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

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<ul><li><p>100</p><p>CHAPTER 7 </p><p>MAXIMUM POWER POINT TRACKING USING </p><p>HILL CLIMBING ALGORITHM </p><p>7.1 INTRODUCTION </p><p> An efficient Photovoltaic system is implemented in any place with </p><p>minimum modifications. The PV energy conversion system implemented in </p><p>this thesis using neural network is trained for MPP depending upon the place </p><p>of installation. The system implemented using fuzzy logic requires prior </p><p>knowledge about the variation in geographical data. The hill climbing method </p><p>of MPPT implemented by Maheshappa et al (1998), dealt with increasing or </p><p>decreasing the array operating voltage and observing its impact on the array </p><p>output power. This algorithm is independent of place of installation and prior </p><p>study of the geographical data is not required. Any system implemented using </p><p>the hill climbing algorithm is considered to be most efficient system. </p><p> Noguchi et al (2000) proposed a novel maximum-power-point </p><p>tracking (MPPT) method with a simple algorithm by using a short-current </p><p>pulse of the PV array to determine an optimum operating current for the </p><p>maximum output power. Here, the optimum operating current was </p><p>instantaneously determined by taking a product of the short-current pulse </p><p>amplitude and a parameter k because the optimum operating current was </p><p>exactly proportional to the short circuit current </p></li><li><p>101</p><p> Nicola Femia et al (2005) proposed that optimization approach lies </p><p>in customization of the perturb and observe MPPT parameters to the dynamic </p><p>behaviour of the PV system. Kasa et al (2000) presented a perturbation and </p><p>observation method with a capacitor identifier for MPPT. The variation of </p><p>duty ratio was determined by considering its circuit parameters. The actual </p><p>capacitance of an electrolytic capacitor in parallel with the photovoltaic array </p><p>has 50% tolerance of its nominal value. Teulings et al (1993) presented a </p><p>digital hill-climbing control strategy combined with a bidirectional current </p><p>mode power cell that makes to get a regulated bus voltage topology, suitable </p><p>for space applications, by means of two converters. MOSFET-based power </p><p>conditioning unit (PCU) along with a control algorithm to track the maximum </p><p>power point was discussed. Maximum power from each PV array was </p><p>extracted in spite of mismatch in the array characteristics. When the variation </p><p>of duty ratio was determined based on its nominal value, the performance of </p><p>the MPPT was degraded. </p><p>7.2 HILL CLIMBING ALGORITHM </p><p> The hill climbing algorithm locates the maximum power point by </p><p>relating changes in the power to changes in the control variable used to </p><p>control the array. This system includes the perturb and absorb algorithm </p><p>which was proposed by Xiao et al (2004). </p><p> Hill-climbing algorithm involves a perturbation in the duty ratio of </p><p>the power inverter. In the case of a PV array connected to a system, </p><p>perturbing the duty ratio of power inverter perturbs the PV array current and </p><p>consequently perturbs the PV array voltage. Figure 7.1 shows the </p><p>characteristic of PV array curve. In this method, by incrementing the voltage, </p><p>the power increases when operating on the left of the MPP and decreases the </p><p>power when on the right of the MPP. Therefore, if there is an increase in </p></li><li><p>102</p><p>power, the subsequent perturbation is kept at same point to reach the MPP and </p><p>if there is a decrease in power, the perturbation is reversed. This algorithm is </p><p>summarized in Table 7.1. The process is repeated periodically until the MPP </p><p>is reached. The system then oscillates about the MPP. The oscillation is </p><p>minimized by reducing the perturbation step size. </p><p>Figure 7.1 Characteristic PV Array Power Curve </p><p>Table 7.1 Summary of Hill Climbing Algorithm </p><p>Perturbation Change in Power Next Perturbation </p><p>Positive Positive Positive </p><p>Positive