ECE 333 Renewable Energy Systems Lecture 7: Power System Operations, Wind as a Resource Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign

Announcements Start reading Chapter 7; also read Prof. Sauer's article on course website explaining reactive power HW 3 is posted; it will be covered by an in-class quiz on Thursday Feb 13 – Material from Power Systems history and operations will be covered on exams (such as true/false) 1

Power Flow A common power system analysis tool is the power flow – It shows how real and reactive power flows through a network, from generators to loads Solves sets of non-linear equations enforcing "conservation of power" at each bus in the system (a consequence of KCL) – Loads are usually assumed to be constant power – Used to determine if any transmission lines or transformers are overloaded and system voltages Educational version PowerWorld tool available at – 2

PowerWorld Simulator Three Bus System Load with green arrows indicating amount of MW flow Used to control output of generator Direction of arrow is used to indicate direction of real power (MW) flow Note the power balance at each bus 3

Area Control Error (ACE) The area control error is the difference between the actual flow out of an area, and the scheduled flow. Ideally the ACE should always be zero. Because the load is constantly changing, each utility must constantly change its generation to “chase” the ACE. MISO ACE| (in MW) from 9/19/12. At the time the MISO load was about 65GW 4

Automatic Generation Control BAs use automatic generation control (AGC) to automatically change their generation to keep their ACE close to zero. Usually the BA control center calculates ACE based upon tie-line flows; then the AGC module sends control signals out to the generators every couple seconds. 5

Three Bus Case on AGC 6

Generator Costs There are many fixed and variable costs associated with power system operation. The major variable cost is associated with generation. Cost to generate a MWh can vary widely. For some types of units (such as hydro and nuclear) it is difficult to quantify. Many markets have moved from cost-based to price- based generator costs 7

Economic Dispatch Economic dispatch (ED) determines the least cost dispatch of generation for an area. For a lossless system, the ED occurs when all the generators have equal marginal costs. IC1(PG,1) = IC2(PG,2) = … = ICm(PG,m) 8

Power Transactions Power transactions are contracts between areas to do power transactions. Contracts can be for any amount of time at any price for any amount of power. Scheduled power transactions are implemented by modifying the area ACE: ACE = Pactual,tie-flow - Psched 9

100 MW Transaction Scheduled 100 MW Transaction from Left to Right Net tie-line flow is now 100 MW 10

Security Constrained Economic Dispatch Transmission constraints often limit system economics. Such limits required a constrained dispatch in order to maintain system security. In three bus case the generation at bus 3 must be constrained to avoid overloading the line from bus 2 to bus 3. 11

Security Constrained Dispatch Dispatch is no longer optimal due to need to keep Line from bus 2 to bus 3 from overloading 12

Multiple Area Operation If Areas have direct interconnections, then they may directly transact up to the capacity of their tie-lines. Actual power flows through the entire network according to the impedance of the transmission lines. Flow through other areas is known as “parallel path” or “loop flows.” 13

Seven Bus Case One-line Diagram System has three areas Area left has one bus Area right has one bus Area top has five buses 14

Seven Bus Case: Area View Actual flow between areas Loop flow can result in higher losses System has 40 MW of “Loop Flow” Scheduled flow 15

Seven Bus System – Loop Flow? 100 MW Transaction between Left and Right Note that Top’s Losses have increased from 7.09MW to 9.44 MW Transaction has actually decreased the loop flow 16

Pricing Electricity Cost to supply electricity to bus is called the locational marginal price (LMP) Presently PJM and MISO post LMPs on the web In an ideal electricity market with no transmission limitations the LMPs are equal Transmission constraints can segment a market, resulting in differing LMP Determination of LMPs requires the solution on an Optimal Power Flow (OPF) 17

Three Bus Case LMPs: Line Limit NOT Enforced Line from Bus 1 to Bus 3 is over-loaded; all buses have same marginal cost Gen 1’s cost is $10 per MWh Gen 2’s cost is $12 per MWh 18

Three Bus Case LMPS: Line Limits Enforced Line from 1 to 3 is no longer overloaded, but now the marginal cost of electricity at 3 is $14 / MWh 19

