PSERC SeminarFebruary 9, 2010

Iowa State University

Frequency control (MW-Hz) with wind

James D. McCalley

Harpole Professor of Electrical & Computer Engineering

1

Outline1. MW-Hz time frames

2. Transient frequency control

3. Variability

4. Regulation

5. Load following

6. Storage

7. Conclusions

2

MW-Hz Time Frames

0+<t<2s; Inertial

t=0+; Proximity

2s<t<10s; Speed-governors 10s<t<3m; AGC

1m<t; AGC, ED

3

MW-Hz Time Frames

Source: H. Holttinen, “The impact of large‐scale power production on the Nordic electricity system,” VTT Publications 554, PhD Dissert, Helsinki U. of Technology, 2004.

Transient frequency response

Inertia & governor

RegulationGovernor & AGC

Load followingAGC and ED

SchedulingED & UC

4

MW-Hz Time Frames

-100

-80

-60

-40

-20

0

20

40

60

80

100

07:00 07:20 07:40 08:00 08:20 08:40 09:00 09:20 09:40 10:00

REGU

LATI

ON IN

MEG

AWAT

TS

Regulation

=

+

Load Following Regulation

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

5

Transient frequency control

What can happen if frequency dips too low?• For f<59.75 Hz, underfrequency relays can trip load.• For f<59 Hz, loss of life on turbine blades• Violation of NERC criteria with penalties

• N-1: Frequency not below 59.6 Hz for >6 cycles at load buses• N-2: Frequency not below 59.0 Hz for >6 cycles at load buses

6

Transient frequency control

fn

i

i

L m

H

fP

dt

fd

1

Re

2

60

t1

mf1

mf2

mf3

Time (sec)

Frequency(Hz)

60-mf1t1

60-mf2t1

60-mf3t1

60

t1

mf1

mf2

mf3

Time (sec)

Frequency(Hz)

60-mf1t1

60-mf2t1

60-mf3t1

The greater the rate of change of frequency (ROCOF) following loss of a generator ∆PL, the lower will be the frequency dip. ROCOF increases as total system inertia ΣHi decreases.Therefore, frequency dip increases as ΣHi decreases.

7

Transient frequency control

49.35

Nadir

2.75 sec

sec/227.0475*2

)50(32.4

21

Re Hz

H

fP

dt

fdm

n

i

i

Lf

Example: Ireland: ∆PL =432 MW=4.32 pu. ΣHi =475 sec

1. Governors2. Load frequency sensitivity

50-0.227*2.75=49.38Hz

8

Transient frequency controlExample: Estrn Interconnection: ∆PL =2900 MW=29 pu. ΣHi =32286 sec

Nadir59.9828 Hz

59.9725z

sec/0269.032286*2

)60(29

21

Re

Hz

H

fP

dt

fdm

n

i

i

Lf

60-0.0269*1.5=59.9597Hz

9

Transient frequency control

So what is the issue with wind….?1. Increasing wind penetrations tend to displace

(decommit) conventional generation.2. DFIGs, without specialized control, do not contribute

inertia. This “lightens” the system(decreases denominator) fn

i

i

L m

H

fP

dt

fd

1

Re

2

Let’s see an example….

10

Transient frequency control

• Green: Base Case

• Dark Blue: 2% Wind Penetration

• Light Blue: 4% Wind Penetration

• Red: 8% Wind Penetration

Estrn Interconnection: Frequency dip after 2.9GW Gen drop for Unit De-Commitment scenario at different wind penetration levels (0.6, 2, 4, 8%)

11

Transient frequency controlWhy do DFIGs not contribute inertia?

They do not decelerate in response to a frequency drop.

FUELSteam Boiler

Generator

CONTROL SYSTEM

Steam valve controlFuel supply control

MVAR-voltage control

Wind speed

Gear Box

Generator

CONTROL SYSTEM

MVAR-voltage control

Real power output control

STEAM-TURBINE

WIND-TURBINE

The ability to control mech torque applied to the generator using pitch control & electromagnetic torque using rotor current control (to optimize Cp and to avoid gusting) enables avoidance of mismatch between mechanical torque and electromagnetic torque and, therefore, also avoidance of rotor deceleration under network frequency decline.

12

Transient frequency control

Ireland sees significant ∆f for loss of largest unit. Estrninterconnection (EI) sees small ∆f for loss of largest unit.

Ireland total system inertia is 475 sec. Estrn interconnection total system inertia is 32286 sec.(ERCOT and WECC are between these extremes.)The “heavier” the system, the less frequency moves.

Hard to cause trans freq dip problem in the EI with N-1 outage, but frequency stability is still of concern because:1. Islanding conditions can be susceptible2. Control performance standards may be impacted.

