generator basics1

7/30/2019 Generator Basics1

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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


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


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

generator basics1

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