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Increasing solar generation reduces the net load especially during periods of relatively low loads and high generation such as in Spring. CAISO has produced a plot of “net load” which is the difference between forecasted load and expected electricity production from variable generation sources such as solar PV systems. In certain times of the year, these curves produce a “belly” like appearance in the mid‐afternoon that quickly ramps up to produce an “arch” similar to the neck of a duck – hence the popular industry given name “The Duck Chart” shown in Figure 1.13 [51]. The more solar generation increases, the more the curve looks like the belly of a duck. This problem mainly occurs during springtime when the weather is sunny but still cool. At this time, solar generation is high but electricity demand is less as air‐conditioning loads are low [52].

Figure 1.13 California ISO (CAISO) Duck Chart.

Source: CAISO [52]. Licensed with permission from the California ISO. Any statements, conclusions, summaries, or other commentaries expressed herein do not reflect the opinions or endorsement of the California ISO.

This behavior of net load creates special problems for system operator to balance generation and load. As the sun sets and the solar generation reduces to zero, a significant net demand ramp (the arch of duck’s neck) is experienced by the system operator. It must, therefore, dispatch significant generation resource to meet the demand ramp. Further, as the net demand decreases in late evening, when a downward ramp is experienced the system operator must rapidly reduce or shut down the generation to offset the downward ramp.

Another major problem is the oversupply during midday which may lead to the system operator curtailing solar power generation thus reducing its economic and environmental benefits. Several mitigative measures which minimize the risk of overgeneration are proposed in [51].