11 - Transient Stability

ETAP 5.0 Transient Stability

Copyright 2003 Operation Technology, Inc.

Time Frame of Power System Dynamic Phenomena

Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability

Slide 2

Introduction • TS is also called Rotor Stability, Dynamic Stability • Electromechanical Phenomenon • All synchronous machines must remain in synchronism with one another • TS is no longer only the utility’s concern • Co-generation plants face TS problems


Slide 3

Analogy

• • • •

Which vehicles will pushed hardest? How much energy gained by each vehicle? Which direction will they move? Height of the hill must they climb to go over?


Slide 4

Introduction (cont’d) • System protection requires consideration of: ¾Critical Fault Clearing Time (CFCT) ¾Critical Separation Time (CST) ¾Fast load transferring ¾Load Shedding ¾…


Slide 5

Causes of Instability • • • • • • •

Short-circuits Loss of utility connections Loss of a portion of in-plant generation Starting of a large motor Switching operations (lines or capacitors) Impact loading on motors Sudden large change in load and generation


Slide 6

Consequences of Instability • Synchronous machine slip poles – generator tripping • Power swing • Misoperation of protective devices • Interruption of critical loads • Low-voltage conditions – motor drop-offs • Damage to equipment • Area wide blackout • … Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability

Slide 7

Synchronous Machines • Torque Equation (generator case)

T = mechanical torque P = number of poles

φair = air-gap flux Fr = rotor field MMF

δ = rotor angle


Slide 8

Swing Equation


Slide 9

Synchronous Machines (cont’d) • Swing Equation

M D Pmech Pelec

= inertia constant = damping constant = input mechanical power = output electrical power


Slide 10

Rotor Angle Responses

•

Case 1: Steady-state stable

•

Case 2: Transient stable

•

Case 3: Small-signal unstable

•

Case 4: First swing unstable


Slide 11

Power and Rotor Angle (Classical 2-Machine Example)


Slide 12

Power and Rotor Angle (cont’d)


Slide 13

Power and Rotor Angle (Parallel Lines)


Slide 14

Both Lines In Service


Slide 15

One Line Out of Service


Slide 16

Equal Area Criterion


Slide 17

Equal Area Criterion


Slide 18

Equal Area - Stable


Slide 19

Equal Area – Unstable


Slide 20

Equal Area - Unstable


Slide 21

Power System Stability Limit • Steady-State Stability Limit ¾ After small disturbance, the synchronous generator reaches a steady state operating condition identical or close to the predisturbance ¾ Limit: δ < 90°


Slide 22

Power System Stability Limit (con’d) • Transient and Dynamic Stability Limit ¾ After a severe disturbance, the synchronous generator reaches a steady-state operating condition without a prolonged loss of synchronism ¾ Limit: δ < 180° during swing


Slide 23

Generator Modeling

• Machine Equivalent Model / Transient Model / Subtransient Model

• Exciter and Automatic Voltage Regulator (AVR) • Prime Mover and Speed Governor • Power System Stabilizer (PSS) Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability

Slide 24

Generator Modeling (con’d) • Typical synchronous machine data


Slide 25

Factors Influencing TS • Post-Disturbance Reactance seen from generator. Reactance ↓ Pmax ↓

• Duration of the fault clearing time. Fault time ↑ Rotor Acceleration ↑ Kinetic Energy ↑ Dissipation Time during deceleration ↑

• Generator Inertia. Inertia ↑ Rate of change of Angle ↓ Kinetic Energy ↓ • Generator Internal Voltage Internal Voltage ↓ Pmax ↓


Slide 26

Factors Influencing TS • Generator Loading Prior To Disturbance Loading ↑ Closer to Pmax. Unstable during acceleration

• Generator Internal Reactance Reactance ↓ Peak Power ↑ Initial Rotor Angle ↓ Dissipation Time during deceleration ↑

• Generator Output During Fault Function of Fault Location and Type of Fault


Slide 27

Solution to Stability Problems • Improve system design ¾ Increase synchronizing power

• Design and selection of rotating equipment ¾ ¾ ¾ ¾

Use of induction machines Increase moment of inertia Reduce transient reactance Improve voltage regulator and exciter characteristics


Slide 28

Solution to Stability Problems • Reduction of Transmission System Reactance • High Speed Fault Clearing • Dynamic Braking • Regulate Shunt Compensation • Steam Turbine Fast Valving • Generator Tripping • Adjustable Speed Synchronous Machines Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability

Slide 29

Solution to Stability Problems • HVDC Link Control • Current Injection from VSI devices • Application of Power System Stabilizer (PSS) • Add system protections ¾ Fast fault clearance ¾ Load Shedding ¾ System separation Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability

Slide 30

11 - Transient Stability

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