Output Voltage [V] 70 75 80 85 90 95 100 Efficiency [%] Vin = 8 V Iout = 1 A Iout = 2 A Iout = 3 A Iout = 4 A Iout = 5 A Iout = 6 A Iout = 7 A Iout = 8 A
Figure 8.16: Measured efficiency versus output voltage, parameterized by output current for an input voltage of 8 V.
Figures 8.16 and 8.17 show the corresponding efficiency versus output voltage for input voltages of 8 V and 16 V, respectively. The input voltage is primarily a function of the number of cells in a given panel, and the voltage range we investigate here (8-16 V) is selected to match that of sub-modules for commercially available solar PV panels.
8.9
Experimental Laboratory Results
In order to properly test the distributed MPPT architecture, the laboratory test setup of Figure 8.18 was constructed. It comprises 18 halogen work lights suspended over the solar panel, with a total electric power output rating of 10 kW. Combined, the lights were able to produce enough irradiation in the wavelengths of interests to approximate the effect of a full sun. For our testing purposes, we need continuous irradiation for long periods of time to evaluate the MPPT algorithms and the power architecture. Conventional test platforms used to evaluate PV panels are typically of the flash type, which only provide a brief light
Solar Photovoltaic Applications 0 2 4 6 8 10 12 14 16 Output Voltage [V] 70 75 80 85 90 95 100 Efficiency [%] Vin = 16 V Iout = 1 A Iout = 2 A Iout = 3 A Iout = 4 A Iout = 5 A Iout = 6 A Iout = 7 A Iout = 8 A
Figure 8.17: Measured efficiency versus output voltage, parameterized by output current for an input voltage of 16 V.
of high intensity (and of a spectrum carefully designed to match that of the sun) to evaluate the instantaneous efficiency and power output of the panel. The test platform of Figure 8.18 is designed only to provide enough total irradiation to test the power electronics, without trying to mimic the spectrum of the sun. In fact, our test setup emits significantly more irradiation in the infra-red regime than the sun, requiring additional fan cooling to keep the PV panel temperature down. In addition, since the test setup of Figure 8.18 was connected to 3-phase AC power with individual lamps allocated to a particular single phase, a 120 Hz AC power ripple could be observed when performing high accuracy measurements of the MPPT output power, so this system is not suitable for measuring very high MPPT tracking efficiency.
Shown in Figure 8.19 is a schematic drawing of a PV solar panel, which indicates the locations of each of the three sub-modules. Also indicated is the shading method used to simulate a cell that performs worse than the others due to issues such as: aging, soiling, shading, manufacturing defect, or external damage.
8.9 Experimental Laboratory Results
Figure 8.18: Photograph of the bench setup for testing of shading effects on solar panel output power. The setup enables repeatable adjustment of light intensity and shading pattern, as well as easy access to measurement instruments.
Solar Photovoltaic Applications
Sub−Module 3
Sub−Module 1
Sub−Module 2
Figure 8.19: Drawing of the solar panel illustrating the physical location of the three sections that are accessible through the junction box (corresponding to the electrical wiring schematic shown in Fig. 8.7a). The bottom right cell in Sub-Module 3 is partially shaded in this experiment. The solar panel used in this experiment was the STP175S-24/Ab01 72-cell monocrystalline Si panel from Suntech
Shown in Figure 8.20 is a power versus string current plot for an experiment when one cell in sub-module 3 (as shown in Figure 8.19) is shaded. Table 8.3 provides the MPPT tracking parameters used for this and all subsequent MPPT tests. The minimum achievable duty cycle step-size with the hardware we implemented was 0.1%, but the 0.6% step-size provided a good trade-off between conversion speed and steady-state accuracy. The effect of the partial shading of a cell in sub-module 3 of the panel can be clearly seen in the reduced output power of sub-module 3. Note also that due to the non-uniform irradiation of our test setup, sub-module 1 produces less power than sub-module 2, even though both of them remain un-shaded in this experiment. The solid turquoise line shows the resulting output power when all three sub-modules are connected in series, as would be done in a conventional solar panel. The effect of the bypass diodes conducting can be clearly seen by the three local maxima in the P-I plot. The maximum output power of the panel configured conventionally is approximately 70 W as seen in the plot.
8.9 Experimental Laboratory Results
Table 8.3: MPPT Tracking Parameters
MPPT Duty Cycle Step-Size 0.6%
MPPT Startup Sweep Step-Size 5%
Minimum Duty Cycle 10%
Maximum Duty Cycle 99 %
ADC Resolution 10 bit
ADC Samples Per Measurement (Overampling) 100
0 1 2 3 Current [A]4 5 6 7 8 9 0 20 40 60 80 100 Power [W]
Max Conventional Panel Power: 70 W Max Distributed MPPT Power: 85 W Improvement with Distributed MPPT: 21 %
50 percent shading of Section 3
Panel Section 1
Panel Section 2
Panel Section 3
All Panel
With Distributed MPPT
Figure 8.20: Plot showing power versus current characteristics of the different panel sections (as shown in Fig. 8.19) under partial shading conditions. In this case, a single cell of sub-module 3 was shaded by 50%. The solid turqoise line shows the maximum output power of a conventional panel, where the effects can be clearly seen in the multiple local maxima (caused by conducting bypass diodes). Also shown is the experimentally-measured output power when the distributed MPPTs of Fig. 8.11 are used, which show increase in output power of approximately 15 watts (21%) for this particular shading scenario.
Solar Photovoltaic Applications
Also shown in Figure 8.20 is the experimentally measured output power when the con- verters of Fig. 8.11 are connected to the panel in a distributed MPPT architecture (as previously described in Figure 8.10b). In this case, the distributed MPPTs allow each of the sub-modules to operate at their individual maximum power points, irrespective of the operating currents of the other sub-modules of the panel. This enables a substantial in- crease in output power, as seen in the plot. We also note that near maximum power can be extracted across a wide range of string currents.
Shown in Figure 8.21 is data from each MPPT during the experiment that generated Figure 8.20. The top plot shows how the load current is stepped in time, and correspond to the discrete current measurements of Figure 8.20. The middle plot shows the corresponding change in duty ratio, as the converters begin with startup sweeps, and adjust their operation each time the load current changes. In this implementation, each MPPT performs two startup sweeps, since the operating points of each MPPT is affected by the other converters. By running two staggered startup sweeps each converter finds the approximate MPP before the perturb and observe algorithm is started. It can be seen in the middle plot that MPPT 2 reaches its maximum duty cycle (1000) first, at a load current of slightly below 4 A. This is consistent with the I-V sweeps of Figure 8.20, which indicates that IM P P of sub-module
2 (the strongest sub-module) is slightly above 4 A. When the string current is below this value, the converter will hit its maximum duty cycle, and no longer operates at the MPP.
The bottom plot of Figure 8.21 shows the output power of each MPPT over time. At each time when the load current changes, the MPPTs adjust their duty cycles to find the new MPP, as can be seen from the increasing ramp waveforms after each current step change. Note also that as the string current is reduced below 4 A, the maximum output powers of MPPT 1 and 2 are reduced, as they cannot operate at their MPP past this point, since their IM P P is higher than 4 A.
A listing of the microcontroller code used for this (and all following) experiments is pro- vided in Appendix I. The Python code used for performing the MPPT algorithm and