7.3 Intermittency - observations and impacts

An example of the impact of solar intermittency on generation output was presented by PEPCO Holdings Inc. for the Atlantic City Convention Centre PV system, as shown in Figure 68 [58]. The red line shows irradiance (scaled) and the blue line shows total system output (in kW) with data obtained every one minute. A direct relationship between solar irradiance and corresponding PV generation output can be seen for the three plots.

Figure 68 PV power output for three consecutive days at the Atlantic City Convention Centre [58]

The net load on a particular electrical feeder, both before and after the installation of a 1.7 MW PV plant, is shown in Figure 69. The variability of net load on a partly cloudy day after the installation of the PV plant, as seen in the middle plot, is much larger than the variability due to load alone, shown in the top plot of Figure 69. It is also worth noting the difference in net load between the clear and partly cloudy days (two bottom plots of Figure 69) with PV having minimal impact on the net load variability for the clear day.

Figure 69 Net load before and after PV on PEPCO feeder [58]

Ramp Rates and Flicker

As the output of any solar system is insolation dependent, curtailment of output is often required to maintain system balance. Figure 70 from [7] shows the kind of occurrences of various curtailment and ramp-up rates for a 500 kW PV system. One-second data was used in this analysis. The penetration percentage is not given here. The highest number of occurrences in the 500 kW system were for ramp rates between 8 kW/sec and 10 kW/sec, which correspond to 0.48 MW/ min and 0.6 MW/min respectively. An interesting comparison between the fluctuations caused by PV due to cloud transits and other loads is shown on a flicker curve for incandescent lamps in Figure 71 [58]. PV sits at around 30 fluctuations per hour with a 0.6% voltage change, just below the boundary of visibility.

Figure 70 Curtailment and ramp-up rates for a 500 kW system [58]

Figure 71 Flicker guidelines [58]

Impacts of increased PV penetration

PEPCO reported that the severity of the impact of distributed PV generation on a feeder depends on a number of factors, including [58]:

  • electrical characteristics of the feeder looking back at the electric system from the location of the DG (distributed generation)
  • daily load profiles during various times of the year
  • maximum output of DG
  • substation transformer ratings
  • locations and settings of regulators, capacitors, and reclosers.

The issues arising from increased PV penetration in Hawaii [52] are:

  • displacement of generation performing critical grid services
  • increased variability making frequency control more challenging
  • lack of visibility and control
  • aggregate loss of PV during faults and contingencies (due to under-voltage and under-frequency)
  • excess energy from non-dispatchable sources.

Over-voltage is seen as a major issue caused by distributed PV generation. An example was obtained from the Ohta City Project in Japan where 73% of possible energy output from a PV system had to be curtailed for the purpose of voltage regulation, as shown in Figure 72 [53].

Figure 72 PV Curtailment Ohta City Project, Japan [53]

In general, the issues Japan is currently experiencing, and expects to see more of with increased PV penetration, are:

  • distribution level
    • voltage and power flow fluctuations
    • electrical islanding
  • system level
    • frequency instability
    • difficulty in scheduling generation
    • transmission expansion requirements for new renewable energy sites.

The reduction in variability from spatial diversity in the Osaka area of Japan is shown in Figure 73 [53]. A significant amount of reduction in variability is seen for spatial diversity of 20 kms or more.

Figure 73 Smoothing effect due to spatial diversity in the Osaka area of Japan [53]

Spain is currently facing a number of challenges integrating RES into their electric system and these include [55]:

  • generation is not well correlated with consumption
  • balancing generation and demand during off-peak hours
  • variability and predictability
  • dynamic behaviour during disturbances
  • provision of ancillary services is impacted with increased non-dispatchable generation.

To solve some of the abovementioned issues associated with an increased penetration of solar power in Spain, the following needs to be achieved [59]:

  • load demand balance will need to be satisfied at the distribution level
  • technical constraints to increased penetration are voltage regulation and reverse power flow
  • new technologies require complex communication and control but may reduce required investment in network reinforcement.