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Generator Basics
Generator is a machine which converts
Mechanical energy to Electrical energy
Magnetic Induction Principle
An emf is induced in a coil whenever
a) a coil cuts through a magnetic field
b) a magnetic field cuts through a coil
Generator Structure
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Generator Basics
GENERATOR BASICS
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Generator Basics
emf induction is due to relative motion
between two parts ( coil & magnetic field)
Relative motion is by Rotation
Two mechanical parts :
Field - Part which produces magnetism (Rotor)
Armature part where emf is induced (Stator)
Generator Structure
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Generator Basics
Generator Structure
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Generator Basics
Prime Mover
Rotor (Exciter)
Stator
Generator components
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Generator Basics
Source of mechanical power for
relative motion
Two classes of Prime movers :
High Speed : Steam & Gas TurbinesLow Speed : IC engines, Water turbines and
DC motors
Generator components- Prime mover
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Generator Basics
Type of prime mover plays an important role
in a generator installation
Many characteristics of alternator
and its construction depend upon
Speed at which the rotor is turned.
Generator components- Prime mover
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Generator Basics
Two types are used in rotating field alternators
- Turbine driven rotor
( used where prime mover is a high speed turbine)
- salient pole rotor( used with low speed prime movers)
Generator components- Rotors
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Generator BasicsGenerator components- Rotors
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Generator Basics
Many types based on
- Power out put
- Voltage output
- Type of cooling- Number of phases
Generator components- Stators
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Generator BasicsGenerator components-
Simplified schematic of 3 ph. Stator
Three single phase windings
displaced by 120 degrees
Star or Delta connection
Neutral may or may not be
brought out
No. of terminals can be
3,4, or 6
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Generator Basics
Out-puts from a Generator
Three phase ac power at :
-Rated Voltage
-Rated Frequency
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Generator Basics
Out-puts from a GeneratorCommon Ratings
Small Generators : 25 kVA to 300 kVA
Medium Size Generators : 500 kVA to 1 MVA
Large size Generators : 1 MVA to 25 MVA
Power Plant Generators: 110 MW to 500 MW
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Generator Basics
Out-puts from a Generator - Frequency
Frequency depends up on the
Speed of Rotation ( direct proportion)
F = PN / 120 where
F = frequency in Hz
P = No. of poles
N = speed of rotation in RPM
( 120 is a constant to convert minutes to seconds
and poles to pole pairs)
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Generator Basics
Out-puts from a Generator - Frequency
Nominal frequency = 50 Hz
Generator frequency can vary due toload fluctuations
Frequency Regulation is mainly
by adjusting the speed
speed is controlled by varying the fuel
input to the prime mover ( Governor control)
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Generator Basics
Out-puts from a Generator - Governors
Governor ensures frequency at a set value
by increasing / decreasing the fuel input
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Generator Basics
Out-puts from a Generator - Governors
Two types of Governors :
Pneumatic : Fuel flow is controlled by
valve position control
Control Input : DC Voltage
Electronic : Fuel flow is controlled by
a total servo mechanism
Control input : DC Voltage pulses
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Generator Basics
Droop Characteristics of Governors
Load
Speed
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Generator Basics
Out-puts from a Generator - Voltage
Voltage is induced in the stator winding
Terminal voltage depends upon :
1) No. of conductors in series per winding
2) Speed at which magnetic field cuts the winding3) Strength of the magnetic field
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Generator Basics
Out-puts from a Generator
Voltage Control
Nominal Voltage = 415 V, 11 kV Voltage can vary due to load fluctuations
Terminal voltage is controlled by
varying the Strength of magnetic field= varying the current through the field coil
= varying the voltage applied to the field coil
(Exciter control)
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Generator BasicsTypical Exciter
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Generator Basics
Out-puts from a Generator
Exciters
External to Exciter Transformer
Generator Rectifier Unit
Controls
Built on the Small DC machine
Generator attached to same shaftof generator
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Generator Basics
Typical Exciter
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Generator Basics
Neutral terminal
Very important from the point of view ofa) reducing the fault currents
b) providing effective protection
should be used for medium size and above
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Generator Basics
Neutral Grounding Resistors
Protect generators from excessive fault currents
Provides safety for operators
Provides protection for connected equipment
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Generator Basics
Generator Impedance
R
X
Synchronous ReactanceTransient reactance
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GENERATOR PROTECTION
= 0o (360O) FORWARD RESISTIVE
= +30o FORWARD RESISTIVE + FORWARD CAPACITIVE
= +60o FORWARD RESISTIVE + FORWARD CAPACITIVE
= +90o FORWARD CAPACITIVE (LOSS OF FIELD)
= +120o REVERSE RESISTIVE + FORWADRD CAPACITIVE
= +150o REVERSE RESISTIVE + FORWARD CAPACITIVE
= +180o REVERSE RESISTIVE
= +210o (-150O) REVERSE RESISTIVE + FORWARD INDUCTIVE
= +240o (-120O) REVERSE RESISTIVE + FORWARD INDUCTIVE
= +270o (-90O) FORWARD INDUCTIVE
= +300o (-60O) FORWARD INDUCTIVE + FORWARD RESISTIVE
= +330o (-30O) FORWARD INDUCTIVE + FORWARD RESISTIVE
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Generator Basics
Generator Behaviour
Influence of Rotor on StatorInfluence of Stator on Rotor
Effect of Load on Generator
Effect of excitation on Generator Impedance
Influence of generator on prime mover
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Generator Basics
Generator Behaviour
Influence of Rotor on Stator
Only variable is excitation current
Terminal voltage is proportional to
Excitation current Will influence Generator impedance
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Generator Basics
Generator Behaviour
Influence of Stator on Rotor
Only variable is the load
Unbalance loads can cause rotor heating
Excessive loads can cause out of step
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Generator Basics
Generator Behaviour
Effect of Load on Generator
Load will decide the pF of generator
Over loads can cause speed reduction
Unbalance loads can cause over heating
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Generator Basics
Generator Behaviour
Effect of excitation on Generator Impedance
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Generator Basics
Power output from Generator
Active power kW Controlled by Governor
Reactive Power kVA Controlled by Exciter