Synchronus Machine Stability

5/21/2018 Synchronus Machine Stability

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Synchronous MachineAll conventional power plants use synchronous generators

for converting power to electrical form

In Synchronous Generator, a DC current is applied torotor winding to produce rotor magnetic field.

The rotor is turned by prime-mover producing a rotatingmagnetic field.

The rotating magnetic field produce three phase sets ofvoltages within the stator.

Armature winding [in stator]

Field winding [in rotor]


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Synchronous Machine Construction


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Equivalent Circuit

Eph= Vt+ IaZs ---> Vector Sum

As rais very small

Eph= Vt+ IaXs ---> Vector Sum


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Phasor DiagramFig. (a): For alternator

Fig. (b): For sync. Motor: Angle between Vtand Ia: Angle between Vtand Eph


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Load Angle ()

Angle between Vtand Eph

Depends on the magnitude of load proportionally

Considered as +ve for alternator andve for sync.motor

Basically load angle is the angle between rotormagnetic field and stator magnetic field.


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Load Angle ()(cont...)

Basically load angle is the angle between rotormagnetic field and stator magnetic field.

Reference machine Loaded machine


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Power Relationships

mindconvP cos3

AAIE Where is the angle between EAand IA:

If the armature resistance RAis ignored (Xs>> RA),

S

A

A

X

EI sincos

Substituting this equation into Pout,

S

A

X

EVP

sin3


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Power Angle Characteristics

S

A

X

EVP

3

max

The P() curve shows that the

increase of power increases the anglebetween the induced voltage and the

terminal voltage.

The power is maximum when =900

The further increase of input power

forces the generator out of

synchronism.

The maximum power is the static

stability limitof the system.

Safe operation requires a 15-20%

power reverse.


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Stability of Electrical System

Power System Stability denotes the ability of an electricpower system, for a given initial operating condition, toregain a state of operating equilibrium after beingsubjected to a physical disturbance, with all system

variables bounded so that the system integrity ispreserved.

Stability is a condition of equilibrium between opposing

forces.

Instability results when a disturbance leads to a sustained

imbalance between the opposing forces.


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Division of stability

Rotor angle stability

Frequency stability

Voltage stability


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Rotor Angle StabilityA simple two machine system has been considered.

XG: Generator reactanceXM : Motor/Load reactance

XL: Line reactance

So total reactance, X = XG+ XM+ XL


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Rotor Angle Stability(Contd...)

The system is stable if and only if -900


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Power Swing

Synchronus Machine Stability

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