CHAPTER 5 MPPT OF PV MODULE BY CONVENTIONAL METHODS

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1 85 CHAPTER 5 MPPT OF PV MODULE BY CONVENTIONAL METHODS 5.1 PERTURB AND OBSERVE METHOD It is well known that the output voltage and current and also the output power of PV panels vary with atmospheric conditions (solar irradiation level, temperature). The usual approach for maximizing the power drawn from PV panels under varying atmospheric conditions is to use a maximum power point tracking (MPPT) method that provides a reference current or voltage for the power electronic converter that interfaces the PV array to a battery or load. The output voltage or current of the PV panel is then varied according to the atmospheric conditions so that the PV panel always operates at or close to the maximum power point (MPP). Off-line or open loop methods do not compute the actual power of the PV panel to define the reference signal. They are based on detailed prior knowledge of the PV panel and measurements of solar irradiation, short circuit current or open circuit voltage of the PV array. Conversely, the online methods track the maximum power point all the time, irrespective of the atmospheric conditions, type of PV panel, and even aging, by processing actual values of PV voltage and current. P&O method is widely used in PV systems because of its simplicity and ease of implementation Implementation In a typical P&O MPPT method, the operating voltage of the PV array is perturbed by changing the quantity in a given direction and the power drawn from the PV array is probed. If it increases, then the operating voltage is further perturbed in the same direction whereas, if it decreases, then the direction of operating voltage perturbation is reversed. The drawbacks of this method are that the operating point oscillates around the MPP, even during sunny days when the irradiance is slowly varying,

2 86 giving rise to the waste of some amount of available energy, slow response speed, and even tracking in wrong way under rapidly changing atmospheric conditions. In this method, first the values of voltage and current are initialized and the array power for these values is found. Next the power corresponding to the perturbed voltage is found and compared with the previous power. If it results in increase of the power, then the perturbation is continued in the same direction by moderating the voltage. Otherwise, it is perturbed in the reverse direction. The sequence of operations performed is shown in the flowchart given below in Fig 5.1. A Matlab program is written for the implementation of the method on 60W solar panel and the result is presented. Fig: 5.1 Flow chart for P&O method In the implementation of this method, the change can be considered either as the perturbation in the voltage or the perturbation in the current. In this work, a

3 87 perturbation in the voltage is considered for the implementation of P&O method, and then the power changes are sensed and accordingly the voltage modifications are implemented Results v1 = 16 i1 = v2 = 16 i2 = p1 = p2 = err = v1 = 17 i1 = v2 = 17

4 88 i2 = p1 = p2 = err = v1 = 18 i1 = v2 = 18 i2 = p1 = p2 = err = v1 = 17

5 89 i1 = pact = pact is the final maximum power obtained after implementation of P&O method on 60W panel. The P&O method of tracking maximum power point is implemented in Matlab environment. Maximum power obtained is found as W. 5.2 INCREMENTAL CONDUCTANCE METHOD The incremental conductance method is used as an MPPT method. The advantage of using this method to track MPP is that it is more efficient than the P&O method in a way that it is able to correctly locate the operating point of the PV array. There is a trade off between the power efficiency and reliability of tracking MPP. Since the P&O method will move away from the power operating point under rapidly changing light condition and not be able to go back to the maximum operating point quickly, this will lead to the inefficient use of the PV array and hence this affects the whole system performance of tracking MPP. Other advantage of using this method is it does not depend on the device physics. This method uses the source incremental conductance for its MPP search method. It is more efficient than Perturb and Observe method and independent of device physics. The output voltage and current from the source are monitored upon which the MPPT controller relies to calculate the conductance and incremental conductance, and to make its decision to increase or decrease duty ratio output. Mathematics of the Incremental Conductance method is discussed below. The output power from the source can be expressed as

6 90 P = V*I --- (5.1) The fact that P = V*I and the chain rule for the derivative of product yields dp/dv = d (V I) / dv = I dv / dv + V di / dv = I + V di / dv (1/V) dp/dv = (I/V) + di/dv -- (5.2) Let us define the source conductance G as G = I/V --- (5.3) and the source incremental conductance as G = di/dv --- (5.4) It is learnt that the operating voltage is below the voltage at the maximum power point if the conductance is larger than the incremental conductance and vice versa. The job of this method is therefore to search the voltage operating point at which the conductance is equal to the incremental conductance. These ideas are expressed by equations 5.5, 5.6, 5.7 and are graphically shown in Fig 5.2. dp/dv < 0, if G < G --- (5.5) dp/dv = 0, if G = G --- (5.6) dp/dv > 0, if G > G --- (5.7) Fig: 5.2 The P-V curve

