In discussing the penetration of PV systems, two different definitions emerge [44]: “Installed penetration“ is the ratio of installed PV capacity to total installed generating capacity of a utility system. Installed penetration changes only when new conventional or PV is installed or removed from service. “Operating penetration” is the ratio of operating PV capacity to total capacity in operation at any specific time. This changes continuously as load and insolation change. With summer-peaking utilities, the load during fall and spring is typically much lower than during summer and therefore the operating penetration of PV will be much higher during spring and fall as compared to summer. Today, the operating penetration has not reached a level significant enough to trigger operational difficulties for
• reverse power flows at the distribution and subtransmission levels, leading to misoperation of protection schemes;
• ramp rates of existing conventional generators not able to cope with rapid changes in PV system outputs due to moving clouds and weather fronts; and
• the need for a larger spinning reserve to maintain overall system reliability and security at desired levels.