FROM CELLS TO MODULES
The voltage output from a single crystalline silicon solar cell is about 0.5 V. Current is directly proportional to the cell's surface area, and designers count on about 7-A maximum output for a 6-in.2 multicrystalline cell. The typical panel module has 30 to 36 cells that are wired in series to generate a nominal 12-V output, which is actually about 17 V at peak power.
Because the cells are connected in series, and cells that are partially or completely shaded by tree branches, chimneys, or even guy wires have a high internal resistance, shading is a problem for photovoltaic modules. If even one full cell is completely shaded, the output voltage of the array will drop to half of its unshaded value to protect itself. If it didn't, the cell would be destroyed by the need to carry the current produced by the rest of the array while in its high-resistance state. If enough cells are shaded by nearby objects that throw hard-edge shadows, the whole module will drop out.
To deal with the shading problem, some modules place bypass diodes across each cell to carry the current when the cells themselves cannot. Otherwise, they would dissipate power from the unshaded cells. A separate blocking diode in each array in a parallel configuration of modules could isolate that panel if it becomes severely shaded. It would prevent other panels' current from flowing through it and being turned into heat.
INVERTERS
An inverter is only required to be totally disconnected from the utility for service or maintenance. At all other times, whether the power circuit to the utility is open or not, the control circuits remain connected to the mains to monitor that voltage. To completely disconnect the inverter from the utility for maintenance, one must open the ac-disconnect switch required by the National Electrical Code (NEC).
Modules and inverters can be configured in several ways. With a central converter, modules are connected in series strings, and the strings are paralleled and connected to a single converter. A more efficient approach uses separate string inverters for each series string of modules, with the outputs of the string inverters paralleled. Large systems that have shading concerns may include a small inverter for each module, with their outputs connected in parallel.
One of the most critical considerations in inverter design is what to do about "islanding." An island is what happens when there's a utility blackout and the distributed-resource system is still generating power. An unexpected island can kill a utility worker or damage equipment.
Most inverters today are certified as "non-island inverters." They cease to energize the utility line within 10 cycles if the real component of the power load seen by the inverter goes to less than 50% or more than 150% of its output, or if the islanded load power factor goes below 0.95, either leading or lagging.
POWER OUTPUT SPECIFICATIONS
An inverter must synchronize its phase with the utility, as long as the utility line frequency is within a range of 59.3 to 60.6 Hz. If the utility frequency exceeds those limits, the inverter must disconnect itself within six cycles. The standards allow anything down to, but not including, 0.85 PF. They also acknowledge special cases where an inverter's design may be greater to provide reactive power compensation.
Today's inverters switch at frequencies as high as 14 kHz, so their output is much closer to a pure sinusoid. However, the standards do require less than 5% total harmonic distortion and provide tables of distortion limits for harmonics even beyond the thirty-third.
IEEE 929-2000 informs readers that most grid-tie inverters are designed as current sources. They use the utility voltage as a reference and supply the current available from the photovoltaic array at whatever voltage and frequency presented by the utility.
MPP
One area not covered by the standards concerns the inverter's task of maintaining the module string that its connected to at the maximum power point (MPP). A PV module can be operated at any combination of current and voltage found on its characteristic curve. But the inverter determines the point on the curve at which it operates, and the target is generally the MPP (though it can sometimes be the maximum voltage point). To do this, the inverter self-calibrates the panels connected to it and determines the MPP empirically (Fig. 4).
METERING AND MONITORING
Although present incentives are based simply on installing PV systems, RWE Schott's Dan Magni says that incentives are about to move to algorithms based on meeting production quotas. Thus, measurement and monitoring become particularly critical in evolving designs. While some inverter systems monitor and record data, there aren't any consistent standards yet (Fig. 5). When asked for their wish list of future enhancements, Magni and McGoney both responded that such standards took the top spot, even ahead of higher cell efficiency.