Transient Stability 2

Transient Stability

©1996-2010 Operation Technology, Inc. – Workshop Notes: Transient Stability

Topics • What is Transient Stability (TS) • What Causes System Unstable • Effects When System y Is Instable • Transient Stability Definition • Modeling and Data Preparation • ETAP TS Study Outputs • Power System TS Studies • Solutions to Stability Problems ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 2

What is Transient Stability • TS is also called Rotor Angle Stability ¾Something ¾S hi b between mechanical h i l system and d electrical system – energy conversion

• It is a Electromechanical Phenomenon ¾Time frame in milliseconds

• All Synchronous Machines Must Remain in y with One Another Synchronism ¾Synchronous generators and motors ¾This is what system stable or unstable means ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 3

What is Transient Stability • Torque Equation (generator case)

T = mechanical torque P = number of poles

φair = air-gap flux Fr = rotor field MMF

δ = rotor angle

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 4

What is Transient Stability • Swing Equation

M D Pmech Pelec

= = = =

inertia constant damping constant input mechanical power output electrical power

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 5

What Causes System Unstable • From Torque Equation ¾T (prime ( i mover)) ¾Rotor MMF (field winding) ¾Air-Gap Flux (electrical system)

• From Swing g Equation q ¾Pmech ¾Pelec ¾Different time constants in mechanical and electrical systems ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 6

What Causes System Unstable • In real operation ¾Short-circuit ¾Sh i i ¾Loss of excitation ¾Prime mover failure ¾Loss of utility connections ¾Loss of a portion of in-plant generation ¾Starting g of a large g motor ¾Switching operations ¾Impact loading on motors ¾Sudden large change in load and generation ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 7

Effects When System Is Instable • Swing in Rotor Angle (as well as in V, I, P, Q and f)

Case 1: Steady-state stable Case 2: Transient stable g unstable Case 3: Small-signal Case 4: First swing unstable ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 8

Effects When System Is Instable • A 2-Machine Example

• At δ = -180º 180 (Out-of-Step, Slip the Pole)

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 9

Effects When System Is Instable • 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 • … ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 10

Transient Stability Definition • Examine One Generator

• Power Output Capability Curve

• δ is limited to 180º ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 11

Transient Stability Definition • 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

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 12

Modeling and Data Preparation • Synchronous Machine

¾ ¾ ¾ ¾

Machine Exciter and AVR Prime Mover and Governor / Load Torque Power System Stabilizer (PSS) (Generator)

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 13

Modeling and Data Preparation

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 14

Modeling and Data Preparation • Typical synchronous machine data

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 15

Modeling and Data Preparation • Induction Machine ¾ Machine ¾ Load Torque q

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 16

Modeling and Data Preparation • Power Grid ¾ Short-Circuit Capability ¾ Fixed internal voltage g and infinite inertia

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 17

Modeling and Data Preparation • Load ¾ Voltage dependency ¾ Frequency q y dependency p y

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 18

Modeling and Data Preparation • Load

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 19

Modeling and Data Preparation • Events and Actions

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 20

Modeling and Data Preparation Device Type

Action

Bus

3-P Fault

L-G Fault

Branch

Fraction Fault

Clear Fault

PD

Trip

Close

Generator

Droop / Isoch

Start

Loss Exc.

Grid

P Change

V Change

Delete

M t Motor

A Accelerate l t

Load L d Change

D l t Delete

Lumped Load Load Change g

Clear Fault

P Change

V Change

Delete

Delete

MOV

Start

Wind Turbine

Disturbance

Gust

MG Set

Emergency

Main

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Ramp

Slide 21

Power System TS Studies • Fault ¾ 3-phase and single phase fault ¾ Clear fault ¾ Critical Fault Clearing Time (CFCT) ¾ Critical System Separation Time (CSST)

• Bus Transfer ¾ Fast load transferring g

• Load Shedding ¾ Under-frequency ¾ Under-voltage U d lt

• Motor Dynamic Acceleration ¾ Induction motor ¾ Synchronous motor ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 22

Power System TS Studies • Critical Fault Clearing Time (CFCT) Fault

Clear fault Clear fault

1 cycle

1 cycle unstable

unstable

unstable

stable

CFCT

Clear fault Clear fault

Cycle

• Critical Separation Time (CSST) Fault

Separation Separation

1 cycle

1 cycle unsstable

unsstable

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

unsstable

sta able

CSST

Separation Separation

Cycle

Slide 23

Power System TS Studies • Fast Bus Transfer Motor residual voltage 1 0.8 Vmotor 0.6 0.4 0.2 0 -1

-0.8

-0.6

-0.4

-0.2

0

s

0.2

0.4

0.6

0.8

1

-0.2 -0.4 -0.6 -0.8 -1

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 24

Power System TS Studies • Fast Bus Transfer δ

ES = System equivalent per unit volts per hertz EM = Motor residual per unit per hertz ER = Resultant vectorial voltage in per unit volts per hertz

¾Ttransfer ≤ 10 cycles ¾δ ≤ 90 degrees ¾ER ≤ 1.33 1 33 per unit (133%) ©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 25

Power System TS Studies • Load Shedding

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 26

Power System TS Studies • Motor Dynamic Acceleration ¾Important ¾I t t for f islanded i l d d system t operation ti ¾Motor starting impact ¾Generator AVR action ¾Reacceleration

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 27

Solution to Stability Problems • Improve System Design ¾ Increase I synchronizing h i i 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

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

Slide 28

Solution to Stability Problems • Application of Power System Stabilizer (PSS) • Add System Protections ¾ Fast fault clearance ¾ Load shedding ¾ System separation ¾Out-Of-Step ¾Out Of Step relay ¾…

©1996-2010 Operation Technology, Inc. - Workshop Notes: Transient Stability

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