IEEE Power & Energy Magazine - November/December 2015 - 83

energy to provide system support. Wind curtailment is
achieved by either automated market-based mechanisms,
such as the MISO dispatchable intermittent resource (DIR)
economic dispatch or through manual directives by the system operator. Usually the most expensive plant is curtailed
first to alleviate system congestion or maintain system balance. The economic compensation provided to the wind
power plant varies and depends on the specifics of the power
contract. Output from individual turbines can be reduced
in seconds to provide downward reserves and ramping support. Output from curtailed wind turbines can be increased
quickly, on the order of seconds to tens of seconds, to provide frequency, upward reserve, and ramping services.
Curtailment from PVs is achieved by controlling inverter
output, and therefore it acts on very short timescales of fractions of seconds. This turns wind and PVs into dispatchable
resources, but it is at the economic cost of reduced energy
output. Therefore, the economic choice to curtail VG at any
given moment reflects the trade-off between the instantaneous value of the energy produced and the value of upward
reserves provided. This economic trade-off is further complicated for wind (in the United States) for projects utilizing
production-based subsidies. However, some positive feedback may exist. For example, thermal generators that stay
online to provide upward reserves and subsequently force
wind to curtail could instead be decommitted if the curtailed
wind provides the upward reserves.

VG Can Be Economically Dispatched
There can be times when the system is constrained with too
much generation and not enough demand and/or transmission
to move the energy. Managing this type of overgeneration

condition is most effective if the solution is incorporated
directly into the economic dispatch process, which (under normal conditions) requires no direct operator intervention and
can be done very quickly and cost effectively. MISO manages
more than 14 GW of wind generation in its market footprint.
The continued growth of wind generation, especially in the
western areas of its territory, has created local transmission
congestion issues during certain periods that were traditionally managed by manual curtailments of specific wind generation facilities. In 2011, MISO implemented the DIR protocol,
which effectively places wind power plants on AGC under
control of the MISO real-time market systems. This resulted
is an overall reduction of curtailed renewable energy delivery
along with a much higher level of operational efficiency and
transparency. Figure 3 shows the monthly downward dispatch
of energy as a percent of the economic maximum. According
to MISO, approximately 95% of wind energy's potential can
be captured through economic dispatch. All new wind generation facilities in MISO must register as DIRs, and more
than 80% of wind generation in MISO is dispatchable.

VG Can Contribute to
Operational Security
As the instantaneous penetration of VG resources increases,
there is greater interest in wind's potential impacts on maintaining the operational stability and security of the bulk
power system during and after major disturbances. Although
this has not been studied as extensively as balancing issues
or widely incorporated into system operation, there is
increased understanding that VG can provide frequency,
inertia, and voltage control capabilities that have been traditionally provided by synchronous generators.

3,500

20
DIR Dispatched Energy (Left Axis)
DIR Dispatch Below Economic Maximum (Left Axis)
% of DIR Dispatched Down (Right Axis)

18
16

GWh

2,500

14
12

2,000

10
1,500

8
6

1,000

Dispatch Down (%)

3,000

4
500

2
Apr. 2013

Mar. 2013

Jan. 2013

Feb. 2013

Dec. 2012

Oct. 2012

Nov. 2012

Sep. 2012

July 2012

Aug. 2012

May 2012

June 2012

Apr. 2012

Mar. 2012

Jan. 2012

Feb. 2012

Dec. 2011

Oct. 2011

Nov. 2011

Sep. 2011

July 2011

Aug. 2011

0
June 2011

figure 3. Wind power plant dispatch in MISO with DIR protocol.
november/december 2015

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