

    microcontroller for determining the variation in power. The voltage is sensed

    by a voltage divider circuit and the current is sensed using shunt.

    7.6.1.2 Voltage sensing

    The voltage divider circuit consists of resistances R1 and R2.The

    output of the voltage divider is given by the equation (7.2).

    21 2

    *( )vo s

    RV VR R

    (7.2)

    where Vs = Output from solar panel in volts

    Vvo = Voltage proportional to output voltage from solar panel

    in volts

    R1,R2 = Resistances in ohms

    The zener diode Z1 is used to limit the voltage input to the

    microcontroller to 5V.

    7.6.1.3 Current sensing

    The dc current from the panel is sensed using a shunt which gives

    the output of 75 mV when a current of 10A flows. The voltage obtained from

    the shunt is five scaled using the operational amplifier connected in non

    inverting mode. The output across the zener diode is given by the

    equation (7.3).

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    3 54

    * 75*10 * ( 1)10

    sio

    I RVR

    (7.3)

    where Is = Output current from solar panel in ampere

    Vio = Voltage proportional to output current from solar panel

    in volts

    R5, R4 = Resistances in ohms

    7.6.1.4 Microcontroller Logic Circuit

    The hill climbing algorithm for the traction of maximum power

    point depending on variation in solar intensity is implemented using

    microcontroller PIC18F452.

    In this circuit, the reset switch is used to reset all the registers in the

    microcontroller whenever it is necessary. The variation in voltage and current

    is sensed and based on that, the modulation index value (m) is produced as an

    eight bit digital output in the port B of the microcontroller.

    7.6.1.5 Digital to Analog Converter

    The output from the microcontroller is converted into analog form

    using the digital to analog converter DAC 0808. The reference signal given to

    the DAC is 5V .The output of the DAC is given by the equation (7.4).

    1 2 3 4 5 6 7 85 *2 4 8 16 32 64 128 256A A A A A A A AModulation index

    (7.4)

    where A1 to A8 is MSB to LSB of digital modulation index

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    7.6.1.6 Modulating signal generation

    The modulating signal is used for the generation of inverter gate

    signals. In order to maintain the output of the inverter as 50Hz, the signal is

    tapped from the grid and stepped down to 5V. Then it is multiplied with the

    modulation index using analog multiplier AD532 to get the required

    modulating signal. The 6V signal generated from the transformer is converted

    into 5V using operational amplifier U1 and U2. The resultant modulating

    signal from the output of the multiplier is converted into two sinusoidal

    signals each phase shifted by 180 degree.

    7.6.1.7 Triangular carrier signal generation

    The triangular wave is generated using the combination of

    operational amplifier operated as a square wave generator and integrator.

    7.6.2 Gate Pulse Generation Circuit Figure 7.10 shows the gate pulse generating circuit. In this G1, G2,

    G3 and G4 represents gate pulses and R1, R2, R3 and R4 represents the

    respective references. The variation in the modulation index given to the

    PWM generation generates gate pulses to trigger the inverter and produces a

    constant output. The bridge inverter circuit requires four gate pulses in order

    to trigger the MOSFET. The first two pulses for one arm of the inverter

    bridge are generated by comparing the triangular and the zero phase shifted

    sine wave obtained from the control circuit, such that each pulse is an

    complement of the other. In the similar way pulses for the second arm is

    generated by comparing the triangular wave and 180 degree phase shifted sine

    wave. All the four pulses are then given to the opto-coupler MCT 2E which

    act as an isolator to prevent the controlling circuit from the surges arising in

    the inverter.

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    Figure 7.10 Gate Pulse Generation Circuit

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    Figure 7.11 shows the gate pulses generated in the gate pulse

    generating circuit. The pulses G1, G2, G3, G4 are obtained using

    oscilloscope. According to the variation in solar insolation the response of the

    modulation index varies. Based on the change in m value the gate pulses

    generated also varies its time duration. Gate pulses generated using simulation

    model presented in Figure 7.7 matches with the hardware gate pulse

    generation circuit waveforms.

    Figure 7.11 Gate Pulses G1, G2, G3 and G4 given to the Inverter

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    7.6.3 Inverter Circuit

    The generated gate pulses from the driver circuit are connected to

    the MOSFET IRF 640 which is connected in bridge configuration. The

    snubber circuit is connected in parallel to all the four MOSFET in order to

    avoid device damage due to surges. The inverter circuit is shown in the

    Figure 7.12.

    Figure 7.12 Inverter Circuit

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    The output voltage waveform of the inverter is connected to the

    step up transformer to obtain a constant secondary output voltage for a load of

    15W lamp. The output of the inverter is connected to the primary side of the

    step up transformer which is provided with tappings of 10V, 20V, 30V, 40V.

    According to the inverters output the corresponding tappings are used in the

    transformer in order to produce constant secondary voltage. Figure 7.13

    shows the output voltage waveform of the inverter given to the primary of the

    transformer and the step up voltage output given to the load.

    Primary side Secondary side

    Figure 7.13 Voltage Waveform of Inverter with 15W Lamp Load

    Figures 7.14 and 7.15 shows the experimental setup for different

    load conditions. Depending on the solar insolation, the control circuit PC

    board senses and drives the gate pulse circuit which is given to the inverter

    circuit. The output of the inverter is given to the suitable transformer tapping

    in the primary side in order to produce a constant secondary output required

    by the load.

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    Figure 7.14 Experimental Setup with 15W Lamp Load

    Figure 7.15 Experimental Setup with 60W Lamp Load

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    Figures 7.16 and 7.17 shows the output waveform of array power

    versus time and array current. The graph indicates the curve drawn for two

    sets of datas one with MPPT control and the other without MPPT control.

    The graph drawn between the array power and time shows the difference in

    variation in the power generated with MPPT and without MPPT during the

    morning and evenings. By using the hill climbing algorithm, it is observed

    that the amount of power produced by the PV generator trained using MPPT

    follows the pattern of irradiance.

    Figure 7.16 Output waveform of Array Power Vs Time

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    Figure 7.17 Output Waveform of Array Power Vs Array Current

    7.7 CONCLUSION

    The panel is trained statically using hill climbing algorithm for

    maximum radiation. It predicts the maximum power point voltage and the

    modulation index value. The pulse width modulation scheme is used to trigger

    the gate of the switching device, which reduces the lower order harmonics at

    the output of the inverter. The simulation results match with the

    implementation results. The implementation complexity of the system is low

    compared to the above two algorithms. The main advantage is, the system is

    not array dependent and periodic tuning is not required.

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