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Power system stabIlity

AndReliabIlity

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CONTENTy STABLITYy CASSIFICATION

i) STEADYSTATESTABILITY

ii) TRANSIENTSTATESTABILITYy POWER ANGLE DIAGRAMy SWING EQUATIONy SWING CURVEy EQUAL AREACRITERIONy

RELIABILITYy METHODS OF IMPROVING RELIABLITYy METHODS OF IMPROVING STABLITY

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What is Stability?y In power system large number of synchronus machine

operating in synchronism and they should work in steadystate condition.

y When they are subjected to some disturbances there istendency for a system to develop forces to bring them tostable condition.

y The ablity of a system to reach a normal or stable condition

after being disturbed is called STABILITY.

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Classification of Stability

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Steady state stabilityy It is the ability of a system to bring stable

condition after small disturbance.

y Concerned with the effect of gradual infinitesimalpower changes.

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Types of Steady State Stabilityy Static Stability-

(Without automatic control devices).

y Dynamic Stability-(With automatic Control devices).

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Steady state stability criteriony The criterion of stability holds the following

assumptions;

yA Generators are represented by constantimpedence in series with no-load voltages.

y The mechanical power input is constant.

y Damping is negligible.

y Load angle varitions are small.

y Speed variation are negligible.

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POWER ANGLE DIAGRAM

o The graphical representation of power P(e) and the

load angle is called power angle diagram. Maximum power transferred when power angle = 90

It becomes zero at power angle =180

o

Maximum value of P(e) is called pull out power orsteady state limit.

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Formula usedy The maximum steady state power transfer is given

by

P = [EVSIN(90)]/X

Where P= POWERTRANSFER

E=VOLTAGE BEHIND DIRECTAXIS

SYNCHRONUS REACTANCE OF THE MACHINE

V= VOLTAGE OF INFINITE BUS

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Power Angle Diagram

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SWINGEQUATIONy SWING EQUATION is basically used to determine the

transient stability.

y It describes swing in the power angle during transients.

y It is second order diffrential equation.

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SWINGCURVESy SWING CURVE is the graph between the torque angle

(in electrical radians)versus time (in seconds) .

yThey provide information regarding stability.

ifTorque angle increases continuously with time system

is stable.

if torque angle starts decreasing after reaching amaximum value then the system will remain stable.

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Swing Curve

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Equal area criterion of stabilityThis is used to assess the stability of two

machine connected on infinte bus

without actually solving the swingequation.

used for quickly predict the stablity of the system after

distubance

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Examples ofRecent Major System

Disturbances/Blackouts1. July 2, 1996 disturbance of WSCC (Western North

American Interconnected) System

2.August 10, 1996 disturbance of WSCC system3. 1998 power failure of Auckland business districts,

New Zealand

4. March 11, 1999 Brazil blackout

5.July 29, 1999 Taiwan disturbance

6.August 14, 2003 blackout of Northeast U.S. andOntario

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HERECOMESTHECONCEPT OF

RELIABLITY

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WHATRELIABILITY IS?Reliability:

1. The probability of satisfactoryoperation over the long run.

2. Denotes the ability to supplyadequate electric service on a

nearly continuous basis, withfew interruptions over anextended period.

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Methods ofREDUCINGTHESE

BLACKOUTSy Impractical to achieve complete immunity to blackouts

y need to strike a balance between economy and security

y Good design and operating practices could significantlyminimize the occurrence and impact of widespread outages

y Reliability criteria

y On-line security assessment

y

Robust stability controlsy Coordinated emergency controls

y Real-time system system monitoring and control

y Wide-spread use of distributed generation

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Reliability CriteriayAt present, systems designed and operated to

withstand

y loss of any single element preceded by single-, double-,or three-phase fault

y referred to as "N-1 criterion"

y Need for using risk-based security assessment

y consider multiple outages

y account for probability and consequences of instability

Built-in overall strength or robustness best defense against

catastrophic failures!

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Dynamic Security Assessment (DSA)y Practical tools with the required accuracy, speed and

robustnessy a variety of analytical techniques integrated

y distributed hardware architecture using low cost PCsy integrated with energy management system

y Capable of assessing rotorangle stabilityandvoltagestabilityy

determine critical contingencies automaticallyy security limits/margins for all desired energy transactions

y identify remedial measures

y Theindustryhasyettotake fulladvantageofthesedevelopments!

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DSA Using Intelligent Systems

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Real-Time Monitoring and Control:

An Emerging Technologyy Advances in communications technology have made it

possible to

y monitor power systems over a wide area

y remotely control many functions

y Research on use of multisensor data fusion technology

y process data from different monitors, integrate and processinformation

y

identify phenomenon associated with impending emergencyy make intelligent control decisions

y A fast and effective way to predict onset of emergencyconditions and take remedial actions

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METHODS OF IMPROVING STABILITYy Use of double circuit lines.

y Use of bundled conductors.

y

High speed excitation system.y Load shedding.

y Braking resistor.

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THANKYOU