J. McCalley

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J. McCalley

• Power System Operation, and Handling Wind
  Power Variability and Uncertainty in the Grid

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Outline

1. Basic problems, potential solutions
2. Wind power equation
3. Variability
4. System Control
5. Comments on potential solutions

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Basic problems with wind power balance

• Wind is a variable resource when it is controlled
  to maximize its power production
• Definition NETLOAD.MWLOAD.MWLOSSES.MW-WIND.MW
• Fact Wind increases NETLOAD.MW variability in
  grid
• Fact Grid requires GEN.MWNETLOAD.MW always
• Fact Expensive (based on marginal cost) gens
  move (ramp) quickly, cheap gens dont, some
  gens do not ramp at all.
• Problem Increasing wind increases need for more
  and faster resources to meet variability,
  increasing cost of wind.
• Wind is an uncertain resource
• Fact Market makes day-ahead decisions for unit
  commitment (UC) based on NETLOAD.MW forecast.
• Fact Large forecast error requires available
  units compensate.
• Problem Too many (under-forecast) or too few
  (over-forecast) units may be available,
  increasing the cost of wind.

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Solutions to variability uncertainty

1. We have always dealt with variability and
   uncertainty in the load, so no changes are
   needed.
2. Increase MW control capability during periods of
   expected high variability via control of the wind
   power.
3. Increase MW control capability during periods of
   expected high variability via more conventional
   generation.
4. Increase MW control capability during periods of
   expected high variability using demand control.
5. Increase MW control capability during periods of
   expected high variability using storage.

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Power production Wind power equation
Mass flow rate is the mass of substance which
passes through a given surface per unit time.

• The disks have larger cross sectional area from
  left to right because
• v1 gt vt gt v2 and
• the mass flow rate must be the same everywhere
  within the streamtube (conservation of mass)
• ?air density (kg/m3)
• Therefore, A 1 lt At lt A 2

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Power production Wind power equation
3. Mass flow rate at swept area
4b. Force on turbine blades
4a. Kinetic energy change
5b. Power extracted
5a. Power extracted
6b. Substitute (3) into (5b)
6a. Substitute (3) into (5a)
8. Substitute (7) into (6b)
9. Factor out v13
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Power production Wind power equation
10. Define wind stream speed ratio, a
This ratio is fixed for a given turbine control
condition.
11. Substitute a into power expression of (9)
12. Differentiate and find a which maximizes
function
13. Find the maximum power by substituting a1/3
into (11)
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Power production Wind power equation
14. Define Cp, the power (or performance)
coefficient, which gives the ratio of the power
extracted by the converter, P, to the power of
the air stream, Pin.
power extracted by the converter
power of the air stream
15. The maximum value of Cp occurs when its
numerator is maximum, i.e., when a1/3
The Betz Limit!
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Power production Cp vs. ? and ?
u tangential velocity of blade tip
Tip-speed ratio
? rotational velocity of blade
R rotor radius
v1 wind speed
Pitch ?
GE SLE 1.5 MW
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Power production Wind Power Equation

• So power extracted depends on
• Design factors
• Swept area, At
• Environmental factors
• Air density, ? (1.225kg/m3 at sea level)
• Wind speed v3
• 3. Control factors affecting performance
  coefficient CP
• Tip speed ratio through the rotor speed ?
• Pitch ?

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Power production Cp vs. ? and ?
u tangential velocity of blade tip
Tip-speed ratio
? rotational velocity of blade
R rotor radius

• Important concept 1
• The control strategy of all US turbines today is
  to operate turbine at point of maximum energy
  extraction, as indicated by the locus of points
  on the black solid line in the figure.
• Important concept 2
• This strategy maximizes the energy produced by a
  given wind turbine.
• Any other strategy spills wind !!!
• Important concept 3
• Cut-in speedgt0 because blades need minimum
  torque to rotate.
• Generator should not exceed rated power
• Cut-out speed protects turbine in high winds

v1 wind speed
GE SLE 1.5 MW
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Power production Usable speed range
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Wind Power Temporal Spatial Variability
JULY2006 JANUARY2006
BlueVERY LOW POWER RedVERY HIGH POWER

• Notice the temporal variability
• lots of cycling between blue and red
• January has a lot more high-wind power (red)
  than July
• Notice the spatial variability
• waves of wind power move through the entire
  Eastern Interconnection
• red occurs more in the Midwest than in the East

