1. How the Power Grid Behaves
Tom Overbye
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign

2. Presentation Overview
Goal is to demonstrate operation of large scale power grid.
Emphasis on the impact of the transmission syste.
Introduce basic power flow concepts through small system examples.
Finish with simulation of Eastern U.S. System.

3. PowerWorld Simulator
PowerWorld Simulator is an interactive, Windows based simulation program, originally designed at University of Illinois for teaching basics of power system operations to non-power engineers.
PowerWorld Simulator can now study systems of just about any size.

4. Eastern Interconnect Operating Areas
Ovals represent operating areas
Arrows indicate power flow in MW between areas

5. Zoomed View of Midwest

6. Power System Basics
All power systems have three major components: Generation, Load and Transmission.
Generation: Creates electric power.
Load: Consumes electric power.
Transmission: Transmits electric power from generation to load.

7. One-line Diagram Most power systems are balanced three phase systems.
A balanced three phase system can be modeled as a single (or one) line.
One-lines show the major power system components, such as generators, loads, transmission lines.
Components join together at a bus.

8. Eastern North American High Voltage Transmission Grid
Figure shows transmission lines at 345 kV or above in Eastern U.S.

9. Zoomed View of Midwest
Arrows indicate MW flow on the lines; piecharts show percentage loading of lines

10. Example Three Bus System
Pie charts show percentage loading of lines
Generator
Load
Bus
Circuit Breaker

11. Generation Large plants predominate, with sizes up to about 1500 MW.
Coal is most common source, followed by hydro, nuclear and gas.
Gas is now most economical.
Generated at about 20 kV.

12. Loads Can range in size from less than a single watt to 10’s of MW.
Loads are usually aggregated.
The aggregate load changes with time, with strong daily, weekly and seasonal cycles.

13. Transmission
Goal is to move electric power from generation to load with as low of losses and cost as possible.
P = V I or P/V = I
Losses are I2 R
Less losses at higher voltages, but more costly to construct and insulate.

14. Transmission and Distribution
Typical high voltage transmission voltages are 500, 345, 230, 161, 138 and 69 kV.
Transmission tends to be a grid system, so each bus is supplied from two or more directions.
Lower voltage lines are used for distribution, with a typical voltage of 12.4 kV.
Distribution systems tend to be radial.
Transformers are used to change the voltage.

15. Other One-line Objects
Circuit Breakers - Used to open/close devices; red is closed, green is open.
Pie Charts - Show percentage loading of transmission lines.
Up/down arrows - Used to control devices.
Values - Show current values for different quantities.

16. Power Balance Constraints
Power flow refers to how the power is moving through the system.
At all times the total power flowing into any bus MUST be zero!
This is know as Kirchhoff’s law. And it can not be repealed or modified.
Power is lost in the transmission system.

17. Basic Power Control
Opening a circuit breaker causes the power flow to instantaneously(nearly) change.
No other way to directly control power flow in a transmission line.
By changing generation we can indirectly change this flow.

18. Flow Redistribution Following Opening Line Circuit Breaker
No flow on
open line
Power Balance must
be satisfied at each bus

19. Indirect Control of Line Flow
Generator change indirectly changes line flow
Generator MW
output changed

20. Transmission Line Limits
Power flow in transmission line is limited by a number of considerations.
Losses (I2 R) can heat up the line, causing it to sag. This gives line an upper thermal limit.
Thermal limits depend upon ambient conditions. Many utilities use winter/summer limits.

21. Overloaded Transmission Line
Thermal limit
of 150 MVA

22. Interconnected Operation
Power systems are interconnected across large distances. For example most of North American east of the Rockies is one system, with most of Texas and Quebec being major exceptions
Individual utilities only own and operate a small portion of the system, which is referred to an operating area (or an area).

23. Operating Areas Areas constitute a structure imposed on grid.
Transmission lines that join two areas are known as tie-lines.
The net power out of an area is the sum of the flow on its tie-lines.
The flow out of an area is equal to total gen - total load - total losses = tie-flow

24. Three Bus System Split into Two Areas
Initially area flow
is not
controlled
Net tie flow
is NOT zero

25. Area Control Error (ACE)
The area control error mostly the difference between the actual flow out of area, and scheduled flow.
ACE also includes a frequency component.
Ideally the ACE should always be zero.
Because the load is constantly changing, each utility must constantly change its generation to “chase” the ACE.

26. Home Area ACE ACE changes with time 06:30 AM 06:15 AM Time -20.0 -10.0
Area Control Error (MW)
ACE changes with time

27. Inadvertent Interchange
ACE can never be held exactly at zero.
Integrating the ACE gives the inadvertent interchange, expressed in MWh.
Utilities keep track of this value. If it gets sufficiently negative they will “pay back” the accumulated energy.
In extreme cases inadvertent energy is purchased at a negotiated price.

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

29. Three Bus Case on AGC With AGC on, net tie flow is zero, but
individual line flows
are not zero

30. Generator Costs
There are many fixed and variable costs associated with power system operation.
Generation is major variable cost.
For some types of units (such as hydro and nuclear) it is difficult to quantify.
For thermal units it is much easier. There are four major curves, each expressing a quantity as a function of the MW output of the unit.

