IEEE Power & Energy Magazine - November/December 2015 - 81

There can be times when the system is constrained
with too much generation and not enough demand
and/or transmission to move the energy.

System Load (MW)

minimum output level and additional wind energy is available,
wind energy must be curtailed or
a base-load unit must be decommited. The economically rational
solution is usually to decommit
the base-load unit because it burns
fuel and wind energy does not.
However, if the base-load unit has
a long shutdown/start-up period,
it may be impossible to decommit
0
4
8
16
12
20
24
the unit and have it available, if
Time (Hour of day)
needed, the next day.
Wind down-ramps can also be
challenging because the operator needs adequate up-reserves
Day
Tens of Minutes
Seconds to
to compensate for the reduced
to
Hours
Minutes
generation. Wind forecasting can
Scheduling
Load Following
Regulation
often predict down-ramps but
there may be a timing error (the
down-ramp may occur sometime
before or after its prediction).
The geographic diversity of wind
is very helpful for smoothing out
these issues. On the other hand,
solar output is easier to predict-
Days
we know when the sun will rise
Unit Commitment
and fall every day, although partly
cloudy days can make forecastfigure 1. The demand for electricity is constantly changing, so operators have proing difficult. The geographic dicesses that allow generation output to change while maintaining the required balance
versity of solar helps with variabilbetween generation and load.
ity caused by cloud movement,
but the sunrise and sunset ramps
require power system operation throughout much wider geo- solar energy penetration on its system. With a 33% renewgraphic regions for mitigation by diversity. Managing large able portfolio standard by 2020, these levels are expected
amounts of PV generation requires being able to manage the to increase. This system is fairly small; it has a peak load of
sunrise and sunset ramps while reducing other generation 7 GW, and more than half of the energy comes from coal.
midday to accept the PV generation.
It can be challenging to balance a small system with large
amounts of coal generation and high VG penetrations. For
example, PSCO reported that wind served more than 50%
VG Can Provide Regulation/AGC Services
More sophisticated approaches have emerged and been of the load during 6% of the hours in October 2014, and it
implemented in recent years to reduce the incremental bur- reached a peak of 60% on 24 May 2013, between 1 and 2 a.m.
den that VG may bring to system balancing. For example, When loads are low at night and wind is high, PSCO has to
wind power plants currently provide regulating reserves decide between deeply cycling their coal-powered plants or
in the Xcel/Public Service of Colorado (PSCO) balanc- curtailing wind power output. In 2011, PSCO quantified the
ing authority area. In 2014, PSCO had 19% wind and 1% trade-offs between these options and found that total costs
november/december 2015

ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015

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