7 Implementation Implementation of Incremental Conductance method is done through Matlab programming. The output voltage and current from the source are monitored upon which the MPPT controller relies to calculate the conductance and incremental conductance and makes its decision by increasing or decreasing duty ratio output. The program flow chart for this algorithm is shown in Fig 5.3. The operating output current (I in (k)) and voltage (V in (k)) are measured from the solar panel. The incremental changes dv and di are approximated by comparing the most recent measured values for (V in (k)) and (I in (k)) with those measured in the previous cycle (V in (k - 1)) and (I in (k - 1)). Then G and G are computed as per the equations 5.3 and 5.4. From equation 5.6, if dp/dv = 0 (i.e. G = G) is true, then the system operates at the MPP and no change in operating voltage is necessary, thus the adjustment step is bypassed i.e. no adjustment for the duty ratio and the current cycle ends. If equation 5.6 is false, equation 5.5 and 5.7 are used to determine whether the system is operating at a voltage greater or less than the MPP voltage and hence to increase or decrease the duty ratio by a step-size of some value accordingly. If the system is operating at the MPP during the previous cycle, the incremental change of the operating voltage is zero (dv = 0). This would lead to a division by zero i.e. G = di dv = di 0, which is impossible for calculation. To avoid this, the condition (dv = 0) is checked first and if true leads to another branch in the algorithm with further tests on possible changes of the panel's operating conditions. Since the voltage dv = 0, that means the voltage has not changed; now the only useful information about possible changes are found from the current measurement. If di is equal to zero, the operating conditions have not changed and therefore the adjustment of the system voltage is bypassed. If di > 0, the duty ratio is increased by step size

8 92 and if di < 0, the duty ratio is decreased by step size. The program then returns and starts tracking again until the MPP is reached. The maximum duty cycle ratio is set at 90% and the minimum is at 10% and hence contributes to the efficient power transfer of the converter. The programming results are also presented. Fig: 5.3 Flow chart of Incremental Conductance method

9 5.2.2 Results 93

10 94 Fig: 5.4 Power Vs duty ratio curve of Incremental Conductance method The incremental conductance method is implemented using Matlab programming. The maximum power tracked using Incremental Conductance method is W. The oscillations around operating point in P&O method are eliminated in this method. 5.3 CONSTANT CURRENT MPPT METHOD The constant current method is based on the observation from I V curves that the ratio of the array s maximum power current Imp, to its short-circuit current, I sc, is approximately constant: The following equation characterizes the main idea of current based peak power point tracking technique. I mp /I SC = M C < 1 (5.8) Where M C is called the current factor.

11 95 Constant current MPPT method approximates the MPP current as a constant percentage of the short-circuit current. To implement this method, a switch is placed across the input terminals of the converter and switched on momentarily. The shortcircuit current I SC is measured and the MPPT calculates the correct operating point using equation 5.8 and the preset value of M C and adjusts the array s current until the calculated Imp is reached. This operation is repeated periodically to track the position of the MPP. Fig: 5.5 Steps involved in constant current method Although this method is one of the fastest methods for maximum power point estimation, it is difficult to choose the optimal value of the constant M C. The literature reports success with M C values ranging from 73 to 86%. However, MPPT tracking efficiency of this method is low relative to those of other algorithms. Reasons for this

12 96 include the error in the value of M C and the fact that measuring the short circuit current is not as easier compared to that of measuring the voltage. Circuit losses are larger for the constant current peak power point tracking system due to the complicated nature of constant current MPPT hardware. However, in constant current algorithm, it is not practically possible to short-circuit the array (i.e., to establish zero resistance across the array terminals) and still make a current measurement. The presence of a MPPT tracker increases the (system) time constant due to the variation of system equivalent resistance Implentation This method is based on the observation that MPP current (I mp ) has almost a linear relation with short- circuit current (I SC ) of the PV panel. I mp = M C* I SC (5.9) Where M C is called the current factor and is equal to 0.86 for the silicon panel and has different values for different solar panels ranging from 71% to 86%. In this method, first the short circuit current, I SC is computed for the considered 60 watt solar module. Then, it is multiplied by a current factor, M C of suitable value (0.86 in this case) to obtain the current (I mp ) corresponding to the maximum power. For this fixed value of current, the power is computed for different voltages. If the difference between the power computed and the peak power is larger than the tolerance value, then value of voltage is either incremented or decremented depending on the power obtained. For that corresponding voltage and MPP current (I mp ), this process is repeated till the difference is in the tolerance range. The program flow chart for constant current algorithm is shown in Fig 5.6.

13 97 Fig: 5.6 Flow chart for constant current MPPT method Results v = 15 pold = dp = v = 16 pold = dp = v = 17

14 98 pold = dp = v = 18 pold = dp = v = 19 pold = dp = v = 20 pold = dp = v = 19 pold = vmp = 19 dp = The constant current method is implemented in Matlab environment. Maximum power obtained is found as W.

