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# Generator Constraints Notes | EduRev

## : Generator Constraints Notes | EduRev

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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.
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.

Page 2

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.
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.

Voltage Limits
As mentioned in the previous slide, the voltage limit of a generator is mainly due to excess flux
considerations. While the maximum continuous limit is about 1.05 pu at nominal frequency, a larger
voltage can be tolerated for a short while (why?).
The typical allowable voltage (at nominal frequency) for different intervals of time are given below
V/Hz (pu) 1.25 1.2 1.15 1.10 1.05
Damage Time
(in min.)
GEN 0.2 1.0 6.0 20.0 Infinity
Typical Name Plate Data of a Generator
Typical Data for a 247 MVA generator is given below:
MVA maximum continuous rating: 247 MVA
MW maximum continuous rating: 210 MW
Rated voltage: 15.75 kV
Rated stator current : 9050 A
Rated power factor : 0.85 lagging
Field current (at maximum continuous rating) : 2600 A
Field Voltage (at maximum continuous rating) : 315 V
Efficiency (at maximum continuous rating) : 98.55 %
Speed : 3000 rpm
Cooling System : Hydrogen at 3.5 kg/cm^2
Recap
In this lecture you have learnt the following
Generators are constrained by heating limits on the armature windings, field winding and core end region

Field winding heating limit decides the capability in the over-excited region (lagging power factor) while
core end heating determines the under-excitation capability (leading power factor)

Congratulations, you have finished Lecture 5. To view the next lecture select it from the left hand side menu of
the page.

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