POWERMANAGEMENT
modules have the exact same IV output characteristic and are exposed to the same level of solar irradiation.
Nevertheless, if PV modules’ are different, or current generated by one or more of the modules differs due to shade, the whole string output is impacted because the modules have limited compliance to accommodate any differences between the common string current and their own individual output current. This causes the output voltage from a shaded module to be significantly reduced or even reversed. Protective diodes are placed across the PV modules to prevent this. As a result, the total amount of power produced by the string drops significantly for even one module under partial shade.
Adding a micro-converter to PV modules provides two benefits. It continually operates each module at maximum power point (MPP) by increasing or decreasing its module current so the product of module current and voltage remains at maximum value. The second benefit is how it converts the module’s variable power produced to a variable output voltage, accommodating whatever current is flowing in the series string. This allows the MPP of each PV module to be reflected through the voltage it contributes to the string, irrespective of its IV characteristic or solar irradiance level.
Figure 5 shows a typical micro-converter block diagram that attaches to a single solar PV module. Three principal elements are: H-bridge power circuit, micro controller, and bias supply. The H- bridge power circuit provides a micro-converter the flexibility to operate in both buck (voltage step- down) and boost (voltage step-up) switching conversion modes. The buck leg is formed by the synchronous switch pair, S1–S2, and the boost leg
by S3–S4. When VPV ≥ VO, the buck leg is active. When VO ≥ VPV, the boost leg is active. Whichever leg is active, the opposite side is idle with its top switch permanently turned on.
Controlling the H-bridge is complex. It has to provide a seamless transition between buck and boost conversion modes, each of which has its own distinct control compensation. It must continually monitor the voltage and current at both the input and output terminals. This determines the operating mode, as well as the IV condition that correlates to the module’s MPP. Performing maximum power point tracking (MPPT) makes this an appropriate application for embedded digital control. A micro-controller unit (MCU) is in Figure 5. The power conversion must be done at a very high efficiency. Several methods are used to identify the MPP for a PV module[4]. Techniques include simple constant voltage operation, perturb
& observe (P&O), and methods that calculate the MPP using additional module temperature and solar irradiance measurements. The two most popular methods provide good accuracy using just the VPV and IPV module measurements during normal operation: the optimized P&O and incremental conductance.
Optimized P&O averages several module power samples, while perturbing the module current. It uses this information to identify the operating point and dynamically adjust the perturbation magnitude and direction. This method provides the best performance versus cost.
The incremental conductance method compares
the instantaneous conductance (IPV/VPV) to the incremental conductance (dIPV/dVPV). When the
IPV/VPV = –dIPV/dVPV, the PV module is operating at its MPP. Basically, the system increments and decrements the module current to find the point where there’s a proportionate change in the module voltage. This method offers the best performance at high irradiance levels, and has a fast response to rapidly changing conditions.
Figure 3. IV curve under different
irradiance conditions
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Figure 4. Mismatch at 1015 W/m2
www.solar-pv-management.com Issue III 2010
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