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

J. McCalley

Outline

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

2

Basic problems with wind & power balance

1.

Wind is a variable resource when it is controlled to maximize its power production a.

b.

c.

d.

e.

Definition: NETLOAD.MW=LOAD.MW+LOSSES.MW-WIND.MW

Fact: Wind increases NETLOAD.MW variability in grid Fact: Grid requires GEN.MW=NETLOAD.MW always Fact: “Expensive” (based on marginal cost) gens move (ramp) quickly, “cheap” gens don’t, some gens do not ramp at all.

Problem: Increasing wind increases need for more and “faster” resources to meet variability, increasing cost of wind.

2.

a.

b.

c.

Wind is an uncertain resource Fact: Market makes day-ahead decisions commitment” (UC) based on NETLOAD.MW forecast.

for “unit 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.

3

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.

4 4

v

Power production Wind power equation

Swept area A t of turbine blades:

Mass flow rate

is the mass of substance which passes through a given surface per unit time.

v 1 v t v 2

The disks have larger cross sectional area from left to

• •

right because v 1 > v t > v 2 and the mass flow rate must be the same everywhere within the streamtube (conservation of mass):

Q

1 

Q t



Q

2 

A

1

v

1  

A t v t

 

A

2

v

2

ρ=air density (kg/m 3 )

x 

Therefore, A 1 < A t < A 2

5

Power production Wind power equation

1. Wind velocity:

v

  

t x

2. Air mass flowing: 3. Mass flow rate at swept area:

Q t

 

m



t

 

A t



t



x

 

m

 

A t v t



A



x

4a. Kinetic energy change:



KE

 1 2

m



v

1 2 

v

2 2 

P

5a. Power extracted:

 

KE



t

 1 2

m



t



v

1 2

v

2 2   1 2

Q t



v

1 2 

v

2 2 

F

4b. Force on turbine blades:



ma



m



v



t



m



t



v



Q t

5b. Power extracted:



v

1 

v

2 

P



Fv t



Q t v t



v

1 

v

2 

6a. Substitute (3) into (5a):

P

 ( 1 / 2 ) 

A t v t

(

v

1 2 

v

2 2 )

7. Equate 6b. Substitute (3) into (5b):

P

 

A t v t

2 (

v

1 

v

2 )  ( 1 / 2 )

v t

(

v

1 2 

v

2 2 ) 

v t

2 (

v

1 

v

2 )  ( 1 / 2 )

v t

(

v

1 

v

2 )(

v

1

8. Substitute (7) into (6b):

P

9. Factor out v 1 3 :

P

 

A t

4

v

3 1 ( 1  (  

A t v

2 )

v

1 2 (( 1 )( 1 /  2 )(

v

2

v

1 )

v

1 

v

2 ) 

v t

2 

v

2 )) 2 (

v

1 (

v

1 

v

2 ) 

v

2 )   

A t

( 1 ( /

v

1 2 4 2 )( 

v

2

v

2 2 

v

)(

v

1 1 ) 

v t



v

2 ) 6

Power production Wind power equation

10. Define wind stream speed ratio, a:

a



v

2

v

1

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:

P

 

A t v

3 1 ( 1 

a

2 )( 1 

a

) 4 

P



a

 

A t v

3 1 4   2

a

(

a

 1 )  ( 1 

a

2  )  0  2

a

2  2

a

 1 

a

2   3

a

2  2

a

 1  0 (  3

a

 1 )(

a

 1 )  0 

a

 1 / 3 ,

a

  1

13. Find the maximum power by substituting a=1/3 into (11):

P

 

A t v

1 3 4 ( 1  1 9 )( 4 3 )  

A t v

3 1 4 8 4 9 3  8 

A t v

3 1 27 7

Power production Wind power equation

14. Define C p , the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, P in .

power extracted by the converter

P

 

A t v

3 1 4 ( 1 

a

2 )( 1 

a

)

C p



P P in

power of the air stream



P in

 

KE



t

 1 2

m



t



A t v

3 1 ( 1 

a

2 )( 1 

a

) 4 1 2 

A t v

3 1  1 2 ( 1 

a

2 )( 1 

a

)  1 2

Q

1

v

1 2  1 2 

A t v

1

v

1 2  1 2 

A t v

3 1

P



C p P in

 1 2

C P



A t v

1 3

15. The maximum value of C p is maximum, i.e., when a=1/3: occurs when its numerator The Betz Limit!

C p



P P in

 1 2 ( 8 9 )( 4 3 )  16 27  0 .

5926 8

Power production Cp vs. λ and θ

Tip-speed ratio:

 

u v

1  

R v

1 u: tangential velocity of blade tip ω: rotational velocity of blade R: rotor radius v 1 : wind speed

Pitch: θ

GE SLE 1.5 MW 9

Power production Wind Power Equation

P



C p P in

 1

C P

(  ,  ) 

A t v

1 3 2 So power extracted depends on 1. Design factors: • Swept area, • Air density,

ρ A t

2. Environmental factors: (~1.225kg/m 3 • Wind speed

v 3

at sea level) 3. Control factors affecting performance coefficient

C P

: • Tip speed ratio through the rotor speed

ω

• Pitch

θ

10

Power production Cp vs. λ and θ

Tip-speed ratio:

 

u

Important concept #1

:

v

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. 