13

Transient frequency control – CPS1, CPS2

2min min 1

min10ute ute

ute

ACECF f

B

min min1 2 100%ute uteCPS CF

10 10

2 1010

1.65 10 10

1i sL B B

CF ACEL

2

2

Number of intervals that CF 1Total number of intervals100(1 )%

R

CPS R

1 10min 11

*10

i

i

ACEC AVG F

B

2 10 min ( )ute iC AVG ACE

2

1min{ } periodRMS dF AVG F

14

Transient frequency controlWhat is the fix for this? Consider DFIG control system

Source: J. Ekanayake, L. Holdsworth, and N. Jenkins, “Control of DFIG Wind Turbines,” Proc. Instl Electr. Eng., Power Eng., vol. 17, no. 1, pp. 28-32, Feb 2003.

15

Transient frequency controlAdd “inertial emulation,” a signal dω/dt scaled by 2H

-2H

dω / dt

16

Transient frequency controlSeveral European grid operators have imposed requirements on wind plants in regards to frequency contributions, including Nordic countries [1,2]. North American interconnections have so far not imposed requirements on wind farms in regards to frequency contributions, with the exception of Hydro-Quebec. (We better fix this before installing 600 GW of wind!!!!)

Hydro Quebec requires that wind farms be able to contribute to reduce large (>0.5 Hz), short-term (< 10 sec) frequency deviation [3]. The Hydro-Quebec requirement states [4], “The frequency control system must reduce large, short-term frequency deviations at least as much as does the inertial response of a conventional generator whose inertia (H) equals 3.5 sec.”[1] “Wind Turbines Connected to Grids with Voltages above 100 kV – Technical Regulation for the Properties and the Regulation of Wind Turbines, Elkraft System and Eltra Regulation, Draft version TF 3.2.5, Dec., 2004. [2] “Nordic Grid Code 2007 (Nordic Collection of Rules), Nordel. Tech. Rep., Jan 2004, updated 2007. [3] N. Ullah, T. Thiringer, and D. Karlsson, “Temporary Primary Frequency Control Support by Variable Speed Wind Turbines – Potential and Applications,” IEEE Transactions on Power Systems, Vol. 23, No. 2, May 2008. [4] “Technical Requirements for the Connection of Generation Facilities to the Hydro-Quebec Transmission System: Supplementary Requirements for Wind Generation,” Hydro Quebec, Tech. Rp., May 2003, revised 2005.

17

Temporal Variability

Source: Task 25 of the International Energy Agency (IEA), “Design and operation of power systems with large amounts of wind power: State-of-the-art report,” available at www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf.

18

Spatial Variability (geo-diversity)

Source: Task 25 of the International Energy Agency (IEA), “Design and operation of power systems with large amounts of wind power: State-of-the-art report,” available at www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf.

19

Spatial Variability (geo-diversity)

Source: Task 25 of the International Energy Agency (IEA), “Design and operation of power systems with large amounts of wind power: State-of-the-art report,” available at www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf.

20

Variability of net load

1 hour 10 minσ max σ max

Load (MW) 123 400 22 135Net load (MW) 130 499 23.6 158

Source:V. Vittal, J. McCalley, V. Ajjarapu, “Impact of Increased DFIGWind Penetration On Power Systems And Markets,” Final report, Power System Engineering Research Center, 2009.

21

Evaluating regulation share of a gen or load

T

TLwX2

222

ORNL Method – allocation of regulation [1]: The basic concept of this method stems from the following: • If a wind farm’s natural diurnal cycle is positively-correlated

with the 24 hour load cycle, then ; the wind will ramp with the load, and there will be less need for load following.• If a wind farm’s natural diurnal cycle is negatively-correlated

with the 24 hour load cycle, then ; the wind will ramp against the load, and there will be less need for load following.[1] B. Kirby, M. Milligan, Y. Makarov, D. Hawkins, K. Jackson, H. Shiu “California Renewables Portfolio Standard Renewable Generation Integration Cost Analysis, Phase I: One Year Analysis Of Existing Resources, Results And Recommendations, Final Report,” Dec. 10, 2003, available at http://www.consultkirby.com/files/RPS_Int_Cost_PhaseI_Final.pdf.

wind.less load totalofVar :

load. totalofVar :

plant windofVar :

2

2

2:

T

L

w

22LT

22LT

Contribution of wind variability to net load variability

22

Solutions to increased variability

1. Increase control of the wind generation via pitch controla. Provide regulation and/or load following capabilityb. 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

2. Increase non-wind MW ramping capability during periods of expected high variability using one or more of the below:a. Conventional generation b. Storage (e.g., pumped storage, CAES, batteries…)c. Load control

23

Ensure availability of high-ramp rate units

Source: www.xcelenergy.com/COMPANY/ABOUT_ENERGY_AND_RATES/RESOURCE%20AND%20RENEWABLE%20ENERGY%20PLANS/Pages/2007_Minnesota_Resource_Plan.aspx

