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How the Power Grid Behaves


Transformers are used to change the voltage. 15. Other One-line Objects ... Load-tap-changing (LTC) transformers: vary their off-nominal tap ratio to keep a ... – PowerPoint PPT presentation

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Title: How the Power Grid Behaves

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

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.

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.

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

One-line Diagram
  • Most power systems are balanced three phase
  • 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.

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

  • 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.

  • 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.

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.

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

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.

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.

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

Overloaded Transmission Line
Thermal limit of 150 MVA
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).

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

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

Home Area ACE
ACE changes with time
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.

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.

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

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.

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

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 -

Implementation of 100 MW Transaction
Overloaded line
Net tie flow is now 100 MW from left to right
Scheduled Transaction
Security Constrained ED
  • Transmission constraints often limit system
  • 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.

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

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

Eastern Interconnect Example
Arrows indicate the basecase flow between areas
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
  • All transmission players in network could be
    impacted, to a greater or lesser extent.

PTDFs for Transfer from Wisconsin Electric to TVA
Piecharts indicate percentage of transfer that
will flow between specified areas
PTDF for Transfer from WE to TVA
100 of transfer leaves Wisconsin Electric (WE)
PTDFs for Transfer from WE to TVA
About 100 of transfer arrives at TVA
But flow does NOT follow contract path
  • 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
  • First contingency refers to loss of any one
  • Contingencies can have major impact on Power
    Transfer Distribution Factors (PTDFs).

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.

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

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.

Reactive Power Example
Reactive power must also sum to zero at each bus
Note reactive line losses are about 13 Mvar
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
  • Voltage regulation is a vital part of system

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

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.

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

  • 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.