Module 2 : Equipment and Stability Constraints in System Operation
Lecture 5 : Generator Constraints
Objectives
In this lecture you will learn the following
Nature of generator constraints.
Generator Capability curves.
Typical rating of a large synchronous generator.
The Generator Capability Curves
Generator Limits: Generator Capability is constrained mainly by the following limits:
Voltage limits: The terminal voltage of a generator is limited due to 2 reasons : 1) Dielectric 2) Heating in
core due to excess magnetic flux. However, the maximum continuous limit due to excess flux is lower than
that due to dielectric breakdown considerations. Therefore the limit due to excess flux is the "determining"
limit. The flux in the core is also affected by the frequency ( core flux is proportional to voltage/frequency).
Armature Winding (heating) Limit: Armature winding heating results due to the resistive loss in armature
windings.
Field Winding (heating) Limit: Ohmic loss and consequent heating in the field winding, imposes a restriction
on the maximum field current. Since field winding current is proportional to the field voltage (after electrical
transients have died down), this limit is equivalent to a field voltage limit. Field current is higher when the
generator supplies reactive power and is over-excited.
Core-end heating limit: Core-end heating results when field current is low (under-excitation). During under-
excitation conditions, the axial flux in the end region is enhanced. This results in heating which may limit the
capability of a generator.
The heating limits are dependent on the efficacy of cooling. A higher pressure of the cooling medium (hydrogen)
results in higher heating limits. Armature winding current limit is essentially an MVA limit since terminal voltage
magnitude is maintained near the rated value. Therefore armature winding limit locus is a circle on the P-Q plane with
origin as the center(why ?)
In practice, the field current of a generator is measured or estimated, and the excitation system of a generator is
controlled so as to avoid exceeding field current limits. Excitation system controls are covered later in the course.
We shall now visualize a typical capability curve of a generator by an example.
Lecture 5 : Generator Constraints
Objectives
In this lecture you will learn the following
Nature of generator constraints.
Generator Capability curves.
Typical rating of a large synchronous generator.
The Generator Capability Curves
Generator Limits: Generator Capability is constrained mainly by the following limits:
Voltage limits: The terminal voltage of a generator is limited due to 2 reasons : 1) Dielectric 2) Heating in
core due to excess magnetic flux. However, the maximum continuous limit due to excess flux is lower than
that due to dielectric breakdown considerations. Therefore the limit due to excess flux is the "determining"
limit. The flux in the core is also affected by the frequency ( core flux is proportional to voltage/frequency).
Armature Winding (heating) Limit: Armature winding heating results due to the resistive loss in armature
windings.
Field Winding (heating) Limit: Ohmic loss and consequent heating in the field winding, imposes a restriction
on the maximum field current. Since field winding current is proportional to the field voltage (after electrical
transients have died down), this limit is equivalent to a field voltage limit. Field current is higher when the
generator supplies reactive power and is over-excited.
Core-end heating limit: Core-end heating results when field current is low (under-excitation). During under-
excitation conditions, the axial flux in the end region is enhanced. This results in heating which may limit the
capability of a generator.
The heating limits are dependent on the efficacy of cooling. A higher pressure of the cooling medium (hydrogen)
results in higher heating limits. Armature winding current limit is essentially an MVA limit since terminal voltage
magnitude is maintained near the rated value. Therefore armature winding limit locus is a circle on the P-Q plane with
origin as the center(why ?)
In practice, the field current of a generator is measured or estimated, and the excitation system of a generator is
controlled so as to avoid exceeding field current limits. Excitation system controls are covered later in the course.
We shall now visualize a typical capability curve of a generator by an example.