Steam turbine plants 1- 5 %/minNuclear plants 1- 5 %/minGT Combined Cycle 5 -10 %/min Combustion turbines 20 %/min Diesel engines 40 %/min

“Coal units typically have ramp rates that are in the range of 1% to 1.5% of their nameplate rating per minute between minimum load and maximum load set points. Coal unit minimum load set-points range from 20% to 50% of nameplate, depending on the design of the air quality control system being used. For example, a 500 MW coal plant may have a minimum load of 100 MW and would be able to ramp up at the rate of 5 MW per minute. In addition, it can take a day or more to bring a coal plant up to full load from a cold start condition. Natural gas-fired combustion turbines, on the other hand, can normally be at full load from a cold start in 10 to 30 minutes (which results in an effective ramp rate of 3.3% to 10% of their nameplate rating per minute).”

24

Regulation via rotor speed & pitch control

FUELSteam Boiler

Generator

CONTROL SYSTEM

Steam valve controlFuel supply control

MVAR-voltage control

Wind speed

Gear Box

Generator

CONTROL SYSTEM

MVAR-voltage control

Real power output control

STEAM-TURBINE

WIND-TURBINE

Whereas speed control may be well suited for continuous, fine, frequency regulation, blade pitch control can provide fast acting, coarse control both for frequency regulation as well as emergency spinning reserve.

Pitch control

Rotor speed control

Sources: Rogério G. de Almeida and J. A. Peças Lopes, “Participation of Doubly Fed Induction Wind Generators in System Frequency Regulation,” IEEE Trans On Pwr Sys, Vol. 22, No. 3, Aug. 2007. B. Fox, D. Flynn, L. Bryans, N. Jenkins, D. Milborrow, M. O’Malley, R. Watson, and O. Anaya-Lara, “Wind Power Integration: Connection and system operational aspects,” Institution of engineering and technology, 2007.

25

Regulation via rotor speed & pitch control

[1] “Wind Generation Interconnection Requirements,” Technical Workshop, November 7, 2007, available at www.bctc.com/NR/rdonlyres/13465E96-E02C-47C2-B634-F3BCC715D602/0/November7WindInterconnectionWorkshop.pdf. [2] [North American Electric Reliability Corporation, “Special Report: Accommodating High Levels of Variable Generation,” April 2009, available at http://www.nerc.com/files/IVGTF_Report_041609.pdf.

Review of the websites from TSOs (in Europe), reliability councils (i.e., NERC and regional organizations) and ISOs (in North America) suggest that there are no hard requirements regarding use of primary frequency control in wind turbines.There are soft requirements [1]:•BCTC will specify “on a site by site basis,” •Hydro Quebec requires that wind turbines be “designed so that they can be equipped with a frequency control system (>10MW)”•Manitoba Hydro “reserves the right for future wind generators”NERC [2], said, “Interconnection procedures and standards should be enhanced to address voltage and frequency ride-through, reactive and real power control, frequency and inertial response and must be applied in a consistent manner to all generation technologies.”

26

Regulation via rotor speed & pitch control

[15] Draft White Paper, “Wind Generation White Paper: Governor Response Requirement,” Feb, 2009, available at www.ercot.com/content/meetings/ros/keydocs/2009/0331/WIND_GENERATION_GOVERNOR_RESPONSE_REQUIREMENT_draft.doc..

ERCOT says [1], “…as wind generation becomes a bigger percentage of the on line generation, wind generation will have to contribute to automatic frequency control. Wind generator control systems can provide an automatic response to frequency that is similar to governor response on steam turbine generators. The following draft protocol/operating guide concept is proposed for all new wind generators: All WGRs with signed interconnect agreements dated after March 1, 2009 shall have an automatic response to frequency deviations. …”

27

But are we sure….?First, primary 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, primary 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 conventional generation to provide the frequency control? Answer: Need to compare system economics between increased production costs from spilled wind, and increased production and investment costs from using storage and conventional generation.

28

Advanced Dispatch SCADA

Wind ProductionForecasting

Wind Production ForecastMeasured Hybrid Wind System Production Data

Storage Status

System-level Dispatch

Plant-level Control

Production Smoothing Control Command

Hybrid Wind Farm Control

Component-level Control

Energy Storage

Wind Plant

Power Storage

- Pumped Hydro + Hydraulic Turbine - CAES + Gas Turbine- Biomass + Gas Turbine- Hydrogen + Fuel Cell

- Super Capacitor + PE- SMES + PE- Flywheel + PE- Battery + PE PE: Power Electronics

Production Firming Control Command

Hybrid Wind Systems

"Energy" Storage "Power" Storage Storage

Technology

Conversion

Technology

Storage

Technology

Conversion

Technology

Pumped

Hydro

Hydraulic

Turbine (HT)Ultracapacitor

Power

Electronics

(PE)

