Environmental Engineering Reference
In-Depth Information
selected days are July 2, 2008, and September 29, 2008.
6
Using the hourly data
for days, it is possible to examine in detail how coal, gas, and wind interact
and the resulting emissions implications.
July 2, 2008, Wind Event
The first wind event began at 4:15 a.m. and continued through 7:45 a.m.
on July 2, 2008. During that period, total wind generation jumped 400%
from approximately 200 MW to approximately 800 MW over a 90-minute
interval, then dropped back around 200 MW in the next 90 minutes. This
event is depicted in the PSCO training manual, shown in Figure 2.7. Coal
generation is shown in the light dotted line, wind generation in solid dark
line, and gas in light solid line. The jagged black line illustrates the area con-
trol error (ACE) used by the National Electric Reliability Council (NERC) to
measure system reliability. ACE measures too much or too little power on
the system to safely serve total load. In short, it is a measure of reliability. As
wind comes online rapidly, ACE spikes upward. Coal generation must be
decreased to bring the ACE measure down to the appropriate level.
At the beginning of the event, gas-fired generation accounted for approxi-
mately 400 MW or 10% of total load. Coal-fired generation accounted for 2,500
MW or 60% of total load. When the wind commenced, PSCO had to curtail
generation at its coal or gas plants to accommodate the incremental wind
generation. As shown in Figure 2.7, PSCO chose to curtail generation from
coal rather than gas. The motivation for this approach is not clear, but the
most likely explanation is that the gas units were operating at near-minimum
levels and could not be curtailed further without significant risk to the facili-
ties. To maintain the system margin standards required by NERC, the sudden
availability of wind forced PSCO to decrease total coal generation from 2,500
to 1,800 MW, then, back to 2,500 MW in a matter of 180 minutes.
To draw coal-fired generation down, PSCO cycled its Cherokee, Pawnee,
and Comanche plants. Figure 2.8 shows the hour-to-hour changes in gen-
eration between 4:00 and 5:00 a.m. on July 2, 2008. All PSCO's plants can
increase or decrease generation from hour to hour; this hour-to-hour change
is known as ramp rate.
As noted earlier, exceeding the designed ramp rate places significant stress
on the equipment, renders operations unstable, and potentially shortens
equipment life. The hour-to-hour changes shown in Figure 2.8 are compared
to the published design ramp rates for PSCO's coal-fired plants as shown
in Table 2.1. Cherokee's performance during the incident was within its
designed ramp rate; Pawnee operated outside its design rate.
Ramp Rates for Selected PSCO Plants
Operation of the Cherokee coal plant during this wind event illustrates the
emission impacts on cycling coal units. The Cherokee plant was chosen due
