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6/16/2016 Peak Power Tracking Control for Photovoltaic Array Charles W. Davidson College of Engineering One Washington Square San José, CA 95192-0080 www.engr.sjsu.edu Ping Hsu Electrical Engineering San Jose State University Outline • Basic photovoltaic cell theory of operation • Characteristic of a photovoltaic cell • Examples of PV panel power controller • Maximum Power Point Tracking control 1 6/16/2016 Basic Photovoltaic Cell Theory Silicon atom has 14 electrons and 14 protons. Heat or light e- ee- ee- e- 14+ 14‐ 14+ 14‐ 14+ 14‐ 14+ 13‐ 14+ 14‐ e-eN14+ e- ee- ee- e- e ee- e- e- ee- e- 14+ 14‐ 1‐ 14+ 14‐ 14+ 14‐ 14+ 14‐ e- Freed electron When sunlight strikes a piece of Silicon, the solar energy knocks and frees electrons from their atom structure. 4 2 6/16/2016 The freed electrons randomly move within the material. This random motion of charge cannot be utilized for power generation. In order to utilize the energy from the sun, this flow of charges must be directed in one direction. Heat or light Freed electron 5 Two types of semiconductors are used Positive type (Ptype) and Negative type (N-type) A small amount of Phosphorus (impurity) is mixed into a Silicon base and this forms an N-type material. Due to the impurity, some atoms in this material have one electron extra for a stable atom configuration. In other words, it has one loosely bonded electron. N-type material 6 3 6/16/2016 • A small amount of Boron (impurity) is mixed into a Silicon base and forms a P-type material. With this impurity, some atoms in this type of material have one electron short for a stable configuration. In other words, it can easily accept one electron. • For simplicity, this characteristic of ‘easily accepting electron’ is represented by a “hole” with a positive charge and a corresponding negative charge at the nucleus. P-type material hole 7 Now put an N-type material in contact with a P-type material. Before making the contact: P-type (neutral) N-type (neutral) 8 4 6/16/2016 P-type (Negatively charged!) boundary layer N-type (Positively charged!) Note that in the boundary layer, the free electrons in the N-type materials combine with the holes in the P-type. Consequently, the P-type side of the boundary layer is negatively charged and N-type side is positively charged. This negative charge in P-type material prevents the free electrons in the rest of the N-type materials continue to migrate into the P-type. (Negative charge repels negative charged free electrons.) The boundary lay is called PN-junction or depletion region. 9 sunlight P-type (Negatively charged) N-type (Positively charged) When sunlight strikes atoms in the P-N Junction and knockouts more electrons (and creates corresponding holes), the free electrons are expelled by the negative charge on the P-type side and hence move to wards the Ntype side. 10 5 6/16/2016 If a load is connected across the cell, a complete circuit is formed. A steady flow of electrons (i.e., current) passes through the load and hence the energy is transmitted to the load. sunlight P-type N-type http://www.youtube.com/watch?v=xLGOagKiXqg 11 Photovoltaic cell characteristics 6 6/16/2016 The current and voltage relationship of a single solar cell can be expressed by the following equation: q (V ) I I L I o exp 1 nKT where I V load IL : Light current, proportional to irradiance (W/m2), Io : Saturation current q: electron charge n : diode quality factor K: Boltzman constant T : temperature in oK, ≈0 ≈∞ IV curves of a solar cell at different levels of irradiance. =1000 W/m2 =800 W/m2 =600 W/m2 Cell current (Amp) =400 W/m2 =200 W/m2 I Cell voltage (volts) V 7 6/16/2016 Output power of a solar panel at different levels of irradiance. Maximum Power Point =1000 W/m2 =800 W/m2 =600 W/m2 Current Power 400 W/m2 200 W/m2 Voltage 1.6m 1m Efficiency 235W 15% (1.6 1.0)m 2 1000(W / m 2 ) 8 6/16/2016 Locus of peak power point =1000 W/m2 =800 W/m2 W =600 W/m2 Panel current (Amp) =400 W W/m2 W =200 W/m2 W W W Panel voltage (volts) The following figure shows the dependence of the peak-power locus on the panel temperature. As shown in the figure, as the temperature rises, the peak-power locus shifts to a lower voltage level. 50oC 70oC o 90 C 10 30oC Panel current (Amp) 5 0 20 25 Panel voltage (volts) 30 9 6/16/2016 Examples of PV panel power controller An example of a Grid-tie Photovoltaic System Panel power controller The panel power controller serves the following functions: (1) Regulating the I-V characteristic to achieve peak power tracking (2) Boosting the voltage (DC bus voltage must be at a certain level for the proper grid-side inverter operation) (3) Electrical isolation (for safety). 