Strawman Proposal for a Taxonomy of Renewable Penetration Levels

This strawman taxonomy provides clear distinctions between different levels of renewable penetration.  By basing the criteria on the architecture and control of the power system, it avoids the ambiguity that is inherent in a taxonomy based on the percentage of renewable penetration or contribution.  Not only is the renewable penetration or contribution subject to multiple definitions, but it can vary widely depending on the load shape, characteristics of specific components, and the resource, which can also vary from year to year.

Low Penetration: In a low penetration hybrid power system, the solar/wind contribution has no effect on the rest of the power system. Solar/wind power is delivered on an as-available basis with no controls beyond the minimum protection devices required for safe operation. The variability introduced by the solar/wind power is on the same order as the variability of the load.

As soon as the solar/wind contribution reaches a point where it is necessary to either curtail some solar/wind output or “dump” some of the power through a secondary load then it is a Medium Penetration system.  Simple controls are necessary to maintain power quality and enforce the minimum load and ramp rate constraints of the conventional fossil generators in the system.  In a medium penetration system, the operation of the conventional fossil generators is not altered by the presence of the solar/wind components. The use of a generator redesigned to accommodate low load operation can increase the contribution of renewable power in a medium penetration system.

High Penetration systems achieve O&M savings in addition to fuel savings by reducing the run-time of one or more of the conventional fossil generators. This requires the system operator to have confidence that additional generation can be brought on-line fast enough to accommodate a drop in the renewable resource.  Typical ways to accommodate this are through storage, load management, a large amount of excess renewable output, or some combination of these approaches.

Very High Penetration systems are able to create a stable grid without the rotating mass of a conventional fossil generator.  These very high penetration systems can run in “diesel-off” mode.  Any small high penetration system that does not synchronize multiple generators in parallel operation qualifies as very high penetration under this criterion.

There is increasing discussion of 100% Renewable Systems. These systems either have an extremely large amount of storage, very aggressive load management, or use a renewable resource other than solar or wind, such as hydro, geothermal, or a backup generator using biofuels.  Very small systems and systems dedicated to specific loads like water pumping are the most likely candidates to be 100% renewable.