Wind turbine electricity production is
negatively correlated with system load; that is, whenever the system load is
high, wind production tends to be low. The result is that any analysis or
modeling of system performance and capacity credit needs to pay careful
attention to wind-load correlations.

The Bonneville Power Administration (BPA)
conducted statistical analysis of wind production vs ambient temperature in the
Pacific Northwest. BPA wind turbines are located in eastern Oregon and eastern
Washington. The distribution function showed strong negative correlations.
Specifically, whenever the temperature was extreme, the probability that wind
production was less than 10% of nameplate was 80% (see the
paper). Remarkably it did not matter whether
the temperature was extremely high or low, the result was the same. BPA was
less successful with direct wind-load correlation in part because weekends
interrupt load-temperature correlations.

Peak system load is a primary driver of system size
and capital investment. The adjacent figure presents system load during daily peak
hour for PJM-2015 (black dots). [Hourly load data is searched each day to
determine the daily peak hour.]

The figure also presents the daily
residual load peak (red X). Residual
load is the hourly system load minus the corresponding hour wind production. [Published
wind data is scaled so that its annual average is 50% of system load. For each
hour, scaled wind is subtracted from system load, to produce residual load.
Then hourly residual load data is searched each day to determine the daily residual
load peak.] Residual load is the power that is required by backup generators after
wind is subtracted from the primary system load.

From the figure, wind is strong and
variable from January through April. For July and August, there is very little
wind during peak load hours. Indeed there are some days when residual load is
almost the same as system load. The addition of wind does very little to reduce
system peak power requirements and nearly full power system backup would be
required to maintain system reliability.

This analysis is based on published PJM
data. The main assumption is that wind added to bring wind production up to 50%
of total load has production similar to existing wind [this would be strictly true
is wind addition maintained the same footprint, the same location of existing wind
farms]. Wind-load correlations an important part of wind system performance. Many
academic wind system models assume implicitly or explicitly that wind is
independent of load.