GENERATOR
CAPABILITY CURVE
By
Prof. C. Radhakrishna
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CONTENTS
GENERATOR CAPABILITY CURVE
Reactive Capability Curves
Armature current limit
Field current limit
End region heating limit
Generator Characteristics
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GENERATOR CAPABILITY CURVE
Reactive Capability Curves
• Synchronous generators are rated in terms of the
maximum MVA output at a specified voltage and power
factor (usually 0.85 or 0.9 lagging) which they can carry
continuously without overheating.
• The active power output is limited by the prime mover
capability to a value within the MVA rating.
• The continuous reactive power output capability is limited
by three considerations: armature current limit, field
current limit, and end region heating limit.
Armature current limit
• One of the limitations on generator rating is the maximum
current that can be carried by the armature without
exceeding the heating limitations.
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Figure 1 Armature current heating limit
Armature current limit

and
E
q
(equal to X
ad
i
fd
). The corresponding phasor diagram, with R
a
neglected, is shown in Figure 2.
.
Field current limit
2
fdfd
iR
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Field current limit
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Field current limit
cos sin
ad
tt tfd i
s
X
PEI Ei
X



2

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End region heating limit
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• 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.
• This is illustrated in Figure 4, which also includes the limit imposed by
the armature current heating effects.
• The field current and armature current heating limits when plotted on a
P-Q plane depend on the armature voltage.
End region heating limit
Figure 4 End region heating limit
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Generator Characteristics
The above limits on the operation of the generator are those
imposed by the capabilities of the machine itself and are determined
by the design of the machine. Additional limits may be imposed by
power system stability limits.
• During conditions of low-system voltages, the reactive power demand on
generators may exceed their field current and/or armature current limits.
• When the reactive power output is limited, the terminal voltage is no
longer maintained constant.
• On most generators, the armature current limit is realized manually by
operators responding to alarms.
• The operator reduces reactive and/or active power output to bring the