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. [1] 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. [2]

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. [3] 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. [4] 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? [5] 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. [6] 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.

References:

1. http://www1.eere.energy.gov/solar/sunshot/vision_study.html

2.  For a good discussion of the SunShot Vision study, see IEEE Power and Energy  magazine, March/April 2013, pgs. 22-32.

3.  See IEEE power & energy magazine, March/April 2013, p. 55-6.

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

5.  For more details, see the IEEE power and energy magazine article previously  cited.

6.  By  power conditioning, we mean transformation of power from one  voltage/current/frequency/wave form to a different one

Related articles

• German Renewables Reach 25 Percent (spectrum.ieee.org)
• Utilities Facing a “Mortal Threat” From Solar (blogs.wsj.com)