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Time frame 1 Transient control
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Time frame 1 Transient control
1-20 seconds
Source FERC Office of Electric Reliability
available at www.ferc.gov/EventCalendar/Files/201
00923101022-Complete20list20of20all20slides.pd
f
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Time frames 2 3 Regulation and Load following
4 seconds to 3 minutes
Every 5 minutes
Source Steve Enyeart, Large Wind Integration
Challenges for Operations / System Reliability,
presentation by Bonneville Power Administration,
Feb 12, 2008, available at http//cialab.ee.washin
gton.edu/nwess/2008/presentations/stephen.ppt.
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Analogy for supply-demand-frequency relationship
Inflow ?Supply Outflow ?Demand Water
leve l?Frequency
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How Does Power System Handle Variability
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How Does Power System Handle Variability
PtieP1P2P3
ACE ?Ptie B?f ?Ptie B?f
REST OF THE INTERCONNECTION
?PtiePtie,act-Ptie,sch
BA
P3
P1
P2
?ffact-60
If ?Ptie0, ?f 0, then ACE0, and generation
does not change If ?Ptiegt0 which means the
actual export exceeds the scheduled export, then
this component would make ACE more positive
therefore tending to reduce generation If ?fgt0
which means the actual frequency exceeds the
scheduled frequency of 60 Hz, then this component
would make ACE more positive therefore tending to
reduce generation.
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Power Balance Control Levels
Control level Name Time frame Control objectives Function
1 Primary control, governor 1-20 seconds Power balance and transient frequency Transient control
2 Secondary control, AGC 4 secs-3 mins Power balance and steady-state frequency Regulation
3 Real-time market Every 5 mins Power balance and economic-dispatch Load following and reserve provision
4 Day-ahead market Every day, 24 hrs at a time Power balance and economic-unit commitment Unit commitment and reserve provision
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Why Does Variability Matter?

• NERC penalties for poor-performance
• Consequences of increased frequency variblty
• Some loads may lose performance (induction
  motors)
• Relays can operate to trip loads (UFLS), and gen
  (V/Hz)
• Lifetime reduction of turbine blades
• Frequency dip may increase for given loss of
  generation
• Areas without wind may regulate for windy areas
• Consequences of increased ACE variability (more
  frequent MW corrections)
• Increased inadvertent flows
• Increase control action of generators
• Regulation moves gen down the stack cycling!

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Power Balance Control Levels
Regulation component varies about the mean and
tends to go up as much as it goes down and is
therefore normal with 0 mean.
Load regulation component
Load following component
Load
?t2 min, 28 min rolling average, so T7.
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Power Balance Control Levels
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Characterizing Netload Variability
?T HISTOGRAM Measure each ?T variation for 1 yr
(?T1min, 5min, 1 hr) Identify variability bins
in MW Count of intervals in each variability
bin Plot against variability bin Compute
standard deviation s.
Regulation
Load following
Loads 2011 12600 MW 2013 12900 MW 2018 13700
MW
Ref Growing Wind Final Report of the NYISO 2010
Wind Generation Study, Sep 2010. www.nyiso.com/pub
lic/webdocs/newsroom/press_releases/2010/GROWING_W
IND_-_Final_Report_of_the_NYISO_2010_Wind_Generati
on_Study.pdf
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Solutions to variability uncertainty

• Do nothing fossil-plants provide reg LF (and
  die ?).
• Increase control of the wind generation
• Provide wind with primary control
• Reg down (4/sec), but spills wind following the
  control
• Reg up, but spills wind continuously
• Limit wind generation ramp rates
• Limit of increasing ramp is easy to do
• Limit of decreasing ramp is harder, but good
  forecasting can warn of impending decrease and
  plant can begin decreasing in advance
• Increase non-wind MW ramping capability during
  periods of expected high variability using one or
  more of the below
• Conventional generation
• Load control
• Storage
• Expand control areas

/min /mbtu /kw LCOE,/mwhr
Coal 1-5 2.27 2450 64
Nuclear 1-5 0.70 3820 73
NGCC 5-10 5.05 984 80
CT 20 5.05 685 95
Diesel 40 13.81
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How to decide?
First, frequency control for over-frequency
conditions, which requires generation reduction,
can be effectively handled by pitching the blades
and thus reducing the power output of the
machine. Although this action spills wind, it
is effective in providing the necessary frequency
control. Second, frequency control for
under-frequency conditions requires some
headroom so that the wind turbine can increase
its power output. This means that it must be
operating below its maximum power production
capability on a continuous basis. This also
implies a spilling of wind. Question Should we
spill wind in order to provide frequency
control, in contrast to using all wind energy and
relying on some other means to provide the
frequency control? Answer Need to compare
system economics between increased production
costs from spilled wind, and increased
investment, maint, production costs from using
storage conventional gen.
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