Transient Stability 2

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(2)

Topics

• What is Transient Stability (TS)

• What Causes System Unstable

• Effects When System Is Instable

y

• Transient Stability Definition

• Modeling and Data Preparation

• ETAP TS Study Outputs

• Power System TS Studies

(3)

What is Transient Stability

• TS is also called Rotor Angle Stability

¾S

hi

b

h

i

l

d

¾Something between mechanical system and

electrical system – energy conversion

• It is a Electromechanical Phenomenon

¾Time frame in milliseconds

• All Synchronous Machines Must Remain in

Synchronism with One Another

y

¾Synchronous generators and motors

¾This is what system stable or unstable means

(4)

What is Transient Stability

• Torque Equation (generator case)

T = mechanical torque

P = number of poles

P

number of poles

φ

_air

= air-gap flux

F

_r

= rotor field MMF

(5)

What is Transient Stability

• Swing Equation

M

= inertia constant

M

inertia constant

D

= damping constant

P

= input mechanical power

P

_mech

= input mechanical power

(6)

What Causes System Unstable

• From Torque Equation

¾T ( i

)

¾T (prime mover)

¾Rotor MMF (field winding)

¾Air-Gap Flux (electrical system)

• From Swing Equation

g

q

¾Pmech

¾Pelec

¾Different time constants in mechanical and

electrical systems

(7)

What Causes System Unstable

• In real operation

¾Sh

i

¾Short-circuit

¾Loss of excitation

¾Prime mover failure

¾Loss of utility connections

¾Loss of a portion of in-plant generation

¾Starting of a large motor

g

¾Switching operations

¾Impact loading on motors

(8)

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

Case 3: Small-signal unstable

g

Case 4: First swing unstable

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Effects When System Is Instable

• A 2-Machine

Example

• At

At

δ = -180º

δ 180

(Out-of-Step,

Slip the Pole)

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Effects When System Is Instable

• Synchronous machine slip poles –

generator tripping

• Power swing

• Misoperation of protective devices

• Interruption of critical loads

Interruption of critical loads

• Low-voltage conditions – motor drop-offs

• Damage to equipment

• Area wide blackout

• …

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Transient Stability Definition

• Examine One Generator

• Power Output Capability Curve

• δ

is limited to 180º

• δ

is limited to 180º

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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

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Modeling and Data Preparation

• Synchronous Machine

¾ Machine

¾ Exciter and AVR

¾ Prime Mover and Governor / Load Torque

¾ Power System Stabilizer (PSS) (Generator)

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Modeling and Data Preparation

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Modeling and Data Preparation

• Induction Machine

¾ Machine

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Modeling and Data Preparation

• Power Grid

¾ Short-Circuit Capability

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Modeling and Data Preparation

• Load

¾ Voltage dependency

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Modeling and Data Preparation

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Modeling and Data Preparation

(21)

Modeling and Data Preparation

Device Type

Action

Bus

3-P Fault

L-G Fault

Clear Fault

Branch

Fraction

Fault

Clear

Fault

PD

Trip

Close

Generator

Droop /

Isoch

Start

Loss Exc.

P Change

V Change

Delete

Grid

P Change

V Change

Delete

M t

A

l

t

L

d

D l t

Motor

Accelerate

Load

Change

Delete

Lumped Load

Load

Change

Delete

g

MOV

Start

Wind Turbine

Disturbance

Gust

Ramp

MG Set

Emergency

Main

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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 U d

lt

¾ Under-voltage

• Motor Dynamic Acceleration

¾ Induction motor

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Power System TS Studies

• Critical Fault Clearing Time (CFCT)

Clear fault

1 cycle

Clear fault

Fault

unstable

Cycle

unstable

stable

CFCT

• Critical Separation Time (CSST)

Separation

Fault

un

s

un

s

_Cycle

1 cycle

un

s

st

a

1 cycle

s

table

s

table

Cycle

s

table

a

ble

CSST

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Power System TS Studies

• Fast Bus Transfer

1

Motor residual voltage

0 0.2 0.4 0.6 0.8 Vmotor -0.8 -0.6 -0.4 -0.2 -1 -0.8 -0.6 -0.4 -0.2 0s 0.2 0.4 0.6 0.8 1 -1

(25)

Power System TS Studies

• Fast Bus Transfer

E

_S

= System equivalent per unit

volts per hertz

E = Motor residual per unit per

δ

E

_M

= Motor residual per unit per

hertz

E

_R

= Resultant vectorial voltage

in per unit volts per hertz

¾T

_transfer

≤ 10 cycles

in per unit volts per hertz

¾T

_transfer

≤ 10 cycles

¾

δ

≤ 90 degrees

¾E

≤ 1 33 per unit (133%)

¾E

_R

≤ 1.33 per unit (133%)

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Power System TS Studies

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Power System TS Studies

• Motor Dynamic Acceleration

¾I

t

t f

i l

d d

t

ti

¾Important for islanded system operation

¾Motor starting impact

¾Generator AVR action

¾Reacceleration

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Solution to Stability Problems

• Improve System Design

¾ I

h

i i

¾ Increase synchronizing power

• Design and Selection of Rotating Equipment

¾ Use of induction machines

¾ Increase moment of inertia

¾ Reduce transient reactance

¾ Improve voltage regulator and exciter

(29)

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 relay

¾Out Of Step relay

¾…