Negative Negative </p><p>Negative Positive Negative </p><p>Negative Negative Positive </p><p>V (Volt) </p><p>Max.Power Point (Slope is Zero) </p><p>Slope =P/ v </p><p>P (Watt) </p></li><li><p>103</p><p>7.3 BLOCK DIAGRAM OF THE PROPOSED SYSTEM </p><p> Figure 7.2 shows the entire block diagram of the proposed system. </p><p>In this, the PV array output vary with temperature, insolation, angle of </p><p>incidence and the PV characteristics of the PV cell or array which is used. So </p><p>in order to track the maximum power point for a particular condition, the </p><p>voltage and current is sensed and is scaled to 5V through an operational </p><p>amplifier and is given as an input to the analog channel of the PIC </p><p>microcontroller for making necessary control action. The PIC microcontroller </p><p>tracks the variation of dp/dv which is either positive, negative or zero. If it is </p><p>zero, it doesnt make any change in control signal. Whereas if it is positive, it </p><p>increments the modulation index and if it is negative, it decrements the </p><p>modulation index. The PIC microcontroller sends necessary signal to PWM </p><p>generator which generates gate pulses for triggering the inverter. </p><p>Figure 7.2 Block Diagram of Entire System </p><p>Output AC </p><p>PV Array </p><p> Single Phase </p><p>Inverter </p><p> Transformer </p><p>Control unit for Implementing Hill </p><p>Climbing Algorithm </p><p>PWM Generation and Driving Circuits </p><p>Modulation index </p><p>Voltage and </p><p>Current Sensing </p><p>Gate Pulses </p></li><li><p>104</p><p> Chihchiang Hua et al (1998) proposed to track, the maximum </p><p>power point of the PV panel in real time using a simple algorithm based on </p><p>perturbation and observation (P&amp;O) method which was widely used because </p><p>of its simple feed back structure and fewer parameters. Nobuyuki Kasa et al </p><p>(2005) proposed the digital signal processing kit to control power </p><p>conditioning unit and MPPT including the PV current and pulse width </p><p>modulation calculation. Figure 7.3 shows the flow chart of the implemented </p><p>algorithm by measuring the array voltage and array current information. The </p><p>PV array output is calculated and compared to the previous PV array output </p><p>power. Initially the modulation index (m) value is set and if the final output </p><p>power is equal to the initially measured output power, the control circuit </p><p>maintains the same m value. If it is greater, then m value is increased and </p><p>vice versa. </p><p> Eftichios Koutroulis et al (2001), proposed a simple method in </p><p>which the PV array output power delivered to a load was maximized using </p><p>MPPT control systems, in which the control unit drive the power conditioner </p><p>such that it extracted the maximum power from a PV array. In this method, a </p><p>Buck-type dc/dc converter was used where the duty cycle variation was not </p><p>analysed. To overcome this, PWM technique is implemented to switch on the </p><p>inverter circuit. </p><p> The level of power flow depends on the desired array voltage value </p><p>determined by the MPPT algorithm. There are two possible situations that </p><p>need to be addressed. First, an increase in the array voltage is required, and </p><p>secondly, a decrease is required. The voltage output of the voltage source </p><p>inverter (VSI) is fixed, the power flow is varied by altering the VSI output </p><p>current. If the MPPT algorithm requires a decrease in the array voltage, the </p><p>output current is increased in phase with the grid voltage to a stable </p><p>magnitude determined by modulation index using PWM generator. </p></li><li><p>105</p><p>Figure 7.3 Flow Chart for Calculating Modulation Index Value </p><p>Start </p><p>Measure voltage and current </p><p>Initialize modulation index (m) </p><p>Power (Pin) = voltage * current </p><p>Increases the m </p><p>Measure voltage and current input </p><p>Power (Pfin) = voltage * current </p><p>If Pin =Pfin </p><p>If Pin &lt; Pfin </p><p>If Pin &gt;Pfin </p><p>m = m+1 m = m-1 m = m </p></li><li><p>106</p><p> This causes an increase in the positive power flow towards the load. </p><p>The extra power comes from the