10 6/16/2016 The boost converter shown in the previous page has limited voltage boost range and it does not provide electrical isolation. A more commonly used circuit configuration consists of an Hbridge and a transformer as shown above. This circuit offers electrical isolation and its voltage boost is achieved by the transformer’s winding ratio. Peak Power Tracking Control Methods (1) Constant voltage with temperature compensation (2) Constant voltage (3) Open circuit voltage (4) short circuit current (5) Incremental conductance (6) Perturb-and-observe (P&O) 11 6/16/2016 (1) Constant voltage with temperature compensation If the panel voltage is regulated at a certain voltage level, the operating point will be near the maximum power point at all irradiance levels. Constant voltage load =1000 W/m2 Resistive load =800 W/m2 W =600 W/m2 Panel current (Amp) =400 W W/m2 W =200 W/m2 W W W Panel voltage (volts) The load characteristic is determined by the panel power controller’s control law. For example, For resistive load: Icmd= K*Vp (K=1/Req) For constant voltage load: Icmd= [Kp+Ki (1/s)] (Vp-Vcmd) where Vcmd is the set panel voltage. 12 6/16/2016 Example of constant voltage Mathod The MPPC circuit controls the inductor current to maintain VIN at the voltage on the MPPC pin. The MPPC pin voltage is set by connecting a resistor between the MPPC pin and GND The MPPC voltage is determined by the equation: VMPPC = 10μA • RMPPC 13 6/16/2016 A 2-cell panel Li-Ion battery charger example VMPPC = 10μA • 75k = 0.75v. Note that the open circuit voltage of this 2-cell panel is 1v. The panel voltage can be varied according to the panel temperature to achieve maximum power control. Panel voltage is varied according to the panel temperature. 50oC 70oC o 90 C 10 30oC Panel current (Amp) 5 0 20 25 Panel voltage (volts) 30 14 6/16/2016 A diode can be used to set the MPPC threshold so that it tracks the cell voltage over temperature. (2) Open circuit voltage method The maximum power operating voltage is closely proportional to the open circuit voltage of the PV panel. The proportionality constant is about 76%. The control circuit momentarily draws zero current from the panel (open circuit) and then measure the voltage. The reference voltage is then set to 76% of the voltage. =1000 W/m2 =800 W/m2 =600 W/m2 Panel current (Amp) W W =400 W/m2 W =200 W/m2 W W W Panel voltage (volts) Open circuit voltage 15 6/16/2016 (3) Short circuit current method The maximum power operating current is closely proportional to the short circuit current of the plane. The proportionality constant is about 78% ~ 92% of the short circuit current. The control circuit momentarily short circuit the panel and measure the current. The reference current is set to 78% of the short circuit current. The controller in this case should be configured as a current source. Short circuit currents =1000 W/m2 =800 W/m2 W =600 W/m2 Panel current (Amp) =400 W W/m2 W =200 W/m2 W W W Panel voltage (volts) (4) Incremental Conductance It is easy to see that at the maximum power point, the derivative of power (P) with respect to voltage should be zero. dP 0 dV =1000 W/m2 =800 W/m2 =600 W/m2 Current Powe r 400 W/m2 200 W/m2 Voltage 16 6/16/2016 (3) Incremental Conductance dP d VI dV dI I V 0 dV dV dV dV I dI V 0 dV dI I 1 4 2dV4 3 1 44 2V4 43 incremental conductance conductance This equation shows that at the maximum power point, the incremental conductance should be the same as the negative of the conductance of the panel. If the operating point is on the ‘left’ of the maximum power point. dP d VI dV dI I V 0 dV dV dV dV I dI V 0 dV dI I dV V 1 4 2 4 3 1 44 2 4 43 incremental conductance conductance If the operating point is on the ‘right’ of the maximum power point. dI I 1 4 2dV4 3 1 44 2V4 43 incremental conductance conductance 17 6/16/2016 Logic flow of the incremental conductance maximum power point tracking algorithm Read I(k) and V(k) dI(k) =I(k)-I(k-1) dV(k) =V(k)-V(k-1) I(k-1) = I(k) V(k-1) = V(k) dI I dV V Increase ref voltage dI I dV V decrease ref voltage Perturb-and-observe (P&O) method The P&O algorithm constantly varies the voltage set-point by a small amount (V). The power level before and after the perturbation are compared. If the power level is higher after a positive voltage perturbation V, the operating voltage is increased by V. 18 6/16/2016 “Implementation of Maximum Power Point Tracking Control” Boztepe, Colak, IEEE CPE 2007 International Conference workshop This is also known as the Hill climbing method. Variation of the P&O schemes: • Dynamically changing perturbation size. • Three-point P&O. Drawbacks: • Oscillation about the maximum power point. • Confused by fast changing irradiance or noise. 19 6/16/2016 Simulation results from : “Energy Comparison of Seven MPPT Techniques for PV Systems” A. Dolara, R. Faranda, S. Leva, J. Electromagnetic Analysis & Applications, 2009, 3: 152-162 Two irradiances profiles are used in the simulation. Case 1 Case 2 Simulation results from : “Energy Comparison of Seven MPPT Techniques for PV Systems,” A. Dolara, R. Faranda, S. Leva, J. Electromagnetic Analysis & Applications, 2009, 3: 152-162 Case 1 Energy (J) Ideal 4493 P&O 4282~4278 Case 2 Rank Energy (J) 1~3 3212 ~ 3134 Rank 3298 1~3 Inc. cond. 4215 4 3117 4 Cost. Voltage 4210 5 3100 6 OC voltage 4200 6 3104 5 SC current 4088 7 2942 7 20 6/16/2016 Thank you! 21