by Bob Shively, Enerdynamics President and Lead Instructor
Though in 2012 solar power made up approximately 0.1% of the electric generation in the United States, solar output has increased sevenfold over the last five years. Is it possible that this growth could keep going to the point that solar becomes an important part of our generation mix?
The recent SunShot Vision study by the U.S. Department of Energy (DOE) suggests yes.  Using a model that creates an economic dispatch stack for all types of generation resources, and assuming that current government support such as the production tax credit will phase out as currently scheduled, the study concluded that solar energy could meet as much as 14% of U.S. electricity needs by 2030 and 27% by 2050. It should be noted that for this to occur, significant cost reductions, grid improvement, and regulatory changes will be required. But even less optimistic assumptions for cost reductions in the study still led to solar being 4% to 17% of U.S. generation output. 
Source: Energy Information Administration (EIA)
To think about whether such projections are realistic, it is useful to look elsewhere in the world for models. A good example of an electric grid where solar power has become a large contributor is Germany, where during some hours solar power already contributes as much as 40% of peak power demand and by 2012 was about 19% of Germany’s installed capacity.  This rapid growth in solar power was stimulated by significant government subsidies, but the point of this article isn’t to discuss whether or not subsidies are merited. Rather the point here is to examine the technical feasibility of such a large percentage of solar power connected to our grid.
Source: BP Statistical Review of World Energy 2012, except for 2012 which is taken from IEEE article cited in references
Issues with connecting solar power
Key needs from transmission and system operations standpoints include:
- increased flexibility of non-solar generation (to handle supply variability due to movement in cloud cover);
- increased size of the operational and planning area so that geographical diversity smooths variability of output;
- new transmission construction to bring in centralized solar energy from remote locations;
- and potentially the addition of storage.
The impact of these needs have been well studied in the U.S.  with the conclusion that addressing the issues is feasible.
Less studied are the impacts on the distribution system for solar photovoltaics (PV) connected to the local grid. Issues include reverse flows in the distribution system, flows from distribution into transmission, and local grid stability. When solar output on a specific distribution circuit exceeds load on that circuit, electricity flows back into transformers and substations. Often the equipment has not been designed for these flows, and equipment trips and/or damage can result. In extreme cases, reverse power flow can even result in power flowing from a distribution substation into the transmission grid. In almost no cases are systems in the U.S. currently designed for such flows. And with large flows on local grids, frequency or voltage instabilities can occur, especially since most distribution lines are designed with the assumption that voltage falls as the line gets further away from the substation. Such assumptions no longer work if PV systems are injecting large amounts of power along the distribution line.
Solutions to large amounts of distribution PV
So what has Germany learned about how to handle large amounts of distributed PV?  Some problems can be solved by redesigning and modifying the distribution system to handle reverse flows. This usually involves replacing transformers and/or reinforcing distribution lines. But in some cases upgrades may not be the most economic solution. Many of the issues can be dealt with by power conditioning at the PV source.  For instance, interconnection requirements in Germany mandate that PV systems support a smooth response to frequency deviations through electronics installed on the PV system. Also, PV systems must be able to control active and reactive power output to help support local voltage.
To improve performance, distribution companies are experimenting with a number of potential solutions that may include decentralized or centralized control strategies that optimize through communication among multiple PV systems. Optimal solutions are an issue for ongoing development. But the conclusion is that the German grid has, and will continue to, work with high penetrations. It just takes a lot of distribution engineering to pull it off.
What this means for the U.S.
The U.S. distribution system is designed a bit differently than systems in Europe. For instance, the U.S. tends to locate transformers closer to provide residential power at a lower voltage. So solutions developed in countries such as Germany will have to be reviewed. But there is no reason to think that what Germany can do, the U.S. can’t. If economics do result in a significant build out of solar power, there is reason to believe that hard work by distribution engineers can result in a system that continues to be highly reliable.
4. See for instance, Transmission System Performance Analysis for High Penetration Photovolatics, available at http://www1.eere.energy.gov/solar/pdfs/42300.pdf and Impact of High Solar Penetration in the Western Interconnection, available at http://www.nrel.gov/electricity/transmission/western_wind.html