The document discusses the excitation and capability curve of a synchronous generator. It contains the following key points: 1. The generator's capability curve plots active power (P) versus reactive power (Q) and is bounded by limits including the armature current limit which determines the MVA rating, the field current limit set by rotor heating, and the prime mover power limit. 2. For stable operation, the load angle (delta) between the generator voltage and current must be less than 90 degrees, which is the theoretical stability limit. 3. The capability curve defines the operating zone of a synchronous generator in the P-Q plane and ensures it operates within thermal and stability limits.

1. Excitation and capability curve
Shashi Om Katiyar
NTPC Consultant
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2. Typical Configuration of a Power Plant
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3. 3
G
G
T
Earth 0 volts
V-
I-
Excitation
system
Due to fault in transmission line voltage will sink
At this moment excitation system will boost the voltage to remain synchronized

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18. Capability Curve
of
Synchronous
Generator
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19.  The MVA loading should not exceed the generator rating.
This limit is determined by the armature of the stator
heating by the armature current.
 The MW loading should not exceed the rating of the prime
mover.
 The field current should not be allowed to exceed a
specified value determined by the heating of the field.
 For steady state or stable operation, the load angle δ must be
less than 90 degrees. The theoretical stability limit
of the stable condition occurs when δ = 90⁰.
GENERATOR CAPABILITY DIAGRAM
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20. GENERATOR CAPABILITY DIAGRAM
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21. GENERATOR CAPABILITY DIAGRAM
Armature current limit
Field current limit
Prime Mover Limit
End region heating limit
(1) Armature Current Limit/ Stator Copper Loss (stator heating): The maximum
allowable heating of the stator sets a maximum phase current IA for the machine. It’s
equivalent tomax apparent power for the machine. (Power factor is irrelevant).
PSCL = 3 IA.RS
(1) Field Current Limit/ Rotor Copper Loss (rotor heating): The maximum allowable heating
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of the rotor sets a maximum field current IF for the machine. It’s equivalent to set a
maximum EA for the machine. PRCL = IF RF
(3)Prime-mover’s Power Limit: The active power output is limited by the prime mover
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capability to a value within the MVA rating. P =|Esinδ|
The limit is related to the mechanical input and the ability of the generator to
electromagnetically create a torque equal and opposite to the driving mechanical torque.
DEFINES THE OPERATING ZONE OF
A SYNCHRONOUS GENERATOR IN
A P-Q PLANE
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22. END REGION HEATING LIMIT
(4) The localized heating in the end region of the armature imposes
a third limit on the operation of a synchronous machine.
This limit affects
the capability of
the machine in the
under excited
condition.
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23. • At a given excitation, if mechanical torque increases, rotor
accelerates, increasing δ and electromagnetic torque.
• This negative feedback continues until electromagnetic and
mechanical torques balance.
• However, if generator is operating with δ close to 90° when rotor
speeds up, δ increases past 90°, electromagnetic torque falls and
positive feedback occurs, causing rotor to accelerate further,
pull out of synchronism and result in zero output power and
possibly catastrophic failure. The static stability limit is set at
δ=90°.
GENERATOR CAPABILITY DIAGRAM
Power = 3 Vph I Cos φ = 3 Vph E Sin δ/ X.
The power or torque can be thought of as
cross product of two electromagnetic
fields or a function of the sine of angle
between V and E.
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24. FUNDAMENTALS
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25. FUNDAMENTALS
Higher stator current will heat the copper of
stator winding and insulation is at stress.
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26. • Generator capability curve a plot of P versus Q
GENERATOR CAPABILITY DIAGRAM
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27. GENERATOR CAPABILITY DIAGRAM
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28. SYN. GENERATOR RATING
 In any balanced design, the thermal limits for the
field and armature intersect at a point, which
represents the machine
power factor rating.
 Capability Diagram gives
nameplate MV
A and
information about full
load rotor (excitation current) & maximum rotor
angle during steady state P
.F.
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29. Thanks
Thanks
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