Generation Supply Curve Base Load Coal and Nuclear Generation Natural Gas Generation As the load goes up so does the price Renewable Sources Such as Wind Have Low Marginal Cost, but they are Intermittent 20

MISO LMPs on Sept 19, 2012 (11:50am EST which is CDT) Available on-line at 21

MISO LMPs on Feb 6, 2015, 1pm Central 22 Available on-line at

MISO Annual Load Duration Curves %20Annual%20Market%20Assessment%20Report.pdf

MISO Average Prices and Wind Output 13%20Annual%20Market%20Assessment%20Report.pdf 24

Wind Power Systems Photos taken Kate Davis near Moraine View State Park, IL 25

Historical Development of Wind Power The first known wind turbine for producing electricity was by Charles F. Brush turbine, in Cleveland, Ohio in kW Used electricity to charge batteries in the cellar of the owner’s mansion Note the person 26

Historical Development of Wind Power First wind turbine outside of the US to generate electricity was built by Poul la Cour in 1891 in Denmark Used electricity from his wind turbines to electrolyze water to make hydrogen for the gas lights at the schoolhouse 27

Historical Development of Wind Power In the US - first wind-electric systems built in the late 1890’s By 1930s and 1940s, large numbers in rural areas not served by the grid for pumping water and sometimes electricity generation Interest in wind power declined as the utility grid expanded and as reliable, inexpensive electricity could be purchased Oil crisis in 1970s created a renewed interest in wind until US government stopped giving tax Renewed interest again since the 1990s Photo: 28

Global Installed Wind Capacity Source: Annual Market Update 2013, Global Wind Energy Council, Total worldwide electric capacity is 4500GW, so wind, at almost 250GW, is 5.6% of total 29

Wind Capacity Additions by Region Source: Annual Market Update 2013, Global Wind Energy Council, 30

Top 10 Countries - Installed Wind Capacity (as of the end of 2013) Source: Annual Market Update 2013, Global Wind Energy Council, 31

US Wind Resources

US Wind Capacity by State, 12/31/14 33

Wind Map for Illinois at 80m 34

Worldwide Wind Resource Map Source: 35

Types of Wind Turbines “Windmill”- used to grind grain into flour or pump water Many different names - “wind-driven generator”, “wind generator”, “wind turbine”, “wind-turbine generator (WTG)”, “wind energy conversion system (WECS)” Can have be horizontal axis wind turbines (HAWT) or vertical axis wind turbines (VAWT) Groups of wind turbines are located in what is called either a “wind farm” or a “wind park” 36

Vertical Axis Wind Turbines Darrieus rotor - the only vertical axis machine with any commercial success Wind hitting the vertical blades, called aerofoils, generates lift to create rotation No yaw (rotation about vertical axis) control needed to keep them facing into the wind Heavy machinery in the nacelle is located on the ground Blades are closer to ground where windspeeds are lower 37

Horizontal Axis Wind Turbines “Downwind” HAWT – a turbine with the blades behind (downwind from) the tower No yaw control needed- they naturally orient themselves in line with the wind Shadowing effect – when a blade swings behind the tower, the wind it encounters is briefly reduced and the blade flexes 38

Horizontal Axis Wind Turbines “Upwind” HAWT – blades are in front of (upwind of) the tower Most modern wind turbines are this type Blades are “upwind” of the tower Require somewhat complex yaw control to keep them facing into the wind – Need to search for the wind to start turning Operate more smoothly and deliver more power Largest turbines are on the order of 6 MW with 1.5 MW a quite common design 39

Number of Rotating Blades Windmills have multiple blades – need to provide high starting torque to overcome weight of the pumping rod – must be able to operate at low wind speeds to provide nearly continuous water pumping – a larger area of the rotor faces the wind – Note, most seem to write “wind speed” as two words Turbines with many blades operate at much lower rotational speeds - as the speed increases, the turbulence caused by one blade impacts the other blades Most modern wind turbines have two or three blades 40

Worldwide Wind Energy Company Market Share, 2013 Installations Source: 41

Vestas Stock Price 42