Accommodating High Levels of Variable Generation - NERC

Transmission Planning & Resource Adequacy
While planners are accustomed to accounting for conventional generating units which may be
forced out of service, they also must consider the additional uncertainty in available capacity
when a large portion of the total supply portfolio is supplied from variable generation.
Traditionally (and primarily for simplicity), resource planning has been a capacity-focused
process. However, with high penetrations of variable generation resources in the system, existing
planning methods will have to adapt to ensure that adequate resources are available to maintain
bulk power system reliability.
The calculation of the capacity contribution of conventional generating units to reserve margins
is somewhat straightforward, based on the unit performance rating, forced outage rate, and
annual unforced maintenance cycle. However, the capacity contribution of variable generation is
not intuitive due to its inherent characteristics of variability and uncertainty.
Current approaches used by resource planners 53 fall into two basic categories:
• A rigorous LOLE/LOLP - based calculation of the Effective Load Carrying Capability
(ELCC) of the variable generation relative to a benchmark conventional unit; and
• Calculation of the capacity factor (CF) of the variable generation during specified time
periods that represent high-risk reliability periods (typically peak hours).
The ELCC approach considers all hours in a given planning period (typically a year) and the
contribution of the variable generation output to capacity requirements during all time intervals
of that period. ELCC calculations are typically conducted through reliability simulations that
consider conventional generating outage and maintenance characteristics and the hourly annual
demand shape. In order to appropriately consider the capacity contribution of variable
generation, the output of the variable generation should be represented by hourly primary fuel
(e.g. wind or solar) data and characteristics of the generator. Care should be taken to account for
the correlation between hourly variable generation and the hourly demand series. To perform
this analysis, a significant amount of time-synchronized 8,760 hourly wind generation and
demand data is required and this data is needed for variable generation plants in the specific
geographic regions being studied. Further, in the near-term, this data will also be required for
variable generation plants that are yet to be built. Currently, for wind generation, the best
approach for obtaining such data is through large-scale Numerical Weather Prediction (NWP)
models. While limited efforts at validating NWP models for specific regional studies have shown
that these models can provide good representations of wind output and variability, work is ongoing
to validate these models for broader use. At the same time, the implementation of the
53 Load Serving Entities (LSEs), Independent System Operators (ISO) and Regional Transmission Operators (RTO)
Accommodating High Levels of Variable Generation 38