POWERMANAGEMENT
1000 W/m2 and 25o C. AC power from the inverters is sent back to the local grid. The behavior of each module can be monitored for MPP operation.
Figure 2 shows an IV (current voltage) curve and a PV (power voltage) curve generated from one of the modules (SP08) for an irradiance condition of 1050 W/m2. The MPP is located close to the knee of the IV curve where the current and voltage product is maximized. Changing the irradiance
condition reduces the ISC and, to a lesser extent, the VOC (see Figure 3). Most commercial IV units take two to three seconds to sweep [3] This
prevents any false peaks and valleys in the curve that could occur from any moving cloud condition.
Module to module mismatch
Figure 1. PV array and testing block diagram
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power delivery occurs at the point of the IV curve where the IV product is maximized. This is the maximum power point (MPP). Assuming one MPP for a string may be incorrect due to mismatches and varying irradiance from module shading. Module electronics can address this performance mismatch. To design and build the module electronics optimally, it is important to characterize and understand the behavior of PV module parameters under different irradiance and ambient conditions.
Figure 2. IV and PV curve for a PV module shows maximum power delivered for that condition
Photovoltaic array configuration
Before understanding the behavior of the module parameters, you must understand the PV array configuration (see Figure 1). Eight modules (SP01–SP08) are connected in series that are connected to a string inverter 1. Similarly, another eight modules are connected to a string inverter 2. All sixteen modules are made of mono-crystalline silicon solar cells from the same manufacturer, and rated at 215 W at standard operating conditions of
Figure 4 shows the mismatch for all modules for a given irradiance condition. The mismatch varies from –0.9% to +1.25%. A static or passing cloud or any other condition could increase the mismatch. This is where module electronics can pay off. Under no mismatch condition, total maximum power from an array (eight modules connected in a string) is equal to the sum of the individual module power. Without module MPP and mismatch condition, the maximum power delivered by the array is less than the sum of the individual module’s maximum power because some modules operate below their maximum power point. Under irradiance condition of 1015 W/m2, taking the array connecting modules SP01–SP08, the mismatch is 2.4 percent, or a loss of 37W.
Module Electronics Advantages
Many small and some large companies use module electronics to improve energy performance and monitor / manage the solar module. Module electronics fall into two categories: micro-inverters (dc/ac), or micro-converters (dc/dc). A micro- inverter converts the power so it can be applied directly to the ac-line to connect to the utility grid. Alternatively, a micro-converter converts the variable current from the PV module to match the common output current shared with other modules in a series string connection. The combined string output is used by a larger string or central inverter to deliver ac-line power to the grid. For either a micro-inverter or a larger central inverter, an operating grid offers infinite demand. Irrespective of the module electronics type, the principle is the same: operate each individual solar module at an IV point that delivers maximum electrical power for a given level of solar irradiation. PV modules produce a relatively constant output voltage for a given temperature. However, their output current varies with irradiation. When connected in a typical series string configuration, the string current is constant for each module. This is not a problem, if all the
www.solar-pv-management.com Issue III 2010
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