Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
Several forms of solar thermal generation, including dish-Stirling and “solar chimney” projects,
have been proposed for utility scale application. Proposed dish-Stirling projects are a collection
of thousands of individual turbine-generators with individual ratings from 10-50 kW. Several
projects have been proposed to be as large as 300 MW in terms of collective plant output. The
ramping characteristics of dish-Stirling plants are expected to be similar to those of PV as the
inertia of an individual Stirling engine is considered nearly zero, though there is some energy
stored in the rotating mass of multiple turbine generators. It is unknown whether the large
geographic areas (one square mile or more) will reduce the ramp severity for the collective
output of a fully deployed Stirling project. The “solar chimney” is expected to yield a solar plant
with a 75% capacity factor with essentially zero variability in minute-to-minute output. Turbine
generators for solar chimney are being developed using existing designs for large hydro plants.
2.4.2.2. Photovoltaic (PV) Technology
PV technology converts the electromagnetic energy in sunlight directly into direct current (DC).
PV (except for concentrating PV) can use both diffuse solar radiation and DNI. As a result, PV
installations are deployed throughout North America and are not limited to regions with superior
DNI resources such as the southwestern U.S., southern California and northern Mexico. PV does
not require larger plant sizes to achieve economies of scale and is often deployed as distributed
generation.
In order to interconnect with the AC power system, a PV system must use a power electronic
inverter (much like wind turbine generators Types 4) to convert its DC output at the terminals of
the PV panel into AC. As with solar thermal there are many forms of PV. This section describes
technical characteristics that are applicable to all forms of PV.
The nature of PV is such that PV does not involve a rotating mass and therefore does not have
inertia. 36 As a result, operating PV systems have demonstrated the potential for substantial
ramps during partially cloudy days. PV systems can experience variations in output of +/- 50%
in to 30 to 90 second time frame and +/- 70% in a five to ten minute time frame. Furthermore,
the ramps of this magnitude can be experienced many times in a single day during certain
weather conditions. This phenomenon has been observed on some of the largest PV arrays
(ranging from 3-10 MW) deployed in the U.S. located in Arizona and Nevada. Figures 2.11 and
2.12 demonstrate the potential for significant ramps in output from a PV plant located in
Nevada. 37
36 Energy storage such as batteries can be added to PV however the inertial response of a PV plant will be driven by
characteristics of the inverter.
37 NV Energy (former Nevada Power Company), Renewable Energy Department.
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