15 CONSTANT VOLTAGE MPPT METHOD The constant voltage method is based on the observation from I V curves that the ratio of the array s maximum power voltage, Vmp, to its open-circuit voltage, Voc, is approximately constant: Vmp / Voc = K < 1 (5.10) The solar array is temporarily isolated from the MPPT and a Voc measurement is taken. Next, the MPPT calculates the correct operating point using equation 5.10 and the preset value of K, and adjusts the array s voltage until the calculated Vmp is reached. This operation is repeated periodically to track the position of the MPP. Although this method is extremely simple, it is difficult to choose the optimal value of the constant K. The literature reports success with K values ranging from 73 to 80%. Its MPPT tracking efficiency is low relative to those of other methods. Reasons for this include the error in the value of K and the fact that measuring the open-circuit voltage requires a momentary interruption of PV power. Fig: 5.7 Instruction flow for constant voltage method

16 100 However, constant voltage control is normally favored because of the relative ease of measuring voltages, and because open-circuiting the array is simple to accomplish, but it is not practically possible to short-circuit the array and still make a current measurement. The constant voltage method is implemented using the flowchart shown in Fig 5.8. Fig: 5.8 Flow chart of constant voltage method Implementation This method is based on the observation that MPP voltage (Vmp) has almost a linear relation with open-circuit voltage (Voc) of the PV panel. Vmp = K*Voc (5.11) where K called the voltage factor is equal to 0.71 for the silicon panel and has different values for different solar panels ranging from 71% to 86%.

17 101 The PV panel is locked at the reference voltage given by equation The opencircuit voltage required to determine the MPP voltage is measured by disconnecting load from the PV panel after regular intervals. The measured value of Voc and K are stored and used for determination of the PV panel voltage V. To operate the panel at MPP, the actual PV panel voltage V is compared with the reference voltage Vref, which corresponds to the MPP voltage Vmp. The error signal is processed to make V = Vref. The error signal is used to change the duty cycle of a dc-dc converter, interfaced between the PV panel and the load, so as to make the PV panel voltage equal to the MPP voltage as shown in Fig 5.8. This method is very simple to implement, but it is not accurate. A problem with this method is that the available energy is wasted when the load is disconnected from the PV array; and also the MPP is not always located at 71% of the array s open circuit voltage. There is substantial power wastage, as it does not take into account the effects of changes in solar insolation and temperature Simulation And Results The PV panel is formed by the combination of many PV cells connected in series and parallel to provide the desired output voltage and current. The PV panel exhibits a nonlinear insolation dependent V-I characteristic, mathematically expressed for the solar cell array consisting of N s cells in series and N p cells in parallel as shown in equation I N I N I [exp( q( V IR ) / mktn ) 1]... (5.12) p ph p s Equation 5.12 can be rewritten in terms of array voltage as: s s V N mkt / q)log(( N I I N I ) / N I ) ( IR N / N ) (5.13) ( s p ph p s p s s s p where q - Electric charge

18 102 m Diode ideality factor k Boltzmann s constant T Absolute temperature R s Cell series resistance I ph Photo Current I s Cell reverse saturation current N p Number of parallel strings N s Number of series cells I and V are the panel current and voltage respectively. To determine the operating point corresponding to maximum power, equation 5.13 is used in simulation. In the proposed MPPT, shown in Fig 5.9, for the solar cell equivalent circuit, a block called PV source is created as shown in Fig 5.10, which simulates the nonlinear V-I characteristics of solar panel as per the equation 5.13, employing the cell short circuit (I sc ) as a measure of insolation level. A delay function is introduced to limit the fast current response of the controlled voltage source and to improve the convergence of solution. For the voltage-based PPT equivalent circuit, a block called Vmppt is used as shown in Fig This block computes cell opencircuit voltage using I no-load and equation 5.13, compares it with the PV output voltage using equation 5.11 and calculates the firing commands for the pulse-widthmodulation block. The pulse width modulated output is used to drive the MOSFET of a step-down dc-dc converter. The duty cycle of the converter changes till the PV panel voltage becomes equal to the MPP voltage. For the simulation of the MPPT system a step down converter model is developed in Simulink. The values of the components selected are: L=5.6mH, C=2200µF, R L =5Ω.

19 103 The switch used is an ideal switch, with low switching and ON state loss. The Simulink setup is shown in Fig 5.9. The simulation results are shown below in Fig 5.13, Fig 5.14 and Fig Fig: 5.9 Circuit diagram for simulation of CV technique Fig: 5.10 PV source block

20 104 Fig: 5.11 Vmppt block Fig: 5.12 PWM block Fig: 5.13 Power characteristic curve with CV method

21 105 Time (seconds) Fig: 5.15 Voltage characteristic curve with CV method Time (seconds) Fig: 5.15 Voltage characteristic curve with CV method Constant voltage maximum power point tracking is implemented on a 60 W panel using Matlab Simulink and the graphical results are presented. 5.5 CONCLUSIONS P&O method gave maximum power of W. In this method oscillations around MPP will be there. Incremental conductance method gave W. This method involves calculation of conductance and incremental conductance. Constant current gave W. This method involves the measurement of short circuit current of the PV array. Constant voltage method gave W. In this method measurement of open circuit voltage of PV array is required.