R v

1 u: tangential velocity of blade tip ω: rotational velocity of blade R: rotor radius v 1 : wind speed

Important concept #2

: • This strategy maximizes the energy produced by a given wind turbine.

• Any other strategy “spills” wind !!!

Important concept #3

: • Cut-in speed>0 because blades need minimum torque to rotate.

• Generator should not exceed rated power • Cut-out speed protects turbine in high winds GE SLE 1.5 MW 11

Power production Usable speed range

Cut-in speed (6.7 mph) Cut-out speed (55 mph) 12

Wind Power Temporal & Spatial Variability

JULY2006 JANUARY2006 Blue ~VERY LOW POWER; Red ~VERY 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 13 13

Time frame 1: Transient control

14

Time frame 1: Transient control

1-20 seconds Source: FERC Office of Electric Reliability available at: www.ferc.gov/EventCalendar/Files/20100923101022-Complete%20list%20of%20all%20slides.pdf

15

Time frames 2 & 3: Regulation and Load following

= +

100 80 60 40 20 0 -20 -40 -60 -80 -100 07:00 07:20 07:40 08:00 09:00 09:20 09:40 10:00 08:20 08:40 Regulation

Load Following Regulation

Every 5 minutes 4 seconds to 3 minutes

16

Source: Steve Enyeart, “Large Wind Integration Challenges for Operations / System Reliability,” presentation by Bonneville Power Administration, Feb 12, 2008, available at http://cialab.ee.washington.edu/nwess/2008/presentations/stephen.ppt

. 16

Analogy for supply-demand frequency relationship

Inflow Outflow Water leve  Supply  Demand l  Frequency

17

17

How Does Power System Handle Variability

Turbine-Gen N Turbine Gen … Turbine-Gen 2 Turbine-Gen 1 ACE= ∆P tie 10B∆f Secondary control provides regulation Primary control controls output in response to transient frequency deviations ∆P tie ∆f B is BA’s frequency bias in MW/0.1Hz.

B is negative.

18

How Does Power System Handle Variability

ACE= ΔP tie – BΔf

=

ΔP tie +| B|Δf ΔP tie =P tie,act -P tie,sch P tie =P1+P2+P3

REST OF THE INTERCONNECTION

P1

BA

P3

Δf=f act -60

If

ΔP tie

=0,

Δf

=0, then ACE=0, and generation does not change; If

ΔP tie

>0 which means the actual export exceeds the scheduled export, then this component would make ACE more positive therefore tending to reduce generation; If

Δf

>0 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.

19

Power Balance Control Levels

Control level

1 2 3 4

Name

Primary control, governor Secondary control, AGC Real-time market Day-ahead market

Time frame

1-20 seconds 4 secs-3 mins Every 5 mins Every day, 24 hrs at a time

Control objectives

Power balance and transient

frequency

Power balance and steady-state

frequency

Power balance and economic- dispatch Power balance and economic- unit commitment

Function

Transient control Regulation Load following and reserve provision Unit commitment and reserve provision 20

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!

21

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.

LR k

Load regulation component 

L k

Load 

LF k

Load following component

LF k

 2

T

1  1

i k



T

 

k



T L i



L k



T



L k



T

 1  ...

 2

T L k

  1 ...



L k



T

 1 

L k



T

Δt=2 min, 28 min rolling average, so T=7.

LF k



L k

 7 

L k

 6  ...



L k

15  ...



L k

 6 

L k

 7 22

Power Balance Control Levels

 2

x

 1

n i n

  1 (

x i

 

x

) 2 Consider two random variables, X and Y.

If Z=X+Y, then 4000 3500 3000 2500 2000 1500 1000 500 0 

z

2  

x

2   2

y

Hourly Load Variability and Load-Wind Variability When Wind Penetration is 10% Load and Load-Wind Hourly Variability (MW)

Load Hourly Variability Load-Wind Hourly Variability 23

Characterizing Netload Variability

∆T HISTOGRAM

Identify “variability bins” in MW

Regulation

Measure each ∆T variation for 1 yr (∆T=1min, 5min, 1 hr)

Load following

Count # of intervals in each variability bin Plot # against variability bin Compute standard deviation σ.

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/public/webdocs/newsroom/press_releases/2010/GROWING_WIND_ _Final_Report_of_the_NYISO_2010_Wind_Generation_Study.pd

f

24

Solutions to variability & uncertainty 1. Do nothing: fossil-plants provide reg & LF (and die



).

2. Increase control of the wind generation a. Provide wind with primary control

•

Reg down (4%/sec), but spills wind following the control

•

Reg up, but spills wind continuously b. 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 3. Increase non-wind MW ramping capability during periods of expected high variability using one or more of the below: a. Conventional generation %/min $/mbtu $/kw LCOE,$/mwhr b. Load control c. Storage d. Expand control areas

Coal 1-5 Nuclear 1-5 NGCC 5-10 2.27

0.70

5.05

2450 64 3820 73 984 80 25 CT 20 40 5.05

13.81

685 95

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? 26

Answer:

Need to compare

system

economics between increased production costs from spilled wind, and increased investment, maint, & production costs from using storage & conventional gen.

26