1. ECEN 460 Power System Operation and Control
Lecture 14: Power System Control, Economic Dispatch
Prof. Tom Overbye
Dept. of Electrical and Computer Engineering
Texas A&M University

2. Announcements Finish reading Chapter 6
Homework 5 is due Tuesday Oct 24
Homework 6 is 6.58, 6.59, 6.60, 6.62, 6.63; it is due on Tuesday Oct 31
Lab 6 and the following labs will be back in WEB 115

3. Power System Control
A major issue with power system operation is the limited capacity of the transmission system
lines/transformers have limits (usually thermal)
no direct way of controlling flow down a transmission line (e.g., there are no valves to close to limit flow)
open transmission system access associated with industry restructuring is stressing the system in new ways
We need to indirectly control transmission line flow by changing the generator outputs
Similar control issues with voltage

4. Indirect Transmission Line Control
What we would like to determine is how a change in
generation at bus k affects the power flow on a line
from bus i to bus j.
The assumption is
that the change
in generation is
absorbed by the
slack bus 3

5. Power Flow Simulation - Before
One way to determine the impact of a generator change is to compare a before/after power flow.
For example below is a three bus case with an overload 4

6. Power Flow Simulation - After
Increasing the generation at bus 3 by 95 MW (and hence decreasing it at bus 1 by a corresponding amount), results in a 30.3 MW drop in the MW flow on the line from bus 1 to 2, and a MW drop on the flow from 1 to 3.
Expressed as a percent, 30.3/95 =32% and 64.7/95=68% 5

7. Analytic Calculation of Sensitivities
Calculating control sensitivities by repeat power flow solutions is tedious and would require many power flow solutions. An alternative approach is to analytically calculate these values 6

8. Analytic Sensitivities7

9. Three Bus Sensitivity Example8

10. Larger System Example: Lab Six 37 Bus System With Two Lines Out
With two lines out, there is an overload on a line between Pear69 and Pecan69.
We will approximate the impact of generation changes on this line flow by looking at the results of changing the generation in the power flow 9

11. Lab Six Example
Increase generation at Orange69 by 10 MW, noting that this change is absorbed at the slack bus
The change in the line flow is MW, so the sensitivity is Of course this is just an approximation! 10

12. Lab Six Example
Decrease the generation at Pear69 by 10 MW, noting that this change is also absorbed at the slack bus
The change in the line flow is MW, so the sensitivity is 0.39 (note sign is positive because increasing the generation would increase the flow) 11

13. Lab Six Example
So shifting 10 MWs from Pear69 to Orange69, should decrease the line flow ( )*10= 9.2 MW
This is very close to the actual change with little impact on the slack bus 12

14. PowerWorld Analytic Sensitivities
Select Tools, Sensitivities, Flow and Voltage Sensitivities to see lots of sensitivities. The below image shows values for our example. 13

15. Contour of the Line Flow to Bus Injection Sensitivities
Image shows how a change in injection at a bus affects the flow on the Pear69-Pecan69 line
This is a contour of the bus field: Sensitivity\Injection Value dValue/dP 14

16. Lab 6, Part B: Can You Save Texas?
You’ll be using a 2000 bus model of a synthetic grid that is supplying a load that matches the actual Texas population. A tornado takes out two lines, and you need to fix the overloads before the grid fails! 15

17. Reactive Power Optimization
Reactive power controllers include switched shunts, static var compensators (SVCs), LTC transformers, sometimes generator voltage setpoints
Goal is to maintain adequate system voltages and reduce losses
Reactive power control is much less linear than real power control; this is partially due to the much higher reactive power losses because for high voltage transmission lines X is usually much larger than R
Losses are vary nonlinear 16

18. Zion Nuclear Power Plant
The Zion nuclear power plant, located on Lake Michigan, on the Illinois/Wisconsin border, use to be a 2000 MW generator, commissioned in
In 1997 a control-room operator inserted control rods too far during a shut down, then withdrew them without following procedures
NRC also said there were too many people in the control room
ComEd ended up shutting down both units because it was too costly to fix the damage (estimated at $435 million!)
However, the plant was used for many years as a source of reactive power for North Illinois 17

19. Zion Nuclear Power Plant18

20. Lab 6 Reactive Power Optimization19

21. Power Transactions
Power transactions are contracts between generators and loads 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 value of Psched used in the ACE calculation 20

22. PTDFs
Power transfer distribution factors (PTDFs) show the linear impact of a transfer of power.
PTDFs can be calculated using the fast decoupled power flow B matrix 21

23. Nine Bus PTDF Example
Figure shows initial flows for a nine bus power system 22

24. Nine Bus PTDF Example, cont'd
Figure now shows percentage PTDF flows from A to I 23

25. WE to TVA PTDFs

26. Line Outage Distribution Factors (LODFS)
LODFs are used to approximate the change in the flow on one line caused by the outage of a second line
typically they are only used to determine the change in the MW flow
LODFs are used extensively in real-time operations
LODFs are state-independent but do dependent on the assumed network topology

27. Flowgates
The real-time loading of the power grid is accessed via “flowgates”
A flowgate “flow” is the real power flow on one or more transmission element for either base case conditions or a single contingency
contingent flows are determined using LODFs
Flowgates are used as proxies for other types of limits, such as voltage or stability limits
Flowgates could be calculated using a spreadsheet
A list of the MISO flowgates is given at

28. NERC Regional Reliability Councils
NERC is the North American Electric Reliability Council
Image:

29. Generation Dispatch
Since the load is variable and there must be enough generation to meet the load, almost always there is more generation capacity available than load
Optimally determining which generators to use can be a complicated task due to many different constraints
For generators with low or no cost fuel (e.g., wind and solar PV) it is “use it or lose it”
For others like hydro there may be limited energy for the year
Some fossil has shut down and start times of many hours
Economic dispatch looks at the best way to instantaneously dispatch the generation 28

30. Generator types
Traditionally utilities have had three broad groups of generators
baseload units: large coal/nuclear; always on at max.
midload units: smaller coal that cycle on/off daily
peaker units: combustion turbines used only for several hours during periods of high demand
Wind and solar PV can be quite variable; usually they are operated at max. available power 29

31. Thermal versus Hydro Generation
The two main types of generating units are thermal and hydro, with wind rapidly growing
For hydro the fuel (water) is free but there may be many constraints on operation
fixed amounts of water available
reservoir levels must be managed and coordinated
downstream flow rates for fish and navigation
Hydro optimization is typically longer term (many months or years)
In 460 we will concentrate on thermal units and some wind, looking at short-term optimization 30