Compressed

Air Energy

Storage

(CAES)

Gas Turbine

(GT)

Superconducti

ng Magnetic

Energy Storage

(SMES)

Power

Electronics

(PE)

BiomassGas Turbine

(GT)Flywheel

Power

Electronics

(PE)

Hydrogen Full Cell (FC) Battery

Power

Electronics

(PE)

Hybrid Wind Systems – Architecture

Storage and Conversion Technologies

A suite of models facilitate a plug-and-

play modular approach to configure

hybrid wind systems.

i

iwiw

i

igig PCPCpf ,,,,mini

i

i

i RCSP

Stochastic SCOPF

Plant-Level Inter-Device Control

Predictive Control

Grid,Wind turbine, Storage devices, per table right

Component Modeling and Control

Design and Control Hybrid Wind System to Firm and Smooth Wind Variability

29

Hybrid Wind Systems –Save Money, Enhance Frequency Regulation

HOLDEN REDBRIDG CHENAUX CHFALLSMARTDALE

HUNTVILL

NANTCOKE

WALDEN COBDEN MTOWN

GOLDEN BVILLE STRATFRDJVILLE

WVILLE

STINSONPICTON

CEYLON RICHVIEWLAKEVIEW

MITCHELL

PARKHILL

BRIGHTON

HANOVER KINCARD

HEARN

DOUGLAS

Number of buses 60Number of generators 25Number of branches 96Peak Load 6,110MWTotal Generation Capacity 10,995MW

Wind Power Capacity 545MWCAES

Power CapacityCompressor 30MWGas Turbine 75MW

CAES Energy Capacity 17,000MWhNaS Battery Power Capacity 5.5MWNaS Battery Energy Capacity 1.25MWh

0 200 400 600 800 1000 1200 1400 1600 1800-50

0

50

100

150

200

250

300

350

400

Time (s)

Po

we

r C

om

ma

nd

(M

W)

Wind Power

CAES Power NaS Battery Power ×10

0 200 400 600 800 1000 1200 1400 1600 180059.96

59.97

59.98

59.99

60

60.01

60.02

60.03

60.04

Time (S)

Syste

m F

req

ue

ncy (

Hz)

Wind plant

Hybrid Wind Systems

0 200 400 600 800 1000 1200 1400 1600 18002

4

6

8

10

12

Time (s)

Win

d S

pe

ed

(s)

Cost ($M) Saving ($M)Investment Cost Operation Cost

155.15 221.83 481.40

Life time: 20 years 0 200 400 600 800 1000 1200 1400 1600 1800-100

-80

-60

-40

-20

0

20

40

60

80

100

Mis

matc

h (M

W)

With StorageNo Storage

30

Conclusion: Select solution portfolioWind energy attrbute

Grid prblemcaused by wind attrbute

SolutionsDFIG Control Inc.

reservesStorage Load Cntrl Stoch-

asticUnit Cmmtprgrm

Dec fore-cast error

Wind plant remote trip (SPS)

HVDC control

Geo-diversity of wind

Inrtialemu-lation

Freq reg via pitch+ cnvrtr

Fast rmping

Spnng/10 min

1 hour Fast Slow Fast Slow

Estimated relative costs/MW of solution technology (to be refined)5 5 6 10 10 9 9 9 9 4 4 6 10 10

Decreased inertia

Transient frequency dips, CPS2 perfrmance

√ √ √ √Increased 1 min MW variability

CPS2 perfrmance √ √ √ √ √ √

Increased 10 min MW variability

CPS1, CPS2 perfrmance √ √ √ √ √ √ √ √

Increased 1 hr MW variability

Balancing market perfrmance √ √ √ √ √ √ √

Increased day-ahead MW variability

Day-ahead market perfrmance √ √ √ √ √ √ √

Increased transmission loading

Increased need for transmssion

√ √ √Low, variable capacity factor

More planning uncertainty √ √ √ √

31

Solar, 1.0

Nuclear, 15

Hydro, 2.95

Wind, 8.1

Geothermal 3.04

Natural Gas 23.84

Old Coal10.42

Biomass 3.88

Petroleum15.13

26.33

8.58

25.7

8.5

Unused Energy

(Losses)43.0

Electric Generation

49.72

12.68

Used Energy42.15

Residential11.48

Commercial8.58

Industrial23.94

Trans-portation

15.5

15

6.82

20.54

6.95

Reducing 2008 CO2 by 35% sees wind at 34%

INCREASE Non-CO2

12Q to 30Q

32

IGCC, 3

REDUCE PETROLEUM 37Q15Q LightDuty: 8.56QFreight: 3.75QAviation: 3.19Q

32