3.6 Summary

The current state of research worldwide on renewable generation intermittency, including centralised and distributed PV, concentrating solar thermal and wind, is summarised in this chapter. One of the main challenges to the power system is associated with the instantaneous penetration of intermittent solar generation. Solar generation is viewed as negative load. When this is combined with the actual system load it yields a ‘net load’, which corresponds to the power that must be supplied by other resources on the system. Of particular concern to utilities is load and solar power moving in opposite directions at the same time creating large changes in the net load. A study on the Californian grid reported an increase in net load variability, especially during low load periods, with the integration of 33% intermittent renewable energy penetration. Various other studies concluded that a high penetration of intermittent generation results in greater variability in the net load compared to the variability in the original load alone without solar or wind. In addition to the magnitude of net load variability, data on ramp rates of such variations would assist in determining the degree of flexibility required in the network to compensate for the variability of PV systems. Studies on the effect of spatial diversity on intermittency seen in output of PV systems showed reduced variability when the aggregate of multiple PV plants and also output of large-scale PV plants were considered.

The impacts of PV power plants are associated with voltage profiles, electrical losses, power factor, capacity planning, power quality, systems operations and protection. Normal functions of conventional generation operations that could be impacted by high penetration solar intermittency, which may need more conventional generation units to be brought on-line or put on regulating duty thus increasing system operating cost, include:

  • load-frequency control
  • load following
  • ramping rate of on-line generators
  • unloadable generation to accommodate maximum output from intermittent generating technologies
  • operating reserve.

There is very little published literature which discusses observed impacts of high penetration solar intermittency. The majority of work discovered focussed on modelling impacts rather than actual observation of impacts. Studies have shown that adequate system flexibility is a key requirement for managing increased levels of intermittent renewable generation and that conventional generators are forced to be more flexible with their output, resulting in a higher per unit cost. A case study at Gardner, Massachusetts, analysing the impact of a passing cloud on the irradiance and net load of the site (53 residential properties and 28 PV installations) shows how rapidly the net load of a system can vary significantly. Large variations in PV output for systems with high penetration PV will result in proportionally large variations in net load puffing added pressure on conventional generating resources on the system to vary their output rapidly. Existing methods for managing large-scale intermittent generation might not be sufficient. A good example of output variations that can be expected from a large-scale PV system can be seen from the output of a large existing array in the US, a 4.6MW PV system located in Springerville, Arizona. The PV output data was sampled every 10 seconds and large abrupt power output drops, from about 4000 kW to 500 kW, were seen to occur over extremely short timeframes. It is suggested in the literature that cheaper less flexible plants will need to be replaced with more flexible expensive plants to accommodate high penetrations of solar generation. Otherwise, a significantly larger amount of ancillary services or additional generation may be required to manage PV power output fluctuations.

A number of studies looked into the likely impacts of increased levels of intermittent renewable generation (IRG), estimating impacts through simulation and modelling. The predicted impacts of high penetration intermittent renewable generation include the displacement of conventional generation units, the need to curtail renewable generation output, load frequency control and costs associated with these. Some general conclusions can be drawn from these studies:

  • The amount of solar generation that can be integrated into the utility power system without compromising grid stability and reliability varies widely. The determining factors are the amount of a utility’s load fluctuation and the regulating capability of existing conventional generating units. This observation indicates the effect of solar generation intermittency on the power system is not uniform and is case sensitive. Hence, a general cause-effect conclusion cannot be drawn.
  • Although high penetration levels of solar generation have the potential to cause adverse network impacts, corrective measures are available, such as assigning more generating units to regulating duty or installing fast-response combined-cycle generators. These measures can be effective if carefully planned. High penetration limits have been shown to be possible (in simulations) after adding fast-response, combined-cycle generation units to the existing generation mix. Another method is to control the solar generation output under intermittent cloud coverage during periods of peak system demand when the network has fewer generating units on standby and less on-line regulating capacity. These corrective measures, however, may cause the system to deviate from its optimal operating condition, thus adversely affecting the economics of solar generation.

The successful integration of large amounts of intermittent solar generation depends highly on the essential element of accurate forecasting. Forecasting at a range of timescales is necessary. More accurate day-ahead prediction of renewable resources is required for more accurate unit commitment. Forecasting at short timescales is needed to predict rapid power dips in the solar system output, while numerical weather models can be used to predict insolation out to a number of days. Increasing system flexibility or decreasing the flexibility requirements of the system is another important determinant for increasing levels of intermittent generation. Some ways by which the system flexibility can be increased include balancing the generation portfolio, introducing more flexible conventional generation and redesigning the power system to enable it to handle reverse power flow from distributed PV. Net load variability can be reduced in order to reduce the required flexibility of the system, the literature indicates, by such means as the use of energy storage, load control, increased control and communication, ability to curtail intermittent generation and spatial diversity of the resource.