How the Power Grid Behaves

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How the Power Grid Behaves

• Tom Overbye
• Department of Electrical and Computer Engineering
• University of Illinois at Urbana-Champaign

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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.

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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.

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Eastern Interconnect Operating Areas
Ovals represent operating areas
Arrows indicate power flow in MW between areas
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Zoomed View of Midwest
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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.

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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.

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Eastern North American High Voltage Transmission
Grid
Figure shows transmission lines at 345 kV or
above in Eastern U.S.
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Zoomed View of Midwest
Arrows indicate MW flow on the lines piecharts
show percentage loading of lines
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Example Three Bus System
Pie charts show percentage loading of lines
Generator
Load
Bus
Circuit Breaker
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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.

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Loads

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

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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.

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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.

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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.

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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 Kirchhoffs law. And it can not
  be repealed or modified.
• Power is lost in the transmission system.

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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.

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Flow Redistribution Following Opening Line
Circuit Breaker
No flow on open line
Power Balance must be satisfied at each bus
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Indirect Control of Line Flow
Generator change indirectly changes line flow
Generator MW output changed
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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.

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Overloaded Transmission Line
Thermal limit of 150 MVA
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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).

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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

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

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Home Area ACE
ACE changes with time
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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.

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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.

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

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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.

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Example Generator Fuel-Cost Curve
Y-axis tells cost to produce specified power (MW)
in /hr
Current generator operating point
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Example Generator Marginal Cost Curve
Y-axis tells marginal cost to produce one more
MWhr in /MWhr
Current generator operating point
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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)

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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 ACEACE Pactual,tie-flow -
  Psched

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

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

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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
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Seven Bus Case Area View
Actual flow between areas
Scheduled flow between areas
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Seven Bus Case with 100 MW Transfer
Losses went up from 7.09 MW
100 MW Scheduled Transfer from Left to Right
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Seven Bus Case One-line
Transfer also overloads line in Top
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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.

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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.

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Eastern Interconnect Example
Arrows indicate the basecase flow between areas
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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.

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PTDFs for Transfer from Wisconsin Electric to TVA
Piecharts indicate percentage of transfer that
will flow between specified areas
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PTDF for Transfer from WE to TVA
100 of transfer leaves Wisconsin Electric (WE)
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PTDFs for Transfer from WE to TVA
About 100 of transfer arrives at TVA
But flow does NOT follow contract path
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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).

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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.

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Reactive Power

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

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Reactive Power

• Reactive power doesnt travel well - must be
  supplied locally.
• Reactive must also satisfy Kirchhoffs law -
  total reactive power into a bus MUST be zero.

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

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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.

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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.

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

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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
• www.powerworld.com
• 60,000 bus commercial version is also available.