11 - Transient Stability
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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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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?
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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…
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 8
Swing Equation
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 9
Synchronous Machines (cont’d) • Swing Equation
M D Pmech Pelec
= inertia constant = damping constant = input mechanical power = output electrical power
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 10
Rotor Angle Responses
•
Case 1: Steady-state stable
•
Case 2: Transient stable
•
Case 3: Small-signal unstable
•
Case 4: First swing unstable
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 11
Power and Rotor Angle (Classical 2-Machine Example)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 12
Power and Rotor Angle (cont’d)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 13
Power and Rotor Angle (Parallel Lines)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 14
Both Lines In Service
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 15
One Line Out of Service
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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Equal Area Criterion
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 17
Equal Area Criterion
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 18
Equal Area - Stable
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 19
Equal Area – Unstable
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
Slide 20
Equal Area - Unstable
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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°
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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 ↓
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Transient Stability
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
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