IEEE Power & Energy Magazine - November/December 2015 - 82

This more mature approach to assessing the impacts and benefits
of VG recognizes the ongoing challenges to integration
at increased penetration levels.

not help the system further; this is similar to a thermal unit
running at full power output and therefore unable to provide
up-regulation. This demonstrates that putting wind on AGC
can provide a significant improvement compared to manually curtailing wind. It simultaneously minimizes wind curtailment (note that from 3:20 a.m. to 4:10 a.m., the ACE was
negative and the block curtailment could have ended if the
operator had finer control over block curtailment) and allows
wind to provide effective ancillary services to help balance
the system.
As wind can provide fast up and down responses without
the wear and tear that thermal generators incur, it may be in
PSCO's interest to use wind to provide as much regulation as
possible (when it is curtailed) and use the thermal generators
for any remaining requirements. Wind has been so effective at providing regulation that 2,172 MW of Xcel's wind
generation are now capable of providing AGC when needed.
The industry has much less experience with PVs on AGC,
but in principle the mechanics would be the same. Several
markets [such as the Southwest Power Pool, the Electric Reliability Council of Texas (ERCOT), the California Independent
System Operator, PJM, the New York Independent System
Operator, and the Bonneville Power Administration] allow VG
to provide downward (and upward, in some cases) regulating
reserves, provided they meet eligibility requirements. Many markets
[Independent System Operator
600
Park Potential
New England, Midcontinent Independent System Operator (MISO),
500
At 2:45 a.m., RT
and Independent Electricity SysWind Farm
operator initiates
400
tem Operator of Ontario] still do
curtailment to 300 MW
Metered Generation
due to high ACE.
not allow VG to provide regulation,
300
and some (Alberta Electric System
Operator) have rules (regulation is
At 4 a.m., RT operator initiates
200
procured day ahead, and suppliers
AGC regulation. Note that the
ACE
ACE stays within ±50.
must provide it for 60 min) that
100
preclude VG from providing it.
For regions anticipating high pen0
etrations of VG, it would be wise
to consider making adjustments to
-100
market rules to allow VG (or any
other resource, such as load, as
-200
long as it meets eligibility requirements) to provide regulation.
figure 2. A wind power plant in the Xcel/PSCO area is the first to manually block curThere is a cost and limit,
tailed wind and then put it on AGC regulation. The y axis is in megawatts. The resulting
however,
to using this curtailed
ACE is shown in yellow.
2:25:00 a.m.
2:35:00 a.m.
2:45:00 a.m.
2:55:00 a.m.
3:05:00 a.m.
3:15:00 a.m.
3:25:00 a.m.
3:35:00 a.m.
3:45:00 a.m.
3:55:00 a.m.
4:05:00 a.m.
4:15:00 a.m.
4:25:00 a.m.
4:35:00 a.m.
4:45:00 a.m.
4:55:00 a.m.
5:05:00 a.m.
5:15:00 a.m.
5:25:00 a.m.
5:35:00 a.m.
5:45:00 a.m.
5:55:00 a.m.
6:05:00 a.m.
6:15:00 a.m.
6:25:00 a.m.
6:35:00 a.m.

were remarkably similar. The company chose the wind curtailment protocol, which provides two benefits:
✔ Coal cycling costs have high uncertainty, and reduced
cycling can avoid potential high-impact, low-probability events.
✔ Curtailed wind can provide an upward regulating reserve.
Wind can be curtailed manually through a block curtailment, which is a reduction in wind power plant output by a
fixed amount for a period of time with little if any changes
in wind output. Figure 2 shows wind power plant potential
(pink), actual wind power plant output (blue), and area control error (ACE, in yellow) during a windy night. The ACE
was high (the PSCO balancing authority area was generating more than its load), so, at 2:45 a.m., the system operator block-curtailed the wind from its output of more than
500 MW down to 300 MW. The ACE immediately dropped
toward zero but then dropped too low (PSCO was consuming more than it was generating). At 4 a.m., the operator
put the wind power plant on AGC regulation. The curtailed
wind power plant was able to adjust output up and down as
appropriate to help keep the ACE within specified limits. At
6 a.m., the wind power plant was released from AGC. After
6 a.m., the ACE dropped low again, but at that point the
wind power plant was producing at its potential, and it could

82

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november/december 2015



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