array, which causes the array voltage to fall </p><p>to the desired value as suggested by Krein et al (2003). The desired voltage is </p><p>reached, the output current goes down to a level that the power on the array </p><p>and load are equal once again. If an increase in the array voltage is required, </p><p>the opposite effect occurs by which a constant voltage is maintained. </p><p> Algorithm and flow chart: </p><p> The algorithm used for MPPT is discussed below: </p><p>Step 1: Sensing and measuring the voltage and current of PV </p><p>array </p><p>Step 2: Initialize the modulation index to a particular value </p><p>Step 3: The initial power Pin is calculated </p><p>Step 4: Increase the value of m </p><p>Step 5: Sense the PV array voltage and current </p><p>Step 6: Calculate the modified power Pfin </p><p>Step 7: If the change in power is positive, increase m value; if it </p><p>is negative, decrease m value and if there is no change </p><p>in power, m value is retained. </p><p>Step 8: Repeat step 5. </p><p> The above algorithm for MPPT is incorporated in PIC </p><p>microcontroller 18F452 using MPLAB IDE. </p></li><li><p>107</p><p>7.4 SIMULATION MODEL </p><p> Figure 7.4 shows the simulation model of the proposed system. The </p><p>input parameters from the PV array are sensed through hill climbing block. </p><p>According to the variations in the PV array parameters, the corresponding </p><p>modulation index (m) value is obtained. Salas et al (2006) implemented a new </p><p>algorithm for MPPT. The algorithm was programmed in a PIC </p><p>microcontroller and according to the panel input parameters, the duty cycle is </p><p>varied in order to track the maximum power output. Based on this technique </p><p>in this proposed work, the m value is varied using hill climbing algorithm and </p><p>the corresponding PWM pulses are produced to trigger the inverter. </p><p>Figure 7.4 Simulation Model of the PV System Using Hill Climbing </p><p>Algorithm </p></li><li><p>108</p><p> Figure 7.5 shows the simulation block of hill climbing algorithm. </p><p>The input parameters current and voltage is sensed from the PV panel. Any </p><p>positive change in power indicates increase in m value; and any negative </p><p>change in power indicates decrease in m value and no change in power </p><p>indicates to retain the m value. </p><p> Figure 7.5 Hill Climbing Simulation Block </p><p> Based on the variation in m value obtained using hill climbing </p><p>algorithm, the corresponding gate pulses are produced in the PWM circuit. </p><p>These pulses are used to trigger the MOSFET used in the inverter circuit. The </p><p>output voltage generated by the inverter is given by the equation (7.1). </p><p> *2dc</p><p>acVV m (7.1) </p><p>where m is modulation index, the ratio of amplitude of sine wave to </p><p>triangular </p><p> Vdc is the dc supply given to inverter. </p></li><li><p>109</p><p>7.5 SIMULATION RESULTS </p><p> The bridge inverter circuit requires four gate pulses in order to </p><p>trigger the MOSFET. The first two pulses for one arm of the inverter bridge </p><p>are generated by comparing the triangular and the zero phase shifted sine </p><p>wave such that each pulse is a complement of the other. In the similar way </p><p>pulses for the second arm is generated by comparing the triangular wave and </p><p>180 degree phase shifted sine wave as suggested by Krein et al (2004). </p><p> The variation in the modulation index given to the PWM generation </p><p>generates gate pulses to trigger the inverter and produces a constant output. </p><p>Figure 7.6 shows the output waveform of PWM generator. In this, the gate </p><p>pulses G1, G2, G3 and G4 are given to the corresponding MOSFETS T1, T2, </p><p>T3 and T4 respectively. </p><p> Figure 7.6 Gate Pulse output </p></li><li><p>110</p><p>The variation in the modulation index given to the PWM generation </p><p>generates gate pulses to trigger the inverter and produces a constant output. </p><p>The gate pulses G1, G2, G3 and G4 are given to the corresponding </p><p>MOSFETS T1, T2, T3 and T4 respectively. Figure 7.7 represents the inverter </p><p>output current and voltage waveforms. </p><p> Figure 7.7 Inverter Output Current and Voltage Waveform </p><p>7.6 HARDWARE IMPLEMENTATION </p><p> The hardware is implemented using hill climbing algorithm for </p><p>tracking the maximum power from the solar panel. Figure 7.8 shows the solar </p><p>panel used for this work. The output of the panel namely voltage and current </p><p>is sensed and given to the PIC microcontroller for determining the variation in </p><p>power. The hill climbing algorithm for traction of maximum power point </p><p>depending on variation in solar intensity is implemented using this </p><p>microcontroller. </p></li><li><p>111</p><p>Figure 7.8 Solar Panel (1812=216 cells) </p><p> The first requirement in designing the hardware is solar panel and </p><p>its specifications. The specification of the panel used is represented in the </p><p>Table 7.2. It indicates the rating of open circuit voltage, short circuit current, </p><p>peak power delivered by the panel, etc. </p><p>Table 7.2 Solar Panel Specification (1812 = 216 cells) </p><p>S.No. Parameter Value </p><p>1 Open circuit voltage (Voc) 62.8 V dc </p><p>2 Peak voltage (Vp) 50.7 V dc </p><p>3 Short circuit current (Isc) 6.4 A dc </p><p>4 Peak current (Ip) 5.8 A dc </p><p>5 Peak power (Pp) 295 W </p><p> Based on the specifications of the PV panel rating listed in </p><p>Table 7.2 the control circuit parameters are designed. </p></li><li><p>112</p><p>7.6.1 Control Circuit </p><p>Figure 7.9 Control Circuit of the PV System </p></li><li><p>113</p><p>7.6.1.1 Sensing Panel Parameters </p><p> The control circuit is shown in Figure 7.9. The panel parameters </p><p>like voltage and current are sensed and transferred as an input to the </p><p>microcontroller for determining the variation in power. The voltage is sensed </p><p>by a voltage divider circuit and the current is sensed using shunt. </p><p>7.6.1.2 Voltage sensing </p><p> The voltage divider circuit consists of resistances R1 and R2.The </p><p>output of the voltage divider is given by the equation (7.2). </p><p> 21 2</p><p>*( )vo s</p><p>RV VR R</p><p> (7.2) </p><p>where Vs = Output from solar panel in volts </p><p> Vvo = Voltage proportional to output voltage from solar panel </p><p>in volts </p><p> R1,R2 = Resistances in ohms </p><p> The zener diode Z1 is used to limit the voltage input to the </p><p>microcontroller to 5V. </p><p>7.6.1.3 Current sensing </p><p> The dc current from the panel is sensed using a shunt which gives </p><p>the output of 75 mV when a current of 10A flows. The voltage obtained from </p><p>the shunt is five scaled using the operational amplifier connected in non </p><p>inverting mode. The output across the zener diode is given by the </p><p>equation (7.3). </p></li><li><p>114</p><p> 3 54</p><p>* 75*10 * ( 1)10</p><p>sio</p><p>I RVR</p><p> (7.3) </p><p>where Is = Output current from solar panel in ampere </p><p> Vio = Voltage proportional to output current from solar panel </p><p>in volts </p><p> R5, R4 = Resistances in ohms </p><p>7.6.1.4 Microcontroller Logic Circuit </p><p> The hill climbing algorithm for the traction of maximum power </p><p>point depending on variation in solar intensity is implemented using </p><p>microcontroller PIC18F452. </p><p> In this circuit, the reset switch is used to reset all the registers in the </p><p>microcontroller whenever it is necessary. The variation in voltage and current </p><p>is sensed and based on that, the modulation index value (m) is produced as an </p><p>eight bit digital output in the port B of the microcontroller. </p><p>7.6.1.5 Digital to Analog Converter </p><p> The output from the microcontroller is converted into analog form </p><p>using the digital to analog converter DAC 0808. The reference signal given to </p><p>the DAC is 5V .The output of the DAC is given by the equation (7.4). </p><p>1 2 3 4 5 6 7 85 *2 4 8 16 32 64 128 256A A A A A A A AModulation index </p><p> (7.4) </p><p>where A1 to A8 is MSB to LSB of digital modulation index </p></li><li><p>115</p><p>7.6.1.6 Modulating signal generation </p><p> The modulating signal is used for the generation of inverter gate </p><p>signals. In order to maintain the output of the inverter as 50Hz, the signal is </p><p>tapped from the grid and...</p></li></ul>

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