Alternator Basics 1

1 © Wärtsilä May 3, 2023 Presentation name / Author

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WLSA Alternator Basic TheoryAlternator Basic Theory

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2 © Wärtsilä 19/01/2007 GKT/TS-TRAINING/ALTER BASICS

For generating electricity/induction we require

Magnet

Relative motion between the two

Coil

N S

FundamentalsALTERNATOR BASICS

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Bicycle Dynamo

Simplest & most basic form of alternator is bicycle dynamo

Rotor is a Two Pole Permanent Magnet

Voltage Speed

Voltage Strength of the magnet. (Preset, not adjustable)

Principle

Fundamentals

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Brush System

Advantages

Permanent magnet replaced by an electromagnet

Output voltage can be adjusted without changing prime mover speed by regulating DC supply to main field

Disadvantages

Two Carbon brushes are required to be used

Principle

Fundamentals

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Brushless Separate excitation System

Advantages

A small alternator named ‘Exciter’ is used to avoid the brushes

Main field, exciter armature & rectifier are mounted on same shaft

Excitation supply comes to exciter field instead of main field

DC power required for excitation is much lesser

Disadvantages

This is an open loop system & input to exciter does not vary for variation in output voltage

Principle

Fundamentals

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Self Excited & Regulated Shunt System

In order to give correction in the exciter input according to the voltage output, AVR are being used

Different types of AVR’s are being used in DG sets

Analog

Digital

Principle

Fundamentals

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MainArmature

MainField

ExciterArmature

RotatingRectifiers

ExciterField

SurgeSuppressor

Short CircuitBack Up CT’s

Rotor

FundamentalsUtility

PT

AVR

Droop CT

DCT SCC

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MainArmature

MainField

ExciterArmature

RotatingRectifiers

SurgeSuppressor

Short CircuitBack Up CT’s

Rotor

FundamentalsUtility

PT

AVR

Booster Unit

ExciterField

End BOOST

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StatorWinding

N

S

MainField

V

AC

Fundamentals

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Separate Excitation Brush System

ACDC

SlipRings

StatorWinding

V

MainField

S

N

+ -

Fundamentals

CarbonBrushes

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StatorWinding

MainFieldN

S

V

ACDC

ExciterRotatingRectifiers

+ -

Fundamentals

Separate Excitation Brushless System

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ACDC

RotatingRectifiers

Exciter

StatorWinding

MainFieldN

S

V

Fundamentals

Self Excitation Brushless System

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Function Of Droop CTFunction Of Droop CT

Droop CT is normally connected in the second ( V or Y or B) phase

Output of the CT secondary is connected to the AVR , and the AVR gets the load information through this input

Helps the AVR to maintain voltage as per the Droop curve in solo mode

Helps for load sharing according to droop curve during parallel mode

Generally disabled during solo mode by single/parallel selector switch/ relay

If enabled in solo mode causes generator output voltage to drop as per droop curve

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Droop CT

Caution about Droop CT connectionCaution about Droop CT connection

In Solo mode

Change in polarity results in increase of voltage with increase in load

In Parallel mode

Results in erratic Reactive load sharing

Power factor can not be maintained

Might result in cascade tripping due to over current because of unequal KVAr load sharing

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Short Circuit Back up CT

Function Of SC back up CT- External faultsFunction Of SC back up CT- External faults During an external faults the terminal voltage tends to reach

zero, which makes the fault current sensed by a protection relay also tend to zero, & this results in the defeat of the protection

To sustain the fault current during the time delay period set in the relay, the terminal voltage also should sustain & hence additional excitation power is needed

This additional power requirement might exceed the capacity of the AVR & might damage the AVR

Since the current during faults is very high, and the secondary output of the SCC CT is also high

AVR Circuit is so designed that during the fault conditions, the additional excitation power needed is supplied from the high output of the SCC CT, while the AVR supplies the minimum power

As a result, the AVR is protected and the voltage and hence the fault current are sustained to operate the protection relay

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FunctionFunction Of SC back up CT- Heavy Motor starts Of SC back up CT- Heavy Motor starts When a large Induction motor drawing a very large starting

current is started, the alternator, in addition to the existing base load, is forced to supply a large current , even though the terminal voltage of alternator does not tend to zero

This increased current demand and the very low lagging power factor during motor start, requires field forcing i.e increase in excitation power

The high secondary output of the SC back up CT, due to large primary current , is used to supply the field forcing power

Field forcing during motor starts also helps the voltage to recover much faster helping the motor to attain the rated speed

As the motor speed increases the back emf produced by the motor also increases and hence the current drawn by motor reduces, reducing the excitation power

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As the motor speed increases the back emf produced by the motor also increases and hence the current drawn by motor reduces, reducing the excitation power

Function Of SC back up CT- Heavy Motor starts Function Of SC back up CT- Heavy Motor starts

The net result is that during heavy motor starts or during application of heavy loads, the additional excitation power required for field

forcing is drawn from the high starting or load current in proportion through the SCC CT, and the power supplied by the AVR directly through it`s internal circuitry remains within its rated value

Generally the output of the SCC CT is connected to the excitation circuit by a voltage controlled relay, whose contacts maintain the secondary of the SCC shorted when the voltage of the alternator is above a preset value, normally 70%

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INTERNAL USE ONLY


Booster

In some of the excitation systems e.g. Leroy Somer RBS 6000,the field forcing by the SCC CTs is permanently connected to support

the output of the AVR, through a rectifier arrangement known as Booster

Booster circuitBooster circuit

Booster consists of a three phase bridge rectifier, a filter capacitor, and an adjustable bleeder resistor

The secondary output of the SCC CT is rectified and filtered to get a smooth DC output and connected to the field circuit in parallel with adjustable bleeder resistor. The bleeder resistor is adjusted in such a way that under normal operation, the excitation

The bleeder resistor is adjusted in such a way that under normal operation, the excitation power is supplied from the booster, and

voltage variations are corrected by the Pulse width modulated (PWM) output of the AVR

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Questions ??

Questions

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Alternator Basics 1

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