Most of today's solar photovoltaic (PV) power sources are utility connected. About 75% of these installations are residential rooftop systems with less than 2 kW capability. These systems typically comprise a number of PV modules arranged in series configuration to supply a power converter, commonly called an inverter, which changes the direct current (DC) from the modules to alternating current (AC) to match the local electrical utility supply.
There is a difficulty with small solar power systems on residential rooftops. Gables and multiple roof angles make it difficult on some houses to obtain enough area having the same exposure angle to the sun for a system of 2 kW. A similar problem arises where trees or gables shadow one portion of an array, but not another. In these cases the DC output of the series string of modules is reduced to the lowest current available from any cell in the entire string. This occurs because the PV array is a constant current source unlike the electric utility, which is a constant voltage source.
An inverter that economically links each PV module to the utility grid can solve these problems as the current limitation will then exist only on the module that is shaded, or at a less efficient angle and does not spread to other fully illuminated modules. This arrangement can increase total array output by as much as two times for some configurations. Such a combination of a single module and a microinverter is referred to as a PV AC module. The AC output of the microinverter will be a constant-current AC source that permits additional units to be added in parallel.
PV AC modules now available suffer poor reliability owing to early failure of the electrolytic capacitors that are used to store the solar cell energy before it is converted to AC. The capacitor aging is a direct consequence of the high temperature inherent in rooftop installations.
The electrolytic capacitors in the power circuit perform two functions. First, the capacitors hold the output voltage of the PV modules close to the maximum power point (MPP) output despite variations in sunlight, temperature or power line conditions and second, the capacitors store energy at the input and even out the DC voltage variations at the power-line frequency that result from changing the DC to AC. These functions place an additional stress on the capacitor causing internal heating that adds to the effects of high external temperature.
The high temperature reached by PV system inverters is a natural consequence of their outdoor mounting. This requires a rainproof enclosure that complicates the heat removal process. The coincidence of maximum power throughput and losses at exactly the time of maximum heating by the sun on both the enclosure and the ambient air exacerbates the condition.
Existing inverter topologies have made the electrolytic capacitor an integral part of the inverter circuit because of the high capacitance value required to store energy from the PV module. If high capacitance is required, the only economic choice is the electrolytic capacitor. Plastic film capacitors are recognized as superior in aging characteristics, but are much more expensive for the same capacitance. Thus, a means to avoid use of electrolytic capacitors can contribute to the reliability of PV power sources.
FIG. 2 illustrates the control system for a conventional photovoltaic (PV) DC-to-AC power converter. This power converter has a pulse width modulated, voltage regulating boost stage and a pulse width modulated, current regulating buck stage. Sinusoidal reference 62 follows AC line 90 voltage and frequency. AC line current reference 63 is generated by multiplying sinusoidal reference 62 by scaling factor 66. Actual AC line current 64 is compared to AC line current reference 63 to create error signal 65. Error signal 65 drives buck stage 60 as part of this servo loop. Current 41 and voltage 42 of PV source 10 are sensed and multiplied to provide 43, a measure of PV source 10 output power. Scaling factor 66 is periodically adjusted to determine the amount of energy sourced onto AC line 90. A control means is used to periodically perturb (45) scaling factor 66 and observe (44) the effect on PV output power 43. If an increase in scaling factor 66 results in an increase in PV power 43, scaling factor 66 is incrementally increased every perturb cycle until an increase in scaling factor 66 results in a decrease in PV power 43. This is how the maximum power point (MPP) of PV source 10 is established. Boost stage 40 is transparent to this perturb and observe function and serves as a typical voltage regulator to maintain the voltage at energy storage capacitor 50 at a regulated voltage higher that the peak voltage of the AC line. Fixed reference voltage 48 is compared to feedback voltage 49 creating error signal 47 to drive boost stage 40. In some inverters designed to work with PV voltages higher than the peak AC line voltages, boost stage 40 is not required.
The problem with this prior art control method is instability and poor dynamic response. If current reference 63 requests a current and therefore power to be delivered into the AC line that PV source 10 cannot supply, the control loop becomes unstable, PV source 10 voltage collapses and cannot be recovered without restarting the power converter and the perturb and observe process. This prior art control method is unstable when operating on the lower-voltage side of the PV source maximum power point. The maximum power point of a photovoltaic source usually changes slowly but moving cloud cover, wind gusts and partial, momentary PV source shadowing can abruptly push the maximum power point into an unstable region for this control method.
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