31. Generator Cost Curves
Input-output (IO) curve: Shows relationship between MW output and energy input in Mbtu/hr.
Fuel-cost curve: Input-output curve scaled by a fuel cost expressed in $ / Mbtu.
Heat-rate curve: shows relationship between MW output and energy input (Mbtu / MWhr).
Incremental (marginal) cost curve shows the cost to produce the next MWhr.

32. Example Generator Fuel-Cost Curve
150
300
450
600
Generator Power (MW)
2500
5000
7500
10000
Fuel-cost ($/hr)
Y-axis tells
cost to
produce specified power (MW) in $/hr
Current generator
operating point

33. Example Generator Marginal Cost Curve
150
300
450
600
Generator Power (MW)
0.0
5.0
10.0
15.0
20.0
Incremental cost ($/MWH)
Y-axis tells
marginal cost to
produce one more MWhr in $/MWhr
Current generator
operating point

34. 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)

35. 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

36. Implementation of 100 MW Transaction
Overloaded
line
Net tie flow is
now 100 MW from
left to right
Scheduled Transaction

37. Security Constrained ED
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.

38. Security Constrained Dispatch
Gens 2 &3
changed to remove
overload
Net tie flow is
still 100 MW from
left to right

39. Multi-Area Operation
The electrons are not concerned with area boundaries. Actual power flows through the entire network according to impedance of the transmission lines.
If Areas have direct interconnections, then they can directly transact up their tie-line capacity.
Flow through other areas is known as “parallel path” or “loop flows.”

40. Seven Bus, Thee Area Case One-line
Area “Top”
has 5 buses
ACE for
each area
is zero
Area “Left” has one bus
Area “Right” has one bus

41. Seven Bus Case: Area View
Actual flow between areas
Scheduled flow between areas

42. Seven Bus Case with 100 MW Transfer
Losses
went up
from
7.09 MW
100 MW Scheduled Transfer from Left to Right

43. Seven Bus Case One-line
Transfer also overloads
line in Top

44. Transmission Service
FERC Order No. 888 requires utilities provide non-discriminatory open transmission access through tariffs of general applicability.
FERC Order No. 889 requires transmission providers set up OASIS (Open Access Same-Time Information System) to show available transmission.

45. Transmission Service
If areas (or pools) are not directly interconnected, they must first obtain a contiguous “contract path.”
This is NOT a physical requirement.
Utilities on the contract path are compensated for wheeling the power.

46. Eastern Interconnect Example
Arrows indicate the basecase flow between areas

47. Power Transfer Distribution Factors (PTDFs)
PTDFs are used to show how a particular transaction will affect the system.
Power transfers through the system according to the impedances of the lines, without respect to ownership.
All transmission players in network could be impacted, to a greater or lesser extent.

48. PTDFs for Transfer from Wisconsin Electric to TVA
Piecharts indicate percentage of transfer that will flow between specified areas

49. PTDF for Transfer from WE to TVA
100% of transfer leaves Wisconsin Electric (WE)

50. PTDFs for Transfer from WE to TVA
About 100% of transfer arrives at TVA
But flow does NOT follow contract path

51. Contingencies
Contingencies are the unexpected loss of a significant device, such as a transmission line or a generator.
No power system can survive a large number of contingencies.
First contingency refers to loss of any one device.
Contingencies can have major impact on Power Transfer Distribution Factors (PTDFs).

52. Available Transfer Capability
Determines the amount of transmission capability available to transfer power from point A to point B without causing any overloads in basecase and first contingencies.
Depends upon assumed system loading, transmission configuration and existing transactions.

53. Reactive Power Reactive power is supplied by
generators
capacitors
transmission lines
loads
Reactive power is consumed by
transmission lines and transformers (very high losses

54. Reactive Power
Reactive power doesn’t travel well - must be supplied locally.
Reactive must also satisfy Kirchhoff’s law - total reactive power into a bus MUST be zero.

55. Reactive Power Example
Reactive power must also sum to zero at each bus
Note reactive line losses are about 13 Mvar

56. Voltage Magnitude
Power systems must supply electric power within a narrow voltage range, typically with 5% of a nominal value.
For example, wall outlet should supply 120 volts, with an acceptable range from 114 to 126 volts.
Voltage regulation is a vital part of system operations.

57. Reactive Power and Voltage
Reactive power and voltage magnitude are tightly coupled.
Greater reactive demand decreases the bus voltage, while reactive generation increases the bus voltage.

58. Voltage Regulation
A number of different types of devices participate in system voltage regulation
generators: reactive power output is automatically changed to keep terminal voltage within range.
capacitors: switched either manually or automatically to keep the voltage within a range.
Load-tap-changing (LTC) transformers: vary their off-nominal tap ratio to keep a voltage within a specified range.

59. Five Bus Reactive Power Example
Voltage magnitude is controlled by
capacitor
LTC Transformer is controlling load voltage

60. Voltage Control
Voltage control is necessary to keep system voltages within an acceptable range.
Because reactive power does not travel well, it would be difficult for it to be supplied by a third party.
It is very difficult to assign reactive power and voltage control to particular transactions.

61. Conclusion
Talk has provided brief overview of how power grid operates.
Educational Version of PowerWorld Simulator, capable of solving systems with up to 12 buses, can be downloaded for free at
60,000 bus commercial version is also available.